Page 339 - Mechanical Engineers' Handbook (Volume 4)

328 Heat Exchangers, Vaporizers, Condensers

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also are sometimes limited to a value corresponding to v less than 4000, where is in lb/
ft and v is in ft/sec.
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Geometry Data
It is necessary for the program user to make a large number of geometry decisions, starting
with the type of exchanger, which decides the type of program to be used. Only a brief list
of suggestions can be accommodated in this chapter, so recommendations will be limited to
some of the main shell-and-tube geometries mentioned in Section 5.4.
TEMA Shell Style. The types E, J, and X are selected based on available pressure drop,
highest E, lowest X, and intermediate J. Types G and H are used mostly for horizontal
thermosiphon reboilers, although they also obtain a slightly better MTD correction factor
than the E-type shell and are sometimes used even for single phase for that purpose. Pressure
drop for G and E shells are about the same. For horizontal thermosiphon reboilers, the
longitudinal baffle above the inlet nozzle prevents the light vaporizing component to shortcut
directly to the exit nozzle. If pressure drop for the less expensive G-shell is too high, H-
shell (two G’s in parallel) is used. Type F is used when it is required to have a combination
of countercurrent flow and two tube passes in a single shell. This type has the disadvantage
of leakage around the longitudinal baffle, which severely decreases performance. A welded
baffle prevents this but prevents bundle removal. Type K is used only for kettle reboilers.

TEMA Front and Rear Head Types. These are selected based on pressure and/or mainte-
nance considerations. TEMA Standards should be consulted. With respect to maintenance,
rear heads permitting bundle removal should be specified for shellside fouling fluids. These
are the split ring and pull-through types.

Baffle Types. These are selected based on a combination of pressure drop and vibration
considerations. In general, the less expensive, higher-velocity segmental baffle is tried first,
going to the double segmental and possibly the triple segmental types if necessary to lower
pressure drop. Allowable pressure drop is a very important design parameter and should not
be allocated arbitrarily. In the absence of other process limits, the allowable pressure drop
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should be about 10% of the operating pressure or the v should be less than about 4000
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(lb/ft )(ft/sec) , whichever gives the lower velocity. However, vibration limits override these
limits. Good thermal design programs also check for tube vibration and warn the user if
vibration problems are likely due to high velocity or insufficient tube support. In case of
potential vibration problems, it is necessary to decrease velocity or provide more tube sup-
port, the latter being preferable. The two best ways of eliminating vibration problems within
allowable pressure drop limitations are 1) no-tube-in-window baffles, or 2) RoDbaffles, as
discussed in Section 4.2. As mentioned in Section 4.3, the ABB Lummus Company offers
software, based on Heat Transfer Research, Inc. technology, containing a helical baffle op-
tion. Helical baffles can both decrease vibration tendencies and improve shellside flow dis-
tribution.
Tube Types. For low temperature differences and low heat-transfer coefficients, low-finned
or enhanced tubes should be investigated. In proper applications these can decrease the size
of the exchanger dramatically. Previously, enhanced tubes were considered only for very
clean streams. However, recent research is beginning to indicate that finned tubes fare as
well in fouling services as plain tubes, and sometimes much better, providing longer on-
stream time and often even easier cleaning. In addition, the trend in the future will be to
stop assigning arbitrary fouling factors, but rather to design for conditions minimizing foul-