Ask the Expert Question-and-Answer Archive
by Ron Joseph
August, 2005
Air Velocity - Air Flow Rate in Paint Spray Booth
Q. I have a Binks cross draft booth over here, and the industrial hygenist told
me that it is pulling 80 CFM right now. And in order to Paint polyurethane paint,
I need to pull 100 CFM. The booth is 25'x14'and has a 5 horsepower exhaust motor
1735 rpm. The filters on the doors look like filters from an a/c unit at your
house and the exhaust filters are paper/cardboard with holes . What would I
have to do to raise the CFM to 100. Would I have to replace the exaust motor,
maybe get a 7.5 or 10 hp motor? I know the motor is a 60 hz motor and here they
have 220/50 hz power supply so it might be slowing down the motor by running
off of 50 hz. Any advice or help you have would be awesome. The paint we use
now turns into chalk in the sun and it would be great to shoot the poly !!
A. First, I need to know whether your industrial hygienist (IH) measured the
air velocity at the face of the filters, or in various locations inside the
paint spray booth. The difference is enormous, and if he/she measured the velocities
at the face of the filters, I can assure you that the velocity inside the spray
booth where the painters stand, is well below the 80 ft/min reported.
Second, I assume that your IH is referring to the linear air velocity (ft/min)
and not the volumetric air flow rate (cubic ft/min)?
There are several reasons why you are not pulling 100 ft/min. However, based
on what you've told me, you are using very inefficient particulate filters to
remove the overspray. My guess is that a huge amount of overspray has deposited
on the sheet metal walls and ducts of the exhaust system. In fact, it is possible
that overspray has also deposited on the fan blades thus slowing them down.
My fist recommendation is that you ask your IH to tell you whether the 80 ft/min
was measured at the face of the filters or inside the booth. Depending on the
answer you might need to increase the air flow rate considerably or only marginally.
Then, I suggest that you look inside the exhaust plenum and especially on the
fan blades to see if they are being hindered by overspray. If so, clean out
the booth, but TAKE ALL SAFETY PRECAUTIONS to prevent sparks and don't undertake
this until you have evacuated all solvent vapors from the booth.
If the air velocity after cleaning out the exhaust plenum is still inadequate,
you might need to increase the fan speed and/or increase the horsepower to the
motor. Your fan supplier can calculate the energy required to increase the RPM
using your existing motor, but changing out the pulley to a smaller diameter
one. He can also calculate the energy and capital cost for changing to a higher
HP motor. Depending on how much more air velocity you require, one option might
be more cost effective than the other.
If you are interested, please visit www.ronjoseph.com/po.htm
for information about a class I'm giving on this topic.
Best wishes,
Ron Joseph