Selecting TEMA Type Heat Exchangers
Defining the major TEMA constructions and identifying their advantages, limitations and applications suitable for each type can be advantageous to your process.
TEMA is a set of standards that defines the heat exchanger style and machining and assembly tolerances to be employed in the manufacture of a given exchanger. TEMA stands for Tubular Exchanger Manufacturers Association, the Tarrytown, N.Y.-based association formed by the group of heat exchanger manufacturers who developed the standards more than 60 years ago. TEMA specifications comprise industry standards that directly relate to recognized quality practices for manufacturing. Vendors who build to TEMA standards can be competitively compared because tolerances and construction methods should be very similar for a given design.
This article defines TEMA constructions and identifies the advantages and limitations for each type. TEMA designations refer to the front-head, shell and rear-head designs. For example, a TEMA type BEM has a type B front head, a type E shell and a type M rear head design. There are special conditions such as high vapor flows, high pressure and temperature crossing where a combination of TEMA features is advantageous. For example, K type shells allow for proper liquid disengagement for reboilers, and J and H type shells accommodate high vapor flow.
Straight Tube, Fixed Tubesheet. Types in this category include BEM, AEM and NEN. 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 head are welded to the tubesheet. Typically, a cover-plate design is provided to facilitate tube cleaning. This TEMA category is the lowest cost TEMA design per square foot of heat transfer surface.
This design has several advantages. It provides maximum amount of surface for a given shell and tube diameter, and it provides for single- and multiple-tube passes to ensure proper velocity. Heat exchangers of this design may be interchangeable with other manufacturers of the same TEMA type.
This design also has limitations. Its shell side can be cleaned only by chemical methods. No provision exists to allow for differential thermal expansion between the outer shell and tubes, so an expansion joint must be used.
Removable Bundle, Externally Sealed Floating Tubesheet. Types in this category include AEW and BEW. This design allows for the removal, inspection and cleaning of the shell circuit and shell interior. The special floating tubesheet prevents intermixing of fluids.
This design offers several advantages. The floating tubesheet allows for differential thermal expansion between the shell and tube bundle. The shell circuit can be inspected and steam or mechanically cleaned. The tube bundle can be repaired or replaced without disturbing shell-side piping. For a removable bundle design, it provides the maximum surface for a given shell diameter.
This design also has limitations. The fluids in both the shell and tube circuits must be nonvolatile and nontoxic. Tube-side passes are limited to one- or two-pass designs. In addition, all tubes are attached to two tubesheets. Tubes cannot expand independently, so large temperature differential applications should be avoided. Finally, its packing materials produce limits on design pressure and temperature.
Removable Bundle, Outside Packed Floating Head. This type includes BEP and AEP. Its design allows for easy tube bundle removal and inspection and cleaning of the shell circuit and shell interior. Its special floating tubesheet prevents fluid intermixing.
Advantages of this type include the removable bundle, externally sealed floating tubesheet design. This TEMA design's floating tubesheet allows for differential thermal expansion be-tween the shell and the tube bundle. Its shell circuit can be inspected and steam cleaned. In addition, if the tube bundle has a square tube pitch, passing a brush between the rows of tubes can clean them. The tube bundle can be repaired or replaced without disturbing shell piping. Only the shell-side fluids are exposed to packing, so toxic or volatile fluids can be cooled in the tube-side circuit. It provides a large bundle entrance area, reducing the need for entrance domes for proper fluid distribution.
This design also has several limitations. Shell-side fluids are limited to nonvolatile, nontoxic materials. Its packing limits the shell-side design temperature and pressure. All of the tubes are attached to two tubesheets, and the tubes cannot expand independently, so that large thermal shock applications should be avoided. Finally it provides less surface per given shell and tube diameter than AEW or BEW.
Removable Bundle, Floating Head With Backing Device. This type includes AES and BES. It is well suited for applications requiring frequent tube-bundle removal for inspection and cleaning. This heat exchanger type uses a straight-tube design suitable for large differential temperatures between the shell and tube fluids. It is more forgiving to thermal shock than AEW or BEW designs.
The design provides several advantages. Its floating head allows for differential thermal expansion between the shell and the tube bundle. Its shell circuit can be inspected and steam cleaned, and if it has a square tube layout, tubes can be mechanically cleaned. It provides higher surface per given shell and tube diameter than pull-through designs such as AET and BET. Also, it provides a multipass tube circuit arrangement.
One limitation is that the shell cover, backing device and floating head cover must be removed to remove the tube bundle.
Removable Bundle, Pull-Through Floating Head. Types in this category include AET and BET, which are suitable for applications that require 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 to pull the tube bundle.
This design has several advantages. A floating head design allows for differential thermal expansion between the shell and the tube bundle. The shell circuit can be inspected and steam or mechanically cleaned. There is a large bundle entrance area for proper fluid distribution and a multipass tube circuit arrangement.
For a given set of conditions, this TEMA style is the most expensive design. Also, there is less surface per given shell-and-tube diamater than other removable designs.
Removable Bundle, U-Tube. This type includes BEU and AEU. Because each tube can expand and contract independently, this design is suitable for larger thermal shock applications.
The U-tube design allows for differential thermal expansion between the shell and the tube bundle as well as for individual tubes. Other advantages include a shell circuit that can be inspected and steam or mechanically cleaned. This design also provides a multipass tube circuit arrangement. The bundle can be removed from one end for cleaning or replacement.
This design also has several limitations. Because of U-bend, tubes can be cleaned only by chemical means. Also, because of U-tube nesting, the individual tubes are difficult to replace. No single tube pass or true countercurrent flow is possible, and tube wall thickness at the U-bend is thinner than at straight portion of tubes. Draining the tube circuit is difficult when mounted with the vertical position with the head side up.
This article was provided by API Heat Transfer Inc., Buffalo, N.Y. To learn more about shell-and-tube heat exchangers from API Heat Transfer, call (716) 684-6700 or visit www.apiheattransfer.com.