Solving the Case of the Whistling Oven
The Mystery. A manufacturer of specialized motors had an oven that was way out of calibration. The mid-sized electric oven, used to cure epoxy for motor windings, was 30°F degrees out of range, a huge discrepancy for an oven that’s rated ±2.5°F (±1.1°C).
The Clues. The first thing ACES’ control system investigator (CSI) noticed was that the temperature fluctuations seemed totally random. This led him to inspect the oven airflow. A draft fan removed any gas buildup by keeping a negative pressure on the oven. The pressure should have been just enough to ensure that no gas escaped into the surrounding room. When the CSI heard air whistling through the thermocouple wells, he suspected that swirling cool-air drafts were contaminating the thermocouple readings, which can mimic a faulty controller. Why was there so much draw on the oven?
The next line of investigation was the dampers. The CSI discovered that the exhaust dampers were cranked all the way open and the intake damper about half open, creating the big, windy draw. He adjusted the dampers to a more moderate draw, checked for air leaks and continued troubleshooting.
The Perp. After some sleuthing, the CSI determined that the controller - a basic PID controller - had indeed gone bad and was irreparable. In addition, the oven operators were in the habit of cranking open the exhaust damper when running at low temperatures of about 180°F (82°C), and closing them back down to run at higher temperatures of about 450°F (232°C). Because the oven had too much heat input on the low end and not enough on the high end, the operators found it necessary to make the manual adjustments in order to maintain their desired temperatures. Unfortunately, the maneuver contributed to temperature fluctuations by sucking in drafts of cool air.
The Solution. The CSI installed a new, more robust controller complete with PID scheduling. Compensating for variable temperature targets puts the burden on the controller, where it should be, instead of forcing the operators to manually tweak the dampers. With the new controller, the oven is purring along within its range of ±2.5°F (±1.1°C). And as for the dampers, the operators just “set it and forget it.”
All is now quiet. Case closed. For more information on ACES, go to www.theacesinc.com.