Level 1, Unit 3, Section 4 – 4.2 Fundamental Processes

Unit 3 — Refrigeration System Fundamentals & Maintenance

Section 4 — Vapour Compression Cycle

4.2 — Fundamental Processes in the Cycle

Six processes define the vapour compression cycle. Each one changes the refrigerant’s pressure, temperature, or state — and understanding all six is what lets a technician interpret any pressure or temperature reading in the field.

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4.2.1 — The Complete Cycle — One Revolution at a Glance

The vapour compression cycle moves refrigerant continuously around a closed loop. Each of the four major components changes the refrigerant’s condition before passing it to the next component.

Low Side

① Evaporator

Liquid/vapour mixture absorbs latent heat. Refrigerant boils at low pressure, low temperature.

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High Side

② Compressor

Superheated vapour is compressed to high pressure and high temperature.

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High Side

③ Condenser

High-pressure vapour rejects heat and condenses to subcooled liquid.

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Transition

④ Metering Device

Liquid drops in pressure and temperature, forming a cold liquid/vapour mixture.

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The dividing line

The high side runs from the compressor discharge through the condenser to the inlet of the metering device. The low side runs from the metering device outlet through the evaporator back to the compressor suction. This dividing line is where you connect your high- and low-side manifold gauges.

4.2.2 — Compression

The compressor draws in low-pressure superheated vapour from the evaporator and compresses it to a high pressure and temperature. This is the only process in the cycle that requires external energy input.

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What Compression Does

Raises discharge pressure so the refrigerant can reject heat to a warmer environment (outdoor air or cooling tower water) in the condenser. Without compression, heat flow would reverse.

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Heat of Compression

Mechanical work done by the compressor appears as additional heat in the discharge gas. This is why the total heat rejected at the condenser is greater than the heat absorbed in the evaporator.

Q_condenser = Q_evaporator + W_compressor

Heat rejected = heat absorbed + work of compression (all in BTU/hr or kW)

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Liquid in the compressor = damage

Compressors are designed to compress vapour only. Liquid refrigerant is incompressible. Even a small amount of liquid entering the cylinder can bend valve reeds, crack pistons, or destroy connecting rods in a single stroke — which is why minimum superheat at the compressor inlet is not optional.

4.2.3 — Expansion

The metering device (TXV, EEV, capillary tube, or fixed orifice) creates a sudden pressure drop between the high-side liquid line and the low-side evaporator. This drop reduces the refrigerant’s boiling point so it can absorb heat from a cold space.

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Isenthalpic Process

Expansion across a metering device is essentially a constant-enthalpy process — enthalpy does not change, but temperature drops because pressure drops. A portion of the liquid flashes to vapour to provide the cooling effect.

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Flash Gas

When liquid refrigerant is throttled, some immediately vaporises — this is flash gas. It absorbs no useful heat from the cooled space; it only provides the temperature drop. Subcooling the liquid before expansion reduces flash gas and improves efficiency.

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Why subcooling saves capacity

Every extra degree of subcooling means less flash gas at the metering device outlet — so more of the refrigerant enters the evaporator as liquid and is available to absorb useful latent heat. 10°F of subcooling can improve system capacity by 1–2%.

4.2.4 — Phase Change — The Engine of Refrigeration

Phase change — specifically the latent heat transferred during evaporation and condensation — is what makes refrigeration efficient. Enormous amounts of heat move at nearly constant temperature, allowing a small refrigerant circuit to transfer heat loads that would be impractical with sensible heat exchange alone.

Evaporation (in the Evaporator)

Refrigerant absorbs latent heat from surroundings
Changes from liquid/vapour mixture to vapour
Temperature stays constant at saturation point
Heat flows from warm space into cold refrigerant
This is the useful cooling effect

Condensation (in the Condenser)

Refrigerant releases latent heat to surroundings
Changes from vapour to liquid
Temperature stays constant at saturation point
Heat flows from hot refrigerant into warm outdoor air
Rejects all heat absorbed + heat of compression

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Connection to Section 2

The latent heat values from Section 2 apply directly here. Refrigerant latent heats are in the same order of magnitude as water (hundreds of BTU/lb), which is why phase-change cycles are so much more effective than air-based sensible heat exchange.

4.2.5 — Saturation, Subcooling & Superheat in the Cycle

Understanding where each refrigerant state exists in the cycle makes it possible to interpret any set of gauge and temperature readings instantly.

Location in Cycle	Refrigerant State	Pressure	What to Measure
Evaporator (most of coil)	Saturated mixture (boiling)	Low	Suction pressure → P–T chart → evap temp
Evaporator outlet / suction line	Superheated vapour	Low	Suction temp − sat temp = evaporator superheat
Compressor discharge	Hot superheated vapour	High	Discharge temp (expected: sat + 50–100°F above condensing temp)
Condenser (most of coil)	Saturated mixture (condensing)	High	Discharge pressure → P–T chart → condensing temp
Condenser outlet / liquid line	Subcooled liquid	High	Sat temp − liquid line temp = subcooling
Metering device outlet	Cold liquid/vapour mixture	Low	Visible as frost on distribution header (normal at low temps)

4.2.6 — Discharge & Suction — The Compressor’s Two Sides

The compressor’s suction and discharge ports are the boundary between the low side and high side of the system. Monitoring pressures at these two points gives an immediate picture of system performance.

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Suction (Low Side)

Low-pressure, low-temperature superheated vapour entering the compressor. Suction pressure corresponds to evaporator saturation temperature via the P–T chart. Low suction pressure often indicates low charge, restricted metering device, or low load.

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Discharge (High Side)

High-pressure, high-temperature superheated vapour leaving the compressor. Discharge pressure corresponds to condensing temperature via the P–T chart. High discharge pressure often indicates dirty condenser coil, inadequate airflow, or overcharge.

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Pressure Ratio — A Compressor Health Indicator

Pressure ratio = Absolute discharge pressure ÷ Absolute suction pressure (both in psia).

Low-temperature systems have high pressure ratios (often 6:1 to 10:1), which stresses compressors and reduces volumetric efficiency. High-temperature systems typically run 3:1 to 5:1 — much more compressor-friendly.

An unusually high pressure ratio (high discharge, low suction) often signals a problem: low charge, restricted metering device, insufficient evaporator airflow, or dirty condenser.