Vapor Compression Refrigeration Cycle

The vapour compression refrigeration cycle consists of four main processes:

1. Compression – 1 → 2
2. Condensation – 2 → 3
3. Expansion – 3 → 4
4. Evaporation – 4 → 1

1. T-S Diagram

• 1 → 2: Compression: Refrigerant vapour from the evaporator is compressed in the compressor. Ideally, compression is isentropic, so entropy remains constant.
• 2 → 3: Condensation: High-pressure, high-temperature vapour passes through the condenser and rejects heat to the surroundings. The refrigerant changes from vapour to liquid.
• 3 → 4: Expansion: High-pressure liquid passes through the expansion valve. Pressure and temperature drop suddenly. The process is approximately constant enthalpy (isenthalpic).
• 4 → 1: Evaporation: The low-pressure refrigerant absorbs heat from the refrigerated space and evaporates, producing the cooling effect.

2. P-H Diagram

• 1 → 2: Pressure and enthalpy increase during compression.
• 2 → 3: Pressure remains approximately constant while heat is rejected and the refrigerant condenses.
• 3 → 4: Pressure drops through the expansion valve, while enthalpy remains approximately constant.
• 4 → 1: Pressure remains approximately constant while the refrigerant absorbs heat and evaporates.

The expansion valve:

• Reduces the pressure of the liquid refrigerant from condenser pressure to evaporator pressure.
• Causes a corresponding drop in refrigerant temperature.
• Meters the correct quantity of refrigerant entering the evaporator.
• Produces a mixture of liquid and vapour at the evaporator inlet.
• The expansion process is approximately isenthalpic, i.e. h₃ = h₄.

Purpose: To provide the required pressure reduction and control the refrigerant flow into the evaporator.

The room thermostat controls the temperature of the refrigerated space.

• It senses the room/cold-space temperature.
• When the temperature rises above the set value, it starts or keeps the compressor running.
• When the required temperature is reached, it stops the compressor or signals the control system to stop it.
• It therefore prevents excessive cooling and maintains the required room temperature.

Purpose: To automatically maintain the refrigerated space at the desired temperature.

The high-pressure cut-out is a safety device fitted on the high-pressure side of the refrigeration system.

• It senses the discharge/condenser pressure.
• If the pressure rises above the preset safe limit, it stops the compressor.
• It protects the compressor, condenser and other components from excessive pressure.
• Causes of high pressure may include poor condenser cooling, dirty condenser, inadequate cooling-water/air flow, overcharging or non-condensable gases.
• The fault should be investigated and rectified before restarting the system.

Purpose: To protect the refrigeration plant against dangerously high discharge pressure.

The equalizing line is normally associated with a thermostatic expansion valve (TXV).

• It connects the evaporator outlet/suction line to the pressure-sensing side of the TXV.
• It allows the TXV to sense the actual evaporator outlet pressure.
• This pressure is used together with the sensing-bulb temperature to control the refrigerant flow and maintain the required superheat.
• It is particularly important where there is a significant pressure drop between the evaporator outlet and the TXV sensing point.

Purpose: To transmit the actual evaporator pressure to the TXV so that the valve can correctly control refrigerant flow and maintain proper superheat.