Refrigeration cycle
Refrigeration is a process requiring energy and work to move heat out of a cooler space to a warmer space for some benefit such as comfort, manufacturing processes, or food preservation and storage.
In the refrigeration cycle, work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Heat naturally flows in the opposite direction. Insulation is used as means to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space.
The most common types of refrigeration systems employ a working fluid called a refrigerant in a cycle where the refrigerant repeatedly changes state from liquid to vapor and back to liquid. The refrigerant is condensed to release heat in one part of the cycle and is boiled (or evaporated) to absorb heat in another part of the cycle.
The most common refrigeration cycle uses an electric motor to drive a mechanical compressor. The compressor does not create a cooling effect directly. The cooling effect is created when a suitable refrigerant boils and absorbs heat from the cooled space through a heat exchanger. You must understanding the refrigerant flow and changes of state to understand the refrigeration cycle:
The first part of the cycle causes refrigerant vapor to be recycled into its liquid form by extracting heat from a high temperature vapor. The compressor compresses a relatively low-pressure and low-temperature refrigerant vapor drawn from the evaporator coil. During compression, the refrigerant vapor is heated by compression itself (PV=RT) and the work of compression to create a high-temperature and high-pressure vapor. Then the vapor is pushed into a heat exchanger known as a condenser located in a higher temperature heat sink that is located outside of the space being cooled. In the condenser, heat is removed from the refrigerant so that it condenses to a liquid state.
The second part of the cycle begins after the liquid refrigerant leaves the condenser as a high temperature, high-pressure liquid and passes through a refrigerant metering device into the cooling coil or evaporator on the low-pressure side of the system. The compressor pumps the refrigerant out of the evaporator at a rate sufficient to cause the pressure and temperature of the refrigerant to drop well below its boiling point as it moves through an interior heat exchanger coil known as the evaporator. This boiling liquid refrigerant absorbs heat energy from the interior space through the walls of this evaporator. The system is designed to completely evaporate liquid refrigerant into a low-pressure vapor within the interior coil before it returns to the compressor to repeat the cycle.
The four essential components of the mechanical refrigeration cycle are the compressor, the condenser, the refrigerant metering device and the evaporator. These four components must be selected or matched for the application and to each other in order for the system to work well and efficiently. None of these parts produce a refrigeration effect. Boiling (or rapidly evaporating) refrigerant absorbs heat and creates the benefit of refrigeration. The refigeration cycle allows a small amount of refrigerant to be cycled and recycled for decades of use.
