Wikipedia, the beloved resource of the internet research system, tells us that pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere. It’s apparently a Greek word origin where pyro is a misspelling of “gyro” and “lysis” might mean burning up. Or something like that.
What you do need to know is that a burn off oven basically heats materials to high temperature until is catches on fire and burns to ash.
For our purpose, a burn off oven is used primarily to remove paint, primer, powder coat, e-coat or other coatings, almost always from steel, but sometimes from aluminum or iron parts. It can also be used to remove some residues or to soften materials to aid in dry ice blasting where high temperatures might damage the underlying structure, such as specialty metal formulations parts.
The ovens operate by heating the chamber to a flashpoint of the substance you want removed, which then ignites and burns to ash. Temperature is regulated by the flow of air into the oven, the gas burners being on or off, and the introduction of water to regulate higher temperatures when the first two cannot control the fire. Beware of the words “runaway burn”, which is a synonym for “your factory building is going to burn down”. Water stops the fire quickly, brings the temperature under control and the burn off can continue to operate.
Temperatures usually top out at 800 degrees, but can be higher during heavy fuel burns, or lower depending on the flash point desired, the length of time or the substrate concerns (soft metals) that are being processed. But more on this later.
Any lesson on burn off ovens starts with limitations, which are in three main categories.
- Oven size
- Fuel Density
- Cycle Time
First – OVEN SIZE. Indigo has two ovens, a smaller oven that is about a five-foot cube, and a large oven that is roughly seven feet wide by eight feet high and deep. I use this as an example to demonstrate that If you have sixteen-foot-long pipes with powder coated paint, these ovens are not an option. Very long/tall/deep items just don’t fit, the same as irregularly shaped parts that take up lots of space in the oven, the expense of the oven operation for a single part usually is not cost effective.
Second, – FUEL DENSITY. Here’s where some sophisticated science concepts make an appearance. Actually, more like common sense science. Too much paint or powder coat, which is the fuel, in the oven can cause the oven temperature to exceed the oven’s safety limits, hence the water system to knock down the fire and control the temperature. If we put heavily coated powder coat racks, that have dozens of layers of powder coat on their surfaces (we’ve seen them inches thick), and pack them tightly into the oven, we have created a situation where the oven will not operate properly because the temperature will continually spike and the water system will come on to lower the temperature. At the end of the cycle you have a failed burn since the parts needed enough time to ash the powder coat. Failed burns are understandably bad for business. Too much fuel also presents burn down the factor hazards as discussed above, so best to avoid that issue entirely. We regulate the amount of fuel we load in a single batch with this in mind.
Finally, CYCLE TIME. Our oven cycles are usually between 4 hours and 10 hours. You have time to ramp up to temp, to achieve the proper “soak” which is the burning time, then you have a cool down period before you can open the doors and handle the parts. This impacts how many parts can be done in a given period. If you have 900 four foot cube bins, given our oven sizes, you are looking at weeks of burn off batches, which might be too long for a customer to wait, or too expensive to do on a per part basis, or both.
Our best jobs are a weekly or monthly set of racks with a consistent quantity of paint or powder on them, that can be put on racks that fit our oven dimensions and easily forklifted in and out of the ovens. Of course, not all jobs are this smooth, lots of times a customer has a batch of small parts with a one time bad paint, or the wrong color on them, so we just need to run them a single time and let them be repainted. Either way we are always happy to see burn off work come through our doors.
This is a basic discussion of how a burn off oven works, with a few of the concerns we look at when we operate it.
In future posts we will discuss how we maximize space and clean off the ash after the cycle is complete so stay tuned!