Project Development and Documentation: Refrigeration Cycles

Refrigeration cycles are used in many mechanical systems and can be somewhat difficult to understand at first. There are four main components to the refrigeration cycle: compressor, condenser, expansion valve, and evaporator. A refrigerant flows within the lines (pipes) of the system and enters through those components. To start at a point (since it is a cycle), the vapor leaves the low-pressure side of the evaporator to the high-pressure side via the compressor. The compressor compresses the refrigerant, making the vapor the hottest at that point. After the vapor leaves the compressor, it enters the condenser where the condenser rejects the heat and the vapor turns into liquid state. The liquid then travels to the expansion valve/thermal expansion valve where it enters the low-pressure side of the system and through the expansion valve to turn back to liquid state. It then travels through the evaporator, where heat is absorbed and continues in the loop to the compressor. There is a phase change between liquid and gas with the mediating substance.

Why a refrigeration cycle? For one, it creates a closed loop system that supplies both heating and cooling-potentially. It is most common in smaller systems like a through-wall air-conditioning unit. The compression and expansion of the refrigerant (which is designed to do this efficiently) work to create cooling by removing the heat from the system. That is an important concept in cooling systems. Cooling is created by removing the heat from the system, not by adding actual cooling, which is a common misconception. The refrigeration cycle removes the heat from the interior to the exterior (this works the same in refrigerators). Pressure is employed to create this system-when the pressure compresses, heat is created. When pressure is released and the agent is expanded, cooling occurs. This all happens within the coils of the system. Fans are employed to run air over the coils and expel cool or warm air, depending on the cycle. Often, units with a refrigerant cycle work in one direction, supplying heating or cooling. However, there are reversible systems that supply both heating and cooling by reversing the loop.

The medium within the coils-the refrigerant-is an engineered substance in which the properties of the substance meet operating pressures. Traditionally, these substances have been considered toxic or harmful for the environment as well as contributing to the depletion of the ozone layer. Because of this, refrigerants are assigned classifications and ratings of OPD (Ozone Depletion Potential) and (GWP) Global Warming Potential.

In regard to the larger system/unit, a COP (or coefficient of performance) is determined, which is a measure of efficiency for the mechanical unit. The COP is measured by the amount of heat removed as a ratio to the amount of work needed to do so; however, the COP differs between the heating and cooling cycles. Often, the COP or other energy-efficient measurement (SEER) is requested by AHJs to prove the efficiency of mechanical equipment.