Centrifugal Compressors

Centrifugal compressors use rotating impellers to convert kinetic energy into pressure rise, providing efficient compression for large refrigeration systems. These dynamic compressors dominate applications above 300 tons cooling capacity, where their efficiency, reliability, and stepless capacity control offer superior performance compared to positive displacement alternatives.

Operating Principles

Refrigerant enters the impeller eye at the center and accelerates radially outward through the impeller passages. The high-velocity discharge passes through the diffuser section, where velocity converts to static pressure. In multistage configurations, the discharge feeds subsequent stages to achieve higher pressure ratios.

The compression process follows polytropic relationships rather than the adiabatic assumptions applicable to reciprocating compressors. Polytropic efficiency typically ranges from 75% to 82% for modern centrifugal designs, accounting for continuous heat transfer during compression.

Impeller Design Classifications

Open Impellers

Open impeller designs feature blades without a front shroud, exposing the blade surfaces. This configuration permits easier manufacturing and field balancing but requires tight clearances between blade tips and the stationary housing.

Key characteristics:

Clearance range: 0.015 to 0.025 inches
Lower manufacturing cost
Reduced efficiency at part-load due to tip leakage
Common in smaller centrifugal units (300-500 tons)
Field serviceability advantage for blade repair

Semi-Open Impellers

Semi-open impellers incorporate a back shroud with exposed blade tips on the front surface. This design balances manufacturing complexity against aerodynamic performance.

Performance attributes:

Tip clearance sensitivity: 1% increase reduces efficiency by 0.5-0.8%
Improved part-load efficiency versus open designs
Easier balancing than closed impellers
Typical efficiency: 76-79% polytropic
Widespread use in 500-1000 ton chillers

Closed Impellers

Closed impellers feature both front and back shrouds, creating fully enclosed flow passages. This configuration delivers maximum aerodynamic efficiency through elimination of tip leakage losses.

Design advantages:

Highest polytropic efficiency: 79-82%
Reduced sensitivity to wear and fouling
Lower noise generation
Required for oil-free magnetic bearing designs
Standard for units above 1000 tons

The enclosed passages create more complex three-dimensional flow fields, requiring computational fluid dynamics analysis for optimization. Blade loading distributions determine pressure rise and efficiency across the operating range.

Stage Configurations

Single-Stage Systems

Single-stage centrifugal compressors operate with pressure ratios from 2.5:1 to 4.5:1, depending on refrigerant properties and impeller tip speed. These systems serve low-lift applications or low-pressure refrigerants (R-123, R-1233zd).

Tip speed limitations govern maximum pressure ratio:

Conventional bearings: 850-950 ft/s
Magnetic bearings: 1000-1100 ft/s
Structural limits: stress = ρω²r²/2

Two-Stage Systems

Two-stage configurations dominate the 300-2000 ton capacity range, providing pressure ratios from 6:1 to 12:1. Inter-stage economizer connections improve efficiency by subcooling liquid refrigerant before expansion.

The flash gas generated during economization returns to the second-stage suction, reducing the work required for vapor compression. Thermodynamic analysis shows coefficient of performance improvements of 8-15% compared to single-stage operation at identical conditions.

Three-Stage and Higher

Three-stage centrifugal compressors serve applications requiring pressure ratios exceeding 12:1 or very high lift conditions. These configurations appear in heat pump applications, low-temperature refrigeration, and process cooling systems.

Each additional stage adds complexity but enables:

Higher discharge temperatures without approaching refrigerant thermal decomposition limits
Multiple economizer ports for enhanced capacity control
Improved part-load efficiency through stage sequencing

Surge and Choke Limits

Surge Phenomenon

Surge represents a flow instability where the compressor cannot maintain stable pressure rise, resulting in flow reversal and severe mechanical vibration. The surge line defines the minimum flow limit on the compressor performance map.

