How to Select a Burn-off Oven
Heat-cleaning ovens, commonly known as burn-off ovens, remove organic materials such as varnish, paint, oil and plastic from metal parts, allowing them to be reused. Burn-off ovens have been used for years in the motor rewind industry and in all industries where products are painted electrostatically or powder coated. They are gaining acceptance in the automotive parts rebuilding industry and more recently in the plastics industry as an economical method of cleaning metal parts.
Emissions from burn-off ovens are low, and the small amount of ash generated usually can go in the trash. With burn-off ovens, environmental impact is minimized and operator safety is maximized. Several applications are well suited to using burn-off ovens, such as:
Paint and Powder. The hooks and racks, or fixtures, that carry parts through a coating line accumulate overspray, losing the conductive surface required for a proper electrical ground. Loss of ground results in poor transfer efficiency, uneven paint coverage and the potential for arcing, which can cause a fire in the powder booth. The severity of these problems varies with the type of coating, type and shape of the parts and design of the fixtures. Cleaning fixtures on a regular basis is the solution.
Electric Motor Rewinding. Electric motor rewinders remove the insulating resin from copper windings and soften the copper wire so that it can be removed easily, allowing the motors to be rebuilt.
Automotive Engine and Parts Rebuilding. Many automotive engine rebuilders have replaced chemical-cleaning systems with burn-off ovens to remove the oil and grease from blocks, heads and other components prior to rebuilding. The ovens eliminate disposal problems and the potential liability associated with chemicals. Burn-off ovens also are used for de-bonding brake shoes and stripping insulating resin from electric alternators.
Plastics Processing. Extrusion and injection-molding equipment used in the production of plastic products can become covered with plastic. Traditional cleaning methods are chemical stripping or using a torch to burn off the unwanted plastic residue. Chemical stripping results in disposal cost and presents potential liability if hazardous chemicals are used. Torching creates smoke, which can be harmful to the environment and to the employees doing the stripping. Burn-off ovens do a good job of removing the plastic, even in the small holes found in dies.
Over the years, many improvements have been made in burn-off oven technology. Modern high-performance afterburners produce very low emissions, and some control systems can sense the amount of vapor in the oven and control it at a safe level to prevent fires. It is now possible to remove large amounts of combustible materials quickly and safely. Some ovens can determine when the process is over, shutting off automatically. Today’s burn-off ovens run no longer than necessary to do the job. Be sure that the oven you select has an up-to-date control system.
Sizing the UnitTo select a burn-off oven, determine the correct size and appropriate features for your application.
The oven must be sized to accommodate the largest pieces to be cleaned and the quantity of parts necessary to keep up with production. The oven must be large enough to allow spaces between the parts for hot burner gases and cooling-water spray to circulate. The manufacturer should be able to help select the correct size for your application. Computer-aided design systems are useful for laying out complex shapes.
For paint and powder coating, cycle times generally run 2 to 4 hr for paint and powder fixtures, depending on the amount and type of coating on the fixtures and the oven temperature. Allow an additional 30 min for loading, unloading and washing. If 100 fixtures must be cleaned each hour, then the oven should be sized to hold 250 to 450 pieces. Allow a space equal to one diameter between fixtures for hot gas and cooling-water spray to circulate.
Generally, it is more time and energy efficient to do large loads rather than small ones because of the additional time required for loading, heating, cooling and unloading multiple loads. If extra fixtures are available for production runs, a larger oven may be more economical than a smaller one.
In electric motor rewind applications, the oven should be large enough to hold the largest core to be stripped and to allow 12" between cores when more than one is loaded. Proper spacing prevents the heat that is released from a core from damaging the ones next to or above it.
When cleaning parts in automotive engine and parts rebuilding, allow sufficient space for hot gases to circulate between parts. Stack parts with flat surfaces, like heads, randomly or rotate alternate layers 90o for best circulation. It is faster to do smaller loads with adequate spacing than full loads where the gas circulation is impeded.
For plastics processing, allow 1" or more between parts for circulation and 12" or more for items with a heavy coating of plastic.
Important features for burn-off ovens include fire prevention systems, cycle-time control, heating method, combustion chamber location, afterburner design, burner type, stack construction, wall construction, pressure-relief door design and processing speed.
Fire PreventionThe most important burn-off oven feature is the fire prevention system. The ovens do not really burn off the coating and other contaminants -- this would cause the parts to get extremely hot and could cause them to warp or melt. Instead, the ovens thermally decompose the volatile solids into a combustible vapor in a low oxygen atmosphere to inhibit combustion, and then destroy this vapor in an afterburner. If vapor is produced too rapidly, an ignition may occur in the oven, causing the release of smoke into the plant. Too much vapor also can overload the afterburner, forcing smoke out exhaust stack. Even a very-large-capacity afterburner can be overloaded if a fire occurs in the oven.
Fire prevention systems fall into three groups: single setpoint, ramp-and-soak, and dynamic control.
Single Setpoint Control. This system allows the combustible vapor to ignite, which drives the oven temperature above the maximum temperature setting (figure 1). A temperature switch turns on water sprays to put out the fire. This method allows a fire to start, and once the fire occurs, it could be difficult to control, and smoke could overload the afterburner or leak out of the doors. These systems should be used only for very light combustible loads. The danger is that sometime during the life of the oven, someone will overload it.
Ramp-and-Soak Control. This system is a refinement of the fixed-setpoint system in that a ramp-and-soak type controller increases the oven temperature over a fixed time period (ramp) until reaching the final processing temperature (figure 2). It then holds that temperature for a predetermined time (soak) to complete the decomposition process.
The ramp-and-soak profile usually is programmed by an operator or selected from a menu that includes a number of profiles. The operator must estimate the amount of combustible material in the load, which may be difficult to do. Formulas are not available to tell the operator how fast to ramp and how long to soak. If the wrong profile is selected, the temperature may increase too rapidly, causing a fire, or the oven may shut off too soon, allowing smoke out the stack. The ramp-and-soak system allows a fire to start before responding and, because it does not monitor the stack temperature, a stack fire is also possible.
Dynamic Response System. This type of system actually monitors the combustible vapor concentration in the oven and controls it at a safe level. The system responds to the load and does not require the operator to make any decisions. If the load does not produce excessive vapor, cooling does not occur. As a result, cycle times are as short as possible.
Two dynamic response systems have been developed. The first, devised in the early 1980s, uses the afterburner temperature as a measure of the vapor concentration in the oven. The vapor produced in the oven causes the afterburner temperature to rise as the vapor burns. If the afterburner temperature exceeds a preset limit, indicating that the vapor concentration has reached the maximum safe level, water sprays turn on to cool the load and slow the process. Because combustible vapor can be produced early in the process before the afterburner reaches its normal temperature, the processing rate must be controlled at a relatively low level to prevent fires.
The second dynamic response system, rate-of-change control, is factory programmed and continuously monitors the heatup rate of both the oven chamber and the afterburner (figure 3). If either heats up too rapidly, indicating excessive combustible vapor, the load is cooled until the heatup rate is acceptable. The rate-of-change unit works from the moment the process starts, giving a better indication of vapor concentration. The system allows the controls to be set to give much greater processing rates.
Most fire prevention systems use water sprays for cooling, with the fine droplets in the spray turning to steam to cool the load and slow vapor production. The systems usually use fine misting sprays that can clog if there are contaminants in the pipe. Consequently, a backup system with a large-diameter nozzle that will not become clogged should be used. The primary burner should automatically shut off to stop the process if the water fails to cool the load. PH