“Air Make-up 101”

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.

Air flow

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.

Balancing Air Flow…

A “Top of the Line” AMU will have an “Automatic Balancing” system. Via an air pressure sensor placed in the booth the AMU will sense the amount of air needed in the booth and automatically control the amount of air entering the booth. In that way, no mater what the booth’s volume or as the filters become clogged and less air can pass thru them, the booth will always sense the necessary amount of air needed and balance itself off.

“Recirculation” vs. “Recycle”

Watching paint dry…the faster the paint dries the better the production, the less the booth runs and the lower the energy costs. Paint dries because the solvent evaporates off the painted surface, thereby, leaving the hardened paint.Twenty some years ago Air Make-up units were designed to heat the incoming air, pass it over the item to be painted only once, and then exhausted. This, “fresh air”, method was excellent because the solvent would quickly evaporate and be discharged out the exhaust and the paint would dry fast, leave a hard finish and the desired gloss… But, because of the tremendous energy consumed by this method the costs were prohibitive.

AMU systems were quickly designed that would “Re-circulate” the heated air thru the booth than back thru the AMU Heater and back to the booth…over…and over…and over again. This reduced energy costs but the air contained evaporated solvents and the solvents were also being re-circulated…over…and over…and over. The problem was the air is acting as a sponge, absorbing the solvent. In re-circulating the same air, this air, (sponge) has a limit and can only absorb so much…you can see the problem. Longer drying times, meaning high energy costs and no “fresh air” over the painted item.

The new Millennium brought a new technology…”Recycle”. In this technology the air would recycle itself in the burner before entering the booth, allowing much higher air temperatures and use far less energy. Once the air passes thru the booth, and becomes “solvent laden” and it is exhausted…100%. This method produces the quickest dry times, the hardest finishes and, by far, the lowest energy costs.

“Cost of Operation”…

Cost of operation is always a major concern with any piece of equipment. Knowing the cost of operation for an AMU is simple to calculate. If you know the following criteria you can easily calculate the “Cycle Cost” (the cost of energy to cycle an AMU thru a complete spray cycle combined with a complete bake cycle). (1) The cost of fuel, (2) the temperature outside, (3) the desired spray and bake temperature and (4) the time the booth is in operation. With that information any AMU representative can give you the cost. Knowing the “Cost of Operation” can be more important than the cost of the equipment. Know one expects the cost of energy to go down and an AMU will last 20 years.