Dp08b Design of Process Heaters

ExxonMobil Proprietary F IRED IRED H EATERS EATERS

DESIGN OF PROCESS HEATERS DESIGN PRACTICES

Section

Page

VIII-B

1 of 79

December, 2000 Changes shown by ➧

CONTENTS Section

Page

SCOPE ............. ............................ ............................. ............................ ............................. ............................. ............................. ............................. ........................... ........................... .............................4 ...............4 REFERENCES ............. ........................... ............................. ............................. ............................ ............................. ............................. ............................. ............................ ........................... ................. ... 4 DEFINITIONS............... DEFINITIONS ............................. ............................. ............................. ............................ ............................. ............................. ............................. .......................... ......................... ................... .....4 4 DETERMINING HEATER EFFICIENCY.............. EFFICIENCY ............................. ............................. ............................. ............................. ............................ ............................. .................... .....4 4 TOTAL HEAT DUTY Q a ............. ........................... ............................. ............................. ............................. ............................. ............................. ............................. .....................4 .......4 STACK TEMPERATURE T s .............. ............................ ............................. ............................. ............................. ............................. ............................. .............................4 ..............4 DESIGN EXCESS AIR RATE ............ .......................... ........................... ........................... ........................... ........................... ........................... ......................... ....................4 ........4 HEAT AVAILABLE FROM FUEL.............. FUEL............................ ........................... ........................... ............................ ............................ ............................ ......................... ............. 5 NET FUEL REQUIREMENT Fn .............. ............................ ............................. ............................. ............................ ............................. ............................. ........................5 ..........5 GROSS FUEL REQUIREMENT Fg ............... ............................. ............................. ............................. ............................ ............................. ............................. ................. ... 5 HEAT FIRED Qf .............. ............................. ............................. ............................. ............................. ............................. ............................. ............................ .......................... ...................5 .......5 HEATER EFFICIENCY ELHV .............. ............................. ............................. ............................. ............................. ............................ ............................. ...........................5 ............5 LAYOUT OF RADIANT SECTION SECTION............. ........................... ............................. ............................. ............................. ............................. ............................. .............................6 ..............6 TUBE SIZE AND NUMBER OF PASSES ............. .......................... ........................... ........................... .......................... ........................... ........................... ............... .. 6 ECONOMICAL TUBE SIZE AND NUMBER OF PASSES ........... ........................ .......................... .......................... .......................... ....................6 .......6 DESIGN RADIANT HEAT DENSITY φr .............. ............................. ............................. ............................. ............................. ............................ ...........................6 .............6 CALCULATION OF TOTAL RADIANT SURFACE.................. SURFACE................................ ........................... ........................... ........................... .......................6 ..........6 TOTAL RADIANT HEAT DUTY Qtr ............. ............................ ............................. ............................. ............................. ............................ ............................. .................... .....7 7 RADIANT SECTION HEAT DUTY Qr .............. ............................ ............................. ............................. ............................. ............................. .............................7 ...............7 RADIANT SECTION SURFACE Ar ............. ............................ ............................. ............................. ............................. ............................ ............................. .................... .....7 7 RADIANT SECTION LAYOUT .............. ............................ ............................ ........................... ........................... ............................ ............................ .......................... ............... ... 7 NUMBER OF RADIANT SECTION TUBES N r .............. ............................ ............................. ............................. ............................. ............................. ................ 8 HORIZONTAL TUBE CABIN HEATER .............. ........................... ........................... ........................... ........................... ............................ ........................... ............... .. 10 VERTICAL-CYLINDRICAL VERTICAL-CYLINDRI CAL HEATER ............. .......................... ........................... ........................... ........................... ........................... ........................... ....................11 ......11 BURNER ARRANGEMENT .............. ........................... .......................... ........................... ........................... .......................... ........................... ........................... ....................11 .......11 VERTICAL TUBE BOX HEATER HEATER.............. ........................... ........................... ............................ ............................ ............................ ........................... .......................13 ..........13 HOOP TUBE CABIN HEATER........ HEATER..................... ........................... ............................ ........................... ........................... ........................... ........................... .....................16 .......16 PRESSURE DROP THROUGH THE COIL.............. COIL ............................ ............................. ............................. ............................. ............................. ...........................17 .............17 TUBE DESIGN ............. ........................... ............................. ............................. ............................ ............................. ............................. ............................. ............................ ........................... ................ 18 MATERIALS........................ MATERIALS......... ............................. ............................. ............................. ............................. ............................. ............................ ........................ ......................... .................. ... 18 DESIGN TEMPERATURE ............. ........................... ............................ ............................ ........................... ........................... ............................ ........................... ....................19 .......19 DESIGN PRESSURE............. PRESSURE.......................... ........................... ............................ ............................ ........................... ........................... ............................ .......................... ............... ... 20 TUBE WALL THICKNESS .............. ............................ ............................ ............................ ............................ ............................ ............................ .......................... .................. ......21 21

ExxonMobil Research and Engineering Company – Fairfax, VA

ExxonMobil Proprietary Section VIII-B

Page 2 of 79

F IRED IRED H EATERS EATERS


December, 2000

CONTENTS (Cont) Section

Page

INSTRUMENTATION............. INSTRUMENTATION ............................ ............................. ............................. ............................. ............................ ............................. ............................. ........................... .................. ..... 23 PROCESS FLUID FLUID............ .......................... ............................ ............................ ............................ ............................ ............................ ............................ .......................... ................... ....... 23 Flow .............. ............................ ............................. ............................. ............................ ............................. ............................. ............................. .......................... .......................... ..................... ...... 23 Temperature .............. ............................. ............................. ............................. .............................. ............................. ............................. ............................. ............................ .................. .... 23 Pressure .............. ............................. ............................. ............................. ............................. ............................ ............................. ............................. ......................... ......................... .............. 23 TUBE METAL TEMPERAT TEMPERATURE URE ............. ............................ ............................. ............................ ............................ ............................ ............................. ....................... ........ 23 FLUE GAS ............. ........................... ............................. ............................. ............................. ............................. ............................ ............................. ......................... ........................ ................ .. 23 FORCED-DRAFT SYSTEM.................. SYSTEM............................... ........................... ............................ ............................ ............................ ........................... ......................... .............. .. 24 Pressure Indicators.......................... Indicators........................................ ............................. ............................. ............................. .............................. .......................... ......................... .............. 24 FUEL SYSTEM.................. SYSTEM................................ ............................ ............................. ............................. ............................ ............................ ........................... ............................ ................. .. 24 STEAM-AIR DECOKING SYSTEM.... SYSTEM.................. ........................... ........................... ........................... ........................... ............................ ........................... ................. .... 24 SOOTBLOWERS......................... SOOTBLOWERS.......... ............................. ............................ ............................ ............................ ............................. ............................. ......................... .................... ......... 24 MECHANICAL SPECIFICATI SPECIFICATION ON.............. ............................ ............................. ............................. ............................. ............................. ............................. ............................ ............. 24 HOW THE REQUIREMENTS ARE COVERED ............ ......................... ........................... ........................... .......................... ........................... ................... ..... 24 TUBE GUIDES AND SUPPORTS ............ ......................... ........................... ........................... .......................... ........................... ........................... ......................... ............ 24 REFRACTORY .............. ............................. ............................. ............................ ............................ ............................ ............................. ............................. ........................ ................... ......... 25 FIREBOX PURGING ............. ........................... ............................ ............................ ............................ ........................... ........................... ............................ .......................... .............. .. 25 EMERGENCY COIL STEAM STEAM.............. ............................ ............................ ............................ ............................ ............................ ............................ .......................... .............. .. 27 MISCELLANEOUS DETAILS ............ .......................... ............................ ............................ ............................ ............................ ............................ ........................... ............... .. 27 SAMPLE PROBLEMS / CALCULATIONS - CUSTOMARY UNITS ............... .............................. ............................. ............................. ................. .. 29 PROBLEM 1 - HEATER EFFICIENCY ............. ........................... ........................... ........................... ............................ ............................ ........................... ............... .. 29 PROBLEM 2 - VERTICAL TUBE BOX HEATER LAYOUT.................................... LAYOUT.................................................. ........................... ................. .... 29 PROBLEM 3 - HOOP TUBE CABIN HEATER LAYOUT ............. ........................... ........................... .......................... ........................... ................. ... 33 PROBLEM 4 - TUBE METAL TEMPERATURE............................... TEMPERATURE............................................. ............................ ............................. ......................... .......... 39 SAMPLE PROBLEMS PROBLEMS / CALCULATIONS CALCULATIONS METRIC UNITS.............. UNITS............................ ............................. ............................. ............................. ............... 40 PROBLEM 1 - HEATER EFFICIENCY ............. ........................... ........................... ........................... ............................ ............................ ........................... ............... .. 40 PROBLEM 2 - VERTICAL TUBE BOX HEATER LAYOUT.................................... LAYOUT.................................................. ........................... ................. .... 40 PROBLEM 3 - HOOP TUBE CABIN HEATER LAYOUT ............ ......................... ........................... ............................ ........................... ................. .... 44 PROBLEM 4 - TUBE METAL TEMPERATURE .............. ............................ ............................ ............................ ............................ ........................... ............. 49 NOMENCLATURE............. NOMENCLATURE ........................... ............................. ............................. ............................ ............................. ............................. ............................. ............................ ...................... ......... 51 COMPUTER PROGRAMS............... PROGRAMS ............................. ............................. ............................. ............................ ............................. ............................. ............................. ....................... ........ 53 GUIDANCE AND CONSULTING......................... CONSULTING...................................... ........................... ........................... ........................... ........................... ......................... .............. .. 53 LITERATURE .............. ............................ ............................ ............................ ............................ ............................. ............................. ............................ ......................... ...................... ........... 53 AVAILABLE PROGRAMS.......... PROGRAMS........................ ............................ ............................ ............................ ............................ ............................ ............................ ....................... ......... 53



DESIGN OF PROCESS HEATERS

Section

Page

VIII-B

DESIGN PRACTICES

3 of 79

December, 2000

CONTENTS (Cont) Section TABLES Table 1 Table Tabl e2 Table Ta ble 3 Table Ta ble 4 Table Tabl e5 Table Tabl e6

FIGURES Figu igure re 1 Figure Fig ure 2 Figure Fig ure 3 Figure Fig ure 4 Figure 5 Figure Figure Fig ure 6 Figure 7 Figure Fig ure 8 Figure 9A Figure 9B Figure 10 Figure 11 Figure 12 Figure 13 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19

Page

Design Desig n Condit Conditions ions for Proce Process ss Heate Heaters...... rs........... .......... .......... .......... .......... .......... .......... ......... ........ ......... .......... ......... ........ ......... ....... 54 Common Heate Common Heaterr Tube Size Sizes s and Prope Properties....... rties........... ......... .......... .......... .......... ......... ........ ......... ......... ......... .......... .......... ....... .. 55 Allowa All owable ble Elas Elastic tic and Cree Creep p Ruptu Rupture re Stres Stress s for for Typic Typical al Heate Heaterr Tube Tube Mate Materia rials ls ... ...... ...... .....56 ..56 Coeffi Coe fficie cient nt of Therm Thermal al Expan Expansio sion n for Typic Typical al Heater Heater Tube Tube Mater Material ials s ... ...... ...... ...... ...... ...... ...... ...... ...... ..... 57 Modulus Modu lus of Elas Elasticit ticity y for Typi Typical cal Heat Heater er Tube Mate Materials...... rials........... .......... ......... ......... .......... .......... ......... ......... .........58 ....58 Minimum Mini mum Yield Stren Strengths gths for Typi Typical cal Heate Heaterr Tube Tube Mate Material... rial....... ......... ......... ......... .......... ......... ......... .......... ........59 ...59

Average Averag e Radiant Radiant Heat Heat Densit Density y for Cabin Cabin and Verti Vertical cal-Cy -Cylin lindri drical cal Heate Heaters... rs...... ...... ...... ...... ...... .....60 ..60 Averag Av erage e Radi Radiant ant Hea Heatt Dens Densit ity y for for Cab Cabin in Hea Heater ters s With Uns Unshie hielde lded d Cent Center er Refractory Wall ............. ........................... ............................. ............................. ............................. ............................. ............................ .........................61 ...........61 Averag Av erage e Radiant Radiant Hea Heatt Densi Density ty for for Verti Vertical cal Tubes Tubes Box Hea Heater ters s (Conta (Containi ining ng Both Both One-Side and Two-Side Fired Tubes) .............. ............................. .............................. .............................. ...............................62 ................62 Averag Av erage e Radian Radiantt Heat Heat Densi Density ty for for Verti Vertical cal Tub Tubes es Box Box Heat Heaters ers (Tw (Two-S o-Side ide Fire Fired d Tubes Only)........................... Only)......................................... ............................. .............................. ............................. ............................. ..............................63 ...............63 Avera Av erage ge Radian Radiantt Heat Heat Densi Density ty for for Hoop-T Hoop-Tube ube Cab Cabin in Heater Heaters s ... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ..... 64 Avera Av erage ge Radian Radiantt Heat Dens Density ity for for Hor Horizo izonta ntall Tube Tube Box Heat Heater er ... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... .....65 ..65 Cabin Heate Heaterr Pass Pass Arrang Arrangement ements s .... ......... .......... .......... ......... ......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... ........66 ...66 Approx App roxima imate te Tube Tube Lengths Lengths for for Horizont Horizontal al Tube Tube Cabin Heater Heaters s ... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ......67 ...67 Approximate Approx imate Tube Lengt Lengths hs for for VerticalVertical-Cyli Cylindric ndrical al Heaters Heaters (Engl (English ish Units)...... Units)........... ......... .......68 ...68 Approximate Approx imate Tube Length Lengths s for Verti Vertical-C cal-Cylin ylindrica dricall Heaters Heaters (Metric (Metric Units Units)) ..... .......... ......... ......... ....... 69 Typical Typi cal Layout Vertical-Cy Vertical-Cylindri lindrical cal Heater............ Heater................. .......... .......... ......... ........ ......... .......... .......... .......... .......... .......... .........70 ....70 Typical Typi cal Pass Pass Arrangemen Arrangements ts for for Vertical Vertical Tube Tube Box Heater Heaters....... s............ .......... .......... .......... ......... ........ ......... ........71 ...71 Typical Typi cal Tube Tube Guide Guide Details Details for Vertic Vertical al Tube Tube Box Heater .... ......... .......... .......... .......... ......... ......... .......... .......... .......72 ..72 Typical Typi cal Layout Layout for Vertic Vertical al Tube Box Box Heater Heater .... ......... .......... .......... .......... ......... ......... .......... .......... .......... .......... .......... ......... .......73 ...73 Typical Typi cal Layout for Hoop-Tube Heater...... Heater........... .......... .......... .......... .......... ......... ........ ......... .......... ......... ........ ......... .......... ......... ........ ...... 74 Linear Thermal Thermal Expansi Expansion on of Various Various Steels....... Steels............ .......... .......... ......... ........ ......... ......... ........ ......... ......... ......... .......... ........75 ...75 Thermal Therm al Conductivity Conductivity of Various Various Steels......... Steels.............. .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .........76 ....76 Typical Typi cal Tube Guides Guides for for Hoop-Tube Hoop-Tube Heater........ Heater............. .......... .......... .......... .......... .......... ......... ......... .......... .......... .......... .........77 ....77 Corrosion Corros ion Frac Fraction...... tion........... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... ......... ......... .......... .......... ......... ...... 78 Rupture Ruptu re Exp Exponent. onent..... ........ ......... .......... .......... .......... .......... .......... ......... ......... .......... .......... .......... .......... .......... .......... .......... ......... ......... .......... .......... .........79 ....79

Revision Memo 12/00

The highlights of this revision are: Editorial and typographical corrections. corrections. Corrected Eq. 22 and equation equation for nozzle area for stack eductor.


ExxonMobil Proprietary Section

F IRED H EATERS

Page

VIII-B

4 of 79


December, 2000

SCOPE This section gives the calculation procedures to be used for designing all process heaters. Separate procedures required for air preheaters are handled separately in Section VIII-K. Convection section and stack design procedures are given in Section VIII-C. Forced-draft systems, burners, and manifolding and other components that may go into the total specification are handled in other sections. No special distinction is given to auxiliary services, such as steam superheaters, contained in the furnace. These duties are considered to be part of the overall heater duty. However, some of the specific procedures must sometimes be modified to fit the requirements of the particular situation.

REFERENCES DESIGN PRACTICES (BESIDES OTHER SECTIONS OF THIS SECTION) Section XII

Instrumentation

Section XIV

Fluid Flow

Section XV

Safety in Plant Design

INTERNATIONAL PRACTICES IP 3-4-1

Piping for Fired Equipment

IP 7-1-1

Fired Heaters

IP 15-1-1

Instrumentation for Fired Heaters

IP 18-3-2

Statically Cast Steel and Alloy Pressure Containing Parts, and Tube Supports for Fired Heaters

IP 19-3-3

Linings for Fired Heaters

OTHER LITERATURE 1.

EMRE Blue Book , Manual Numbers 012 and 011.

2.

EMRE Refinery Construction Materials Manual , Manual No. EETD 028.

3.

API 530, Calculation of Heater Tube Thickness in Petroleum Refineries.

4.

ASME Standard B31.3, Process Piping.

DEFINITIONS See Design Practice VIII-A, Fired Process Heaters

DETERMINING HEATER EFFICIENCY TOTAL HEAT DUTY Qa From the process requirements, determine the total heat duty. This must include all flexibility requirements. An allowance need not be added to the heater duty to compensate for preheat exchanger fouling, since this is normally covered by the exchanger fouling factors. Qa = Total heat absorbed by the heater, Btu/hr (MW)

STACK TEMPERATURE TS Procedures for determining economical stack temperature are given in Section VIII-A.

DESIGN EXCESS AIR RATE As discussed in Section VIII-A, the design excess air is based on the type of combustion system and the fuel:

• •

Use 15% excess air for forced-draft firing of all fuels and natural draft firing of gas fuels. Use 20% excess air for natural draft oil or combination gas/oil firing.


ExxonMobil Proprietary F IRED H EATERS

Section

DESIGN OF PROCESS HEATERS

Page

VIII-B

DESIGN PRACTICES

5 of 79

December, 2000

DETERMINING HEATER EFFICIENCY (Cont) HEAT AVAILABLE FROM FUEL Heat available from the fuel fired can be determined from the flue gas enthalpy (heat available, HA) curve for the design excess air rate in Figures 4 to 10 of Section VIII-M. The ordinate of these charts (heat available, Btu/lb (MJ/kg) of fuel fired) represents the amount of heat that has been extracted from the flue gas (and absorbed by the process fluid) when the flue gas temperature has been reduced to that shown on the abscissa. The total heat obtained from the fuel is the value read from the chart if the temperature on the abscissa is the stack temperature. The particular heat available chart selected should be the one for a fuel which most closely resembles the design fuel. There is no significant difference between calculations based on different fuels of the same type. Heaters designed for combination gas/oil firing should be based on oil firing. Once a chart is selected, all calculations must be based on heating values and flue gas rates consistent with the selected fuel. For unusual fuels, the heat available curve should be calculated. Computer Program No. 3558 may be used for this purpose. The heat available curves in Section VIII-M are based on combustion air temperatures of 60 °F (15°C), for heater applications involving preheated combustion air, these curves must be adjusted to include the additional heat content of the combustion air.

NET FUEL REQUIREMENT Fn Fn

=

Qa

Eq. (1)

(HA )s

where: Fn Qa (HA)s

= = =

Net fuel required, lb/hr (kg/s) Total heat absorbed in heater, Btu/hr (MW) Heat available at stack temperature, Btu/lb of fuel (MJ/kg of fuel)

Net fuel is used for all flue gas enthalpy calculations in the furnace.

GROSS FUEL REQUIREMENT Fg Fg

=

1.01 Fn for heaters larger than 100 MBtu/hr (29 MW) heat absorbed

Eq. (2a)

=

1.02 Fn for heaters between 15 and 100 MBtu/hr (4 and 29 MW)

Eq. (2b)

=

1.03 Fn for heaters smaller than 15 MBtu/hr (4 MW)

Eq. (2c)

where: Fg

=

Gross fuel requirement, lb/hr (kg/s)

Gross fuel is used to determine total heat fired, efficiency, combustion air requirements, stack gas velocity, and flue gas mass velocity and pressure drop. The difference between net fuel and gross fuel accounts for radiation and other heat losses from the heater (excluding stack losses).

HEAT FIRED Qf Q f

= Fg

x LHV

where: Qf = LHV =

Eq. (3) Heat fired, Btu/hr Fuel lower heating value, Btu/lb (Figures 4 to 10 of Section VIII-M)

By convention, lower heating value is always used in all heater and burner calculations.

