Reciprocating (Piston) VS Rotary 5 Hp. to 25 Hp.
Until fairly recently, single acting one and two stage air compressors were the only economically viable solution in this size range. However, through improvement in design and production, rotary screws may or may not be preferable as a compressed air producer. The variables needed to determine which is best are:
Hours run per month or year?
Is the compressed air demand steady during operations or does it vary greatly?
Is the environment clean or dirty?
What are the pressure requirements on the compressed air system?
In general, reciprocating compressors such as the Curtis ML or CA series, single acting, two stage, air cooled units fill most of the applications in the size range. These compressors do well under wide ranging demands as they are relatively efficient at light load and cost less to buy. However, for maximum service life these compressors should be sized so that the maximum demand placed upon them does not exceed 75% to 80% of their full rated output for periods of more than a half hour. Exceeding this can lower the service life considerably and in extreme cases can lead to valve refurbishing or replacement in less than a year. The use of diester synthetic lubricants will increase the valve service life under high loads, but the life of the compressor is shorter than if was at 75% or lower load factor. In dirty conditions, reciprocating compressors are less affected than rotary compressors. Dirty conditions shorten the service of all air compressors.
Rotary air compressors are best suited for steady loads with long hours of operation. Under 24 hours a day, 7 days a week, a rotary screw will last longer than a reciprocating compressor. However it is not as energy efficient under light loads. This is an important consideration as under full load conditions (80 hours a week) a compressor will consume its purchase price in electricity in one year in most areas on the United States. At higher pressures (175 PSIG), seen in the auto and truck service industries, rotary compressors do not produce as much volume (CFM) as comparable two stage piston compressors. Rotaries generally cost more to buy and annual maintenance costs are higher. However under steady high loads their 100% duty cycle can allow for the purchase of a smaller compressor as compared to a piston machine. As an example under a steady load a 10 Hp rotary maybe able to do the job that would require a 15 Hp at 75% load.
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