Balanced airflow and positive pressure — these are the hallmarks of a superior spray booth design, a design which incorporates an air make-up system to maintain positive pressure within the cabin, and uses an exhaust system, adjustable dampers and diffusion to control the balance.
The exhaust design should promote airflow completely balanced around the object, including the front, rear and sides. Unevenly balanced airflow creates conditions which encourage uncontrolled overspray migration onto the paint finish.
Critical, of course, is the exhaust filter itself. The various exhaust filtration media and their qualities and attributes are reviewed in detail in the Filtration Lesson. In every system configuration, all the parts of the filtration system must be maintained to O.E.M. (original equipment manufacturer) specifications. This is the only way to assure the proper airflow and a particulate-free environment.
The design of the exhaust “box” can vary from an engineered pit underneath the booth, or an exhaust chamber at one end of a booth, or exhaust outlets along the floor on each side of the booth. In downdraft booths, the preferred configuration is a continuous length “pit”, spanning the enclosure end to end, which eliminates dead zones often associated with shorter pits. In other draft styles, exhaust chamber filter assemblies usually cover the entire wall in which they are installed. They can also be designed as “columns” – included within an exhaust yoke, usually configured around a large entrance door. No matter what the configuration of the exhaust, whether pit, chamber or yoke style, research and testing has revealed the necessity to maintain balance through the life of the exhaust filtration. This implies continuous monitoring of pressure, and adjustments and filter replacement when indicated by gauge readings.
Dissfusing The Airflow
Balancing the airflow in the booth is more than the total air in and total air out. There needs to be balance in the distribution within the booth. Complete system control requires engineered baffling and diffusion. This must be designed into the system to achieve the balance required around the painted object, and provide a consistent environment.
Air coming into the booth, whether through a plenum in the ceiling, or through an attached input plenum, or filter door, establishes the airflow in the booth. This entry point represents the first opportunity to influence and control airflow direction.
What is a “baffle”? Baffles are rigid constructions which are pierced or contain holes which permit air to pass through. Baffles can be made of paper, Styrofoam, metal or other materials. Their purpose is to force air to move in the desired patterns. This redirection of air is called “diffusion”. Baffles diffuse airflow.
How can airflow be “engineered”? It takes testing, testing and more testing. In the situation illustrated below, an airflow meter, or velometer, is used to measure the speed of air. The floor of the booth is marked out in a precise grid using masking tape. The velometer’s sensor is tightly taped to a camera tripod fitted with a plumb line, so that measurements will always be at a consistent height, and so that the instrument can be positioned precisely at the various grid intersections. As the tests reveal the results, various openings in the prototype baffles can be covered or uncovered to influence the next set of readings. When a satisfactory result is achieved, manufacturing can then retool or create a new part to conform.
This diffusion device has openings and bendable “wings” to redirect the airflow.
In some industrial situations, a booth can be engineered with an air input plenum, which is fitted with diffuser plates or baffles, which can be moved to alter the airflow
In addition, airflow in a downdraft booth is directly related to passage of air into the exhaust pit. Here is another opportunity to influence the movement of air, through diffuser pans below the pit filter. Here is an example of such a pan. Diffuser pans of engineered porosity lying below the pit filter regulate the flow of air into the exhaust chamber, and thereby, influence the pattern of airflow within the booth environment. Testing determines the optimal density of such pans.
Research has revealed that a downdraft input plenum design should cover as much as possible of the surface area available on the ceiling. The ideal design would be from wall to wall, providing a superior envelope of laminar airflow into the enclosure and around the object to be finished. Anything less than this surface area will result in poor airflow within the spray zone, leading to particulate migration and collection. This migration and collection will, over time, contribute to poor paint finishes requiring rework.
In horizontal flow and semi-downdraft flow booths, air arrives in the booth through an air input plenum or front filter door. The same objective is sought: the best balance possible in the finishing environment. Continuous pressure monitoring in these styles can minimize particulate migration and collection.
The objective is balance. The plenum design is critical in the effort to achieve the desired results.
How do all the parts work together to achieve the results we want? The proper airflow at the proper temperature is the goal we are trying to achieve. All the major components work to enhance, control or motivate the airflow. So understanding the airflow is most important.
Monitoring booth pressure is done through a differential pressure gauge. Pressure sensors are placed immediately in front of and after the filtration assemblies which are being monitored. The sensor should be kept clear of obstructions and paint buildup.
The balancing gauge monitors:
-Effectiveness of post-filtration assembly
-Effectiveness of exhaust or pit filtration assembly
Adjusting Booth Balance
Two types of differential pressure gauges are shown below. Either type can be used to indicate when the booth cabin is balanced and ready for spraying.
Gauges must be calibrated prior to use. To calibrate a gauge, first turn off the booth, open an access door and check location of needle.
With clean filters, the exhaust damper is adjusted to bring the gauge to .05 inches of water. The booth pressure should remain there during the paint cycle. If the supply blower and exhaust fan are adjusted properly with clean filters, the exhaust damper will be at approximately 45 degrees in the exhaust duct when this pressure is achieved in the booth on an Expert or BT system.
If the indicator on the gauge sets at .0, continue to spray cycle. To adjust, place a small screwdriver in the slot and turn right or left to reset indicator to .0. With the booth operating in spray mode, adjust the booth pressure by turning the knob until the indicator is between .0 and .05.
Type B (magnahelic gauge)
With clean filters, the exhaust damper is adjusted to bring the needle to the left side of the marked paint zone. The booth pressure should remain there during the paint cycle. If the supply blower and exhaust fan are adjusted properly with clean filters, the exhaust damper will be at approximately 45 degrees in the exhaust duct when this pressure is achieved in the booth on an Expert or BT system.
If the needle on this gauge is in the Paint Zone, continue to spray cycle. To adjust, place a small screwdriver in slot and turn right or left to reset until the needle is vertical. With the booth operating in spray mode, adjust the booth pressure until the needle is always in the blue spray zone.
As overspray continues to load the exhaust filter, the needle will move out of either spray zone. To bring the cabin pressure back to balance, manually adjust until the indicator reaches the spray zone. If the indicator still remains to the right of the spray zone, it is time to replace the exhaust filters.
Once the new exhaust filters are installed, check the position of the indicator and adjust if necessary.
Causes of Improper Airflow
Equipment out of adjustment or inoperative
- Supply blower
- Main exhaust fan
- Main exhaust damper
- Changeover damper (recirculating system)
- Burner profile plates and / or cold plates
- Discharge damper (forced dry system)
Foreign material blocks operations or airflow
- All ductwork
- Pit and / or tunnel
- All inlet or exhaust parts
Loaded, wrong, missing or incorrectly installed filters
- Plenum filters
- Pit paint arrestor
- Burner pre-filters
- Incorrectly assembled ductwork or equipment
- Too many elbow or transitions causing excessive pressure drop
- Restrictive ductwork or transitions causing excessive velocity of airflow (particularly in the plenum)
- Main exhaust stackhead incorrectly assembled; won’t open completely
- Changeover dampers