LIST OF FIGURES Figure. 2.1 Baffle spacers and tie rods
7
Figure 2.2 Fixed-tube plate (based on figures from BS 3274: 1960)
9
Figure 2.3 U-tube (based on figures from BS 3274: 1960)
9
Figure 2.4 Internal floating head without clamp ring (based on figures from BS 3274:1960)
10
Figure 2.5 Internal floating head with clamp ring (based on figures from BS 3274: 1960)
10
Figure 2.6 External floating head, packed gland (based on figures from BS 3274: 1960)
11
Figure 2.7 Kettle reboiler with U-tube bundle (based on figures from BS 3274: 1960)
11
Figure 3.1 Basic Design Procedure
12
Figure 4.1 Rating process for heat exchanger design
16
Figure 5.2.1. Idealized main stream flow
21
Figure 5.2.2. Shell-side leakage and by-pass paths
22
Figure 5.2.3 Heat-transfer factor for cross-flow tube banks
23
Figure 5.2.4 Tube row correction factor Fn
24
Figure 5.2.5 Window correction factor
25
Figure 5.2.6 Bypass correction factor
26
Figure 5.2.7. Coefficient for FL, heat transfer
26
Figure 5.2.8 Friction factor for cross-flow tube banks
28
Figure 5.2.9. Bypass factor for pressure drop F’b
29
Figure 5.2.10. Coefficient for F’L, pressure drop
29
Figure 6.1.1 Business Example
33
Figure 6.1.2 Engineering Example
33
Figure 7.1.1 Selecting process fluid from dropdown menu
35
Figure 7.1.2 Preliminary manual inputs
36
Figure 7.1.3 Generating physical properties of the process fluids
36
Figure 7.1.4 Assuming the values for different parameters
37
Figure 7.1.5 Selecting the method of design
37
1
Figure 7.1.6 Results
37
Figure 8.1.1 Problem inputs
39
Figure 8.1.2 Physical properties of process fluids
39
Figure 8.1.3 Selecting no. of passes in tube side
40
Figure 8.1.4 Selecting the type of pitch
40
Figure 8.1.5 Selecting the type of head
40
Figure 8.1.6 Selecting the type of shell side & tube side fluid
40
Figure 8.1.7 Results obtained by kern’s method
41
Figure 8.1.8 Selecting other method of design
41
Figure 8.1.9 Results obtained by Bell-Delaware’s method
41
Figure 8.2.1 Selecting the other coolant for same problem
42
Figure 8.2.2 Physical properties of process fluids
42
Figure 8.2.3 Selecting no. of passes in tube side
43
Figure 8.2.4 Selecting the type of pitch
43
Figure 8.2.5 Selecting the type of head
43
Figure 8.2.6 Selecting the type of shell side & tube side fluid
43
Figure 8.2.7 Results obtained by kern’s method
44
Figure 8.2.8 Selecting other method of design
44
Figure 8.2.9 Results obtained by Bell-Delaware’s method
44
LIST OF TABLES Table 7.2.1 Color code in spreadsheet
38
2
UNIT-1 INTRODUCTION (1) The transfer of heat to and from process fluids is an essential part of most chemical processes. The most commonly used type of heat-transfer equipment is the ubiquitous shell and tube heat exchanger; the design of which is the main subject of this report. The word “exchanger” really applies to all types of equipment in which heat is exchanged but is often used specifically to denote equipment in which heat is exchanged between two process streams. Exchangers in which a process fluid is heated or cooled by a plant service stream are referred to as heaters and coolers. If the process stream is vaporized the exchanger is called a vaporizer if the stream is essentially completely vaporized; a reboiler if associated with a distillation column; and an evaporator if used to concentrate a solution. The term fired exchanger is used for exchangers heated by combustion gases, such as boilers; other exchangers are referred to as “unfired exchangers”. The principal types of heat exchanger used in the chemical process and allied industries, 1. Double-pipe exchanger: the simplest type, used for cooling and heating. 2. Shell and tube exchangers: used for all applications. 3. Plate and frame exchangers (plate heat exchangers): used for heating and cooling. 4. Plate-fin exchangers. 5. Spiral heat exchangers. 6. Air cooled: coolers and condensers. 7. Direct contact: cooling and quenching. 8. Agitated vessels. 9. Fired heaters.
3
1.1 Heat Exchanger Type Heat transfer equipment is usually specified both by type of construction and by service. A heat exchanger is a specialized device that assists in the transfer of heat from one fluid to the other. In some cases, a solid wall may separate the fluids and prevent them from mixing. In other designs, the fluids may be in direct contact with each other. In the most efficient heat exchangers, the surface area of the wall between the fluids is maximized while simultaneously minimizing the fluid flow resistance. Fins or corrugations are sometimes used with the wall in order to increase the surface area and to induce turbulence. In heat exchanger design, there are three types of flow arrangements: counter-flow, parallelflow, and cross-flow. In the counter-flow heat exchanger, both fluids entered the exchanger from opposite sides. In the parallel-flow heat exchanger, the fluids come in from the same end and move parallel to each other as they flow to the other side. The cross-flow heat exchanger moves the fluids in a perpendicular fashion. Compare to other flow arrangements counter –flow is the most efficient design because it transfers the greatest amount of heat.
There are two major different designs of heat exchangers: shell and tube, and plate heat exchanger. The most typical type of heat exchanger is the shell and tube design. This heat exchanger can be design with bare tube or finned tubes. One of the fluids runs through the tubes while the other fluid runs over them, causing it to be heated or cooled. In the plate heat exchanger, the fluid flows through baffles. This causes the fluids to be separated by plates with a large surface area. This type of heat exchanger is typically more efficient than the shell and tube design.
