Page 328 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 309
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P in P out ¼ K P ½2N P fG L=g c rD i FŠ: ð10:17Þ drop is cost effective. This is especially true if there is
a relatively higher outside heat transfer coefficient.
0:14 . How can shell side DP be reduced while designing a
F ¼ 1:02ðm =m Þ : ð10:18Þ
b w
shell and tube exchanger?
& Rod baffles, instead of plate baffles, give lower DP on
Where K P is the correction factor for contraction/ex-
the shell side.
pansion/reversal and N P is the number of tube passes.
& Use of rods or tube protectors in top rows instead of
. Will there be any possibility of DP changing during the
plates reduces DP. These create less pressure drop
operation of a heat exchanger when flow rates remain
and better distribution than an impingement plate. An
constant? If so, why?
impingement plate causes an abrupt 90 turn of the

& Fouling reduces flow areas and hence increases DP.
shell stream, which causes extra pressure drop.
. What are the considerations involved in deciding the
. Compare exchangers arranged in parallel and in series.
value of allowable pressure drop for a heat exchanger?
& Parallel: Low DP, low velocities; poorer thermal
& Allowable DP for an exchanger depends on the com-
performance; savings in pumping costs.
binedpressuredropsforthenetworkofequipmentand
& Series: High DP, high velocities; good heat transfer;
associated piping in the circuit, which must be over-
higher pumping costs (may require a booster pump).
come by the pump pumping the fluid through the
network. . How is DP reduced while retrofitting an existing heat
exchanger for increase in its capacity?
& Depending on the nature of each of the equipment
& When an increase in capacity will cause excessive
plus the piping and fittings, the designer distributes
the available DP among the individual pieces of pressure drop, a relatively inexpensive alteration is to
equipment and piping. reduce the number of tube passes. Other possibilities
are arranging the exchangers in parallel or using low
& Pressure drop considerations are crucial for certain
fins or other special tubing.
types of equipment like, for example, vacuum dis-
. Give an expression for a quick estimate of additional DP
tillation columns for which very low pressure drops
are required. resulting from increased number of tube passes in ashell
and tube exchanger.
& Excessive increases in pump discharge pressures
& When the calculated pressure drop inside the tubes is
depend on the nature of the pump and its input power
underutilized, the estimated pressure drop with in-
that also affects mechanical design of the individual
creased number of tube passes can be estimated from
equipment in the first few stages of equipment.
the relationship,
& Therefore, the designer has to carefully examine the
options and distribute the available overall DP,
3
including considerations for use of intermediate New number of passes
ðDPÞ New ¼ðDPÞ Old :
pumping stages. Old number of passes
. Arrange the following in the order of increasing pres- ð10:19Þ
sure drop for the case of flow outside tube banks (for the
& This would be a good estimate if advantage is not
same flow rate):
taken of the increase in heat transfer. Since the
(ii) Tube arrangement with triangular pitch (staggered).
increased number of tube passes gives a higher
(iii) Tube arrangement with square pitch (in-line).
velocity and increases the calculated heat transfer
(iv) Use of finned tubes with staggered arrangement. coefficient, the number of tubes to be used will
& Square pitch (ii). decrease. For a better estimate of the new pressure
& Triangular pitch (i). drop, add 25% if the heat transfer is all due to sensible
& Finned tubes (iii). heat transfer.
. What is the normally recommended DP specified for
shell and tube exchangers handlingheavyhydrocarbons?
10.1.5 Shell Side Versus Tube Side
& Frequently 35–70 kPa (5 or 10 psi) is specified for
allowable pressure drop inside heat exchanger tub- . In the following cases, how do you select which of the
ing. For heavy liquids that have fouling character- fluids isto betakenontheshellside andwhich oneonthe
istics, this is usually not enough. There are cases tube side of a shell and tube heat exchanger? Explain.
where the fouling excludes using turbulence promo- & Gas–Liquid: Gas on tube side as its velocity and
ters and using more than the customary tube pressure hence h can be more easily increased.