Compressor
Positive displacement using pistons driven by a crankshaft. Single or two-stage.
Typical range
1–30 HP, up to 175 PSI (single-stage ~125 PSI)
Best for
Intermittent duty, small shops, portable use.
Operating principle
Pistons driven by a crankshaft draw air through intake valves, compress it on the upstroke, and discharge through discharge valves. Single-stage compresses once; two-stage passes air through an intercooler between stages for higher pressures and cooler running.
Efficiency notes
70–90% isothermal efficiency two-stage; 50–65% single-stage. Volumetric efficiency drops as the pressure ratio rises — multi-staging is needed above ~125 PSIG.
Selection guidance
Choose by duty cycle (intermittent <60%), max pressure, and CFM. Two-stage above 175 PSIG or for continuous shop air. Avoid for >50% duty — a rotary screw is more efficient there.
Components
- Crankcase & crankshaft
- Pistons, rings, cylinders
- Intake & discharge valves (reed/plate)
- Intercooler (two-stage)
- Aftercooler
- Pressure switch
- Unloader valve
- Check valve
- Receiver tank
Common issues
- Worn rings → oil carryover, low pressure
- Stuck/broken valves → loss of capacity, overheating
- Unloader failure → motor stalls on restart
- Loose belt → slow pump-up, squeal
Maintenance
- Daily: drain tank, check oil level, check for leaks
- Monthly: clean intake filter, check belt tension
- 500 hrs / annual: change oil, inspect valves, torque head bolts
Field tips
- Belt-driven units: check sheave alignment at every PM
- Pulsation dampeners reduce pipe vibration on multi-cylinder units
- Synthetic oil extends valve life 2–3× in intermittent service