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

2.5 — Screw 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.

2.5.1 — Screw Compressors

Screw compressors use helical rotors to compress refrigerant and are widely used in industrial and large commercial applications for their high capacity, reliability, and excellent part-load efficiency. Two designs exist: twin-screw and single-screw.

Twin-Screw vs. Single-Screw

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Twin-Screw

Two helical rotors — a male (driving) rotor with lobes and a female (driven) rotor with flutes — mesh within a close-tolerance housing. Refrigerant fills the spaces between lobes and flutes at the suction end; as the rotors turn, the spaces progressively reduce in volume, compressing refrigerant toward the discharge end. The most common screw compressor design in HVAC/R.

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Single-Screw

One helical main rotor with grooves engaged by two star wheels (gate rotors) on opposite sides. The star wheel teeth act as pistons, compressing refrigerant in the grooves as the main rotor turns. The symmetrical design creates balanced radial forces that reduce bearing loads and extend bearing life.

Characteristics

Capacity Range20 to 1,500 tons (70 to 5,275 kW)
EfficiencyExcellent, especially with variable speed drive
ReliabilityExcellent; continuous duty capability
Capacity ControlSmooth, stepless via slide valve or variable speed
ApplicationsIndustrial refrigeration, large chillers, process cooling
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Advantages

  • High reliability and long service life
  • Smooth, pulse-free discharge
  • Excellent part-load efficiency with slide valve
  • Handles liquid refrigerant better than reciprocating
  • Compact for high-capacity output
  • Low vibration and noise
  • Continuous duty capability
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Disadvantages

  • High initial cost
  • Requires significant oil separation equipment
  • Complex manufacturing requirements
  • Oil flooding required for sealing and cooling
  • Specialized service skills required

2.5.2 — Screw-Specific Components

In addition to common compressor components, screw compressors use three elements specific to their helical-rotor design: precision-machined rotors, a slide valve for capacity control, and an oil separator in the discharge stream.

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Rotors

Twin-screw compressors use male and female helical rotors machined to precise profiles. The male rotor typically has 4 lobes; the female rotor has 6 flutes. Rotor clearances are critical — tight enough for efficient compression, but sufficient to prevent metal-to-metal contact under thermal expansion.

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Slide Valve

An axial slide valve provides stepless capacity control from approximately 10% to 100% of full capacity. Moving the slide valve changes the point at which compression begins, effectively varying the active displacement of the compressor without stopping or staging.

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Oil Separator

Screw compressors inject significant quantities of oil into the compression chamber for sealing, lubrication, and cooling. A high-efficiency oil separator in the discharge stream removes oil from the refrigerant before it exits the compressor package, returning it to the compressor sump for recirculation.

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