Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 224 of 331

much the same way as the supply. It is of aluminized metal construction and rec- tangular in shape. The duct is designed for slightly lower velocities. The velocity in the duct is held at 2, 000 fpm and openings are 20 to 25% greater than the supply. It is poor design to count on the recirculation duct for providing any control over the oven environment. The influence of suction pressure is negligible at even short distances from the source. While air naturally moves to the areas of lower pressure, this movement cannot be easily controlled. It is better to place a small amount of re- circulation in the hottest part of the oven and let the supply air do the work. EXHAUST SYSTEM Every oven must be exhausted in order to create a negative environment so that air seals can properly operate and to remove the VOCs and other products of the cure from the oven, plus eliminate the build-up of smoke. These requirements exist in all types of cur- ing ovens, whether powder, electrocoat, high solids, or waterborne. Additionally, the exhaust serves the purpose of purging the oven prior to startup. The requirement for purge is to change to enclosure atmosphere four times in a reasonable period of time (20 minutes) prior to ignition. The location of the exhaust is rarely critical because the supply and recirculation systems mix the oven atmosphere so effectively. As long as the exhaust intake does not improperly influence another part of the oven, such as an opening, one location is as good as the next. It is the flexibility of convection curing that keeps it popular with today's finishers. A convection oven properly designed, installed, and put into operation requires little at- tention relative to pretreatment and application processes. It can run effectively with sim- ple controls, can be combined with other curing methods, and can be operated efficiently. To conserve on factory space, ovens can be elevated, located outside, or on the building roof. This flexibility, not readily available with other applications, will continue to keep direct-fired convection curing the number one choice of general industry. 223

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