Not all whistling is music to the ears, especially when it is coming from an oven. Here is the full story from Rose Hill, Kan.-based American Control & Engineering Service Inc.
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).
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
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
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.
Solving the Case of the Whistling Oven
February 1, 2012