1.1.1 Shell & Tube Exchanger A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. It consists of a tube bundle enclosed in a cylindrical casing called a shell. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.
4
Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes. In this way, waste heat can be put to use. This is a great way to conserve energy.
Typically, the ends of each tube are connected to plenums through holes in tube sheets. The tubes may be straight or bent in the shape of a U, called U-tubes. Most shell-and-tube heat exchangers are 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other.
There are two basic types of shell-and-tube exchangers. The first is the fixed tube sheet unit, in which both tube sheets are fastened to the shell and the tube bundle is not removable. The second type of shell-and-tube unit has one restrained tube sheet, called the stationary tube sheet, located at the channel end. Differential expansion problems are avoided by use of a freely riding floating tube sheet at the other end or the use of U tubes.
This design may be used for single or multiple pass exchangers. The tube bundle is removable from the channel end, for maintenance and mechanical cleaning. There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes.
Counter current heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams. Many companies however do not use single pass heat exchangers because they can break easily in addition to being more expensive to build. Often multiple heat exchangers can be used to simulate the counter current flow of a single large exchanger.
5
Shell-and-tube exchangers are designed and fabricated according to the standards of the Tubular Exchanger Manufacturers Association (TEMA).
1.1.2 Plate Heat Exchangers Plate and frame heat exchanger for general refinery service can be referring as gasketed plate heat exchangers. The plate heat exchanger consists of a frame, which consists of a head, follower, column, carrying bar, guiding bar, and a number of clamping bolts. In between head and follower a varying number of pressed plates are clamped together. Each plate is supplied with a gasket, so that the plates form a closed system of parallel flow channels, through which the Medias flow alternatively at every second interval. The gaskets are glued on the plates, securing tightness between Medias and the atmosphere. Between the different Medias there are double gaskets, which have intermediate drain areas, meaning that mixing of the two Medias is impossible. Every second plate in the stack has to turn 180°, so that the plates form a closed system of parallel flow channels, through which the Medias flow alternatively at every second interval.
The advantage of the gasketed plate heat exchanger: (i) High thermal efficiency due to high film efficiency of heat transfer for fluids, no bypassing and leakage streams, and counter-current operation. (ii) Plate design is feasible with size, chevrons angles and pass arrangements. (iii) Easy maintenance that the plate can be easily disassembled for cleaning. (iv) The plates of the unit can be rearranged, added or removed from the plate rack to suit for difference of service condition. (v) Have very wide range of total surface area up to 15,000 ft2. (vi) Low fouling is encountered due to high turbulence create by plate and the fluid low residence in plate. The disadvantage, (i) Have limitations in service temperature and pressure. Maximum service temperature is 450oF and pressure is 335 psig. (ii) The gaskets impose restrictions on the nature of the fluids which can be handled.
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UNIT- 2 SHELL AND TUBE EXCHANGERS: CONSTRUCTION DETAILS (1) The shell and tube exchanger is by far the most commonly used type of heat-transfer equipment used in the chemical and allied industries. The advantages of this type are: 1. The configuration gives a large surface area in a small volume. 2. Good mechanical layout: a good shape for pressure operation. 3. Uses well-established fabrication techniques. 4. Can be constructed from a wide range of materials. 5. Easily cleaned. 6. Well-established design procedures. Essentially, a shell and tube exchanger consists of a bundle of tubes enclosed in a cylindrical shell. The ends of the tubes are fitted into tube sheets, which separate the shell-side and tubeside fluids. Baffles are provided in the shell to direct the fluid flow and support the tubes. The assembly of baffles and tubes is held together by support rods and spacers, Figure 2.1.
Figure. 2.1 Baffle spacers and tie rods
2.1 Exchanger types The principal types of shell and tube exchanger are shown in Figures 2.2 to 2.7. Diagrams of other types and full details of their construction can be found in the heat exchanger standards (see Section 2.5.1.). The standard nomenclature used for shell and tube exchangers is given below; the numbers refer to the features shown in Figures 2.2 to 2.7.
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Nomenclature
Part number 1. Shell
2. Shell cover
3. Floating-head cover
4. Floating-tube plate
5. Clamp ring
6. Fixed-tube sheet (tube plate)
7. Channel (end-box or header)
8. Channel cover
9. Branch (nozzle)
10. Tie rod and spacer
11. Cross baffle or tube-support plate
12. Impingement baffles
13. Longitudinal baffle
14. Support bracket
15. Floating-head support
16. Weir
17. Split ring
18. Tube
19. Tube bundle
20. Pass partition
21. Floating-head gland
22. Floating-head gland ring (packed gland)
23. Vent connection
24. Drain connection
25. Test connection
26. Expansion bellows
27. Lifting ring
The simplest and cheapest type of shell and tube exchanger is the fixed tube sheet design shown in Figure 2.2. The main disadvantages of this type are that the tube bundle cannot be removed for cleaning and there is no provision for differential expansion of the shell and tubes. As the shell and tubes will be at different temperatures, and may be of different materials, the differential expansion can be considerable and the use of this type is limited to temperature differences up to about 80°C. Some provision for expansion can be made by including an expansion loop in the shell (shown dotted on Figure 12.3) but their use is limited to low shell pressure; up to about 8 bar. In the other types, only one end of the tubes is fixed and the bundle can expand freely.
The U-tube (U-bundle) type shown in Figure 2.3 requires only one tube sheet and is cheaper than the floating-head types; but is limited in use to relatively clean fluids as the tubes and bundle are difficult to clean. It is also more difficult to replace a tube in this type.
