Page 266 - Design and Operation of Heat Exchangers and their Networks

Optimal design of heat exchanger networks 255

in transferred from the system to the cold utility, but no heat flows into the
system. If there is a heat flow across the pinch, the excess external cooling
and excess external heating for the same amount of heat are required, which
increases both the hot and cold utility costs. The pinch principle allows the
designer to keep the level of heat recovery optimized by simply making sure
that the heat crossing the pinch is minimized.
Because any network design that transfers heat across the pinch will cause
both heating and cooling duties larger than their minimum, there are three
principles:
(1) Do not use cold utilities above the pinch.
(2) Do not use hot utilities below the pinch.
(3) Do not transfer heat across the pinch.
For the network design, there are therefore three consequences:
(1) Divide the network at the pinch into two parts.
(2) Design each part separately.
(3) Do not use heaters in the part above the pinch (hot end part); do not use
coolers in the part below the pinch (cold end part).
For the matching of streams, two rules should be followed:
(1) In the part above the pinch, the number of the hot streams (including
their branches) should be less than or equal to that of the cold streams
(including their branches), that is,
N h N c above the pinchð Þ (6.74)
Otherwise, the stream splitting is necessary to ensure that Eq. (6.74)
is fulfilled. Similarly, in the part below the pinch, the inequality is
inversed:
N h N c below the pinchÞ (6.75)
ð
(2) For a match in the part above the pinch, the thermal capacity rate of a
hot stream (or the branch of a hot stream) should be less than or equal to
that of the cold stream (or the branch of a cold stream) to be matched,
that is,
_ _
C h C c above the pinchð Þ (6.76)
Otherwise, the stream splitting is necessary. For a match in the part
below the pinch, the inequality is inversed:
_ _
C h C c below the pinchð Þ (6.77)
The pinch design method is illustrated in detail in Example 6.4.