Although ordinary heat exchangers may be
extremely different in design and construction and may
be of the single- or two-phase type, their modes of operation
and effectiveness are largely determined
by the direction of the fluid flow within the exchanger.
The most common arrangements for flow paths
within a heat exchanger are counter-flow and parallel
flow. A counter-flow heat exchanger is one in which the
direction of the flow of one of
the working fluids is opposite to the direction to the flow
of the other fluid. In a parallel flow exchanger,
both fluids in the heat exchanger flow in the same direction.
Figure 9 represents the directions of fluid
flow in the parallel and counter-flow exchangers. Under
comparable conditions, more heat
is transferred in a counter-flow arrangement than in a
parallel flow heat
exchanger.
The temperature profiles of the two heat
exchangers indicate two major disadvantages in the parallel-flow
design. First, the large temperature difference at the ends
(Figure 10) causes large thermal
stresses. The opposing expansion and contraction of the
construction materials due to diverse
fluid temperatures can lead to eventual material failure.
Second, the temperature of the cold
fluid exiting the heat exchanger never exceeds the lowest
temperature of the hot fluid. This relationship
is a distinct disadvantage if the design purpose is to raise
the temperature of the cold fluid.
The design of a parallel flow heat exchanger
is advantageous when two fluids are required to be brought
to nearly the same temperature.
The counter-flow
heat exchanger has three significant advantages over the
parallel flow design. First,
the more uniform temperature difference between the two
fluids minimizes the thermal stresses
throughout the exchanger. Second, the outlet temperature of
the cold fluid can approach the
highest temperature of the hot fluid (the inlet temperature).
Third, the more uniform temperature
difference produces a more uniform rate of heat transfer
throughout the heat exchanger.
Whether parallel or counter-flow, heat
transfer within the heat exchanger involves both conduction
and convection. One fluid (hot) convectively transfers heat
to the tube wall where conduction
takes place across the tube to the opposite wall. The heat is
then convectively transferred
to the second fluid. Because this process takes place over
the entire length of the exchanger,
the temperature of the fluids as they flow through the
exchanger is not generally constant,
but varies over the entire length, as indicated in Figure 10.
The rate of heat transfer varies
along the length of the exchanger tubes because its value
depends upon the temperature difference
between the hot and the cold fluid at the point being viewed.
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