In a draw-through system, 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. Adding a filtered door — as opposed to using an open-front booth — is important because air carrying dust and dirt particles is filtered prior to entering the working chamber. This removes many of the causes of imperfections and also smooths the airflow by diffusing the air.
In pressurized paint booths, an air replacement system consisting of a fan and possibly an air heater adds replacement air directly to the paint booth in lieu of drawing the air through a filtered door. These systems are difficult to balance, unless the fundamentals of air balancing are understood.
Incorrect Airflow Balance
In this system, more air enters the spray booth than is allowed to leave. When the air systems are first turned on, the exhaust system tries to pull air out of the spray booth. However, over time, the surplus air builds pressure in the middle of the booth, causing a cloud to form. Air is a compressible fluid that bunches up at the point of interference. This cloud is invisible until the painter starts painting. Once the air is colored with paint, the cloud becomes apparent.
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, making it difficult to get a reading on the meter that is stable.
Turbulence is one of the worst things that can happen to a painter. It causes blemishes in the paint, including thinly applied paint, overspray, dirt inclusions and low visibility. Turbulence will appear as paint overspray on the walls and lamps of the paint booth, as well as on the floor and on the painter. 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 fresh, filtered air enters the booth until it leaves through the exhaust filters. This is the same pressure profile as a draw-through system.
It is best to design a booth to move the zero point of pressure in the working 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 a draw-through system.