How to Adjust a Burn-Off Oven To Remove Coatings

Q. We are having issues with our burn-off oven. It doesn’t seem to be adjusting to the amount of coating on the hooks and fixtures. What can I do to fix this?

A. Modern burn-off (heat-cleaning) ovens have come a long way from the “barrel burners” of the 1970s. The biggest improvements have been in controls. Ovens with automatic process control eliminate the possibility of operator error and give the shortest possible cycle times. These ovens are designed to adjust to the parts being processed in a couple of ways:

1. The control system senses the amount of combustible vapor being produced in the oven and uses water sprays to control it at a safe level.
2. The cycle timer adjusts to the mass of the load and the amount of combustible material on the hooks and fixtures so that the process is complete before the oven shuts off, and it doesn’t run any longer than necessary to do the job.

If this process isn’t working, there are several possibilities;

1. The oven does not have a modern control system, meaning the operator will have to guess at the proper cycle time for each load. If the operator doesn’t select enough time, the oven will still be smoking when it shuts off and if too much time is selected, it will waste valuable fuel.
2. The burners are out of tune. This can cause the oven to be slow to heat or cause unnecessary water spray.
3. The door gasket is leaking, allowing air to infiltrate the oven, which slows heat-up and may cause fires.
4. The water sprays are dripping. This keeps the oven cool, which will also slow the process.
5. Low water pressure is turning off the primary burner. The water pressure switch can turn off the primary burner repeatedly during a run. Some ovens have an alarm to alert the operator of this issue. One easy fix for this is to install a pressure tank and check valve close to the oven. This traps pressure at the oven and provides a reliable supply of water for operation.
6. The spray nozzles are plugged. They should put out a fan-shaped mist.
7. The thermocouples need to be replaced.

Modern ovens with PLCs enable you to send data files to the manufacturer for diagnosis. They also have an alarm screen that shows system faults. These tools taken together can help pinpoint the cause of a problem. For example, the data log might show that the flame signal is getting lower than normal, which indicates a dirty flame rod. The alarm screen would show a “Low Flame Signal” alarm to alert the operator that he needs to clean the rod before he gets a failure.

One important thing to consider is that burn-off ovens are very different than most other process ovens. They heat products like other ovens, but then they have to control the process to prevent fires in the oven or excess emissions from the stack. So, while it is fine to hire a local service contractor to work on your oven, they should always contact the factory before making any changes. Most manufacturers have a toll-free number and free service over-the-phone.

Q. Can I use a commercial, double-wall chimney with my burn-off oven?

A. The short answer is no. Burn-off ovens operate at high temperatures up to 2,000°F, and the exhaust stream is sometimes corrosive. Steel-lined stack won’t hold up in this environment.

In burn-off ovens, the exhaust stack works as an exhaust fan to pull the smoke out of the oven and through the afterburner. The environment is much too hot for a conventional exhaust fan. If the stack fails, the oven may not vent properly and noxious smoke may be emitted into the workspace. If the stack shell is breached, the building may be exposed to the high-temperature exhaust, possibly causing a fire.

Burn-off oven manufacturers usually make their own stack with a ridged ceramic fiber liner rather than steel. These specially made stacks easily handle this environment over an extended time. You should always use the stack that was provided by the oven manufacturer.

Other things to consider:

1. If the roof that the stack passes through is combustible or has combustible material, such as a rubber liner, the combustible material should be removed 2 feet around the stack. This can result in a very large opening, so an alternative solution is to use an insulated “thimble,” which goes around the stack and protects the roof in the event of a stack failure. The thimble should be available from the oven manufacturer.
2. Some manufacturers will offer a galvanized shell for their stack, which will eventually need to be replaced. The problem is that often the shell will fail without anyone noticing, which presents a very dangerous situation. Always insist on a stainless-steel shell when you buy a new oven or replacement stack.  

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Originally published in the May 2017 issue.