The Tubular Exchanger Manufacturers Association (TEMA) is an association of manufacturers of shell and tube heat exchangers. TEMA has established a set of construction standards for Shell and Tube Heat Exchangers. The Standards are regularly updated and published; the most recent edition is the seventh, published in 1988. Most shell and tube exchangers ordered by the process industries and for other high-severity applications throughout the world are built to TEMA standards.
Read also: Classification of heat exchangers
The TEMA Mechanical standards are applicable to shell and tube heat exchangers that do not exceed any of the following criteria:
- Inside diameters of 100 inches
- Product of nominal diameter, inches, and design pressure, psi
- Design pressure of 3000psi
The intent of these parameters is to limit the maximum shell wall thickness to approximately 3 in. and the maximum stud diameter to approximately 4 in.
TEMA Class ‘R’ exchangers
The TEMA Standards for Class ‘R’ heat exchangers specify the design and fabrication of unfired shell and tube heat exchangers for the generally severe requirements of petroleum and related processing applications.
TEMA Class ‘C’ Exchangers
The TEMA Standards for Class ‘C’ Heat Exchangers specify the design and fabrication of unfired shell and tube HEAT exchangers for the generally moderate requirements of commercial and general process applications.
TEMA Class ‘B’ exchangers
The TEMA Standards for Class ‘B’ Heat Exchangers specify the design and fabrication of unfired shell and tube HEAT exchangers for chemical process service.
Front Head Types
1. A-Type front header
This is the most common stationary head type and one of the removable bundle exchangers type stationary heads which gives the ease to replace the tube bundle without replacing the shell or bonnets. It does however have two seals (one between the tube sheet and header and the other between the header and the endplate). This increases the risk of leakage and the cost of the header over a B-Type Front Header.
- These are generally less cost-effective than non-removable designs.
- This type of header is easy to repair and replace.
- It also gives access to the tubes for cleaning or repair without having to disturb the pipework.
2. B-Type front header
This is the cheapest type of front header. It also is more suitable than the A-Type Front Header for high-pressure duties because the header has only one seal. This front head features a removable channel with an integral cover. It is used with a fixed tube sheet, U-tube, and removable bundle exchanger design. A disadvantage is that gaining access to the tubes requires disturbance to the pipework in order to remove the header.
3. C-Type front header
The channel with a removable cover is integral to the tube sheet. It is attached to the shell by a flanged joint and is used for U-tube and removable bundles.
- This type of header is for high-pressure applications (>100 bar)
- It does allow access to the tube without disturbing the pipework but is difficult to repair and replace because the tube bundle is an integral part of the header.
4. D-Type front header
This is the most expensive type of front header. It does allow access to the tubes without disturbing the pipework but is difficult to repair and replace because the tube bundle is an integral part of the header.
- It is for very high pressures (> 150 bar).
5. N-Type front header
This type of stationary head is integral to the shell and is used with fixed tube sheet designs. The use of type N with U-tube and removable bundles is not recommended since the channel is integral with the tube bundle, which complicates bundle maintenance.
- This type of header is that the tubes can be accessed without disturbing the pipework
- It is cheaper than an A-Type Front Header.
- They are difficult to maintain and replace as the header.
- Tube sheets are an integral part of the shell.
1. E-Type shell
This is the most commonly used shell type, suitable for most duties and applications. Other shell types only tend to be used for special duties or applications.
2. F-Type shell
This type of shell design provides a two-pass longitudinal flow plate to be installed inside the tube bundle assembly. The pate causes the shell fluid to travel one half of the tube bundle, then down the other half, in fact producing a counter-current flow pattern which is best for heat transfer.
This type of construction can be specified where a close approach temperature is required and when the flow rate permits the use of one half of the shell at a time. In heat recovery applications, or where the application calls for increased thermal length to achieve effective overall heat transfer, shells can be installed with the flows in series. Up to six shorter shells in series is common and results in counter-current flow close to performance as if one long shell in a single pass design were used.
3. G-Type shell
This type of shell is used when there is a phase change in the system. This is used for horizontal thermosiphon re-boilers and applications where the shell side pressure drop needs to be kept small. This is achieved by splitting the shell side flow.
4. H-Type shell
This is used for similar applications to G-Type Shell but tends to be used when larger units are required.
