Chemical Engineering December 2022 - 42

Engineering Practice
New Applications for
Spiral-Tube Heat Exchangers
A decades-old, yet less well-known type of heat exchanger offers advantages for
new and emerging applications
James R. Lines
Graham Corp.
S
piral-tube or helically-coiled
heat exchangers have been
around
for
decades
addressing
sample cooling,
mechanical seal cooling, vent condensers,
vaporization and general
heating or cooling requirements.
They serve niche or unique applications
and are not as well known or
understood as are ubiquitous shelland-tube
or gasketed-plate heat exchangers.
With turnover in engineering
departments and entrance of
younger engineers, a loss of familiarity
with, or awareness of, spiral-tube
heat exchangers is inevitable.
The last several years ushered in
new heat-transfer requirements that
fit spiral-tube heat exchangers perfectly.
The energy transition, applications
involving supercritical-fluid
heat transfer [1] and a focus on removing
or reclaiming volatile organic
compound (VOC) emissions [2], to
name a few drivers, have increased
demand and expanded the applications
where spiral-tube heat exchangers
are used.
This article introduces - or for
some, reintroduces - spiral-tube
heat exchangers and provides an
overview of new applications where
they are being used or are specified
for emerging or developing markets,
such as the hydrogen economy, botanical
extraction, compressed natural-gas
systems, cryogenic vaporization
and vent-emission reduction.
Spiral-tube heat exchanger
A spiral-tube heat exchanger consists
of a number of tubes stacked
and helically coiled (Figure 1). The
coiled tubes at each end are welded,
soldered or brazed into manifolds or
piping that permit fluid to enter and
exit the coil. In heat-exchanger parlance,
this is referred to as the tube
side of the heat exchanger. The coil
42
Graham Corp.
FIGURE 1. On the left is an exploded view of a spiral-tube heat exchanger. The flow path of the heattransfer
fluid is shown on the right
is placed inside a casing or housing
where a baseplate provides for a
sealed enclosure, creating the shell
side, or casing side, that permits fluid
to enter and flow along a pathway exposed
to the exterior of the coil and
then exit the heat exchange area.
A number of advantages are present
with such a configuration [3]:
Compactness. The straight length
of tubing, which can be 45 ft long,
is coiled, resulting in a smaller footprint
as compared to a corresponding
shell-and-tube heat exchanger.
This attribute is ideal for heat-exchanger
integration within a packaged
system. For example, a spiraltube
heat exchanger with 380 ft2 of
heat-exchange area addressing a
3,000 psig operating pressure occupies
a volume of 5 ft × 4 ft × 4
ft. In contrast, a shell-and-tube heat
exchanger with high pressure on the
tube side occupies a volume of 15 ft
× 3 ft × 2 ft. The 15-ft tube length
for a shell-and-tube exchanger
causes integration complexity and
an increase in floor space needed
for the overall packaged system by
approximately 10 ft.
High pressure capability. The coil
is comprised of
cylindrical
parts,
specifically, the tubes and manifolds,
which can withstand high operating
pressures. Pressures of 5,000 psi
(345 bars) are rather routine, and
for hydrogen service, pressures of
15,000 psi (1,000 bars) are economically
possible. This attribute is an
ideal fit for supercritical-fluid service,
where operating pressure is high, or
for hydrogen fueling systems.
Maximized LMTD. Fluid-flow orientation
between hot-side and coldside
fluids
is
fully countercurrent,
thus eliminating logarithmic meantemperature
difference (LMTD) correction
factors for multipass shelland-tube
heat exchangers. Such an
attribute is ideal when heat transfer
requires a temperature cross, more
specifically, when the hot side is
cooled below the cold-side fluidoutlet
temperature.
Large
temperature
differences.
The coiled geometry permits handling
large-temperature variation between
the hot- and cold-side fluid.
It is not uncommon to have a cryogenic
temperature on the tube side,
such as liquid nitrogen at -280°F,
and steam on the casing side at
300°F. This coiled geometry characteristic
is well suited when thermal
growth issues are challenging in
shell-and-tube type heat exchangers
Removable bundle. In most common
geometries, the casing or shell
side is accessible for cleaning or removal
of fouling deposits. Also the coil
can be removed and easily replaced.
Materials of construction (MoC).
MoC for coiled-tube heat exchangers
are comparable to those common
for shell-and-tube exchangers,
including stainless steel, duplex,
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