A producer of carbon fiber molding for the aircraft industry had a large oven (18 x 10 x 10') that the operators thought was working just fine. However, the manufacturer faced an opportunity to land a new contract if it could prove the oven was certified for more rigorous specifications.
The company called ACES to perform an official temperature uniformity survey and system accuracy test. ACES’ control system investigator performed an initial system accuracy test, discovering a discrepancy between the controller’s settings and the oven’s temperature. The customer had been cranking up the temperature 20 to 30°F (11 to 17°C) on the high side in order to reach the correct internal oven temperature for the parts.
To make matters worse, a failed initial temperature uniformity survey revealed a temperature fluctuation of ±35°F (±19.4°C) in an oven that was rated for ±10°F (±5.5°C).
But there were clues to the perpetrators of the dastardly deed. The discrepancy between the controller settings and actual temperature readings pointed to several possible causes such as a damaged thermocouple, faulty wiring or offsets in the controller.
Lots of CulpritsThe failed accuracy test showed that the control thermocouple was out of tolerance, which was one contributor to the wide range in temperature. The CSI also noticed that the gas train was fitted with a solenoid valve, which could only turn the burner full on or full off, further impacting temperature.
After some investigation, the CSI discovered that, although the thermocouple was Type J, it was specified as Type K in the controller. The improper information resulted in erroneous input and drove the customer to jack up the settings in the controller to reach the desired temperature.
To address the wide temperature swings, the CSI recommended trading the solenoid valve for a butterfly valve and a modulating motor to release smaller measures of gas in increments based on voltage input from the controller - not unlike installing a dimmer switch on a light bulb.
He also found some thermocouple wires that were running in the same conduit as power wires, which introduced inaccuracies as voltage from the power wires contaminated the thermocouple temperature readings.
On even further examination, the CSI ascertained the oven had no high-temperature limit device as a safety precaution if the oven were to veer out of control and exceed a pre-determined setpoint. In such an event, the device would shut down the oven and prevent fire and damage.
The CSI’s solutions were many, all necessary. He reprogrammed the controller for the existing Type J thermocouples, which solved the temperature discrepancy.
To address the temperature fluctuations, the CSI made several repairs. He replaced the faulty control thermocouple; traded out the solenoid valve for a butterfly valve; installed a modulating motor; and ran some new external thermocouple wires, rerouting them away from the power wires carrying voltage. In addition, he installed a new high-temperature limit controller.
After the repairs, upgrades and tuning, the CSI performed a followup temperature uniformity survey and system accuracy test. Everything was stable.
The CSI gave the customer the documentation needed to prove an accurate oven with only ±7°F (±3.8°C) temperature fluctuation, opening the door for business expansion and new contracts.
Now the company really does have the reliable, safe and highly efficient oven it thought it had all along. Case closed.
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