8
Figure 2.2 Fixed-tube plate (based on figures from BS 3274: 1960)
Figure 2.3 U-tube (based on figures from BS 3274: 1960)
Exchangers with an internal floating head, Figures 2.4 and 2.5 are more versatile than fixed head and U-tube exchangers. They are suitable for high-temperature differentials and, as the tubes can be rodded from end to end and the bundle removed, are easier to clean and can be used for fouling liquids. A disadvantage of the pull-through design, Figure 2.4, is that the clearance between the outermost tubes in the bundle and the shell must be made greater than in the fixed and U-tube designs to accommodate the floating head flange, allowing fluid to bypass the tubes. The clamp ring (split flange design), Figure 2.5, is used to reduce the clearance needed. There will always be a danger of leakage occurring from the internal flanges in these floating head designs.
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In the external floating head designs, Figure 2.6, the floating-head joint is located outside the shell, and the shell sealed with a sliding gland joint employing a stuffing box. Because of the danger of leaks through the gland, the shell-side pressure in this type is usually limited to about 20 bars, and flammable or toxic materials should not be used on the shell side.
Figure 2.4 Internal floating head without clamp ring (based on figures from BS 3274:1960)
Figure 2.5 Internal floating head with clamp ring (based on figures from BS 3274: 1960)
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Figure 2.6 External floating head, packed gland (based on figures from BS 3274: 1960)
Figure 2.7 Kettle reboiler with U-tube bundle (based on figures from BS 3274: 1960)
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UNIT-3 BASIC DESIGN PROCEDURE FOR A SHELL & TUBE HEAT EXCHANGER A lot has been written about designing heat exchangers, and specifically, shell-and-tube heat exchangers. For example, the book by Kern
(2)
published in 1950 details basic design
procedures for a variety of heat exchangers. An article in 1979 by Taborek
(3)
outlines how
heat exchanger design techniques evolved over the years since the appearance of the book by Kern. More recent developments are discussed in numerous articles in the magazine “Chemical Engineering.”. From here on, references to page numbers, table numbers, and equation numbers are from Coulson & Richardson’s Chemical Engg., vol.6, “Chemical Engineering Design”(1).
3.1 Basic Design Procedure Route
Figure 3.1 Basic Design Procedure
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Usually, the flow rates and the physical properties of the two streams involved are specified, and the temperatures at which the fluids are available are known. If the outgoing temperature of one of the streams is not specified, usually a constraint (e.g. the temperature of the cooling water cannot exceed 99◦C) is given. Then, by an energy balance, the outgoing temperature of the second stream can be calculated along with the heat duty.
1. Select the stream that should be placed on the tube side. The tube side is used for the fluid that is more likely to foul the walls, more toxic or more corrosive, or for the fluid with the higher pressure. Cleaning of the inside of the tubes is easier than cleaning the outside. When a gas or vapor is used as a heat exchange fluid, it is typically introduced on the shell side. Also, high viscosity liquids, for which the pressure drop for flow through the tubes might be prohibitively large, can be introduced on the shell side. 2. The heat duty Q is usually fixed by the required service. The selected heat exchanger has to meet or exceed this requirement. Heat load of a heat exchanger can be estimated from heat balance:Q= (m Cp Δt) = (m Cp ΔT) Where Δt is the temperature difference in the tube side fluid & ΔT is the temperature difference in the shell side fluid. If three of the temperatures are given, the fourth can be calculated using the above equation. The above equation assumes no phase change in any of the fluids.
3. Make an approximate estimate of the size of the heat exchanger by using a reasonable guess for the overall heat transfer coefficient. For typical shell-and-tube heat exchangers in a chemical process or a refinery, Table 12.1 can be used as a starting point for the estimate. Using this estimate, calculate the heat transfer area Aο.
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Where • Ao Outside tube surface area • q Heat duty – heat exchange between tube and shell side • Uo Overall heat transfer coefficient • F Correction factor, F=1.0 for cross flow heat exchanger • Tm True mean temperature, Tm = F Tlm • Tlm Log means temperature difference. This will give you an idea of the approximate size of the heat exchanger, and therefore its cost.
4. The next step is to determine the approximate number of tubes Nt needed to do the job. Because we have an idea of the approximate heat transfer area, we can write Aο = Nt (Π D◦) L Where D◦ is the OD of a tube and L is its length. Both of these are only available in discrete increments. For example, the length is selected as 8, 10, 12, 16, or 20 feet. Likewise, the OD is specified as ¼, 3/8, 1/2, 5/8, 3/4, 1, 1 1/4, 1 1/2 inch. The tubes are typically specified to be 14 BWG. The most common tube lengths are 16 and 20 feet and the most common tube OD values are 3/4 and 1 inch. So, selecting one of the values in each set will get you started in estimating the approximate number of tubes. Check the velocity through a single tube; it should not exceed roughly 1 to 2 m/s for liquids, to keep the pressure drop under reasonable constraints, but it should be at least 4 m/s (the specific choice depends on the viscosity as well) to maintain turbulent flow, and minimize fouling. If necessary, adjust the number of tube passes to get the velocity to fall in this range. You can learn more about tubes and the tube-side construction from Section 12.5.2.
5. Determine the shell size. To do this, once the number of tubes is known, select a pitch and the number of passes. Typical initial guesses are 1 or 2 tube passes. A square pitch is chosen for reasons of convenience in cleaning the outside of the tubes; when the tubes are in-line, cleaning is
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relatively straightforward. The standard choice is a pitch equal to 1.25 inches for 1-inch OD tubes, and a pitch of 1 inch for 3/4 –inch OD tubes. Tubes on a triangular pitch cannot be cleaned by tools, but rather by passing a chemical solution through on the shell-side. Because triangular pitches allow for the packing of more tubes into a given space, they are more common when cleaning the outside is not a major issue. Rectangular pitches are uncommon.
