Managing the airflow is probably the most important element of a spray booth and its design. Creating a laminar airflow envelope in the spray area at an engineered velocity separates a “spray booth” from a “tin box”.
This managed airflow enables a painter to get maximum efficiency of the paint sprayed while directing overspray away from the painted finish. In a superior design, air is controlled to flow in unidirectional layers, either in horizontal, semi-downdraft or downdraft flow patterns, while maintaining an even velocity.
Velocity must be evenly maintained and Balanced!
Booth Air Requirements
A critical step in selecting a spray booth system is establishing the minimum air velocity and volume requirements. The spray booth should be located to allow for proper air entry and flow through the booth.
The graphic at below shows that an open-faced booth should be located with the face at least booth height dimension from any wall. Booth “a” is too close to the building walls. The front of booth “b” is placed at a distance from the wall that is equal to the height of the booth. When this placement is not possible, air input plenums can provide adequate airflow. Booth “c” can be placed next to the wall because it has a direct-connected air input plenum.
A spray booth requires a minimum air draft or velocity, measured in lineal feet per minute (fpm), to carry overspray through the booth, past the operator or the automatic equipment, and deposit it into either the filter pads or water curtain. As a rule, OSHA inspectors rely on the guidelines specified in NFPA-33 requirements in the booth during spraying operations. While the NFPA-33 guideline covers most spray operations, greater airflow may be required when specific types of finishing equipment are used. The high-pressure atomization equipment used to break up higher solids materials, for example, produces high atomization pressures and consequently, high fluid stream velocity, at the tip of the spray gun. This can cause overspray to rebound and may expose the operator to toxic materials present in the paint. Velocity should always be sufficient to carry the overspray away from the operator and into the exhaust chamber.
The velocity possible in a booth depends on the fan size. Most standard booths offered in the market come equipped with fan and motor packages sized to deliver the necessary draft. Draft requirements take into account real world static pressures; that is, resistance to airflow from entry losses, stack filters and ductwork.
Static pressure is the amount of resistance air must overcome while moving from point A to point B. Static pressure in a spray booth is encountered in two areas: intake and exhaust filters and intake and exhaust ductwork.
The static pressure of any filter is determined by how much air will pass through that filter. Air intake filters for downdraft spray booths are denser and pass less air than air intake filters for either cross-draft or semi-downdraft booths. Consequently, air intake filters for downdraft spray booths have a higher static pressure rating than the air intake filters for other booths.
When intake or exhaust filters become clogged with dirt or material overspray, the amount of air that can pass through the filter decreases. When airflow is restricted, the filter’s static pressure or resistance to airflow increases. Air intake and exhaust ducts also influence static pressure.
Air volume and velocity are decreased when elbows, reducers, transitions and long runs are added to ducts. Elbows introduce angles and increase resistance to airflow. Reducers and transitions also increase the static pressure in ductwork. The ideal situation is to keep ductwork to a minimum.
Static pressure is also a factor when choosing an air replacement unit. Because of the similarities to an exhaust booth, pressure drops in and out of the unit must be considered.