Selection of Shell and tube heat exchanger (STHE)
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This presentation describes the considerations involved in selecting the shell and tube exchanger according to TEMA Designations. Also, it helps to identify whether fluid should be sent tube side or shell side
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Selection of Shell and tube heat exchanger (STHE)
- 1. SELECTION OF SHELL AND TUBE HEAT EXCHANGER (STHE) GAUTHAM S PROCESS ENGINEER
- 2. Principal Components of STHE Shell Shell cover Tubes Channel Channel cover Tubesheet Baffles and nozzles
- 3. Other Components Tie-in rods and spacers Pass partition plates / Channel partitions Impingement plates/rods Longitudinal baffles Sealing strips and Supports
- 4. SCHEMATIC
- 7. SHELL SIDE OR TUBE SIDE? The General method of Selection is based on the necessity to reduce cost and make maintenance easy PARAMETER SHELL SIDE TUBE SIDE Mechanical Cleaning Fouling fluids Pressure High Pressure Viscosity High Viscous Fluids (Doubt) Corrosion Very corrosive fluids Metallurgy Expensive metallurgy
- 8. Straight Tube and Fixed Tubesheets Examples such as BEM, AEM, NEN, etc., This TEMA type is the simplest design and is constructed without packed or gasketed joints on the shell side. The tubesheet is welded to the shell and the heads are bolted to the tubesheet On the NEN heat exchanger, the shell and the head is welded to the tubesheet. Typically, a cover plate design is provided to facilitate tube cleaning. NEN is the lowest cost TEMA design per square foot of heat transfer surface.
- 9. Straight Tube and Fixed Tubesheets ADVANTAGES LIMITATIONS APPLICATIONS Less costly than removable bundle designs Shell side can be cleaned only by chemical methods Oil Coolers, Liquid to Liquid, Vapor condensers, reboilers, gas coolers Provides maximum amount of surface No provision to allow for differential thermal expansion, must use an expansion joint Provides for single and multiple tube passes to assure proper velocity
- 10. Removable Bundle, Externally Sealed Floating Tubesheet Example such as, AEW, BEW This design allows for the removal, inspection and cleaning of the shell circuit and shell interior. Special floating tubesheet prevents intermixing of fluids. Maximum surface for a given shell diameter for removable bundle design Tubes can be cleaned in AEW models without removing piping. Packing materials produce limits on design pressure and temperature
- 11. Removable Bundle, Externally Sealed Floating Tubesheet ADVANTAGES LIMITATIONS APPLICATIONS Floating tubesheet allows for differential thermal expansion between the shell and the tube bundle. Fluids in both the shell and tube circuits must be non-volatile, non- toxic Intercoolers and after coolers, air inside the tubes Shell circuit can be steam or mechanically cleaned Tube side passes limited to single or two pass design Jacket water coolers or other high differential temperature duty The tube bundle can be repaired or replaced without disturbing shell pipe All tubes are attached to two tubesheets. Tubes cannot expand independently so that large thermal shock applications should be avoided
- 12. Removable Bundle, Outside Packed Head, Example such as, BEP, AEP, etc., This design allows for the easy removal, inspection and cleaning of the shell circuit and shell interior without removing the floating head cover. Special floating tubesheet prevents intermixing of fluids. In most cases, straight tube removable design is more costly than U-tube designs. On AEP design, tubes can be serviced without disturbing tubeside piping Less costly than TEMA type BES or BET designs
- 13. Removable Bundle, Outside Packed Head ADVANTAGES LIMITATIONS Floating tubesheet allows for differential thermal expansion between the shell and the tube bundle. Shell fluids limited to non volatile, non toxic materials Shell circuit can be inspected and steam cleaned. If the tube bundle has a square tube pitch, tubes can be mechanically cleaned by passing a brush between rows of tubes. All tubes are attached to two tubesheets. Tubes cannot expand independently so that large thermal shock applications should be avoided The tube bundle can be repaired or replaced without disturbing shell piping Packing limits shell side design temperature and pressure
- 14. Removable Bundle, Internal Split Ring Floating Head Example such as, AES, BES, Ideal for applications requiring frequent tube bundle removal for inspection and cleaning Uses straight-tube design suitable for large differential temperatures between the shell and tube fluids More forgiving to thermal shock than AEW or BEW designs. Suitable for cooling volatile or toxic fluids. Higher surface per given shell and tube diameter than “pull-through” designs such as AET, BET, etc.
- 15. Removable Bundle, Internal Split Ring Floating Head ADVANTAGES LIMITATIONS Floating head design allows for differential thermal expansion between the shell and the tube bundle. Shell cover, split ring and floating head cover must be removed to remove the tube bundle, results in higher maintenance cost than pull-through Provides multi-pass tube circuit arrangement More costly per square foot of surface than fixed tubesheet or U-tube designs Shell circuit can be inspected and steam cleaned. If it has a square tube layout, tubes can be mechanically cleaned
- 16. Removable Bundle, Pull-Through Floating Head, Example such as, AET, BET Ideal for applications requiring frequent tube bundle removal for inspection and cleaning as the floating head is bolted directly to the floating tubesheet. This prevents having to remove the floating head in order to pull the tube bundle
- 17. Removable Bundle, Pull-Through Floating Head, ADVANTAGES LIMITATIONS Floating head design allows for differential thermal expansion between the shell and the tube bundle. For a given set of conditions, this TEMA style is the most expensive design Shell circuit can be inspected and steam or mechanically cleaned Less surface per given shell and tube diameter than other removable designs Provides large bundle entrance area for proper fluid distribution Provides multi-pass tube circuit arrangement.