HEATER EFFICIENCY ELHV ELHV

=

100 Qa Q f

where: ELHV=

Eq. (4)

Thermal efficiency of heater, based on lower heater value, percent



F IRED H EATERS

Page 6 of 79


December, 2000

LAYOUT OF RADIANT SECTION TUBE SIZE AND NUMBER OF PASSES Mass Velocity G - As discussed in Section VIII-A, adequate flow rate inside the heater tubes is necessary to develop a satisfactory film coefficient so that heat transfer from the tube wall to the fluid is obtained with a reasonable differential temperature across the film. Table 1 gives suggested design fluid mass velocities for different services. Since the heater throughput (lb/sec, kg/s) is determined by the process requirements, the total tube internal cross-sectional area required for the flowing fluid is the throughput divided by the mass velocity. This cross-sectional area determines the inside diameters of the tubes and the number of parallel passes through the radiant section (and usually also through the convection section): G =

W p A x

where: G W p Ax

Eq. (5)

= = = =

Fluid mass velocity through the coil, lb/sec ft 2 (kg/s m2) Flow rate through the heater, lb/sec (kg/s) Number of parallel passes Cross-sectional flow area through one tube, ft2 (m2)

Available Tube Sizes - Wherever possible, tube diameters should be selected from standard nominal pipe sizes in the range of 2 to 8 in. (50 to 200 mm). These sizes are listed in Section XIV-A, Fluid Flow . Non-standard sizes should be used only on special occasions, when design parameters cannot be met with standard sizes. In such cases, tubing can be obtained in 1/8 in. (3 mm) (or sometimes even smaller) increments of outside diameter, with 5.0, 6.0, and 7.625 in. O.D. (127, 152, and 194 mm O.D.) being some of the more common sizes. Most heaters are designed using 4 to 8 in. (100 to 200 mm) tubes. These tubes, with the most commonly specified wall thicknesses, are listed in Table 2. Unless experience indicates otherwise, a first assumption of a 0.285 in. (7.24 mm) minimum wall thickness should be made for alloy radiant section tubes. Note that the wall thickness for carbon steel should be specified as standard schedule sizes, whereas alloy tubes should be specified for the actual minimum wall thickness required. Table 2 merely lists the most commonly used sizes for the convenience of the designer.

ECONOMICAL TUBE SIZE AND NUMBER OF PASSES 1.

The most economical size tubes to use are 4, 5, and 6 in. (100, 125, and 150 mm) pipe sizes. In some cases, to obtain the required mass velocities, it may be necessary to use smaller sizes, but more than one pass of a smaller size should usually be avoided.

2.

In vaporizing or all-liquid services, the difficulty of uniformly distributing flow to multiple passes increases with the number of passes. Therefore, the number of passes should be minimized, consistent with the heater arrangement. This tends to favor selection of larger size tubes. The same number of passes should be maintained throughout the heater.

3.

In all-vapor services, even distribution of flow to individual passes is obtained by proper manifold design (see Section VIII-J, Fired Heater Manifolds). Therefore, the selection of tube size and number of passes should be based on layout considerations. A different number of passes and different tube sizes can be used for the radiant and convection sections, since the convection section outlets can be combined and then redistributed at the radiant section inlets.

DESIGN RADIANT HEAT DENSITY φr Process Considerations - As discussed in Section VIII-A, the permissible radiant heat density from process considerations is a function of several factors which include heater geometry, feed stock, service, and oil outlet temperature. Maximum allowable heat densities are, therefore, established by experience, as well as from theoretical considerations. Table 1 lists are recommended average radiant heat densities for most services. These heat densities, along with other recommended heater design criteria, will insure satisfactory maximum heat densities. Mechanical Considerations - Using average radiant heat densities dictated by process conditions will usually result in flue gas temperatures of about 1600 to 1800°F (870 to 980°C) leaving the radiant section (bridgewall temperature). At this temperature level, good mechanical service can be expected from conventional tube supports and refractory materials. A lower radiant heat density should be used if the bridgewall temperature exceeds 1800 °F (980°C).

CALCULATION OF TOTAL RADIANT SURFACE Bridgewall Temperature Tbw - The “bridgewall temperature” is the temperature of the flue gas leaving the radiant section. Since the shield tubes “see” the radiant section, they absorb part of the total radiant heat to be transferred. This shield radiant duty is included in the heat absorbed in cooling the flue gas down to the bridgewall temperature.




Section VIII-B

Page 7 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) The bridgewall temperature is primarily a function of average radiant heat density, average radiant tube metal temperatures and the basic shape of the heater. Figures 1 to 5 present bridgewall temperature as a function of these variables for the various types of fired heaters designed by ExxonMobil Engineering. Figure 6 presents bridgewall temperature information for horizontal tube box heaters which, although n o longer specified for new heater applications, are still in use at many locations. In reality bridgewall temperature is also a function of fuel type, excess air level, combustion air temperature, and radiant tube spacing. However, for typical heater designs the effect of these variables is small and can be ignored for design purposes. On the other hand, if any of these variables are outside the recommended limits summarized below, the Heat Transfer Equipment Section of EETD should be contacted so appropriate bridgewall temperature correction factors can be developed and applied. Fuel Type - Figures 1 to 6 are only valid for typical refinery fuel gases and fuel oils. Unusual fuels with higher or lower than normal adiabatic flame temperatures may require adjustment of the bridgewall temperature curves. Excess Air - Figures 1 to 6 are valid for excess air levels from 15 to 50%. Combustion Air Temperature - Figures 1 to 6 are valid for combustion air temperatures from ambient up to about 850 °F (450°C). Radiant Tube Spacing - Figures 1 to 6 are based on the assumption that the majority of the radiant tubes will have a tube spacing of two times the n ominal diameter. When starting a design, assume that the average radiant tube metal temperature is about 100 °F (50°C) above the average bulk fluid temperature. The “average” radiant tube metal temperature is usually based on “uncoked” radiant tubes. Corrections listed on these figures must be made to the temperature read from the curves, since the bridgewall temperature depends on heater height. This requires a trial-and-error solution to determine final bridgewall temperature, radiant heat density, tube surface, etc.

TOTAL RADIANT HEAT DUTY Qtr Q tr

= (HA )bw Fn

Eq. (6)

where: Qtr = (HA)bw = Fn =

Total radiant section heat duty (including shield section radiant duty), Btu/hr (MW) Heat available at bridgewall temperature, Btu/lb of fuel (MJ/kg of fuel) Net fuel required, lb/hr (kg/s) [see Eq. (1)]

n

Shield Radiant Heat Duty Qsr - This is the radiant heat absorbed by the shield section Btu/hr (MW). Calculation procedures for shield radiant duty are found in Section VIII-C, Design of Convection Sections and Stacks.

RADIANT SECTION HEAT DUTY Qr Qr

= Q tr − Q sr

where: Qr

=

Eq. (7) Heat absorbed by the radiant section surface, Btu/hr (MW)

RADIANT SECTION SURFACE Ar Ar

=

Qr

Eq. (8)

φr

where: Ar

φr

= =

Radiant section tube surface, ft 2 (m2) Radiant section average heat density, Btu/hr ft2 (W/m2)

RADIANT SECTION LAYOUT General - The radiant section layout is developed from a number of requirements, as outlined below. Since these are interrelated to some extent, a trial layout must first be developed and then modified as the design progresses. The radiant section layout must provide sufficient space for mounting burners and installing the required heat transfer surface. The required clearances between burners and tubes (see Burner Clearances, below) determine the minimum tube envelope around the burners. In many heaters, this envelope is governing, and either the required tubes must be spread out on this envelope or more tubes must be added to the minimum number required.



F IRED H EATERS

Page 8 of 79


December, 2000

LAYOUT OF RADIANT SECTION (Cont) The tube size and number of passes are selected to give the desired mass velocity. The number of passes must be consistent with the heater type, so that each pass receives the same amount of heat. While vertical cylindrical heaters can be designed for almost any number of passes, cabin heaters usually require an even number of passes so that they can be symmetrically arranged in the heater. The convection section layout must be determined before the radiant section can be finished, since the length of each section depends upon the other. Also, the radiant duty in the shield section depends upon the size of the convection section. Therefore, design of the convection section (see Section VIII-C) must be done concurrently with design of the radiant section. The following general layout criteria should be used with all heater configurations (see sketch below): 1.

Centerline-to-centerline spacing of adjacent radiant tubes should be two times the nominal diameter (short radius U-bend).

2.

Radiant wall tube centerline should be located a distance of 1-1/2 time the nominal diameter away from the wall.

3.

Corner tubes in the radiant section should be located so that there is no “dead” tube, which is partially shielded by adjacent tubes and receives substantially less than the average amount of heat. See the sketch below:

Avoid "Dead" Tube

2 x IPS

1 1/2 x IPS Preferred

Avoid

DP8BFa

4.

To assure adequate visibility from radiant section observation doors, tube spacing at these doors shall be 3 times the nominal diameter (long radius U-bend).

5.

Compatibility with the layout of inlet distribution and outlet collection piping should be kept in mind when the tube layout is being set.

Tube Lengths - The proper choice of tube length has a marked effect on the cost of any heater being designed. Guidelines for selecting economical tube lengths are presented below for cabin and vertical-cylindrical heaters. The particular lengths given are approximations and can vary in individual cases. For vertical tube box heaters, the number of tubes and layout are usually developed first, with the tube length being determined afterwards. Maximum tube lengths for convection sections or for horizontal radiant sections should be limited to about 100 ft (30 m), because of the difficulty in handling longer tubes. Maximum lengths of vertical tubes should be limited to 45 ft (13.7 m) (preferably shorter), because the vertical maldistribution of heat input is excessive in longer tubes.

NUMBER OF RADIANT SECTION TUBES Nr N r

=

A r

Eq. (9)

A o Lr

where: Nr Ao Do Lr

= = = =

Number of tubes in the radiant section Tube outside surface, ft2/ft = 0.262 D o (m2/m = π Do x 10-3)(see Table 2) Tube outside diameter, in. (mm) Radiant tube effective length, ft (m). This is the exposed tube length for heat transfer and does not include return bends. While return bends located inside the firebox will actually absorb some heat, EMRE practice is not to count bends in the effective heat transfer surface of process heaters. Note that for tubes with return bends located in header boxes, the actual length of the tubes will be longer than the effective length.



Section

Page

VIII-B


9 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) This calculated number of tubes should be adjusted so that there are an equal number in each pass. Each pass must also have a number of tubes consistent with the type of heater being designed, as well as with the locations of inlets and outlets (e.g., a top inlet and bottom outlet from the radiant section requires an odd number of tubes per pass). Note that in special cases, it may be desirable to use an unequal number of tubes per pass in order to compensate for a severe maldistribution of heat input to the passes. Consideration must be given to flow control in such cases, since pressure drops in the passes will be unequal. For heater designs where the number of tubes is known, based on layout, etc. (e.g., vertical tube box), Eq. (9) is used to determine tube lengths. Burner Clearances - Listed below are minimum clearances that should be provided around the burners. 1.

Natural Draft For natural draft operation with burners firing vertically or horizontally, the following minimum clearances shall be provided:

DISTANCE in ft (mm) MAXIMUM HEAT RELEASE PER BURNER MBtu/hr (MW)

A Burner Exit to Centerline of Roof Tubes or Refractory (Vertical Firing Only)

B Burner Centerline to Centerline of Tubes

C Burner Centerline to Unshielded Refractory

D Between Opposing Burners (Horizontal Firing Only)

OIL

4 (1.2) 6 (1.7) 8 (2.3) 10 (2.9)

12 ft (3700) 16 ft (4900) 20 ft (6100) 24 ft (7300)

3 ft, 3 in. (1000) 3 ft, 9 in. (1100) 4 ft, 3 in. (1300) 4 ft, 9 in. (1400)

2 ft, 0 in. (600) 2 ft, 6 in. (800) 3 ft, 0 in. (900) 3 ft, 6 in. (1100)

16 ft (4900) 22 ft (6700) 28 ft (8500) 32 ft (8700)

GAS

2 (0.6) 4 (1.2) 6 (1.7) 8 (2.3) 10 (2.9) 12 (3.5) 14 (4.1)

7 ft (2100) 10 ft (3100) 13 ft (4000) 16 ft (4900) 19 ft (5800) 22 ft (6700) 25 ft (7600)

2 ft, 3 in. (700) 2 ft, 9 in. (850) 3 ft, 3 in. (1000) 3 ft, 9 in. (1100) 4 ft, 3 in. (1300) 4 ft, 9 in. (1400) 5 ft, 3 in. (1600)

1 ft, 6 in. (500) 2 ft, 0 in. (600) 2 ft, 6 in. (800) 3 ft, 0 in. (900) 3 ft, 6 in. (1100) 4 ft, 0 in. (1200) 4 ft, 6 in. (1400)

8 ft (2500) 12 ft (3700) 16 ft (4900) 20 ft (6100) 24 ft (7300) 26 ft (7900) 28 ft (8500)

Notes:

2.

(1)

For horizontal firing, the distance between the burner centerline and the closest roof or sloped arch tube centerline or refractory shall be 50 percent greater than the distances in Column B.

(2)

For combination liquid and gas burners, the clearances will be based on liquid fuel firing, except when liquid fuel is used for start-up only.

(3)

This table applies to burners capable of natural draft operation, even though they may often operate with a small forced and/or induced draft.

(4)

Clearances for liquid fuel service govern for combination gas/oil services, or where provisions are made for future oil firing.

(5)

No minimum burner-to-burner clearances are set. Allow at least 1 in. (25 mm) between adjacent burners. If a plenum chamber is to be used for muffling combustion noise (the usual case), increased spacing is required mainly to ensure proper air distribution around the burner. See Section VIII-F, Burners.

Forced Draft Upfiring a.

The following clearances are required for typical large, forced-draft burners. BURNER HEAT RELEASE MBtu/hr (MW)

b.

MINIMUM DISTANCE FROM BURNER CENTERLINE FOR BOTH GAS AND OIL FIRING TO: Centerline of Radiant Tubes

Unshielded Refractory Walls

Centerline of Adjacent Burners

20 (5.9)

4 ft, 6 in. (1370 mm)

3 ft, 6 in. (1070 mm)

4 ft, 6 in. (1370 mm)

30 (8.8)

5 ft, 0 in. (1520 mm)

4 ft, 0 in. (1220 mm)

5 ft, 0 in. (1520 mm)

40 (11.7)

5 ft, 8 in. (1730 mm)

4 ft, 8 in. (1420 mm)

5 ft, 8 in. (1730 mm)

Provide vertical clearances between burner and refractory, tubes, stack entrance, etc. in line of the burner throw. Consult burner manufacturer for clearances of a particular burner model. Typirovisions are made for future oil firing.



Page

Section VIII-B

10 of 79


December, 2000

LAYOUT OF RADIANT SECTION (Cont) HORIZONTAL TUBE CABIN HEATER 1.

Number of Passes and Arrangements a.

Typical pass arrangements are shown in Figure 7. Combinations of these arrangements can be used for more than 4 passes (Figures 7A and 7D).

b.

Use Figure 7C for severe coking services (vacuum pipestill heaters), rather than Figure 7A. The use of jump-over return fittings are very expensive and should not be used unless they are necessary.

2.

Tube Length - Figure 8 may be used as a guide in choosing approximate radiant tube effective length for the typical horizontal tube cabin heaters shown in Figure 7. This approximate length may require adjustment, based on the number of tubes and passes, burner clearances, etc. For most applications, the actual length used can be expected to be 80 to 100% of this approximate length.

3.

Heater Length - The locations of return bends will affect the actual length of radiant and convection tubes. The possible arrangements are shown below: (a)

(b)

Convection Section

Radiant Section

DP8BFb

4.

a.

Sketch (a) is preferred. The effective length of the convection tubes is approximately 3 ft (0.9 m) longer than that of the radiant tubes. Clearance for thermal expansion must be provided between radiant section return bends and the end wall. Clearances must also be provided between convection section return bends and the header box, but this affects only the detailed mechanical design.

b.

Sketch (b) should be used when radiant section return bends with removable plugs (plug headers) are used. Radiant and convection tubes have the same effective length in this case.

c.

Convection section return bends should always be located in header boxes to prevent flue gas by passing around the end of the convection section.

Radiant Section and Burner Layout a.

Assume a single row of burners (one row in each cell for Figure 7D).

b.

Select burner to tube clearances as previously stated. For the preliminary layout, assume a burner to tube center-tocenter spacing of 51 in. (1300 mm) for natural draft and 60 in. (1520 mm) for forced draft.

c.

Leave a 1 ft (300 mm) clearance between the outside diameter of the lowest radiant wall tube and the floor.

d.

Space tubes (center-to-center) on two nominal diameters (2 times IPS).

e.

Hip sections (transition between radiant and convection sections) should be assumed to be at a 45-degree angle.

f.

Determine the number of burners required, based on the minimum clearances previously stated. Determine if burners can be physically laid out in a single row as assumed. If burners cannot be placed in single row:

g.

i.

For natural draft, use a staggered row (triangular pitch) or double row of burners and increase the width of the radiant sections accordingly.

ii.

For forced draft, revise the layout to fit.

The height/width ratio of the radiant section in general should not exceed 3.5:1 unless necessary to comply with plot space or specific burner flame length requirements. Height should be taken as the distance between the floor and the centerline of the uppermost tube in the vertical wall section. Width should be taken as the tube centerline spacing across the firebox.




Section

Page

VIII-B

11 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) VERTICAL-CYLINDRICAL HEATER 1.

General - Most vertical-cylindrical heaters will be designed with horizontal tube convection sections. Heaters should not be designed with integral convection sections. (See Section VIII-A, Figure 1B.)

2.

Number of Passes and Arrangement - Any number of passes can be used in vertical-cylindrical furnaces, since the radiant section layout is always symmetrical. However, certain number of passes (1, 2, 4 or 8) tend to result in simpler outlet piping. Other arrangements may require an expensive ring manifold, not otherwise required, to collect the heater outlets and should be avoided unless process considerations dictate their use.

3.

Radiant Tube Length - Figure 9 may be used to determine the approximate range of possible tube lengths. Consistent with the criteria listed below, economical radiant tube lengths are usually the longest possible. The final length will only rarely fall below the indicated range (Lr / Dt < 1.6). To avoid excessive longitudinal maldistribution of heat input, maximum tube length should normally be 35 to 40 ft (10.7 to 12.2 m). In no case should tubes longer than 45 ft (13.7 m) be used.

4.

Radiant Section Layout a.

The circumference of the tube circle is determined by multiplying the number of tubes by the center-to-center tube spacing.

b.

The ratio of tube length to tube circle diameter (Lr / Dt) should be no greater than 2.6 and no smaller than 1.2.

c.

Check burner clearances as developed below.

d.

If necessary, the radiant layout can be revised by the following: i.

Increase the number of tubes. This decreases L r and increases D t. Thus, Lr / D t is reduced, and burner-to-tube clearances are increased. However, clearance in line of burner throw is also reduced and may in turn require smaller burners.

ii.

Increase the spacing between radiant tube passes. This reduces Lr / Dt and increases burner-to-tube clearances, but does not decrease burner throw clearance. Since this approach causes a far greater increase in heater cost than that from increasing the number of tubes, it should not be used until these possibilities are exhausted.

e.

The welded return bends are normally located inside the firebox and the overall height of the box must allow for thermal expansion of the coil.

f.

The radiant coil may be either top-supported and guided at the bottom or bottom-supported and guided at the top. With an even number of radiant tubes per pass, the outlet will be at the top of the radiant section (since the inlet from the convection section is at the top). In this case, the radiant coil should usually be hung to eliminate vertical growth at the heater outlet nozzle. Conversely, with an odd number of radiant tubes per pass, the coil should normally be supported from floor level, since the outlet nozzle is at the bottom of the heater. Convection to radiant crossover designs is less difficult with top supported coils, since the crossover requires much less flexibility. However, both arrangements are routinely done successfully. Consideration should be given to the likely arrangement of the outlet manifold and transfer line since these are also affected by locations of supports and tube outlets.

g.

Crossovers should normally be located outside the heater rather than inside. This permits better support and permits increased flexibility, thereby avoiding potential mechanical problems. This also provides a place for temperature indicators between the radiant and convection sections.

BURNER ARRANGEMENT a.

Select the number and size of burners. The minimum clearances specified above must be maintained. When possible, the number of burners should also be a multiple of the number of passes to provide symmetry.

b.

Avoid using two burners in a vertical-cylindrical heater. The use of two burners produces unsymmetrical heat patterns and may result in poor operations.

c.

The use of only one burner requires that the heater be shut down every time the burner must be cleaned or otherwise maintained. Therefore, the use of three small burners is preferable to one large burner in heaters in continuous services.

d.

The burners are normally laid out in one burner circle.


ExxonMobil Proprietary Page

Section VIII-B

12 of 79

F IRED H EATERS


December, 2000

LAYOUT OF RADIANT SECTION (Cont) 6.

7.

8.

9.

Convection Section Layout - In addition to the general section on convection section design, the following are specific to the vertical-cylindrical heater (see Figure 10): a.

