Chemical Engineering July 2022 - 37

Engineering Practice
More Specification Tips to Maximize
Heat Transfer
Presented here are practical points to help in the selection of plate-and-frame and
air-cooled heat exchangers
John Boyer and Jim Klimek
Xylem Inc.
W
ith a wider variety of heat
exchangers to choose
from than ever before,
there are a number of
variables to be considered when assessing
options.
As energy-intensive industries increasingly
look for ways to improve
efficiency, choosing the right heat
exchanger to fit the needs of specific
applications and systems is critical.
In the second part of this twopart
series, we look at specification
tips to maximize heat transfer
in plate-and-frame and air-cooled
style heat exchangers in order to
boost heat-exchanger performance
and increase efficiency. (For part 1,
which covered shell-and-tube heat
exchangers, see Chem. Eng. February
2022, pp. 42-44).
Plate-and-frame exchangers
The plate-and-frame heat exchanger
has emerged as a viable alternative to
shell-and-tube exchangers for many
applications. Such units are composed
of a series of plates, mounted
in a frame and bolted together. Space
between adjacent plates form flow
channels, and the system is arranged
so that hot and cold fluids enter and
exit through ports at the four corners
(Figures 1 and 2).
Within the exchanger, an alternating
gasket arrangement diverts the
hot and cold fluids from each inlet
into an alternating sequence of flow
channels. In this arrangement, each
cell of heat-transfer media is separated
by a thin metal wall, allowing
heat to transfer easily from one
media to the other.
A corrugated chevron or herringbone
pattern is pressed into each
plate to give the exchanger strength
and rigidity, to extend the effective
surface area of plates and to increase
turbulence in the flow channels,
which combined, boost heat
transfer (Figure 2).
The plate and frame's highly efficient
countercurrent flow typically
yields heat-transfer coefficients three
to five times greater than other types
of exchangers. As a result, a morecompact
design is possible.
Depending on the applications,
plate selection is optimized to yield
the fewest total number of channel
plates. Because the plates can be
easily removed, service and maintenance
costs are typically lower than
that of shell-and-tube exchangers.
Although the plate-and-frame
heat exchanger can be used in a
broad range of applications, the
following selection criteria may be
used as a guideline:
* Maximum design or working pressure
is limited to 450 psi
FIGURE 1. When the plates of a plate-and-frame
exchanger are assembled, the holes in the
corners form a continuous channel. Alternating
gasket patterns direct the hot and cold fluids into
alternating passes. Heat transfer then takes place
across the plates
* Temperature limits and fluids must
be compatible with gasket materials,
typically limited to 300°F
* Plate materials must be compatible
with process media
* The narrow passageways in the
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FIGURE 2. A corrugated chevron or herringbone
pattern is pressed into each plate to give the
exchanger strength and rigidity, to extend the effective
surface area of plates and to increase turbulence
in the flow channels, which combined, boost
heat transfer
plate and frame can result in high
pressure drops, making the exchanger
incompatible with low-pressure,
high-volume gas applications
* Rapid fluctuations in steam pressures
and temperatures can be
detrimental to gasket life. For this
reason, applications that use steam
favor shell-and-tube exchangers
* In applications where process
media contain particulate matter,
careful consideration should be
given to the free channel space
between adjacent plates. Maximum
weight percentage of 20%
solids with particle size not exceeding
50% of the free channel
space are typical limits
* The plate-and-frame design is best
suited for applications with a large
temperature cross or small temperature
approach. A temperature
approach of 2°F is often achievable
Early designs of plate-and-frame
heat exchangers used glue to attach
gaskets to plates. The glue was
often applied unevenly, greatly
creasing the chance of process fluid
in37
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Chemical Engineering July 2022

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