Page 340 - Fluid mechanics, heat transfer, and mass transfer

THERMAL DESIGN OF SHELL AND TUBE HEAT EXCHANGERS 321

& The process fluid leaks into the shell increasing shell - For a plate heat exchanger, the uncertainty can
side pressure. be 10–30%.
& The shell, being much weaker than the tubes, might - For a plate-fin exchanger, it can be 20%.
not be able to withstand the increased pressure and ➢ Variations in process and ambient conditions.
might fail spilling the steam and the tube side fluid
- Process Conditions: Capacity increases can be
if adequate safety relief is not provided on shell side.
the primary cause for increased erosion and
& There will be possibilities for explosion.
vibration problems that can cut down the life
& Shell side condensate gets contaminated and results of an exchanger. Flow decreases increase foul-
in pollution problems with discharged condensate. ing rates.
& Flow/pressure variations on tube/shell side might - Ambient Conditions: Results in variations in
result. MTD and hence performance of an exchanger.
& Corrosion, fouling, and other problems can arise. Effects are particularly high with air-cooled
exchangers.
➢ Lessons from previous experience.
➢ Risks associated with an exchanger that does not
10.2 THERMAL DESIGN OF SHELL AND TUBE
meet the process requirements.
HEAT EXCHANGERS
. Define heat exchanger design margin.
. What are the criteria to be considered for designing shell
& Exchanger design margin is defined as any heat
and tube heat exchangers?
transfer area exceeding the required area for a clean
& The process requirements for heat transfer within the
exchanger to satisfy a specified duty.
allowable pressure drops, effects of fouling and
& The following equations complete the definition:
cleaning requirements, must be fulfilled.
➢
& The heat exchanger must withstand the service con- Percent excess area from fouling
ditions of the plant environment. ¼ 100½ðU clean =U actual Þ 1Š: ð10:33Þ
& Minimum or maximum flow velocities.
➢ Percent over design
& The exchanger must be maintainable. In other words,
a configuration that permits replacement of any ¼ 100½ðU actual =U required Þ 1Š: ð10:34Þ
component that is especially vulnerable to thermal
expansion, corrosion, erosion, vibration, or aging ➢ Percent total excess area
must be chosen.
¼ 100½ðU clean =U required Þ 1Š: ð10:35Þ
& Consideration of the advantages of a multishell
arrangement with flexible piping and valving pro-
U clean U actual U required : ð10:36Þ
vided to allow one unit to be taken out of service for
maintenance without disturbing the rest of the plant.
. Name the important methods used in designing shell
& Cost considerations while satisfying the above
and tube heat exchangers.
criteria.
& Limitations on the heat exchanger length, diameter, & Kern Method: Simplest and long established. Re-
weight, and/or tube specifications due to site require- stricted to a fixed baffle cut (25%) and cannot ade-
ments, lifting, and servicing capabilities must be all quately account for baffle-to-shell and tube-to-baffle
taken into consideration in the design. leakages. However, the Kern equation is not partic-
ularly accurate, it does allow a very simple and rapid
. What are the reasons for adding margins while design-
calculation of shell side coefficients and pressure
ing a heat exchanger?
drop to be carried out and has been successfully used
& To account for
since its inception.
➢ Fouling.
& Bell Delaware Method: The most widely accepted
➢ Uncertainties in the methods used and fluid prop-
method. Used as a rating method.
erties. Uncertainties in estimation of heat transfer
coefficients depend on exchanger geometry and
phase condition of the fluids.
10.2.1 Kern Method
- For a shell and tube exchanger, on the tube side,
normally variation is within 10%, while on . Describe Kern method for the design of shell and tube
shell side it can be as high as 20–50%. heat exchanger.