The convection section length Lc and width W c depend upon the radiant tube circle diameter. Therefore, they must be worked out in conjunction (trial and error) with the radiant layout.

b.

For the preliminary layout, assume Lc = 0.9 Dt. Final design lengths will usually fall in the range of 0.8 Dt to 1.0 Dt.

c.

The convection section box is supported by the radiant section cylindrical shell and supporting structure. The inside four corners of the convection section refractory should be located just above the inside of the radiant section refractory wall. This then sets the convection tube effective length as well as convection section width, since the combination of both is a function of radiant section diameter.

d.

The convection section header boxes usually extend beyond the radiant shell.

e.

In the rare case where the radiant tubes have cleanout plugs, the convection tubes (including headers and header boxes) cannot extend beyond the radiant tube circle diameter, since access must be provided for cleaning of the radiant tubes.

All-Radiant Heaters a.

For an all-radiant type heater, no means are provided for cooling flue gases leaving the radiant section. The stack temperature is the bridgewall temperature. Stack temperature and efficiency depend solely on the choice of radiant heat density.

b.

Radiant surface is obtained directly by dividing the heater duty by the heat density.

c.

All-radiant vertical tube heaters usually have an even number of tubes per pass, with the coil bottom-supported.

Very Small Heaters - Meeting normal established design criteria is often difficult with the design o f very small heaters [less than about 10 MBtu/hr (3 MW) heat duty]. The following should be considered: a.

These heaters are usually all-radiant. However, a convection section may be economical, particularly with only gas fuel.

b.

Higher-than-normal allowances should be provided for radiation and other heat losses. Multiply the net fuel by 1.03 to determine the gross fuel required.

c.

The minimum height of the radiant section should be about 15 ft (4.6 m).

d.

Minimum burner-to-tube clearances should be maintained.

e.

Increase the spacing between passes (as previously discussed) and/or decrease the heat density, as necessary. Since the heater shell dimensions will largely be determined by minimum burner clearances, decreasing heat density has the added b enefit of increasing heater efficiency.

f.

Helical coils are sometimes used in small heaters, instead of the usual serpentine coil. i.

The coil is supported at three points on each turn. Each support usually consists of a high alloy pipe extending the entire height of the radiant section, with the coil supported from this pipe by means of U-bolts.

ii.

No return bends are required for the coil, reducing heater cost. Pressure drop through the coil is also reduced by the elimination of return bends. Pressure drop is approximately 150% of that of a straight tube of the same length.

iii.

A maximum of two tube passes should be used.

Multi-Service Vertical-Cylindrical (V.C.) Heaters - The radiant section of V.C. heaters can be divided into two or three separate, but compatible, services. However, when possible, this arrangement should be avoided. a.

A refractory brick wall can be used to divide the radiant section into separate zones. Although each zone has its own firing controls, the heat input is influenced to some extent by the other zones. Since the maximum height of the free standing interior walls is about 25 ft (7.6 m), this influence will be substantial in tall heaters. See the discussion on HOOP-TUBE CABIN HEATERS for more details on the dividing wall.

b.

Separate services can also be installed in the same radiant section, but without the interior wall. However, this arrangement can be used only in special cases, since there is no way of varying the relative heat input to the individual services once the amount of surface in each service has been selected.




Section VIII-B

Page 13 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) VERTICAL TUBE BOX HEATER Note:

1.

2.

3.

The vertical tube box heater is an ExxonMobil proprietary design and has substantial advantages over the design of our competitors. To protect this proprietary technology, care must be taken to ensure that only contractors and heater vendors who have signed appropriate confidentiality agreements are given information on the vertical tube box heater.

General a.

Vertical tube box heaters use a combination of one-side fired tubes along the end and side walls of the radiant box, plus two-side fired tubes in rows across the box. The two-side fired (or center) tubes divide the box into a number of cells.

b.

Since equal heat input per pass is desired, each pass must contain the same number of one-side fired tubes and of two-side fired tubes.

Typical Arrangements - Figure 11 shows sketches of a number of generalized arrangements, with various pass and cell configurations. This group of arrangements is not intended to cover all possible combinations. Other arrangements may be used to meet special requirements. a.

Figures 11a to 11c show the most commonly used arrangements. Similar arrangements can be used for 4 and 10 passes. This type arrangement should be used as first preference. As shown, the outlet tubes should be located at the sides of the heater, which is the region of less than average heat density. This layout tends to reduce the coking rate of the hottest tubes.

b.

Figures 11d and 11e show other arrangements that have been used. However, some variations in heat input per pass can be expected because of the non-symmetrical layout of the two-side fired tubes. Also, the heaters tend to be wider than shown in Figures 11a to 11c, leading to a short, wide convection section (perhaps too wide). Similar arrangements can be made for 9 passes (4 cells) and 12 passes (4 or 5 cells).

c.

Figure 11h shows the use of only high heat density two-side fired tubes, which minimize coil length at the expense of a larger box. This arrangement should be considered when very expensive tubes are required (e.g., stainless steel) or where conditions require a short residence time or uniform radiant heat density around the tube. Note that heat release in the end cells is one-half of that in the center cells. Figure 4 presents bridgewall temperature curves for this type heater. Some variations in the layout procedures described below are required for heaters with only two-side fired tubes, since these procedures were p rimarily developed for combinations of one- and two-side fired tubes.

Layout Considerations - The following considerations should be kept in mind when the vertical box layout is being developed. In special cases, the guidelines should be modified as required. a.

Most heaters will have a top inlet to the radiant section (from the convection section) and a bottom outlet. Therefore, an odd number of tubes per pass is normally required.

b.

Jumpovers (i.e., between wall tubes and center tubes) may be located at the top or bottom of the radiant section, although they are most commonly located at the bottom. They are usually located outside the firebox.

c.

Tubes are bottom-supported and guided at the top. Wall tubes over 35 ft (10.7 m) long should also have a midpoint guide to restrict tube bowing caused by peripheral maldistribution of heat input.

d.

Center tubes under the convection section must be about 3 ft (0.9 m) longer than the other tubes to accommodate the top tube guide system. See Figure 12 for typical guide details.

e.

The location of center tubes relative to the convection section side walls will determine the type of tube guide required. Care must be exercised in locating these tubes to ensure that they can be adequately guided. The use of 1 or 2 long radius U-bends may be required for proper locations of these tubes. See Figure 12 for details of typical center tube guides.

f.

Allow a clear path for access and tube replacement through the end walls and from cell to cell. This is usually located down to the center of the furnace between outlet tubes of adjacent passes. Allow a 24 in. (610 mm) tube center-tocenter spacing for 6 in. (150 mm) or smaller tubes [17 in. ( 430 mm) minimum clear access].

g.

Location of the transfer line will depend upon heater and burner arrangement. Typically, one transfer line will run along each side of the heater, as shown in Figure 11f . Should this arrangement not be possible, a less desirable arrangement with outlet tubes in the center and a single transfer line may be used, as shown in Figure 11g.

h.

As nearly as possible, the number of end wall tubes and center tubes per row should be equal, to take full advantage of available space for tubes. Since the design will not always permit this, it is preferable to use fewer tubes on the end walls and maximize the use of center tubes. If this results in wide spaces on the end walls, a few long radius U-bends should be used to spread out the end wall tubes.



Page

Section VIII-B

14 of 79


December, 2000

LAYOUT OF RADIANT SECTION (Cont) i.

While all cells should be the same size, it is acceptable to make the center cell (or two) slightly wider than the others to accommodate one or two extra wall tubes if these tubes are necessary to obtain an equal number of wall tubes per pass. Conversely, it is also acceptable to leave a wall tube out of one or more cells if necessary. However, cell size should not be reduced since this is probably set by minimum burner clearances.

j.

4.

A minimum of one long radius return bend (or equivalent space) should be provided in each side of each cell. These should be located at diagonally opposite corners of the cells. These larger spaces are necessary for peepholes which permit viewing of tubes and burners (see Figure 13).

Radiant Section Design a.

Determine the proposed arrangement from typical arrangements shown in Figure 11, as previously discussed.

b.

Determine the number of burners. The heat release, number and size of burners will be identical in each cell (except for Figure 11h, where heat release in end cells is one-half that of the other cells).

c.

Determine the minimum tube envelope surrounding the burners, based on previously stated minimum burner-to-tube and burner-to-burner clearances. Keep in mind that additional clearance between burners may be required at the center of the heater if the transfer line arrangement shown in Figure 11f is used.

d.

The preliminary layout should be based upon the minimum tube envelope surrounding the burners, unless experience indicates that a larger envelope should be used for the specific case. In many designs, this minimum envelope has been used in the final layout with virtually no modifications.

e.

Determine the approximately number of tubes. The following method can be used as a short-cut aid in the trial-anderror procedure for determining the proper number of radiant section tubes. See Figure 13. The following nomenclature is used in this method: B B′ Lt Lt Li Li Wt Wt′ Wi Wi a a′ b c e n p x y

= = = = = = = = = = = = = = = = = = =

Cell width (tube center-to-tube center), ft (m) Preliminary approximate width, based on minimum tube envelope. Heater length (tube center-to-tube center), ft (m) ΣB Heater radiant section overall length, inside refractory, ft (m) Lt + 3 times tube size (IPS) Heater width (tube center-to-center), ft (m) Preliminary approximate width, based on minimum tube envelope Heater radiant section overall width, inside refractory, ft (m) Wt + 3 times tube size (IPS) Number of wall tubes at one end of heater Preliminary approximate number of wall tubes at one end of heater Number of wall tubes at one side of one cell Tube center-to-center spacing, ft (m) Number of center tubes between adjacent cells Number of cells Number of passes Number of wall tubes (one-side fired) per pass Number of center tubes (two-side fired) per pass

a′, b′, e′, x′, y′ are first approximations based on the following equations. These equations assume that the corner tubes of each cell are arranged at a 45° angle, and one long radius return bend is included in each side of each cell. i.

End wall tubes a′

=

 ′= a 

Wt '

−

2

c Wt '

− c

+

0.586

0.610

+

(Customary)

 

0.586

(Metric)

Let a = whole even number based on a′


Eq. (10)

Eq. (10)M



Section

Page

VIII-B

15 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) Determine: (a′ – a)c If this is positive, this is the increase in center access clearance above 2 ft (0.6 m). If it is negative, this is the increase in tube envelope over the minimum. ii.

Side wall tubes b′

=

B′ c

−

0.957

Eq. (11)

Let b = next higher whole number gre ater than b′ iii.

One-side fired tubes per pass x′

=

2a

+

2nb

Eq. (12)

p

Since x must be a whole number, some adjustments may be required:

• • • • iv.

Decrease (a) by 2 or 4 tubes. remaining tubes.

Long radius U-bends should be used as necessary to spread out the

Add to (a). Note that this also will require addition to (e). Add to (b) in each cell. Add or subtract from (b) 1 or 2 tubes in only the center cell(s). If subtracting tubes, leave the resulting space blank and don't decrease the cell size.

Two-side fired tubes per pass e = a (approximately) y =

Eq. (13a)

e (usually, when there are 2 pass outlets per row of center tubes) 2

Eq (13b)

e (n − 1) (adjust as required to obtain whole number) p

Eq. (13c)

or y v. f.

=

Total tubes per pass - The total number of tubes per pass (x + y) should be an odd number for most arrangements. Make slight adjustments to the number of tubes to modify this total as required.

Determine the radiant surface. Eq. (8) is used to determine the amount of radiant tube surface required. However, this equation must be modified for the vertical tube box heater arrangement, because of the usage of both one-side and two-side fire tubes. The amount of equivalent surface required based on one-side firing can be determined. Since for standard tube-totube spacings (2 IPS) the average heat density on two-side fire tubes ( φ2-side) is 1.5 times the average heat density for one-side fire tubes (φ1-side) the heat absorbed by each two-side fire tube used in the layout will be equivalent to that of 1.5 one-side fire tubes. A1− side

=

Qr

φ1− side

Eq. (8a)

where: A1-side = Equivalent radiant section tube surface, based on one-side fired heat density, ft 2 (m2) The bridgewall temperature used for this calculation depends on the radiant section height, so a trial-and-error solution must be made to determine the final bridgewall temperature. However, for the initial approximation of radiant tube length, assume no height correction of bridgewall temperature. The convection section layout and shield duty are estimated based upon the radiant layout developed up to this point.



Page

Section VIII-B

16 of 79


December, 2000

LAYOUT OF RADIANT SECTION (Cont) g.

Determine the radiant tube length Lr . Lr

=

A1− side p (x

+

1.5 y ) A o

Eq. (14)

Radiant tube length Lr should be within the range of 30 to 40 ft (9.1 to 12.2 m), with the optimum length being 35 to 40 ft (10.7 to 12.2 m). It should not exceed 45 ft (13.7 m). If L r > 40 ft (12.2 m), consider adding to the number of tubes (x & y) to reduce L r . Heater width should be increased first, then length. If Lr < 30 ft (9.1 m), review the design basis, since this is an unusual case and may possibly be uneconomical. As discussed above, the final L r must be used in determining the final bridgewall temperature. 5.

Multi-Service Box Furnaces - The vertical tube box heater can be used for two services by splitting the radiant section; one end of the heater in one service and the other end in a second service. The convection section may be in one service over the entire radiant section or in two services, one above each radiant section. The two services must operate together at similar heat densities.

HOOP TUBE CABIN HEATER (SEE SECTION VIII-A, FIGURES 4C AND 4D) 1.

General - The double hoop ar rangement shown in Figure 4D of Section VIII-A is customarily used for POWERFORMING heaters but can also be used for other all-vapor services. These heaters are usually multi-service, with each radiant zone separated from the others by free standing brick walls. The single hoop arrangement shown in Figure 4C of Section VIII-A can be used for two separate duties, one in each of the radiant hoop sections. A free standing brick wall should be placed between the two hoops to permit a turndown of one of the sections. Additional zones can be provided in the radiant section by joining this brick wall with another running to the heater side wall.

2.

Typical Arrangement - A typical arrangement of double hoop tube cabin heater is shown in Figure 14. This arrangement shows three separate radiant zones. Note that vertical sections of tubes in center rows are on a staggered pitch and ar e connected by standard U-bends. These tubes are considered to be one-side fired, the same as if they were backed by a refractory wall. The hoops are also counted as equivalent to one-side fire tubes. Although Figure 14 shows inlet and outlet manifolds parallel to the convection section, manifolds can also be arranged normal to the convection section. Whatever manifolds arrangement is used, the goal is to reduce pressure drop by simplifying piping between the heater and the reactors. Refer also to Section VIII-J for additional details on manifold layout and design.

3.

Number of Passes, Tube Size and Length - Special considerations are required for selecting the number of parallel passes and tube size, since tube length per pass must also be considered. (Note that all references are to Figure 14.) a.

Various combinations of tube size and number of passes should be considered. Since each tube size has a different ratio of tube surface to flow area, each combination will result in a different tube length requirement.

b.

Choice of coil length is relatively limited, when compared to other designs. i.

Minimum height Ht is based upon required burner clearances discussed earlier.

ii.

Maximum height is based upon maintaining uniform heat transfer, as well as mechanical considerations. Height should be limited to straight tube length L s of:

• • iii.

4.

30 ft for 5 in. IPS (9.1 m for 125 mm NPS) and larger tube diameter. 25 ft for 4 in. IPS (7.6 m for 100 mm NPS) and smaller.

Cell width between tubes (B) is based on minimum burner-to-tube clearances. However, minimum cell width should be 10 ft, 0 in. (3.05 m) tube center to tube center.

c.

Although it is desirable to use the same tube size in all zones, this is not necessary; and in practice, different tube sizes can be used in the individual cells.

d.

Coil length can be increased by a factor of two by increasing the number of series passes through the zone. This is done by providing blinds, or baffles, in the manifolds. Zones A and C are shown with one series pass, while Zone B has two series passes. Any number of additional series passes can be used, if necessary.

Convection Section Length - The convection section extends the entire length of the heater, across all radiant zones. The convection section length is equal to the sum of the lengths of all radiant zones, plus the width of the internal dividing walls.




Section VIII-B

Page 17 of 79

December, 2000

LAYOUT OF RADIANT SECTION (Cont) 5.

Radiant Section Duty - As in other heater designs, overall radiant section duty can be determined from heat available charts, given the net fuel rate and bridgewall temperature. Each radiant zone has its own bridgewall temperature. Average bridgewall temperature is based on the combined flue gas streams lea ving each radiant section. Convection section design is based on this average bridgewall temperature. For services which are composed evenly of all-radiant duty, bridgewall temperature is a function of radiant heat density and tube metal temperature. The complete design of these zones in straightforward. Bridgewall temperature is determined from Figure 5. Heat transfer to the hoop has been included in this curve. For services which combine radiant and convection duties, the radiant duty must be determined by subtracting the other, all-radiant duties from the overall radiant duty. Before this radiant duty can be determined, the heater layout must be estimated (and later corrected as required), so that the shield radiant duty can be calculated. Each radiant zone will contribute to the total shield section radiant duty. This contribution will depend on the radiant section heat density of each zone and on the portion of the convection section located over each radiant zone. The calculation procedure for shield section radiant duty is given in Section VIII-C. Variations in relative duties over the run length must be considered in selecting design radiant heat densities for each zone. For example, in the typical POWERFORMER heater (Figure 14), Zones A and C are all-radiant reheat services, while Zone B and the convection section are in preheat service. Since A and C contribute a smaller amount of cooler flue gas to the convection section during operations when these reheat duties are reduced, Zone B must be fired harder to make up for this reduced convection heat input. Therefore, Zone B may have to be increased in the size to avoid high radiant heat densities during the periods when Zones A and C operate at low rate.

6.

Burner Layout - Burners should be located to give uniform heat distribution to the tubes, consistent with minimum burnerto-wall clearances. The burners should be located so that the burner-to-wall distance is approximately half the burner-toburner distance. Adequate burner space is not always available in hoop tube heaters and additional clearances are occasionally required. For natural draft burners, these may be obtained through use of a double row of burners (Zone A). In other heaters, it may be necessary to provide a longer cell length than otherwise required.

7.

Dividing Walls - Free standing refractory brick walls divide the radiant zones. These walls should be 2 ft, 3 in. (690 mm) wide at the base [three 9 in. (230 mm) bricks], with a stepped construction and a maximum height of about 25 ft (7.6 m). The radiant section layout should provide for a minimum clearance between this interior wall and the adjacent hoop tubes of 1.5 times the tube size, the same as between the tubes and the exterior walls.

8.

Radiant Section Manifolds - These manifolds are located below each radiant zone, at each side of the heater. Various methods of sealing the heater in this area are used: an external header box to enclose the manifold, a sliding seal plate arrangement or a foil seal around each individual tube connection to the manifold. The latter method is not always possible, particularly in designs with small tubes, where manifold thermal expansion is greater than the space between adjacent tubes (Figure 15). The sealing arrangement used must also ensure that the manifold is not directly exposed to radiant heat transfer.

PRESSURE DROP THROUGH THE COIL Computer Program No. 3660 can be used to calculate heater coil pressure drop for both single and two-phase flow conditions. This program initially calculates thermodynamic property information for the heater feed stream and then using this data calculates the pressure drop for the coil geometry submitted. Heater pressure drop can be calculated by hand using the procedures and equations presented in Section XIV which covers fluid flow. For all-liquid and all-vapor systems, pressure drops can be easily and quickly calculated by hand since specific volume changes through the coil are small. To facilitate this calculation, the Fluid Flow Module of the Pegasys computer program can be used for single-phase pressure drop calculations. However, with two-phase systems the pressure drop per unit length of coil changes continuously with changes in gas-liquid ratio. Calculation of pressure drop is, therefore, a complex point-to-point trial-and-error calculation. As a result, hand calculation of two-phase pressure drop is very time-consuming and is best done by using a computer program such as 3660.



F IRED H EATERS

Page 18 of 79


December, 2000

PRESSURE DROP THROUGH THE COIL (Cont) The internal diameter of the tube used in pressure drop calculations should be the average instead of the minimum tube wall thickness. [Minimum wall thickness = 0.875 (average wall thickness).] Since the tube diameter greatly affects pressure drop (1 / ∆P varies approximately as the 2.5 power of tube diameter in vaporizing services), it is important that any anticipated coke buildup be allowed for, when the internal diameter of the tube is figured for pressure drop calculations. The following are suggested thicknesses of coke to be used in the radiant section (no coking is assumed in the convection section, unless reverse decoking facilities are also provided):

• •

1/8 in. (3 mm) for light coking services (atmospheric pipestills, etc.). 1/4 in. (6 mm) for heavy coking services (vacuum pipestills, etc.).

In addition, the designer may have to estimate the pressure drop through the transfer line downstream of the radiant section. In this case, refer to Section VIII-J for sonic velocity considerations.

