The refrigeration cycle is the basis of compression heat pumps. During this cycle, a chemical compound called the refrigerant circulates around a closed piping loop passing through all major components of the heat pump. These major components are named based on how they affect the refrigerant passing through them. They are as follows:
• Thermal expansion valve (TXV)
The basic arrangement of these components to form a complete refrigeration circuit are shown in Figure 2-1.
To describe how this cycle works, a quantity of refrigerant will be followed through the complete cycle.
The cycle begins at station (1) as cold liquid refrigerant within the evaporator. At this point, the refrigerant is colder than the source media (e.g., air or water) passing across the evaporator. Because of this temperature difference, heat moves from the higher-temperature source media into the lower-temperature refrigerant. As the refrigerant absorbs this heat, it changes from a liquid to a vapor (e.g., it evaporates). The vaporized refrigerant continues to absorb heat until it is slightly warmer than the temperature at which it evaporates. The additional heat required to raise the temperature of the refrigerant above its saturation temperature (e.g., where it vaporizes) is called superheat, which also comes from the source media.
This vaporized refrigerant then flows on to the compressor at station (2). Here a reciprocating piston or an orbiting scroll driven by an electric motor compresses the vaporized refrigerant. This causes a large increase in both pressure and temperature. The electrical energy used to operate the compressor is also converted to heat and added to the refrigerant. The temperature of the refrigerant gas leaving the compressor is usually in the range of 120º to 170ºF depending on the operating conditions.
The hot refrigerant gas then flows into the condenser at station (3). Here it transfers heat to a stream of water or air (e.g., the sink media) that carries the heat away to the load. As it gives up heat, the refrigerant changes from a high-pressure, high-temperature vapor into a high-pressure, somewhat cooler liquid (e.g., it condenses).
The high-pressure liquid refrigerant then flows through the thermal expansion valve at station (4), where its pressure is greatly reduced. The drop in pressure causes a corresponding drop in temperature, restoring the refrigerant to the same condition it was in when the cycle began. The refrigerant is now ready to repeat the cycle.
The refrigeration cycle remains in continuous operation whenever the compressor is running. This cycle is not unique to heat pumps. It is used in refrigerators, freezers, room air conditioners, dehumidifiers, water coolers, vending machines and other heat-moving machines.
Figure 2-2 shows the three primary energy flows involved in the refrigeration cycle. The first energy input is low-temperature heat absorbed from the source media into the refrigerant at the evaporator. The second energy input is electrical energy flowing into the compressor whenever it is operating. The third energy flow is the heat output into the sink media at the condenser.
The first law of thermodynamics dictates that, under steady state conditions, the total energy input rate to the heat pump must equal the total energy output rate. Thus, the sum of the low-temperature heat absorption rate into the refrigerant at the evaporator, plus the rate of electrical energy input to the compressor, must equal the rate of energy dissipation from the refrigerant at the condenser. This is depicted by the arrows in Figure 2-2.