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Types of Heat Exchanger Construction

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Types of Heat Exchanger Construction
Although heat exchangers come in every shape and size imaginable, the construction of most heat
exchangers fall into one of two categories: tube and shell, or plate. As in all mechanical devices,
each type has its advantages and disadvantages.

Tube and Shell
The most basic and the most common type of heat exchanger construction is the tube and
shell, as shown in Figure 1. This type of heat exchanger consists of a set of tubes in a
container called a shell. The fluid flowing inside the tubes is called the tube side fluid
and the fluid flowing on the outside of the tubes is the shell side fluid. At the ends of
the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s).
The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight
seal. In systems where the two fluids are at vastly different pressures, the higher pressure
fluid is typically directed through the tubes and the lower pressure fluid is circulated on
the shell side. This is due to economy, because the heat exchanger tubes can be made
to withstand higher pressures than the shell of the heat exchanger for a much lower cost.
The support plates shown on Figure 1 also act as baffles to direct the flow of fluid within
the shell back and forth across the tubes.
Plate
A plate type heat exchanger, as illustrated in Figure 2, consists of plates instead of tubes
to separate the hot and cold fluids. The hot and cold fluids alternate between each of the
plates. Baffles direct the flow of fluid between plates. Because each of the plates has
a very large surface area, the plates provide each of the fluids with an extremely large
heat transfer area. Therefore a plate type heat exchanger, as compared to a similarly
sized tube and shell heat exchanger, is capable of transferring much more heat. This is
due to the larger area the plates provide over tubes. Due to the high heat transfer
efficiency of the plates, plate type heat exchangers are usually very small when compared
to a tube and shell type heat exchanger with the same heat transfer capacity. Plate type
heat exchangers are not widely used because of the inability to reliably seal the large
gaskets between each of the plates. Because of this problem, plate type heat exchangers
have only been used in small, low pressure applications such as on oil coolers for
engines. However, new improvements in gasket design and overall heat exchanger
design have allowed some large scale applications of the plate type heat exchanger. As
older facilities are upgraded or newly designed facilities are built, large plate type heat
exchangers are replacing tube and shell heat exchangers and becoming more common.
Types of Heat Exchangers
Because heat exchangers come in so many shapes, sizes, makes, and models, they are categorized
according to common characteristics. One common characteristic that can be used to categorize
them is the direction of flow the two fluids have relative to each other. The three categories are
parallel flow, counter flow and cross flow.
Parallel flow, as illustrated in Figure 3, exists when both the tube side fluid and the shell
side fluid flow in the same direction. In this case, the two fluids enter the heat
exchanger from the same end with a large temperature difference. As the fluids transfer
heat, hotter to cooler, the temperatures of the two fluids approach each other. Note that
the hottest cold-fluid temperature is always less than the coldest hot-fluid temperature.
Counter flow, as illustrated in Figure 4, exists when the two fluids flow in opposite
directions. Each of the fluids enters the heat exchanger at opposite ends. Because the
cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters
the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid.
Counter flow heat exchangers are the most efficient of the three types. In contrast to the
parallel flow heat exchanger, the counter flow heat exchanger can have the hottest coldfluid
temperature greater than the coldest hot-fluid temperatue.
Cross flow, as illustrated in Figure 5, exists when one fluid flows perpendicular to the
second fluid; that is, one fluid flows through tubes and the second fluid passes around the
tubes at 90° angle. Cross flow heat exchangers are usually found in applications where
one of the fluids changes state (2-phase flow). An example is a steam system's
condenser, in which the steam exiting the turbine enters the condenser shell side, and the
cool water flowing in the tubes absorbs the heat from the steam, condensing it into water.
Large volumes of vapor may be condensed using this type of heat exchanger flow.
Comparison of the Types of Heat Exchangers
Each of the three types of heat exchangers has advantages and disadvantages. But of the three,
the counter flow heat exchanger design is the most efficient when comparing heat transfer rate
per unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that the
average   T (difference in temperature) between the two fluids over the length of the heat
exchanger is maximized, as shown in Figure 4. Therefore the log mean temperature for a
counter flow heat exchanger is larger than the log mean temperature for a similar parallel or
cross flow heat exchanger. (See the Thermodynamics, Heat Transfer, and Fluid Flow
Fundamentals Handbook for a review of log mean temperature). This can be seen by comparing
the graphs in Figure 3, Figure 4, and Figure 5. The following exercise demonstrates how the
higher log mean temperature of the counter flow heat exchanger results in a larger heat transfer
rate. The log mean temperature for a heat exchanger is calculated using the following equation.
The results demonstrate that given the same operating conditions, operating the same heat
exchanger in a counter flow manner will result in a greater heat transfer rate than
operating in parallel flow.

In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow;
they are usually a combination of the two or all three types of heat exchangers. This is due to
the fact that actual heat exchangers are more complex than the simple components shown in the
idealized figures used above to depict each type of heat exchanger. The reason for the
combination of the various types is to maximize the efficiency of the heat exchanger within the
restrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids,
operating pressures, and temperatures, all help determine the complexity of a specific heat
exchanger.

One method that combines the characteristics of two or more heat exchangers and improves the
performance of a heat exchanger is to have the two fluids pass each other several times within
a single heat exchanger. When a heat exchanger's fluids pass each other more than once, a heat
exchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heat
exchanger is called a single-pass heat exchanger. See Figure 6 for an example of both types.
Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more
sets of "U" bends in the tubes. The "U" bends allow the fluid to flow back and forth across the
length of the heat exchanger. A second method to achieve multiple passes is to insert baffles
on the shell side of the heat exchanger. These direct the shell side fluid back and forth across
the tubes to achieve the multi-pass effect.
Heat exchangers are also classified by their function in a particular system. One common
classification is regenerative or nonregenerative. A regenerative heat exchanger is one in which
the same fluid is both the cooling fluid and the cooled fluid, as illustrated in Figure 7. That is,
the hot fluid leaving a system gives up its heat to "regenerate" or heat up the fluid returning to
the system. Regenerative heat exchangers are usually found in high temperature systems where
a portion of the system's fluid is removed from the main process, and then returned. Because
the fluid removed from the main process contains energy (heat), the heat from the fluid leaving
the main system is used to reheat (regenerate) the returning fluid instead of being rejected to an
external cooling medium to improve efficiency. It is important to remember that the term
regenerative/nonregenerative only refers to "how" a heat exchanger functions in a system, and
does not indicate any single type (tube and shell, plate, parallel flow, counter flow, etc.).

In a nonregenerative heat exchanger, as illustrated in Figure 7, the hot fluid is cooled by fluid
from a separate system and the energy (heat) removed is not returned to the system.

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