Painting Q&A: Recirculating Air

Q. When and why should you consider recirculating air in a paint booth?

A. The first responsibility of a paint booth is to contain over-sprayed material inside the booth and collect the over-sprayed material into filters. The air moves through these filters, arresting the airborne particles enabling the air to continue through the filter bank and extracting any solvent and/or volatile organic compounds (VOC) that may be in the air stream, evacuating them out through the duct work outside the building.

When you think about air, understand how it is measured. We measure air mainly in three ways:

• Pounds per square inch (psi), which measures the compressed pressure of air.
• Cubic feet per minute (cfm), which measures volume.
• Lineal feet per minute (lfpm or fpm), which measures the velocity of air.

To assure your booth is designed properly, calculate the space you are moving air through to determine the correct volume of air (cfm) needed to reach your desired velocity of air (lfpm). Typically this air velocity runs around 100 lfpm. Once you understand the volume of air needed, based on the design velocity, you can determine the size of the fan required, the number of filters needed and the size of ductwork required to move the appropriate amount of air.

When we understand these principles, we should also keep in mind that when the paint booth draws the air through its filters, it is extracting from the atmosphere around it and does not replace that air. If there is not enough atmospheric air replenishment, an air makeup unit (AMU) must be incorporated into the design. The AMU is used to replace the air the paint booth removed so the booth can maintain its designed air velocity. It is also common to incorporate heat and/or cooling of the returned air for flashing or force drying of coatings or to simply create a controlled environment.

But when is recirculation a viable option? The idea behind recirculation is to reuse or recirculate a percentage of the total air moving into the spray area so that less than 100 percent is actually exhausted to atmosphere. A portion or percentage is exhausted, and the remaining percentage is returned back into the air stream, reducing the amount of air that is extracted from the atmosphere.

Recirculating a percentage of the spray booth air has two key advantages:

The lower amount of air that is exhausted from the building reduces the size of the equipment needed.

If the exhaust stream has enough solvent content to require abatement (destruction of VOCs), recirculation will concentrate the VOC and provide a more fuel-rich airflow.

Both of these air volume reduction issues will reduce the cost of operating the system. Lower AMU on the front end reduces the capital cost and the cost associated with heating air makeup, and a more fuel-rich, lower volume exhaust stream reduces capital cost and operating cost in a system that uses abatement equipment for VOC destruction.

The recirculated air volume does have a higher concentration of solvent than a booth with a typical exhaust arrangement that exhausts 100 percent of the air volume. From a safety standpoint­—especially in manned spray areas­—keep in mind that you are extracting contaminated air which contains flashing solvents in the air stream and must be measured to assure a safe environment. The spray booth will need a monitor located in the ductwork to measure the volume of solvent in the airstream. Rules regarding concentration of solvent in the booth require that the volume measured must always be below 25 percent of the lower flammable level. Recirculation is a very viable and energy-efficient option when your system is properly designed.    

Steve Houston has been a business leader for more than 30 years.  He is currently the chief marketing officer at Col-Met Engineered Finishing Solutions in Rockwall, Texas.

Originally published in the December 2015 issue.