Metal Finishing Guide Book

2013

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usually found in automotive plants and are described as grain-count booths, meaning that they are considered to have a higher collection efficiency than standard waterFig. 6. Semidowndraft air flow. wash booths because of higher internal static pressure and scrubbing action. For a booth to be considered a grain-count booth, it should not release more than 3 grains (weight) to the atmosphere per 1,000 cfm of exhaust air. Test procedures are necessary to measure washer efficiency. Pumpless booths also come in two forms, those requiring high pressure and those with low pressure. In pumpless types that require high internal static pressure as a means to circulate and scrub water, high velocity air moves water up through the exhaust chamber. It is then released at a high point and returns to the water tank through an exposed water curtain. Pumpless types with low static pressure usually are fitted with a water holding pan and little or no water movement through the exhaust plenum. Collection depends on an abrupt change of air direction to impinge overspray onto the water surface. Draft Classifications Booths are also classified by the method of draft. Cross-draft booths are characterized as having air flow designed in a horizontal movement (Fig. 4). Air travels parallel to the floor, from the face of the booth to the rear of the exhaust chamber. The majority of booths are designed as cross-draft. The booth can have the face open to the atmosphere, closed with input plenum, or closed with filter doors. In the downdraft booth, the air flow is from overhead and moves down toward the building floor (Fig. 5).The building floor normally has a sunken pit to accept either dry-filter or water-wash exhaust. A bar-type grating is laid over the pit opening. The booth can also be placed on an elevated platform to avoid pit construction. The top of the booth may be open or enclosed with a filter input plenum. Most downdraft booths have overhead, filtered input plenums. A booth with a filter plenum is normally used in conjunction with a heated air make-up unit. This is considered a must for a clean paint job. A semidowndraft booth combines features of the cross-draft and downdraft booths. The method of inputting the air to the booth makes it a semidowndraft. Air is introduced to the booth through the ceiling in the first 25% to 30% of booth length (Fig. 6). This input air may be introduced by relying on the suction of the exhaust fan or it can be pressurized. For the best results, air make-up should be used and the booth should be positively pressurized. The exhaust is placed at the booth rear as would be the case in a normal cross-draft booth. A second style of semidowndraft places a floor level filtered exhaust plenum down each side of the booth. A full air input plenum is located in the booth ceil775

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