Hermetic compressors are characterized by their permanently sealed steel shell, their internal motor-and-pump assembly, and a set of measurable performance, electrical, thermal, acoustic, and mechanical properties. Understanding these characteristics helps when selecting, sizing, or troubleshooting compressors in refrigeration and air conditioning systems.
Sealed Shell Construction
The defining physical characteristic of a hermetic compressor is its welded, gas-tight enclosure. The shell is made from steel sheet, with a top cover welded to a bottom housing to create a permanently sealed unit. This means the refrigerant circuit is completely closed off from the atmosphere, eliminating the shaft seals that open-type compressors require and removing the risk of refrigerant leakage at those points.
Inside the shell, the mechanical assembly (motor, cylinder, and valve plate) is suspended on four springs. These springs keep the moving parts centered within the housing and absorb vibration so it doesn’t transfer directly to the shell walls and into the surrounding structure. Because the shell is welded shut, a hermetic compressor cannot be opened for internal repair. If an internal component fails, the entire compressor must be replaced, which is the primary trade-off for the leak-proof design. Semi-hermetic compressors, by contrast, use bolted housings that technicians can disassemble for maintenance.
Compression Mechanism Types
Hermetic compressors are further characterized by which mechanical method they use to compress refrigerant. The three most common types are reciprocating, scroll, and rotary.
- Reciprocating: One or more pistons driven by a crankshaft move back and forth inside cylinders. Cylinders can be arranged in inline, V, or W configurations, ranging from 1 to 16 cylinders. Small reciprocating compressors draw less than 10 kW, medium units 10 to 50 kW, and large machines 50 kW and above.
- Scroll: Two spiral-shaped scrolls nest together. One remains stationary while the other orbits around a fixed point, trapping and compressing gas between the scroll surfaces. Scroll compressors typically operate in the 5 to 35 kW range and are common in residential air conditioning, heat pumps, and automotive climate systems.
- Rotary: A rolling piston or rotating vane compresses gas against the cylinder wall. Rotary hermetic compressors are compact and widely used in smaller residential refrigeration and window air conditioners.
Motor and Electrical Characteristics
Because the electric motor sits inside the sealed shell, it runs in direct contact with the refrigerant. Hermetic compressors use several types of single-phase induction motors, each offering different starting torque. The most common configurations are PSC (permanent split capacitor), CSR (capacitor start, capacitor run), CSIR (capacitor start, induction run), and RSIR (resistance start, induction run). PSC motors are the simplest and least expensive but produce the lowest starting torque. CSR motors, which add a start capacitor, deliver significantly higher torque for applications with greater startup loads.
Two key electrical ratings define how a hermetic compressor interacts with the electrical system. Rated Load Amps (RLA) is the current draw at full cooling capacity and is used to size starters, disconnects, and circuit breakers. Locked Rotor Amps (LRA) is the current the motor would draw if its rotor were held completely still while energized. LRA is typically five to seven times the motor’s full load amps. This ratio matters when selecting starter types: a star-delta starter draws roughly 33% of LRA during startup, while a solid-state starter draws about 45%.
Thermal Management Through Refrigerant Cooling
With the motor sealed inside the compressor shell, waste heat from motor losses has to go somewhere. In hermetic and semi-hermetic compressors, the suction gas itself serves as the coolant. Refrigerant flows over and around the motor windings on its way to the compression chamber, absorbing heat before entering the suction port. This eliminates the need for a separate motor cooling system and prevents the refrigerant and oil leakage that can occur at shaft seals in open-type designs.
The cooling passages inside the motor need to be carefully arranged to match where heat is generated most intensely. If suction gas temperature rises too much as it passes the motor, the compressor loses efficiency because it’s compressing hotter, less dense gas. Designers balance cooling effectiveness against pressure drop through the motor cavity, since any restriction in the suction gas path reduces the volume of refrigerant reaching the cylinder.
Efficiency and Performance Metrics
Hermetic compressor performance is most commonly characterized using Energy Efficiency Ratio (EER) and Coefficient of Performance (COP). EER expresses cooling output in BTU per hour divided by electrical input in watts. COP uses the same concept but in consistent units (watts out divided by watts in).
For domestic refrigerator and freezer compressors, efficiency has improved substantially over the decades. In 1980, the best large hermetic compressors (above 750 BTU/hr nominal capacity) achieved an EER of about 4.0. By the early 1990s, production models reached 5.0 to 5.3, and units with electronically commutated motors pushed to 6.0. Smaller compressors (around 200 BTU/hr) tend to have lower EER values, around 3.5, because the fixed losses in the motor and valve system represent a larger share of total energy use at low capacities.
Lubrication in a Sealed System
Oil management is a distinctive challenge in hermetic compressors because the lubricant shares the sealed space with the refrigerant and must remain compatible with it. Most hermetic reciprocating compressors use a combination of splash and forced-feed lubrication. Splash lubrication relies on a paddle or slinger on the crankshaft dipping into the oil sump, while forced feed uses an oil pump to deliver lubricant directly to bearings and cylinder walls.
The refrigerant itself affects which oil can be used. Older systems running CFC refrigerants used mineral oils, but modern HFC refrigerants like R-134a are immiscible with mineral oil. The industry initially turned to polyalkylene glycol (PAG) oils for their compatibility, then shifted largely to polyol ester (POE) oils due to better chemical stability. Oil viscosity plays a critical role: low-viscosity lubricants can allow metal-to-metal contact at bearing surfaces, causing severe wear, while higher-viscosity oils maintain a protective film that reduces friction and extends component life.
Noise and Vibration Profile
Acoustic output is another important characteristic, especially for compressors in household refrigerators and residential HVAC systems where noise matters to the end user. The dominant noise sources in hermetic compressors are pressure pulsations from the suction and discharge processes and the physical motion of internal mechanical parts.
Low-frequency noise in the 500 to 630 Hz range is particularly problematic. In refrigerator compressors, this band can range from about 26 dBA at lower operating temperatures to 42 dBA at higher temperatures. Much of this variation comes from cavity resonances inside the shell. When a natural resonance frequency of the gas cavity lines up with a harmonic of the motor’s rotational frequency, it amplifies that frequency dramatically. Engineers address this by tuning the cavity geometry so resonance frequencies don’t coincide with rotational harmonics. In one well-documented design improvement, shifting the cavity resonance brought the 500 Hz noise level in the normal refrigerator operating range down to 30 to 38 dBA and reduced unit-to-unit variation significantly.
Sizing and Selection Parameters
When characterizing a hermetic compressor for a specific application, several parameters come together. Displacement volume (measured in cubic meters per hour) defines the gas-handling capacity. A typical hermetic scroll compressor for small air conditioning might have a displacement of around 2.08 m³/hr. Cooling capacity in BTU/hr or kilowatts describes how much heat the compressor can move at rated conditions. Electrical requirements include voltage, phase, frequency, RLA, and LRA. Physical dimensions and weight matter for installation, and the refrigerant type dictates both the oil and the operating pressures the compressor must handle.
Together, these characteristics (shell construction, compression type, motor configuration, electrical load profile, cooling mechanism, efficiency rating, lubrication method, and acoustic output) form a complete picture of how any hermetic compressor performs, how it fits into a system, and what its operational limits are.