of this basic design, some of which provide the rotor
with blades to trap and compress the vapor.
Centrifugal compressors are used in large
refrigeration and air-conditioning systems, handling
large volumes of refrigerants at low-pressure
differentials. Their operating principles are based on
the use of centrifugal force as a means of compressing
and discharging the vaporized refrigerant. Figure 7-28
is a cutaway view of one type of centrifugal
compressor. In this application, one or two
compression stages are used, and the condenser and
evaporator are integral parts of the unit. The heart of
this type of compressor is the impeller wheel.
A scroll compressor has two different offset spiral
disks to compress the refrigerant vapor. The upper
scroll is stationary, while the lower scroll is the driven
scroll. Intake of refrigerant is at the outer edge of the
driven scroll, and the discharge of the refrigerant is at
the center of the stationary scroll. The driven scroll is
rotated around the stationary or "fixed" scroll in an
orbiting motion. During this movement, the refrigerant
vapor is trapped between the two scrolls. As the driven
scroll rotates, it compresses the refrigerant vapor
through the discharge port. Scroll compressors have
few moving parts and have a very smooth and quiet
Controls used in air conditioning are generally the
same as for refrigeration systemsthermostats,
humidistats, pressure and flow controllers, and motor
overload protectors (fig. 7-29).
The thermostat is an adjustable temperature-
sensitive device, which through the opening and
closing of its contacts controls the operation of the
Figure 7-28.Cutaway view of one type of centrifugal compressor.