Much thought has been given to paint booth pressures. Usually this discussion centers on whether the booth is under a negative or positive pressure. Some discussions insist that the booth be held at a positive pressure relative to the outside so that dirt and debris cannot enter the painting chamber and soil the object being painted. This is a serious consideration in booth design and is generally solved by having very good seals on the doors and completely caulking the panels. Others insist the booth be at a negative pressure relative to the outside. This prevents emissions and VOCs from entering the room adjacent to the spraying chamber. Once again, having very good seals on the doors and caulking the panels solve this problem.
The aim of the booth designer is to achieve laminar airflow at all relevant points in the paint booth. This can be accomplished by managing the airflow. Using a sheet metal shroud to manage the airflow is an effective means and is approved by NFPA-33. Laminar airflow does not necessarily mean the airflow is constant. In fact, if left at a constant level, the airstream has a tendency to degrade and develop vortices (room effects), which are damaging to the painter’s environment. A slight acceleration of air speed from front to back is helpful in controlling the vortices.
Room effects are hard to see and to understand. It is better to follow simple empirical practices to negate the effects of the room. Acceleration of the airflow is one such way to manage the problem. The other is by understanding booth pressure.
This negative/positive pressure discussion generally assumes that the paint booth is a place that is at a uniform pressure level. This is not true and the science of balancing paint booths, especially so-called “big rooms”, is easy to understand.
Pressure Differences Move Air
In order for air to move, a pressure difference must exist from one end of the booth to the other. In a draw-thru booth (Figure 1) the pressure at the entrance will be near zero, and it will get progressively more negative as it approaches the filter bank and exhaust fan. One may think of the spraying chamber as a section of ductwork, since it is completely enclosed.
Notice the gradation of pressure as the air moves from one end to the other. If the room pressure were to be measured relative to the outside area at the center of the booth, you would find that the pressure would be different at the end of the booth nearest the exhaust filters.