Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 223 of 331

sidered. These losses, expressed in BTUs per hour, are used for selection of the burner and corresponding electrical devices necessary for burner control. The burner, most often a direct-flame device, provides the energy for the cure. The heat-load calculation also provides information for the selection of the oven supply fan. The heat required to maintain a good oven temperature is delivered by heat- ing the supply air to no more than 100°F above the oven operating temperature and distributing this air to the oven proper. The fan volume must be expanded for the el- evated temperatures. The supply fan should turn over the oven volume approxi- mately two times every minute. Because the fan is a constant-volume device, the fan motor is sized for cold starts to avoid overloading. These rules will provide an oven temperature profile + 10°F throughout the enclosure. Another feature of many heater units is filtration to continuously clean the oven en- vironment. The efficiency of the filters varies with the application, but the most effec- tive are the types used to final filter make-up air, modified for the elevated temperatures. Filters require velocities, which are much lower than in normal heater units. Including these means increasing the size of the heater unit to accommodate this requirement. Oven filters continuously clean the air and, as a result, load very slowly. It is not necessary to prefilter high-efficiency filters. Sometimes, the products of combustion are not compatible with the coating. In these cases, indirectly fired heater units are an option. These use air-to-air heat ex- changers and are applied at the cost of the loss of efficiency. In practical applica- tions, indirect heating equipment can require a third more energy. As the heater unit discharges the supply air, it is directed into the oven supply system. The purpose of the supply system is to deliver and distribute the energy de- veloped in the heater unit. The supply duct is constructed of aluminized metal and is rectangular in shape. For proper operation, velocities in the duct should not exceed 2, 500 fpm. This assures good laminar flow in the duct and good temperature control. AIR SUPPLY SYSTEM The actual delivery of the supply air into the oven is achieved through some type of discharge device. The simplest of these is a hole in the side or top of the duct; how- ever, this provides no control over the air. A better design is to provide a control de- vice or slide damper over the opening. The slide allows the size of the opening to be adjusted to change the amount of air leaving the supply duct at a particular open- ing. The total area of these openings should approximate the cross-sectional area of the ductwork. Because of the poor control available with these devices, more discharge area is not better as the air will leave the duct at the point of the highest pressure differential. Too many openings will allow a large volume of air to escape the duct near the heater, leaving very little air to do the work in remote locations. Simple openings in the duct have a second problem. Simply allowing it to escape the duct does not assure that it will change directions, mix with the oven environment, and find its way back to the recirculation system. One effective tool to correct or change the direction of discharged air is a discharge nozzle. These devices are inserted over the dis- charge openings and give the air a new direction, away from problem areas. RECIRCULATION SYSTEM The purpose of the recirculating system is to return the oven air to the heater unit so the process of adding energy to the oven can continue. This is accomplished by us- ing the duct with the supply fan to create a negative pressure condition within the en- closure. The oven air will naturally migrate to the areas of low pressure, be captured by the duct system, and be returned to the heater. Recirculation duct is fabricated in 222

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