Indirect Heaters Indirect heaters use an intermediate bath of heating uid to transfer heat energy from the heat source to the process uid. Typically, a ame and hot exhaust gases are conned within a retube, which is surrounded by the heating uid. Exhaust gases from engines and turbines are are also commonly used to heat the bath uid. The process process uid ows through a pipe coil that that is also surro surrounded unded by the heating heating uid. Heat energy is transferred through through the retube or exhaust exhaust pipe wall to the heating uid and then through the surrounding heating uid to the process uid. Within the heating uid either natural or forced conection may be used to distribute the heat energy. !eeral types of indirect indirect heaters are aailable, di"ering chiey in the heating uid used to transfer the heat. Table # lists the most common types of indirect heaters, with normal bath temperatures temperatu res and typical bath properties listed for each. Typical retube $et Thermal E%ciency &$TE', which indicates the useful heat transferred per net heat input, is on the order of () to *+ percent.
Table 1: Typical Bath Properties for Firetube Heaters Heater
ath Temp /utside 0oil - undle ho tu1 &hr2ft32-'
iretube lux &415' tu1&hr2 ft3'
Water ath
#*+2#7)
#(+
#+,+++2 #8,+++
9)+27++
9(2*3
)+ percent Ethylene :lycol
#7)23+)
##)
*,+++2#+,+++
*++27++
9(2*+
;ow
3=)23)+
#+++
#),+++2 #*,+++
*++27++
9(2*+
Hot /il or HT
8++2))+
=+
(,+++2*,+++
7++2##++
9#29(
>olten !alt
=++2*++
3++
#),+++2 #*,+++
#+++ #+ ++2# 2#3+ 3++ + (* (*29 29= =
TE: ?eboiler ?eboiler
8)+2=++
22
(,+++2*,+++
*++
9)2*+
3=)239+
22
(,)++2#+,+++
7++
9)2*+
5mine ?eboiler
!tac6 Temp Temp -
iretube E%ciency $TE percent
Common Types of Indirect Heaters Water Bath Heaters Water bath heaters are the most common form of indirect red heater. ;ine heaters for heating well uids to preent hydrate formation are almost exclusiely water bath heaters. The heater is designed to operate at atmospheric pressure, and it is typically a hori@ontal cylinder anged at both ends with saddle type supports, as shown in igure #. The rebox assembly inserts through one end and anges to the shell. The process coil anges to the opposite end. The retubes are located in the lower section of the shell, with the process coil aboe the retube.
igure # Water heated by the retubes rises due to density di"erences and transfers heat to the process coil. The cooled water then drops bac6 toward the retubes, and the cycle is repeated. This type of natural conectie circulation is used in all indirect heaters. The rebox consists of single or multiple A2shaped retubes, a coer ange, the burner or burners, and the exhaust stac6s. igure # shows a single retube with one burner and one stac6. Heat transfer from the retube surface to the bath uid occurs by natural conection. iretubes are designed for a specic heat ux rate,
the rate of heat per unit area of retube, aboe which the retube surface will not stay wetted &apor or bubbles will form on the retube surface'. The retube heat ux rate is a function of the heater uid. iretubes in water bath heaters are normally designed for a heat ux rate of #+,+++ to #8,+++ tu1hr2ft. The lower ale is recommended. Besigns that exceed the recommended retube heat ux rate will experience a loss in heat transfer e%ciency, an increase in eaporation losses, and an increase in retube hot spots and burnout. The process coil is the only portion of a water bath heater designed for pressures aboe atmospheric. The coil consists of a series of straight tubes connected with #*+ degree return bends and a coer ange. !eamless pipe is normally used for the coil. abrication of the coil should be in accordance with 5!>E !E0 CIII B#D 5!>E 8#.8D 5E 8#.8 is reFuired in the A.!. and adds a margin of safety in the design of the process coil. The fuel gas supply to the burner is normally controlled by a temperature controller on the water bath temperature. To preent the loss of water from the bath the operating temperature is normally limited to #7+-. 5t higher temperatures excessie amounts of water will be lost through the atmospheric ent. 5lternatiely, a temperature controller may be used to maintain a certain process uid outlet temperature. In this case the bath temperature would uctuate, depending on the process uid inlet temperature. /ne common method uses a temperature controller on the water bath which oerrides the temperature controller on the process uid to assure that the bath uid is controlled at less than #7+-. 5 separate high temperature shutdown shall be considered on the water bath regardless of the temperature control method. /ther sources of heat can be used in place of the retube in igure #. The most common is to route the exhaust piping from an engine or turbine through the water bath. In such a system, the exhaust is dierted through the pipe in the bath, as reFuired by the process temperature controller, and allowed to go straight to atmosphere through its normal exhaust stac6 when heat is not reFuired from the bath. A coil of either steam or a heat medium uid, or an electrical immersion heater can also be used to add heat to the bath uid.
