When purchasing an air compressor — especially a piston or reciprocating type — you may have run across the term “duty cycle”. What exactly does duty cycle mean when it comes to your air compressor? And why does the duty cycle matter? Read on for the answers — plus, how understanding duty cycles can help you extend the life of your air compressor.
What Is the Duty Cycle on an Air Compressor?
What does duty cycle mean? Put simply, the duty cycle of an air compressor is the amount of time it spends producing air vs. the amount of time it spends resting (also known as load/unload time or run/rest time). For example, the compressor may spend 40 seconds compressing air (loaded) and 20 seconds resting or idling (unloaded) over the course of a one-minute cycle (60% duty cycle), or five minutes loaded and five minutes unloaded over a ten-minute cycle (50% duty cycle). The total cycle time refers to the complete loading/unloading cycle.
You will most commonly see duty cycle referred to for reciprocating (piston-style) air compressors but it also applies to any compressor that cycles such as a rotary tyle that loads/unloads. All compressors are controlled by a pressure switch. When pressure (PSI) in the tank drops to a certain level (usually 15 to 30 PSI below the maximum pressure rating for the tank), the pressure switch turns the compressor on or loads. The compressor will keep making air until it reaches the maximum PSI it is set for (cut-out pressure). At that point, the compressor rests until pressure drops back down to the minimum level (cut-in pressure). The difference between the cut-in pressure and cut-out pressure is known as the “pressure band” that the compressor is operating within. Loading and unloading of the compressor may be accomplished by simply turning the motor on and off or opening and closing the inlet valve. For certain 100% duty compressors, loading and unloading is accomplished via a valve system that allows the motor to keep running without compression.
How to Calculate Air Compressor Duty Cycle
The formula of the compressor's duty cycle refers to:
Compressor Time On/(Time On + Time Off) = Duty Cycle Percentage
The Time On + Time Off is the total cycle length for the compressor, or the time from the start of loading in one cycle to the time of the start of loading in the next cycle. The Compressor On Time is the time out of that cycle that the compressor motor is actually running (the compressor is making air). So, if the total cycle time is ten minutes, and the time the compressor spends making air is four minutes, the duty cycle refers:
4 minutes/10 minutes = 40% duty cycle
Air Compressor Duty Cycles Rating Percentages Compared
Different air compressors are rated for different air compressor duty cycles. This determines the amount of time that the compressor can run-make air-out of a total cycle. The duty cycle is expressed as a percentage. The higher the duty cycle rating, the more time the compressor can spend loaded and making air. Air compressors with shorter duty cycles are more suitable for lighter-duty intermittent applications, while longer air compressor duty cycles are more appropriate for heavier-duty applications.
25% Duty Cycle Rating
A 25% duty cycle means the compressor is making air one-fourth of the time. So:
- 15 seconds run time out of a 60-second cycle
- 2.5 minutes run time out of a ten-minute cycle
- 7.5 minutes run time out of a 30-minute cycle
An air compressor with a 25% duty cycle ratings is best for applications requiring light, intermittent use, such as powering light hand air tools. A short duty cycle in the 25-30% range is more typical of the light-duty consumer air compressors sold for home use or very small shops.
50% Duty Cycle Rating
An air compressor with a 50% duty cycle can make air for half of the total cycle time. For example:
- 30 seconds out of a 60-second cycle
- 5 minutes out of a 10-minute cycle
- 15 minutes out of a 30-minute cycle
A 50% duty cycle compressor is suitable for small- to medium-sized operations that use air intermittently, such as a garage or mechanic shop. 50% capacity would be suitable for moderate use of air-powered hand tools such as nail guns and pneumatic wrenches or for occasional tire filling and pneumatic lifting. However, if air tools are used more or less continuously, it is probably best to look for a machine with a higher duty cycle ratings.
75% Duty Cycle Rating
A 75% duty cycle air compressor can make air for three-quarters of its cycle time. That translates to:
- 45 seconds out of a 60-second cycle
- 7.5 minutes out of a 10-minute cycle
- 22.5 minutes out of a 30-minute cycle
With a 75% duty cycle, the air compressor is better able to keep up with busy operations and heavy use. Pneumatic hand tools like nailers, drivers, wrenches, and hammers can be used more or less continuously; the small periods of rest in between each burst of air use are enough to keep operation within the duty cycle. A 75% duty cycle machine also works for manual spray painting and other medium-duty applications.
100% Duty Cycle Rating
An air compressor rated for a 100% duty cycle can deliver air continuously over the whole cycle time. This allows the compressor to be used for applications that require a continuous supply of air for many minutes or even hours, such as paint lines or conveyor systems. Essentially, the compressor can make air as fast as you are using it (up to its CFM rating).
Keep in mind that just because a compressor is rated for a 100% duty cycle does not mean that it can run 24/7. There is a difference between having a 100% duty cycle rating and run continuously. Most 100% duty cycle piston compressors still need time to cool down and rest between periods of heavy use. Check the owner’s manual for guidance on the maximum run time for your compressor.
Intermittent vs. Continuous Duty Cycle Air Compressors
An intermittent duty cycle compressor must have time to rest and cool down in between periods of making air. A continuous-run air compressor is designed to run for long periods of time — even indefinitely — without breaks.
