Introduction to Commercial Refrigeration Systems : The Refrigeration Cycle

 Stages of the Refrigeration Cycle:

1. Compression:
   • Process: Refrigerant enters the compressor as low-pressure, low-temperature superheated vapor.
   • Function: The compressor raises the pressure and temperature of the refrigerant, pumping it through the system.
2. Condensation:
   • Process: Refrigerant vapor travels to the condenser where heat is rejected.
   • Function: The vapor desuperheats (cools to saturation temperature), condenses into a liquid, and then subcools below its saturation temperature.
   • Condenser: Acts as a heat exchanger, transferring heat from refrigerant to another substance.
3. Metering:
   • Process: High-pressure liquid from the condenser travels to the metering device.
   • Function: The metering device reduces pressure, causing a small amount of refrigerant to flash into vapor (flash gas) and lowering its temperature.
   • Types: Thermostatic expansion valve (TXV), electronic expansion valve (EEV), and fixed bore devices like capillary tubes.
4. Evaporation:
   • Process: Refrigerant enters the evaporator at reduced pressure and temperature, causing it to boil and absorb heat from the air or water passing over the coil.
   • Function: The refrigerant fully vaporizes, and the resulting superheated vapor returns to the compressor to continue the cycle.

Analogy:

• The refrigeration cycle carries heat like a sponge carries water.
  • Evaporator: Soaks up heat like a sponge soaks up water.
  • Condenser: Rejects heat like a hand squeezing water out of a sponge.

System Components:

• Pressure Sides:
  • Low Pressure: Evaporator.
  • High Pressure: Condenser.
• Dividing Points: Compressor and metering device separate high and low-pressure sides.
• Component Variability:
  • Evaporators: Can be designed for specific functions, like maintaining or lowering humidity.
    • Example: Gravity coil evaporators in supermarkets prevent meat from drying out.
  • Defrost Systems: Important for low and medium temperature applications to prevent frost accumulation on evaporator coils.
    • Methods: Internal hot gas or external electric heaters.