Chemical Engineering May 2013 - 50

Table 1. Typical fouling facTors
Engineering Practice
ture gaskets or floating heads, the
shellside typically is not a suitable
location for fluids that are hazardous,
corrosive or especially valuable,
because the risk of leaks is too
high. Such fluids should therefore
normally go on the tubeside. Exchangers
featuring all-welded construction
can safely carry hazardous
fluids on the shellside, though you
should remember the difficulty of
cleaning the shellside.
* Thermal expansion may be an issue
if one of the fluids undergoes a temperature
change of more than 150-
200°C (300-400°F). In this case you
would normally put the high-temperature-change
fluid on the shellside,
which is better able to handle
large temperature changes in certain
exchanger designs.
* In summary, the fluids preferred on
the tubeside are the following:
- Cooling water
- The more-fouling, erosive or corrosive
fluid
- The less-viscous fluid
- The fluid at higher pressure
- The hotter fluid
- The smaller volumetric flowrate.
Remember, however, that none of the
suggestions above is definitive. Use
them as a starting point, but if they
indicate a different fluid arrangement
from what has been used in the past
in your plant or industry, you may find
that there is a good reason. If two suggestions
conflict, or the performance
of your initial configuration looks unsatisfactory
- because the predicted
pressure drop or heat-transfer performance
does not meet your requirements
- do not be afraid to reverse
the arrangement of the two fluids and
see whether that improves matters.
More key decisions
Allowable pressure drop. You will
have to understand the process thoroughly
before you can attempt to
specify the pressure drop on each side
of the heat exchanger. As a rule of
thumb, start with 10 psi on both the
shellside and the tubeside. If there
is a pump upstream of the heat exchanger,
there probably will be no concern
about pressure drop as long as
the pump can handle this. For gases, if
there is a compressor upstream, check
fluid
Fuel oil
with your equipment-design
engineer that it can provide
the necessary pressure drop.
For cooling water, check for
constraints on the allowable
return pressure at the battery
limit of the unit.
Sometimes the need to
optimize the heat exchanger
means that you will have to
take a higher pressure drop
than originally specified. A
higher pressure drop means
higher velocity, which in
turns gives a higher Reynolds
number and a higher
heat-transfer coefficient.
Give the heat exchanger
vendor an allowable pressure
drop as high as realistically
possible to allow
flexibility in optimizing the
design. Once the designer
has confirmed the calculated
pressure drop, pass
this value on to your rotary
equipment engineer, who
will need it for sizing pumps
and compressors.
Fouling factors. These are
very important in sizing the
heat exchanger. Do not expect
the vendor to provide
you with fouling factors. A
higher fouling factor translates
to a lower design heattransfer
coefficient (Ud) and
a larger required surface
area. Fouling factors can
often be taken from existing
plant data. If these are not available,
you will have to assume a value taken
from company guidelines or published
sources (Table 1). Make sure that your
customer - whether internal or external
- is in agreement with your assumed
fouling factor. Designing with
a too-high fouling factor will result in
an oversized heat exchanger that will
cost you more and probably will not
work as intended.
Excess area. The difference between
the design heat-transfer coefficient
and the service heat-transfer coefficient
provides a safety factor, often
known as " excess area " because it is
equivalent to specifying a larger heattransfer
area than necessary. The
excess area is usually a minimum of
48 ChemiCal engineering www.Che.Com may 2013
Steam (clean)
Exhaust steam (oil bearing)
Typical fouling
factor
(ft2·°f·h/btu)
0.005
0.0005
0.001
Refrigerant vapors (oil bearing) 0.002
Compressed air
0.002
Industrial organic heat-transfer
media
Refrigerant liquids
Hydraulic fluid
Molten heat-transfer salts
Acid gas
Solvent vapors
MEA and DEA solutions
DEG and TEG solutions
Caustic solutions
Vegetable oils
Lean oil
Cooling water
Natural gas
Atmospheric tower overhead
vapors
Vacuum overhead vapors
0.001
0.001
0.001
0.0005
0.001
0.001
0.002
0.002
0.002
0.003
0.002
0.001
0.001
0.001
0.002
Specifying appropriate fouling factors is important
but not always easy. In the absence of
operating experience, pick figures from reliable
published sources.
Source: TEMA
10%, but can be up to 30%. Choose a
value from your plant's or unit's design
basis, or ask your customer.
Heating and cooling curve. If the
heat exchanger will be used to condense
or vaporize process fluids, the
vendor will require a corresponding
heating or cooling curve showing how
the vapor fraction varies with temperature,
and the corresponding thermal
properties of the liquid and vapor fractions.
A heating or cooling curve with
8-10 points can easily be generated
using simulation software.
Design temperature and pressure.
Calculate the design temperature and
pressure on both the shellside and the
tubeside by adding an appropriate
safety margin to the maximum values
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Chemical Engineering May 2013

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