Compressed Air

The significant impact of compressed air systems on workflow and product quality is often overlooked.

Compressor Location

Compressors are often installed where their noise, vibration, and heat will least bother staff and customers rather than where they will best perform and be easily serviced.

Rotary compressors offer much lower operating temperatures, up to 150 F cooler than the typical piston — and much lower noise levels, allowing for more flexibility on compressor location.

Maintenance and Long-Term Performance

Maintenance requirements and long-term compressor performance are essential factors to consider. Piston compressors and rotary compressors have different maintenance and service requirements.

Piston compressors have proven to be very reliable and require relatively little preventive maintenance other than periodic oil changes, replacing the air inlet filter, and maintaining proper belt tension. Rotary compressors also require these as well as oil filter and air/oil separator changes.

However, the pistons, cylinders, rings, and valves in reciprocating units wear over time, causing the compressor to deliver less air, and send more lubricating oil past the rings into the compressed air system. Without proper filtration and more frequent filter maintenance, this will cause contamination issues. Preventive maintenance will slow this process, but rebuilding a piston compressor periodically may be necessary to reduce the oil carry-over and reverse the gradual loss of performance. However, rotary screw compressors are designed so that the rotors do not touch each other or the rotor housing, therefore performance does not change over time.

Air Treatment

Many sources recommend improving paint finish quality by eliminating contamination in the spray booth, but compressed air quality plays an equally important role as well. Here are three basic types of air system contaminants and their effects: Water/moisture in the form of vapor, mist, and liquid may:

• Cause defects in finishes (“fish eyes”)

• Contaminate the repair surface

• Cause excessive wear in tools

• Cause rust in iron pipe

• Accumulate in storage tanks, reducing the volume available for storage and causing the compressor (piston) to run beyond its recommended duty cycle

For example: Assuming 24-hour operation, on a 75 F day with 75 percent relative humidity, a 10-hp compressor can introduce 7 gallons of water into a compressed air system. On a 90 F day with 90 percent humidity, the same compressor can introduce 15 gallons.

Particulates (dirt, dust, rust, etc.) build up in piping causing:

• Defects in paint finishes

• Surface contamination

• Pressure drop

• Excessive tool wear

Oil, usually in the form of vapors or mists, causes defects in finishes and can combine with particulate to clog tools and spray guns. It may also build up in piping and cause significant pressure drop.

In summary, these contaminants reduce system pressure, increase maintenance costs, and cause product defects (both cosmetic and safety critical). They increase labor and material costs for each aircraft worked on.

Air storage

Air storage tanks (receiver tanks) are a critical air system component. Almost every facility has one, and it is important to understand some of its functions and benefits:

• Provides the first stage of moisture separation • Stores air for short periods of high air demand • Allows the compressor to shut off to save energy and prevent piston units from overheating (if properly sized)

Here are a few guidelines for tanks:

• Allow at least 3 to 4 gallons per cfm • Pressure rating must exceed highest possible system pressure • Must have safety relief valve, pressure gauge, and drain to remove liquids • Must meet ASME or other required code (check with local authorities)


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