5. J-Type shell
This shell design offers a divided flow arrangement.
TEMA J Shells are typically specified for phase change duties where significantly reduced shell side pressure drops are required. They are commonly used in stacked sets with single nozzles used as the inlet and outlet. A special type of J-shell is used for the flooded evaporation of shell-side fluids. A separate vapor disengagement vessel without tubes is installed above the main J shell with the vapor outlet at the top of this vessel.
6. K-Type shell
This is used only for re-boilers to provide a large disengagement space in order to minimize shell-side liquid carryover. Alternatively, a K-Type Shell may be used as a chiller. In this case, the main process is to cool the tube side fluid by boiling a fluid on the shell side.
7. X-Type shell
The TEMA X shell or crossflow shell is most commonly used in vapor condensing applications, though it can also be used effectively in low-pressure gas cooling or heating. It produces a very low shell side pressure drop and is, therefore, most suitable for vacuum service condensing. In order to assure adequate distribution of vapors, X-shell designs typically feature an area free of tubes along the top of the exchanger. It is also typical to design X shell condensers with a flow area at the bottom of the tube bundle to allow free condensate flow to the exit nozzle. Careful attention to the effective removal of non-condensable is vital to X-shell constructions.
Rear Head Types
1. Type L
This rear head type is similar to the type A stationary head. This type of header is for use with fixed tube sheets only, since the tube sheet is welded to the shell, and access to the outside of the tubes is not possible. The main advantages of this type of header are that access can be gained to the inside of the tubes without having to remove any pipework and the bundle to shell clearances are small. The main disadvantage is that a bellows or an expansion roll are required to allow for large thermal expansions and this limits the permitted operating temperature and pressure.
2. Type M
This is similar in construction to the type B stationary head and also similar to the L-type rear head but is slightly cheaper. It is used with fixed tube sheet exchangers. However, the header has to be removed to gain access to the inside of the tubes. Again, special measures have to be taken to cope with large thermal expansions and this limits the permitted operating temperature and pressure.
3. Type N
This is similar in construction to the type N stationary head. It is also used with fixed tube sheet exchangers. The advantage of this type of header is that the tubes can be accessed without disturbing the pipework. However, they are difficult to maintain and replace since the header and tube sheet are an integral part of the shell.
4. Type P
This is an outside-packed floating rear header. It is, in theory, a low-cost floating head design that allows access to the inside of the tubes for cleaning and also allows the bundle to be removed for cleaning. The main problems with this type of header are large bundle to shell clearances required in order to pull the bundle; it is limited to low-pressure nonhazardous fluids (not suitable for hydrocarbons and toxic fluids on shell side) because it is possible for the shell side fluid to leak via the packing rings and only small thermal expansions are permitted.
In practice, it is not a low-cost design, because the shell has to be rolled to small tolerances for the packing to be effective.
5. Type S
This is a floating rear header with a backing device. It is the most expensive of the floating head types but does allow the bundle to be removed and unlimited thermal expansion is possible. It also has a smaller shell to bundle clearances than the other floating head types. However, it is difficult to dismantle for bundle pulling and the shell diameter and bundle to shell clearances are larger than for fixed head type exchangers.
6. Type T
This is a pull-through floating head. It is cheaper and easier to remove the bundle than with the S-Type Rear Header but still allows for unlimited thermal expansion. It does, however, have the largest bundle to shell clearance of all the floating head types and is more expensive than a fixed header and U-tube types.
7. Type U
This is the cheapest of all removable bundle designs but is generally slightly more expensive than a fixed tube sheet design at low pressures. However, it permits unlimited thermal expansion, allows the bundle to be removed to clean the outside of the tubes, has the tightest bundle to shell clearances, and is the simplest design. A disadvantage of the U-tube design is that it cannot normally have pure counter-flow unless an F-Type Shell is used. Also, U-tube designs are limited to even numbers of tube passes.
8. Type W
This is a packed floating tube sheet with a lantern ring. It is the cheapest of the floating head designs, allows for unlimited thermal expansion, and allows the tube bundle to be removed for cleaning. The main problems with this type of head are the large bundle to shell clearances required to pull the bundle and; the limitation to low-pressure nonhazardous fluids (because it is possible for both the fluids to leak via the packing rings).
It is also possible for the shell and tube side fluids to become mixed if leakage occurs.