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MECHANICAL DESIGN OF HEAT EXCHANGERS
MECHANICAL DESIGN OF HEAT
EXCHANGERS
Morris, M.
DOI:
10.1615/AtoZ.m.mechanical_design_of_heat_exchange
Article added: 2 February 2011
Article last modified: 14 February 2011
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The Shell and Tube, the Air Cooled and the Plate-type Exchanger are the three most
commonly used types of exchangers in the chemical and process industries. With
increasing effort in recent years to reduce weight and size and increase efficiency, other
types of exchangers are increasingly used. However the mechanical (and thermal) design
of these alternative exchangers tends to be of a proprietary nature which may explain why
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MECHANICAL DESIGN OF HEAT EXCHANGERS
many clients prefer the tried-and-tested shell and tube exchanger type which still
predominates in most plants.
The general principles of the mechanical design of the following types of exchangers are
given in the Heat Exchanger Design Handbook (1994), and full descriptions of each, are
given under the corresponding entries in this encyclopedia:
a. Shell and Tube Exchangers
b. Air Cooled Exchangers
c. Plate Type Exchangers
d. Plate Fin Type Exchangers
e. Double Pipe Exchangers
f. Graphite Block Exchangers
g. Spiral Plate Type Exchangers
h. Direct Contact Exchangers
i. Heat Pipes.
The shell and tube exchanger basically consists of a number of connected components,
some of which are also used in the construction of other types of exchangers. The
pressurized components of the shell and tube exchanger are designed to be in accordance
with a pressure vessel design code such as ASME VIII (1993) or BS5500 (1994).
To meet the relevant regulations (see Pressure Vessels) the pressurized components of
alternative types of exchangers must meet at least the principles of a relevant pressure
vessel design code.
A pressure vessel design code alone cannot be expected to cover all the special features of
heat exchangers. To give guidance and protection to designers, manufacturers and
purchasers, a supplementary code is desirable. A universally accepted code for shell and
tube exchangers is TEMA (1988), which although designed to supplement ASME VIII, can
be used in conjunction with other pressure vessel codes. TEMA specifies minimum
thicknesses, corrosion allowances, particular design requirements, tolerances, testing
requirements, aspects of operation, maintenance and guarantees. (See also TEMA
Standards.)
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MECHANICAL DESIGN OF HEAT EXCHANGERS
One of the most useful functions of TEMA is to provide a simple three-letter system that
completely defines all shell and tube exchangers with respect to exchanger type, stationary
end head, rear end head and shell side nozzle configuration. This system is shown in
Figure 1. The first letter defines the stationary end head, the middle letter defines the shell
type and the last letter the rear end type.
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MECHANICAL DESIGN OF HEAT EXCHANGERS
Figure 1. TEMA Designation System. © 1988 by Tubular Exchanger Manufacturers
Association.
In specifying TEMA (Fig. 1) the purchaser will choose one of the three classes:
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Class R for "generally severe requirements of petroleum and related processing
applications,"
Class C for "generally moderate requirements of commercial and general process
applications,"
Class B for "chemical process service."
Heat transfer equipment may be designated by type or function it performs, such as chiller,
condenser, cooler reboiler, etc. The choice of shell and tube type is determined chiefly by
factors such as the need for the provision for differential movement between shell and
tubes, the design pressure, the design temperature, and the fouling nature of the fluids
rather than the function. More information on the choice of types, their main features and
their design, is given in Saunders (1988).
A common type of shell and tube exchanger is the fixed tubesheet type. This is shown in
Figure 2, and has the TEMA designation AEM. The main components of the exchanger
shown in Figure 2 feature in most shell and tube exchangers and are given a reference
number which relates to the component descriptions below.
Figure 2. Fixed tubesheet exchanger.
The following components perform a function mainly related to fluid flow:
1. Tubes. The usual outside diameter range for petroleum and petrochemical applications
is 15 to 32 mm, with 19 and 25 being the most common. Tubes may be purchased to
minimum or average wall thickness. The thickness tolerances for minimum wall tubes
are minus zero, plus 18% to 22% of the nominal thickness, while those of average wall
tubes are plus and minus 8% to 10% of the nominal wall thickness. Tube thickness must
be checked against internal and external pressure but the dimensions of the most
commonly used tubes can withstand appreciable pressures. The most common tube
length range is 3600 to 9000 mm for removable bundles and 3600 to 15000 mm for the
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fixed tube type. Removable bundle weights are often limited to 20 tons. TEMA specifies
minimum tube pitch/ outside diameter ratios and minimum gaps between tubes.
2. Channel partition plates. For exchangers with multiple tube passes, the channels are
fitted with flat metal plates which divide the head into separate compartments. The
thickness of these plates depends on channel diameter but is usually 9 to 16 mm for
carbon and low alloy steels and 6 to 13 mm for the more expensive alloys. Except for
special high pressure heads, the partition plates are always welded to the channel barrel
and also to the adjacent tubesheet or cover if either of these components is in turn
welded to the channel. If the tubesheet or cover is not welded to the channel, the
tubesheet or cover is grooved and the edge of the partition plate sealed by a gasket
embedded in the grooves.
