Metal Finishing Guide Book

2013

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that for the amount of WD-CARC you intend to use, the air velocity would only need to be 20 ft/min to achieve 25% LEL, then the velocity would be so low that all the overspray would fall onto the floor and not be carried to the filters. The smaller the filter area, the higher the velocity of air that passes through the filters. This then increases the filter resistance, and you will need to purchase a stronger fan to pull the air through. On the other hand, the larger the filter area, the more efficiently the filters will be able to capture the overspray. Bear in mind, the spray booth is not only there to withdraw the VOCs, but almost equally important is the filters' ability to capture the sticky, messy paint particles. For a nice clean paint job, you would like the overspray to be quickly carried to the filters; otherwise, they can deposit on freshly painted surfaces and result in dry spray. You most certainly can bring air into the booth from the main building, but you must exhaust the air to the outside. You can pull it into the booth with a single exhaust fan or push it in with an air make-up fan, whichever you prefer. MEASURING SPRAY BOOTH AIR VELOCITY Q: I need to confirm that our spray booth meets OSHA requirements of 100 fpm per 29 CFR 1910.107. Where do you take the measurements in a large spray painting booth (25 ft wide x 15 ft high x 60 ft deep); at the face of the booth, midway inside the booth or at the open end? A: When I measure booth velocity, I stand a few feet from the filters and measure in various locations so that I can calculate an average. You will find that as you get close to the side walls the velocity will usually be close to zero. Therefore, I don't stand within 3 ft from the walls. If you have poor seals in the double doors at the entrance of the booth, you will find that there is a spike in air velocity in the center of the booth. Therefore, try to avoid measuring air flow along the centerline between the doors. My method measures the air flow at the points where the painter stands, which is, after all, the intention of the regulation. On the other hand, you can measure the velocity directly in front of each filter and then average your readings. Multiply this by the area of all the filters to get the volumetric flow rate (CFM) passing through the booth. Since your filter plenum is probably narrower than the width of the booth, you can now divide the volumetric flow rate by the cross-sectional area of the booth (in your case 25 ft W x 15 ft H) to arrive at the average velocity across the booth. This velocity will usually be considerably higher than my measurement, because air often travels along the ceiling of the booth and not where the painter stands. As you probably know, 29 CFR 1910.107 provides Table G10 that lists the air velocities for different situations. However, for a typical USCG side-draft spray booth, 100 ft/min is a good number to work with. AIRFLOW RATES FOR DOWN-DRAFT SPRAY BOOTHS Q: I have a project where an existing down-draft spray booth 54ft (L) x 19.3ft (W) x 16ft (H) (with exhaust pit) is used to spray helicopters. It has an airflow rate of 33,000 CFM and provides a down-draft velocity of approx 32 fpm. I have been told that the velocity should be 100 fpm. In reviewing NFPA and industrial ventilation standards, I have found little information about reduced flow in down-draft booths. It appears that the airflow meets the LEL. 557

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