Spray booths need to be used correctly

gregwilliamsThe purpose of a spray booth, room or cabin is to provide a safe and efficient facility in which to spray apply coatings to a surface. It should be of sufficient size to readily accommodate the product to be finished, but not excessively large, as the cost of operation goes up with volume. For safety, a clear space of not less than 3’ around the booth should be kept free of storage or combustible construction. The booth should be of such construction as to discourage the spread of fire and contain minor explosions.

The booth should have power ventilation to confine and limit the escape of spray, vapor and other residue, both liquid and solid, of the coating applied. This is necessary for the safety of the spray operator and for the efficiency of the coating application. The air stream from inlet to exhaust should be of such a configuration that overspray and solvent vapors are carried away from the item being coated before the overspray can settle on the object and in such a manner that the operator is not enveloped in vapor or suspended particulate.

Proper ventilation

The ventilation system brings air into the booth. This “makeup” air should be clean, dry, and of a temperature well tolerated by the coating (65 to 90 F is typical; 19 to 32 C). It should be free of oil, vapors and dust. Generally, it will pass through a particulate filter before entering the spray booth. In a passive makeup system, the exhaust fan will draw air into the booth through the filters, across the item being sprayed and out of the booth through the exhaust filters. In this case the exhaust fan should be capable of maintaining sufficient flow (volume, velocity) even when the filters are partially clogged.

With powered inlet air, a separate fan or air mover provides a positive pressure in the booth, moving the airstream toward the exhaust filters. In this case, the exhaust fan could be somewhat smaller.

Filtering the outgoing air removes much of the wet or dry particulate before exhausting the air to the outside. This filtration does not remove solvent vapors. Depending on location, it could be desirable — or mandated — that the vapors be exhausted through a vertical stack to avoid offending others in the area.

The airstream should be sufficiently powerful to move the particulate as described, but not so powerful as to materially deflect the spray pattern from the gun. Nor should it be so powerful that it causes excessively fast drying of the coating or excessive evaporative cooling. The velocity of the airstream is measured in feet per minute (fpm), and the volume of the airstream is measured in cubic feet per minute (cfm). For most furniture coating liquids, 125 fpm is sufficient and at least 100 fpm is often required to meet regulatory mandates.

Let there be lots of light

The booth should be so configured as to provide sufficient and appropriate lighting, both ambient and task, for the operator. If color matching is required, the color temperature and Color Rendering Index should be considered. Light should illuminate, without shadows, the surface at the point where the liquid coating contacts the surface of the object so that the operator can clearly see the wet edge as it is forming. The industry standard for booth lighting intensity is 100 foot candles 3’ above the work surface.

Dust and overspray accumulation on lighting sources will diminish the amount and quality of light available, so they should be kept clean and replaced when they have dimmed or changed color appreciably. Explosion-proof fixtures might be mandated and are always a good idea when spraying any flammable solvents. A safety interlock is often incorporated to ensure that the air for the spray apparatus can’t be on while the exhaust fan is not operating.

Any electrical power supply to the booth might also be regulated and should be similarly explosion-proof and sealed. Even coatings, such as water-based, which are nonflammable in their wet form, could be flammable as dry dust. Overspray might collect in small spaces, such as electrical switches and junction boxes, and can be ignited by the spark of a switch contact or broken circuit. When possible, switches should be kept outside the booth.

And air, too

Compressed air is necessary for most coating spray operations. The compressor should be outside the booth and dry, oil-free, clean, filtered air should enter the booth. An operator adjustable pressure regulator with an easily read pressure gauge should be mounted at eye level to the rear or side of the operator, out of the airstream for the exhaust. The compressor should be sized to run all necessary tools at the same time. The temperature of the incoming air should be acceptable to the operator and the coating material, typically between 65 and 90 F (19 to 32 C).

A moisture and oil separator should be mounted before the regulator. The transmission lines (pipes through which the air flows) should be of such a size that no more than a 10 percent drop in air pressure will result at the furthest point in the system when at maximum workload. The transmission lines should pitch downward 1/2 degree in the direction of the air flow and moisture traps should be installed below the main line at the low point as well as on downlines or drops supplying tools. PVC should not be used for air lines and local regulations might further limit the material options. Copper, steel, iron and aluminum are often used. A flexible air hose could be used to carry air from the regulator to the spray gun and should be as short as is practical and of a sufficiently large diameter for the use. Smaller diameter and coils or loops in the hose cause a pressure drop from the regulator to the tool.

Keeping the booth clean reduces the chance of fire or explosion, improves the working environment for the operator and reduces debris in the coating. Filters should be cleaned and replaced when they become sufficiently loaded to cause excessive pressure differential between the opposite sides of the filter. Manometers or draft gauges can measure the air pressure differentials across a filter, which can indicate when the filter needs to be changed. This applies to both incoming and exhaust filters. Booth paper and strippable coatings can help to prevent the accumulation of dried coating on wall and floor surfaces.

Fire suppression measures, from simply having a fire extinguisher handy to automatic gas, liquid or dry-chemical-dispensing equipment are necessary. An automatic suppression system can cut off the air and fluid to spray equipment when a door is opened or a fire event is detected and cut off makeup air when the suppression system is deployed.

As accidental sparks can ignite many solvents, all tools, wheels, material-handling equipment or other metal items used in the spray booth should be non-sparking. All metal parts of spray booths, exhaust ducts and piping systems conveying flammable or combustible liquids or aerated solids should be properly grounded. Only the paint necessary to complete the job is permitted within the booth or room. Proper grounding is also required for cans of flammable paint and solvent, since the buildup of a static electrical charge could cause a spark. UL-listed waste cans with self-closing metal lids must be used for rags and other solid waste and separate UL-listed safety cans for liquids must be used for solvents and liquid waste.

In addition to preventing fire and exhausting harmful vapors, personal respiratory, eyesight, hearing and skin-protection equipment and training should be provided. A comprehensive protocol should be developed and enforced.

In conclusion

This article is intended to make the reader aware of the general purpose of the spray booth and of the many considerations necessary to make it perform in a safe and efficient manner. There are many variations that can accomplish the reader’s purpose and there are many codes and regulations that prescribe or prohibit certain materials or practices. You will need to contact regulatory agencies in your area for details. While many finishers see the requirement to have a spray booth as intrusive and costly, it is a tool as much as the spray gun. Designed, built, maintained and used effectively, it will save time, material and repair or rework, contributing to an enhanced bottom line. It could also improve your health and even save your life.

Greg Williams, formerly senior touchup and finishing instructor for Mohawk Finishing Products, is now a freelance instructor and consultant for finishing and touchup. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

This article originally appeared in the October 2012 issue.

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