Chemical Engineering February 2022 - 42

Engineering Practice
Specification Tips to Maximize Heat Transfer
Presented here are some practical points to help in the selection of a shell-and-tube
heat-exchanger design for various applications
John Boyer and Jim Klimek
Xylem Inc.
H
eat exchangers are widely
used in various sectors of
the chemical process industries
(CPI), including:
chemical; oil and gas; power generation;
food and beverages; heating,
ventilation and air conditioning
(HVAC) and refrigeration; pulp and
paper to name a few. Their use is
expecting strong growth over the
coming years, driven, in part, by a
rising focus on environmental impact
and improving efficiency standards.
Although used for varied applications,
a heat exchanger's primary
role is to transfer heat from one fluid
or gas to another in order to control
the temperature of a system and
manage waste heat. Growing environmental
concerns and regulations
are motivating energy-intensive industries
to look at ways to improve
heat exchanger performance and
maximize energy efficiency use.
Choosing the right heat exchanger
to fit the needs of specific applications
and systems is critical in
achieving optimal efficiency.
Operating conditions, ease of
access for inspection and maintenance,
and compatibility with
process fluids are just some of the
variables to be considered when
assessing heat exchanger options.
Other factors include the following:
* Maximum design pressure and
temperature
* Heating or cooling applications
* Maintenance requirements
* Material compatibility with process
fluids
* Gasket compatibility with process
fluids
* Cleanliness of the streams
* Temperature approach
A properly selected, installed and
maintained heat exchanger can help
enhance the reliability and efficiency
of a fluid system by optimizing energy
consumption and reducing associated
operating costs.
In this article, we look at the spec42
ification
tips
to
maximize
heat
in
transfer
shell-andtube
(S&T)
heat exchangers
in order
to boost heat
exchanger
performance
and increase
efficiency.
S&T
exchangers
The shell-andtube
heat exchanger's
flexible
design,
high
and
pressure
temperaFIGURE
1. In a fixed tubesheet exchanger, straight runs of tubing are attached to
two perforated tubesheets. The design has no shell-side gasket or packed joints.
This minimizes the potential for leakage, and makes this exchanger ideal for
high-pressure operations, or those handling potentially lethal fluids
ture capabilities, and its ability to
handle high levels of particulate
material make it the most common
heat exchanger type used. Mechanically
simple in design and a proven
technology, the shell-and-tube design
offers a low-cost method of
heat exchange for many process
operations. The following is a brief
description of each of the most common
shell-and-tube configurations:
Straight-tube, fixed-tubesheet exchangers.
The fixed-tubesheet exchanger
is the most common, and
typically has the lowest capital cost
per square foot of heat-transfer surface
area. Fixed tubesheet exchangers
consist of a series of straight
tubes sealed between flat, perforated
metal tubesheets (Figure 1).
Because there are no packed or
gasketed joints on the shell side,
potential leak points are eliminated,
making the design suitable for higher
pressure or potentially lethal service.
However, because the tube bundle
cannot be removed, the shell side
of the exchanger (outside the tubes)
can only be cleaned by chemical
means. The inside surfaces of the
individual tubes can be cleaned
mechanically, after the channel covers
have been removed. The fixed
tubesheet exchanger is limited to
applications where the shellside fluid
is non-fouling; fouling fluids must be
routed through the tubes.
In line with the Tubular Exchanger
Manufacturers Association (TEMA)
heat exchanger standards and nomenclature,
shell-and-tube
heatexchanger
types have a three-letter
designation which refers to the specific
type of stationary head at the
front end, the shell type, and the
rear-end head type, respectively.
Common TEMA designations for
the straight-tube, fixed-tubesheet exchangers
are BEM, AEM, NEN. Common
applications include the following:
* Vapor condensers
* Liquid-liquid exchangers
* Reboilers
* Gas coolers
Removable-bundle, externally
sealed, floating-head exchangers.
Floating-head heat exchangers
are so named because they have
one tubesheet that is fixed relative
to the shell, and another that is attached
to the tubes, but not to the
shell, so it is allowed to " float " within
the shell. Unlike fixed tubesheet designs,
whose dimensions are fixed
at a given dimension relative to the
shell wall, floating-head exchangers
are able to compensate for differenCHEMICAL
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Chemical Engineering February 2022

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