Page 463 - Handbook of Energy Engineering Calculations

FIGURE 14 Spiral-tube heat exchanger. (Chemical Engineering.)

The coils, which are stacked on top of each other, are held together by the
cover plate and casing. Spacing is maintained evenly between each turn of
the coil to create a uniform, spiral-flow path for the shellside fluid.
Coils can be formed from almost any material of construction, with some

of the more common ones being carbon steel, copper and copper alloys,
stainless steels, and nickel and nickel alloys. Tubes may have extended
surfaces. Casings are made of cast iron, cast bronzes, and carbon and
stainless steel.

Tubes may be attached to the manifolds by soldering, brazing, welding, or,
in some cases, rolling. Draining or venting can be facilitated by various
manifold arrangements and casing connections. Flow through both the coil
and casing may be single-or multipass (the latter by means of baffling).
2
Spiral-tube exchangers are available in sizes up to 325 ft , and pressures up
to 600 psi. Tubeside pressures may be even higher.
The spiral-tube exchanger offers the following advantages over the shell-

and-tube exchanger: (1) it is especially suited for low flows or small heat
loads; (2) it is particularly effective for heating or cooling viscous fluids;
since L/D ratios are much lower than those of straight-tube exchangers,
laminar-flow heat transfer is much higher with spiral tubes; (3) its flows can
be countercurrent (as with the spiral-plate exchanger, flows are not truly

countercurrent, but again, the correction for this can be ignored); (4) it does
not present the problems usually associated with differential thermal
expansion; and (5) it is compact and easily installed.

The following are the chief limitations of the spiral-tube exchanger: (1) Its
manifolds are usually small, making the repair of leaks at tube-to-manifold
joints difficult (leaks, however, do not occur frequently); (2) it is limited to
services that do not require mechanical cleaning of the inside of tubes (it can
be cleaned mechanically on the shellside, and both sides can be cleaned

chemically); (3) for some of its sizes, stainless steel coils must be provided
with spacers to maintain a uniform shellside flow area—and these spacers
increase pressure drop (this increase is not accounted for in the equations

presented later).
The shortcut rating method given above for spiral-tube exchangers depends
on the same technique as used for shell-and-tube exchangers (which is