Liquid Coatings Clinic: Considerations When Purchasing a Spray Booth
Q. Our company is in the market for a new spray booth and has been looking at several options. We want to make sure it is energy and cost efficient. What should be considered in the design of a spray booth to capture paint overspray?
A. The two primary goals of a spray booth are to capture the paint overspray and remove it from the airstream, and to isolate any potential hazards from the painter and surrounding areas. Typically, the primary factors that will dictate the style of spray booth (i.e. crossdraft booth, downdraft booth or hybrid) are part orientation and the specific surfaces that require painting.
For example, if parts are on an overhead conveyor or moveable rack and only need to be painted on one side or can be turned, a crossdraft spray booth would work best. In this configuration, the painters have the fresh air being drawn across their backs. The overspray is captured and pulled away from them. Even if the part is on a conveyor and cannot be turned, the same configuration can be used with two painters, each in their own booth facing the opposite directions. In a second example, if the part is large and needs to be painted on all sides, it would be best to use a downdraft booth. This way the painter can walk around the part, and the overspray is pulled downward away from them. This would apply to overhead conveyors or hoist and floor type conveyors.
When designing a spray booth, one must follow the requirements of OSHA and the National Fire Protection Association, the two principal guiding agencies. NFPA 33 is an easy document to read and understand. I recommend it as mandatory reading to anyone with the responsibility of specifying a spray booth. The old rule for determining required air exhaust is to measure the cross-sectional area of the spraying chamber in the direction of air travel and multiply by 100 feet per minute (fpm) average velocity. For example, a 10-foot-wide by 7-foot-high crossdraft spray booth would require 7,000 cubic feet per minute (cfm) of exhaust. To ensure you always have the 100 fpm velocity, even when your dry filters start capturing the overspray and load up, it is recommended to start with clean filters and use 125 fpm.
As the filters become full and the static pressure across them increases, the fan will pull less air. In this fashion, the spray booth will always have the minimum velocity of air, whether the filters are new or used. If you have a conveyor carrying the parts through the booth, the conveyor openings must be added to the cross-sectional area. Otherwise, air drawn in through these openings would reduce the air across the parts and not meet the minimum requirements.
It is possible to apply for a variance from the requirements if a large part was always being painted in the booth. Then a lower volume of air might be able to be used. However, this should only be considered after discussion with your local jurisdictional safety or fire department.
Depending on the building design and location, an air make-up unit (AMU) to offset the amount of exhausted air may be desirable. The AMU can be ducted directly to the spray booth or just provide general building make-up air. This is especially important to consider in the northern climates or if the plant is air conditioned, as it is undesirable to pull cold or hot outside air into the plant, creating problems for employees. In addition, some paints require the temperature and humidity to be controlled at specific ranges, which can only be accomplished by installing an AMU.
Other considerations in the booth design include the understanding of the hazard areas as defined in the NFPA 33. The three-foot area in front of a crossdraft spray booth or any other opening is classified as a Class 1 Division 2 for any electrical device located in this area. It must be either explosion proof or intrinsically safe (less than 24V DC). This includes lights, motors, fork trucks or anything that can cause a spark.
It is always recommended to have your design reviewed by a spray booth manufacturer or consultant. The goal is to ensure that the booth selection has been optimized for productivity, meets all OSHA requirements and those of local governing agencies. It’s easier to review these issues in the early planning stages, rather than try and rework or replace equipment after it is installed.
Kevin Coursin is General Manager of Evansville Operations at George Koch Sons located in Evansville, IN.
Originally published in the December 2016 issue.
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