- 18. Removable Bundle, U-Tube Example such as, BEU, AEU, Especially suitable for severe performance requirements with maximum thermal expansion capability. Because each tube can expand and contract independently, this design is suitable for larger thermal shock applications. While the AEM and AEW are the least expensive, U-tube bundles are an economical TEMA design.
- 19. Removable Bundle, U-Tube ADVANTAGES LIMITATIONS U-tube design allows for differential thermal expansion between the shell and the tube bundle as well as for individual tubes Draining of tube circuit is difficult when mounted with the vertical position with the head side up. Shell circuit can be inspected and steam or mechanically cleaned Because of u-bend, tubes can be cleaned only by chemical means Less costly than floating head or packed floating head designs Because of U-tube nesting, individual tubes are difficult to replace Provides multi-pass tube circuit arrangement. No single tube pass or true countercurrent flow is possible Bundle can be removed from one end for cleaning or replacement Tube wall thickness at the U-bend is thinner than at straight portion of tubes
- 20. Discussion What type of exchanger do we select for a cost-wise economical design? 1. No Fouling/ Fouling condition 2. Non-Corrosive/ Corrosive condition 3. High differential thermal gradients 4. Combination of any 2 5. All the above expect 4
- 22. QUIZ TYPE B TYPE C TYPE N
- 23. Fixed Tubesheet Shell side fluid is non-fouling or the fouling can be chemically cleaned. The Mean temperature differential between shell and tube wall must be less than 50 Deg C. Otherwise, expansion bellow is required. Stress caused by differential expansion between the shell and the tube should not exceed the design stress limits considering winter and start up conditions
- 24. U-Tube Bundle The U-tube is limited to applications where the tube side fluid is non-fouling; any fouling fluid must be routed through shell side only. In this respect, tube side mechanical cleaning is considered possible, if the centre to centre distance between the parallel legs of the U-tube is at least 150mm. However, this later option may be used only if required by specific process requirements. Horizontal U-Tube should be used when condensing fluid in the tube side.
- 25. Floating head Split ring (S type), Pull through (T type), Externally sealed tubesheet (W Type) and Outside packed (P Type) Type S and Type T are the common types of Floating head Floating head type or U-tube type heat exchanger should be selected if flexibility is required to avoid overstressing. The maximum shell diameter is based upon tube- bundle removal requirements and is limited by crane capacities. Thus, floating head heat exchangers are often limited to a Shell ID of 1400 to 1500 mm. In S-type as there is an internal joint at the floating head, a careful design is necessary to avoid leakage of one fluid to the other.
- 26. Shell Selection
- 27. QUIZ TYPE E TYPE J TYPE G
- 28. E Shell Commonly used shell type Its limitations are shell side pressure drop or problem due to flow-induced vibrations Shell types such as Type J, Type X ,Type H can be used as alternatives. Special case of E type shell where either entry or exit of the shell side fluid can be split into two parts to reduce pressure drop or flow induced vibrations in the shell side. J Shell
- 29. G Shell G shells have a longitudinal baffle axially “G” shells are used for a maximum tube length of twice the maximum unsupported span as per TEMA as there is one full support plate in the tube bundle. It is used for horizontal thermosyphon reboilers in order to reduce the pressure drop as well as to avoid flow mal-distribution H shells have a longitudinal baffle axially “H” shells are used for a maximum tube length of four times of the maximum unsupported span due to presence of 2 full support plates in the tube bundle. It is used for horizontal thermosyphon reboilers in order to reduce the pressure drop as well as to avoid flow mal-distribution H Shell
- 30. F Shell This shell is used when there is a temperature cross i.e., when the outlet temperature of cold stream is higher than the outlet temperature of the hot stream. “F” shells are prone to leakage across the longitudinal baffle in removable bundle exchangers and hence their use is generally not recommended. If “F” shell is employed in a removable type exchanger, the shell side pressure drop should be limited to 0.35 kg/cm2g. multiple shells in series are to be employed when more number of tube passes is required. This type of shell is called kettle construction where there is an enlarged shell above the tube bundle for the disengagement of the vapor from the boiling liquid. The kettle type is a special application of the U-tube type and pull through type of construction K Shell
- 32. QUIZ TYPE T TYPE S TYPE W
- 33. Rear End Selection Rear End selection depends on a number of factors such as cost, maintenance, Fluid characteristics, application, etc. Normally, Rear End “Type M” is used for “Type A” (Bonnet Type) Front End but for heat exchanger with “Type A” front end stationary head and an odd number of tube passes, “Type L” shall be selected. Rear end head Type T shall be used for a kettle type exchanger with floating head. TYPE M TYPE L TYPE T
- 34. Appendix – A (Baffle Design) Longitudinal baffles, cross baffles and support baffles are the 3 types of baffles. Commonly used baffle is single segmental but if it is necessary to reduce the shell side pressure drop then double segmental baffle is used To reduce flow induced vibrations on the shell side, the no tube in windows design with single segmental baffle can be used. Optimum baffle spacing is between 0.3 to 0.6 of Shell ID. The minimum baffle spacing as per TEMA is one fifth of the shell inside diameter or 4” whichever is lower
- 35. Appendix-B (Tube layout and Pitch) Triangular, rotated triangular, square and rotated square are different tube layouts Triangular or rotated triangular tube layouts is commonly used as it can accommodate more number of tubes but it is limited to clean services Square and rotated square tube layouts is used for dirty shell side service and when mechanical cleaning of shell side is necessary The minimum tube pitch should be 1.25 times tube OD. For square and rotated square layouts, a minimum cleaning lane of 6mm should be provided.
- 36. THANK YOU