By Marcus Lindqvist · Updated September 1, 2026 · 6 min read
Short answer
Hermetic compressors seal motor and compressor in a welded shell and are replaced when they fail. Semi-hermetic compressors use a bolted housing, with the motor inside the refrigerant circuit, so they can be opened and overhauled. Open-drive compressors place the motor outside with a shaft seal, which allows any motor type and is required for ammonia because it attacks copper windings.
The terms hermetic, semi-hermetic and open describe how the motor and the compressor are packaged, not how the compressor compresses gas. A scroll, screw or reciprocating compressor can come in more than one housing. The housing decides serviceability, leak points, how the motor is cooled and which refrigerants the machine can use. For a wider view of compressor technologies, see our guide to refrigeration compressor types.
What is a hermetic compressor?
In a hermetic compressor, the motor and compressor share one welded steel shell. There is no shaft seal, so the main leak path of other designs is gone. The motor is cooled by suction gas flowing past it, and the oil sump sits inside the shell.
- Strengths: compact, quiet, low leak risk, low cost.
- Limits: the shell cannot be opened in the field, so a failed unit is normally replaced. A burned motor spreads contamination through the system; see compressor burnout: causes and testing.
- Typical use: residential and light commercial air conditioning, small refrigeration, most scroll compressors.
What is a semi-hermetic compressor?
A semi-hermetic compressor, sometimes called accessible hermetic, also puts the motor inside the refrigerant circuit, but the housing is bolted together with gaskets rather than welded. Heads, valve plates, bearings and sometimes the motor can be removed and replaced.
- Strengths: serviceable, durable, available in a wide range of sizes; many commercial and mid-size industrial units use this design.
- Limits: the gasketed joints can leak over time, and the motor still sits in refrigerant and oil, so electrical or lubrication faults can contaminate the system.
- Typical use: commercial refrigeration racks, supermarket systems, mid-size chillers, many reciprocating and screw compressors that run on HFC or HFO refrigerants.
What is an open-drive compressor?
An open-drive compressor has its motor outside. A shaft passes through the compressor housing and is sealed by a mechanical shaft seal. The motor connects by a coupling or belt, or the compressor can be driven by an engine or turbine.
- Strengths: the motor is not exposed to refrigerant, any suitable motor can be used and replaced separately, and the compressor can run on refrigerants incompatible with copper windings.
- Limits: the shaft seal is a wear item and a leak point. Alignment between the motor and compressor matters, and the motor needs its own cooling and space.
- Typical use: ammonia plants, large industrial refrigeration, some automotive and mobile systems.
How do the three compare?
| Factor | Hermetic | Semi-hermetic | Open drive |
|---|---|---|---|
| Housing | welded shell | bolted housing | compressor body only, motor separate |
| Motor location | inside refrigerant circuit | inside refrigerant circuit | outside, shaft seal |
| Motor cooling | suction gas | suction gas, sometimes added cooling | air or separate motor cooling |
| Field service | replace the unit | overhaul possible | full service, motor separate |
| Main leak paths | very few | gasketed joints | shaft seal and joints |
| Alignment needs | none | none | coupling alignment required |
| Typical size range | small to mid | mid | mid to very large |
| Ammonia suitable | no | no | yes |
Why does ammonia require open drive?
Ammonia attacks copper and copper alloys. Hermetic and semi-hermetic motors use copper windings, so ammonia would corrode them. Open-drive compressors keep the motor away from the refrigerant, and ammonia compressors are built without copper or brass in contact with the refrigerant. Ammonia systems are covered in ammonia refrigeration system basics.
How is the motor cooled in each design?
- Hermetic and semi-hermetic: suction gas passes over the motor on its way to the compression chamber. This is efficient, but it means motor heat is added to the refrigerant. High suction superheat or low refrigerant flow can overheat the windings.
- Open drive: the motor is cooled like any industrial motor, by fan or ambient air. Cooler suction gas goes into the compression chamber, which is a small efficiency advantage, and the motor heat is rejected to the room instead.
Overheating and loss of oil return are common causes of motor failure in all three types. For other warning signs, see five signs a compressor is wearing out.
How should you choose?
- Refrigerant. Ammonia requires open drive. HFCs and HFOs can use any housing.
- Capacity. Small loads suit hermetic. Mid-size commercial and industrial work uses semi-hermetic. Large plants often use open drive.
- Serviceability. If you want to overhaul rather than replace, choose semi-hermetic or open.
- Leak risk and regulation. Fewer leak paths reduce refrigerant loss and compliance work. Shaft seals need monitoring.
- Space and noise. Hermetic units are compact. Open-drive machines need floor space and alignment.
- Contamination risk. A motor in the refrigerant circuit means an electrical failure contaminates the system, which raises the cost of failure.
Frequently asked questions
Is a semi-hermetic compressor the same as a hermetic one?
No. Both enclose the motor in the refrigerant circuit, but a hermetic shell is welded shut and a semi-hermetic housing is bolted and can be opened.
Do open-drive compressors leak more?
They have an extra leak path, the shaft seal, which wears and needs checks. Modern seals perform well when aligned and maintained.
Can I identify the type from the nameplate?
The model number or catalog tells you the housing type. See how to read a compressor nameplate.
SpecServis engineers in Ukraine work with hermetic, semi-hermetic and open-drive machines. 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.
