Discussions on paint booth pressure center around whether the booth is under negative or positive pressure relative to the environment outside the spray booth. This is a serious consideration in spray booth design and is generally solved by having very good seals on the doors and completely caulking the panels.
- Benefits of Positive Pressure: Dirt and debris cannot enter the working chamber and soil the object being painted
- Benefits of Negative Pressure: Prevents emissions and volatile organic compounds (VOCs) from entering the room adjacent to the working chamber.
The aim of the booth designer is to achieve laminar airflow at all relevant points in the paint booth. Using a sheet metal shroud is an effective way to manage airflow and meets NFPA 33 standards. 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 the air speed from the front of the booth to the back of the booth is helpful in controlling 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 at a uniform pressure level. However, 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-through booth, the pressure at the entrance will be near zero and will get progressively more negative as it approaches the filter bank and exhaust fan.
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 spray booth and at either end, the pressure would be lowest at the end of the booth nearest the exhaust filters.