Forced draft is the best way to introduce air into a paint booth. This is the best possible control of the air quality. Air may be double- or triple-filtered prior to introduction into the space. At the same time, air can be humidity- and temperature-controlled. Airborne dirt and dust from the exterior environment are not permitted to enter the booth. The only way for contaminants to enter the booth is for the painter to carry them in on his body or on the plane.
There are three ways of forcing air into a structure such as an insert:
- Plenum door with good seals
- Sealed plenum area between the booth and hangar doors
- Semi-downdraft air entry
These are listed in decreasing order of preference. The sealed plenum area depends on good seals at the hangar doors and booth doors, as well as at the building walls. This is difficult in practice and leaks do occur.
Use of the semi-downdraft, front-located intake plenums means air is introduced into the booth in a downward trajectory, then the airflow must bounce off the floor and travel horizontally, while also achieving laminar airflow. The bounce or splashing effect represents turbulence, and there is little a designer can do to avoid this. If the air is introduced too slowly, it stratifies and separates into hot and cold bands of air. If it enters too fast, the splash effect is magnified. The use of a plenum door eliminates all of these concerns, however, plenum doors do have design problems. They are huge structures that move as well. A plenum door for a large aircraft facility rises six to seven stories and is 8 to 10 feet thick. It traverses over a (usually) sloping terrain and must be kept level at all times to avoid undue stresses on the building at the hinges.
But the plenum door has the effect of introducing air into the paint booth in a very laminar air pattern. On average, air rates vary about 20 to 30 FPM over the entire door. This is extremely close control of airflow and does not require elaborate dampers or vanes in the interior of the door. The use of high-diffusion filters in the face of the plenum door achieves all of the airflow control that could be expected.
This large machine is powered at the base by drive motors on rubber-tired wheels. The suspension of the wheels is an air-ride suspension favored in the trucking industry for its level ride and gentle handling of cargo. The suspension is further equipped with a terrain-following optical sensor that detects subtle shifts in the terrain and holds the door level.