Level 1, Unit 6, Section 5 – 5.2 Volumetric Efficiency

Unit 6 — Refrigeration System Components

Section 5 — Compressor Performance Metrics

5.2 — Volumetric Efficiency

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.

5.2.1 — Volumetric Efficiency

A compressor never moves as much refrigerant as its displacement suggests. Volumetric efficiency quantifies this gap, and clearance volume is one of its primary causes. Understanding both terms helps a technician diagnose a compressor that is running but not delivering expected capacity.

Volumetric efficiency (VE) is the ratio of the actual volume of refrigerant vapour drawn into the cylinders each cycle to the theoretical displacement. It is expressed as a percentage and is always less than 100% in practice.

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Typical VE Range

Most reciprocating compressors operate at 60–90% volumetric efficiency under normal conditions. Scroll and screw compressors tend toward the upper end of this range. As compression ratio increases, VE falls — the re-expansion of gas trapped in the clearance volume occupies more of the intake stroke before new refrigerant can enter.

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What Reduces VE

Clearance volume re-expansion (primary factor); valve leakage (gas flows back past a worn suction valve); gas heating as it contacts hot cylinder walls; pressure drop across suction valves; and high compression ratio. Worn compressor valves are a common field cause of low VE and reduced capacity that closely mimics a refrigerant undercharge.

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Field Relevance

A compressor with low VE due to worn valves will show: suction pressure higher than expected, discharge pressure lower than expected, high suction superheat, and low amperage draw. These symptoms distinguish a worn compressor from a refrigerant undercharge, where suction and discharge pressures are both abnormally low.

5.2.2 — Clearance Volume

The clearance volume is the small space remaining between the top of the piston at top dead centre (TDC) and the valve plate. This gap is intentional — it prevents mechanical contact between the piston and the valve plate — but it has a direct penalty on volumetric efficiency.

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How Clearance Volume Reduces Volumetric Efficiency

At the end of each compression stroke, high-pressure gas is trapped in the clearance volume. On the intake stroke, this trapped gas must re-expand back to suction pressure before the suction valve can open and admit new refrigerant. The larger the clearance volume and the higher the compression ratio, the greater the portion of the intake stroke consumed by re-expansion before any new refrigerant enters the cylinder.

Small clearance volume = less re-expansion = higher VE — compressor designers minimize clearance volume while maintaining adequate mechanical clearance
At very high compression ratios, the re-expansion of clearance gas can consume the entire intake stroke — the compressor pumps nothing and VE approaches zero
Liquid refrigerant in the clearance volume (liquid slugging) cannot re-expand as a gas and creates a hydraulic pressure spike that can break valve plates, connecting rods, or the crankshaft

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Liquid Slugging — The Leading Cause of Catastrophic Compressor Failure

Refrigerant liquid or oil entering the compressor cylinders cannot be compressed like vapour. The sudden pressure spike damages valve plates, breaks connecting rods, and can crack the compressor shell. Liquid slugging is caused by insufficient suction superheat, liquid floodback from a flooded evaporator, or refrigerant migration to the compressor crankcase during off-cycles. Suction superheat and crankcase heaters are the primary defences.