Page 330 - Fluid mechanics, heat transfer, and mass transfer
P. 330
HEAT EXCHANGERS 311
➢ Available DP depends on the system considera- & Suitability of any particular cleaning solution?
tions (discussed elsewhere). Most heat exchangers
& Space requirements for proper maintenance?
should need between 35 and 100 kPa of pressure
loss to operate effectively.
& Fouling tendencies of the fluids involved? 10.1.6 Specification Sheet
& Either of the fluids non-Newtonian?
. Prepare a typical specification sheet for a heat
& Non-Newtonian fluids have flow characteristics that
exchanger.
dictate viscosity characteristics depending on the & Table 10.10 gives a typical specification sheet for a
forces acting on the fluid. This has implications on
shell and tube heat exchanger.
selection and design.
& Either of the fluids corrosive? Required material of
constructions? 10.1.7 Log Mean Temperature Difference
➢ While selecting material of construction, consid-
. What is LMTD?
erations of relative corrosive characteristics of the
& LMTD stands for log mean temperature difference,
fluids, temperatures, and pH must be addressed.
which is used in the heat exchanger design equation,
For example, fluids with corrosive nature should
be on the tube side.
Q ¼ UADT m ; ð10:20Þ
➢ Where requirements are for expensive alloys, a
where DT m is the mean temperature difference
compact exchanger might be considered as they
expressed as logarithmic mean value.
involve thinner gauge plates. Care should be ex-
ercised. Another point to consider is that just LMTD ¼ DT lm ¼ðDT 1 DT 2 Þ=lnðDT 1 =ðDT 2 Þ:
because a fluid is compatible with a stainless steel
ð10:21Þ
tube, for example, it may not be compatible with a
stainless steel plate that has been prestressed & Sometimes DT lm can be approximated to arithmetic
(during the pressing process). Prestressing of me- mean temperature difference, (DT 1 þ DT 2 )/2, when
tals can make them susceptible to pitting corrosion 1 (DT 1 /DT 2 ) 2.2, the error introduced by consid-
such as chloride attack. ering the arithmetic mean instead of logarithmic
& Compatibility of elastomers and/or compression gas- mean, is within 5%, that is,
kets with the fluids?
DT am =DT lm ¼ 1:05 ð10:22Þ
➢ Elastomer gaskets are commonly offered in ma-
terials such as EPDM, nitrile, PTFE, and FKMG (a where DT am ¼ðDT 1 þ DT 2 Þ=2:
generic form of Viton-G from Dupont). Elastomer . Give expressions for LMTD for (i) countercurrent flow
gaskets can seldom be rated for temperatures in and (ii) cocurrent flow in a heat exchanger.
excess of 150 C.
& Countercurrent Flow: The two fluids flow in oppo-
➢ Selection of nonmetallic or metallic gaskets is to
site directions, each entering the exchanger at oppo-
be evaluated. Nonmetallicgaskets are seldom used site ends. Because the cooler fluid leaves the
at pressures in excess of 8000 kPa and tempera- exchanger at the end where the hot fluid enters, the
tures in excess of 450 C. cooler fluid will approach the inlet temperature of
& Toxicity of either of the fluids? the hot fluid. Counterflow heat exchanger can have
➢ The ASME pressure vessel code stipulates very the hottest cold fluid temperature greater than the
specific pressure vessel requirements for heat coldest hot fluid temperature.
transfer service that are qualified as lethal.If & Temperature profiles for countercurrent flow are
the service requires an ASME L stamp, it must given in Figure 10.36.
be communicated to the heat exchanger
manufacturer. LMTD ¼½ðT 1 t 2 Þ ðT 2 t 1 Þ=ln½ðT 1 t 2 Þ=ðT 2 t 1 Þ:
& Any mechanical cleaning expected for one or both ð10:23Þ
fluids?
& Cocurrent or Parallel Flow: The two fluids enter the
➢ Floating head shell and tube heat exchangers,
heat exchanger from the same end with a large
gasketed plate exchangers, spiral heat exchangers, temperature difference (Figure 10.37). As the fluids
and some welded plate heat exchangers allow transfer heat, hotter to cooler, the temperatures of the
good access for mechanical cleaning. two fluids approach each other. It should be noted that
