Chemical Engineering May 2013 - 49

Engineering Practice
Specifying Shell-andTube
Heat Exchangers
Understand what heat exchanger design
specialists need to know - and remember,
you know your process best
Asif Raza
S
hell-and-tube heat exchangers
are one of the most important
and commonly used process
equipment items in the chemical
process industries (CPI). If you
are working on a project during either
the basic or the detailed engineering
phase, there is a good chance that you
will need to specify one or more shelland-tube
exchangers - and perhaps
many of them.
While the actual design will likely
be done by a specialist at an equipment
vendor or within your own company,
you still need to fill out a process
datasheet for each heat exchanger and
in due course, review the vendor's detailed
proposal. You know your process
best, and it is a bad idea to rely on the
vendor always to make the right decisions.
This article shows you the basics
of specifying and selecting shell-andtube
heat exchangers: the process information
and preliminary design decisions
needed to fill out the datasheet,
and how to check any corresponding
assumptions made by the vendor. Although
it does not go into detail on the
design procedure, the article is also a
good starting point if you intend to design
the heat exchanger yourself.
Datasheet information
Though every company is likely to have
its own heat exchanger datasheet,
most of them look much like the
sample shown in Figure 2 (p. 49). To
complete the datasheet you will need
to know:
1. The composition and normal flowrate
of the process fluid(s), and the
temperature change required.
Refer to heat and
material balances.
2. Process fluid properties -
density, viscosity and thermal conductivity
- at the operating temperature
and pressure.
Figure 1. Which fluid goes on the shellside and
which on the tubeside? There is no straightforward
answer, but the guidelines presented here will help
you decide
Which fluid on which side?
Next comes your first design decision:
Which fluid goes on the shellside
and which on the tubeside (Figure 1)?
There is no straightforward answer,
but some considerations and rules of
thumb outlined in an online reference
(http://smartprocessdesign.com) and
incorporating the author's experience
are summarized here:
* Corrosive fluids are best kept to the
tubeside. Since the tubeside has less
metal than the shellside, this will
minimize the use of expensive metals
that may be needed to withstand
the fluids' corrosive properties.
* Fluids at extreme pressures and
temperatures are preferably kept to
the tubeside, because they are likely
to require a greater metal thickness,
or more expensive materials of construction.
The tubes, being smaller
in diameter than the shell, withstand
higher pressures.
* Fluids that need to be kept at a high
velocity, such as water or propylene
glycol for cooling, should be kept on
the tubeside.
* Dirty fluids, or streams that are otherwise
likely to cause fouling, should
go on the tubeside. This is because
the tubes are easier to clean than
the shell. For instance, it is often possible
to clean the tubes by water jetting,
having simply opened the head
of the exchanger, without needing to
remove the tube bundle. The shell
and the outside of the tube bundle,
on the other hand, are harder to
clean mechanically, and chemical
cleaning is often the only option.
* The shellside offers a larger crosssection
for vapor flow, and hence
lower pressure drops. Process vapors
to be condensed are therefore
normally placed on the shellside,
though the tubeside is generally
used for condensing steam.
* The baffles on the shellside help to
ensure good mixing, which reduces
the effects of laminar flow and therefore
tends to increase heat-transfer
coefficients. Hence you will get better
heat transfer if viscous fluids are
kept on the shellside - I confirmed
this recently on a project involving a
very viscous polymer.
* Twisted tubes, static mixers or tube
inserts increase turbulence and
thus heat-transfer coefficients on
the tubeside by reducing the effects
of laminar flow. Because these are
usually proprietary technologies,
however, your ability to check the
vendor's performance claims may be
limited. If you think you would benefit
from one of these technologies,
work closely with the vendor and be
sure to evaluate all the options.
* In heat exchanger designs that feaChemiCal
engineering www.Che.Com may 2013 47
Shellside
fluid
Tubeside
fluid
http://www.smartprocessdesign.com http://www.Che.Com
Chemical Engineering May 2013

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