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Compressors (Reciprocating, Scroll, Rotary, Screw)

HVAC compressors are grouped by how they compress gas: reciprocating, scroll, rotary and screw. This sheet covers each type, how to read a compressor nameplate and what must match when one is replaced.

Ref HV-0012UL 60335-2-34

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

The compressor raises the pressure and temperature of refrigerant vapor so the system can reject heat. Reciprocating compressors use pistons and valves. Scroll compressors use one fixed and one orbiting spiral. Rotary compressors use a rolling piston or vane in a cylinder. Screw compressors use one or two meshing rotors and are used on larger chillers. Each can be hermetic (sealed in a welded shell), semi-hermetic (bolted shell that can be opened) or open (motor outside the compressor). Capacity can be fixed speed, two-stage, digital or variable speed (inverter).

How to identify

Read the nameplate. It shows maker, model, serial, refrigerant, voltage and phase, rated load amps (RLA), locked rotor amps (LRA) and often the oil type. Shape helps too. Scroll compressors are tall cylinders. Rotary compressors in mini-splits and PTACs are smaller, with an external suction accumulator beside the shell. Semi-hermetic reciprocating compressors have bolted heads and cast bodies. Single-phase compressors have three terminals marked C, S and R.

Compressor types

Type	How it compresses	Typical equipment	Notes
Reciprocating, hermetic	Pistons in a sealed shell	Older residential, small commercial, refrigeration	Valves can fail; tolerates some liquid poorly
Reciprocating, semi-hermetic	Pistons, serviceable bolted body	Commercial refrigeration, older chillers	Can be rebuilt
Scroll	Orbiting spiral against fixed spiral	Residential and light commercial AC and heat pumps	Few moving parts; rotation direction matters on 3-phase
Rotary	Rolling piston or vane	Mini-splits, PTACs, window units	Often inverter driven
Screw	Meshing rotors	Chillers and large refrigeration	Large oil separation systems

Key points

A replacement must match the refrigerant, oil type, voltage and phase, and capacity, and must be approved by the equipment or compressor maker.
Three-phase scroll compressors run backward if two legs are swapped. They are noisy and do not pump.
Inverter compressors are driven by a matched board. They are not run directly on line power.
After a compressor burnout, plan for an acid test, suction line drier and oil change. See filter driers.
RLA and LRA are for wiring and protection decisions by the installer. Overcurrent sizing comes from the nameplate and code, not the counter.

Common mistakes

Replacing a compressor without matching the oil type to the refrigerant.
Ignoring rotation on a three-phase scroll.
Swapping a fixed-speed compressor into an inverter system.
Skipping the start components check on single-phase units.

Governing standards: UL 60335-2-34 (safety of motor-compressors); equipment and compressor maker data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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