To produce air conditioning for large areas, such as department stores and office buildings, it is necessary to use another means of cooling the air. Chilled water is used to produce the cooling needed to reduce the interior temperature of offices and stores. To understand the function of the chilled water, it is necessary to look at the total system. See Fig. 16-14.
The refrigerating machine is the chiller. Water is supplied to the chiller. There, its temperature is reduced to about 48°F (8.9°C). The chilled water then flows to the coils in the fan coil unit. The fan coil unit is located in the space to be conditioned. In some cases, a central air-handling system is used. Pumps are used to move the water between the chiller and the air-handling equipment. The water is heated by the room air that is pulled over the chilled water coils. Thus, the water reaches a temperature of about 55°F (12.8°C). In some installations it reaches 58°F (14.4°C). The water absorbs about 10°F (5.5°C) of heat as it is exposed to the room air being drawn into the unit by blowers.
The heated water is then pumped back to the chiller. There, the water is chilled again by the machine removing the absorbed heat. Once chilled, the 48°F (8.9°C) water is again ready to be pumped back to the fan coil unit or the central air-handling system. This process of recirculation is repeated as needed to reduce the temperature of the space being conditioned.
Figure 16-15 shows the refrigeration cycle of a chiller.