By Marcus Lindqvist · Updated September 4, 2026 · 7 min read
Short answer
A reciprocating compressor uses pistons driven by a crankshaft to draw in refrigerant gas through a suction valve, compress it in a cylinder and push it out through a discharge valve. Cylinder unloaders reduce capacity by taking cylinders out of service. Valves, rings, bearings and oil are the main wear points. The design is rugged and serviceable, so it remains common in commercial refrigeration, low-temperature duty and industrial plants.
A reciprocating compressor compresses refrigerant with pistons that move back and forth in cylinders, like a small engine pump. On the downstroke the piston draws low-pressure gas in through a suction valve. On the upstroke it squeezes the gas, and when the pressure is high enough a discharge valve opens and releases it toward the condenser. A crankshaft turns the motor’s rotation into the piston motion. The design has been used for over a century and is still common because it is robust, handles high pressure ratios and can be repaired.
What are the main parts?
- Crankcase and crankshaft. The housing holds the oil and supports the crankshaft, which has offset journals, one for each connecting rod.
- Connecting rods and pistons. Rods link the crankshaft to the pistons. Piston rings seal against the cylinder wall.
- Cylinders and head. Cylinders can be in-line, V or W arrangements. The head holds the valve plates.
- Suction and discharge valves. Spring-loaded reed or ring-plate valves that open by pressure difference.
- Oil pump. Delivers oil to bearings and the shaft seal on larger machines. Small ones splash-lubricate.
- Motor. Hermetic and semi-hermetic units have the motor inside the shell. Open drive units use an external motor with a shaft seal. See hermetic vs semi-hermetic vs open compressors.
- Unloaders and controls. Mechanisms for capacity control, plus pressure and oil safety switches.
How does the compression cycle work?
- Suction stroke. The piston moves down. Pressure in the cylinder drops below suction pressure and the suction valve opens, letting gas in.
- Compression stroke. The piston moves up. Both valves are closed and the gas is squeezed, raising its pressure and temperature.
- Discharge. When cylinder pressure exceeds discharge pressure, the discharge valve opens and the hot gas flows out.
- Re-expansion. A small volume of gas, called clearance volume, remains at the top of the stroke. It expands again as the piston moves down, delaying the next suction until pressure falls below suction pressure.
Clearance volume and valve losses are the main reasons that volumetric efficiency drops as the pressure ratio increases. That is also why a reciprocating compressor is often selected for low-temperature or high-pressure-ratio work, where it can be staged or fitted with extra cooling.
How is capacity controlled?
| Method | How it works | Notes |
|---|---|---|
| Cylinder unloading | Suction valves are held open on selected cylinders so they do no compression | Common on semi-hermetic and open compressors, gives steps of capacity |
| Speed control | A variable frequency drive changes motor speed | Needs attention to oil return and lubrication at low speeds |
| Multiple compressors | Several machines staged on a rack | Standard in supermarkets and cold storage |
| Hot gas bypass | Discharge gas is routed back to suction | Simple, but wastes energy |
Variable speed is covered in more depth in variable speed and VFD compressors.
How is a reciprocating compressor lubricated?
Oil lubricates the crankshaft and rod bearings, cylinder walls and shaft seal, and also helps seal the rings. Some oil is carried out with the discharge gas, so the system must return it through an oil separator and properly sloped piping. Common oil types, such as mineral, POE and PAG, are matched to the refrigerant, as explained in compressor oil types: POE, PAG and mineral. Oil analysis can show bearing wear and contamination, as we describe in what oil analysis tells you.
What are the common wear points?
- Valves. Broken or leaking reeds and plates reduce capacity and can overheat the gas. Liquid or debris is a frequent cause.
- Piston rings and cylinder walls. Wear leads to blow-by and lower efficiency.
- Bearings. Poor lubrication, dilution of oil by liquid refrigerant or contamination causes scoring.
- Shaft seal. Open drives can leak refrigerant or oil at the seal.
- Motor windings. Electrical stress, heat and acid in the system lead to failure. See compressor burnout: causes and symptoms.
- Liquid slugging. Liquid does not compress. Entering a cylinder, it can break valves, rods or the crankshaft.
Typical warning signs include knocking noises, high discharge temperature, low oil pressure, falling capacity and rising amp draw. For a fuller checklist see five signs a compressor is wearing out and compressor failure causes.
Where are reciprocating compressors used?
- Supermarket and convenience store racks
- Walk-in coolers and freezers and other commercial refrigeration
- Low-temperature and blast freezing processes
- Industrial ammonia plants, especially in older installations
- Some transport refrigeration and process cooling
In air conditioning and small chillers, scroll and screw designs have taken a large share, as compared in refrigeration compressor types guide.
Frequently asked questions
Why do reciprocating compressors get hotter at low suction pressure?
At low suction pressure, the compression ratio rises, so the gas leaves each stroke much hotter. Less returning gas also means less cooling of the motor on suction-cooled machines. Manufacturers set operating envelopes and may require liquid injection or additional cooling for extreme conditions.
Can a reciprocating compressor be rebuilt?
Many semi-hermetic and open reciprocating compressors can be overhauled by replacing valves, rings, bearings and seals. Whether it makes sense depends on the damage, parts availability and cost versus a new or remanufactured unit. We compare the options in compressor rebuild vs replace.
Why are reciprocating compressors noisier than scrolls?
The pistons start and stop twice per revolution and the valves open and close each stroke, so gas flow is pulsating and vibration is higher. Scroll compressors move gas continuously and have no suction valves.
SpecServis engineers in Ukraine work with compressors, chillers and refrigeration plants. We are planning to enter the Central Texas market and are looking for technicians, partners and a company to buy. See our open positions or partnership options.
