Trouble shoot erratic production
Articles 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. Finally, recommended corrective actions are proposed.
Why Fluid velocity has even more effect on heat transfer performance than viscosity?
A food processor began experiencing sporadic production problems
with a multiple-user heat-transfer
system that was used to heat tanks.
Once again, the pump pressures and
temperatures were all within the expected ranges. Because the fluid had
been in service for a number of years,
the likely solution was deemed to be
fluid replacement. The shutdown was
planned and quotes were obtained.
After the costs of the fluid and lost
production were totaled, cooler heads
prevailed, and it was decreed that the
fluid should be tested by the current
fluid suppliers to be sure it really did
need to be replaced. Although the fluid
had not been tested for a number of
years and actually was a blend of several fluids, the supplier was able to determine that the fluid was in acceptable
condition. Now that the “easy solution”
was not applicable, the real investigation started. Particularly confusing,
but overlooked when the fluid was the
prime suspect, was the fact that the
most significant decline in production
occurred when there was the least demand on the heater. Fluid velocity has
even more effect on heat transfer performance than viscosity, so whenever
there is a drop in heat transfer, it’s time
to look at the flowrate.
Liquid-phase heaters require continuous flow to prevent fluid degradation.
Hence these systems need some way
to bypass the heat users when heat is
not required. There are two ways to accomplish this: 1) A backpressure control valve that maintains flow when
the two-way control valves are closed;
and 2) One or more three-way control
valves (depending on the number of
users) with a manual pressure-equalization valve on the bypass port.
Theoretically three-way valves are
superior to a backpressure valve arrangement because they provide a
constant flow through the heater — a
concept that is favored by the purists — if the balancing is done rigorously. This exact balance is difficult to
maintain over time due to changes in
equipment and the ever-present potential for third-shift adjustments. In this
particular case, it was discovered that
the bypass valves on the least-used leg
of the system had been fully opened so
that when that system was not operating, a substantial amount of fluid was
bypassing. When the unit was operating, the bypass volume was reduced,
which in turn increased the pressure
and thus flow to the other units bringing production rates back up. Instead of
attempting to balance all of the bypass
valves (which would have required the
installation of multiple pressure gages)
the solution was to install a back pressure valve between the feed and return header and then close all of the
bypass valves, effectively turning them
into two-way valves. While this control
scheme did allow the heater flow to
vary, it made the system much easier
to control since each user was independent of the others.
Author Jim Oetinger is the director of technology at Paratherm Corp. (4 Portland Rd., West Conshohocken, PA 19428; Phone: 800-222-3611, Fax: 610-941-9191; Website: www. paratherm.com). He has over 30 years experience in the chemical and plastics indus- tries. He has been involved with a wide range of products and processes including pig- ments, refrigerants, consumer plastic recycling, polymer compounding, process instrumentation and spray dried polymers. In addition, Oetinger has over 20 years experience in sales, marketing, and technical support of thermal fluids. He has authored articles on thermal fluid and system troubleshooting for this and other publications. A member of the Delaware Valley Chapter of the AIChE, he holds a B.S.Ch.E. from Clarkson University and a Masters of Management degree from Northwestern University. Oetinger and his family reside in a suburb of Philadelphia, Pa.
content and image courtesy:www.paratherm.com