TUBE DESIGN MATERIALS The primary considerations are the required strength, resistance to corrosion (or erosion), and oxidation (or reduction) characteristics. Bearing upon these characteristics are the temperature level, the heater atmosphere, and corrosive constituents of the process fluid or the fuel. The most commonly used materials are carbon steel, C-1/2 Mo, 1-1/4 Cr-1/2 Mo, 2-1/4 Cr-1 Mo, 5 Cr-1/2 Mo, 9 Cr-1 Mo, 18 Cr8 Ni and 25 Cr-20 Ni. For selecting the proper materials, as well as for establishing corrosion rates, see the Refinery Construction Materials Manual (RCMM) or consult the Materials Engineering Section of EETD. For process heaters, the following tube materials are most commonly selected, based on internal and external conditions: Internal Conditions - Largely based on sulfur corrosion. 1.

2.

3.

Pipestills - See RCMM Corrosion Design Curve Nos. 1-A to 1-J for APS and 4-A to 4-J for VPS. a.

Carbon Steel (CS) is used at low tube metal temperatures [approximately 700 to 900°F (270 to 480°C)], until the corrosion rate becomes excessive.

b.

5% Cr is used for high temperatures. It is generally good up to 1200°F (650°C) tube metal temperature (TMT).

c.

9% Cr is occasionally used where the crude is extremely corrosive and/or the operating conditions result in high tube metal temperatures.

d.

18 Cr-8 Ni is rarely used due to concern over polythionic stress-corrosion cracking (PSCC) during downtimes.

Reboilers, Hydrofiners, etc. - See RCMM Corrosion Design Curves Nos. 2 and 3, A through J. The corrosion rate may be significantly higher than in pipestills. See also RCMM Corrosion Design Curve 5 for hydrocarbon streams containing H2S and H2. a.

CS is used for low temperatures.

b.

5% Cr is used for moderate temperatures (no increased corrosion resistance over CS for H2S / H2 service).

c.

18 Cr-8 Ni is used for higher temperatures or high H2S partial pressure in the feed.

POWERFORMERS - Tube selection depends mainly upon resistance to H 2 attack (RCMM Corrosion Design Curve No. 6) and material strength. There is relatively little corrosion. a.

C-1/2 Mo should not be used in hydrogen service even at low temperatures.

b.

1-1/4 Cr-1/2 Mo is used at low temperatures as would be typical for convection section tubes.

c.

2-1/4 Cr or 5 Cr is used for higher temperatures. 5 Cr is used if external oxidation becomes limiting.

d.

For very high temperature cases, modified 9 Cr-1 Mo material can be used to provide required strength and resistance to external oxidation.

External Oxidation - This normally plays a relatively minor role in tube selection or corrosion allowance, but is the principal concern in the location and selection of materials for convection section extended surface. For high-temperature oxidation rates, see CMM Corrosion Design Curve No. 8.



Section

Page

VIII-B


19 of 79

December, 2000

TUBE DESIGN (Cont) DESIGN TEMPERATURE Unless otherwise indicated, the design temperature is the maximum calculated outside tube metal temperature. Calculation of radiant section tube metal temperature is described below. Calculation of convection section tube temperatures is described in Section VIII-C. Tm

=

Tb

+ ∆Tf + ∆Tc + ∆Tm ;

Eq. (17)

with:

 1   Do  ∆Tf =       ( φr )max  hi   Di 

Eq. (17a)

 t   2 Do   ∆Tc =  c   (φr )max    k c   Di + Di '  ∆Tc = 10−3

x

(Customary)

 tc   2 Do      ( φr )max    k c   Di + Di ' 

(Metric)

  t    2 Do  ∆Tm =  a    (φr )max  k m   Do + Di  ∆Tm = 10−3 where: Tm Tb

x

= =

∆Tf = ∆Tc = ∆Tm = Do Di Di′ ta tc

hi

= = = = = = = =

kc

=

Eq. (17b)

Eq. (17b)M

(Customary)

 ta   2 Do         ( φr )max  k m   Do + Di 

Eq. (17c)

(Metric)

Eq. (17c)M

Maximum tube metal temperature, °F (°C) Temperature of bulk fluid at the point of calculation, °F (°C) (coil outlet or peak temperature is usually used for Tb) Temperature rise across inside oil film, °F (°C) Temperature rise across coke layer (fouling service), °F (°C) Temperature rise across tube wall, °F (°C) Outside tube diameter, in. (mm) Inside tube diameter, based on average tube wall thickness, in. (mm) = Do – 2 ta Inside diameter of coke layer, in. (mm) = Di – 2 tc Average tube wall thickness, in. (mm) Design coke thickness, in. 1/8 in. (3 mm) for light coking service (atmospheric pipestill, etc.) 1/4 in. (6 mm) for heavy coking service (vacuum pipestills, visbreakers, etc.) Inside oil film coefficient, Btu/hr ft 2 °F (W/m2 • °C). See Section VIII-C for recommended calculation procedures. Thermal conductivity of coke,

Btu / hr ft 2

°F / in.

(W/m • °C), as follows:

FILM TEMP. °F

500

1000

1500

Btu / hr ft 2

38

34

30

kc

°F / in.

FILM TEMP. °C

kc

W m

• °C


260

500

800

5.5

5.0

4.4


F IRED H EATERS

Page 20 of 79


December, 2000

TUBE DESIGN (Cont) km

= Thermal conductivity of tube metal temperature at mean wall thickness,

Btu / hr ft 2

°F / in.

(W/m • °C). See Figure 16. (φr )max = Maximum point heat density, based on outside tube diameter and corrected for longitudinal and peripheral maldistribution of heat and flame luminosity, Btu/hr ft2 (W/m2) ( φr )max

=

C1 C2 C3

where: C1

= = =

C2

=

φ r

Eq. (17d)

Dimensionless correction factor for peripheral maldistribution around tube 1.77 for one-side fired tubes 1.18 for two-side fired tubes (These factors are good for process heaters with t a < 0.5 in. (13 mm), and tube center to center of 2 x IPS. For other applications, see Figure 6 in Section VIII-D. Multiply factor from this figure by 1.5 for one-side fired tubes.) Dimensionless correction factor for vertical maldistribution, as follows: FURNACE RADIANT SECTION HEIGHT, ft

FURNACE RADIANT SECTION HEIGHT, m

C2 FACTOR

Up to 25

Up to 7.6

1.20

30

9.0

1.23

35

10.7

1.28

40

12.2

1.33

45

13.7

1.40

(Vertical maldistribution depends upon heater geometry. This factor should be used for all heaters previously described. For unusual applications, consult the Heat Transfer Equipment Section of EETD.) C3

φr

= = = =

Dimensionless correction factor for luminosity, which depends upon the type of flame: 1.0 for non-luminous flames (gas) 1.08 for luminous flames (oil, combination gas/oil) Radiant section average heat density, Btu/hr ft2 (W/m2)

DESIGN PRESSURE The pressure used for the design of fired heater tubes will be one or both of following. Elastic Design Pressure (Pe) - This pressure is the maximum pressure the heater coil could be exposed to for short periods of time. The value of this pressure is generally related to a safety valve setting and must be determined by the individual responsible for the overall unit process design. This pressure is used in an elastic design equation to prevent excessive elastic stresses in the tube when at its maximum pressure. Rupture Design Pressure (Pr ) - This pressure is the normal operating pressure the tube must sustain over long periods of operation. If the operating pressure changes during an operating run, the highest pressure should be used. The rupture design pressure is used in a creep design equation to ensure that any creep damage accumulated under the action of the operating pressure (stress) does not result in a tube failure. In most situations, both elastic and rupture design pressures vary depending upon the section of the total heater coil under consideration. However for low pressure and/or low pressure drop heaters it is common practice to use a constant elastic and rupture design pressure for the whole coil. For high pressure and/or high pressure drop heaters, consideration should be given to establishing different elastic and rupture design pressures for various sections of the heater coil since considerable investment savings on tubes can often be realized.



Section


Page

VIII-B

21 of 79

December, 2000

TUBE DESIGN (Cont) TUBE WALL THICKNESS Calculation of heater tube wall thicknesses is based upon the procedures and stress data outlined in API RP-530. When calculating the minimum tube wall thickness required, three different design bases will generally have to be considered. The most conservative basis will set the minimum required tube wall thickness for the heater. The relevant equations for each case are summarized below. 1.

Elastic Design Basis - Preventing failure by bursting during a maximum, short term pressure condition near the end of the design life when the corrosion allowance has been used up. tm

=

tm

=

Do Pe 2 Se + Pe

TCA

(Customary)

+

(Metric)

Do Pe 2000 Se

where: tm Do Pe Se

+

Pe

TCA

Eq. (19)

Eq. (19)M

= = =

Minimum tube wall thickness, in. (mm) Tube outside diameter, in. (mm) Elastic design pressure, psig (kPa, gage)

=

Allowable elastic design stress [psi (MPa)] at the design tube metal temperature (based on two-thirds of minimum yield strength for ferritic steels and 90% of yield strength for austenitic steels). Table 3 presents stress values for typical materials used in heater design. Refer to API STD-530 for other materials. Total corrosion allowance, in. (mm)

TCA = =

Note:

2.

+

Nominal annual corrosion rate (in. or mm per year) times 5. (Heater tubes are normally placed in the 5 yr corrosion design category.) Nominal annual corrosion rate (including both inside and outside corrosion) is evaluated at the design tube metal temperature using the curves contained in the EMRE Refinery Construction Materials Manual (RCMM). The corrosion data in the RCMM is considered to be conservative, so a 5 year design life normally results in a much longer actual life. If more accurate, measured corrosion rate data is available and used for design, the life should be increased to 10 years.

Rupture Design Basis - Preventing failure by creep rupture during the design life. tm

=

Do Pr 2 Sr + Pr

tm

=

Do Pr 2000 Sr + Pr

where: tm Do Pr Sr

+

f TCA

+

f TCA

(Customary)

(Metric)

= = = =

Eq. (20)

Eq. (20)M

Minimum tube wall thickness, in. (mm) Tube outside diameter, in. (mm) Rupture design pressure, psig (kPa, gage) Allowable creep rupture design stress (psi [MPa]), based on 100% of the minimum stress to produce rupture at the design life and the design tube metal temperature. Design life is to be 100,000 hrs unless otherwise specified. Table 3 presents stress values for 100,000 hours design life for typical materials used in heater design. Refer to API RP-530 for other materials. f = Corrosion fraction, dimensionless. Determined from Figures 18 and 19. This factor is applied to take credit for the reduction in stress which results with the addition of the corrosion allowance. TCA = Total corrosion allowance, in. (mm). Determined exactly the same as for elastic designs, see Item 1 above.



F IRED H EATERS

Page 22 of 79


December, 2000

TUBE DESIGN (Cont) S 3.

Minimum Wall Design Basis - Ensures that a tube near the end of the corrosion life will have sufficient mechanical rigidity to resist sagging and/or damage from external forces or abuse. tm

=

0.125 in. (3.17 mm) + TCA for radiant, shield, and convection tubes.

Eq. (21a)

tm

=

0.22 in. (5.59 mm) for studded tubes to provide mechanical rigidity required for the studding operation.

Eq. (21b)

Tubes in Steam Superheating or Generating Service - Fired heater tubes in these services should be designed according to the procedures outlined above unless local statutes or ordinances specifically require ASME or some other design basis. Thermal Stress Checks for High Pressure or High Temperature Tubes - The tube design equations discussed previously assume that the thermal stress induced by the radial temperature gradient across the tube wall is negligible. However, for high pressure or high temperature heaters, tube wall thicknesses and temperature gradients may be large enough to result in significant thermal stresses. Therefore, tubes designed for these type heaters should be checked to ensure that the thermal stresses are indeed negligible. If thermal stresses are found to be excessive, the tube material can be upgraded or the heat flux decreased to reduce the tube wall thickness and thermal stress to acceptable levels. The equations given below can be used to determine whether thermal stresses are within acceptable limits for tubes operating in the “elastic” region. However no simplified formulas have been developed for checking thermal stresses when tubes operate in the “creep” region. Therefore, any tube designs in the creep range which has a temperature difference across the tube wall of 150 °F (80°C) or greater should be submitted to the Mechanical Engineering Section of EETD for review. The maximum thermal stress for a tube is given by the following equation:

➧

S th

 α E ∆T =  2 (1 − ν ) In

where: Sth =

  Y

 2 Y2   In  2   Y − 1  

(Y)

 − 1 

Eq. (22)

Maximum thermal stress, psig.

Y

=

Do / Di, ratio of outside to inside tube diameter (D i based on average tube wall thickness).

α

=

Coefficient of thermal expansion at mean tube wall temperature, °F-1 (°C-1). Table 4 contains thermal expansion data for the commonly used materials.

=

Modulus of elasticity of mean tube wall temperature, psi (MPa). Table 5 contains values of E for the commonly used materials.

ν

=

Poisson's ratio, dimensionless. Equal to 0.3 for steels.

T

=

Design temperature difference across average tube wall thickness, °F (°C).

For the elastic region, the calculated thermal stress should meet the following criteria: • For ferritic steels: Sth Sth

•

≤ ≤

(2.0 – 0.67 Y) Sy and 1.33 Sy

For austenitic steels: Sth (2.7 – 0.90 Y) Sy and Sth ≤ 1.8 Sy

where: Sy =

Minimum yield strength, psi (MPa). Yield strengths for common materials are presented in Table 6.

External Piping - Heater piping external to the firebox and header boxes, such as external crossovers, should be designed per the Piping Code, ASME B31.3. For most applications, crossovers are made the same thickness and of the material as the tubes they connect (the upstream material if a change of materials occurs).




Section VIII-B

Page 23 of 79

December, 2000

INSTRUMENTATION The following instrumentation should be provided for process heaters. This is usually shown on the design specification flow plan. Instruments requiring connections on the heater itself should also be shown on the heater sketch, so that these connections can be properly located. Additional instrumentation may be required in individual situations. No distinction has been made here between indicators and recorders, the choice depending upon the requirements of the individual situation. See Section XII for details of control schemes.

PROCESS FLUID Flow 1.

Flow measurement and control for each pass in liquid and vaporizing services.

2.

Protection from low flow (or flow stoppage) should be provided in accordance with Section XV-B.

Temperature 1.

Coil Inlet - Average temperature into the fired heater. Also, individual pass indicators if flashing occurs across a control valve.

2.

Crossovers - Temperature of each pass (or of common header) between radiant and convection sections.

3.

Coil Outlet Temperature (COT) of each pass, plus combined temperature in transfer line. Because of the large number of passes in hoop tube type heaters (with nearly uniform outlet temperatures), coil outlet temperatures on individual passes are not normally measured. At most, COTs from a few representative passes should be measured.

Pressure 1.

Coil Inlet - Pressure of each pass, downstream of control valve. Pressure in feed line if valves are not used to control flow to individual passes.

2.

Coil Outlet - Overall pressure in transfer line.

TUBE METAL TEMPERATURE Thermocouples for monitoring tube metal temperatures should normally be provided. The exact number and location of the TIs will vary depending upon the type of heater being designed. As a minimum, the general philosophy should be to provide one TMT thermocouple per pass located at the coil outlet and at an elevation above the heater floor coincident with that of the peak heat density. Refer to IP 15-2-1 for guidelines on using TMT thermocouples.

FLUE GAS Temperature (Instruments and connections required by IP 15-1-1.) 1.

In stack below the damper.

2.

At the bridgewall. When possible, the thermocouples shall be placed in the arch out of view of the shield tubes.

3.

Between different services in the convection section.

Draft Gages (Required by IP 15-1-1.) 1.

At the bridgewall.

2.

Downstream of stack or duct control dampers.

3.

Upstream of stack or duct control dampers.

4.

In the sidewall at the heater floor.

Other Pressure Instruments - A high-pressure alarm (and under certain conditions, a cut-out) at the top of the radiant section is also required for all heaters, per IP 15-1-1. Oxygen/Combustibles Analyzers - One shall be provided to permit continuous analysis of the flue gas in the stack, per IP 15-1-1.



F IRED H EATERS

Page 24 of 79


December, 2000

INSTRUMENTATION (Cont) FORCED-DRAFT SYSTEM Pressure Indicators 1.

Main air duct.

2.

At each burner, downstream of the shut-off damper.

Controls & Safety System - As required by IP 15-1-1. Air Flow Measurement - This can normally be justified when it is planned to regulate the fuel/air ratio by computer control.

FUEL SYSTEM Control & Safety System - Pressure indicators, alarms, cutouts, as required by IP 15-1-1. Atomizing Steam - Provide a pressure indicator on the header, downstream of the control valve. Fuel Oil - Reference off-site DP on Fuel System. Provide a temperature indicator near the end of dead-ended headers, and at the inlet and outlet of return loop headers.

STEAM-AIR DECOKING SYSTEM See Section VIII-I.

SOOTBLOWERS Pressure Indicator on steam header. Flow Indicator on steam header for low-pressure systems (< 250 psi, < 1725 kPa).

MECHANICAL SPECIFICATION HOW THE REQUIREMENTS ARE COVERED International Practices - IP 7-1-1, 7-4-1 and 7-5-1 cover minimum requirements governing the design, fabrication, and inspection of process heaters. Although minimum process requirements are included for application to vendor-designed heaters (duty spec.), the IPs are largely concerned with the common mechanical design requirements that apply to most of our heaters. These requirements are updated periodically, based on our latest experiences and developments. Design Specification - The design specification gives the information shown in the checklist (Table 1) in Section VIII-A. A sketch showing the general heater arrangement is necessary. Special mechanical features for the particular heater are also included in the design specification. Up-to-date details of these mechanical features are available from the Heat Transfer Equipment Section of EETD.

TUBE GUIDES AND SUPPORTS Corrosive Compounds in Fuel - If the fuel contains significant amounts of vanadium and sodium, special consideration must be given to metal parts (mainly tube supports) operating at temperatures above 1200 °F (650°C). The tube support operating temperature is assumed to be the same as the flue gas temperature. Above 1200°F (650°C), molten ash deposits on the supports and fluxes them. The common tube support and guide materials of 25/20 and 18/8 Cr-Ni alloys rapidly deteriorate under these conditions. The following means of reducing this problem must be included in the specification, where applicable: 1.

The designer must specify the quantities of vanadium and sodium in the fuel, so that the appropriate requirements of IP 7-1-1 are invoked. Among these are protective refractory coatings or the use of high chromium-nickel (50 Cr, 50 Ni, and Nb) alloy for the tube supports.

2.

Where practical, heater components can be designed to minimize exposure to hot flue gases. Examples are locating tube supports and guides outside the firebox and behind tubes. Also, convection section intermediate tube sheets can sometimes be eliminated in the case of V.C. heaters by making the convention section shorter and wider.

3.

The vulnerable components can also be made easily removable for replacement during turnarounds. This is practical for the radiant section supports and guides. IP 7-1-1 requires supports for horizontal radiant section tubes to be replaceable without removing the tubes. Making convection section tube supports replaceable without removing the tubes requires substantial extra investment and is rarely done. Sometimes the owner wishes the radiant tube supports to be made replaceable without even shutting down the heater. However, this is normally very expensive and seldom justified. Neither of these two features should be specified unless specifically requested by the owner. ExxonMobil Research and Engineering Company – Fairfax, VA


Section VIII-B


Page 25 of 79

December, 2000

MECHANICAL SPECIFICATION (Cont) Special Mechanical Details - Certain heater designs require inclusion of special mechanical design features in the design specification in order to obtain a satisfactory detailed design. Although typical details are included in this Design Practice, consult the Heat Transfer Equipment Section of EETD for up-to-date details. 1.

Vertical Tube Box Heaters - Details of the tube guide and support system must be included for this type of heater. See Figure 12.

2.

Hoop-Tube - Inclusion of guide details for the radiant tubes is recommended. See Figure 16.

REFRACTORY Materials - The heater refractory materials are affected by the corrosive compounds in the fuel. The most common low temperature problems are caused by sulfur, but high concentrations of metals can also cause deterioration of refractories at high temperatures. The designer must specify the quantities of sulfur and metals in the fuel, so that the appropriate IP 19-3-3 and IP 7-1-1 requirements for refractory materials can be applied. Temperatures - Hot face design temperatures should be specified so that the required refractory thickness can be calculated, based on heat losses. [IP 7-1-1 requires a maximum of 180 °F (82°C) casing wall temperature with no wind and 80 °F (27°C) ambient temperature.] Floor and arch cold face temperatures are chosen so that their heat loss is the same as for the walls. Hot face design temperatures should be realistic estimations of actual temperatures. The following basis should be used to determine these temperatures [round off to nea rest 50°F (30°C)]: 1.

Radiant Section Protected Walls (shielded by tubes) - Average of bridgewall temperature and average tube metal temperature (uncoked), plus 100°F (55°C).

2.

Radiant Section Unprotected Walls - Bridgewall temperature.

3.

Radiant Section Arch - Bridgewall temperature.

4.

