Refrigerant Properties and Heat Pump Cycles

Refrigerant Properties

• They can be toxic.
• They can be highly flammable or pose an explosion risk.
• They must possess low viscosity to facilitate flow and prevent leakage issues.
• They should not be corrosive to metals used in the system.

Refrigeration Cycle Explained

The process achieves a cooling effect through these steps:

1. Energy (W) is supplied to the compressor, compressing the refrigerant gas to high pressure and temperature.
2. The high-pressure, hot gas moves to the condenser, where it releases heat (Q1) to the surroundings (e.g., outdoor air or water) and liquefies.
3. The high-pressure liquid refrigerant then passes through an expansion valve (or throttling device), where its pressure and temperature drop significantly.
4. In the evaporator, the low-pressure, cold refrigerant absorbs heat (Q2) from the space or substance to be cooled. This absorption causes the refrigerant to evaporate back into a low-pressure gas, producing the cooling effect.
5. The low-pressure gas returns to the compressor, completing the cycle.

Heat Pump Operation Principle

A heat pump operates on the same thermodynamic cycle as a refrigerator. However, its primary purpose is to utilize the heat rejected by the condenser (Q1) for heating, rather than focusing on the cooling effect at the evaporator (Q2).

Water-to-Water Heat Pump

This type of heat pump uses water as both the heat source and the heat distribution medium:

• The heat source is often water from a well or ground loop, typically at a temperature between 13°C and 15°C.
• A pump circulates this source water through the evaporator. Here, the refrigerant absorbs heat (Q2) from the water, causing the refrigerant to evaporate.
• The source water exits the evaporator at a lower temperature (e.g., around 5°C) and is typically discharged or recirculated.
• Water circulating through the building's heating system (e.g., radiators or underfloor heating) passes through the condenser. This water absorbs the heat (Q1) released by the refrigerant as it condenses back into a liquid.
• The result is hot water output, often around 50°C, suitable for space heating.

Air-to-Air Heat Pump

This type of heat pump uses air as both the heat source and the heat distribution medium:

• The heat source (in heating mode) is the ambient outdoor air, potentially at a temperature like 15°C (though they can operate at much lower temperatures). A fan draws this air over the outdoor coil, which functions as the evaporator.
• The refrigerant absorbs heat (Q2) from the outdoor air and evaporates. The air leaving the outdoor coil is colder (e.g., around 5°C).
• Inside the building, indoor air (e.g., at 20°C) is circulated by another fan over the indoor coil, which functions as the condenser.
• The hot refrigerant gas releases its heat (Q1) to the indoor air as it condenses.
• The heated air (e.g., up to 50°C or so) is then distributed through ducts or vents to warm the indoor space.

Thermodynamic Cycle Stages

The ideal vapor-compression refrigeration cycle can be broken down into four main thermodynamic processes, often visualized on pressure-enthalpy or temperature-entropy diagrams:

• AB: Compression - In the compressor, the low-pressure refrigerant vapor is compressed isentropically (ideally) to a high pressure and high temperature. Volume decreases, pressure increases, and work (W) is input.
• BC: Condensation - In the condenser, the high-pressure, high-temperature vapor rejects heat (Q1) to the surroundings at constant pressure, condensing into a high-pressure liquid. Volume decreases significantly.
• CD: Expansion - The high-pressure liquid passes through an expansion valve, undergoing a throttling process (constant enthalpy, ideally). Pressure and temperature drop significantly, and a portion of the liquid flashes into vapor, resulting in a low-temperature, low-pressure liquid-vapor mixture. Volume increases.
• DA: Evaporation - In the evaporator, the low-pressure, low-temperature mixture absorbs heat (Q2) from the space being cooled at constant pressure. The liquid evaporates completely into a low-pressure vapor. Volume increases sharply.