Thermal Transfer Systems TEMA Designations of Heat Exchangers User Manual

TEMA DESIGNATIONS OF HEAT EXCHANGERS
Because of the number of variations in mechanical designs for front and rear heads and shells, and for commercial reasons, TEMA has designated a system of notations that correspond to each major type of front head, shell style and rear head. The first letter identifies the front head, the second letter identifies the shell type and the third letter identifies the rear head type.
REMOVABLE BUNDLE EXCHANGERS
Removable bundle exchangers give the customer the ability to replace the tube bundle without replacing the shell or bonnets. They are generally less cost effective than non removable designs.
BEU/AEU- U Bundle Exchangers are generally the most cost effective design style of removable bundle exchanger. Tubes may be water blasted, steam or chemically cleaned. These units must have an even number of tube passes, sometimes limiting their applicability to a service(e.g. they generally can not be used when a temperature cross occurs).
CEU- This design has the tubesheet welded to the bonnet. You can remove the bundle from the shell, however to replace the bundle, the inlet bonnet is included or you must cut off the tubesheet. Tubes may be chemically cleaned, water blasted or steam cleaned.
BEW/AEW- These are straight tube units with one floating head and one stationary head. The floating head is generally sealed with an O-Ring. These units are most often used as oil coolers or air coolers. Cleaning can be performed either by chemical, mechanical method, water blast or steam cleaning.
AEP/BEP- These are straight tube units with one inside packed floating head and one stationary head. The floating head is generally sealed with packing. These units are most often used as intercoolers and aftercoolers with the gas on the tube side. They are also the most common style for oxygen service exchangers. These units have been used in services with tube side design pressures in excess of 2000 PSIG.
AES/AET- These units are the most expensive of the removable bundle designed units. The floating head is internal to the shell. Tubes can be cleaned mechanically, chemically, water blasted or steam cleaned. The design of these units forces an even number of tube side passes therefore they suffer the same service restrictions as U bundles. Although in theory one pass unit can be designed, this is rarely done. These units are generally used in services where U bundles are not desired and the service may be too corrosive/damaging to the packing used in AEP/BEP units.
NON REMOVABLE BUNDLE EXCHANGERS
These types of units are often used in high pressure services and services where you wish to avoid leakage problems at gasketed joints. Another advantage is that they are generally more cost effective than removable bundle designs.
NEU- The most cost effective design available. The tubesheet is welded to both the shell and bonnet. There is no access to the shell. Tubes may be chemically cleaned, water blasted or steam cleaned from inside only. These units are commonly used in high pressure services (such as feedwater heaters), where process conditions allow for even pass exchangers.
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NEN- Tubesheets are welded to both the Shell & Bonnets. Access to the tubes is through covers on the channels. These units are favored in very high pressure designs as their construction minimizes the tubesheet thickness and number of high pressure retaining flanges.
AEM/BEM/AEL-Shell side is completely welded up, however, the bonnets are removable. Chemical, mechanical, and water blast cleaning of the tubes is possible, however you do not have access to the shell.
You should avoid using Steam cleaning on a fixed tube sheet unit unless the unit has a shell side expansion joint. The steam will cause the tubes to expand and pull out of the Tube Sheet causing failure at startup.
DIFFERENTIAL THERMAL EXPANSION
Since the duty of heat exchangers includes the handling of fluids of differing temperature, flow rate and thermal properties, differential expansion of the metals will take place. When the terminal temperature difference between the fluids is substantial, over 50-60 degrees, these stresses can become severe, causing shells to become deformed and damage mounting supports, tubes to deform the tube sheet or tubes to become broken or dislodged from the tube sheet. Fixed tube sheet designs are most vulnerable to differential thermal expansion, because there is no inherent provision to absorb the stresses. One approach in common use is installing an expansion joint in the shell pipe of such designs. This is a cost effective approach for pipe-size shells. An expansion joint can also be installed in the tube side of floating head designs, but
Diagram of U-Tube Heat Exchanger
manufacturing costs are much higher. Alternative approaches involve the design of a U-tube bundle so that each tube can independently expand and contract as needed or by using a rear floating internal tube sheet design which allows the entire bundle as a unit to expand and contract. The floating head is typically sealed against the interior of the shell by means of packing or O-ring designs.
U-tube designs while offering the best answer for differential thermal expansion have some drawbacks. Individual tubes can be difficult of expensive to replace, especially for interior tubes. Also, the tube interior cannot be effectively cleaned in the u-bends. Erosion damage is also frequently seen in the u-bends in high tube side velocity applications. In large diameter shells, the long length of unsupported tube in the u-bends of outer tubes can lead to vibration induced damage.
FLOATING HEAD DESIGNS OF HEAD EXCHANGERS
In an effort to reduce thermal stresses and provide a means to remove the tube bundle for cleaning, several floating rear head designs have been established. The simplest is a "pull-through" design which allows the tube bundle to be pulled entirely through the shell for service or replacement. In order to accommodate the rear head bolt circle, tubes must be removed resulting in a less efficient use of shell size. In addition, the
SOURCE: WWW.WERMAC.ORG/
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