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The heat exchanger is one of the most fundamental pieces of
equipment used in thermal systems. At the same time, though, it's also one of
the most widespread and varied, taking on many forms depending on the
application or system it's used for.
At its core, a heat exchanger is simply a device that allows
for the transfer of heat from one medium to another. In many cases, the two
mediums are two different liquids, although gases are sometimes utilized as
well. Basic heat exchanger applications include controlling temperature,
capturing waste heat from a process, and producing steam to run a turbine.
Classifying heat exchangers can be a daunting task when
considering all their characteristics. For instance, they can be distinguished
based on the number of fluids used, their flow arrangements, or the heat
transfer mechanism. But luckily, these details are often more important for the
designer or manufacturer to focus on. For you (the engineer looking to source a
heat exchanger), the first step to consider is the type, or how it is
constructed.
Types
There are three basic types of heat exchangers: shell &
tube, plated, and air-cooled.
The aptly named shell
& tube heat exchanger (pictured right) uses (you guessed it) tubes encased in a shell. One fluid enters through the tubes and another flows around
them in the shell, with heat transfer taking place through the tube wall. Flow
and performance will vary based on the design of the shell and the arrangement
of the tubes and baffles inside of it. Shell and tube exchangers are the most
standard heat exchanger design, favored for their familiarity, versatility,
wide operating ranges, and rugged construction. Unfortunately these designs are
typically the most thermally efficient and in some cases are subject to a
variety of flow problems such as vibration and mal-distribution.
 Next up we have plated
heat exchangers, which use plates
(see the trend, yet?) instead of tubes as the heat transfer boundary. Typically
the two fluids are directed by baffles to flow on alternating sides of a number
of plates (as pictured left), usually contained in a casing. Performance varies
based on the type of plate design and how the plates are connected (welded,
semi-welded, gasketed). The corrugation and larger surface areas of plates
make them more thermally efficient than tubes, and the overall design lends
itself better to temperature cross conditions. However, plated heat exchangers
can be difficult to seal reliably, often limiting them to lower capacity
operations. They also have a narrower range of operating conditions
(temperatures and pressures).
 Lastly, air-cooled
heat exchangers (pictured right) reject heat directly by (surprise surprise)
air flow. Instead of pumping water or another liquid, these devices are
designed specifically to use fans to push air over the heat transfer surface
(tube bundles or plates) to remove heat. Induced draft designs, which pull air
through the equipment, are more thermally efficient but have higher power
consumption than forced draft designs which instead push the air. In cooling
systems where water is in short supply or is too costly, these types of
exchangers are a lower-cost alternative. They are easy to operate and maintain,
and are well suited for high temperature process streams.
Performance
If only it were enough to throw all heat exchangers into
three broad categories and be done with it. Unfortunately, it's not quite that
simple. Heat exchanger designs and applications vary widely in scope, and a complete
classification would be exhaustive, if not nearly impossible, and not much fun
to read.
In the end, what should be most important to you is that the
exchanger fits the needs of the system and gets the job done right. The primary
performance factor is heat transfer rate (heat load), which defines how much
heat energy the unit can transfer over time. Other key system specifications are
designated for each fluid; they include flow rate, allowable pressure drop, and
the design temperature and pressure. When consulting a manufacturer, these are
the specifications required for proper custom heat exchanger design.
And So Much More…
Other things you might want to consider for your heat
exchanger include materials of construction, proper connections and fittings, standards,
and much more. But if you haven't heard enough about heat exchangers yet, check
out GlobalSpec's
How to Select Heat Exchangers page, where you can get a more detailed look
at what heat exchanger selection is all about. And while you're at it, share a
comment here about your own endeavors with these essential pieces of equipment.
Image Sources
EngineeringSpec.com
Diversified
Heat Transfer
Heat Exchanger Manufacturers
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Re: Selecting a Heat Exchanger
