Page 168 - Chemical process engineering design and economics

Process Heat Transfer 151

Heat-Exchanger Evaluation and Selection

The process engineer must be familiar with the types of equipment that are avail-
able for the various process units. Because the evaluation and selection of equip-
ment occur frequently, we will first establish general criteria that applies to most
equipment. These criteria are to determine:
1. operating principles
2. equipment type
3. sealing
4. thermal expansion
5. maintenance
6. materials of construction - shell, tubes, and seals
7. temperature-pressure rating
8. economics
There may also be other special considerations that do not fit in the above criteria.
The most commonly used heat exchangers are the coil and double pipe for
small heat-exchange areas and the shell-and-tube design for large areas. Devore et
al. [13] recommend that if:
2
2
1. A < 2m (21.5 ft ) select a coiled heat exchanger
2
2
2
2. 2 m < A < 50 m (538 ft ) select a double-pipe heat exchanger
3. A > 50 m 2 select a shell-and-tube heat exchanger
The coiled heat exchanger is very compact, and it is frequently used when space is
limited. The decision between the heat-exchanger types is not as distinct as indi-
cated. At the boundary of each category, a detailed analysis is required to arrive at
the most economical choice. Walas [5] discusses other heat-exchanger designs.
The most frequently used heat exchanger is the shell-and-tube heat ex-
changer, which is available in several designs. Figure 4.1 shows some of the more
common ones. Each heat exchanger consists of entrance and exit piping, called
nozzles, and hundreds of lengths of tubing contained in a shell. Usually, the out-
side diameter of the tubes are 0.75, 1.0, 1.5, and 2.0 in (1.9, 2.5, 3.8, 5.1 cm) [14].
The tubes are arranged in parallel and joined to metal plates, called tube sheets, as
shown in Figure 4.1. The tubes are joined to the tube sheet by either welding or
expanding the ends of the tube - called rolling. These methods of joining make
very reliable seals. Tube diameters less than 0.75 in (1.9 cm) are difficult to clean
and therefore should be used with clean fluids. The tubes are arranged in standard
patterns, as shown in Figure 4.2. Although the triangular pitch is a more compact
arrangement, resulting in a larger surface area per unit volume of heat exchanger,
the other tube layouts are more accessible for cleaning. Also, the square pitch has
a lower shell pressure drop than the triangular pitch, if the flow is in the direction
indicated in Figure 4.2. Normally, tube lengths are 8, 12, 16, and 20 ft (2.44, 3.66,
4.88, 6.10m) [5].

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