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What is the difference: Reciprocating vs Rotary Screw Compressors?

by Nick Calvo on May 06, 2020

Reciprocating vs Rotary Screw

Reciprocating or Piston Type

Reciprocating or Piston-Type compressors pump air by action of pistons. Drawing in air on the down stroke and discharging air on its upstroke. Reciprocating compressors can be single-stage or two-stage, while two-stages of compression creates greater efficiency. These compressors are designed to run for a period, and then have down time to cool. High pressures are achievable with piston compressors because the compression cycle is positive displacement. Multiple stages allow for higher pressures to be reached. An example of this would be breathing air compressors that must recharge air tanks rated for 4500psi.

The maximum operating duty cycle on a reciprocating compressor is 70%, optimal is 50%.
The maximum operating duty cycle on an oil free reciprocating compressor is 50%, optimal is 30%.

 

Rotary Screw

Rotary Screw uses two helical screws that rotate with close clearances next to each other. Air travels through the grooves and is compressed as the channels volume decreases, which increases pressure. Rotary compressors have fewer moving parts and excellent cooling capabilities, meaning they can run 100% of the time without needing down time to cool. Oil is used to cool, lubricate, and seal the screws – which must be separated from the air after the compression cycle. Due to the nature of lubricating the screws, there is more oil present in the discharged air. Separation is achieved by use of an Air/Oil separator.

The maximum operating duty cycle on a rotary screw compressor is 100%, optimal is 70%.



Comparison by compressor type

Reciprocating Rotary Screw
Advantages
  • Simple design
  • Low initial cost
  • Two-stage models makes them more efficient
  • Higher discharge pressure
  • Excellent cooling capabilities
  • Quiet Operation
  • Designed to run 100% of the time (100% duty cycle)
Disadvantages
  • Loud operation
  • Heat caused by friction of pistons requires down time to cool
  • Higher maintenance cost
  • Not designed to run 100% of the time
  • High rotational speeds
  • Oil injected results in oil carryover
  • Difficult to operate in dirty environments

 

Average CFM per Horse Power by compressor type

 Reciprocating CFM @ 175 PSI Rotary Screw CFM @ 150 PSI CFM @ 125 PSI
5HP 18 5HP 16 18
7.5HP 24 7.5HP 26 38
10HP 35 10HP 38 40
15HP 50 15HP 54 60
20HP 80 20HP 78 85
25HP 95 25HP 102 108
30HP 125 130
40HP 155 160
50HP 185 200
60HP 210 235
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