Carnot and Vapor Compression Refrigeration Cycles

Posted on Dec 9, 2024 in Technology

Carnot Refrigerator

The operating principle of refrigeration circuits is based on the reversed Carnot cycle, unlike internal combustion engines. External work extracts heat from a cold source and transfers it to a warm reservoir. A Carnot refrigeration system consists of: a condenser (hot reservoir, T1), an evaporator (heat sink, T2), a compressor (increases pressure and temperature), and an expander/turbine (decreases pressure and temperature). The four stages of the cycle are:

  • The compressor, driven by a motor, increases the coolant’s pressure and temperature adiabatically from P1 to P2 and Tf to Tc (Stage 1-2).
  • The fluid reaches the condenser, liquefies, and releases heat, Q1, to the cooling system at a constant temperature, Tc (Stage 2-3).
  • In the expander, the fluid expands adiabatically, cooling down by decreasing pressure and temperature to Tc, leading to evaporation (Stage 3-4).
  • The fluid reaches the evaporator, expands isothermally at Tc, vaporizes almost entirely at constant pressure, absorbing heat, Q2, from the cooling system (Stage 4-1).

Practical cooling cycles differ from the Carnot cycle in two ways: actual processes are not reversible, and compression involves saturated vapor rather than a liquid-vapor mixture. Instead of a turbine, a throttle valve or expansion valve is used for expansion. This modified cycle is closer to real systems.

There are two main types of cooling systems: vapor and gas. In vapor systems, the refrigerant alternates between vaporization and condensation. In gas systems, the refrigerant remains gaseous.


Vapor Cooling Systems (Gas and Steam)

In vapor cooling systems, the processes are:

  • Adiabatic compression of the liquid-vapor mixture entering the compressor until it becomes saturated steam. Pressure and temperature increase, requiring work input (Stage 1-2).
  • Heat transfer from the refrigerant to the outside (hot reservoir). The fluid condenses, releasing latent heat of vaporization at constant temperature and pressure (Stage 2-3).
  • Adiabatic expansion in a turbine (or expansion valve), converting saturated liquid to a liquid-vapor mixture, producing work and decreasing temperature and pressure. An accumulator stores fluid from the condenser (Stage 3-4).
  • Heat absorption from the cold source by the coolant in the evaporator. The liquid-vapor mixture becomes enriched in vapor, returning to the initial conditions to start a new cycle (Stage 4-1).

In gas cooling systems, the cycle starts with adiabatic expansion in the turbine, producing work (Stage 3-4). Then, the gas absorbs heat from the cold source, increasing its temperature (Stage 4-1). Next, the gas is compressed adiabatically, increasing pressure and temperature (Stage 1-2). Finally, the gas releases heat to the outside, returning to the initial conditions (Stage 2-3).

Refrigeration applications include: cooling chambers for food preservation, refrigerated transport, air conditioning, etc. The environment serves as the heat sink and hot reservoir.