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Unit 6 — Refrigeration System Components
Section 2 — Types of Compressors

2.6 — Centrifugal Compressors

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.

2.6.1 — Centrifugal Compressors

Centrifugal compressors are dynamic compressors that use rotating impellers to accelerate refrigerant vapour, then convert this velocity to pressure in a diffuser section. They are the compressors of choice for large-capacity applications, particularly water chillers in commercial and industrial buildings.

Operating Principle

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Velocity-to-Pressure Conversion

  • Refrigerant vapour enters the eye (centre) of a rapidly rotating impeller
  • Impeller blades accelerate the vapour outward, imparting kinetic energy
  • High-velocity vapour passes through a diffuser section where the flow passage expands, velocity decreases, and pressure rises
  • Multiple stages (impellers in series) may be used to achieve higher pressure ratios
  • Capacity is varied by inlet guide vanes, variable speed drive, or both

Characteristics

Capacity Range100 to 10,000+ tons (352 to 35,170+ kW)
EfficiencyHighest at design conditions; excellent with variable speed
Noise / VibrationLow; smooth continuous operation
ApplicationsLarge water chillers, district cooling, process cooling
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Advantages

  • Very high capacity in a compact package
  • Oil-free designs available (magnetic bearings)
  • Highest efficiency at large capacities
  • Smooth, continuous operation
  • Long service life with proper maintenance
  • Infinitely variable capacity with inlet guide vanes or VSD
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Disadvantages

  • Limited to low-pressure refrigerants (typically)
  • Susceptible to surge at low loads
  • Not practical for small capacities
  • High initial cost
  • Sensitive to liquid refrigerant carryover
  • Limited pressure ratio per stage
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Surge — Critical Operating Risk for Centrifugal Compressors

Surge occurs when refrigerant flow drops below a critical point. High-pressure discharge gas reverses flow back through the compressor, causing violent oscillations, extreme mechanical stresses, and potential damage. Centrifugal compressors must always operate above the surge line, which varies with speed and refrigerant conditions. Modern chillers include surge detection and automatic capacity controls to prevent surge from occurring.

2.6.2 — Centrifugal-Specific Components

Centrifugal compressors use a fundamentally different compression mechanism and therefore have unique components: an impeller to accelerate refrigerant, a diffuser to convert velocity to pressure, inlet guide vanes for capacity control, and a gear system to achieve the high impeller speeds required.

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Impeller

A rotating disc with curved blades that accelerates refrigerant vapour outward, imparting kinetic energy. Impeller design significantly affects efficiency, capacity, and surge characteristics. Materials include aluminum, steel, and titanium depending on application requirements and refrigerant.

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Diffuser

A stationary element surrounding the impeller where high-velocity gas decelerates and kinetic energy is converted to pressure. Diffusers may be vaneless (a simple annular space) or vaned (with guide vanes for higher efficiency at design conditions).

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Inlet Guide Vanes (IGVs)

Adjustable vanes at the impeller inlet that provide capacity modulation. By changing the angle of the vanes, the volume of refrigerant admitted to the impeller is varied, allowing the compressor to match load conditions without speed changes. Often combined with variable speed drive for broader control range.

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Gear System

Many centrifugal compressors include a step-up gear system to raise impeller speed above motor speed. Impeller tip speeds of 700 ft/s (213 m/s) or higher are often required, necessitating gear ratios of 3:1 to 10:1. Magnetic-bearing designs may use high-speed motors directly without gears.

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