A fixed-tubesheetheat exchanger
(Figure 2) has straight tubes that are secured at both ends to tubesheets
welded to the shell. The construction may have removable channel covers (e.g.,
AEL), bonnet-type channel covers (e.g., BEM), or integral tubesheets(e.g.,
NEN).
U-tube.
As the name implies, the tubes of
a U-tube heat exchanger (Figure ) are bent in the shape of a U. There is only
one tube sheet in a U-tube heat exchanger. However, the lower cost for the
single tubesheet is offset by the additional costs incurred for the bending of
the tubes and the somewhat larger shell diameter (due to the minimum U-bend
radius), making the cost of a U-tube heat exchanger comparable to that of a
fixed tube sheet exchanger. The advantage of a U-tube heat exchanger is that
because one end is free, the bundle can expand or contract in response to
stress differentials. In addition, the outsides of the tubes can be cleaned, as
the tube bundle can be removed.The disadvantage of the U-tube construction is
that the insides of the tubes cannot be cleaned effectively, since the U-bends
would require flexible-end drill shafts for cleaning. Thus, U-tube heat
exchangers should not be used for services with a dirty fluid inside tubes.
Floating head:
The floating-head heat exchanger
is the most versatile type of STHE, and also the costliest. In this design, one
tubesheet is fixed relative to the shell, and the other is free to “float” within
the shell. This permits free expansion
of the tube bundle, as well as cleaning of both the insides and outsides of the
tubes. Thus, floating-head SHTEs can be used for services where both the
shellside and the tubeside fluids are dirty — making this the standard
construction type used in dirty services, such as in petroleum refineries. There
are various types of floating-head construction. The two most common are the
pull-through with backing device (TEMA S) and pull through (TEMA T) designs.
End Channels and Bonnets:
End channels or bonnets are
typically fabricated or cast and control the flow of the tubeside fluid in the
tube circuit. They are attached to the tube sheets by bolting with a gasket
between the two metal surfaces. In some cases, effective sealing can be
obtained by installing an O-ring in a machined groove in the tube sheet. The
head may have pass ribs that dictate if the tube fluid makes one or more passes
through the tube bundle sections. Front and rear head pass ribs and gaskets are
matched to provide effective fluid velocities by forcing the flow through
various numbers of tubes at a time. Generally, passes are designed to provide
roughly equal tube-number access and to assure even fluid velocity and pressure
drop throughout the bundle. Even fluid velocities also affect the film
coefficients and heat transfer rate so that accurate prediction of performance
can be readily made. Designs for up to six tube passes are common. Pass ribs
for cast heads are integrally cast and then machined flat. Pass ribs for
fabricated heads are welded into place. The tube sheets and tube layout in
multipass heat exchangers must have provision for the pass ribs. This requires
either removing tubes to allow a low cost straight pass rib, or machining the
pass rib with curves around the tubes, which is more costly to manufacture.
Where a full bundle tube count is required to satisfy the thermal requirements,
this machined pass rib approach may prevent having to consider the next larger
shell diameter.
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