Knowing the number of tubes to be used and the number of passes, you can select the required shell size. For this, you need to know about the clearance that must be allowed between the tube bundle and the shell inside diameter.
A 1-pass shell is the most common in use, but sometimes a 2-pass shell can be specified to improve thermal effectiveness. Shells are made from commercial steel pipes up to an outside diameter of 24 inches. Shells with a larger OD are made by rolling steel plate and welding.
6. You need to estimate the number of baffles to be used and the spacing among them. You can read about baffles from 12.5.7. Normally, baffles are equally spaced. The minimum baffle spacing is one-fifth of the shell diameter, but not less than 2 inches, and the maximum is determined by considerations involving supporting the tube bundle.
7. Now, we are ready to check the thermal performance of the selected heat exchanger. Calculate the tube-side and shell-side heat transfer coefficients, the tube wall contribution to the resistance, and the appropriate fouling resistances.
See if the calculated Uº matches the required Uº that you used for estimating the heat transfer area. If it is too small, start all over again! If it is too large, then the heat exchanger is overspecified for the required thermal duty.
If the calculated Uº is too small, you need to examine whether the tube-side or the shell-side resistance is controlling (sometimes they are comparable).
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UNIT-4 RATING OF THE HEAT EXCHANGER DESIGN (4) Rating an exchanger means to evaluate the thermo-hydraulic performance of a fully specified exchanger. v Input to the rating process is heat exchanger geometry (constructional design parameters), process conditions (flow rate, temperature, pressure) and material/fluid properties (density, thermal conductivity) Ø First output from the rating process is either the outlet temperature for fixed tube length or the tube length itself to meet the outlet temperature requirement. Ø Second output from the rating process is the pressure drop for both fluid streams hence the pumping energy requirements and size.
Figure 4.1 Rating process for heat exchanger design v If the output of the rating analysis is not acceptable, a geometrical modification should be made Ø If the required amount of heat cannot be transferred to satisfy specific outlet temperature, one should find a way to increase the heat transfer coefficient or increase exchanger surface area ·
One can increase the tube side heat transfer coefficient by increasing the fluid velocity - Increase number of tube passes.
·
One can increase the shell side heat transfer coefficient by decreasing baffle spacing and/or baffle cut.
16
·
One can increase the surface area by, § Increasing the heat exchanger length § Increasing the shell diameter § Multiple shells in series
v If the pressure drop on the tube side is greater than the allowable pressure drop, then §
the number of tube passes can be decreased or
§
the tube diameter can be increased which may result to • decrease the tube length – (Same surface area) • increase the shell diameter and the number of tubes
Ø If the shell side pressure drop is greater than the allowable pressure drop then baffle spacing, tube pitch, and baffle cut can be increased or one can change the baffle type. THERE IS ALWAYS A TRADE-OFF BETWEEN THERMAL & PRESSURE DROP RATINGS.
Different approaches for Shell Side heat transfer Coefficients v There are three rating methods to calculate the shell side heat transfer coefficient: Ø Kern method is a simplified approach suitable for shell side flow without baffles Ø Taborek method Ø Bell Delaware method is the most complex but accurate way of rating a heat exchanger with baffles.
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UNIT-5 DIFFERENT DESIGN APPROACHES FOR SHELL & TUBE HEAT EXCHANGER (1) 1. Kern’s method This method was based on experimental work on commercial exchangers with standard tolerances and will give a reasonably satisfactory prediction of the heat-transfer coefficient for standard designs. The prediction of pressure drop is less satisfactory, as pressure drop is more affected by leakage and bypassing than heat transfer. The shell-side heat transfer and friction factors are correlated in a similar manner to those for tube-side flow by using a hypothetical shell velocity and shell diameter. As the cross-sectional area for flow will vary across the shell diameter, the linear and mass velocities are based on the maximum area for cross-flow: that at the shell equator. The shell equivalent diameter is calculated using the flow area between the tubes taken in the axial direction (parallel to the tubes) and the wetted perimeter of the tubes.
The procedure for calculating the shell-side heat-transfer coefficient and pressure drop for a single shell pass exchanger is given below:
1.
Calculate the area for cross-flow As for the hypothetical row of tubes at the shell equator, given by:
Where pt = tube pitch, do = tube outside diameter, Ds = shell inside diameter, m, lB = baffle spacing, m. The term (pt –do)/pt is the ratio of the clearance between tubes and the total distance between tube centers.
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2. Calculate the shell-side mass velocity Gs and the linear velocity us:
Where Ws = fluid flow-rate on the shell-side, kg/s, ƍ = shell-side fluid density, kg/m3.
3. Calculate the shell-side equivalent diameter (hydraulic diameter), For a square pitch arrangement:
For an equilateral triangular pitch arrangement:
Where de = equivalent diameter, m. 4.
Calculate the shell-side Reynolds number, given by:
5. For the calculated Reynolds number, read the value of jh for the selected baffle cut and tube arrangement, and calculate the shell-side heat transfer coefficient hs from:
The tube wall temperature can be estimated using the method given for the tube-side.
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6. For the calculated shell-side Reynolds number, read the friction factor and calculate the shell-side pressure drop from:
Where L = tube length, lB = baffle spacing. The term (L/lB) is the number of times the flow crosses the tube bundle = (Nb +1) Where Nb is the number of baffles.
