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Unit 6 — Refrigeration System Components
Section 5 — Compressor Performance Metrics

5.1 — Compression Ratio

Mastery of compressor terminology enables clear communication among technicians, engineers, and other professionals in the refrigeration and air conditioning field. This lesson defines the fundamental mechanical terms — compression ratio, displacement, and volumetric efficiency — along with refrigerant state terms, discharge temperature, oil charge, and the performance metrics used to rate and compare refrigeration equipment.

5.1.1 — Compression Ratio

Compression ratio and displacement are the two most fundamental mechanical descriptors of a compressor. Together they define how hard the compressor is working and how much refrigerant it can theoretically move — both directly influence system capacity, efficiency, and the operating conditions the compressor experiences.

The compression ratio is the relationship between the absolute discharge pressure (high side) and the absolute suction pressure (low side). It must be calculated using absolute pressures — gauge pressure plus atmospheric pressure (approximately 101 kPa / 14.7 psi at sea level).

Compression Ratio
CR = Absolute Discharge Pressure ÷ Absolute Suction Pressure
Both pressures must be in the same unit (kPa absolute or psia)
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Effect of High Compression Ratio

A higher compression ratio increases the work the compressor must do per unit of refrigerant moved. Discharge temperature rises, volumetric efficiency falls, and oil breakdown risk increases. Compression ratios above approximately 10:1 indicate abnormal operating conditions — low suction pressure, high head pressure, or both — and require investigation.

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Typical Operating Range

Well-designed refrigeration systems typically operate with compression ratios between 2:1 and 7:1 depending on refrigerant and application. A ratio near 2:1 represents a mild duty cycle (high suction, moderate discharge). A ratio above 7:1 stresses the compressor and should prompt a review of operating pressures, refrigerant charge, and condenser/evaporator performance.

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Worked Example

Suction pressure: 69 kPa gauge (10 psig) → absolute = 170 kPa (24.7 psia).
Discharge pressure: 1,138 kPa gauge (165 psig) → absolute = 1,239 kPa (179.7 psia).
CR = 1,239 ÷ 170 = 7.3 : 1. This is at the upper end of the normal range — worth monitoring condenser performance.

5.1.2 — Displacement

Displacement refers to the volume of refrigerant vapour that a compressor can theoretically move through its cylinders in a given time, typically expressed in cubic metres per hour (m³/h) or cubic feet per minute (CFM). It is a fixed mechanical characteristic of the compressor design and does not change with operating conditions.

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What Determines Displacement

  • Cylinder bore: the internal diameter of each cylinder; a larger bore increases displacement per stroke
  • Stroke length: the distance the piston travels from bottom dead centre (BDC) to top dead centre (TDC); a longer stroke increases volume swept per revolution
  • Number of cylinders: total displacement scales directly with the number of cylinders operating
  • Rotational speed (RPM): displacement per unit time increases with shaft speed; a compressor running at 1,750 RPM moves more refrigerant per hour than the same compressor at 1,450 RPM
  • Displacement alone does not describe actual compressor performance — it must be combined with volumetric efficiency to determine actual refrigerant flow
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