Self-contained refrigerative air-conditioning units are either window units (figs. 4-8 and 4-9) or larger

Figure 4-11.-Refrigerating cycle of a package type air-conditioning unit.

floor-mounted units (fig. 4-10). Both types of units contain a complete system of refrigeration components. The window units need not be installed in windows. They also can be installed in transoms, or they can be framed into outside walls. The use of outside walls is important for proper performance. When the unit is operating, the compressor (fig. 4-11) forces a high-- pressure (high-temperature) refrigerant gas to the condenser. The condenser fan draws in and blows outside air over the condenser coils. This movement of the relatively cooler outside air over the hot condenser coils changes the gas to a liquid, giving off heat that is exhausted to the outside. The liquid then passes through a control device that regulates the flow of the liquid to the evaporator. In the evaporator, the liquid changes to a low-pressure (low-temperature) gas that is circulated through the evaporator coils. The inside or room air is then circulated by an evaporator fan over the cold evaporator coils. This action removes heat from the air and returns the cooled air back to the room. A variation of this type of unit is the heat pump. In a heat pump, the roles of the condenser and the evaporator can be reversed so that the unit draws in and heats outside air and expels cold inside air. In this way, the unit functions as a heating unit, rather than a cooling unit.