:: Fluids --- ID :: 16478
Troubleshooting Heat-Transfer Fluid Systems - Part2
Trouble shoot erratic production

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