Hot il or Heat Transfer Fluid !HTF" Bath Heaters HT bath heaters use organic heat transfer uids such as BowthermG, and TherminolG as heating media. These uids hae low apor pressures at eleated temperatures, allowing operating temperatures up to ))+- at atmospheric pressure. Therefore, HT bath heaters are ery similar to water bath heaters but are normally used for temperatures greater than #7+-. The particular HT selected for an HT bath heater a"ects the heater design. The physical properties of the HT shall be obtained at the intended operating
temperature. The HTs apor pressure and thermal expansion characteristics are important factors to consider when a heater is designed. To preent the loss of these expensie HTs to the atmosphere the apor pressure shall be low. Typically, a small expansion tan6 is attached to an HT bath heater so that at low operating temperatures the shell may be full or nearly full of HT. 5s the temperature rises, the expansion tan6 allows for the thermal expansion of the HT. The rebox for an HT bath heater is the same as for a water bath heater except that the heat ux is normally limited to *,+++ tu1hr2ft. This protects the HT from hot spots and excessie lm temperatures that cause thermal degradation of the HT. The process coil may be made of seamless pipe &as with a water bath heater', or it may be a tube bundle arrangement fabricated of heat exchanger tubing to proide a large surface area. This allows a close approach between the HT bath and the process uid temperatures. Thus, the HT bath may be operated at a minimum temperature that reduces heat losses to the atmosphere and protects the bath uid from thermal degradation. 5 typical HT bath heater with an expansion tan6 is shown in igure 3.
igure 3
%irect Fired Heaters Birect red heaters transfer heat directly from the combustion gases through a tube wall to the process uid. Thus only one pipe or tube wall instead of two separates the process uid from the ame and the heating medium. The transfer of heat to the process uid is more e%cient, but the danger of exposing the process uid to the ame is greater. Birect red heaters may be designed to contain either the combustion gases or the process uid within tubes. or low pressure applications a heater may be used with retubes surrounded by the process uid. This type of heater is ery similar to indirect heaters. 5lternately, the process uid may be contained within tubes past which the combustion gases are reFuired to ow. With this type of direct heater the process uid can be at high pressures within the tubes. ?egardless of the function of the heater, either retube or process tube direct heaters may be used. Three maor types of direct red heaters are classied as ;iFuid Heaters, !team :enerators and ?eboilers. The description is restricted to ;iFuid Heaters and ?eboilers
&i'uid Heaters 5 direct red liFuid heater that has a retube for heating a crude oil stream prior to processing is shown in igure 8. $ote that the retube is surrounded by the process uid and the heat energy is transferred directly to the process uid. 5s with indirect heaters, other sources of heat could be used to heat the process uid &e.g., exhaust gas, steam coils, heat transfer uid coils, etc'.
igure 8 If the direct heaterJs shell is designed to contain pressures of #) psig or more, it shall be designed and fabricated in accordance with 5!>E !E0 CIII B# or other acceptable codes. iretubes shall be of the remoable u2tube type. The shell for this type of heater is typically limited to #3) psig for economic reasons. 5t higher pressures it is usually economical to conne the process uid within high pressure tubes surrounded by the combustion gases. Birect red heaters may be used for a ariety of heating reFuirements such asK #. Heating crude oil to brea6 water emulsions 3. Heating a uid that can then be pumped to a number of heat users 8. Heating a glycol to drie o" water apor =. Heating an amine or other chemical solent to reerse a chemical reaction ). Heating crude oil to reduce the oil iscosity prior to pumping (. Heating bottoms of stabili@ers or fractionators.
)eboilers Birect red 6ettle reboilers may be used to regenerate glycols or amines, or to boil a ariety of liFuids for separation processes. These reboilers are ery similar to retube steam generators, but typically a liFuid weir is added to maintain the liFuid leel in the retube section. Excess liFuid spills oer this weir and is remoed.
Table 2: Criteria for +election bet,een Indirect and %irect Fired Heaters Criteria Heat lux
Thermal E%cienc
Indirect
%irect Fired
)emar-s
#8,+++2#*,+++ tu1 &hr2ft3'
*,+++2#+,+++ tu1&hr2 ft3'
Birect red heaters need to operate at lower heat ux due to the followingK a. :reater probability of hot spot formation b. 0hemical degradation of the liFuid &co6e formation' c. Excessie locali@ed thermal stresses in the retube d. 5ccumulation of co6e or scale on the retube surfaces e. Enhancing corrosion from uids containing H 3! or 0/3
*+2*)L
M7+L
y
Ease of !6id >ounting
:enerally ;arger
:enerally !maller
0ase Nto2case basis depending on the application :reater
0ase Nto2case basis depending on the application ;ower
This is specically applicable to any heater which has a retube conguration.