2. Bell’s method In Bell’s method the heat-transfer coefficient and pressure drop are estimated from correlations for flow over ideal tube-banks, and the effects of leakage, bypassing and flow in the window zone are allowed for by applying correction factors. This approach will give more satisfactory predictions of the heat-transfer coefficient and pressure drop than Kern’s method; and, as it takes into account the effects of leakage and bypassing, can be used to investigate the effects of constructional tolerances and the use of sealing strips. The procedure in a simplified and modified form to that given by Bell (1963) is outlined below. The method is not recommended when the by-pass flow area is greater than 30% of the cross-flow area, unless sealing strips are used.
2.1. Flow pattern The flow pattern in the shell of a segmentally baffled heat exchanger is complex, and this makes the prediction of the shell-side heat-transfer coefficient and pressure drop very much more difficult than for the tube-side. Though the baffles are installed to direct the flow across the tubes, the actual flow of the main stream of fluid will be a mixture of cross flow between the baffles, coupled with axial (parallel) flow in the baffle windows; as shown in Figure 5.2.1. Not all the fluid flow follows the path shown in Figure 5.2.1; some will leak through
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gaps formed by the clearances that have to be allowed for fabrication and assembly of the exchanger. These leakage and bypass streams are shown in Figure 5.2.2., which is based on the flow model proposed by Tinker (1951, 1958). In Figure 5.2.2., Tinker’s nomenclature is used to identify the various streams, as follows: §
Stream A is the tube-to-baffle leakage stream. The fluid flowing through the clearance between the tube outside diameter and the tube hole in the baffle.
Figure 5.2.1. Idealized main stream flow §
Stream B is the actual cross-flow stream.
§
Stream C is the bundle-to-shell bypass stream. The fluid flowing in the clearance area between the outer tubes in the bundle (bundle diameter) and the shell.
§
Stream E is the baffle-to-shell leakage stream. The fluid flowing through the clearance between the edge of a baffle and the shell wall.
§
Stream F is the pass-partition stream. The fluid flowing through the gap in the tube arrangement due to the pass partition plates. Where the gap is vertical it will provide a low-pressure drop path for fluid flow.
Note. There is no stream D. The fluid in streams C, E and F bypasses the tubes, which reduces the effective heat transfer area. Stream C is the main bypass stream and will be particularly significant in pull-through bundle exchangers, where the clearance between the shell and bundle is of necessity large. Stream C can be considerably reduced by using sealing strips; horizontal strips that block the gap between the bundle and the shell. Dummy tubes are also sometimes used to block the pass-partition leakage stream F.
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The tube-to-baffle leakages stream A does not bypass the tubes and their main effects are on pressure drop rather than heat transfer.
Figure 5.2.2. Shell-side leakage and by-pass paths
2.2. Heat-transfer coefficient The shell-side heat transfer coefficient is given by: hs = hoc Fn Fw Fb FL Where hoc = heat transfer coefficient calculated for cross-flow over an ideal tube bank, no leakage or bypassing. Fn = correction factor to allow for the effect of the number of vertical tube rows, Fw = window effect correction factor, Fb = bypass stream correction factor, FL = leakage correction factor. The total correction will vary from 0.6 for a poorly designed exchanger with large clearances to 0.9 for a well-designed exchanger.
22
hoc, ideal cross-flow coefficient The heat-transfer coefficient for an ideal cross-flow tube bank can be calculated using the heat transfer factors jh given in Figure 5.2.3. Figure 5.2.3 has been adapted from a similar figure given by Mueller (1973). Mueller includes values for more tube arrangements than are shown in Figure 5.2.3. As an alternative to Figure 5.2.3, the comprehensive data given in the Engineering Sciences Data Unit Design Guide on heat transfer during cross-flow of fluids over tube banks, ESDU 73031 (1973), can be used; see Butterworth (1977).
Figure 5.2.3 Heat-transfer factor for cross-flow tube banks
The Reynolds number for cross-flow through a tube bank is given by:
Where Gs = mass flow rate per unit area, based on the total flow and free area at the bundle equator. This is the same as Gs calculated for Kern’s method, do = tube outside diameter. The heat-transfer coefficient is given by:
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Fn, tube row correction factor The mean heat-transfer coefficient will depend on the number of tubes crossed. Figure 5.2.3 is based on data for ten rows of tubes. For turbulent flow the correction factor
Fn is close to 1.0. In laminar flow the heat-transfer coefficient may decrease with increasing rows of tubes crossed, due to the build up of the temperature boundary layer. The factors given below can be used for the various flow regimes; the factors for turbulent flow are based on those given by Bell (1963). Ncv is number of constrictions crossed = number of tube rows between the baffle tips; see Figure. 1. Re > 2000, turbulent; take Fn from Figure 5.2.4
Figure 5.2.4 Tube row correction factor Fn
2. Re > 100 to 2000, transition region, take Fn = 1.0; 3. Re < 100, laminar region, Fn α (Nc’) -.18 Where Nc’ is the number of rows crossed in series from end to end of the shell, and depends on the number of baffles. The correction factor in the laminar region is not well established, and Bell’s paper, or the summary given by Mueller (1973), should be consulted if the design falls in this region.
Fw, window correction factor This factor corrects for the effect of flow through the baffle window, and is a function of the heat-transfer area in the window zones and the total heat-transfer area. The correction factor is shown in Figure 5.2.5 plotted versus Rw, the ratio of the number of tubes in the window zones to the total number in the bundle, determined from the tube layout diagram.
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For preliminary calculations Rw can be estimated from the bundle and window crosssectional areas.
Figure 5.2.5 Window correction factor
Fb, bypass correction factor This factor corrects for the main bypass stream, the flow between the tube bundle and the shell wall, and is a function of the shell to bundle clearance, and whether sealing strips are used:
Where α = 1.5 for laminar flow, Re < 100, α = 1.35 for transitional and turbulent flow Re > 100, Ab = clearance area between the bundle and the shell, As = maximum area for cross-flow, equation 12.21, Ns = number of sealing strips encountered by the bypass stream in the cross-flow zone, Ncv = the number of constrictions, tube rows, encountered in the cross-flow section. Equation applies for Ns < Ncv/2. Where no sealing strips are used, Fb can be obtained from Figure 5.5.6.
