Air Make-up (AMU)
is defined as…A mechanical means of replacing air that has been exhausted.
In relation to a paint spray booth, when the exhaust fan of the booth is turned on it creates a “negative” air pressure in the booth cabin. To counter this “negative” pressure an AMU (air make-up) unit is used to supply air to replace the air being exhausted. If the AMU is designed to force more air into the booth than is exhausted…the air pressure (or booth) is said to be “positive”.
Controlling the air going into the booth, with relation to the air being exhausted from the booth, is critically important to maintain the proper “flow” of air over the item being painted and remove the overspray effectively and efficiently.
“Tube axial” is a propeller blade type fan. Tube axial is the best fan to be used on the exhaust of the booth because they are designed to “pull” the air, just as an airplane’s propeller pulls air over the aircraft’s wing. Aluminum blades are used because they will not spark (causing a fire) if the blades become dislodged and scrape against the steel fan housing.
A well designed AMU will use a “Blower” type fan (squirrel cage) because blowers are designed to “push” the air (into the booth). The heater in your car has a blower which is used to push the heat thru the heater vents.
Controlling the airflow and pressure is a delicate balance between input air and exhausted air.
Example…If the booth is exhausting 10,000 CFM (cubic feet per minute) and the input blower of the AMU is forcing in 10,000 CFM the booth is said to be perfectly “Balanced”. But, if the painter opens the door to enter the booth, it will cause the booth pressure to go negative and suck in dirt and contamination thru the open door. To combat this, more air is pushed into the booth (say 12,000 CFM) than is being exhausted…creating a “positive” condition, by which, when the painter enters the booth, a slight amount of air is pushed out controlling contamination.
The difficulty occurs when the item to be painted is placed into the booth. The size of the item will take up a certain amount of volume in the booth. If you continue to force the same amount of air (12,000 CFM) into a booth, which now has a reduced volume, you will over-pressurize the booth.
Note…Over-pressurization is the # 1 cause of dirt contamination in a booth. It severely disrupts the designed airflow in the booth and causes uncontrolled turbulence.
Another difficulty occurs as the exhaust filters become clogged with overspray. As the filters clog the exhaust fan can not remove enough air from the booth. The example of 10,000 CFM may reduce down to 8,000 or less. If the AMU continues to force the same 12,000 CFM in the booth, it will again over-pressurize and contaminate the finish.