Fundamentals of Vapor-Compression Refrigeration Systems

1. Principle of Operation of Cooling Machines

Cooling machines operate based on a closed thermal cycle. Unlike a heat engine, a cooling machine requires an external energy input (Work, W) to move heat from a cold source to a hot source.

In this cycle:

• Heat quantity Q₂ is absorbed from the cold source (the space being cooled) in the evaporator.
• Heat quantity Q₁ is rejected to the hot source (the environment) in the condenser.

Since the system requires work input, the total heat rejected (Q₁) must be greater than the heat absorbed (Q₂), following the relationship: Q₁ = Q₂ + W. The cycle is closed, meaning the working fluid returns to its initial conditions after completion, allowing continuous operation.

2. Refrigerant Fluid Characteristics

The refrigerant fluid is the working substance that facilitates heat transfer. It is designed to easily change state (liquid to gas and back) at specific temperatures and pressures. Refrigerants must meet several critical requirements:

Required Properties of Refrigerants

• They must not be toxic or flammable, as these characteristics are highly unsuitable and can cause explosions.
• Viscosity must be appropriate to ensure efficient flow and prevent system escape.
• They must be non-corrosive to the metals used in the refrigeration system components.

Common Types of Refrigerants

Historically, various fluids have been used, though many are now restricted due to environmental or safety concerns:

• Propane and Butane: These are inexpensive but dangerous due to their high flammability.
• Sulfur Dioxide (SO₂): Highly toxic.
• Chlorofluorocarbons (CFCs): These substances, such as Freon or R113, severely damage the ozone layer and are largely phased out.
• Hydrochlorofluorocarbons (HCFCs): These are widely used today, serving as transitional refrigerants while the industry moves toward more environmentally friendly alternatives.

3. Vapor-Compression Refrigeration System Configuration

Vapor-compression cooling machines are composed of four basic elements that facilitate the continuous thermodynamic cycle:

The Four Basic Elements

Compressor

The compressor receives the refrigerant fluid in a vapor state and compresses it, increasing its pressure and temperature. This operation requires the input of energy (W), typically provided by an electric motor.

Condenser

The high-pressure, high-temperature refrigerant vapor is cooled, causing it to condense back into a liquid state. During this phase change, the heat quantity Q₁ is released to the surrounding environment.

Expansion System (or Valve)

This component reduces the pressure of the liquid refrigerant, preparing it to enter the evaporator at a much lower pressure suitable for absorbing heat at the desired cold temperature.

Evaporator

The low-pressure liquid refrigerant evaporates (boils) inside the evaporator. This phase change absorbs the heat quantity Q₂ from the surrounding area (the cooling chamber). This process effectively cools the chamber, transferring the heat to the refrigerant fluid.

After evaporation, the fluid returns to its initial state (low-pressure vapor) and passes back to the compressor, completing the cycle and initiating a new one.