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FL, Leakage correction factor This factor corrects for the leakage through the tube-to-baffle clearance and the baffle-to shell clearance.
Figure 5.2.6 Bypass correction factor
Figure 5.2.7. Coefficient for FL, heat transfer
Where βL = a factor obtained from Figure 5.2.7, Atb = the tube to baffle clearance area, per baffle. Asb = shell-to-baffle clearance area, per baffle. AL = total leakage area = (Atb + Asb).
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2.3 Pressure drop The pressure drops in the cross-flow and window zones are determined separately, and summed to give the total shell-side pressure drop.
Cross-flow zones The pressure drop in the cross-flow zones between the baffle tips is calculated from correlations for ideal tube banks, and corrected for leakage and bypassing. ∆Pc = ∆Pi F’b F’L Where ∆Pc = the pressure drop in a cross-flow zone between the baffle tips, corrected for by-passing and leakage, ∆Pi = the pressure drop calculated for an equivalent ideal tube bank, F’b = by-pass correction factor, F’L = leakage correction factor.
∆ Pi ideal tube bank pressure drop The number of tube rows has little effect on the friction factor and is ignored. Any suitable correlation for the cross-flow friction factor can be used; for that given in Figure 5.2.8, the pressure drop across the ideal tube bank is given by:
Where Ncv = number of tube rows crossed (in the cross-flow region), us = shell side velocity, based on the clearance area at the bundle equator. jf = friction factor obtained from Figure 5.2.8, at the appropriate Reynolds number, Re = (ρusdo/μ).
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Figure 5.2.8 Friction factor for cross-flow tube banks
F’b, bypass correction factor for pressure drop Bypassing will affect the pressure drop only in the cross-flow zones. The correction factor is calculated from the equation used to calculate the bypass correction factor for heat transfer, but with the following values for the constant ˛. Laminar region, Re < 100, α = 5.0 Transition and turbulent region, Re > 100, α = 4.0 The correction factor for exchangers without sealing strips is shown in Figure 5.2.9.
F’L, leakage factor for pressure drop Leakages will affect the pressure drop in both the cross-flow and window zones. The factor is calculated using the equation for the heat-transfer leakage-correction factor, with the values for the coefficient β’L taken from Figure 5.2.10.
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Window-zone pressure drop Any suitable method can be used to determine the pressure drop in the window area; see Butterworth (1977). Bell used a method proposed by Colburn. Corrected for leakage, the window drop for turbulent flow is given by:
Where uz = the geometric mean velocity,
uw = the velocity in the window zone, based on the window area less the area occupied by the tubes Aw
Ws = shell-side fluid mass flow, kg/s, Nwv = number of restrictions for cross-flow in window zone, approximately
Figure 5.2.9. Bypass factor for
Figure 5.2.10. Coefficient for F’L,
pressure drop F’b
pressure drop
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End zone pressure drop There will be no leakage paths in an end zone (the zone between tube sheet and baffle). Also, there will only be one baffle window in these zones; so the total number of restrictions in the cross-flow zone will be Ncv + Nwv. The end zone pressure drop ∆Pe will therefore be given by:
Total shell-side pressure drop Summing the pressure drops over all the zones in series from inlet to outlet gives: ∆Ps = 2 end zones + (Nb – 1) cross-flow zones + Nb window zones ∆Ps = 2∆Pe + ∆Pc (Nb – 1) + N b ∆Pw Where Nb is the number of baffles = [(L/lB) -1].
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UNIT-6 SPREADSHEET – A COST-EFFECTIVE & TRANSPARENT TOOL (5) Nowadays there are commercial applications such as HYSYS ® and ASPEN PLUS ® that allows the user to simulate chemical plants in a very realistic way. Generally speaking, these applications are very expensive and do not indicate exactly the simplifications upon which the simulation models are based. However, using a low cost tool as Spreadsheet i.e. MS Excel ®, it is possible to build and solve simulation models that duplicate the results obtained using commercial simulators. In order to develop practical simulations in Excel®, engineers must use detailed mathematical models of unit operations, computer code for the calculation of thermodynamic properties, and a computational tool designed to solve the highly nonlinear equation systems involved in such models.
Presently, there are some free computer programs designed to construct and solve simulation models. Among them, Ascend IV (6) is powerful mathematical modeling software with some thermodynamics and distillation libraries; however, its interface is not very user friendly, an aspect that causes some problems to the beginner.
Chemical process simulation involves the integration of three basic elements: • Mathematical models of unit operations, • Thermodynamic properties calculation methods, • Numerical methods for the solution of non-linear equations systems. Many papers present mathematical models for particular units in particular conditions, but it is difficult to find works grouping general models that shear the same style and notation. Additionally, even though there are many theoretical presentations (7), there are only a few low cost tools for the calculation of thermodynamic properties of mixtures. Among them, BibPhy®-ProSim and ProdeProperties®- Prode are two examples that allow thermodynamic properties calculations using Excel®.
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Finally, some commercial tools such as MATLAB ® and MAPLE ® allow the solution of general equations systems; however, these packages are relatively expensive and use traditional numerical methods that can be inadequate for the solution of the highly non-linear equations involved in the simulation of chemical processes (8).
Due to its low cost and highly acceptance in industry and academy, MS Excel is and ideal platform for the construction and solution of mathematical models. The present work presents how the three elements descried above can be integrated in MS Excel, offering a practical method that allow the simulation of unit operations and complete processes at a fraction of the cost of commercial process simulators.
