Unit 6 — Refrigeration System Components
Section 2 — Types of Compressors
2.1 — Common Compressor Components
Refrigeration and air conditioning systems use several compressor designs, each suited
to different capacity ranges, applications, and operating conditions. This lesson covers
the two broad categories — positive displacement and dynamic — and examines
the five main types: reciprocating, scroll, rotary, screw, and centrifugal.
2.1.2 — Common Compressor Components
While specific components vary among compressor types, certain elements are common across
most designs. These include the housing, mechanical drive train, valves, motor, lubrication
system, and bearings.
Structural Mechanics
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Housing / Shell
Contains and protects internal components while withstanding system pressures. In
hermetic and semi-hermetic compressors, the shell also serves as the oil sump and may
be designed to separate oil from returning suction gas. Materials include cast iron,
steel, and aluminum, selected for strength, cost, and refrigerant compatibility.
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Crankshaft
In reciprocating compressors, the crankshaft converts rotary motor motion into
reciprocating piston motion. It is supported by main bearings and includes
precision-machined journals for connecting rod bearings. Proper crankshaft
lubrication is critical for compressor longevity.
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Pistons & Connecting Rods
Pistons (typically aluminum or cast iron) are designed for appropriate clearance and
wear characteristics. Connecting rods link the pistons to the crankshaft and must
withstand significant forces while maintaining precise alignment. Piston rings provide
sealing between the piston and cylinder wall.
Valves
Suction and discharge valves control refrigerant flow into and out of reciprocating
compressor cylinders. Valves must open and close quickly, seal tightly, and resist
wear and fatigue — their condition directly determines compressor efficiency
and reliability.
- Reed valves: thin, flexible metal strips that flex to open and close; simple, reliable, and the most common type in hermetic compressors
- Ring valves: concentric rings that lift off a seat; used in larger, open-drive or semi-hermetic compressors where higher refrigerant flow is required
- Disc valves: circular plates that lift off seats; used in high-capacity applications where large valve opening area and low pressure drop are required
Cylinder, Valve Plate & Motor
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Cylinder & Valve Plate
The cylinder provides the bore in which the piston operates. The valve plate
assembly contains suction and discharge valves and separates the cylinder from the
high and low-pressure sides. Proper valve plate gasket installation is critical to
prevent refrigerant leakage between pressure zones.
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Motor & Stator
In hermetic and semi-hermetic compressors, the electric motor is integral to the
assembly. The stator (stationary windings) is mounted to the shell; the rotor
(rotating element) is mounted on the crankshaft. Motor cooling is typically provided
by suction gas flowing over the motor windings.
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Oil Pump
Larger compressors use positive-displacement oil pumps (gear or eccentric/crescent
type) to maintain adequate lubrication pressure. Oil pressure must exceed crankcase
pressure to force oil through bearings and lubricated surfaces. Oil pressure safety
controls monitor pressure and shut down the compressor if it falls below the minimum.
Bearings
- Sleeve bearings: metal or composite sleeves providing a sliding surface; rely on an oil film to separate surfaces; common in reciprocating compressor crankshafts
- Ball bearings: use steel balls between inner and outer races; common in scroll compressors and small hermetic units; handle both radial and axial loads
- Roller bearings: use cylindrical or tapered rollers between races; handle higher radial loads than ball bearings; used in larger open-drive compressors
- Magnetic bearings: use electromagnetic forces for contactless support; no wear, no lubrication required; found in some centrifugal compressors with oil-free designs