Physical mechanism:

Reduced flow causes increased impeller incidence angle
Flow separation occurs on blade suction surfaces
Pressure rise collapses, allowing high-pressure refrigerant to reverse flow
System pressure drops, flow attempts to re-establish
Cycle repeats at 1-5 Hz frequency

Operating in surge conditions produces:

Thrust bearing damage from axial load reversals
Seal damage from pressure fluctuations
Efficiency degradation from flow separation
Potential motor overload from rapid power swings

Surge control systems maintain operation at 10-15% above the surge line through hot gas bypass, inlet guide vane positioning, or VFD speed reduction.

Choke Phenomenon

Choke occurs when refrigerant velocity approaches sonic conditions in the impeller throat or diffuser inlet, limiting maximum flow regardless of pressure differential. The choke line establishes the right-side boundary of the compressor map.

Sonic velocity for typical refrigerants:

R-134a at 40°F: 580 ft/s
R-1234ze at 40°F: 520 ft/s
R-513A at 40°F: 545 ft/s

Approaching choke conditions creates:

Dramatic efficiency loss from shock wave formation
Increased noise levels
Flow limitation preventing capacity increase
Potential mechanical vibration

Modern compressor maps include 5-8% margin between rated maximum flow and choke line to ensure stable operation under all load conditions.

Inlet Guide Vanes

Inlet guide vanes (IGVs) provide pre-rotation to refrigerant entering the impeller eye, effectively reducing the velocity seen by the impeller and shifting the performance curve. This mechanism enables efficient capacity reduction while maintaining stable operation above surge.

Vane Positioning and Flow Control

IGVs rotate from fully open (0°) to closed positions (60-90°), imparting tangential velocity component that reduces the relative velocity entering impeller passages. The velocity triangle relationship shows:

W = C - U

Where W is relative velocity, C is absolute velocity, and U is impeller tip speed.

Pre-rotation in the direction of impeller rotation reduces W, decreasing the work input per unit mass and shifting operation toward lower flow and pressure rise. Capacity modulation from 100% down to 15-20% becomes possible while maintaining polytropic efficiency above 70%.

Control Integration

IGV position responds to cooling load through the building automation system or chiller controller. Position versus capacity relationships are non-linear:

0-30° closure: minimal capacity reduction (90-100%)
30-60° closure: proportional control range (40-90%)
60-90° closure: approach to minimum stable operation (15-40%)

Hot gas bypass activates below minimum IGV position to prevent surge during extreme part-load conditions. Combined IGV and bypass control maintains stable operation across the full load range.

Variable Frequency Drive Operation

VFD control varies compressor speed to match system load, providing superior part-load efficiency compared to IGV modulation alone. Power consumption follows affinity law relationships:

Flow ∝ Speed
Pressure Rise ∝ Speed²
Power ∝ Speed³

Reducing speed to 60% for 60% capacity decreases power to approximately 22% of full-load consumption, assuming constant efficiency. Actual performance accounts for motor and drive losses, resulting in power consumption of 25-30% at 60% capacity.

Speed Range and Control

VFD centrifugal chillers typically operate from 30% to 100% speed, with minimum speed limited by:

Magnetic bearing control requirements (10,000-12,000 RPM minimum)
Motor cooling requirements (air velocity across windings)
Lubrication system requirements (oil pressure at low speed)
Surge margin at reduced pressure rise capability

The combination of VFD speed control and IGV positioning provides maximum efficiency across all load points. Control algorithms optimize between speed reduction and IGV closure based on real-time efficiency calculations.

Magnetic Bearing Technology

Magnetic bearings support the compressor shaft through electromagnetic forces, eliminating physical contact between rotating and stationary components. This technology enables oil-free operation and ultra-high efficiency.

Operating Principles

Radial magnetic bearings use electromagnetic coils to levitate the shaft within the bearing housing. Position sensors monitor shaft location at 20-50 kHz sampling rates. Digital control systems adjust coil current to maintain shaft position within ±0.001 inches.