Radiant Section Floor - 1800°F (980°C).

5.

Convection Section Protected Walls - Divide the section into two parts (bare tubes and extended surface tubes). Use temperature of 200°F (110°C) below inlet flue gas temperature into each section.

FIREBOX PURGING Each heater design should include facilities for purging the heater firebox before light-off of the first burner pilot. The purge facilities should as a minimum allow for one volume change of the heater box atmosphere (radiant section) every 5 minutes. Acceptable means of purging include: (1) use of forced or induced draft fans when provided, (2) steam purge connection(s) in the radiant section floor or lower wall, and (3) an eductor located in the heater stack or flue gas duct. A stack eductor is the preferred option over steam purging when fans are not provided and the burners are equipped with flame ionization rods. This is because of concerns with moisture condensing on the flame rods rendering them ineffective. Design of Steam Purge Facilities - A restriction orifice should be provided in the piping system to limit steam flow to the required rate mentioned above. Otherwise, a much higher actual steam rate is likely, with possibly harmful effects to the entire steam system. In calculating steam rate, remember that the heater box is at atmospheric pressure. Purging steam connections are preferably located in the heater floor. This avoids direct impingement of steam on the tubes. Enough connections should be specified to distribute the steam uniformly throughout the firebox, about one connection per 150 ft2 (14 m2) of floor area. Design of Stack Eductors - A stack eductor induces purging air flow through the burners by creating a draft in a cold heater. Either steam or air can be used as the motive fluid in the eductor, the choice being based on economics. The steam supply chosen should not be less than 50 psig and should have a superheat of at least 100 F (55°C) to ensure good operation. Purging is achieved by activating the eductor for 15 minutes with the stack dampers and air registers fully open. °

The eductor consists of a single nozzle/orifice positioned in the center of the stack or duct. The eductor nozzle flow is directed in the same direction as the flue gas flow. The nozzles and piping inside the flue gas system should be made of 304 SS.



F IRED H EATERS

Page 26 of 79


December, 2000

MECHANICAL SPECIFICATION (Cont) To determine the necessary nozzle diameter and steam or air flow rate, first calculate the draft required from the eductor: Draft = ∆Pburner + ∆PC.S. where:

∆Pburner = ∆PC.S. =

Pressure drop through burners, in H2O (Pa) Pressure drop though convection section, in H2O (Pa)

Both of these pressures should be based on a flow rate equivalent to one consecutive radiant section volume change in 5 minutes. The burner manufacturer's burner curves should be consulted to obtain the pressure drop through the burners. Refer to Section VIII-C to calculate the pressure drop through the convection section. Next, calculate the critical pressure and density, as well as the sonic velocity at the throat of the nozzle by using the following equations: Pthroat = 0.546 Psteam

or

Pthroat = 0.528 Pair

ρthroat =

or

ρthroat

0.628 ρsteam

v sonic

=

68.1

v sonic

=

K

K

(Customary)

⋅ Pthroat ρthroat

where: K P

ρ vsonic ➧

⋅ Pthroat ρthroat

= = = =

= 0.634 ρair

(Metric)

Specific heat ratio, 1.4 for air and 1.31 for steam Pressure, psia (Pa) Density, lb/ft 3 (kg/m3) Sonic velocity at eductor throat, ft/s (m/s)

Finally, the area of eductor nozzle discharge is found by using: A nozzle

=

A nozzle

=

167.4 Draft x A stack v 2sonic

ρthroat

+

4637 (Pthroat

− Patm )

Draft x A stack v 2sonic

ρthroat

where: Draft= A = P =

+

(Pthroat

−

Patm )

(Customary)

(Metric)

Draft required from eductor, in H2O (Pa) Area, in.2 (cm2) Pressure, psia (Pa)

To allow for vena contracta effects and inefficiencies, multiply the area of the nozzle by a factor of 1.2. So that the nozzle diameter shall be: Dnozzle

=

1.2 x 4 x A nozzle

π

And the flow rate (in lb m/hr or kg/hr) of the steam or air is found by: W = 25 x 1.2 x

ρthroat

W = 0.36 x 1.2 x

x Athroat x vsonic

ρthroat

x Athroat x vsonic

(Customary) (Metric)




Section VIII-B

Page 27 of 79

December, 2000

MECHANICAL SPECIFICATION (Cont) EMERGENCY COIL STEAM Facilities for steam purging the coil in the event of a loss of flow are occasionally specified. Since they have very limited value, they should not normally be provided. This coil purge is intended to prevent the high temperature residual heat in the heater refractory from coking the hydrocarbon remaining in the coil. Coil purge steam should never be considered as a substitute for immediately shutting off the fuel upon loss of flow in the coil. A steam purge has little or no value in services containing light hydrocarbons (reboilers, etc.) or mixtures of hydrocarbons and hydrogen (POWERFORMERS, HYDROFINERS). Experience has also shown that it is not even required in heavy hydrocarbon services if firing is immediately stopped upon loss of flow in the coil. If coil purge steam is to be specified, a steam rate equivalent to a mass velocity of about 5 lb/sec ft 2 (25 kg/s/m) should be adequate for low-pressure systems. This will evacuate the coil in less than 1 to 2 minutes. The steam supply pressure must be higher than the downstream system pressure. For pipestills, 125 to 150 psig (860 to 1040 kPa gage) steam can be used through the decoking facilities. Also, when coil purge steam is specified, consideration must be given to the effect of the steam on the downstream equipment.

MISCELLANEOUS DETAILS Observation Doors - Although IP 7-1-1 requires that sufficient observation and inspection doors be provided, the number and location should be spelled out in the design specification, since vendors will usually provide only marginal viewing of the heater interior. The following are typical observation door requirements: 1.

2.

3.

4.

V.C. Heaters a.

Two peep holes in floor for viewing radiant and shield tubes.

b.

At the lower platform level - About 4 ft, 6 in. (1.4 m) above the floor, mainly for observing burner operation. One for every burner.

c.

Upper radiant section - One to two under the convection section for viewing the arch, shield tubes and supports, and radiant tube supports.

d.

Arch - One to two for viewing radiant tubes and burners.

Cabin Heaters a.

At the lower platform level for observing the burners. One for each burner along the side(s) of the heater.

b.

At each end of the heater. Two per cell at the lower platform; one to three in the area of the upper radiant section.

c.

On the side of hoop tube heaters at the beginning of the hip section, for viewing tubes and tube guides (Figure 14).

Vertical Tube Box Heaters a.

Two peep holes in the floor of each cell, at diagonally opposite corners.

b.

At the lower platform level. One per burner or one per burner row in each cell. All doors provided must be located between tubes which are on 3 IPS spacing.

c.

At the upper platform level. One to two per cell on each side of the furnace. Two to three at each end.

d.

One in the arch over each cell.

Convection Section - All Heaters - Provide at least one set of inspection doors to view representative tubes and supports, so that deterioration and fouling can be detected. These should be vertically aligned to permit inspection of each convection section tube row and located adjacent to an intermediate tubesheet if one is provided. A second set of inspection doors should be provided for convection sections over 50 ft (15.2 m) long and should be located adjacent to an intermediate tube sheet also.



F IRED H EATERS

Page 28 of 79


December, 2000

MECHANICAL SPECIFICATION (Cont) Platforms and Ladders - As with observation doors, although covered by IP 7-1-1, platforms and ladders should be specified to minimize problems. 1.

2.

Typical platforms are provided as follows: a.

Around the heater floor (burner observation) level.

b.

As required around the heater at the upper observation door level.

c.

At both ends of the convection section for access to the header boxes.

d.

Along one side of the convection section for access to sootblowers, if used. This platform should be wide enough to extend beyond the outboard end of the sootblowers (this requirement is adequately covered by IP 7-1-1).

e.

Access to peep door locations other than those mentioned above should be by ladder, stairway or platform, as dictated by the heater arr angement.

f.

Access to dampers is normally provided.

g.

Access to instrument connections is normally provided, but in most cases these connections can be located so they are accessible from platforms provided for other reasons.

Stairways are customarily specified for access from grade to the burner control platform (floor level) and on up to the sootblower level. An additional ladder or stairway is usually provided from grade to the sootblower level at the opposite end of a heater to serve as an alternate means of escape.



Section VIII-B


Page 29 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS - CUSTOMARY UNITS PROBLEM 1 - HEATER EFFICIENCY Given:

Find:

Atmospheric pipestill service Inlet Temperature = 450°F Outlet Temperature = 725°F Throughput = 1,610,000 lb/hr Heat Absorbed = 353 MBtu/hr Fuel: Atmospheric pipestill bottoms Heater design efficiency, assuming forced-draft burners, 150 °F stack approach temperature. (Note: With current fuel costs, approach temperatures significantly below 150 °F are frequently justified.)

Solution: Ts

=

450°F

+ 150°F =

600°F

Use 20% excess air Fn

=

353 MBtu / hr = 23,460 lb / hr 15,050 Btu / hr

from Eq. (1)

(Section VIII-M, Figure 6 at 600°F and 20% excess air) Fg

= 1.01 x

Q f

=

23,460 lb / hr = 23,690 lb / hr

23,690 lb / hr x 17,500 Btu / lb

−

from Eq. (2)

414.6 MBtu / hr

from Eq. (3)

(Lower Heating Value, Section VIII-M, Figure 6)

LHV Efficiency

=

353 MBtu / hr x 100 414.6 MBtu / hr

=

85.14%

from Eq. (4)

PROBLEM 2 - VERTICAL TUBE BOX HEATER LAYOUT Given:

Same as in Problem 1

Find:

Radiant section layout

Solution:

Determine possible combinations of tube size and number of passes: W

=

1,610,000 lb / hr = 447 lb / sec 3,600 sec/ hr

TUBE SIZE IPS

ASSUME tm, in.

Ax, ft2 (TABLE 2)

p

G, lb/sec ft2 (Eq. (5)]

6 in.

0.285

0.1946

6 8 10

383 288 230

5 in.

0.258

0.1315

8 10 12

425 340 283


from Eq. (5)


F IRED H EATERS

Page 30 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Assume: 8 pass, 6 in. IPS as best combination pass arrangement per Figure 11b. Determine number of burners: 5 cells at 2 to 4 burners/cell = 10 to 20 burners Normal heat release per burner: 414.6 MBtu / hr 10

=

41.5 MBtu / hr ; 414.6 M ( too high) 15

=

27.6 M ; (OK )

414.6 M 20

=

20.7 M (OK )

From possible burner sizes, select 27.6 MBtu/hr. Minimum tube envelope around burner:

5 Ft. - 8 In. B' = 11 Ft. - 4 In.

5 Ft. - 8 In.

5 Ft. - 8 In.

W' = 22 Ft. - 8 In.

DP8BFc

Determine number of tubes per pass: a′

=

22.67

−

2

b′

=

11.33

−

0.914

x′

=

2( 20)

+ 0.586 = 21.26; a = 20 or 22 1 12 in. tube center-to-center = 2 x 6 in. IPS

1

+ 8

2(5)11

= 10.42; =

150 8

b

= 11

= 18.8

Let x = 19 tubes per pass 19 x 8 = 152 total wall tubes


from Eq. (10)

from Eq. (11)

from Eq. (12)


Section VIII-B


Page 31 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Possible arrangements: (1)

a = 20 x 2 ends =

40

b = 11 x 4 cells x 2 sides =

88

12 x 1 (center) cell x 2 =

24 152 tubes

or (2)

a = 22 x 2

44

b = 11 x 4 cells x 2 =

88

c = 10 x 1 cell x 2 =

20 152 tubes

Assume first arrangement is chosen. Center tubes: e = 20

y

=

20 2

from Eq. (13a)

= 10

from Eq. (13b)

Total tubes per pass = 19 wall + 10 center = 29 Layout (21.256 - 20) + 2 = 3.256 Ft. = 3 Ft. - 3 In. (This Space Can Be Reduced To 2 Ft. - 3 In. By Using One Long Radius U-band on Each Side of End Wall) 10 End Tubes

10

9 11 Side Wall Tubes 2

11

10 Center Tubes

10

11

6 12

11

11 DP8BFd



Page

Section VIII-B

32 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Wt

(0.707 x 12 in. = 8.5 in.) x 2

=

1 ft, 5 in.

+ (9 tube spaces at 12 in.) x 2

=

18 ft

+ Center space

=

3 ft, 3 in.

[or use Eq. (10)]

22 ft, 8 in. vs. 22 ft, 8 in. required (W′)

Wi = 22 ft, 8 in. + 3 (6 in.) = 24 ft, 2 in. B

(for 4 cells) = (0.707 x 12 in. - 8.5 in.) x 2

=

1 ft, 5 in.

+ (9 tube spaces at 12 in.)

=

9 ft

+ (1 tube space at 18 in.)

=

1 ft, 6 in. 11 ft,11 in. vs. 11 ft, 4 in. required (B ′)

[or use (Eq. 11)] B

(for center cell) = (0.708 x 12 in. = 8.5 in.) x 2

=

1 ft, 5 in.

+ (10 tube spaces at 12 in.)

=

10 ft

+ (1 tube space at 18 in.)

=

1 ft, 6 in. 12 ft, 11 in.

Lt = 4 (11 ft, 11 in.) + 12 ft, 11 in. = 60 ft, 7 in. Li = 60 ft, 7 in. + 18 in. = 62 ft, 1 in. Determine Radiant Tube Length: Choose φ1-side = 12,000 Btu/hr ft 2

see Table 1

Determine Tbw 1st Trial: Let Tbw = 1650°F (Figure 3, assuming 750 °F avg. TMT and no height correction) Qtr

=

9700 Btu / lb x 23,460 lb / hr = 227.6 MBtu / hr

from Eq. (6)

(Section VIII-M, Figure 6 at 1650°F and 20% excess air) Qr

=

A1-side =

Lr

=

227.6 MBtu / hr − 16.4 M = 211.2 MBtu / hr (Qsr from Problem 1 of Section VIII-C) 211.2 MBtu / hr 12,000 Btu / hr ft

2

= 17,000

+

∴ Tbw

from Eq. (8a)

2

1.5 x 10) 1.734 ft / ft

272 equivalent tubes, Tbw

ft 2

17,600 ft 2 8 (19

from Eq. (7)

=

17,000 471.6 ft 2 / ft

=

37.3 ft

outside surface of NPS 6

Requires height correction of (37.3 - 35 ft) 8°/ft = 18 °F = 1650°F – 18°F = 1632°F assumed


from Eq. (14)

see Table 2


Section VIII-B


Page 33 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS 2nd Trial: Assume Lr

=

38 ft

Tbw

=

1650°F – (38 - 35 ft) 8 °/ft = 1626°F

Qtr

=

9850 Btu/lb x 23,460 lb/hr = 231.1 MBtu/hr

Qr

=

231.1 MBtu/hr – 16.4 M = 214.7 MBtu/hr 214.7 MBtu / hr

A1-side =

Lr

=

Tbw

=

12,000 Btu / hr ft 17,900 ft 2 471.6 ft 2 / ft

=

2

= 17,900

ft 2

38.0 ft (checks)

(say) 1625°F

PROBLEM 3 - HOOP TUBE CABIN HEATER LAYOUT Given:

The following high pressure POWERFORMER service heating requirements for two limiting simultaneous conditions. PREHEAT I (II)

OPERATION: Throughput, lb/hr

2nd REHEAT I (II)

DRIER

TOTAL I (II)

--------------------- 376,000 (305,000) ---------------------

25,000

—

Temperature, Inlet, °F Outlet, °F

Heat Duty, MBtu/hr

Find:

1st REHEAT I (II)

512 (390)

923 (767)

970 (875)

250

995 (905)

995 (925)

995 (945)

700

152 (137)

26.5 (45.5)

9.5 (20.0)

10.0

— 198 (212.5)

Radiant section double hoop tube layout, assuming 700 °F stack temperature and forced draft burners to be used (20% excess air).

Solution: Choose Services: Preheat will be r adiant and convection 1st and 2nd Reheat will be all-radiant Drier will be all-convection Limiting Conditions: Case II is limiting for Reheats (maximum duty) Case I is limiting for Preheat and Drier (maximum duty and minimum contribution of Reheats to convection duty) ∴ Design Reheats first based on Case II, then design overall furnace based on Case I Determine Design 1st and 2nd Reheat Services Approximate radiant tube surface required: Assume: φr = 12,000 Btu/hr ft 2 1st Reheat

A r

=

2nd Reheat

45.5 MBtu / hr ft 2 12,000 Btu / hr ft

= 3790

ft

2

see Table 1

2

20.0 MBtu / hr 12,000 Btu / hr ft 2 1670 ft

2


from Eq. (8)


F IRED H EATERS

Page 34 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Total coil length required: 6 in. IPS:

3,790 ft 2

= 2,180 ft

960 ft

5 in. IPS: 3,790/1.456 = 2,600

1,150

4 in. IPS: 3,790/1.178 = 3,220

1,420

1,734 ft 2

see Table 2

Determine possible combinations of tube size, number of passes and L r W

=

376,000 lb / hr = 104.4 lb / hr (I); 305,000 3,600 sec/ hr 3,600

=

84.7 lb / sec (II)

From Coil Design Pressure (500 psig), and estimated Tm = 1150°F, estimate t m: For 6 in. IPS, t m = 0.285 in. For 4 in. and 5 in. IPS, t m = 0.245 in.; t a = 0.280 in. Desired G = 35 to 60 lb/sec ft2 Lr For chosen burners:

=

Total Coil Length Re quired No. of Passes, p

min Ht = 25 ft, B = 10 ft

∴ min LS = 20 ft Max LS = 25 ft for 4 in. IPS; 30 ft for 5, 6 in. IPS For double hoop tubes desired L r : 4 in., Lr = 4 (20 to 25) +

π (10) = 111 to 131 ft

5, 6 in., L r = 4 (20 to 30) +

π (10) = 111 to 151 ft


see Table 1


Section VIII-B


Page 35 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS TUBE SIZE, NPS

ASSUME tm, in.

Ax, ft2 (BASED ta) (TABLE 2)

6 in.

0.285

0.1946

5 in.

4 in.

0.245

0.245

0.1365

0.0845

P

G, lb/sec ft2 I/II [Eq. (5)]

Lr 1st REHEAT

Lr 2nd REHEAT

(DETERMINE LATER) Lr PREHEAT

120 ft

500 ft

8

67/54

272 ft

10

54/44

218 ←

96 ←

400

12

45/36

182

80

333

14

38/31

156

69

286

12

64/52

216

96

396

14

55/44

186

340

16

48/39

163

298

18

43/34

145

20

38/31

130

58

238

20

62/50

161

71

294

22

56/46

146

268

24

51/42

134

245

26

48/39

124

226 ←

28

44/36

115

30

41/33

107

264

210 47

196

Three possible combinations satisfy the L r and G requirements for 1st Reheat. (6 in. would require two series passes. This has the advantage of inlet and outlet on same side of heater.) Only one combination is available for 2nd Reheat. Since required Lr is less than the minimum, pass has high G for Case I.)

φr will

Select 6 in. - 10 parallel passes for both Reheat services. Use Ls = 20 ft for both services Lr = 4 (20) + π (10) = 111.4 ft EXPRESSION

2nd REHEAT

3,864 ft 2

1,932 ft2

= (Case II)

11,780 Btu/hr ft 2

10,400 Btu/hr ft 2

= (Case I)

16,850 Btu/hr ft 2

4,910 Btu/hr ft 2

Ar = (p) (Lr ) (1.734)

φr φr

1st REHEAT


be reduced. (6 in. - 8


Page

Section VIII-B

36 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Heater Layout Heater Layout

2 x 12 In. = 2 Ft.

Internal Dividing Wall Coil Inlet

9 In. + 1 In. Expansion = 10 In.

10 2 Ft. - 3 In.

6 In.

Preheat

10 Tubes.

9 Spaces @ 12 In. = 9 Ft.

6 In.

9 Ft.

9 Ft.

9 In. + 1/2 In. Expansion = 9 1/2 In. Coil Outlet

10 In. Radiant Cell Length = 22 Ft. - 2 In.

1 Ft. - 1 1/2 In.

Equivalent Shield Tube Area

11 Ft. - 1 In. 12 Ft. - 2 1/2 In.

23 Ft. - 3 1/2 In. 1st Reheat

Note:

2nd Reheat

DP8BFe

Header thermal expansion at 1000°F = 8.9 in./100 ft (see Figure 15).

Determine Reheat Service Firing Rates: Wc = 8.5 ft (from Convection Section Design - not included in sample problem) EXPRESSION

1st REHEAT

“Lc”

2nd REHEAT

23.29 ft

12.12 ft

350 φr

184 φr

2.34/4.12

0.90/1.91 MBtu/hr

Qr =

26.5/45.5

9.5/20.0

Qtr

28.8/49.6

10.4/21.9 MBtu/hr

Tbw (assume Tm = 1000 °°F) (HA) bw

1,520/1,740°F 10,400/9,230

1,415/1,685°F 10,950/9,500 Btu/lb Fig. 6, Section VIII-M

Fn = Qtr / (HA) bw

2,770/5,380

950/2,310 lb/hr

Fg = 1.01 F n (Based Total Heater Size)

2,800/5,430

960/2,330 lb/hr

Qf = Fg x 17,500 Btu/lb

49.0/95.0

16.8/40.8 MBtu/hr

No. Burners Req.

