The Heat Pump Refrigeration Cycle

A refrigerant is a fluid, which vaporizes (boils) at a low temperature. The refrigerant circulates through tubes (refrigerant lines) that travel throughout the heat pump. We'll begin our description of the refrigeration cycle at point A on the illustration below, which describes the heat pump when it is heating the house.


 At point A the refrigerant is a cold liquid, colder than the outdoor air. The refrigerant flows to the outdoor coil (point B). This coil is a "heat exchanger" with a large surface area to absorb heat from the air into the colder refrigerant. The heat added to the refrigerant causes the fluid to vaporize, so this heat exchanger is called the "evaporator coil" during the heating cycle. When materials change state (in this case from liquid to gas), large amounts of energy transfer take place. At point C the refrigerant is a cool gas, having been warmed and vaporized by the outdoor air. It is too cool to warm the house, so that's where the compressor (point D) comes in. The compressor raises the pressure of the gas. When that happens, the gas temperature rises. One way to think about it is that the compressor concentrates the heat energy. The compressor is often thought of as the "heart" of the heat pump, since it does most of the work of forcing heat "uphill." The compressor also forces the now hotgas (point E) further into the cycle. The indoor coil (point F) is where the refrigerant gives up its heat to the indoor air. A fan blows air past the indoor coil to distribute heat to the house. This cools the refrigerant to the point where much of it condenses, forming a liquid. In the heating season, the indoor coil is called the "condenser coil." This change of state results in a large transfer of heat energy. The warm mixture of liquid and gas (point G) continues through the cycle to point H, the expansion device (sometimes called a "metering device"). This device reduces the pressure, causing the refrigerant, to become cold again - cold enough so that it is once again ready to absorb heat from the cool outdoor air and repeat the cycle.