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topic 0921

Calculating Upper / Lower Explosive Limits for paint booth


Can someone please direct me to info on how to calculate upper and lower explosion limits within a automotive paint booth. Any help would be greatly appreciated.

James Rhoades


The information you seek on lower explosive limits can be found in the publication NFPA 33 [link by ed. to spec at TechStreet] Standard for Spray Application Using Flammable or Combustible Materials. If your need relates to powder coating, be aware that a typo exists in the illustrated calculation, the text describing the calculation is correct.

The National Fire Protection Association (NFPA) web page is at

Jeff Hagerlin
paint supplier - Houston, Texas

sidebar 1998

I am looking for mfg's of economically priced LEL Meters, Do you have any suggestions of places to look.Thanks, Joe

Joseph J. Gulley
May 1, 2008

I have found that GDS in santa fe Texas has cost effective lel monitors for any gas I have come across.

john hopkins
- charlotte North Carolina


The book by John Bond, "Sources of Ignition" [link is to product info at Amazon] includes a very good compilation of lower flammable limit data. To make use of it, you will need to know the concentration of flammable vapors in the air of your shop. If you don't know the concentration, then you might make the assumption that the air is saturated by the vapor in equilibrium with the liquid solvent. You would want the concentration of the vapor to be below the lower flammable limit for safety.

If you can't find the lower flammable limit of a particular solvent in Bond's book, then you might be able to estimate it with the LFLAIR program from Space Coast Computers, Inc.

As an alternative, you might simply contact the vendor of the solvent and ask for guidance.

William H. Seaton
- Titusville, Florida


I have to disagree with Mr. Seaton's approach. Most solvents are at or above their LEL when in equilibrium with air. You need to consider the paint application rate and the air rate of the booth. Then there is the issue of drying time. Different solvents evaporate at different rates.

A conservative approach would be to assume that the paint dries quickly. The concentration would be the rate of solvent sprayed inside the booth divided by the air flow rate. However, many paints take hours to dry and will emit solvents for a long time. Thats why its important to keep booth ventilation on. If you turn it off, the solvent vapors will build up and you might exceed the LEL.

For safety reasons, the solvent concentration should never exceed 25% of the LEL. In addition, most booth ventilation rates are set to maintain a safe work environment for the painter. This safe level is orders of magnitude lower than the LEL. Setting the air flow rate to maintain a low exposure level is why few worry about calculating LEL levels in a paint booth.

Mike Callahan


The Lower Explosive Limit calculations of a spray booth do not seem to be the relevant question. Instead, how to keep the volatiles below their limits would seem to be the question. NFPA 33 does give some explanation as to the limits and a safety factor. Specifically a 10,000 cubic foot per gallon airflow rate of 1% LEL solvent gives a 4 to 1 safety factor. Therefore if you take anemometer measurements of the exhaust system and find a 10,000 cubic foot per minute flow rate you can evaporate 1 gallon per minute of a 1% solvent. Most spray guns can deliver about 1 pound per minute maximum. Therefore in a booth that has a 10,000 cubic foot per minute you could have about 7 spray guns going full blast at one time and still be below the 4-1 LEL safety factor. Exhaust systems need to remain energized until the item painted has dried.

John Allen, P.E.
- Atlanta Georgia


I am looking for information or method of calculating the effects of an oxygen-enriched environment on the LEL AND UEL values of solvents or hydrocarbons in an off-gas vent stream. The stream has hydrogen gas, carbon monoxide gas, carboxylic acid vapor, oxygen gas and dilution air. the stream is at 240 F. We have been trying to determine a LEL/UEL range for the stream, and the effects oxygen has on these values.

Ronald Z. Collier
- Cincinnati, Ohio,USA


To the question of needing LEL-UEL for oxygen information. The answer is Normal oxygen content by standards are 19.5-23.5% If you are taking an LEL-UEL reading you have to include a oxygen reading. Your LEL will reduced oxygen content will in most cases not cause a problem. If your LEL has oxygen above 23.5% your probability rises with every %. Remember oxygen oxidizes with other products and explosive ranges are acerbated with this over oxygenation. I would take an air sample and have closed cup test performed to determine your exposures.

Ferman Weedon
- Harker Heights, Texas


LEL and UEL are the same as LFL and UFL, respectively.
Many chemicals list a flash point, which is related to the LEL. For instance, Acetone [linked by editor to product info at Amazon] has an LEL of 2.6% and an UEL of 13.0%, and its flash point is -4 °F. In other words, at -4 °Fahrenheit, the saturation concentration of acetone in air is 2.6% (based on its vapor pressure at that temperature), which is the LEL. At 15 °C, however, the saturation concentration of acetone in air is 15.2%, above the UEL. Of course, it's easy to dilute the acetone with more air and put the concentration below the UEL, creating a flammable environment.

David Gordon
- Kalamazoo, Michigan

November 28, 2011

Paint booth room size is 30 m long x 18 m wide x 13 m height.
Please help us to quote the paint booth with the calculations....
what will be the air flow, air velocity etc...

Binu Thomas

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