Axial magnetic bearings control thrust loads from:

Impeller pressure differentials
System transients during startup and shutdown
Surge events creating instantaneous thrust reversals

Touchdown bearings provide mechanical backup during power loss or control system failure, allowing coast-down without damage.

Advantages Over Oil Lubrication

Oil-free operation delivers:

Zero refrigerant contamination from lubricant carryover
Elimination of oil separators, coalescers, and recovery systems
Reduced maintenance (no oil changes or filter replacements)
Improved heat transfer coefficients in evaporator and condenser
Lower operating costs from efficiency gains (2-4% improvement)

Power consumption for magnetic bearing control systems: 1.5-3 kW continuously, representing 0.3-0.5% of total compressor power at full load.

Oil-Free Centrifugal Designs

Oil-free centrifugal compressors eliminate all lubrication from the refrigerant circuit, requiring either magnetic bearings or dry gas seals for shaft support. These designs achieve the highest efficiency and lowest maintenance requirements.

System Configuration

Complete oil-free systems include:

Magnetic bearing assemblies (radial and thrust)
Variable frequency drive for speed control
Closed impeller designs for maximum efficiency
Advanced control systems with predictive algorithms
Integrated vibration monitoring and diagnostics

The absence of oil permits operation with pure refrigerant, enhancing thermodynamic performance through:

Improved nucleate boiling in evaporator (5-8% heat transfer increase)
Enhanced condensation (3-5% improvement)
Elimination of oil fouling over time
Precise refrigerant charge (no oil volume uncertainty)

Reliability Considerations

Oil-free designs demonstrate mean time between failures exceeding 100,000 hours for magnetic bearing systems. Critical monitoring includes:

Bearing gap sensors (continuous position tracking)
Vibration analysis (bearing health assessment)
Motor winding temperature (thermal protection)
Discharge temperature (refrigerant thermal stability)

Redundant power supplies for magnetic bearing controls ensure safe shutdown during facility power events. Battery backup systems maintain bearing levitation for 30-60 seconds, allowing controlled deceleration.

Applications in Large Chillers

Centrifugal compressors dominate the large chiller market from 300 tons to 10,000+ tons capacity, where their efficiency and reliability outweigh higher initial costs compared to screw compressors.

Capacity Range: 300-1000 Tons

Two-stage centrifugal compressors with semi-open or closed impellers serve this range. Typical specifications:

Full-load efficiency: 0.50-0.58 kW/ton
Part-load IPLV: 0.38-0.48 kW/ton
Sound levels: 80-85 dBA at 10 feet
Operating weight: 8,000-15,000 lbs

Standard features include IGV capacity control, microprocessor controls with communication interfaces, and refrigerant options including low-GWP alternatives (R-1234ze, R-513A).

Capacity Range: 1000-2000 Tons

This segment employs two-stage or three-stage configurations with closed impellers and advanced bearing systems. Performance characteristics:

Full-load efficiency: 0.48-0.55 kW/ton
Part-load IPLV: 0.35-0.45 kW/ton
Magnetic bearings increasingly standard
VFD integration for optimal part-load performance

District cooling plants, large commercial buildings, and industrial process cooling represent primary applications.

Capacity Above 2000 Tons

Ultra-large centrifugal chillers utilize three-stage compression, magnetic bearings, and VFD control as standard features. Custom-engineered designs address specific application requirements:

Multiple compressors in parallel (redundancy and staging)
Heat recovery configurations (simultaneous heating and cooling)
Low-temperature applications (process cooling to -40°F)
High-lift conditions (heat pump operation with 100°F+ lift)

Installation planning addresses:

Rigging requirements (40,000+ lbs shipping weight)
Electrical service (2000+ amps at 480V three-phase)
Condenser water flow (3000+ GPM)
Chilled water flow (4000+ GPM)
Equipment room access and clearances

Performance monitoring systems track real-time efficiency, predict maintenance requirements through vibration and temperature trending, and optimize operation through weather prediction and load forecasting algorithms. Integration with building management systems enables demand response participation and utility incentive program compliance.