3. Shell baffles. Shell cross baffles have the dual purpose of supporting the tubes at
intervals to prevent sag and vibration, and also of forcing the shell side fluid back and
forth across the bundle, from one end of the exchanger to the other. Segmentally single
cut baffles are the most common, however, thermal or pressure drop may dictate baffles
of more complicated shape. Split backing ring and pull through floating head
exchangers have a special support type baffle adjacent to the floating head to take the
weight of the floating head assembly. TEMA specifies the minimum baffle thickness, the
maximum unsupported tube length, the clearances between tubes and holes in the
baffles and between shell inside diameter and baffle outside diameter. Two shell pass
exchangers (see Figure 2 shell types F, G or H) require a longitudinal baffle, which for F
type exchangers is welded to the stationary tubesheet. Leakage of the shell side fluid
between the shell and the longitudinal baffle edges must be minimized. When
removable bundles are used, this leakage gap is sealed by flexible strips or packing
devices. Figure 3 shows a typical flexible strip.
4. Tie rods. Tie rods and spacers are used to hold the tube bundle together and to locate
the shell baffles in the correct position. Tie rods are circular metal rods screwed into the
stationary tubesheet and secured at the farthest baffle by lock nuts. The number of tie
rods depends on shell diameter and is specified, by TEMA.
The following components perform a function mainly related to pressure and fluid
containment. Their design is carried out in accordance with the relevant pressure vessel
code, see Pressure Vessels.
5/6. Shell barrel and channel barrel. TEMA specifies minimum barrel thicknesses
depending on diameter, material and class. Most barrels larger than 450 mm internal
diameter are fabricated from rolled and welded plate. The shell barrel must be straight
and true as a tightly fitting tube bundle must be inserted and particular care has to be
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taken in fabrication. Large nozzles may cause "sinkage" at the nozzle/shell junction due
to weld shrinkage and temporary stiffeners may be needed.
7/8. Dished heads and flat heads. Small diameter, low pressure dished heads are
sometimes cast but most dished heads are fabricated from plate and are of semiellipsoidal, torispherical or hemispherical shape. The minimum thickness of dished
heads is the same as for adjacent barrels. Tube cleaning with a welded channel bonnet
(TEMA front end B) would require the breaking and remaking of the channel nozzle
flanges to enable the channel to be removed. A flat head (TEMA front end A) avoids this
and allows the pipework to remain in place.
9. Nozzles. Most nozzles are sized to match the adjacent schedule piping. The openings
in the barrels require reinforcement in accordance with the relevant pressure vessel
code which in turn will limit the maximum size of nozzle opening. Figure 4 shows a
typical nozzle in moderate service, with reinforcement provided by a reinforcement plate
and with a weld neck nozzle flange.
10. Flanges. Three types of flanges are found in shell and tube exchangers, namely, Girth
flanges for the shell and channel barrels; internal flanges in the floating head exchanger
to allow disassembly of the internals and removal of the tube bundle; and nozzle flanges
where the flange and gasket standards, the size and pressure rating will be set by the
line specification. Figure 5 shows three types of flanges. The weld neck flange type,
which has a tapered hub with a smooth stress transition and accessibility for full
nondestructive examination, provides the highest integrity of the three types. A flange
consists of three subcomponents: the flange ring, the gasket and the bolting. The
successful operation of the flange depends on the correct choice, design and assembly
of these subcomponents. The Heat Exchange Design Handbook contains two chapters
discussing these factors.
11. Tubesheets. Tubesheets less than 100 mm thick are generally made from plate
material. Thicker tubesheets, or for high integrity service, are made from forged discs.
Clad plate is commonly used where high alloy material is required for process reasons. A
clad tubesheet consists of a carbon or low alloy backing plate of sufficient thickness to
satisfy the pressure vessel design code, with a layer of the higher alloy material bonded
onto it by welding or by explosion cladding. TEMA gives design rules to calculate the
tubesheet thickness, which give similar but not identical results to the rules in ASME
and BS5500. It also specifies tolerances for tube hole diameter, ligament width and for
drill drift. Different methods are available for the attachment of the tube end to the
tubesheet. The most common method is roller expansion where the force produced by
an expanding tool deforms the tube radially outward to give a mechanical seal. In
explosive expansion a charge is placed inside the tube within the tubesheet thickness. It
is more expensive than roller expansion but can produce tighter joints. Welded tube
joints can be produced at the "outer" face of the tubesheet or downhole at the "inner"
face of the tubesheet. The success of the tube end joints is highly dependent on the
correct choice of type and the experience of the manufacturer. This is discussed in detail
in Saunders (1988).
12. Expansion bellows. These may be required in the shell of a fixed tubesheet
exchanger or at the floating head of single tube pass floating head exchangers. They are
discussed in more detail in Expansion joints.
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Figure 3. Longitudinal baffle.
Figure 4. Welds neck nozzle.
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Figure 5. Flange types.
Other important heat exchanger components include those in the floating head
assemblies, supports and rectangular headers in air cooled exchangers. These and other
components are described in the Heat Exchanger Design Handbook.
References
1. ASME VIII Division 1. ASME Boiler and Pressure Vessel Code. (1993) Rules for the
construction of pressure vessels. ASME New York.
2. BS5500 British Standard for the Specification for Unfired Pressure Vessels. (1994) BSI
London.
3. Saunders, E. A. D. (1988) Heat Exchangers: Selection, Design and Construction.
Longman, London.
4. Heat Exchanger Design Handbook (1994) Begell House Inc, New York.
5. TEMA Standards of the Tubular Exchanger Manufacturers Association. (1988) TEMA
New York
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