As with electricity and other utilities, compressed air is usually taken for granted. It’s often when there’s a problem and not much can get done in the shop without it that shop owners realize the importance of a good, reliable compressed air system.
Further, interruptions with compressed air in a fleet maintenance shop can be significant, as the financial impact of not having vehicles on the road will be much greater than the cost to repair the air system itself.
The reliability of the system goes beyond the quality of the compressor you buy. It’s about how the system is designed and maintained.
The good news is it isn’t rocket science. If you build and operate your system to avoid common problems – including inadequate flow, leaks and pressure drop – you’ll save time and money in keeping your business going.
Operating at the correct pressure – First, don’t confuse pressure and flow when sizing a compressor. What determines the amount of air you get at your tools is flow, which is expressed in CFM (cubic feet per minute).
Flow is determined by the size and efficiency of the compressor pump and the motor driving it. It’s common to refer to compressors by horsepower, but compressed air system professionals prefer to size compressors by cubic feet per minute (CFM).
A rule of thumb is that piston compressors produce 3 to 4 CFM per horsepower (hp) and rotary compressors produce 4 to 5 CFM per hp.
Any compressor can be configured to operate at a range of pressures. Whether it’s a 5 hp or a 25 hp unit, the typical two-stage piston found in automotive service facilities can run up to 175 pounds per square inch gauge (psig). But you won’t get much more than 15 CFM from the 5 hp one. Similarly, 5 hp rotary screw compressors are available from 80 to over 200 psig, but you won’t get much more than 20 CFM from it.
It’s important to know that the higher the pressure, the less flow you will get from a compressor.
Many shops operate their compressors at 145 or 175 psig. But in fact, very few tools require pressures above 100 psig.
Avoid a common pitfall, select the correct pressure. Don’t over-pressurize. The higher the system pressure, the greater the volume lost through leaks and the higher the energy consumption.
Determining demand –When sizing your air compressor and receiver tank, add together the flow required by the largest air consumer at each work station, then multiply the total by the percentage of time each tool is used. This requires some study of how the different parts of the repair shop operate throughout the day.
For larger shops, it may make sense to have a compressed air professional measure and record compressor usage with electronic data loggers. Though it may sound complicated and expensive, it isn’t.
DUTY CYCLE AND TANK CONSIDERATIONS
Duty cycle considerations – When selecting compressors, you must consider the CFM delivered and the recommended duty cycle. Duty cycle is the percentage of time a compressor can operate without the risk of overheating and causing excessive wear to the compressor.
A piston compressor may provide adequate flow for a short period, but most commercial piston compressors are limited to 60 to 70 percent duty cycle. For this reason, piston compressors are usually oversized to allow the compressor to periodically shut down and cool off because of the high operating temperatures (300 to 400 degrees F).
Rotary compressors, on the other hand, have fluid cooling circuits that allow 100 percent duty cycle (i.e., 24/7 operation) if needed.
Tank considerations – Tanks do two important things: 1.) They store up air so that the compressor can shut off. This saves energy in all compressors, but is also critical for the long-term health of piston compressors because it allows them to cool down. 2.) The tank is used to remove water and other contaminants from the air stream.
Tanks require very little attention or service, but water can accumulate very quickly (especially in warmer months), eliminating space for air storage.
How to choose the appropriate shop air compressor