Chemical Engineering December 2011 - 34

Cover Story
Feature Report
Part 2
Troubleshooting
Heat-Transfer
Fluid Systems
Real examples demonstrate how to analyze
problems in heat transfer systems. The
culprit is oftentimes not the thermal fluid
Jim Oetinger
Paratherm Corp.
rticles about thermal fluid systems
often start with a variation
of the statement that " thermal
fluid systems typically require
little ongoing maintenance for the first
few years of operation " and then go on
to extol the various advantages of indirect
thermal-fluid process heating over
competitive heating methods, such as
direct heat, steam and so on. The corollary
to that statement, however, is that
by the time there is a problem, the operating
personnel that were trained on the
system have moved on, been excessed
or promoted. As a result, when things
do go wrong, the guessing begins. And,
unless there is an obvious cause like a
geyser from the expansion-tank vent or
a pump that sounds like it's moving ball
bearings, someone will likely blame the
thermal fluid for the problem.
There are several problems that seem
to occur with some frequency. This article
reviews a number of real examples
and describes how the symptoms can
be misinterpreted. The suspected fluid
properties and the testing procedures
necessary to determine which of the
fluid properties (if any) is responsible
for the problem are examined (Figure
1). Finally, recommended corrective actions
are proposed.
A
Decrease in production rates
The following three examples explore
production-rate problems, with the
analyses and findings that resulted. A
simple flowchart that can help in this
type of analysis is given in Figure 2.
Example 1: A large facility experienced
reduced productivity from its thermal
fluid system that was operating at
450°F. The heater outlet temperature
was increased to maintain throughput
on several pieces of equipment, but the
process was still losing ground. (Note:
this is an excellent reason to log the
heater outlet temperature so that you
know when changes started, should
you have a problem.) Previous test results
had indicated the presence of carbon
sludge in the fluid, so plans were
made to activate a sidestream filter
to remove the carbon. Before the plan
was implemented, however, a sample of
thermal fluid was taken that immediately
identified the problem.
The fluid property that has the
greatest effect on heat transfer rates
is viscosity. Because the fluid heattransfer
coefficient (which controls
the rate of heat transfer between the
heat exchange surface and the fluid)
is only one element of the overall
heat-transfer coefficient, changes in
the viscosity at elevated temperature
(350°F or more) have to be significant
(on the order of 200%) to cause
a noticeable change in system performance.
In this case, the problem was
obvious and required no testing - a
sample that was extracted at operat34
CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
FIGURE 1. A very high viscosity
thermal fluid can be a reason for
failure of a heat-transfer system
ing temperature went almost solid
when it cooled. This put the fluid well
above the 200% threshold.
Example 2: A chemical plant requested
a sample kit to test its thermal
fluid because one of its vacuum
reactors was taking too long to heat.
Even though the fluid had been in the
system for many years and had recently
been tested, it was assumed that
the fluid must have gone " bad " . This
situation is probably one of the more
common scenarios for heat transfer
systems. The problem comes to light
when someone realizes that the heater
temperature has to be increased to
keep production on schedule. In this
case, the evidence against the fluid
was further strengthened by the relatively
" normal " heater pressure and
temperature readings. This prompted
the request for the sample kit and a
quotation for a complete fluid changeout.
While the latter course of action is
appealing to the fluid supplier, it was
unlikely to solve the problem since the
problem wasn't the fluid.
The overriding evidence in this
specific situation was that there had
been no maintenance required on
the system since the fluid had been
tested. Fluid had not been removed
or added (which eliminates contamination
as a suspect) nor had any of
the operating conditions changed. It
turned out that there was a leak in
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Chemical Engineering December 2011

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