Chemical Engineering May 2013 - 54

Engineering Practice
(Figure 3, p.50), and in this case there
are no right or wrong answers. Here is
a list of criteria that will help you in
making a decision:
1. If the fluids are relatively clean and
the difference in temperature between
the shellside and the tubeside is not
very high (around 100°C / 200°F), then
consider a BEM (fixed tubesheet)
design. Typical applications are condensers;
liquid-liquid, gas-gas, and
gas-liquid heating and cooling; and
vertical thermosyphons.
2. If the heat exchanger must accommodate
a significant amount of thermal
expansion between shell and
tubes (more than 100°C / 200°F), consider
type BEU, in which the tubes
are free to expand. Keep in mind
that BEU exchanger tubes can only
be cleaned chemically, not mechanically,
so these exchangers are best
suited to clean service on both the
shellside and the tubeside.
3. For a chiller with refrigerant evaporating
on the shellside and cooling
a process fluid on the tubeside, consider
a heat exchanger of type BKU.
4. Similarly to Point 2 above, if the difference
in operating temperature
between shellside and tubeside is
more than 100°C (200°F), consider a
design with hairpin tubes, a floating
head or a floating tubesheet (types
P-W). These types are best suited to
dirty fluids, and may be either horizontal
or vertical.
5. If you encounter a temperature
cross - that is, if the outlet temperature
of the hot fluid is below the
outlet temperature of the cold fluid
- then you cannot use a single
BEM or BEU type heat exchanger.
Consider a BFS type with a twopass
shell and a longitudinal baffle,
or two shells in series. Other types
of heat exchanger, such as spiral
and plate types, are fully countercurrent
and so better suited to handling
temperature crosses.
Material of construction. Do not
trust the vendor to pick the right material
of construction for your service.
That is your job. That said, do not take
responsibility for the material of construction
unless you have agreed it
with the user or verified it with an appropriate
expert.
Tube-to-tubesheet joints. These determine
the integrity of your shelland-tube
heat exchanger. The basic
guidelines are the following:
1. For a design pressure of less than
300 psig and a design temperature
below 180°C (350°F), use rolled and
expanded tube-to-tubesheet joints.
These are used primarily for water,
air and oil service.
2. For higher design pressures or temperatures,
use grooved, rolled and
expanded tube joints.
3. When dealing with light hydrocarbons
or other flammable fluids, even
at low pressure and temperature,
consider seal welding.
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52 ChemiCal engineering www.Che.Com may 2013
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Chemical Engineering May 2013

Table of Contents for the Digital Edition of Chemical Engineering May 2013
