1.2 TYPES OF HEAT EXCHANGER
1.2.1 Parallel Flow
Heat Exchangers
In general, parallel flow heat
exchangers considered less efficient than counter flow heat exchangers in terms
of transferring heat from one fluid to another. However, there are applications
where parallel flow has its benefits, such as when limiting the transfer of
heat is recommended.
Another advantage if parallel flow heat
exchangers are used is that outlet temperature of the fluid being cooled can
reach a limiting temperature. If water is kept above 32 deg F, freezing can be
avoided.
While parallel flow arrangement can be
beneficial, under certain conditions that reduce the limiting temperature,
channelling problems can occur or freeze may be caused at shutdown.
Thus, parallel flow
in heat exchangers minimizes the chance of freezing or channelling, but does
not eliminate the possibility of either. Adding supplemental heat is
recommended to solve these problems. 
Figure.1(Parallel
Flow Heat Exchangers)
1.2.2 COUNTER
FLOW
Counter flow, as
illustrated the above
exists when the two fluids flow
in opposite directions. Each of the fluids enters the heat exchanger at
opposite ends. Because the cooler fluid exits the counter flow heat exchanger
at the end where the hot fluid enters the heat exchanger, the cooler fluid will
approach the inlet temperature of the hot fluid. Counter flow heat exchangers
are the most efficient of the three types. In contrast to the parallel flow
heat exchanger, the counter flow heat exchanger can have the hottest cold fluid temperature greater than the coldest hot-fluid temperatue.
Figure.2
(counter flow)
1.2.3 CROSS FLOW
Cross flow, as illustrated below,
exists when one fluid flows perpendicular to the second fluid; that is, one
fluid flows through tubes and the second fluid passes around the tubes at 90
angle. Cross flow heat exchangers are usually found in applications where one
of the fluids changes state (2-phase flow). An example is a steam system's
condenser, in which the steam exiting the turbine enters the condenser shell
side, and the cool water flowing in the tubes absorbs the heat from the steam,
condensing it into water. Large volumes of vapor may be condensed using this
type of heat exchanger flow.
Figure.3
(cross flow)