Q: Are rotary compressors best for applications over 10hp?
A: There is no definitive cutoff point when it comes to horsepower. That’s why some manufacturers offer rotary screw compressors with as little as 3hp or piston compressors with up to 100hp. Although these extreme examples aren’t very common, they do serve a purpose.
A more appropriate place to start for selecting a compressor is to determine the constant air demand, or duty cycle. For example, a job shop with a CNC machine will have fairly steady demand, whereas an automotive shop with an impact wrench may only use air for short, periodic durations.
Although some piston compressors on the market are liquid cooled, more typical ones are air cooled, so they normally have duty cycles up to 75 percent. Even though they have a fan built into the flywheel to help cool the pump, piston compressors must cool at least 25 percent of the time. This prevents overheating, since approximately 2,500 btu/hp is generated when air is compressed.
On the other hand, all rotary compressors are liquid cooled, so they can run continuously to achieve 100-percent duty cycle. Therefore, if an operation’s air tools require up to 100 CFM, and the demand stays fairly constant at 100 CFM throughout the day, a rotary compressor is likely the only option. However, if demand fluctuates wildly from 100 CFM one minute to 10 CFM the next, a piston compressor may also be practical, since it will have time to cool in between periods of compressing air.
Information provided by: Daniel Leiss, president, Jenny Products, Inc.
