Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2003

Spray Booth Air Flow Rate

Q. We are in the process of converting a hanger into a spray booth. We are striving to be compliant with all local and federal rules. I have had a rep from Bonanza air make up systems come and evaluate our needs. After much time waiting for a materials list and prices. He e-mailed to say that his parent company would not let him tell me or even recomend how many cfm the air make-up unit should be for our application. My question is this; Is there an industry standard for the time it takes to replace the air in a booth?

A. You should not be concerned with the time it takes to replace the air. Instead, OSHA and the NFPA requires that you operate the booth at <25% LFL. If you know what coatings you use and their solvent contents, you calculate the minimum airflow that with bring you down to 25% LFL. When you perform this calculation you will probably find that the airflow rate required to keep the solvent concentration down is relatively low. On the other hand you also want the airflow rate to be sufficient to carry the paint particulates to the filters rather than have them fall to the floor. Many spray booth users still work on the basis of having a minimum airflow VELOCITY of 100 fpm.

I have been into many spray booth hangars where the airflow is 60 - 80 fpm, and while this might be sufficient to extract the vapors it is not always adequate to carry the particulates to the filters. Even if you have an air velocity of 100 fpm going through the booth, you will still need to insure that the booth has been designed so that the air will carry the particulates to the filters. Air velocity itself is not the only consideration. You also need to look at the design of the air intake plenum, intake filters, location of the exhaust filters, etc.

To convert velocity (fpm) to cfm, multiply the fpm by the cross-sectional area of the face of the filters.

Q. Our hanger has 43200 cubic feet of air space. We plan to have a direct fired air make-up unit for pressurization and a tube axial fan in the exaust filter unit. in order to maintain a positive booth pressure and still have good air flow what might be the difference in cfm between the two units?

In don't have a definitive answer for you because pressure losses in the system will affect the calculation. Bear in mind that you will want to limit to positive pressure to 0.03-0.07" W.C. If your pressure differential is too high, you will encourage air turbulence in the booth, and that can lead to paint defects in the finished coatings. Frankly, you should consider investing in a Variable Frequency Drive (VFD) so that you can adjust the balance between the air supply and air exhaust to give the positive pressure you require. As the filters build up with overspray the booth dynamics will constantly change; but with a VFD the inlet and outlet air flows should remain in balance.

I would like to give you much more time, but there is only so much I can give you in this column. Since I visit many aerospace spray booths over the course of a year, there is much more I can help you with to ensure that your booth will function properly. For this type of advice I do need to work with you on a fee basis, and if you are interested in pursuing this, please don't hesitate to contact me again.