HVAC (Heating, Ventilation, Air Conditioning)

Monday, June 02, 2008

Paint Shop compliance

A client of mine recently moved his paint shop from one city to another in the Bay Area. Whereas he was in compliance in his old location he needs to prepare drawings to show compliance to the city he is now operating in. When I visited the site I found a ceiling hung gas-fired heater in the warehouse area he has moved his business to. Section 1503.2.2 of California Fire Code requires open flames to be at least 20 ft. away from flammable liquids and vapors. I am going to have the gas-heater relocate to the top of the office space and blow hot air in the direction of the warehouse. [In high ceiling areas, ceiling hung gas-fired air heaters are not a good way to heat the space—hot air rises up and you need to heat the whole indoor air before benefiting the occupants standing below. It is much better to provide spot radiation heaters. These radiation heaters must be placed directly above the heads of the personnel—here the assumption being that the operators don't move a lot from their workstations. But then, why not do it the old-fashion way? Have operators wear very warm clothes in winter, in such high ceiling areas.]

How much air needs to be exhausted from a Spray Paint Booth?
[Other possible headings:
How to design a spray paint booth?
Exhaust requirement for a paint booth.
Paint shop ventilation design.]

The governing code is CAMC 505.1.2.
“In systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the lower flammability limit (LFL).”

LFL is expressed in % by volume. For example LFL for methane is 5% i.e., if 100 cubic feet of air has 5 cubic feet of methane, then you would be able to throw a burning match in this air mixture and see combustion taking place.
So, how do we go from 25% of LFL to spray paint booth exhaust.
Let's look at an example.
Suppose looking at the MSDS for a particular paint you find its LFL to be 0.7%. Then 25% of this LFL is (0.7/4)%. If you are using a spray gun with a discharge rate of 0.4 Gallons/ second, then you really want to dilute 3.2 CFM (conversion from gallons to cu ft and s to min) of paint to a level of (0.7/4)%. Simple math will tell you, you need 1833 CFM of air to reach that level. And that is what your exhaust fan CFM should be.

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