Considerations for Buying A Burn-Off Oven
A time-tested way to keep hooks clean is with a burn-off oven, and Steelman Industries’ Carlton Mann explains considerations for buying one.
Q: Our facility is thinking of adding a burn-off oven. What are some of the considerations?
A: Most coating professionals know the importance of clean fixtures to the coating operation. With clean hooks, paint and powder waste is minimized, safety is maximized (no arcing), and part contamination is eliminated. A time-tested way to keep hooks clean is with a heat-cleaning (burn-off) oven.
Burn-off ovens are available with a wide range of construction materials, components and controls. To prevent your purchase from becoming a 10-year headache, make sure you do your homework and select the right oven for your application.
Often, the purchase decision is made based on lowest cost. You might assume that the cheapest oven will do the job, and you really don’t need those extra features. But those extra features may mean the difference between a reliable oven and one that’s down when you need it, one that’s automatic and one that requires a skilled operator, one that’s safe to run unattended and one that might catch fire.
Here’s what to consider:
Construction materials. Ovens should be constructed of structural steel framing consisting of angles, channel, tubing or I-beams with heavy-gauge (14 or higher) sheet steel panels forming the walls, ceiling and floor. The atmosphere inside a burn-off oven is hostile. High temperatures, water sprays and acid formed from decomposing compounds can damage the oven walls. However, when those walls are protected properly, the oven will last for many years. Usually, a coating such as a paint or an asphalt mastic protects the inside surface of the steel shell. A barrier that prevents corrosive vapors from condensing on the wall will provide additional protection.
The wall coating and vapor barrier also must be protected from the oven heat with insulation. Usually, 2 to 3 inches of mineral wool is used. Most manufacturers also add 1 inch of high-temperature ceramic fiber to the inside surface for additional heat retention and to protect the mineral wool from high temperatures. The insulation should be protected by a steel liner fabricated from perforated or expanded steel or solid steel panels. Solid stainless steel panels are recommended for applications involving polyvinyl chloride, Teflon, sulfur or high combustible loads that will spray a lot of water.
Burners. A wide range of burners are used for heating the oven and the afterburner chambers. This is a demanding application for burners. They are exposed to high temperatures and potentially corrosive materials. Lower-cost ovens may have burners made of sheet metal, while “industrial-quality” burners will be made of rolled steel or cast iron. Burners are available with many different firing rates. Generally, the higher the BTUs per hour, the faster the oven will process material. This important if you must process multiple loads per day, but less-demanding schedules can use less-robust burners.
Controls. Heating the oven is the easy part. It’s what happens after the combustible material is hot that is more complicated. Modern burn-off ovens use water sprays to control the process but have different ways of activating those sprays. The safest system is one that measures the amount of combustible smoke that’s produced in the oven and uses the water sprays to control it at a safe level. These systems also can tell when the process is finished so they can automate the cycle time. Programmable logic controllers are available that help in troubleshooting and allow monitoring and data-logging on a network.
Size. Ovens are available in many different sizes and configurations to fit different requirements. Consider one that is larger than your current needs to allow for growth. Give some consideration to the way the fixtures will be loaded. Generally, hanging the fixtures will prevent them from bending at high temperatures, so be sure the oven is tall enough for hanging. Allow for 2-3 inches between parts for hot air and cooling water to circulate. This will shorten cycle times and actually increase production. If the oven is too small to allow space between the fixtures, the cycle time will be longer, and the parts may not be clean.
Cart. Carts conveniently allow parts to be loaded outside and avoids the risk of damage to the interior of the oven. The downside is that the cart requires a track that takes up space on the shop floor. Some coaters use a flat cart with removable hanging racks. While one rack is processing in the oven, one can be near the line to hold dirty fixtures and another near the line holding clean fixtures. Many operations use fork trucks to move carts in and out of larger ovens. The danger is that if a fixture snags the liner, it will rip it out of the oven without the operator noticing until its too late. Powered cart movers are perfect for these types of applications and can prevent extensive damage to the oven.
Cycle time. it’s difficult to estimate the cycle time for an oven to process a typical load, because there are no typical loads. Cycle times increase with more mass and combustible material and decrease as the firing rate of the burners goes up. Most ovens will process a full load in 2-4 hours, but it’s difficult to compare times of ovens from one manufacturer to another. No oven manufacturer will be able to guarantee a cycle time.
Air permits. Many states require air permits for burn-off ovens, and it’s not a bad idea to apply for a permit before you make a purchase. The oven manufacturer should be able to help with all the technical information required, including stack test data.
Secondary cleaning. Burn-off ovens remove all organic material from fixtures but leave behind inorganic pigment and binders. These materials must be removed by pressure washing or by caustic solutions. Plan on a wash station near the oven, and locate the oven itself away from the coating line to minimize contamination.
Infrared cure is gaining increased attention from coaters as a result of shorter cure cycles and the possibility of smaller floor space requirements when compared to convection oven curing.
Question: What methods are available for removing cured powder coatings, and what are the pros and cons of these methods?
Powder coating is one of the most durable finishes that can be applied to industrial manufactured products, and offers excellent corrosion protection and is very safe because of its lack of volatile organic compounds.