Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2009

Spray Booth Pressure Problem

Q. We have a Garmat 3000 spray booth is this normal? The problem we have is that during start up from off to spray cycle the booth goes negative pressure sucking any dust from the outside. It does last for only 30 or so seconds then the booth equalizes +.05 pressure. The other issue is that on cool down after bake the booth goes positive enough to pop a door open sometimes. It really looks like there is no pressure control during cool down. New filters or dirty does not have an effect it's been like this since new. We recently switched to waterborne and contaminants is becoming a problem. Our local Garmat rep has not been able to fix the problem.

A. It is common that spray booths with two fans; an exhaust and an air supply fan will start off with negative pressure and then stabilize at slightly positive or negative pressure. This is because the exhaust fan comes on before the air supply fan. It is alomst certain that a variable frequency drive (VFD) is installed in a modern paint booth. It might have drifted off its setting or was improperly setup in the first instance.

Exhaust fans are very light mechanisms (low inertia) and air make up fans are heavy and have dampers to be opened. For this reason, the exhaust fans are ramped up in speed slowly to coincide with the reaching of final speed by the supply system. This "tuning" can be accomplished by watching the doors during start up and changes from run to cure or run to stop.

With some older heat groups, this all is done with dampers. Thus damper adjustment and maintenance is critical.
Your vendor should be able to reprogram the control system so that the exhaust fan starts up sooner. It appears that the two fans are well balanced, because the equilibrium pressure is only 0.05 inches W.C. which is excellent.

By the same token, after the cool down cycle the exhaust fan appears to be shutting down very much sooner than the air supply fan. Again, your vendor should be able to change the control program.

I don't know why you are experiencing more dust and dirt problems with the waterborne paints than with the solvent based ones, but perhaps you are not using the correct exhaust filters. Alternatively, you have changed your atomizing air pressure and might be creating more overspray.


Ron Joseph

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