Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2008


Q: I have a single filter bank in my paint booth and recently installed a Dwyer manometer. I'm having difficulty finding info on what the measurements mean. Is .15"wc a good reading or should I order some filters? If it reads 1.5"wc do I need outside screens or inside filters?

A: The manometer simply tells you the pressure difference across the filter(s). If you don't have any filters in the filter bank there will be no pressure difference between the front of and behind the filter bank. The pressure differential will read zero. As you place a resistance, such as a set of filters, into the filter bank the manometer will record a pressure difference, where the higher pressure is in front of the filters and the lower pressure is in the exhaust section of the booth.

The pressure difference increases as the resistance increases. Therefore, as the filters capture more and more overspray, the pressure difference will increase. In addition, the faster the air flows through the filters, the higher will be the difference.

To get the most of the manometer insure that it reads zero when the fan is turned off. There is a zero control knob on the manometer and you might need to turn it to the left or right until the column of red-colored water reads zero. Now install brand new filters, turn on the fan and measure the pressure difference across the filters. Take a colored marker, such as a green crayon or felt pen and mark the position of the water column.

Once you start spraying paint the overspray will collect on the filters and the water column will move ever so slowly up the column to give you higher readings. If you started with 0.15 inches water column (0.15" WC) when the filters were new, you might need to change the filters when they are so plugged that the pressure difference has increased to 1.15" WC.

If your fan has only limited capacity to pull air through the filter, once the overspray starts building up it is possible that the fan will no longer be able to pull air through and you will find that the water level in the manometer no longer increases. This is dangerous, because if air cannot pass through the filters, the air velocity through the booth itself is zero and solvent vapors can build up to flammable levels.

The purpose of the manometer is to forewarn you when the filters should be changed. If you notice that airflow through the booth is low, I suggest you change the filters. Before doing so, however, make another mark on the manometer, perhaps with a red crayon or felt pen so that in future you will know when you are getting close to a changeout.

A simple qualitative method for determining airflow is to take a 2-3 ft strip of masking tape and let it hang from your fingers, about 1 ft in front of the filters. If the tape simply hangs limply, you don't have any airflow, or it too low. You would like to see the tape being pulled toward the filters.

The best way to measure airflow is to purchase an inexpensive airflow meter, such as a vane anemometer. I use one that I purchased for less than $100 from Kestrel. According to OSHA you should always operate your booth at approximately 100 ft/min or higher. When the air velocity drops much blow this level you should change your filters. Suppose you decide that you won't operate the booth at a velocity below 80 ft/min, then place the red mark on your manometer when the velocity is at this low level.

For some spray booths you can change the filters when the manometer reads 1.0" WC, whereas for others you can go all the way up to 2-3" WC. If you go much beyond 3.0" you run the risk that the vacuum in the exhaust section will cause the air ducts and sheet metal to collapse. At the other extreme, the South Coast Air Quality Management District (SCAQMD) requires many spray booth permits operators to change out the filters when the manometer has risen 0.25" WC above the initial value; namely the value when the filters were brand new. To me this is a terrible waste of money and material, because it is quite possible that at this small increase the airflow might still be above 100 ft/min.

In your question you also asked about a reading of 0.15" WC. This is typical of very porous filters, usually the inexpensive ones that allow too much overspray to pass through and settle on the sheet metal, air ducts and fan blades. If you are using such filters I suggest you purchase ones with higher initial resistance. While they will cost slightly more, you will be preserving the exhaust section of the booth and more importantly you will prevent overspray from settling on the fan blades.

I hope this helps.


Ron Joseph

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