The use of the semi-downdraft front-located intake plenums means the air is introduced into the booth in a downward trajectory and then the airflow must bounce off the floor and travel horizontally, while at the same time achieving laminar airflow. In reality, the bounce or splashing effect means turbulence and there is very 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 huge. The use of a plenum door eliminates all of these concerns. This is not to say plenum doors do not have design problems. They are huge structures and they move as well. A plenum door for a large aircraft facility will rise 6-7 stories and be 8-10 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. In a recent test, we checked air velocities over the entire door surface in a huge traverse array and found air rates to vary about 20-30 fpm over the entire door. This is extremely close control of airflow and did not require any elaborate dampers or vanes in the interior of the door. The use of high diffusion filters in the face of the plenum door achieved all of the airflow control that could have been 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 so 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.