Rotary Compressors

Rotary compressors utilize rotating motion to compress refrigerant vapor through continuous volume reduction. These positive displacement machines achieve compression by trapping refrigerant between a rotating element and a stationary housing, progressively reducing the trapped volume as rotation proceeds.

Operating Principle

Rotary compressors compress refrigerant through continuous rotation rather than reciprocating motion. The fundamental mechanism involves an eccentric rotor mounted within a cylindrical chamber. As the rotor rotates, it maintains continuous contact with the cylinder wall, creating isolated pockets of refrigerant that decrease in volume during the compression cycle.

Key Operating Characteristics:

• Continuous compression process
• Minimal vibration compared to reciprocating designs
• Smooth torque delivery to motor
• Inherent vapor sealing through oil film
• Single discharge per revolution

Rolling Piston Design

The rolling piston configuration represents the most common rotary compressor design in residential air conditioning applications. A cylindrical piston rolls eccentrically within a circular cylinder, trapped against a spring-loaded blade that maintains sealing contact.

Construction Elements:

• Eccentric shaft drives rolling piston
• Spring-loaded vane maintains contact with piston
• Suction port admits vapor at low pressure
• Discharge port releases compressed vapor
• Oil sump provides continuous lubrication

The piston divides the cylinder into suction and compression chambers. As the piston rolls, the suction chamber expands while the compression chamber contracts. Refrigerant enters through the suction port, becomes trapped between the piston and vane, undergoes compression, and discharges through the outlet valve.

Performance Parameters:

Rotary Vane Design

Rotary vane compressors employ multiple spring-loaded vanes that slide radially within slots in an eccentric rotor. Centrifugal force and gas pressure maintain vane contact with the cylindrical housing, creating multiple compression chambers.

Vane Configuration:

• 2 to 8 vanes depending on application
• Eccentric rotor creates varying chamber volumes
• Multiple compression events per revolution
• Balanced loading reduces vibration
• Higher capacity than rolling piston designs

Each vane divides adjacent chambers. As the rotor turns, chambers on one side increase in volume (suction), while chambers on the opposite side decrease (compression). This arrangement produces multiple discharge pulses per revolution, reducing pulsation amplitude.

Advantages Over Rolling Piston:

• Higher capacity per unit displacement
• Better dynamic balance
• Lower discharge pulsation
• Multiple compression chambers
• Suitable for higher pressure ratios

Hermetic Construction

Rotary compressors for HVAC applications utilize hermetic or semi-hermetic construction. The motor and compressor share a sealed housing, with refrigerant vapor cooling the motor windings before compression.

Hermetic Design Features:

• Welded steel shell contains motor and pump
• Electric motor directly drives eccentric shaft
• Refrigerant provides motor cooling
• Oil sump integral to shell bottom
• No external shaft seal required

The hermetic configuration eliminates refrigerant leakage through shaft seals while simplifying construction. Refrigerant enters the shell, flows through motor windings (providing cooling), then enters the compression chamber. This motor cooling approach limits operating temperatures and constrains applications to moderate pressure ratios.

Construction Materials:

• Cast iron cylinder and housing
• Steel eccentric shaft
• Spring steel or carbon fiber vanes
• Aluminum or copper motor windings
• Synthetic oil compatible with refrigerant

Lubrication System

Rotary compressors require continuous oil lubrication to seal clearances, reduce friction, and remove heat. Oil circulation occurs through pressure differential and mechanical pumping action.

Oil System Functions:

• Seals clearances between moving parts
• Reduces friction and wear
• Removes compression heat
• Dampens vibration and noise
• Maintains vane spring tension through oil film

The oil sump at shell bottom supplies lubricant to bearing surfaces and compression chambers. Eccentric shaft rotation creates centrifugal pumping that distributes oil to critical interfaces. Oil mixes with refrigerant during compression, requiring oil separation before refrigerant enters the system.

Oil Requirements:

Capacity Range and Applications

Rotary compressors dominate the small capacity range for residential and light commercial applications. Manufacturing advantages and competitive performance make them economical for mass production.

Primary Applications:

• Residential air conditioners (1-5 tons)
• Window and portable AC units
• Residential heat pumps
• Household refrigerators
• Dehumidifiers
• Small commercial systems

Capacity Characteristics:

• 3,000 - 60,000 BTU/hr typical range
• Single or twin-cylinder configurations
• Fixed speed or inverter driven
• Limited to moderate pressure ratios
• Optimized for R-410A and R-32 refrigerants

Twin-cylinder designs utilize two rolling piston cylinders on a common eccentric shaft, doubling capacity while maintaining compact dimensions. This configuration improves dynamic balance and reduces vibration amplitude.

Efficiency Characteristics

Rotary compressor efficiency derives from continuous compression, reduced clearance volume, and smooth gas flow. Volumetric efficiency exceeds reciprocating designs at typical air conditioning conditions.

Efficiency Factors:

• Minimal clearance volume (1-2% of displacement)
• No suction or discharge valves in rolling piston
• Continuous compression without cyclic losses
• Reduced friction from balanced rotating motion
• Effective sealing through oil films

Typical Performance Metrics:

Inverter-driven rotary compressors extend efficiency through variable speed operation. Speed modulation matches capacity to load, maintaining higher part-load efficiency than fixed-speed alternatives.

Comparison with Reciprocating Compressors

Rotary and reciprocating compressors compete in overlapping capacity ranges. Design differences create distinct performance and application characteristics.

Rotary Advantages:

• Lower vibration and noise
• Fewer moving parts
• Smoother torque delivery
• Higher volumetric efficiency at low compression ratios
• More compact for equivalent capacity
• Better suited to inverter drive

Reciprocating Advantages:

• Higher efficiency at high compression ratios
• Better performance with liquid refrigerant slugging
• Established service procedures
• Wider capacity range
• Field-serviceable in semi-hermetic designs

Performance Comparison:

Operational Considerations

Rotary compressor operation requires attention to oil management, liquid refrigerant handling, and proper system charging. Design limitations constrain operating envelope.

Critical Operating Parameters:

• Discharge temperature limit: 220-240°F maximum
• Compression ratio limit: 5.5:1 maximum
• Oil return from evaporator essential
• Liquid refrigerant flooding protection required
• Proper superheat control (10-20°F)

Failure Modes:

• Vane or blade wear from inadequate lubrication
• Bearing failure from oil starvation
• Motor winding burnout from high temperature
• Eccentric shaft scoring from contamination
• Valve failure in discharge port

Liquid refrigerant entering the compressor dilutes oil, reducing lubrication effectiveness and causing rapid wear. Proper system design maintains adequate superheat and includes accumulator protection where necessary.

Design Selection Criteria

Application requirements determine suitability of rotary compressor selection. Matching compressor characteristics to system demands optimizes performance and reliability.

Selection Factors:

• Required capacity range
• Operating temperature range
• Compression ratio range
• Noise limitations
• Physical space constraints
• Inverter drive compatibility
• Refrigerant type
• Cost targets

Rotary compressors excel in applications requiring compact dimensions, quiet operation, and moderate compression ratios. Residential air conditioning represents the ideal application, combining these characteristics with mass production economics.