Heat pumps circulate a fluid refrigerant that cycles
alternatively from its liquid phase to its vapor phase in a
closed loop. The refrigerant, starting as a low-
temperature, low-pressure vapor, enters a compressor
driven by an electric motor. The refrigerant leaves the
compressor as a hot, dense vapor and flows through a
heat exchanger called the condenser, which transfers
heat from the refrigerant to a body of air. Now the
refrigerant, as a high-pressure, cooled liquid, confronts
a flow restriction which causes the pressure to drop. As
the pressure falls, the refrigerant expands and partially
vaporizes, becoming chilled. It then passes through a
second heat exchanger, the evaporator, which transfers
heat from the air to the refrigerant, reducing the
temperature of this second body of air.