6.1
The Uses of Microsoft Excel in Business and Engineering
Microsoft Excel is a spreadsheet tool capable of performing calculations, analyzing data and integrating information from different programs. Microsoft Excel is comprised of organizational units called workbooks. A standard workbook contains worksheets and chart sheets. Worksheets perform calculations, store and organize data, present graphics and controls like a web page; they are extremely versatile. A worksheet in turn is comprised of millions of cells. The job of a cell is to store a formula that performs a calculation or communicates with some other application (i.e. program) such as a database. They also store and present data. A chart sheet's job is to present a chart or graph developed from data stored on a worksheet. ·
A typical business worksheet (A), its elements and the workbook that contains it are presented in the illustration below. This simple example is a shipping status system developed in Microsoft Excel in less than an hour utilizing the conditional formatting feature, drawing shapes and worksheet functions like Vlookup.
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Figure 6.1.1 Business Example ·
A typical engineering worksheet (B), its elements and the workbook that contains it are presented in the illustration below. This simple example is a pressure vessel design developed in Microsoft Excel in less than an hour.
Figure 6.1.2 Engineering Example
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Think of Microsoft Excel as a modular tool set that can be rapidly configured to accomplish a desired task. A key force behind Microsoft Excel's capabilities is a powerful programming language called Visual Basic for Applications (VBA) which comes standard with Microsoft Excel. Using Microsoft Excel and VBA, a professional can accomplish important tasks like: ·
Rapid analysis and charting
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Advanced modeling including numerical simulation
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Automated report generation
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Problem optimization using Solver and Crystal Ball
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Software design
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Team and model integration
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Database communication and control
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Project command and control
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Project constraint monitoring
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Information command and control
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Multi-language programming with FORTRAN and C code (DLL's)
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Real time integration with other applications
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Data sorting and analysis
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UNIT-7 SHELL & TUBE HEAT EXCHANGER DESIGN VIA SPREADSHEET The present work is an initiation to bring both the previously discussed approaches to a common platform and compare the same. MS Excel was selected as a tool to address the above due to its inherent features like easy accessibility, simplicity and transparency. Both the approaches can be very well formulated as CAD (Computer Aided Design) using inbuilt functions like IF condition, VLOOKUP, etc.
7.1 SPREADSHEET PROCEDURE
7.1.1 First of all one has to select the process fluid of shell side & tube side from the dropdown menu. Ø Shell side fluid
Ø Tube side fluid
Figure 7.1.1 Selecting process fluid from dropdown menu
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7.1.2 The process variables which are to be manually supplied are temperature of the process fluid at the inlet & outlet of the shell & tube heat exchanger design, flow rate of the process fluids.
Figure 7.1.2 Preliminary manual inputs
7.1.3 After providing preliminary variables input, the spreadsheet generates the physical properties generated for the systems from the databanks, which is included in this work (9)
Figure 7.1.3 Generating physical properties of the process fluids (10)
7.1.4 Now, shell & tube heat exchanger design is the very iterative process, so we have to give some assumed or initial values for some of the tentative set of parameters. These parameters are no. of passes at the tube side, overall heat transfer coefficient (U0), type of pitch, type of head, etc.
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Figure 7.1.4 Assuming the values for different parameters.
7.1.5 Then the select the type of method for designing the shell & tube heat exchanger.
Figure 7.1.5 Selecting the method of design.
7.1.6 The spreadsheet then computes & validate the necessary and sufficient conditions for the determining the result of design parameters like the area of heat exchanger, no. of tubes required, shell side heat transfer coefficient & pressure drop, tube side heat transfer & pressure drop, etc.
Figure 7.1.6 Results
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7.2 Color code used in spreadsheet: The following color codes are used in the excel sheet for making it more user friendly.
Color
Inference Manual inputs Values obtained from the databank Assumed values Resulting parameters Values of resulting parameters obtained by spreadsheet procedure Different result values due to different approaches
Table 7.2.1 Color code in spreadsheet
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UNIT-8 CASE STUDIES 8.1 CASE STUDY -1 Design an exchanger to sub-cool condensate from a methanol condenser from 95ºC to 40°C. Flow-rate of methanol 100,000 kg/h. brackish water will be used as the coolant, with a temperature rise from 25°C to 40°C.
Figure 8.1.1 Problem inputs
Now, the spreadsheet will estimate the values of necessary physical & chemical properties of methanol and water from its databank. The follwing figures are displaying the same thing.
Figure 8.1.2 Physical properties of process fluids (9) & (10) + Now take the initial values for the tentative set of parameters, like No. of passes in the tube side = 2, Overall heat transfer coefficient = 600 W/m2 °c, Type of pitch = triangular pitch, Type of head = split ring floating head, Type of shell side fluid = organic liquid, Type of tube side fluid = sea water, etc.
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The follwing figures are displaying the same thing.
Figure 8.1.3 Selecting no. of passes in tube side
Figure 8.1.4 Selecting the type of pitch
Figure 8.1.5 Selecting the type of head
Figure 8.1.6 Selecting the type of shell side & tube side fluid
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Next, the spreadsheet will compute and validate the necessary and sufficient conditions for determining the design parameter values based on both the approaches refer to the following figures.
Figure 8.1.7 Results obtained by kern’s method Now, choose other method of design the shell & tube heat exchanger, we get the different result values for the shell side heat transfer coefficient & pressure drop. The follwing figures are displaying the same thing.
