Page 68 - Design and Operation of Heat Exchangers and their Networks

Basic thermal design theory for heat exchangers 55

simultaneously, because it is directly related to the pumping power. Take a
shell-and-tube heat exchanger as an example. We can decrease the inner
diameter of tubes or the number of tubes to increase the fluid velocity,
which will increase the heat transfer coefficient and reduce the size and
weight of the heat exchanger. However, the frictional pressure drop will
increase quadratically with the velocity, against which a much larger pump
with much higher pumping power consumption might be required. The
benefit of saving exchanger capital cost by increasing fluid velocities might
be canceled by a more expensive pump or might be lost by increased oper-
ating costs in a short period.
In some cases, the pressure drop will influence the heat transfer directly,
especially in two-phase flow heat transfer. In such cases, the saturation tem-
perature of a fluid decreases with the pressure decrease along the heat
exchanger. The pressure drop analysis can help us judge a flow arrangement
in a heat exchanger. For example, in an upward-flow evaporator, the boiling
fluid flows upward with a pressure decrease in the flow direction, which
results in a temperature decrease. Therefore, it is reasonable that the heating
fluid also flows upward, so that the required mean temperature difference
can be decreased slightly, which yields a little higher heat exchanger
effectiveness.
For the heat exchangers with parallel-flow passages, different flow and
heat transfer conditions among the passages might introduce nonuniform
flow rate distribution. A typical example is the flow rate distribution in a
Z-type arrangement plate heat exchanger (Bassiouny and Martin, 1984).
In the Z-type arrangement plate heat exchanger, the fluid velocity decreases
in the intake conduit and increases in the exhaust conduit. According to the
Bernoulli equation, it will make the pressure rise in the intake conduit and
fall in the exhaust conduit. Such a pressure distribution might lead to a non-
uniform flow in the channels. Whether this nonuniformity is significant
should be estimated by the pressure drop analysis.
The pressure drop of flows through heat exchangers is composed of three
components: the frictional pressure drop, static pressure drop, and acceler-
ation pressure drop;

Δp ¼ Δp f + Δp g + Δp a (2.126)
For flow through valves and pipeline fittings,
1 2
Δp ¼ ζ ρu (2.127)
2
with ζ as the drag coefficient.