Chemical Engineering December 2011 - 30

Cover Story
Feature Report
Part 1
Thermal Fluid Systems:
Design Considerations
Single-fluid heat-transfer
systems that both heat
and cool a process require
special considerations for
trouble-free operation
Jay Hudson
J.G. Hudson & Associates
hermal fluid systems that both
heat and cool are often utilized
in batch processes. They are also
often found in continuous processes
where heating is required at
one point in the process and cooling
(often for the purpose of heat recovery)
is required in a different part of
the process. Because these systems
utilize the same fluid for both the
heating and cooling process, they are
collectively referred to as " single fluid
systems. " This article discusses considerations
specific to the design and
operation of single fluid systems, although
some of these considerations
are more widely applicable.
Single fluid systems can operate in
an efficient and trouble-free manner
or they can subject the owner to operational
inefficiencies and maintenance
problems, depending on whether or
not certain design and operational pitfalls
are avoided. These heat-transfer
systems can operate over a wide range
of temperatures, from as high as 400°C
(750°F) to temperatures well below
-40°C (-40°F). A few heat-transfer
fluids can actually accommodate most
of this broad temperature range.
In this article, I discuss pitfalls and
considerations for the very hot and
very cold aspects of single fluid systems
with the understanding that, as
operating temperatures trend closer
T
Heat Exchange and Transfer
to ambient temperatures, operational
concerns will be lessened; however attention
paid to these considerations
for all systems can improve performance
and reliability.
Single fluid systems can be heated
directly, as with electrically heated
systems (Figure 1) or indirectly, as
with secondary loop designs that
receive heat, normally by direct injection
of hot thermal fluid from a
central source (Figure 2). Careful attention
to the components of the system
can contribute to reliable operation
with good performance and low
maintenance costs.
Thermal fluid selection
I have maintained for a long time that
the selection of the thermal fluid is
the most-important single decision
to be made in the design and specification
of a thermal fluid system, and
nowhere is it more important than in
single fluid systems.
For heating-only systems, it is
common practice to evaluate potential
thermal fluids by comparing
maximum-allowable bulk- and
fluid-film temperatures. The fluid's
30 CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
vapor pressure should be considered
for expansion tank and pump NPSH
(net positive suction head) considerations.
Resistance to oxidation is
also important.
The evaluation of the thermal fluid
for low temperature operation is also
extremely important. Here, we are
looking beyond the basic lowest temperature
where the fluid is " pumpable " .
Pumpability is a consideration
for start-up in heating-only systems,
but when the system is expected to
efficiently cool the process, the fluid
should be significantly warmer than its
minimum pumpability temperature.
The consideration here is viscosity. A
fluid may be pumpable at a viscosity
of one hundred to several hundred
centistokes viscosity, but high viscosity
makes for a low Reynolds number
and a poor fluid-film heat-transfer coefficient.
Significant attention should
be paid to the viscosity and flow characteristics
of the fluid at the minimum
temperature that the fluid is expected
to cool the process.
Careful attention to the fluids available,
and evaluation of fluid properties
as compared to the individual
FIGURE 1. This
packaged singlefluid
system heats
fluid directly with
the two electrically
heated heat exchangers
in the foreground.
The cooling
heat exchanger
(horizontally
mounted behind
the heating units)
cools fluid that is
diverted through it
by a control valve.
Fluid is motivated by
a pump that is not
visible in this image.
When installed,
this unit will have
an expansion tank
mounted, either to
the unit or nearby
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Chemical Engineering December 2011

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