4

2

Minimum Cell Length Req.

13 ft vs. 22 ft actual

8 ft vs. 11 ft actual

Qsr = 1.77 φr Lc W c = Case I/II


from Eq. (9a) of Section VIII-C

see Figure 5


Section VIII-B


Page 37 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Design of Preheat Radiant Section Overall Furnace Performance Ts (HA )s

= =

700°F (given)

see Figure 6,

14,560 Btu / lb

Section VIII-M

=

Total Fn

198 MBtu / hr 14,560 Btu / lb

=

Fn (preheat zone)

= 13,600 lb / hr

13,600

− (2,770 + 950) =

from Eq. (1)

9,880 lb / hr

Estimate Radiant Section

φr

∴Tbw = 1750°F

(HA)bw

= =

12,000 Btu/hr ft2; 9,150 Btu/lb

Qtr

=

9,880 x 9150

= 90.5 MBtu/hr

Assume “Lc” Qsr

≈ =

40 ft 1.77 (12,000) (40) (8.5)

= 7.2 MBtu/hr

= =

Qgr – Qsr 83.3 x 106 / 12,000 = 6940 ft2

Assume

∴Qr Ar

Figure 5 Figure 6, Section VIII-M from Eq. (6)

from Eq. (9a), Section VIII-C from Eq. (7) from Eq. (8)

= 83.3 MBtu/hr

Total Coil Length Required 6 in. IPS:

6,940/1.734

=

4,000 ft

5 in.

6,940/1.456

=

4,760 ft

4 in.

6,940/1.178

=

5,890 ft

Determine Possible Combinations of Tube Size, Passes, L r . These are summarized on the previous tabulation. Use of 5-in. or 6-in. tubes would require several series passes. Two possible combinations of 4-in. tubes are apparent, with 26 passes preferred due to slightly higher G and required L r for φr = 12,000 Btu/hr ft 2, slightly over the minimum.

∴ Use 4 in. IPS, 2 series passes/26 parallel passes

6 In. = 2 In. Expansion = 8 In.

26 Tubes 25 Spaces @ 8 In. = 16 Ft. - 8 In.

2 x 8 In. = 16 In. 4 In.

8 In.

Radiant Cell Length = 36 Ft. - 4 In.

1st Reheat

Shield Area 38 Ft. - 7 In.

2nd Reheat DP8BFf



F IRED H EATERS

Page

VIII-B

38 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS Determine actual Lr for

φr = 12,000 Btu/hr ft 2:

Qsr =

1.77 (12,000) (38.58) (8.5) = 6.95 MBtu/hr

Qr

=

90.5 MBtu/hr - 7.0 M - 83.6 MBtu/hr

Ar

=

83.6 x 106 / 12,000 Btu/hr ft 2 = 6,960 ft2

Lr

=

Lr

=

Ls

=

6,960 ft 2 1.178 ft 2 / ft x 52 tubes 4 (Ls )

+ π (10)

113.9

−

31.4

4

=

20.62 ft

= 113.9

=

from Eq. (9a), Section VIII-C

ft

20 ft , 7 − 1 / 2 in.

Determine Overall Radiant Section T bw (For Convection Section Design) RADIANT SECTION

Qtr

Ar

Shield Lc

Preheat

=

90.5 MBtu/hr

6,960 ft 2

38.58 ft

1st Reheat

=

28.8

3,864

23.29

2nd Reheat

=

10.4

1,932

12.21

129.7

12,756 ft2

74.08 ft

Total

(HA)bw = Qtr / Fn = 129.7 MBtu/hr/13,600 lb/hr = 9,540 Btu/lb Tbw = 1680°F

see Figure 6, Section VIII-M

Check by Bridgewall Temperature Curve: Equivalent Shield Ar Total A =

= 1.77

(74.08 ) (8.5)

=

1,114 ft 2

12,756 + 1,114 13,870 ft2

see Figure 5

φr = 129.7 x 106 / 13,870 = 9,350 Btu/hr ft 2 Tbw = 1640°F Checks closely with heat available curve. Principal reason for difference is the method of apportioning the shield tube surface. For Convection Section Calculations use T bw = 1680°F as determined from Heat Available Curve. Use overall

φs based on Σ

Qsr / Lc W c

φs = [6.95 + 2.3 + 0.9 = 10.15 MBtu/hr] / 74.08 x 8.5 = 16,100 Btu/hr ft 2



Section


VIII-B

Page 39 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) CUSTOMARY UNITS PROBLEM 4 - TUBE METAL TEMPERATURE Given: Find:

Atmospheric Pipestill Heater Design per Problems 1 and 2. Design tube metal temperature

Solution: Do = 6.625 in., assume t m = 0.285 in., t a = 0.326 in., Di = 5.973 in. =

0.125 in., D′i = 5.973 - 0.250 = 5.723 in.

Use hi

=

300 Btu/hr° F ft2 (Calculation procedure per Section VIII-C; not included)

φr

=

12,000 Btu/hr ft2 (1-side fired basis)

C1

=

1.77

C2

=

1.33 for 40-ft radiant section

C3

=

1.08 for oil firing

(φr )max

=

12,000 x 1.77 x 1.33 x 1.08 = 30,500 Btu/hr ft2

Tb

=

725°F

Assume tc

a.

b.

∆Tf ∆Tc

=

=

1 6.625 x x 30,500 300 5.723 0.125 35

x

 2 x 6.625     x 5 . 973 5 . 723 +  

30,500

=

122°F

=

124°F

=

54°F

at 850°F,

c.

∆Tm

0.326 194

=

x

 2 x 6.625     x  6.625 + 5.973 

30,500

5% Cr at 1000°F, Tm

see Table 2

=

1025


from Eq. (17)


F IRED H EATERS

Page 40 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS METRIC UNITS PROBLEM 1 - HEATER EFFICIENCY Given:

Find:

Atmospheric pipestill service Inlet Temperature = 230°C Outlet Temperature = 385°C Throughput = 203 kg/s Heat Absorbed = 103.5 MW Fuel: Atmospheric pipestill bottoms Heater design efficiency, assuming forced-draft burners, 85 °C stack approach temperature. (Note: With current fuel costs, approach temperatures significantly below 85 °C are frequently justified.)

Solution: Ts = 230°C + 85°C = 315°C Use 20% excess air (based on ER&E forced-draft burners) Fn

=

103.5 MW 35.0 MJ / kg

=

29.6 kg / s

from Eq. (1)

(Section VIII-M, Figure 6 at 315°C and 20% excess air) Fg = 1.01 x 29.6 kg/s = 2.98 kg/s

from Eq. (2)

Qf = 103 x 2.98 kg/s x 4.07 MJ/kg = 121.5 MW (Lower Heating Value, Section VIII-M, Figure 6)

from Eq. (3)

LHV Efficiency

=

103.5 MW x 100 121.5 MW

=

85.1%

from Eq. (4)

PROBLEM 2 - VERTICAL TUBE BOX HEATER LAYOUT Given:

Same as in Problem 1

Find:

Radiant section layout

Solution:

Determine possible combinations of tube size and number of passes: W = 203 kg/s NOM. TUBE SIZE

Assume:

ASSUME

Ax, m2

from Eq. (5)

mm

(IPS, in.)

tm, mm

(TABLE 2)

p

G, kg/s•m2

150

(6)

7.24

0.01808

6 8 10

1,870 1,400 1,120

125

(5)

7.24

0.0122

8 10 12

2,080 1,660 1,380

8 pass, 150 mm NPS as best combination pass arrangement per Figure 11b.



Section VIII-B


Page 41 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Determine number of burners: 5 cells at 2 to 4 burners/cell = 10 to 20 burners Normal heat release per burner: 121.5 MW 10

=

121.5 MW 12.2 MW ; ( too high) 15

=

8.13 MW ; 121.5 MW (OK ) 20

=

6.10 MW (OK )

From possible burner sizes, select 8.13 MW. Minimum tube envelop around burner:

1730 mm B' = 3460 mm

1730 mm

1730 mm

W' = 6920 mm

DP8BFg

Determine number of tubes per pass: 6.908

−

0.610

b′ =

3.454

−

0.914

x′ =

2 (20)

+

2 (5) 11

a′ =

+ 0.586 = 21.58; a = 20 or 22 0.300 300 mm tube center-to-center = 2 x 150 mm NPS

0.300

= 10.60 ;

8

Let x

= 19 tubes per pass

19 x 8

= 152 total wall tubes

=

150 8

b

= 11

= 18.8

Possible arrangements: (1)

a = 20 x 2 ends =

40 tubes

b = 11 x 4 cells 2 sides =

88

12 x 1 (center) cell x 2 =

24 152 tubes


from Eq. (10)

from Eq. (11)

from Eq. (12)


F IRED H EATERS

Page 42 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS or (2)

a = 22 x 2

44

b = 11 x 4 cells x 2 =

88

c = 10 x 1 cell x 2 =

20 152 tubes

Assume first arrangement is chosen. Center tubes: e = 20

y

=

20 2

from Eq. (13a)

=

20

from Eq. (13b)

Total tubes per pass = 19 wall + 10 center = 29 Layout (21.58 - 20) x 0.305 + 0.61 = 1.09 m (This Space Can Be Reduced To 0.79 m By Using One Long Radius U-band on Each Side of End Wall) 10

10 End Tubes 9 11 Side Wall Tubes 2

11

10

10 Center Tubes

11

6 12

11

11 DP8BFh

W=

(0.707 x .300 m = .212 m) x 2 + (9 tube spaces at .300 m) x 2 + center space

= = =

6.91 m vs. 6.91 m required (W ′)

[or use Eq. (10)] W=

6.91 m + 3 (.150 m)

.42 m 5.40 m 1.09 m

=

7.366 m



Section VIII-B


Page 43 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS B (for 4 cells) = (0.707 x .300 m = .212 m) x 2 + (9 tube spaces at .300 m) + (1 tube space at .450 m)

= = =

.42 m 2.70 m .45 m 3.57 m vs. 3.45 m required (B ′)

[or use Eq. (11)] B (for center cell) = (0.707 x .300 m = .212 m) x 2 + (10 tube spaces at .300 m) + (1 tube space at .450 m)

Lt =

4 (3.57 m) + 3.87 m = 18.14 m

Li =

18.14 m + 0.45 m = 18.59 m

= = =

.42 m 3.00 m .45 m 3.87 m

Determine Radiant Tube Length: Choose φ1-side = 37,900 W/m2

see Table 1

Determine Tbw 1st Trial: Let Tbw = 900°C (Figure 3, assuming 400°C avg. TMT and no height correction) Qtr

=

22.6 MJ / kg x 2.96 kg / s

=

66.71 MW

from Eq. (6)

(Section VIII-G, Figure 6 at 900°C and 20% excess air) Qr

=

A1-side =

Lr

=

66.71 MW − 4.84 MW = 61.90 (Qsr from Problem 1 of Section VIII-C) 61.90 MW 37,900 W / m

2

x 106

m2

1,635 m2 8 (19

+ 1.5

x 10)

272 equivalent tubes

0.5285 m2 / m

from Eq. (8a)

=

1,635 143.7 m2 / m

= 11.4

m

outside surface of 150 mm NPS

Requires height correction of (11.4 - 10.7) 14.6 °C/m = 10°C

Tbw Tbw

= 1,635

from Eq. (7)

=

900° – 10° = 890°C vs. 900°C assumed


from Eq. (14)


Page

Section VIII-B

44 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS 2nd Trial: Assume Lr

=

11.6 m

Tbw

=

900°C – (11.6 m – 10.7 m) 14.6°C/m = 887°C

Qtr

=

22.21 MJ/kg x 2.96 kg/s = 67.74 MW

Qr

=

67.74 MW – 4.84 MW = 62.90 MW

A1-side =

62.90 MW 37,900 W / m 1,600 m

Lr

=

Tbw

= (say) 885°C

143.7 m 2 / m

2

x 10 6

= 1,660 m2

= 11.6 m (checks)

PROBLEM 3 - HOOP TUBE CABIN HEATER LAYOUT Given:

The following POWERFORMER service heating requirements for two limiting simultaneous conditions: PREHEAT I (II)

OPERATION Throughput, l b/hr

2nd REHEAT I (II)

-------------------------------- 4 7.4 ( 38.4) - ---------------------------

Temperature, Inlet, °C Outlet, °C Heat Duty, MBtu/hr

Find:

1st REHEAT I (II)

DRIER

TOTAL I (II)

3.15

267 (199) 535 (485)

495 (408) 535 (496)

521 (468) 535 (507)

121 371

44.5 (40.1) (62.3)

7.76 (13.3)

2.8 (5.9)

2.9

58.1

Radiant section double hoop tube layout, assuming 700 °F (370°C) stack temperature and forced draft burners to be used (20% excess air).

Solution: Choose Services: Preheat will be radiant and convection 1st and 2nd Reheat will be all-radiant Drier will be all-convection Limiting Conditions: Case II is limiting for Reheats (maximum duty) Case I is limiting for Preheat and Drier (maximum duty and minimum contribution of Reheats to convection duty) ∴ Design Reheats first based on Case II, then design overall furnace based on Case I Design 1st and 2nd Reheat Services Determine approximate radiant tube surface required Assume:

A r

=

φr = 37,900 W/m2

see Table 1

1st Reheat

2nd Reheat

13.3 MW x 10 6

5.9 MW x106

37,900 W / m 2

37,900 W / m2

= 351 m2

156 m2


from Eq. (8)


Section VIII-B


Page 45 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Total coil length required: 150 mm NPS:

351 m2

=

664 m

295 m

Table 2 125 mm NPS: 351 / 0.4438

=

791

352

100 mm NPS: 351 / 0.3591

=

977

434

0.5285 m 2 / m

Determine possible combinations of tube size, number of passes and L r W = 47.4 kg/s (I),

38.4 kb/s (II)

From Coil Design Pressure (3450 kPa, gage), and estimated Tm = 620°C, estimate t m: For 150 mm NPS, t m = 7.24 mm For 100 and 125 mm NPS, t m = 6.22 mm; t a = 7.11 mm Desired G = 170 to 290 kg/s .m2 Lr For chosen burners:

=

see Table 1

Total Coil Length Re quired No. of Passes, p

min Ht = 7.6 m, B = 3.0 m

∴ min LS = 6.1 m Max LS = 7.6 m for 100 mm NPS; 9.1 m for 125 and 150 mm NPS For double hoop tubes desired L r : 100 mm, Lr = 4 (6.1 to 7.6) + π (3.0) = 33.8 to 39.8 m 125, 150 mm, L r = 4 (6.1 to 9.1) + π (3.0) = 33.8 to 45.8 m

TUBE SIZE mm

ASSUME Tm, in.

Ax, ft2 (BASED ta) (TABLE 2)

P

G, lb/sec ft 2 I/II [Eq. (5)]

150

7.24

0.01808

8

328/266

82.9 m

10 12 14

262/213 218/177 187/152

66.4 55.5 47.5

12 14 16 18 20

312/252 267/216 234/189 208/168 187/151

65.8 56.7 49.7 44.2 39.6

29.3

20 22 24 26 28 30

302/245 274/222 252/204 232/188 216/175 201/163

49.1 44.5 40.8 37.8 35.1 32.6

21.6

125

100

6.22

6.22

0.01268

0.00785

Lr 1st REHEAT

←

Lr 2nd REHEAT

(DETERMINE LATER) Lr PREHEAT

36.6 m

152.4 m

29.3 ← 24.4 21.0

121.9 101.5 87.2

17.7

14.3


120.7 103.6 90.8 80.5 72.5 89.6 81.7 74.7 68.9 ← 64.0 59.7


F IRED H EATERS

Page

VIII-B

46 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Three possible combinations satisfy the L r and G requirements for 1st Reheat. (150 mm would require two series passes. This has the advantage of inlet and outlet on same side of heater.) Only one combination is available for 2nd Reheat. Since required Lr is less than the minimum, 8 pass has high G for Case I).

φr will be reduced (150 mm –

Select 150 mm - 10 parallel passes for both Reheat services. Use Ls = 6.1 m for both services Lr = 4 (6.1) + (3.0) = 33.8 m EXPRESSION

1st REHEAT

2nd REHEAT

360 m2

180 m2

(Case I)

21,700 W/m2

15,600 W/m2

(Case II)

36,000 W/m2

32,800 W/m2

Ar

=

(p) (Lr ) (0.5285)

φr φr

= =

Heater Layout Heater Layout

2 x 300 mm = 600 mm

225 mm + 25 mm Expansion = 250 mm

Internal Dividing Wall Coil Inlet

10 690 mm

150 mm

Preheat

10 Tubes.

9 Spaces @ 300 mm = 2700 mm

150 mm

2700 mm

2700 mm

225 mm + 13 mm Expansion = 238 mm Coil Outlet

250 mm Radiant Cell Length = 6650 mm

345 mm

Equivalent Shield Tube Area

3330 mm 3670 mm

7000 mm 1st Reheat

Note:

2nd Reheat

Header thermal expansion at 540°C = 7.4 mm/m (see Figure 15).


DP8BFi


Section


VIII-B

Page 47 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Determine Reheat Service Firing Rates: Wc = 2.59 m (from Convection Section Design - not included in sample problem) EXPRESSION

1st REHEAT

2nd REHEAT

“Lc”

6.99 m

3.67 m

= 1.77 φr Lc W c

32.1 φr

16.8 φr

= Case I/II

0.70/1.18

0.26/0.55 MW

=

7.76/13.33

2.78/5.86 MW

Qtr

8.46/14.51

3.04/6.41 MW

Tbw (assume T m = 540°C)

825/950°C

770/920°C

(HA)bw

24.2/21.5

25.5/22.2 MJ/kg

Fn = Qtr / (HA)bw

0.350/0.675

0.119/0.290 kg/s

Fg = 1.01 F n (Based Total Heater Size)

0.353/0.682

0.120/0.293 kg/s

14.4/27.8

4.88/11.9 MW

4

2

4.0 m vs. 6.7m

2.4 m vs. 3.4 m

Qsr

Qr

Qf = Fg x 40.7 MJ/kg No. Burners Req. Minimum Cell Length Req.


see Figure 5 see Figure 6, Section VIII-M

Design of Preheat Radiant Section Overall Furnace Performance Ts (HA)s

= =

Total Fn =

370°C (given) 33.9 MJ/kg 58.1 MW 33.9 MJ / kg


= 1.714 kg / s

from Eq. (1)

Fn (preheat zone) = 1.714 – (0.350 + 0.119) = 1,245 kg/s Estimate Radiant Section Assume

φr

=

∴

37,900 W/m2;

Tbw = 955°C

(HA)bw =

9,150 Btu/lb

Qtr

=

1.245 kg/s x 21.3 MJ/kg

Assume “Lc”

≈

12.2 m

Qsr

=

∴ Qr

=

Ar

see Figure 5 see Figure 6, Section VIII-M =

26.5 MW

from Eq. (6)

1.77 (37,900) x 12.2 x (2.59)

=

2.12 MW


Qtr – Qsr

=

24.4 MW

from Eq. (7)

=

24.4 x 106 W 2

37,900 W / m

=

664 m2

Total Coil Length Required 150 mm NPS: 125 mm NPS: 100 mm NPS:

644/0.5285 = 1,220 m 644/0.4438 = 1,450 m 644/0.3591 = 1,790 m


from Eq. (8)


F IRED H EATERS

Page 48 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Determine Possible Combinations of Tube Size, Passes, L r . These are summarized on the previous tabulation. Use of 125 or 150 mm nominal diameters tubes would require several series passes. Two possible combinations of 100 mm nominal diameter tubes are apparent, with 26 passes preferred due to slightly higher G and required L r for φr = 37,900 W/m2, slightly over the minimum.

