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The following was reprinted in its entirety from the November 1999 issue of Chemical Engineering.
Synthetic heat-transfer fluid reduces drying times and boosts production at this custom fabric facility
Reliable heat transfer is crucial when operating a process that depends on a consistent operating temperature. Heat-transfer fluids, however, can degrade over time, resulting in a diminished ability to transport heat, resulting in increased heating times or even an inability to heat up the target material. Consequently, these problems can reduce productivity, and thus, profits.
This was the challenge facing the Farmville, Va. plant of Weavexx Corp. (Wake Forest, N.C.), a producer of custom fabrics used in paper making. Weavexx uses heat-transfer-fluid systems to heat-set (i.e., thermally join) several layers of nylon-based stock into so-called press felts that are used in high-quality paper manufacturing. Weavexx sets the felts by heating them from the inside via a heated roll and from the outside by means of a hot cap.
In the spring of 1998, the company observed a decline in the performance of one of its two heat-transfer systems at Farmville. "The mineral-oil product we were using caused substantial carbon deposits, which led to excessive wear in pumps and valves and forced us to replace this fluid every 18 months," says Billy Stanfill, Weavexx's manager of engineering and maintenance. The roll was taking more than 30 minutes to reach the appropriate temperature, and the hot cap was simply not heating up. As a result, each felt took much longer to set, decreasing productivity to three felts per day.
In order to keep up with the demand for its product, Weavexx had to take action and solve this problem. The company was considering cooling down the system to replace the older fluid with new mineral oil. Doing this, however, would likely cause the degraded mineral oil to solidify, due to its high viscosity, and the pipes would need to be mechanically cleaned - a process that would have cost $30,000-$40,000 and about 1,000 man-hours to complete. This option, therefore, was unacceptable.
After an extensive search for an alternative solution, the company decided to get help from Solutia, Inc. (St. Louis, Mo.) Solutia analyzed Weavexx's dilemma and determined that the system had likely been contaminated with a petroleum-based fluid and that the heat-transfer fluid should be replaced immediately. Solutia flushed the Weavexx system with Therminol FF flushing fluid, and then installed Therminol 66 as the heat transfer fluid, which Solutia says will last the lifetime of the system.
After the installation, Weavexx was able to decrease roll heating time to 20 minutes, thereby increasing productivity to five felts per day and matching the demand for the company's product. Although Weavexx will not disclose the cost of the Therminol changeover, the payback period for the installation was less than six months.
Weavexx consulted with Solutia to develop a customized, system-cleaning procedure to allow the refill to be done by Weavexx maintenance personnel. The company purchased 400 gallons of Therminol FF Flushing Fluid from Solutia. Personnel began the change-over process by stabilizing the spent heat transfer fluid at about 200°F, turning off the heat input, pumping out the fluid and sending it to Solutia for disposal. The Weavexx team then introduced Therminol FF and circulated it throughout the system.
Next, the group gradually heated the fluid to about 225°F, and vented the system through the system's expansion tank. This action released any moisture that may have entered during the emptying of the system and the introduction of the Therminol FF. The team then heated the fluid to about 300°F and circulated it for about 24 hours. At that point, they added more fluid and continued circulation for an additional 16 hours. The Weavexx team then flushed the Therminol FF out of the system and returned it to Solutia for final disposal. As the final step, Weavexx re-started the system after filling it with Therminol 66.
With Therminol 66, Weavexx has not experienced fouling problems and should not have to change the fluid for the life of the heat transfer fluid system. Such improvements led Weavexx to convert its second heat-transfer-fluid system to Therminol, resulting in similar benefits.
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