Page 100 - Design and Operation of Heat Exchangers and their Networks

88 Design and operation of heat exchangers and their networks

3.2.4 Sizing problem
In sizing case, if the heat load or one of the outlet fluid temperature is
specified, the unknown outlet fluid temperatures and heat load can be found
directly from the energy balance:

_
0
Q ¼ C h t t 00 (3.95)
h h

_
00
Q ¼ C c t t 0 (3.96)
c c
The overall heat transfer coefficient k should be estimated (see
Chapter 2). Then, we can use Eq. (3.54) to calculate the heat transfer area
A and determine the size of the exchanger. According to the new size of the
exchanger, the heat transfer coefficients of hot and cold fluids might be mod-
ified, and therefore, the overall heat transfer coefficient k and the heat trans-
fer area A will be newly calculated.

3.3 ε-NTU analysis of crossflow heat exchangers

Crossflow heat exchangers have been widely used in industries essentially
due to the structural considerations. For example, the automobile radiators
are usually of crossflow type so that they have enough large frontal area
and can be efficiently cooled by the air flowing through the radiators. Small
plate-fin heat exchangers often use the crossflow arrangement. By the
crossflow arrangement, the distributor sections at the inlets and outlets of
the exchanger core are not necessary, and the exchanger structure can be
simplified.
In a crossflow heat exchanger, each fluid can be treated as laterally
unmixed (e.g., airside fluid in an automobile radiator) or laterally mixed
(e.g., tube-side fluid of the automobile radiator). For the unmixed case,
the fluid temperature distribution is two dimensional and varies in its
own flow direction and the flow direction of the other fluid perpendicular
to its own flow direction.

3.3.1 Crossflow with both fluids unmixed
The crossflow with both fluids unmixed is the fundamental configuration of
the crossflow heat exchangers and is called sometimes as pure crossflow heat
exchangers. A solution to the problem was first obtained by Nusselt (1911)
in the form of analytical series expansions. Since then, many other workers
have sought improved solutions, and Baclic and Heggs (1985) showed that