Intermittent Duty Cycle Compressors
An intermittent duty cycle compressor needs to cycle down and cool off on a regular basis to prevent overheating and avoid excess wear and tear on the machine. Most piston (reciprocating) air compressors are rated for intermittent duty, even if they have a 100% duty cycle. A 100% duty cycle piston compressor may be able to provide air continuously at its rated CFM for ten minutes at a time, 30 minutes at a time, or even a couple of hours at a time before it needs to cool down. If the compressor has a built-in cooling system, it will be able to sustain longer run times. Some compressors are pressure lubricated for better cooling. Some 100% duty cycle compressors have a valve system that allows the electric motors to take air into the cylinders and exhaust for added cooling while the compressor is unloaded. Compressors with shorter air compressor duty cycles (e.g., 25%, 50%, 75%) have their required rest time built into the cycle.
Continuous Duty Cycle Compressors
A continuous-duty air compressor generally has a built-in cooling system and other features that make it safe to run for long periods of time without breaks to cool down. Very few piston air compressors are designed to handle continuous operation. If you need your compressor to make air for hours at a time or for 24/7 operation, an industrial rotary screw air compressor is the best choice. These air compressors are designed to handle large-scale conveyor systems, robotic machinery, and other applications requiring a consistent supply of air over long periods of time.
Why Does Duty Cycle Matter?
Understanding the duty cycle is important when sizing your air compressor and deciding what kinds of applications it is good for. The duty cycle impacts how much air your compressor is able to deliver over a period of time. If you use a lot of air over the course of a shift, an air compressor with a 25% or 50% duty cycle may not be able to keep up with your total air use, even if it has an adequate CFM rating.
Relationship Between Air Compressor Duty Cycle and CFM
An air compressor is rated for a certain volume of airflow (CFM) at a certain pressure (PSI). At 100% duty cycle or continuous run, that’s all you need to know: if the compressor is rated for 100 CFM at 100 PSI, that’s what it will deliver the entire time it is running.
The situation is more complicated for compressors with an intermittent duty cycle. Say you use a total of 10 cubic feet of air each minute for all of your air-using tools, on average, for a total of 600 cubic feet of air in an hour. A compressor rated for 10 CFM at a 50% duty cycle will only make air half the time, or up to 300 cubic feet of air in an hour. To make sure the compressor can keep up with your air use, you would need an air compressor rated at 10 CFM and a 100% duty cycle OR a compressor rated at 20 CFM and a 50% duty cycle. For compressors with a duty cycle of less than 100%, look at your total air use over a period of time and multiply the CFM rating by the total minutes of use and the duty cycle percentage (e.g., 0.5 for a 50% duty cycle).
Not sure how much air you need? We have tips for calculating compressed air requirements.
How Often Should an Air Compressor Cycle?
Different air compressors are designed with different optimal cycle times. Check your user’s guide for the recommended duty cycle. The duty cycle is usually based on how long it takes the compressor to fill the tank; in other words, the length of time the compressor must run between the cut-in pressure and the cut-out pressure. If it takes 5 minutes to completely fill the tank after cut-in and the compressor is rated for a 50% duty cycle, it should also have 5 minutes of rest before it loads again, for a total cycle time of 10 minutes. Factors that impact the cycle time for the compressor include the tank size, the CFM rating and the rate at which you are using air.
In general, longer duty cycles are better for the compressor than shorter cycles. Turning on and off frequently adds more wear and tear to your compressor motor and shortens the motor life. It also uses more energy, driving up operating costs. Avoid operating conditions that cause your compressor to cycle more frequently than recommended.
How to Prevent Air Compressor Over-Cycling
If air is being used faster than the compressor can keep up, it will spend more time loaded (making air) than recommended — a condition known as “over-cycling.” If a compressor is rated for a 50% duty cycle, but is running 75-100% of the time to try to keep up with air use, that is over-cycling, and it is very bad for your machine. Over-cycling will cause the machine to overheat and cause parts to wear out prematurely, substantially reducing the lifespan of your air compressor.
To prevent over-cycling of the air compressor, limit air use to the CFM that the compressor can keep up with within its rated duty cycle. If the compressor has been running for longer than its recommended duty cycle, stop air-using applications until the compressor has time to complete a full cycle with cool-down. If you find that your compressor is frequently over-cycling, that is a sign that you need a larger air compressor to keep up with your air use.
How to Extend the Cycle Time of a Compressor
To extend the cycle time of your air compressor, there are a few things you can do. Lengthening the cycle time (reducing the number of times your compressor turns on or off) will make your compressor more efficient and reduce wear and tear on the motor, starter, and other components. You can accomplish this in a couple of different ways:
- Get a larger storage tank. A larger tank will take longer for the compressor to fill, but the air will also last longer, extending the overall cycle time.
- Use a wider pressure band. If you drop the cut-in pressure, the tank will empty more before the compressor loads. It will take longer for the tank to load, but it will also take longer for PSI to drop to the cut-in pressure. Raising the cut-out pressure can have a similar effect, causing the compressor to make more air before cutting out. However, there usually isn’t much room to do this, as cut-out pressure is typically already set close to the maximum recommended PSI for the air tank. It is not safe to exceed the maximum PSI rating for the tank.
Conclusion: Which Air Compressor Is Right for You?
Understanding air compressor duty cycles and the difference between intermittent and continuous operation air compressors can help you make the right choice for your facility. For most medium size and larger industrial applications, that will mean a rotary screw air compressor rated for 100% duty cycle and continuous use.
Fluid-Aire Dynamics can help you calculate your CFM requirements and select the right reciprocating or rotary screw compressor for your operations. We sell, install, and service industrial reciprocating and rotary screw compressors and compressed air equipment. Talk to us about your compressed air system questions.