What Affects Balance?
When a balanced spray booth is disturbed by the object being painted, the painter and the spray apparatus, the balance may change as well.
The Effects of Changing Balance
The most noticeable thing that happens is the filters quickly become soiled with paint and particles. 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 result 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 is visible when the painter is spraying. A velometer is also 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 frequency drive (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-through and pressurized systems.
A partially perforated plate with a pressure sensor is installed in the ductwork after the exhaust fan. The differential pressure across this plate is directly proportional to the airflow within the desired range to control the speed of the fan. As the differential pressure builds, the pressure sensor and pressure controller increase the speed of the fan using the VFD.
A sensor and controller measure the room’s 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 minus-.03 inches is held at the center of the booth, holding the pressure to minus-.05 inches nearer the exhaust filters can have the same good results.
Adaptations During Spraying
The object being painted introduces challenges that can frustrate a painter. But by experimenting with stance and the location of the spray, painters can overcome most of the negative effects of object interferences. Mostly, these effects show up in turbulence, as the overspray causes a soiled area on the object being painted. By carefully selecting the method of applying paint, many of these effects can be mitigated. For example, the painter may find that by spraying from back to front or re-orienting, the object will modify the turbulence to the point that it is no longer a problem.