Understanding the Vapour Compression Refrigerator Cycle

A vapour compression refrigerator is a system used for cooling. It works by circulating a refrigerant through a cycle of phase changes and pressure variations. The fundamental operation of such a system is based on a specific thermodynamic cycle.

Let's look at the cycles mentioned in the options:

• Rankine Cycle: This is a thermodynamic cycle used to generate mechanical power from heat. It is the fundamental operating cycle for steam power plants. It involves processes like pumping, boiling, expansion through a turbine, and condensation. This is a power generation cycle, not a refrigeration cycle.
• Carnot Cycle: This is an ideal thermodynamic cycle that represents the most efficient possible engine or refrigerator operating between two temperature reservoirs. As a power cycle, it involves isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. As a reversed cycle (Reversed Carnot), it represents the theoretical maximum efficiency for refrigeration. However, achieving the isothermal heat transfer processes perfectly in practice is difficult due to the need for extremely slow heat transfer rates, making it an impractical cycle for real refrigerators.
• Reversed Rankine Cycle: This cycle is essentially the Rankine cycle running in reverse. Instead of generating power by using heat, it uses power (input work) to transfer heat from a low-temperature reservoir to a high-temperature reservoir, achieving refrigeration. This cycle typically involves four main processes:
  1. Evaporation (Refrigerant absorbs heat at low temperature and pressure)
  2. Compression (Work is done on the refrigerant, increasing its pressure and temperature)
  3. Condensation (Refrigerant releases heat at high temperature and pressure)
  4. Expansion (Refrigerant's pressure drops, causing a temperature decrease)
  This cycle closely models the practical processes occurring in a vapour compression refrigeration system.
• Reversed Carnot Cycle: As mentioned, this is the ideal, theoretically most efficient refrigeration cycle. While the Reversed Rankine cycle aims to approach the performance of the Reversed Carnot cycle, it deviates in key areas, such as using an expansion valve instead of an expander (turbine) and non-isentropic compression and expansion processes in real systems.

The vapour compression refrigeration system utilizes processes that align most closely with the Reversed Rankine cycle. It is a practical realization of a refrigeration cycle that can be implemented using standard components like compressors, condensers, expansion valves, and evaporators.

Therefore, the vapour compression refrigerator employs the Reversed Rankine cycle.