∴ Use 100 mm NPS, 2 series passes/26 parallel passes

150 mm + 50 mm Expansion = 200 mm

26 Tubes 25 Spaces @ 200 mm = 5000 mm

2 x 200 mm = 400 mm 100 mm

200 mm

Radiant Cell Length = 10900 mm Shield Area 11590 mm

1st Reheat

2nd Reheat DP8BFj

Determine actual Lr for

φr = 37,900 W/m2:

Qsr

=

1.77 (37,900) (11.6) (2.59) = 2.02 MW

Qr

=

26.5 MW – 2.0 MW = 24.5 MW

Ar

=

24.5 MW x 106 / 37,900 W/m 2 = 646 m2

Lr

=

Lr

=

Ls

=

646 m 2 0.3591 m2 / m x 52 tubes 4 (Ls ) 34.6

= 113.9

ft

+ π (3.05)

− 9.6 = 6.25 m

4




Section VIII-B


Page 49 of 79

December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS Determine Overall Radiant Section T bw (For Convection Section Design) RADIANT SECTION

Qtr 26.52 MW

Ar 646 m 2

Shield Lc

Preheat

=

1st Reheat

=

8.46

360

6.99 m

2nd Reheat

=

3.04

180

3.67 m

Total

38.0 MW

1,186 m 2

11.59 m

22.25 m

(HA)bw = Qtr / Fn = 38.0 MW / 1.714 kg/s = 22.2 MJ/kg Tbw = 915°C


Check by Bridgewall Temperature Curve: Equivalent Shield Ar

=

1.77 Lc Wc = 1.77 (22.25) (2.59) = 102 m2 1,186

Total A

=

+ 102 1,286 m2

φr

=

38.0 MW / 1,288 m2 = 29,500 W/m2

Tbw

=

895°C

see Figure 5

Checks closely with heat available curve. Principal reason for difference is the method of apportioning the shield tube surface. For Convection Section Calculations use T bw = 915°C as determined from Heat Available Curve. Use overall

φs based on Σ Qsr / LcW c

φs = [2.02 + 0.70 + 0.26 = 2.98 MW] / 22.25 x 2.59 = 51,700 W/m 2 Problem 4 - Tube Metal Temperature Given: Find:

Atmospheric Pipestill heater design per Problems 1 and 2 Design tube metal temperature

Solution: Do = 168.3 mm, assume t m = 7.24 mm, t a = 8.28 mm, D i = 151.7 mm Assume tc Use

hi

φr C1 C2 C3 (φr )max Tb

see Table 2

=

3.17 mm, Di′ = 151.7 – 6.35 = 145.3 mm

= = = = = = =

1,700 W/m2°C (Calculation procedure per Section VIII-C; not included) 37,900 W/m2 (one-side fired basis) 1.77 1.33 for 12.2 m radiant section 1.08 for oil firing 37,900 x 1.77 x 1.33 x 1.08 = 96,300 W/m2 385°C



F IRED H EATERS

Page

VIII-B

50 of 79


December, 2000

SAMPLE PROBLEMS / CALCULATIONS (Cont) METRIC UNITS a.

∆Tf =

b.

∆Tc =

1 168.3 x x 96,300 300 145.3

10− 3 x

3.17 5.05

x

 2 x 168.3      151.7 + 145.3 

x 96,300

=

66°

=

68°C

=

30°F

=

549°C

at 455°C

c.

∆Tm =

10−3 x

8.28 28

x

 2 x 168.3      168.3 + 151.7 

x 96,300

5% Cr at 540°C, Figure 14 Tm


from Eq. (17)



Section VIII-B

Page 51 of 79

December, 2000

NOMENCLATURE Ao

=

Total outside surface area, ft2/ft (m2/m)

Ar

=

Radiant section tube surface, ft 2 (m2)

Ax

=

Cross-sectional flow area through tube, ft2 (m2)

A1-side

=

Equivalent radiant section tube surface, based on one-side fired heat density, ft 2 (m2)

a

=

Number of wall tubes at one end of furnace, dimensionless

B

=

Cell width - tube center to tube center, ft (m)

b

=

Number of wall tubes at one side of one cell

C1

=

Factor for peripheral maldistribution of heat transfer around radiant tube

C2

=

Factor for vertical maldistribution of heat transfer

C3

=

Luminosity factor

c

=

Tube center-to-center spacing, ft (m)

Di

=

Tube inside diameter, based on average wall thickness, in. (mm)

Di′

=

Inside diameter of coke layer, in. (mm)

Do

=

Tube outside diameter, in. (mm)

Dr

=

Radiant section diameter (inside refractory), ft (m)

Dt

=

Radiant section tube circle diameter, ft (m)

E

=

Modulus of elasticity, psi (MPa)

ELHV

=

Thermal efficiency based on lower heater value, percent

e

=

Number of center tubes between adjacent cells

Fg

=

Gross fuel required, lb/hr (kg/s)

Fn

=

Net fuel required, lb/hr (kg/s)

FG

=

Lb flue gas/lb fuel (kg flue gas/kg fuel)

f

=

Corrosion fraction, dimensionless

G

=

Fluid mass velocity through the coil, lb/sec. ft 2 (kg/s m 2)

Ht

=

Height of hoop tube above floor, ft (m)

HA

=

Heat available from fuel, Btu/lb (MJ/kg)

(HA)bw =

Heat available at bridgewall temperature, Btu/lb (MJ/kg)

(HA)s

=

Heat available at stack temperature, Btu/lb (MJ/kg)

hi

=

Inside film coefficient, Btu/hr ft 2 °F (W/m2 °C)

kc

=

Thermal conductivity of coke,

km

=

Thermal conductivity of tube wall,

Lc

=

Convection section inside length, ft (m)

Li

=

Radiant section overall length (inside refractory), ft (m)

Lr

=

Radiant tube effective length, ft (m)

Ls

=

Exposed straight tube length of hoop tube, ft (m)

Lt

=

Radiant section length, tube center to tube center, ft (m)

LHV

=

Fuel lower heating value, Btu/lb (MJ/kg)

Nc

=

Number of tubes in convection section

Nr

=

Number of tubes in radiant section

n

=

Number of cells in radiant section

Btu / hr ft 2 ( W / m°C) °F / in. Btu / hr ft 2 ( W / m°C) °F / in.



Page

Section VIII-B

52 of 79


December, 2000

NOMENCLATURE (Cont) Pa

=

Absolute pressure, psia (kPa-abs)

Pe

=

Elastic design pressure, psig (kPa-gage)

Pr

=

Rupture design pressure, psig (kPa-gage)

p

=

Number of parallel passes

Qa

=

Total furnace heat absorbed, Btu/hr (MW)

Qc

=

Heat absorbed in convection section, excluding shield section radiant duty, Btu/hr (MW)

Qf

=

Heat fired, Btu/hr (MW)

Qr

=

Heat absorbed by radiant section surface, Btu/hr (MW)

Qsr

=

Radiant heat absorbed by the shield section, Btu/hr (MW)

Qtc

=

Total convection section duty, Btu/hr (MW)

Qtr

=

Total furnace radiant duty, Btu/hr (MW)

Se

=

Allowable elastic design stress, psi (MPa)

Sr

=

Allowable creep rupture design stress, psi (MPa)

Sth

=

Allowable thermal stress, psi (MPa)

Sy

=

Minimum yield strength, psi (MPa)

Tb

=

Bulk fluid temperature, °F (°C)

Tbw

=

Bridgewall temperature, °F (°C)

Tg

=

Flue gas temperature, °F (°C)

Tm

=

Maximum tube metal temperature, °F (°C)

Tr

=

Fluid temperature, °F (°K)

Ts

=

Stack temperature, °F (°C)

TCA

=

Total corrosion allowance, in. (mm)

ta

=

Average thickness of tube wall, in. (mm)

tc

=

Design coke thickness, in. (mm)

tm

=

Minimum thickness of tube wall, in. (mm)

W

=

Fluid flow rate through the furnace, lb/sec (kg/s)

Wc

=

Convection section inside width, ft (m)

Wf

=

Flue gas rate, lb/sec (kg/s)

Wi

=

Radiant section overall width inside refractory, ft (m)

Wt

=

Radiant section width, tube center to tube center, ft (m)

x

=

Number of wall tubes (1-side fired) per pass

Y

=

Ratio of outside to inside tube diameter, dimensionless

y

=

Number of center tubes (2-side fired) per pass

∆Tc ∆Tf ∆Tm α γ

=

Temperature rise across coke layer, °F (°C)

=

Temperature rise across inside oil film, °F (°C)

=

Temperature rise across tube wall, °F (°C)

=

Poissons ration, dimensionless

φr (φr )max φ1-side φ2-side

=

Radiant section average heat density, Btu/hr ft 2 (W/m2)

=

Maximum point radiant heat density, Btu/hr ft 2 (W/m2)

=

Radiant section average heat density for tubes fired from one side only, Btu/hr ft 2 (W/m2)

=

Radiant section average heat density for tubes fired from two sides, Btu/hr ft 2 (W/m2)

ν

=

Poisson’s ratio, dimensionless

=

Coefficient of thermal expansion, °F-1 (°C-1)




Section VIII-B

Page 53 of 79

December, 2000

COMPUTER PROGRAMS GUIDANCE AND CONSULTING For up-to-date information on available programs and how to use them, affiliate personnel should get in touch with their Affiliate Library Contact. Fairfax personnel should consult either the ExxonMobil Engineering Section responsible for the technology involved and/or the ExxonMobil Engineering Technical Program Contact.

LITERATURE The following references are available:

•

ExxonMobil Engineering Computer User's Manual, published and maintained by the Computer Technology and Services Division.

•

Computer Program Library Catalog, published and maintained by ExxonMobil's Communications and Computer Sciences (CCS) Department - Computer Technology and Training.

AVAILABLE PROGRAMS The applicable programs available at the time of this writing are listed below: PROGRAM NO.

TITLE AND DESCRIPTION

3558

Radiant and Convection Section Design. Also used for fuel heat available.

3660

Heater pressure drop for all-liquid, all-vapor or vaporizing services.



Page

Section VIII-B

54 of 79


December, 2000

TABLE 1 DESIGN CONDITIONS FOR PROCESS HEATERS Suggested Average Heat Density φr (Based on One-Side Firing)* Btu/hr ft2 [W/m2]

Services Atmospheric Pipestill Preheaters Vacuum Pipestill Preheaters

Light Ends Units a. Preheaters and Reboilers b. Rich and Lean Oil Heating Lube Rerun Still Preheaters

Suggested Design Mass Velocity G lb/sec ft2 [kg/s • m2]

Remarks

12,000 [37,900]

300 (min) to 400 [1,450 (min) to 1,950]

Salt content less than 40 lb/1000 barrels.

10,000 [31,600]

350 (min) to 450 [1,700 (min) to 2,200]

See below for outlet tubes. Timetemperature limitations for lube vacuum pipestill.

12,000 [37,900] 12,000 [37,900]

250 (min) to 350 [1,200 (min) to 1,700] 250 (min) to 350 [1,200 (min) to 1,700]

12,000 [37,900]

250 (min) to 350 [1,200 (min) to 1,700]

12,000 [37,900] 12,000 [37,900]

250 (min) to 350 [1,200 (min) to 1,700] 250 (min) to 350 [1,200 (min) to 1,700]

6,000 [18,900]

250 to 350 [1,200 (min) to 1,700]

Low heat density due to poor film coefficient.

10,000 [31,500]

250 to 350 [1,200 (min) to 1,700]

Require long runs and dependable operation. Must handle dirty, heavy materials.

12,000 [37,900] 12,000 [37,900] 12,000 [37,900]

35 to 60 [170 to 290] 100 to 200 [500 to 1,000] 15 to 40 [75 to 200] 35 to 60 [175 to 300]

Less than 80% vaporization. No highly unsaturated materials which polymerize.

—

Distillate & Gas Oil Heaters a.

FCCU Preheat

b.

Hydrofiner Preheat

Asphalt Heaters Residuum Heaters in Fuel Oil Circulating Systems Catalytic Reforming a. Gas Preheat b. Naphtha Preheat c. Mixed Gas and Naphtha (Low Pressure Powerformer) (High Pressure Powerformer) Lube Treating (Phenolfiner) a. Extract Solution b. Raffinate Solution Cracking Service (Visbreaker) a. Heater b. Soaker Desulfurization Preheat (GO-Finer)

12,000 [37,900] 7,000 [22,100]

All-vapor services. Low Pressure: Approx. 300 - 450 psi High Pressure: Approx. 450 - 600 psi

100 to 200 [500 to 1,000] 200 to 300 [1,000 to 1,500]

See Note 1

12,000 [37,900]

—

—

600 to 900 [2,900 to 4,400]

Based on absence of cracking coil tar and only moderate salt content. Design to 910°F max. Film based on cleaned tube.

250 to 350 [1,200 to 1,700]

Mixture of feed and recycle gas at high pressure.

Notes:

* (1)

Average heat densities for two-side fired tubes are 1.5 times the heat densities listed for one-side fired tubes. Design heat fluxes are set as required to achieve film temperature criteria of 910 °F maximum. Typical average heat fluxes are 10,000 Btu/hr ft 2 [31,600 W/m 2] for heater and 5,000 Btu/hr ft 2 [15,800 W/m 2] for soaker.



Section

Page

VIII-B


55 of 79

December, 2000

TABLE 2 COMMON HEATER TUBE SIZES AND PROPERTIES(1) Nominal Size (NPS), in.

Outside Diameter Do, in.

Avg. Inside Diameter Di, in.

Pipe Schedule

Average Wall Thickness ta, in.(2)

Minimum Wall Thickness tm, in.(2)

Flow Area Ax, ft2(3)

Inside Surface Area Ai, ft2 per ft(3)

Outside Surface Area Ao, ft2 per ft

8

8.625

7.981 7.973 7.939 7.767 7.625

40 — — — 80

0.322 0.326 0.343 0.429 0.500

0.282 0.285 0.300 0.375 0.437

0.3480 0.3467 0.3435 0.3290 0.3171

2.089 2.087 2.080 2.033 1.996

2.258

6

6.625

6.065 5.973 5.939 5.767 5.761

40 — — — 80

0.280 0.326 0.343 0.429 0.432

0.245 0.285 0.300 0.375 0.378

0.2006 0.1946 0.1922 0.1814 0.1810

1.587 1.564 1.555 1.510 1.508

1.734

5

5.563

5.047 4.911 4.877 4.813 4.705

40 — — 80 —

0.258 0.326 0.343 0.375 0.429

0.226 0.285 0.300 0.328 0.375

0.1390 0.1315 0.1296 0.1265 0.1207

1.321 1.286 1.277 1.260 1.232

1.456

4

4,500

4.026 3.848 3.826 3.814 3.642

40 — 80 — —

0.237 0.326 0.337 0.343 0.429

0.207 0.285 0.295 0.300 0.375

0.0884 0.0808 0.0798 0.0793 0.0723

1.055 1.007 1.002 0.998 0.953

1.178

Avg. Inside Diameter Di, mm

Pipe Schedule

Average Wall Thickness ta, mm(2)

Minimum Wall Thickness tm, mm(2)

Flow Area Ax, m2(3)

Inside Surface Area Ai, m2 /m(3)

Outside Surface Area Ao, m2 /m

Nominal Pipe Size, mm

Outside Diameter Do, mm

200

219.1

202.7 202.5 201.6 197.3 197.3

40 — — — 80

8.18 8.28 8.71 10.90 12.70

7.16 7.24 7.62 9.53 11.10

0.03233 0.03221 0.03191 0.03057 0.02946

0.6367 0.6361 0.6340 0.6197 0.6084

0.6882

150

168.3

154.1 151.7 150.9 146.5 146.3

40 — — — 80

7.11 8.28 8.71 10.90 10.97

6.22 7.24 7.62 9.53 9.60

0.01864 0.01808 0.01786 0.01685 0.01682

0.4837 0.4767 0.4740 0.4602 0.4596

0.5285

125

141.3

128.2 124.7 123.9 122.3 119.5

40 — — 80 —

6.55 8.28 8.71 9.53 10.90

5.74 7.24 7.62 8.33 9.53

0.01291 0.01222 0.01204 0.01175 0.01121

0.4026 0.3920 0.3892 0.3840 0.3755

0.4438

100

114.3

102.3 97.7 97.2 96.9 92.5

40 — 80 — —

6.02 8.28 8.56 8.71 10.90

5.26 7.24 7.49 7.62 9.53

0.00821 0.00751 0.00741 0.00737 0.00672

0.3216 0.3069 0.3054 0.3042 0.2905

0.3590

Notes: (1)

This table lists common tube sized for the convenience of the designer. Intermediate tube thickness can and should be used whenever appropriate.

(2)

Specify wall thickness given in bold type (specify t m, unless t a corresponds to a standard pipe schedule, in which case specify t a). Note that t m = 0.875 t a.

(3)

Based on average wall thickness.



F IRED H EATERS

Page

VIII-B

56 of 79


December, 2000

TABLE 3 ALLOWABLE ELASTIC AND CREEP RUPTURE STRESS FOR TYPICAL HEATER TUBE MATERIALS

Elastic and Creep Rupture Stress, psi Medium Carbon Steel

C-1/2 Mo

1-1/4 Cr-1/2 Mo

2-1/4 Cr-1 Mo

5 Cr-1/2 Mo

9 Cr-1 Mo

18 Cr-8 Ni

Temp. °F(1)

Elastic Stress

Creep Stress

Elastic Stress

700

15,800

20,800

15,700

15,250

18,000

16,800

15,900

16,000

750

15,500

16,900

15,400

15,000

18,000

16,500

15,500

15,700

800

15,000

13,250

15,000

14,600

17,900

15,900

15,100

15,400

850

14,250

10,200

14,500

14,250

17,500

15,200

14,500

15,100

900

13,500

7,500

14,000

17,000

13,800

17,500

17,000

16,700

14,400

13,250

13,750

20,500

14,750

950

12,600

5,400

13,400

10,250

13,300

10,900

16,500

12,100

13,500

9,600

13,000

13,750

14,400

1,000

11,500

3,700

12,700

5,900

12,800

6,700

15,750

8,700

12,400

7,000

12,100

9,300

14,200

1,050

11,900

3,400

12,100

4,150

14,750

6,400

11,300

5,100

11,200

6,200

13,800

15,500

1,100

10,900

2,000

11,400

2,600

13,600

4,600

10,250

3,700

10,100

4,150

13,500

12,200

1,150

12,300

3,150

9,200

2,700

9,000

2,750

13,100

9,600

1,200

10,700

1,750

8,200

1,950

7,700

1,860

12,700

7,500

6,450

1,250

12,300

5,900

Creep Stress

Elastic Stress

Creep Stress

Elastic Stress

Creep Stress

Elastic Stress

Creep Stress

1,250

Elastic Stress

Creep Stress

Elastic Stress

Creep Stress

Elastic and Creep Rupture Stress, MPa Medium Carbon Steel

C-1/2 Mo

1-1/4 Cr-1/2 Mo

2-1/4 Cr-1 Mo

5 Cr-1/2 Mo

9 Cr-1 Mo

18 Cr-8 Ni

Temp. °C(1)

Elastic Stress

Creep Stress

Elastic Stress

375

108.0

135.0

108.0

104.0

125.0

115.0

110.0

110.0

400

105.0

115.0

106.0

102.0

124.5

112.5

107.5

108.0

425

102.5

93.0

104.0

100.0

123.0

110.0

105.0

106.0

450

99.0

73.5

101.0

98.0

121.0

105.0

100.0

104.0

475

95.0

56.5

97.0

132.5

96.0

135.0

119.0

125.0

100.0

100.0

96.0

156.0

102.0

500

89.0

42.0

94.0

86.0

93.5

88.0

116.0

93.0

95.0

75.0

91.0

110.0

100.0

525

82.5

30.5

90.0

52.5

90.0

57.0

112.0

70.0

89.5

56.0

86.0

77.0

98.0

550

85.0

32.0

86.0

37.0

106.0

53.0

82.0

42.0

80.0

53.5

96.0

575

80.0

20.0

82.0

28.0

100.0

40.0

75.5

31.5

74.5

37.0

94.0

98.0

600

73.0

12.0

77.0

15.7

92.5

29.5

69.0

23.6

67.5

26.0

92.0

80.0

625

83.5

20.5

62.5

17.7

60.0

18.0

90.0

64.0

650

73.0

11.5

56.5

13.2

53.0

12.5

88.0

52.0

45.0

8.8

85.0

41.5

Creep Stress

Elastic Stress

Creep Stress

Elastic Stress

Creep Stress

Elastic Stress

Creep Stress

Elastic Stress

675 Note: (1)

For intermediate temperatures, stresses can be obtained by graphical interpolation. Source: API Standard 530


Creep Stress

Elastic Stress

Creep Stress


Section VIII-B


Page 57 of 79

December, 2000

TABLE 4 COEFFICIENT OF THERMAL EXPANSION FOR TYPICAL HEATER TUBE MATERIALS Mean Coefficient of Thermal Expansion Between 70 °F and Indicated Temperature, °F-1 (Multiply Table Value by 10 -6) Temp, °F

Carbon Steel thru 3 Cr-1 Mb

5 Cr-1/2 Mo thru 9 Cr-1 Mo

18 Cr-8 Ni Type 304 SS

700

7.44

6.80

9.92

750

7.54

6.88

9.99

800

7.65

6.96

10.05

850

7.57

7.03

10.11

900

7.84

7.10

10.16

950

7.91

7.16

10.23

1000

7.97

7.22

10.29

1050

8.05

7.27

10.34

1100

8.12

7.32

10.39

1150

8.16

7.37

10.44

1200

8.19

7.41

10.48

1250

8.24

7.45

10.51

1300

8.28

7.49

10.54

Source: ExxonMobil Engineering Materials Data Book

Mean Coefficient of Thermal Expansion Between 20 °C and Indicated Temperature, °C-1 (Multiply Table Value by 10 -6) Temp, °C

