Notice that in the draw-thru system, the air is drawn through a filter door or wall. This is usually the product entry door because it has sufficient area to allow for good filtration. It is an important addition to a booth to add a filter door (as opposed to using an open front booth) because air carrying dust and dirt particles is filtered prior to entering the painting chamber removing most of the cause of imperfections. Filters will also smooth out the airflow by diffusing the air.
In pressurized systems an air replacement system consisting of a fan and maybe a heater system adds replacement air directly to the booth in lieu of drawing the air thru a filter door. These systems are difficult to balance, but once the fundamentals are understood the balancing is very simple. Let us look at a system that is incorrectly balanced.
Incorrect Airflow Balance
In this system, more air enters the booth than is allowed to leave. When the air systems are first turned on, the suction from the exhaust tries to evacuate the room, but in time the surplus air builds up pressure in the middle of the booth causing a cloud to form. Air is a compressible fluid and will just “bunch up” at the point of interference. This cloud is invisible until the painter starts his painting operation. Then the air is colored with paint and the cloud is very visible.
Turbulence is also obvious when using a small handheld velometer. Since the meter is a vane type device, air entering from one side causes the vane and its needle to move to the appropriate place on the scale. When air enters from both sides such as at a location of turbulence, the needle and vane oscillate and it is difficult to get a reading on the meter that is stable.
In any case, turbulence is one of the worst things that can happen to a painter. It causes all manner of blemishes in the paint from thinly applied paint to overspray and dirt inclusions, to low visibility. Turbulence will show up as paint overspray on the walls and lamps of the paint booth, on the floor, and on the painter himself. It makes the working conditions very difficult and the chances of customer dissatisfaction very likely.
Looking at a well-balanced system (Figure 4), the booth is at a negative pressure (relative to the outside) from the time the fresh filtered air enters the booth until it leaves through the exhaust filters. This is the same pressure profile as the draw-thru system. It is as if the exhaust fan were reaching out to the zero point of pressure in the booth and gathering the air, while the supply fan is pushing the air just enough to move it to the zero point of pressure in the booth.
It is best to design a booth to move the zero point of pressure in the painting chamber to the intake filter section of the booth. In this manner the fan reaches out to the filters and pulls the air through them into the booth. This is the same principle used in the draw-thru system.