Chemical Engineering December 2011 - 31

Heat Exchange and Transfer
the spectrum, these fluids can be more
stable at high temperatures and allow
heating to temperatures as high as
400°C (750°F). There is no fluid that
can achieve heat transfer performance
at both -100°C and +400°C, but aro
-
matic fluids are available with usable
temperature ranges as broad as 400°C
(720°F). These fluids are often more ex
pensive than petroleum-based fluids,
but their extended performance and
higher thermal stability offer value to
many users.
-
FIGURE 2. This packaged single-fluid system is heated indirectly by direct injection
of hot fluid from a central source. The cooling heat exchanger (horizontally mounted
behind the pump) cools fluid that is diverted through it by a control valve. The pump
maintains alignment by having the casing supported at its centerline. The pump is
also air-cooled (note the fins), and the mechanical seal is protected from extreme
temperatures by being located at the rear of the pump
Temperature range of various heat transfer fluids
Temperature, °C
Fluid type
Petroleum fluid
(46CST)
Terphenyl / biphenyl
Diphenyl ethane
Diethyl benzene
Methyl cyclohexane
Silicone
(high temperature)
Silicone
(medium temperature)
-150
FP 47 °C
FIGURE 3. This chart shows the temperature range of a typical petroleum-based
" hot oil " plus several synthetic aromatic fluids and two silicone-based fluids (FP
is flashpoint)
demands of the single fluid system
can have a significant effect on the
longterm performance of the system
(Figure 3). Classes of fluids include
the following:
Petroleum-based fluids: The socalled
" hot oils " are widely used in
industry and can be used successfully
in single fluid systems. There is
a cost advantage to using these fluids,
but most are limited (at the hot
end) to 300°C (572°F) and do not give
good cooling performance below 40°C
(100°F). There are petroleum fluids
that perform well above 300°C and
other formulations that perform at
lower temperatures.
Synthetic aromatic fluids: These
fluids are manufactured rather than
refined, and offer extended temperature
performance at both high and
low temperatures. Some of these fluids
have very low viscosities at low temperatures,
which allows high Reynolds
numbers (and relatively high heattransfer
coefficients) to be maintained
at low temperatures. Low temperature
performance can be extended to
as low as -115°C (-175°F) with these
fluids. At the high-temperature end of
-9
FP 132 °C
-50
FP 146 °C
-70
FP 57 °C
-115
FP -7 °C
-40
FP 160 °C
260
400
175
Pumps
The pump is almost always the component
requiring the most maintenance
in a thermal fluid system, and
is deserving of considerable attention
when a new single-fluid system is
being specified, or an existing system
is being upgraded.
There are three pump applications
in single fluid systems:
1. The heating (or hot- fluid-circulating)
pump
2. The cooling pump, which may circulate
thermal fluid or cooling water,
depending on temperatures and system
design
3. The single fluid pump, which will
undergo the temperature swings
associated with heating and cooling
the process
Not all single-fluid systems have
three pumps, because the applications
are often combined. For instance, on
smaller systems, the hot-fluid pumping
application and the single-fluid
CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011 31
315
315
-150 -100
40
FP 227 °C
380
100
200
300
300
400
Silicone fluids: Silicone fluids
offer advantages of very wide,
useable temperature ranges and
low toxicity. One fluid has a range of
almost 800°F (-40 to +750°F). Silicone
fluids are high performance fluids
and carry a price to match. They are
very manageable in properly designed
systems. Silicone fluids can develop
significant vapor pressure at high
temperatures and, in the author's experience,
can be more prone to leakage
than other fluids. However, proper
system design and a good piping-material
specification can help make a
very reliable and robust system.
offer advantages of very wide,
useable temperature ranges and
There are other classes of fluids, including
glycol-based fluids and fluorocarbon-based
fluids, but they are not
discussed in detail in this article.
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Chemical Engineering December 2011

Table of Contents for the Digital Edition of Chemical Engineering December 2011
