Unit 6 — Refrigeration System Components
Section 1 — Compressors
1.1 — Compressor’s Critical Functions
The compressor does more than simply raise pressure — it performs four interdependent
functions that keep the refrigeration cycle operating. This lesson examines each function
and explains how the compressor creates the conditions that allow heat to be absorbed in
the evaporator and rejected in the condenser.
Pressure Differential
Refrigerant Circulation
Heat of Compression
Vapour Removal
313A / 313D
1.1.2 — Circulating Refrigerant
By continuously drawing in low-pressure vapour and discharging high-pressure vapour,
the compressor acts as a pump that circulates refrigerant throughout the system. The
mass flow rate of refrigerant determines the system’s cooling capacity.
- A higher refrigerant mass flow rate means more refrigerant passes through the evaporator per unit time, absorbing more heat and increasing system capacity
- Mass flow rate depends on compressor displacement, volumetric efficiency, and the density of the suction vapour at operating conditions
- Any factor that reduces refrigerant circulation — low suction pressure, worn valves, refrigerant undercharge — directly reduces system cooling capacity
- The compressor does not create refrigerant; it only moves it — adequate refrigerant charge is a prerequisite for the compressor to circulate the intended flow rate
1.1.3 — Adding Heat of Compression
The compression process adds energy to the refrigerant in the form of heat. This heat
of compression raises the refrigerant temperature above the condensing temperature,
enabling efficient heat transfer in the condenser. The compression work input also
represents the energy cost of operating the refrigeration cycle.
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Why Superheating Is Necessary
The refrigerant must arrive at the condenser hotter than the condensing medium (air
or water) to drive heat transfer. The heat of compression provides this temperature
margin. Without it, the refrigerant could not reject heat to a medium that is, at times,
nearly as warm as the refrigerant’s saturation temperature.
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Energy Cost of Compression
The work input to the compressor is the primary operating cost of the refrigeration
system. This work appears as heat of compression and is rejected through the condenser
along with the heat absorbed from the cooled space. Reducing compression ratio —
by keeping condensers and evaporators clean — directly reduces this energy cost.
1.1.4 — Removing Vapour from the Evaporator
By maintaining low suction pressure, the compressor enables complete evaporation of
refrigerant in the evaporator. This ensures maximum heat absorption and prevents liquid
refrigerant from returning to the compressor.
- As refrigerant evaporates in the evaporator, it must be continuously removed as vapour to make room for incoming liquid refrigerant from the metering device
- If the compressor cannot remove vapour fast enough, suction pressure rises and the refrigerant’s saturation temperature increases, reducing the temperature difference driving heat absorption
- Effective vapour removal ensures liquid refrigerant fully evaporates before it can travel back along the suction line to the compressor, protecting against liquid slugging
- Suction superheat — the temperature of vapour above its saturation point at suction pressure — is the primary indicator that vapour removal is complete before the compressor draws it in
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Liquid Refrigerant Return — Critical Risk
If the compressor fails to maintain low suction pressure — or if the metering
device floods the evaporator — liquid refrigerant can travel back to the compressor.
A compressor is designed to compress vapour, not liquid. Liquid entering the cylinders
causes hydraulic pressure spikes that can destroy valve plates, bend connecting rods,
and crack the compressor housing.