Page 329 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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& Viscous Liquid–Nonviscous Liquid: The decision of & Large Volumetric Flow Rate and Small Volumetric
whether the moreviscous fluid should be taken on the Flow Rate Fluids: Large volume flows on shell side
tube side or shell side involves the evaluation of as shell side volume is more than tube side volume.
many, often contradictory factors. It will be easier & Condensing fluids are typically placed on the shell
to increase turbulence on the shell side by the ad- side.
justment of baffle spacing, baffle cut, and staggered & Steam and Cooling Water: Steam on shell side as it
tubepitchthanonthetubeside.Also,tomaintainflow
occupies higher volume.
of viscous liquids through tubes will involve prohib-
& High-TemperatureandNormal-TemperatureLiquid:
itively high pressure drops. However, if the high-
High-temperature fluid is on tube side as otherwise
viscosity fluid results in shell side N Re < 200 and is
heat losses will be more.
being cooled, unstable operation becomes a possi-
& Toxic and hazardous fluids are taken on tube side, as
bility, and it would be better to pass such a fluid on the
containment is easier on tube side than on shell side.
tube side. This has additional advantages such as ease
of cleaning and increasing tube velocities with in- & For applications where complete drainability for
crease of tube passes more easily than shell passes. servicing or process fluid changes is required, the
The overall heat transfer coefficient is controlled by critical fluid should be placed on the tube side.
the heat transfer coefficient for the viscous fluid. . Why generally a fouling fluid is placed on the tube side
& High-Pressure Fluid and Low-Pressure Fluid: of a heat exchanger?
High-pressure fluids are usually placed in the tubes. & For better control over design, fluid velocity and
Placing the high-pressure fluid in the tubes will higherallowablevelocityintubeswillreducefouling.
minimize the cost associated with the exchanger . What are the questions to be addressed before preparing
because the cost of thicker tube walls is generally a specification sheet for a shell and tube exchanger?
less expensive than a thick shell. With their small & Any phase changes expected?
diameter and nominal wall thicknesses, tubes are
➢ In liquid–liquid exchangers, no vaporization must
better able to withstand high pressures. This approach
be ensured by use of proper operating pressures for
avoids having to design more expensive, larger di-
the fluids.
ametercomponents for highpressure.Ifitisnecessary
& Any dissolved gases in either stream?
to put the higher pressure fluid stream in the shell, it
should be placed in a small diameter, long shell. ➢ Dissolved gases reduce heat transfer, these must be
removed in a separation vessel prior to admitting
& Corrosive and Noncorrosive Fluids: Corrosive
to the exchanger. Example is in gas absorption
fluids that require a higher alloy are best placed in
systems.
the tubes so that the shell does not have to be cladded
with or fabricated from an expensive material. It is ➢ Anydissolvedorsuspendedsolidsineitherstream?
much less expensive to use the special alloys suitable For example, cooling water should not be allowed

to resist corrosion for the tubes than for the shell. to exceed 50 C to preventreversesolubility salts to
Other tube side components can be clad with corro- deposit on the heat transfer surfaces. Amounts and
sion-resistant materials or epoxy coated. nature (particle size distribution and relative hard-
ness) of suspended solids are to be ascertained to
& Slurries/Fouling Liquids and Nonfouling Liquids:
take appropriate measures such as velocities, ero-
Fouling/scaling liquids inside tubes. Tubes are easier
sion aspects, and so on.
to clean using common mechanical methods than the
shell, the later being very difficult to clean. & Operating pressure and available DP in the exchanger
(for existing pumps)?
& Air and Flue Gas: Air on shell side and flue gas on
tube side. Flue gas is dirty and cleaning is easy on ➢ The design pressure must be a certain factor above
tube side. the highest operating pressure. For example, if the
highest operating pressure for a heat exchanger is
& Many possible designs and configurations, affecting
to be 700 kPa, then a reasonable design pressure
tube pitch, baffle use and spacing, and multiple
may be 1000 or 1500 kPa, considering the higher
nozzles, to name a few, can be used when laying out
the design pressure, the more expensive the
the shell circuit. Because of this, it is best to place
exchanger will be.
fluids requiring low pressure drops in the shell circuit.
➢ Leakage considerations, with the consequences of
& The fluid with the lower heat transfer coefficient is
contamination of the two fluids with one another
admitted in the shell circuit, if low-fin tubing, which
and safety aspects, determine which fluid should
will increase available surface area, can be used to
be at higher pressure.
offset the low heat transfer rate.