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Refrigeration Cycle

The refrigeration cycle uses a fluid, called a refrigerant, to move heat from one place to another. The key to understanding how it works is recognizing that at the same pressure, the refrigerant boils at a much lower temperature than water. For example, the refrigerant commonly used in home refrigerators boils between 40 and 50°F as compared to water's boiling point of 212°F. Let's look at the process to see how boiling and condensing a refrigerant can move heat. The process is the same whether it is operating a refrigerator, an air conditioner or a heat pump. This example illustrates a closed-loop system.

We'll begin with the cool, liquid refrigerant entering the indoor coil, operating as the evaporator during cooling. As its name implies, refrigerant in the evaporator "evaporates." Upon entering the evaporator, the liquid refrigerant's temperature is between 40 and 50°F and without changing its temperature, it absorbs heat as it changes state from a liquid to a vapor. The heat comes from the warm, moist room air blown across the evaporator coil. As it passes over the cool coil, it gives up some of its heat and moisture may condense from it. The cooler, drier room air is re-circulated by a blower into the space to be cooled.

The vapor refrigerant now moves into the compressor, which is basically a pump that raises the pressure so it will move through the system. Once it passes through the compressor, the refrigerant is said to be on the "high" side of the system. Like anything that is put under pressure, the increased pressure from the compressor causes the temperature of the refrigerant to rise. As it leaves the compressor, the refrigerant is a hot vapor, roughly 120 to 140°F.

It now flows into the refrigerant-to-water heat exchanger, operating as the condenser during cooling. Again, as the name suggests, the refrigerant condenses here. As it condenses, it gives up heat to the loop, which is circulated by a pump. The loop water is able to pick up heat from the coils because it is still cooler than the 120 degree coils.

As the refrigerant leaves the condenser, it is cooler, but still under pressure provided by the compressor. It then reaches the expansion valve. The expansion valve allows the high pressure refrigerant to "flash" through becoming a lower pressure, cooled liquid. When pressure is reduced, as with spraying an aerosol can or a fire extinguisher, it cools. The cycle is complete as the cool, liquid refrigerant re-enters the evaporator to pick up room heat. In winter, the reversing valve switches the indoor coil to operate as the condenser and the heat exchanger as the evaporator.

In summary, the indoor coil and refrigerant-to-water heat exchanger is where the refrigerant changes phase, absorbing or releasing heat through boiling and condensing. The compressor and expansion valve facilitate the pressure changes, increased by the compressor and reduced by the expansion valve.