Carbon Steel thru 3 Cr-1 Mo



375

13.42

12.26

17.87

400

13.59

12.39

17.99

425

13.76

12.52

18.09

450

13.92

12.64

18.18

475

14.07

12.75

18.28

500

14.19

12.85

18.37

525

14.31

12.95

18.47

550

14.42

13.04

18.56

575

14.53

13.12

18.65

600

14.62

13.20

18.73

625

14.69

13.28

18.80

650

14.76

13.34

18.86

675

14.82

13.41

18.92

700

14.90

13.47

18.96




F IRED H EATERS

Page 58 of 79


December, 2000

TABLE 5 MODULUS OF ELASTICITY FOR TYPICAL HEATER TUBE MATERIALS Modulus of Elasticity, psi (Multiply Table Value by 10 6)

Temp, °F

Medium Carbon Steel

C-1/2 Mo thru 3 Cr-1 Mo



700

25.4

26.6

24.9

24.8

750

24.6

26.1

24.5

24.4

800

23.8

25.7

24.2

24.1

850

22.7

25.1

23.8

23.7

900

21.5

24.5

23.5

23.4

950

20.1

23.7

23.1

23.1

1000

18.8

23.0

22.8

22.7

1050

16.9

21.7

22.3

22.3

1100

15.0

20.4

21.9

22.0

1150

13.1

18.0

21.3

21.6

1200

11.2

15.6

20.8

21.3

1250

20.1

21.0

1300

19.5

20.7


Modulus of Elasticity, MPa (Multiply Table Value by 10 3)

Temp, °C

Medium Carbon Steel

C-1/2 Mo thru 3 Cr-1 Mo



375

174

183

171

171

400

169

180

169

168

425

164

177

167

166

450

158

174

165

164

475

150

170

163

162

500

142

166

160

160

525

134

161

158

158

550

124

155

156

155

575

112

147

153

153

600

100

137

150

151

625

89

121

146

149

650

77

107

143

147

675

139

145

700

135

143




Section VIII-B


Page 59 of 79

December, 2000

TABLE 6 MINIMUM YIELD STRENGTHS FOR TYPICAL HEATER TUBE MATERIAL Minimum Yield Strength, psi

Temp, °F

Medium Carbon Steel

C-1/2 Mo

1-1/4 Cr-1/2 Mo

2-1/4 Cr-1 Mo

5 Cr-1/2 Mo

9 Cr-1 Mo

18 Cr-8 Ni

700

23,900

23,800

22,900

27,000

25,000

23,800

17,700

750

23,100

23,100

22,300

27,000

24,500

23,200

17,400

800

22,200

22,500

22,000

26,800

23,800

22,500

17,000

850

21,300

21,900

21,300

26,200

22,900

21,800

16,700

900

20,200

21,000

20,800

25,600

21,500

20,600

16,400

950

18,900

20,100

20,000

24,800

20,100

19,500

16,000

1,000

17,300

19,100

19,100

23,500

18,600

18,100

15,700

1,050

17,900

18,200

22,100

17,000

16,600

15,300

1,100

16,400

17,000

20,400

15,700

15,000

15,000

1,150

18,400

13,700

13,300

14,500

1,200

1,600

12,200

11,500

14,100

9,600

13,600

1,250

Minimum Yield Strength, MPa

Temp, °C

Medium Carbon Steel

C-1/2 Mo

1-1/4 Cr-1/2 Mo

2-1/4 Cr-1 Mo

5 Cr-1/2 Mo

9 Cr-1 Mo

18 Cr-8 Ni

375

162.5

162.5

157.0

186.0

172.5

164.0

122.0

400

158.0

160.0

155.0

185.0

169.0

160.0

120.0

425

155.0

156.0

152.0

184.5

165.0

155.0

117.5

450

148.0

151.0

149.0

181.0

159.0

150.0

115.0

475

141.0

146.0

145.0

178.0

151.0

144.0

113.0

500

134.0

140.0

140.0

174.0

143.0

136.0

111.0

525

125.0

135.0

135.0

167.0

134.0

128.0

109.0

550

128.0

130.0

160.0

124.0

120.0

107.0

575

120.0

123.0

150.0

113.0

110.0

105.0

600

110.0

116.0

138.5

103.0

100.0

103.0

625

125.0

94.0

90.0

100.0

650

110.0

84.0

80.0

97.0

67.5

94.0

675 Source: API Standard 530.



Page

Section VIII-B

60 of 79


December, 2000

FIGURE 1 AVERAGE RADIANT HEAT DENSITY FOR CABIN AND VERTICAL-CYLINDRICAL HEATERS Average Radiant Density φr , W/m2 0

10,000

20,000

30,000

40,000

50,000

60,000 1200

1100

2000

1000 C , °

F ,

w b

°

w b

T e r u t a r e p m e T l l a w e g d i r B

900

AVG TMT 1500 °F (800 °C)

1500

800

Radiant Section Height <25 Ft (7.6m) Reduce Tbw 8 °F/Ft (14.6 C/m) for Height > 25 Ft (7.6m) 700 1000 °F (550 C) 750 °F (400 C) 600

500 °F (250 C) 1000 0

5,000

10,000

15,000

Average Radiant Heat Density φr , Btu/Hr Ft 2 DP8BF01


20,000



Section VIII-B


Page 61 of 79

December, 2000

FIGURE 2 AVERAGE RADIANT HEAT DENSITY FOR CABIN HEATERS WITH UNSHIELDED CENTER REFRACTORY WALL Average Radiant Density φr , W/m2 0

10,000

20,000

30,000

40,000

50,000

60,000 1200

1100

2000

1000 C , °

F ,

w b

°

w b


900

AVG TMT 1500 °F (800 °C)

1500

800

Radiant Section Height <25 Ft (7.6m) Reduce Tbw 8 °F/Ft (14.6 C/m) for Height > 25 Ft (7.6m) 700 1000 °F (550 C) 750 °F (400 C)

600

500 °F (250 C) 1000 0

5,000

10,000

15,000



20,000



Page

Section VIII-B

62 of 79


December, 2000

FIGURE 3 AVERAGE RADIANT HEAT DENSITY FOR VERTICAL TUBES BOX HEATERS (CONTAINING BOTH ONE-SIDE AND TWO-SIDE FIRED TUBES) Average Radiant Density φr , W/m2

φ2-Side

0

15,000

30,000

45,000

60,000

75,000

90,00 0

φ1-Side

0

10,000

20,000

30,000

40,000

50,000

60,000 1200

1100

2000

1000 F ,

C ,

°

°

w b


w b

900 AVG TMT 1500 °F (800 °C)

1500

800

Radiant Section Height = 35Ft (10.7m) Reduce Tbw 8°F/Ft For Height > 35Ft (14.6 C/m for Height > 10.7m) Increase Tbw 8°F/Ft For Height < 35Ft (14.6 C/m For Height < 10.7m)

1000 °F (550 C)

700

750 °F (400 C)

600

500 °F (250 C) 1000

φ1-Side φ2-Side DP8BF03

0 0

5,000 5,000

10,000 10,000

15,000

15,000 20,000

25,000

Average Radiant Heat Density φr , Btu/Hr Ft 2


20,000 30,000



Section VIII-B


Page 63 of 79

December, 2000

FIGURE 4 AVERAGE RADIANT HEAT DENSITY FOR VERTICAL TUBES BOX HEATERS (TWO-SIDE FIRED TUBES ONLY) Average Radiant Heat Density φr = φ Side, W/m2 20,000

30,000

40,000

50,000

60,000

70,000

80,000 1200

1100

2000

1000 F , °

C ,

w b

T e r u t a r e p m e T l l a w e g d i r 1500 B

°

w b

900 AVG TMT 1500 °F (800 °C) 800

700 1000 °F (550 C)

Radiant Section Height = 35Ft (10.7m) Reduce Tbw 8°F/Ft For Height > 35Ft (14.6 C/m for Height > 10.7m) Increase Tbw 8°F/Ft For Height < 35Ft (14.6 C/m For Height < 10.7m)

750 °F (400 C) 500 °F (250 C) 1000 5,000 DP8BF04

10,000

15,000

20,000

Average Radiant Heat Density φr = φ Side, W/m2


600

25,000



Page

Section VIII-B

64 of 79


December, 2000

FIGURE 5 AVERAGE RADIANT HEAT DENSITY FOR HOOP-TUBE CABIN HEATERS Average Radiant Density φr , W/m2 0

10,000

20,000

30,000

40,000

50,000

60,000 1200

1100

2000

1000 C , °

F ,

w b

°

w b


AVG TMT

900

1500 °F (800 °C) 1500 800 Radiant Section Height <25 Ft (7.6m) Reduce T bw 8 °F/Ft (14.6 C/m) for Height > 25 Ft (7.6m) 700

1000 °F (550 C) 750 °F (400 C) 500 °F (250 C)

600

1000 0

5,000

10,000

15,000



20,000



Section VIII-B


Page 65 of 79

December, 2000

FIGURE 6 AVERAGE RADIANT HEAT DENSITY FOR HORIZONTAL TUBE BOX HEATER* Average Radiant Density φr , W/m2 0

10,000

20,000

30,000

40,000

50,000

60,000 1200

1100

2000

1000 C , °

F ,

w b

°

w b


AVG TMT

900

1500 °F (800 °C) 1500 800

700

1000 °F (550 C) 750 °F (400 C) 500 °F (250 C)

600

1000 0

5,000

10,000

15,000

Average Radiant Heat Density Density φr , Btu/Hr Ft 2 DP8BF06

Note:

*

To be used for checking existing heaters.


20,000



F IRED IRED H EATERS EATERS

Page 66 of 79


December, 2000

FIGURE 7 CABIN HEATER PASS ARRANGEMENTS

(A) 4-Pass

(B) 2-Pass

(C) 4-Pass

(D) 4-Pass


DP8BF07


Section

Page

VIII-B


67 of 79

December, 2000

FIGURE 8 APPROXIMATE APPROXIMATE TUBE LENGTHS FOR HORIZONTAL TUBE CABIN HEATERS Tube Length, m 0

10

5

20

15

30

25

15,000

14,000

1400

1300 Double Radiant Zone (Fig. 7-D)

13,000

1200

12,000

1100

11,000 1000 10,000 900 2 t

f 9,000 , e c a f r u S t n 8,000 a i d a R l a 7,000 t o T

800

700

600 6,000 500 5,000

400 4,000 300

3,000

200

2,000

Single Radiant Zone (Fig. 7-A,B,C) 100

1,000

0

0 0 DP8BF08

10

20

30

40

50

60

70

80

Tube Length, ft


90

100

2

m , e c a f r u S t n a i d a R l a t o T


F IRED H EATERS

Page 68 of 79


December, 2000

FIGURE 9A APPROXIMATE TUBE LENGTHS FOR VERTICAL-CYLINDRICAL HEATERS (ENGLISH UNITS) 10,000 9,000 8,000 7,000 6,000

Curve 1 Determine Lr (or a Range of Lr 's) for known Ar Curve 2 Determine Dt For Known Ar and Lr

6 . 1 = t D L r

5,000 6 . 2 = t D / L r

4,000

40

6 1 . = t D / L r 30

3,000

2,500 F , r A e c a f r u S t n a i d a R l a t o T

6 2 . = t D / L r

A r s v L r 1 e v r u C

2 t

2,000

20

1,500

1,000 900 800

2 e r v u C

10

L r v s t - D

9 8 7

700

6

600

500

400

300 250

200 10

DP8BF9a

15

20

25

30

40

50

60

70

Radiant Section Tube Length Lr , Ft


80 90

100

t F , t D r e t e m a i D e l c r i C e b u T n o i t c e S t n a i d a R


Section VIII-B


Page 69 of 79

December, 2000

FIGURE 9B APPROXIMATE TUBE LENGTHS FOR VERTICAL-CYLINDRICAL HEATERS (METRIC UNITS) 900 Curve 1 Determine Lr (or a Range of Lr 's) for known A r

800 700

6 . 1 = t D L r 6 . 2 = t D L r

Curve 2 Determine Dt For Known Ar and Lr

600 500 400

300

20

200

2

m , r A e c a f r u S t n a i d a R l a t o T

r

A s v L r 1 e v r u C

100 90 80 70

6 1 . = D t / L r

10 9 8

6 2 . = D t / L r

7 6

60 50

2 v e u r C

40

5

L r v s t - D

4

3

30

2

20

10

DP8BF9b

1

1.5

2

2.5

3

4

5

6

7

8

9 10

Radiant Section Tube Length L r , m


20

m , t D r e t e m a i D e l c r i C e b u T n o i t c e S t n a i d a R


F IRED H EATERS

Page 70 of 79


December, 2000

FIGURE 10 TYPICAL LAYOUT VERTICAL-CYLINDRICAL HEATER

~

1.5 (IPS) 1.5 (NPS)

~

) D r ( . R ad i an . t T I D ub e C y r i rc l e D ia me t o t e c r ( D a f r t ) e R n i o t c e t S n i a Lc a d R

Convection Section Side Wall

Radiant Section Shell

Convection Section End Tube Sheet (Refractory Lined) DP8BF10

Convection Section Header Box



Section VIII-B


Page 71 of 79

December, 2000

FIGURE 11 TYPICAL PASS ARRANGEMENTS FOR VERTICAL TUBE BOX HEATERS

(a) 6-Pass, 4 Cells

(b) 8-Pass, 5 Cells

(c) 12-Pass, 7 Cells

(h) Two-Side Fired Tubes Only

(d) 6-Pass, 3 Cells

(f) Transfer Line Arrangement

Legend Indicates a Series of Tubes (e) 8-Pass, 3 Cells Indicates external DP8BF11

(g) Transfer Line Arrangement

Indicates Coil Outlet Tube



F IRED H EATERS

Page 72 of 79


December, 2000

FIGURE 12 TYPICAL TUBE GUIDE DETAILS FOR VERTICAL TUBE BOX HEATER Provide adequate clearances to permit removing horizontal guides with stubs attached. These clearances should be provided for both guide pisitions and in both convection section side walls (for interchangeability). Provide pin between guides to prevent rotation.

Horizontal Guide - 5.5" O.D. x 1/2" min. wall, (140mm O.D. x 13mm min. wall) 25 Cr - 20 Ni, Centrifugally Cast

Removable Loose Fitting Cap

Typical Structure

Provide minimum of 2" (50mm) in cold position Note 6

B

B

4" (100mm) min

1/8" (3mm) thick 18 Cr - 8 Ni Sleeve

Guide for Center Tubes Under Convection Section

Note 1 6 min

A

Note 6

A Note 2

Vertical Guide 3-1/2" O.D. x 1/2" min. wall (90mm O.D. x 13 mm min. wall) 25 Cr - 20 Ni

Note 3 Note 4 Note 5

Guide for Center and Wall Tubes Not Under Convection Section

Guide for Center and End Wall Tubes Under Convection Section as Required

Note 6

Note 6 Note 7 Clearance between Guides = Tube O.D. + 1/2" = Tube O.D. + 13mm 1/2" (13mm)

Section A-A

Section B-B

Notes: (1)

Guide - 1/2 in. (13 mm) thick 25 Cr - 20 Ni channel. Fill with refractory.

(2)

Guide Pin – 3-1/2 in. O.D. x 1/2 in. minimum wall (90 mm O.D. x 13 mm min. wall). Cast 25 Cr - 20 Ni.

(3)

Weld between Guide Pin and Socket to be 182 Inconel or equal.

(4)

Socket and weld between socket and return bend: Material for both to be equal to return bend.

(5)

Alternate locations for Guide Pin and Socket as required.

(6)

Provide minimum clearance required for thermal movement of tube guide. 4 in. min. clearance.

(7)

Stubs - 3-1/2 in. O.D. x 1/2 in. minimum wall (90 mm O.D. x 13 mm min. wall). Cast 25 Cr - 20 Ni. DP8Bf12



Section VIII-B


Page 73 of 79

December, 2000

FIGURE 13 TYPICAL LAYOUT FOR VERTICAL TUBE BOX HEATER

Wi Wi 1.5 (IPS) 1.5 (NPS)

(610 mm min.) 2' min.

45i

(typ.)

Peep Hole in Floor

B Peep Hole in Floor

Outlet Tubes External Jumpover

Lt Lt

Typical Burner Layout

Long Radius U-Bend for Observation Door

DP8BF13

Inlet Tubes (From Convection Section)



Page

Section VIII-B

74 of 79


December, 2000

FIGURE 14 TYPICAL LAYOUT FOR HOOP-TUBE HEATER

Convection Section Observation Door in Arch

Typical Locations for Observation Doors

Ht

B Ls

U-bend to be located inside firebox and not buried in floor c Process Headers

c/2 c

END ELEVATION

(610 mm) 2'-3" (Typ.)

Coil Inlet

Coil Inlet 1.5 (NPS) 1.5 (IPS) + Thermal Growth of Header

Coil Outlet

l l a W r o i r e t n I

l l a W r o i r e t n I

Burners

Coil Inlet w

2w

2w

2w

2w

w

Typical Burner Spacing ZONE A DP8BF14

ZONE B

ZONE C

PLAN VIEW


Typical Tube Spacing


Section VIII-B


Page 75 of 79

December, 2000

FIGURE 15 LINEAR THERMAL EXPANSION OF VARIOUS STEELS Temperature °C 0

100

200

300

400

500

600

700

800

900

1000

1100

1200

26

22

24 20

18 Cr - 8 Ni 22 . t F 0 0 1 / s e h 20 c n I , e r u t 18 a r e p m e T d 16 e t a c i d n I d 14 n a F 0 7 n 12 e e w t e B n o 10 i s n a p x E l 8 a m r e h T r a 6 e n i L

25 Cr - Ni

18

16

14

12

°

°

10

Carbon Steel, C - Mo, Low Co

5 Cr - Mo to 9 Cr - Mo

8

6

4 4 2 2

0

DP8BF15

m / m m , e r u t a r e p m e T d e t a c i d n I d n a C 0 2 n e e w t e B n o i s n a p x E l a m r e h T r a e n i L

0 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Temperature °F


2200

2400


Page

Section VIII-B

76 of 79


December, 2000

FIGURE 16 THERMAL CONDUCTIVITY OF VARIOUS STEELS Temperature °C 0

100

200

300

400

500

600

700

800

900

1000

1100

400 55

350

C a r bo n S t e e l

C - 1 / 2 M o

45

300 2 t

. F n r I / h / F u t B °

250 , m k y t i v i t c u d 200 n o C l a m r e h T 150

1 1 / 4 Cr 1 / 2 Mo

2 1/4 Cr - 1 Mo

2 5 u g h t o r W

Mo 5 Cr - 1/ 2 Mo 9 C r - 1

i 2 0 N C r -

N b N i 5 3 C r 2 5 t s C a

/ 35 W , m K y t i v i t c C u d m n o C l 25 a m r e h T °

Cast 25 Cr - 20 Ni (HK40)

N i r - 8 1 8 C

100

15

50 5

0 0

500

1000

1500

Temperature, °F


2000

DP8BF16


Section VIII-B


Page 77 of 79

December, 2000

FIGURE 17 TYPICAL TUBE GUIDES FOR HOOP-TUBE HEATER Guide: 1/2 in. (13mm) thick 25 Cr-20 Ni channel approx. 5 ft. (1.5m) long. Attach to tubes with 25 Cr-20 Ni U-bolts. Two guides per tube. Each series of tubes to be independent of tubes guided by the next two guides (do not interlock). Fill channel with refractory for protection from vanadium attack.

Pads welded on tubes to locate and support channels. Material equal to tubes

A Section A-A

A 45i

Detail Size U-Bolt for 1/8" (3mm) clearance all around tube

Typical Radiant Tube Guides

Shoulder on U-Bolt to maintain clearance around tube

DP8BF17


Tack Weld or Peen


F IRED H EATERS

Page

VIII-B

78 of 79


December, 2000

FIGURE 18 CORROSION FRACTION .90

12 10 9 8 7

tm = ts + f TCA

.85

6 5 4

.80

3

.75 f , n o i t c a r F n o i s o r r o C

.70

.65

.60 e l a c S f o e g n a h C e t o N

.55

B ts CS Do Pr Sr n

= TCA/ts = Pr Do/(2Sr + Pr ) = Corrosion Allowance = Outside Diameter = Rupture Design Pressure = Rupture Allowable = Rupture Exponent (See Figure 19)

.50 0 DP8BF18

0.25

0.50

0.75

1.0

1.5

Parameter B


2.0

2.5

n , t n e n o p x E e r u t p u R

Dp08b Design of Process Heaters

Recommend Documents