Scroll Compressors

Scroll Compressor Operating Principles

Scroll compressors employ two intermeshing spiral-shaped elements to compress refrigerant through continuous reduction of vapor pockets. The compression mechanism consists of a fixed scroll bolted to the compressor housing and an orbiting scroll that executes a circular orbital motion without rotation. This orbital motion creates moving compression pockets that progressively decrease in volume as vapor travels from the outer periphery toward the center discharge port.

The geometry of scroll wraps follows an involute curve, which maintains constant contact lines between the fixed and orbiting scrolls throughout the compression cycle. Refrigerant vapor enters at the outer edge where multiple compression pockets form simultaneously. As the orbiting scroll completes its circular path, these pockets spiral inward, continuously reducing volume and increasing pressure until the compressed gas reaches the central discharge port.

Scroll Geometry and Compression Process

The involute profile ensures that compression occurs smoothly without pulsation, creating multiple compression chambers operating at different pressure ratios simultaneously. This multi-chamber design distributes compression work across several pockets, reducing the pressure differential across any single sealing surface and minimizing leakage paths.

Key geometric parameters include:

The compression ratio capability of scroll compressors depends on the number of wrap turns and the involute geometry. Most HVAC applications utilize scrolls designed for compression ratios between 2:1 and 4:1, optimized for air conditioning and heat pump operating conditions.

Compliance Mechanisms

Scroll compressors incorporate compliance mechanisms to maintain proper sealing between scroll elements while accommodating thermal expansion and manufacturing tolerances. These mechanisms prevent metal-to-metal contact damage and ensure efficient compression across varying operating conditions.

Axial compliance allows the orbiting scroll to move vertically along the scroll axis, maintaining tip sealing between the scroll wrap tips and the opposing base plate. This is typically achieved through:

• Back pressure chambers that apply controlled force against the orbiting scroll
• Gas pressure from intermediate compression pockets
• Spring-loaded mechanisms for startup conditions

Radial compliance maintains flank sealing between the wrap surfaces through:

• Precise manufacturing tolerances (typically 10-25 micrometers)
• Thermal expansion matching between components
• Centrifugal force during operation that presses wraps together

The combination of axial and radial compliance enables scroll compressors to maintain volumetric efficiency above 95% across their operating envelope while preventing mechanical wear.

Capacity Modulation Technologies

Digital Scroll Technology

Digital scroll compressors modulate capacity by cyclically loading and unloading the compression mechanism. An axial compliance mechanism allows the orbiting scroll to separate from the fixed scroll, creating a bypass condition where refrigerant circulates without compression. The loading/unloading cycle operates at frequencies between 10-20 Hz, providing capacity modulation in 10% increments from 10% to 100%.

The digital modulation control varies the ratio of loaded time to total cycle time, enabling precise capacity matching without efficiency losses associated with hot gas bypass. This technology maintains full compression efficiency during loaded periods while consuming minimal power during unloaded periods (approximately 5-8% of full load power).

Variable Speed Scroll Compressors

Inverter-driven variable speed scroll compressors utilize electronic controls to vary compressor speed from 20% to 100% of design RPM. This approach provides continuous capacity modulation while maintaining optimal compression efficiency across the operating range.

Operating characteristics:

Variable speed operation reduces cycling losses, eliminates inrush current surges, and provides superior comfort through continuous temperature control. The compressor maintains efficient operation across the speed range because scroll geometry compression efficiency remains relatively constant regardless of rotational speed.

Efficiency Advantages

Scroll compressors demonstrate superior efficiency compared to reciprocating compressors through several mechanisms:

1. Continuous compression process eliminates suction and discharge valve losses present in reciprocating designs
2. Reduced gas re-expansion due to minimal clearance volume (typically 1-2% vs 4-6% for reciprocating)
3. Lower mechanical friction from fewer moving parts and balanced rotary motion
4. Minimal flow restrictions with direct discharge path and no valve pressure drops
5. Reduced internal leakage through continuous flank and tip sealing

Typical isentropic efficiency ranges:

• Scroll compressors: 65-75% across operating range
• Reciprocating compressors: 55-70% with greater variation by condition

The efficiency advantage becomes more pronounced at part-load conditions where reciprocating compressor clearance volume effects increase and valve dynamics degrade performance.

Hermetic Construction

Most HVAC scroll compressors utilize hermetic or semi-hermetic construction where the motor and compression mechanism share a sealed housing filled with refrigerant. This configuration provides several advantages:

• Refrigerant-cooled motor prevents overheating during low load operation
• Elimination of shaft seals removes potential leak paths
• Sound attenuation from refrigerant mass surrounding components
• Protection of internal components from atmospheric contamination

The hermetic design requires oil management systems to ensure proper lubrication returns to the compressor sump. Oil separators may be employed in larger systems to minimize oil circulation through the refrigeration circuit.

Applications in HVAC Systems

Residential HVAC Applications

Scroll compressors dominate residential air conditioning and heat pump applications in capacities from 1.5 to 5 tons (5-18 kW). Their advantages in residential systems include:

• Quiet operation from continuous compression process (typically 68-76 dBA)
• High seasonal efficiency (SEER ratings of 16-26 with variable speed technology)
• Reliable operation with fewer moving parts than reciprocating designs
• Compact footprint enabling smaller outdoor unit dimensions

Commercial HVAC Applications

Commercial applications utilize scroll compressors ranging from 3 to 60 tons (10-210 kW) in:

• Packaged rooftop units
• Split system air conditioning
• Water-source and ground-source heat pumps
• Process cooling applications
• Condensing units for refrigeration

Tandem and trio scroll compressor configurations provide capacity staging and redundancy in larger commercial systems. Multiple compressors enable efficient part-load operation and continued system function during single compressor failure.

Comparison with Reciprocating Compressors

Scroll compressors demonstrate clear advantages in noise, vibration, and efficiency for HVAC applications below 60 tons. Reciprocating compressors remain competitive in very large capacity applications and industrial refrigeration where robustness and liquid tolerance are priorities.

The primary limitation of scroll compressors involves liquid refrigerant slugging. The minimal clearance volume provides no buffer for liquid ingestion, making proper system design and controls critical to prevent liquid floodback during startup and low ambient conditions.

Variable Speed Integration Benefits

Integration of variable speed drives with scroll compressors creates optimal efficiency and control characteristics for modern HVAC systems. The continuous compression process of scroll geometry combined with variable speed operation enables:

• Capacity matching to within 1-2% of required load
• Elimination of room temperature swings from cycling operation
• Seasonal efficiency improvements of 30-40% compared to fixed speed reciprocating systems
• Reduced electrical demand charges through elimination of high inrush currents
• Extended equipment life through reduced start/stop cycles

This combination positions variable speed scroll compressors as the dominant technology for premium residential HVAC systems and increasingly for commercial applications where efficiency and comfort are valued over first cost considerations.