Requirements for Airflow Through a Spray Booth

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Q: We are having a problem with our spray booth airflow. We are almost certain that the airflow problem is the result of a poor booth design of the exhaust plenum. It appears that the airflow is severely restricted by a third stage 12" deep pocket filter. Since our spray booth permit requires us to change filters when the differential pressure (ΔP) across the three-stage system exceeds 1.2” W.C. we find it necessary to change filters considerably more frequently that we had expected. Also, the airflow drops below the OSHA requirements. What do you suggest we do?

A: There are several issues here. First, some states have a regulatory requirement for changing filters when the ΔP exceeds an arbitrary limit. In my opinion this limit is usually unrealistic for reasons that I will explain.

In the Aerospace NESHAP, for instance, the filter vendor must suggest a ΔP limit that you should not exceed. In most cases, this limit does not correlate to breakthrough of solid paint particulates from the filter media. Because the regulation requires filter vendors to give their customers a “limit”, most vendors provide a conservative value that keeps everyone happy and out of trouble. Technically, there are three criteria that should be met; (i) the air velocity through the spray booth should not fall below 100 fpm, (ii) the vacuum created in the exhaust plenum of the spray booth should not be large enough to cause the duct work and sheet metal to collapse. Typically, spray booth designers do not like to see vacuum’s of more than about 3” W.C. This means that the actual pressure downstream of the filters should not be negative by more than 3” W.C.

The third implied parameter is that the air stream should be capable of carrying the solid paint overspray to the filters.

First, I’ll address the air velocity criterion. In the past, OSHA required a minimum air velocity of 100 fpm for non-electrostatic spray booths. The citation is provided below:

1910.94(c)(6)(i)

Except where a spray booth has an adequate air replacement system, the velocity of air into all openings of a spray booth shall be not less than that specified in Table G-10 for the operating conditions specified. An adequate air replacement system is one which introduces replacement air upstream or above the object being sprayed and is so designed that the velocity of air in the booth cross section is not less than that specified in Table G-10 when measured upstream or above the object being sprayed.

TABLE G-10
MINIMUM MAINTAINED VELOCITIES INTO SPRAY BOOTHS

___________________________________________________________________

                          |             |

                          |             |Airflow velocities, f.p.m.

 Operating conditions for | Crossdraft, |__________________________

   objects completely     |  f.p.m.     |                |

     inside booth         |             |    Design      | Range

__________________________|_____________|________________|_________

                          |             |                |

Electrostatic and         | Negligible. | 50 Large booth |   50-75

  automatic air-less      |             |                |

  operation contained in  |             |                |

  booth without operator. |             |                |

__________________________|_____________|________________|_________

                          |             |                |

Air-operated guns,        |             | 100 small booth|  75-125

  manual or automatic ....| Up to 50 ...| 100 large booth|  75-125

__________________________|_____________|________________|_________

                          |             |                |

Air-operated guns,        |             | 150 small booth| 125-175

  manual or automatic ....| Up to 100 ..| 150 large booth| 125-175

__________________________|_____________|________________|_________

                          |             |                |

                          |             | 200 small booth| 150-250

__________________________|_____________|________________|_________

NOTES:

Attention is invited to the fact that the effectiveness of the spray booth is dependent upon the relationship of the depth of thebooth to its height and width.
Crossdrafts can be eliminated through proper design and such design should be sought. Crossdrafts in excess of 100 fpm (feet per minute) should not be permitted.
Excessive air pressures result in loss of both efficiency and material waste in addition to creating a backlash that may carry overspray and fumes into adjacent work areas.
Booths should be designed with velocities shown in the column headed "Design." However, booths operating with velocities shown in the column headed "Range" are in compliance with this standard.

1910.94(c)(6)(ii)

In addition to the requirements in paragraph (c)(6)(i) of this section, the total air volume exhausted through a spray booth shall be such as to dilute solvent vapor to at least 25 percent of the lower explosive limit of the solvent being sprayed. An example of the method of calculating this volume is given below.

In the 2000 edition of its bulletin NFPA-33, Section 5.2 “Performance Requirements” the National Fire Protection Association has omitted the reference to air velocity, but does require the concentration of vapors and mists in the exhaust stream of the ventilation system to be less than 25% of the lower explosive flammable (LFL). This section also discusses an exception for confined spaces in which airflow might not be capable of providing the necessary ventilation. Under these circumstances NFPA recommends that a properly applied inerting procedure shall be permitted to be used, and they reference the reader to NFPA-69, “Standard on Explosion Prevention Systems“.

Based on the many discussions I’ve had with spray booth manufacturers an air velocity of 100 fpm might not be necessary for facilities that do not spray a large amount of paints, coatings and solvents at any one time. If you know the approximate mixture of solvents used in your spray booth and your worst case coating usage (gals/hour), it is possible for you to perform relatively simple calculations to determine the air velocity required to keep your LFL from exceeding 25%. For a large booth, or one in which you emit small quantities of solvents, the 100 fpm guideline might be overkill. On the other hand, you will still need to insure that sufficient air flows through the booth to capture the overspray and carry it to the filters. This is very much a function of the booth design. For instance, if you have a very deep (long) booth, such as a cross draft truck booth you will need more airflow than if the painter stands close to the filters, such as in a small component booth. Watch what happens to the overspray when a painter does his/her daily work. You will quickly get an idea of how well the overspray is being carried to the filters.

Now that I’ve given you the preamble, let me answer the original questions. Before you simply accept the filter vendor’s ΔP recommended limit for changing the filters, find out form him/her how that value was derived. In many cases you will find that this is nothing more than a standard value that the vendor gives to most customers. If this is so, then ask your vendor to work with you to establish a realistic ΔP, taking your booth design into consideration. You may well find that you can push the limit to a higher value without harming the environment in any way.

If you pull out the third stage filter altogether, you may still meet the filtration efficiency as provided in EPA Method 319. I tackled this in the previous question.

Another alternative is to shop around for other filters. Like everything else, not all filters are made equal, and the ΔP across each stage differs based on filter design. It is quite possible that you are using very dense filters for which the composite ΔP is relatively high, preventing the blower (fan) from pulling sufficient air through the system. Before you rashly purchase filters with lower ΔP, consider that some filters have a higher holding capacity than others and therefore last longer. In other words, a particular high resistance filter might have a higher holding capacity, and you might nevertheless find it cost effective to go with it.

Finally, perhaps it is true that your spray booth was not properly designed or that the fan was improperly sized for the required airflow. Ventilation engineers and some spray booth companies have the capability of making this determination for you.