Decoding the Duty Cycle Rating of Piston Compressors
Selecting the correct air compressor for your application can be a complicated process. Before making a purchase, operators need to specify the quality and amount of air their application requires. Each compressor technology (rotary, piston, centrifugal, etc.) has an optimal flow output, so users should choose compressor technology based on the compressed airflow requirement of their application.
Some compressors, like rotary screw and centrifugal, are designed to run continuously at full speed while maintaining peak airflow (defined in cubic feet per minute, or CFM). The motors and cooling systems of these compressors are engineered to run 100 percent of the time without overheating. However, this isn’t true for all compressors.
Piston compressors do not have the cooling capabilities to run continuously for extended periods of time. Therefore, each piston compressor must have a specified flow output and pressure, as well as the percentage of time it can run without damaging the compressor. This percentage time running and stopped is referred to as the duty cycle of a compressor.
What is duty cycle?
Duty cycle, a term typically used as a rating for electric motors, is the time a compressor runs compared to the total cycle time (total time running and stopped). For example, a compressor with a total cycle time (Tc) of 10 minutes and a running time (Tr) of six minutes has a duty cycle of 60 percent.
Duty cycle is not an industry definition
While the compressed air industry has many standards that manufacturers adhere to, there is not an official definition for the duty cycle of a compressor. This lack of definition can create confusion, and operators may end up purchasing expensive compressors that do not meet their expectations or needs.
Some piston compressors claim to have a 100 percent duty cycle, but that is impossible. So what does is mean when you see a piston compressor with that rating?
Decoding the 100 percent duty cycle rating
Typically, piston compressors can generate over 150 psi of compressed air and have a large storage tank per horsepower compared to a rotary screw compressor of the same horsepower. (Remember, rotary screw compressors can run continuously, so they don’t need a large air receiver.) The piston compressor will fill the tank with high-pressure air that the user will deplete over time. Eventually, the compressor will turn back on to refill the tank.
A piston compressor that specifies a 100 percent duty cycle does not mean it can run continuously, but that it can provide air at a specific pressure and flow 100 percent of the time with the help of a storage tank. Let’s take a look at an example.
A compressor advertises a 100 percent duty cycle rating of 25 cfm at 100 psi. In reality, this compressor can create 42 cfm and has a tank size of 130 gallons.
Piston compressors typically use a pressure switch to control system pressure. This pressure switch controls when the compressor starts and stops depending on the system pressure. The pressure switch will have two pressure settings called a pressure band that uses a lower pressure setting as the cut-in or turn on pressure and a higher pressure setting as the cut-out or shut off pressure. A typical pressure band for a piston compressor is 30psi. For our example, this will allow the unit to turn on at a system pressure of 115 psi and turn off at a 145 psi.
As advertised, 25 cfm can be continuously provided from the storage tank for the application. When the system pressure dips to 115 psi, the compressor turns on and pumps the system up to 145 psi for 125 seconds before turning off.
After 83 seconds of continuous use at 100 psi and a flow of 25 cfm, the system pressure will have dropped back down to 115 psi, and the cycle will start over.
Tr (total running time) = 83 seconds
Tc (total cycle time) = 208 (83+125) seconds
Tr/Tc = Duty cycle 83/208 = .60 = 60%
From the above calculations, we can see that the piston compressor is actually running for 60 percent of the time. 60 percent is the typical running duty cycle for a piston compressor, ensuring a long and reliable life for the machine.
Don’t let duty cycle ratings disrupt your system
It is critical to know that the 100 percent duty cycle on a piston compressor does not mean the machine can run continuously. Doing so can damage the compressor, resulting in premature wear and higher maintenance costs.
If you have an application that requires your piston compressor to run for more than six minutes at a time (60 percent of the ideal cycle time of 10 minutes), consult an expert. You might need a larger compressor or different compression technology to meet the demands of your system.
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