Paint booth air flow measurement

[habs]

I have a customer that needs to maintain equal make up and exhaust through a paint booth, there are vfds on the exhaust fans. They need to speed the drives up when the filters get dirty. The make up should not be a problem, I can get a control to measure the cfm but the exhaust will be harder due to the paint, it will get into the sensor, anyone have any suggestions how I can help this guy out?

[jogas]

How about measuring paint booth differential pressure to control VFD exhaust airflow? The idea would be to maintain a target DP set point between booth and surroundings that would be dictated by application and field testing.
Filters load up, booth goes positive due to reduced airflow, DP changes, exhaust vfd is controlled to a faster speed to overcome higher filter DP.
jogas

[OutOfControlMan]

jogas got it right. This is how I seen it done. I might add to delay any control response (PID Loops) by monitoring the the doors opening and closing to keep your Exhaust VFD from overshooting..

[mechdorn]

I have a customer that needs to maintain equal make up and exhaust through a paint booth, there are vfds on the exhaust fans. They need to speed the drives up when the filters get dirty. The make up should not be a problem, I can get a control to measure the cfm but the exhaust will be harder due to the paint, it will get into the sensor, anyone have any suggestions how I can help this guy out?

In an ideal world I agree with the DP strategy. From my experience, I would put a sensing probe in the exhaust duct with a liberal opening facing the exhaust fan. Measure and maintain duct static pressure not CFM. The negative air pressure is moving air away from the transmitter. Paint booths are highly mistreated rooms, doors are left open (too hot), closures get broken, and seals get ripped, just too many variables to warranty. If a sensing probe needs changing every 2-3 years, not too bad. Out there every month for neglect and poor maintenance, gets old fast

[orion242]

Space pressure, not duct static.

Measure the paint booth room & the surrounds. Modulate the exhaust to keep the paint booth negative.

Duct pressure will not tell you much. Ducts could be different sizes, lengths, grills, etc. There is no good way to ensure that the booth is negative based on duct static. Putting a pickup in the exhaust duct is a sure way to get it clogged in a hurry.