Page 333 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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& The outlet temperature of the cold fluid can approach
the highest temperature of the hot fluid (the inlet
temperature).
& The more uniform temperature difference between
the two fluids minimizes the thermal stresses
throughout the exchanger and produces a more uni-
form rate of heat transfer throughout the heat
exchanger.
& The efficiency of a counterflow heat exchanger is due
to the fact that the average DT between the two fluids,
over the length of the heat exchanger, is maximized
resulting in larger LMTD for a counterflow heat
exchanger than for a similar parallel or cross-flow
heat exchanger.
& If one stream is isothermal, the two cases are equiv-
FIGURE 10.36 Temperature profiles for countercurrent flow. alent and the choice of countercurrent or cocurrent is
immaterial.
. What are the disadvantages of parallel flow
& There will be no heat losses from the exchanger. arrangement?
& There is no longitudinal heat transfer within a given & The large temperature difference at the ends causes
stream. That is, plug flow conditions prevail without large thermal stresses. The opposing expansion and
backmixing. contraction of the materials due to diverse fluid
& The flow is either countercurrent or cocurrent. temperatures can lead to eventual material failure.
. What are the important advantages of countercurrent & The temperature of the cold fluid exiting the heat
flow arrangement over cocurrent arrangement? exchanger never exceeds the lowest temperature of
the hot fluid. This relationship is a distinct disadvan-
& In countercurrent flow arrangement, the maximum
tage if the design purpose is to raise the temperature
temperature change is limited by one of the outlet
of the cold fluid.
temperatures equilibrating with the inlet temperature
of the other stream, giving a basically more efficient . Under what circumstances, parallel flow is
heat transfer for otherwise identical inlet conditions advantageous?
compared to the cocurrent arrangement. Due to this & The design of a parallel flow heat exchanger is
reason, countercurrent flow arrangement is chosen advantageous when the two fluids are required to be
wherever possible. brought to nearly the same temperature.
. Give examples of cases where LMTD is not the correct
mean temperature difference to be used.
& LMTD is equal to true or effective mean temperature
difference, sometimes simply referred to as mean
temperature difference (MTD), only if flow of the
two streams is truly countercurrent or parallel.
& For all other flow arrangements, LMTD 6¼ MTD. For
example, if there is any cross flow, partial or other-
wise, is involved,

MTD ¼ FðLMTDÞ; ð10:25Þ
that is; DT m ¼ FDT lm ; where F is a nondimensional
correction factor.
& LMTD is not correct if U changes appreciably or
when DT is not a linear function of Q. Examples are as
follows:
➢ The exchanger is required to cool and condense a
Temperature profiles for cocurrent flow. superheated vapor or subcool condensate.
FIGURE 10.37