Page 130 - Design and Operation of Heat Exchangers and their Networks

118 Design and operation of heat exchangers and their networks

3.5.1 Concept of plate-fin heat exchangers
Basically, a plate-fin heat exchanger consists of a series of corrugated sheets
(fins) separated by flat plates, clamped together. These corrugations have the
dual purpose of holding the plates (parting sheet) together and of forming a
secondary surface for heat transfer. At the edges of the plates, there are bars to
contain each fluid between the adjacent plates, which formed the primary
surface. The space between two plates forms a channel and is known as a
layer. Also, a fluid is partitioned through adequate distributors in a series
of adjacent layers, and numbers of layers are apportioned to each fluid in
the core of the heat exchanger. An exhibit crossflow plate-fin heat
exchanger core is shown in Fig. 3.12.
Till now, a lot of types of fins have been developed. The commonly used
fins are plain fins, wavy fins, offset strip fins (serrated fins), louvered fins, and
perforated fins. Two of them are shown in Fig. 3.13.
For a two-stream plate-fin heat exchanger, the half-fin-height idealiza-
tion can be used, in which the fins are considered adiabatic at the half height
of the fins. This idealization is reasonable because in the two-stream plate-fin
heat exchanger, the temperature distributions in the plates separating the hot
and cold fluids are almost the same except those of the first and last layers.
Therefore, the fins can be treated as the extended surface (see Section 2.1.1)
at each side of the plates, taking the half height of the fins for the calculation
of the fin efficiency. Thus, the rating and sizing methods for common heat

Sideplate
Side bar Serrated fins

Plate

Plain fins

Side bar

Fig. 3.12 An exhibit crossflow plate-fin heat exchanger core manufactured by
Aeronautical Development Agency, Bangalore, India.