Understanding Refrigerant State in Vapour Compression Cycle

The vapour compression refrigeration cycle is a common method used for cooling. It involves several stages where the refrigerant changes its state (liquid, vapour, or a mixture of both) and properties (pressure, temperature).

Key Stages and Refrigerant States

Let's look at the main components and the state of the refrigerant as it passes through them in a typical vapour compression cycle:

• Evaporator: The refrigerant absorbs heat from the space being cooled, evaporates, and changes from a low-pressure liquid-vapour mixture to a low-pressure vapour. It usually leaves the evaporator as saturated vapour or slightly superheated vapour.
• Compressor: The low-pressure vapour from the evaporator is compressed to a high pressure. This process increases both the pressure and the temperature of the refrigerant. As a result, the refrigerant leaves the compressor as a high-pressure, high-temperature superheated vapour.
• Condenser: The high-pressure, high-temperature superheated vapour enters the condenser. It releases heat to the surroundings (air or water), first desuperheating, then condensing into a high-pressure liquid. It typically leaves the condenser as a saturated liquid or subcooled liquid.
• Expansion Valve (or Throttle Valve): The high-pressure liquid from the condenser passes through the expansion valve. This valve reduces the pressure of the refrigerant significantly. This pressure drop causes a portion of the liquid to flash into vapour, resulting in a low-pressure, low-temperature liquid-vapour mixture before entering the evaporator.

Refrigerant State at Compressor Exit

Based on the cycle description, the process in the compressor takes a low-pressure vapour and increases its pressure and temperature. This compression process, especially when starting from saturated or slightly superheated vapour, results in the refrigerant reaching a state where its temperature is above its saturation temperature at the elevated pressure. This state is known as superheated vapour.

Therefore, the refrigerant exists in a superheated vapour state at the exit of the compressor.

Analyzing the Options

• at the exit of the compressor: As explained above, the compression process leads to a high-pressure, high-temperature superheated vapour state. This matches the description.
• after leaving the expansion valve: The expansion valve causes a pressure drop and flashing, resulting in a low-pressure liquid-vapour mixture. This is not a superheated vapour state.
• after leaving the evaporator: The refrigerant leaves the evaporator as saturated vapour or slightly superheated vapour, not typically a fully superheated vapour compared to the state after compression.
• after passing through the condenser: The refrigerant leaves the condenser as a saturated liquid or subcooled liquid after rejecting heat and condensing. This is not a vapour state.

Thus, the only point where the refrigerant is in a distinct superheated vapour state in the cycle is after leaving the compressor.