Figure 8.1.8 Selecting other method of design
Figure 8.1.9 Results obtained by Bell-Delaware’s method
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8.2 CASE STUDY-2 Design an exchanger to sub-cool condensate from a Methanol condenser from 95ºC to 40°C. Flow-rate of methanol 100,000 kg/h. Now, let’s take the other tube side fluid i.e. Hexane as the coolant, with a temperature rise from 25°C to 40°C.
Figure 8.2.1 Selecting the other coolant for same problem
Now, the spreadsheet will estimate the values of necessary physical & chemical properties of methanol and water from its databank. The follwing figures are displaying the same thing.
Figure 8.2.2 Physical properties of process fluids (9) & (10)
Now take the different initial values for the tentative set of parameters, like No. of passes in the tube side = 4, Overall heat transfer coefficient = 800 W/m2 °c, Type of pitch = square pitch, Type of head = fixed & U-tube head, Type of shell side fluid = organic liquid, Type of tube side fluid = organic liquid, etc.
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The follwing figures are displaying the same thing.
Figure 8.2.3 Selecting no. of passes in tube side
Figure 8.2.4 Selecting the type of pitch
Figure 8.2.5 Selecting the type of head
Figure 8.2.6 Selecting the type of shell side & tube side fluid
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Next, the spreadsheet will compute and validate the necessary and sufficient conditions for determining the design parameter values based on both the approaches refer to the following figures.
Figure 8.2.7 Results obtained by kern’s method Now, choose other method of design the shell & tube heat exchanger, we get the different result values for the shell side heat transfer coefficient & pressure drop. The follwing figures are displaying the same thing.
Figure 8.2.8 Selecting other method of design
Figure 8.2.9 Results obtained by Bell-Delaware’s method
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UNIT-9 SUMMARY Ø A Shell & Tube Heat Exchanger Design Spreadsheet is easy to learn, & offers a quicker way for preliminary calculation of the Shell & Tube Heat Exchanger design. Ø By using spreadsheet method, we obtain a quick view of results in tabulated format, which is easy to visualize the result. Ø A Spreadsheet also eliminates human error in doing the iteration calculation, which is commonly used in design calculation.(11) Ø In addition to this, it can prove to be a vital tool for practicing Chemical Engineers and Designers for the design of Shell & Tube Heat exchanger with the minimum inputs. Ø Here the by using the lookup function of excel, we can easily visualize the physical properties of different compounds, by selecting corresponding compound by the use component dropdown function. Ø This method also offers an easy way for the designer to scale up and optimize the process. Ø The Spreadsheet also provides a cheaper alternative to the designer compared to costly commercial software. & also gets the transparent results of the given process. Ø For lab level or pilot plants, this spreadsheet method provides a less costly path of examining the changes in parameters for changes in process conditions. ·
Here in the case study-1 of methanol in shell side & water in tube side, gets the same area for the two different approaches i.e. kern & Bell-Delaware. But there is difference in pressure drop & heat transfer coefficient on the shell side.
·
Now, in the case study-2, by changing the other tube side fluid i.e. Hexane as a coolant, gets the different values for the design parameters.
Ø Although this is a primitive stage, the calculations can be extended to include more complicated systems and design.
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Ø The present work is an initiation to compare the fundamental and practical approaches on a common platform using spreadsheet. Owing to the salient features of spreadsheet, the computation tool developed can be extensively used for teaching the concepts of certainly augment the traditional classroom teaching. Ø This type of spreadsheet application can serve the dual purpose of being helpful to the students in understandings the effects of different operating variables on the system and the industrial personnel for the verification of the design aspects. Ø Further, the scope can be expanded if spreadsheet application is merged with mechanical designing aspects of Heat Exchanger, tubes, tube sheets, head, nozzles, etc. this will make it a complete spreadsheet design application for the Shell & Tube Heat Exchanger designing.
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REFERENCES 1. Coulson & Richardson’s Chemical Engineering, vol.6., “Chemical Engineering Design”, 4th Edition, R.K.Sinnott, (2005). 2. Kern, D.Q., “Process Heat Transfer”, McGraw-Hill, New York, (1950). 3. Taborek, J., Evolution of heat exchanger design techniques, Heat Transfer Eng. 1, No. 1, 15-29 (1979). 4. Sadik Kakac & Hongtan Liu, “Heat Exchangers, Selection Rating & Design”, CRC Press, 2nd Edition, (2002). 5. Bernard V. Lineage, “A Guide to Microsoft Excel for Scientists and Engineers”, Second Edition, (2000). 6. P. C. Piela, T. G. Epperly, K. M. Westerberg and A. W. Westfzrberg, “Ascend: an object-oriented computer environment for modeling and analysis: The modeling language”, Computers & Chemical Engineering, Vol. 15, No. 1. pp. 53-72, (1991). 7. Prausnitz JM, Lichtenthaler RN, De Azevedo EG, Molecular Thermodynamics of fluid-phase equilibria, 3rd edition, Pretence hall PTR, New Jersey, (1999). 8. M. F. Cardoso, R. L. Salcedo, S. Feyo de Azevedo, D. Barbosa, “Optimization of reactive distillation processes with simulated annealing”, Chemical Engineering Science,55,pp. 5059-5078, (2000). 9. Don W. Green & Robert H. Perry, “Perry’s Chemical Engineer’s Handbook”, 8th Edition, McGraw-Hill, New York, (2008). 10. PERRY, R. H., GREEN, D.W. and MALONEY, J. O. “Perry’s Chemical Engineers Handbook”, 7th edition, (McGraw-Hill), (1997). 11. Jolius Gimbun, A.B.Dayang Radish, T.G. Chuah, Bioreactor design via spreadsheeta study on the monosodium glutamate (MSG) process, journal of food engineering, 64, p.277-283, (2004).
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