Brad Wilson:

Spray Booth Management - Filters

Which type of filter?

There are many different types of overspray. There are also several types of filters used to collect overspray. With so many possible combinations it is likely that whatever particular filter came with the booth is not the most appropriate for the current application. Therefore the first step in spray booth filter maintenance is to determine which filter has the best combination of efficiency, capacity, airflow, and cost for the application.

Impingement vs. Impaction

Spray booth exhaust filters but they fall into two main categories - Impingement and Impaction. Each type has certain efficiency/capacity/air flow tradeoffs.

Impingement type include cardboard baffle style (Maxi-Air) and accordion style (PAF-90) filters. These high capacity filters maintain efficiency level and air flow over the life of the filter. They do not adversely effect airflow until they are 95% or more loaded.

Impaction type filter composition includes paper mesh (Research Products), fiberglass, and polyester pockets (Paint Pockets). As these types of filters load up they become more efficient and airflow is continuously reduced over the life of the filter. In the end the filter is completely "blinded" and air flow virtually non-existent.

Graduated Density Filtration (The Grabber Filter System)

Graduated density filtration (GDF) is the principle which dictates that overspray should meet the highest capacity filter or part of the filter first, followed by the highest efficiency filter or part of the filter. The most common example of this is Research Products 3200 series which has a polyester backing (efficiency) on a paper mesh filter (capacity). This type of filter adheres to the principle of GDF but the filter components are sown together and disposed of at the same time. When this filter is changed out, typically only one of the component's life is over thus losing value in the other component. A two-stage system, which uses an impingement filter in front and an impaction filter in behind, is the most economical means of filtration. This is because it gets full value out of the high capacity impingement primary filter and full value out of the high efficiency impaction secondary filter when they are changed out separately.

The Grabber Filter System is a two stage system utilizing the highest capacity impingement type filter (Maxi-Air or PAF-90) with the highest efficiency impaction filter (Grabber. When fully loaded the Maxi-Air will hold 8-10 pounds of overspray solids. Mean while, the second stage Grabber is twice as efficient as the EPA standard, to a theoretical emission rate of .0114 grams per cubic foot of air. Used as a finishing filter it produces no effectual resistance to airflow.