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Heat Relief Fan Sizing

What size should the HVAC heat relief hood be for a powder coat booth and oven?

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Q. I’m designing the HVAC system for a new industrial shop with a powder coat booth and a powder coat oven. What size hood and cubic feet per minute (cfm) should I use for the heat-relief hood in front of the oven doors? The oven has one set of double doors (5 × 10 ft.) with total door area of 10 × 10 ft. Oven dimensions are 16 × 13.5 × 13 ft., and the burner is 1.5 million Btuh. The exhaust fan on top of the oven is 1,500 cfm, and the oven recirculating fan is 14,000 cfm. Using the ACGIH manual for drying oven ventilation, the cfm is about 13,000, which seems very high. The building height is about 44 ft. —D.L.

A. You will want the hood to extend at least 5 ft. out from the oven wall. Use a large-diameter stack (24" minimum) and run it as straight as possible up through the roof. Install a damper so you can adjust it. I would not use a fan unless your building has significant negative air pressure. It would be best to rely on a normal hot air updraft. If you must use a fan, 13,000 cfm would be a disaster. You would pull way too much air from the plant and the oven. I would suggest a maximum of 3,000 cfm. 

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