Proper Booth Filtration

Q. Hello, we are a job shop that finishes small auto parts. We spray in a 10' wide by 8' deep and 8' tall spray booth. We typically spray acrylic enamels on the parts. Lately, it seems that we have had to change our filters a lot to maintain good airflow through the booth. What is the proper filter that we should be using, and how often should a typical filter be changed?—S.S.

A. Good question. There are several things to evaluate when it comes to proper filters in a spray booth. First, to be compliant, your booth should have an attached manometer to measure proper air pressure. These devices are inexpensive and easy to attach, but many companies have them so covered up by debris that they are of little function. A manometer is filled with a special red fluid and a gauge that serves as a visual reference to measure the atmospheric pressure between the front of the booth filters and the back exhausted side of the filter. As your filters become loaded, the fluid in the J-shaped gauge will rise, indicating it is time to change the filter. (Typically 0.20 to 0.30 range). But you will usually know this by the lack of draw and over spray when applying the product.

Now, let's look at various filter mediums. Have you made any changes to your filters? Often, when a filter needs to be changed a lot, it is due to a less expensive filter material. Less expensive fiberglass filters, for example, will fill up and clog quicker. Also, a cheaper filter will not properly trap spray particulate, which often ends up in the exhaust shaft, and can lead to costly and time-consuming cleanups.

So, what type of filter should you be using? Generally speaking, when spraying solvent-based products, you should look at a good quality single stage, or multi-stage layered filters that will give you a very high efficiency rating (usually 98+ percent) with a high dry holding capacity (usually 8-9.5#). I particularly like the high capacity pocketed type single- and multi-stage filters. They may cost a little more, but they typically last much longer.

Final note: Follow the guidelines of NESHAP (National Emissions Standards for Hazardous Air Pollutants). Issues related to filter clogging are often due to improper spray operator training or transfer efficiency. You would be amazed at how many shops I've visited where the standards are not followed, and spray operators clean their guns in lines directly into the filters—a big no-no!