Page 55 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

3.2 Design 51

• Double-pipe exchangers are perhaps the simplest heat exchanger.
• Flow distribution is not a problem, and dismantling and cleaning are easy.
• As the dimensions are small, these exchangers are suitable when either or both the fluids are at
very high pressure.
• Since double-pipe sections permit true countercurrent or true co-current flow, they may be of
particular advantage when very close temperature approaches are required. Countercurrent flow
2
results in lower surface area requirements, usually below 28 m (300 sq. ft) for services having a
temperature cross.
• In some cases where the thermal resistances of the two fluid films are essentially the same, it is
found that for small heat loads, the installation of double-pipe units is more economical than shell
and tube units, which are economical, mostly in larger sizes.
• Hairpin exchangers are cheaper than shell and tube exchangers at very small sizes and can be
specified for areas from 7 to 150 m 2
• They are easier to fabricate using standard bought out pipes and pipe fittings. Shortened delivery
time results from the use of stock components that can be assembled into standard sections.
• Potential need for expansion joint is eliminated in U-tube construction.
• Double-pipe exchangers are modular and are used in applications requiring adding and dismantling
the modules or the rearrangement of sections for new services, thus ensuring flexibility.

Nevertheless, multiple hairpin sections are not always economically competitive with a single shell
and tube heat exchanger. They are more expensive on a cost per unit area basis and are generally used for
2
small capacity applications where the total heat transfer surface area is less than 50 m .Comparedto
shell and tube exchangers or other more compact heat exchanger types, these require more floor space
and also entail a large number of points at which leakage may occur. In addition, proprietary closure
design requires special gaskets, and a longer length is required to bring about the required heat transfer.

3.2 Design
Double-pipe heat exchanger design involves the estimation of heat transfer area and pressure drop at
the tube and the shell side. After determining the required heat exchanger surface area for counterflow
or parallel flow, the pipe sizes and the number of bends are finalised.

3.2.1 Input data

The input data is the same as that in the case of any other type of exchanger. Table 3.1 contains the
items of input data pertaining to the inner and outer fluids. In addition, information on the nature of the
fluids, e.g., flammability, corrosive nature, fouling tendency, solid concentration, etc., as applicable,
are also considered.

3.2.2 Deliverables
Design output is the details to be filled in the heat exchanger datasheet. A typical datasheet is shown in
Table 3.1.
In addition, the design references: Process calculation references (Methods: Kern, HTRI, etc.);
Mechanical standard class (TEMA, BIS, etc.) are also to be furnished as part of design documentation.
Complete fabrication drawing consisting of the following are required to be included: General
arrangement drawings including stacking plan, if required; shell, nozzles and support details, other
connections (vent, drain, instruments, etc.); tube bundle and its component details, if provided; details
of the head.