contraction. The fuel oil service/transfer heaters,
any substantial increase in the overall size and
commonly used by the Navy, are examples of a
weight of the unit.
double tube heat exchanger.
Type of Construction
STRUT TUBE AND PLATE TUBE.--Strut
Surface heat exchangers are often given
tube and plate tube heat exchangers are noticeably
names that indicate general features of design and
different in design from the other shell-and-tube
construction. Basically, all surface heat ex-
heat exchangers. The tubes in both strut tube and
changers are of SHELL-AND-TUBE construc-
plate tube heat exchangers consist of pairs of flat,
tion. However, the shell-and-tube arrangement is
oblong strips; one fluid flows inside the tubes and
the other flows around the outside. Strut tube and
modified in various ways and in some cases it is
not easy to recognize the basic design. Shell-and-
plate tube heat exchangers are used primarily as
tube heat exchangers include such types as
water coolers and lubricating oil coolers in
(1) straight tube, (2) U-bend tube, (3) helical or
internal-combustion engines; they are also used
spiral tube, (4) double tube, (5) strut tube, and
as lube oil coolers for start air compressors.
(6) plate tube heat exchangers.
Direction of Flow
STRAIGHT TUBE.--In straight tube heat
exchangers, the tubes are usually arranged in a
In surface heat exchangers the fluids may flow
bundle and enclosed in a cylindrical shell. The
parallel to each other, counter to each other, or
at right angles to each other.
ends of the tubes may be expanded into a tube
sheet at each end of the bundle, or they may be
expanded into one tube sheet and packed and
PARALLEL FLOW.--In parallel flow (fig.
ferruled into the other. The ferrules allow the tube
6-33), both fluids flow in the same direction. If
to expand and contract slightly with temperature
a parallel flow heat exchanger has a lengthy heat
changes.
transfer surface, the temperatures of the two
fluids will be practically equal as the fluids leave
U-BEND TUBE.--U-bend tube heat ex-
the heat exchanger.
changers, sometimes called RETURN BEND heat
exchangers, have a bundle of U-shaped tubes
COUNTERFLOW.-- In counterflow (fig.
inside a shell. Since the tubes are U-shaped, there
6-34), the two fluids flow in opposite directions.
is only one tube sheet. The shape of the tubes
Counterflow heat exchangers are used in many
provides enough allowance for expansion and
applications that require large temperature
contraction.
changes in the cooled or heated fluids. Fuel oil
heaters, lube oil coolers, and many internal-
combustion engine coolers are examples of this
HELICAL OR SPIRAL TUBE.--Helical or
spiral tube heat exchangers have one or more coils
type.
of tubing installed inside a shell. The tubes may
communicate with headers at each end of the shell
CROSSFLOW.--In crossflow (fig. 6-35), one
or, if a relatively simple unit, the ends of the
fluid flows at right angles to the other. Crossflow
tubing may pass through the shell and serve as the
is particularly useful for removing latent heat,
inlet and the outlet for the fluid that flows through
thus condensing a vapor to a liquid. Counterflow
the coil of tubing.
DOUBLE TUBE.--Double tube heat ex-
changers have one tube inside another. One fluid
flows through the inner tube and the other fluid
flows between the outer and inner tubes. The
outer tube may thus be regarded as the shell for
each inner tube. The shells, or outer tubes, are
usually arranged in banks and are connected at
one end by a common tube sheet with a
partitioned cover that serves to direct the
flow. Many double tube heat exchangers are of
U-bend construction to allow for expansion and
Figure 6-33.--Parallel flow in heat exchanger.
6-22
