How to recognize a type of air compressor

This article explains how to quickly identify the type of compressor present in an industrial installation. Visual, mechanical and acoustic clues make it possible to accurately assess the technology used: external structure, operating noise, nameplate and key components.

1. General observation

Overall shape: pressurized or non-pressurized oil reservoir, visible cylinders, sound-insulated enclosure.
Cooling: axial fan or large radiator → often a screw compressor.
Number of stages: different cylinder diameters → good indicator of a piston compressor > 150 PSIG.
Nameplate: ACFM/FAD, HP/kW, maximum pressure.
Connections: air outlet, automatic drain, cooler (radiator).

2. Questions to identify the compressor type

Question	Identification clue
Is there a single outlet pipe?	Typically a compressor
Significant ventilation / expelled heat?	Screw compressor
Flow shown in ACFM or FAD? (dryer in SCFM)	Screw, piston or scroll
Reciprocating (pop-pop) noise?	Piston compressor
Very compact and quiet machine?	Screw or scroll
Two pistons of different diameters?	Piston compressor > 150 PSIG
Does the motor speed vary?	VSD (Variable Speed Drive)

3. Confirming the compressor type

Indicator	Interpretation
Air/oil separator	Lubricated screw or vane compressor
Exposed cylinders	Piston compressor
Multiple stages (different diameters)	High-pressure compressor
Two compression blocks / inter-stage cooler	Oil-free screw

4. Modulation modes

Mode	Description	Field observation
Load / Unload	Motor always running, compressing and unloaded	Cyclic noise, pressure switch
Modulation	Inlet restriction, throttling at the intake	Stable pressure, modulating valve
VSD	Variation of the motor speed; the sound level changes slowly	Sound variation, VFD panel
Start / Stop	Motor stops when the pressure is reached, restarts when the pressure drops to the start point	Mainly small compressors

Pressure curve — load/unload modulation — Load / unload.

Pressure curve — start/stop modulation — Start / stop.

Pressure curve — inlet modulation — Inlet modulation.

Pressure curve — variable speed (VSD) — Variable speed (VSD).

5. Summary method to recognize a compressor

Observe the external structure (cylinders, oil reservoir, fan).
Listen to how it runs: continuous = screw; reciprocating = often piston.
Analyze the nameplate: ACFM/FAD = industrial compressors.
Identify the key components: air/oil separator, coolers.
Check the modulation: load/unload, VSD or start/stop.

Internal reference: “Identifying an air compressor” fact sheet, Onyx M3.

Frequently asked questions

How can I tell whether a compressor is a screw or a piston type?

A screw compressor runs continuously, has a sound-insulated enclosure, significant ventilation and an air/oil separator. A piston compressor has a reciprocating (pop-pop) sound and exposed cylinders.

How do I recognize a high-pressure compressor (> 150 PSIG)?

Look for several compression stages: two pistons (or blocks) of different diameters with an inter-stage cooler are a good indicator of a piston compressor above 150 PSIG.

How can I tell whether a compressor is variable speed (VSD)?

The motor speed varies: the sound level changes slowly and a VFD (variable frequency drive) panel is present. The other common modes are load/unload, inlet modulation and start/stop.