Determining the Type of Spray Booth Your Shop Needs

Q: I own a business that supplies and installs entry door systems for homeowners. To better control finish quality and delivery, we want to bring the coating process in house. What kind of spray booth do I need? Our most common part is about 3.5' × 7' × 6" and our largest part is 8' × 7' × 6".

A: Spray booths come in many different shapes and sizes, and are normally designated based on the direction of the air flow in the booth. For instance, high-end automotive repair shops use a downdraft style booth as it provides one of the best overall conditions for spraying an automobile. Air is introduced from the top of the booth and exhausted through the bottom. Depending on the exact design, this can involve a significant investment.

A more cost-effective solution would be a crossdraft booth. This booth consists of a vertical chamber with filter media to capture the paint overspray. Mounted on top of the chamber is a fan that draws the air across the part being sprayed. The air is drawn through the filters where the paint overspray will be captured, and the air will exhaust through ductwork to the outside. When sizing the booth, at a minimum you want to have three feet around all sides of the part; optimally, five feet provides more range of motion for the painter.

The parts that you described can be placed in the booth perpendicular to the exhaust chamber. This orientation of the part will allow the operator to have access to both sides of the doors you are coating. A 10-foot-wide booth would offer sufficient space for your painter. A 10-foot-high booth will provide three feet above the part, which would be more than adequate based on the depth of the parts being coated. With a crossdraft booth, the height and width are the most critical dimensions as they are needed to calculate the airflow. Crossdraft booths with manual sprayers are designed to have an airflow between 100 to 125 feet per minute. At this rate, the air movement will be more than adequate to remove overspray. Based on the height (10’) and width (10’) of the booth, there is 100 square feet of surface area. This number then gets multiplied by the desired air flow of 125 feet per minute to arrive at 12,500 cubic feet per minute of air flow. This is the volume of air the booth will remove.

The depth of the booth does not impact the air flow calculation, so you want to ensure that it is deep enough to accommodate the largest part and ample room for the operator. With the largest part being 8’ wide, I would recommend allowing for three feet from the filters and four feet from the entrance of the booth. This yields a total depth of 15 feet. You want to ensure that the part and all spraying is done within the confines of the booth to ensure overspray containment.

Based on the above, we have determined that a booth 10’ by 10’ by 15’ will work for your application. Note that this is the inside dimensions of the booth, the depth of the exhaust chamber will need to be added to get the overall depth. In addition to this, you must factor in the width of the panels which is typically two inches. The overall exterior dimension of the booth would be 10’4” by 10’2” by 18'4". Please keep in mind that spray booth manufacturers can build booths in all shapes and sizes. When looking for a cost-effective solution, look at a manufacturer's catalog booth, which come in a range of standard sizes, and find one that meets your needs. These models are pre-engineered and are available with shorter lead times.

There are a couple of accessories to consider for the booth. Filtered doors at the entrance of the booth will prevent shop debris and contaminants from being drawn inside and improve overall finish quality. You can also consider a variable frequency drive for the booth exhaust fan control. This will enable a more precise control over the exhaust airflow and extend the usable life of the booth filter media.

Three additional pieces of information are required to finalize the booth selection—available voltage/phase, roof height and slope. Common voltage in an industrial facility is 480V/240V three-phase or 240V/120V single-phase power. Regarding roof height, this is needed to calculate the amount of exhaust duct needed. Typically, the exhaust stack will extend six feet above the lowest point of the roof line. Exact requirements for exhaust stack height vary by location based on local regulations.

Now that the booth is properly sized, you must replace the air that is being drawn out by the spray booth. Quite often this step is omitted, creating a negative pressure in the building that can lead to various issues and potentially hazardous conditions. An Air Makeup Unit (AMU) draws in outside air to replace the volume of air exhausted by the spray booth. Because the air is being pulled from the outside, it will need to be heated in the winter months. An AMU can be installed inside or outside of the building. The AMU can be ducted to disperse the air into an open area adjacent to the spray booth or directly coupled to the spray booth. When coupled directly to the spray booth, this is called a pressurized booth. In this case, the inbound and exhaust air need to be balanced.

Contact the local authority having jurisdiction (AHJ) regarding specific requirements they may have relative to the installation and operation of a spray booth. There are various codes and regulations that NFPA and OSHA have in place regarding operation, maintenance, monitoring and fire protection. Therefore, it is best to check with the AHJ to understand requirements in your area. You will also want to contact your insurance company to understand any requirements they may have.