- Describe, with the aid of a sketch, a vapour-compression refrigeration cycle, showing clearly the physical state and condition of the refrigerant in the system.(10)
A vapour-compression refrigeration cycle
Let’s break down a typical vapor-compression refrigeration cycle, emphasizing the state and condition of the refrigerant as it moves through the system.
Core Components and Refrigerant’s Journey
- Compressor
- Refrigerant Enters: Low-pressure, low-temperature vapor.
- This vapor is slightly superheated, meaning its temperature is a bit above its boiling point at that pressure. This ensures no liquid enters the compressor.
- Compressor’s Action: It compresses the vapor, significantly increasing its pressure and temperature.
- Refrigerant Leaves: High-pressure, high-temperature vapor.
- It’s still in a gaseous state but now has much higher energy content due to the compression.
- Condenser
- Refrigerant Enters: High-pressure, high-temperature vapor.
- It might still be slightly superheated.
- Condenser’s Action: Heat is rejected from the refrigerant to a cooling medium (air or water).
- The refrigerant cools down, reaching its saturation temperature.
- It then condenses, changing phase from vapor to liquid.
- Refrigerant Leaves: High-pressure liquid.
- Ideally, it’s subcooled, meaning its temperature is slightly below its saturation point, ensuring it remains liquid.
- Expansion Valve (or Metering Device)
- Refrigerant Enters: High-pressure liquid.
- Valve’s Action: Creates a restriction, causing a sudden drop in pressure.
- This pressure drop triggers some of the liquid to flash (evaporate) instantly, cooling the remaining liquid.
- Refrigerant Leaves: Low-pressure, low-temperature mixture of liquid and vapor.
- It’s now in a two-phase state, ready to absorb heat in the evaporator.
- Evaporator
- Refrigerant Enters: Low-pressure, low-temperature liquid/vapor mixture.
- Evaporator’s Action: Heat is absorbed from the surroundings (the space being cooled) into the refrigerant.
- The remaining liquid evaporates, turning completely into vapor.
- The vapor also gets slightly superheated to ensure no liquid reaches the compressor.
- Refrigerant Leaves: Low-pressure, low-temperature vapor.
- This vapor is now ready to be compressed again, starting the cycle anew.
Key Takeaways
Heat Exchange: The system absorbs heat at the evaporator and rejects it at the condenser.
Phase Changes: The refrigerant changes between liquid and vapor phases to enable heat transfer.
Pressure & Temperature: These change throughout the cycle, following the refrigerant’s specific properties.
Superheat & Subcooling: Ensure only vapor enters the compressor and only liquid enters the expansion valve for efficiency and safety.