What Affects Balance?
When the “perfectly balanced” environment of a spray booth is disturbed by the object to be painted, as well as by a painter, and spray apparatus, the perfect booth also changes. This is nature, obeying the laws of physics.
The Effects of Changing Balance
The most noticeable thing that happens is the filters become soiled with paint and particulates. This fact of life is a sign that the paint booth is functioning correctly. If the opposite were true, the paint booth would be greatly out of balance, and paint and particulate would soil the object and the floor or walls. As the filters become clogged, the exhaust fan sees a greater resistance to flow and moves less air. The intake fan is less likely to see a change to its resistance and continues to supply the same volume of air.
The effect is the zero point of pressure in the booth starts to migrate toward the center of the booth. Wherever this zero point is located a cloud of paint will be seen when the painter is working at that location. A velometer will also be helpful in demonstrating this effect. To move the zero point back to the intake of the booth, the exhaust fan must run at a faster rate to compensate for the increased static pressure.
Affecting Balance Through Mechanics
The practical way of doing this is to install a variable speed drive system (VFD) on the fan to allow the fan speed to increase as the filters load with overspray. There are different ways to control airflow in draw-thru and pressurized systems.
A partial perforated plate is installed in the ductwork after the exhaust fan. This acts as an orifice plate of sorts. The differential pressure across this plate is directly proportional to the airflow within the range we wish to control the speed of the fan. As the differential pressure builds, the pressure sensor and pressure controller increases the speed of the fan using the VFD.
a sensor and a controller measure the room static pressure relative to the outside pressure and adjust the fan speed by using the VFD. It is not important where the sensor is located since the gradation of pressure is essentially linear. If a pressure of -.03” is held at the center of the booth, holding the pressure to -.05” nearer the exhaust filters can have the same good results.
Adaptations During Spraying
The object (to be painted) introduces challenges that can frustrate a painter at times. But painters are clever creatures and by experimenting with their stance and the location of the spray they can overcome most of the negative effects of the object interferences. Mostly, these effects show up in turbulence and the overspray causes a soiled area on the object to be painted. By choosing the method of applying paint, many of these effects can be mitigated. The painter may find that by spraying from back to front works better than the opposite way. Perhaps re-orienting the object will modify the turbulence to the point that it is no longer a problem.
Balancing a booth can be a difficult job unless the technician understands the principles of air movement and pressure gradients. Once armed with these principles, the job is less an art than a science.