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

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