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HVAC Fundamentals: Refrigeration
COMPRESSORS
A variety of compressor types are available to meet
the many refrigeration and air-conditioning applications.
Several types are discussed in this section, but more
detail can be found in the ASHRAE Handbook—HVAC
Systems and Equipment (ASHRAE 2004a, chapter 34).
Figure 2.4 provides diagrams of two common hermetic
designs. The term hermetic indicates the motor and
compressor are encapsulated into a pressure vessel to
eliminate the compressor drive shaft leakage that occurs
with open compressor designs. The suction port opens
directly into the casing, while the discharge port and
electrical leads enter through pressure-sealed passages.
Motor cooling and lubricating oil circulation are accomplished
with strategic injection of the return gas. The
reciprocating piston design shown in Figure 2.4 places
the motor above the compressor. The orbiting scroll
design has become popular because of the reduction in
moving components. Advances in precision manufacturing
have resulted in scroll compressors with slightly
higher efficiency compared to reciprocating models.
Figure 2.5 shows the scroll compression cycle.
Another type of compressor often used in a variety
of applications is the rotary vane. One design is shown
in Figure 2.6. Larger compressors are typically “open”
drives in that they are connected through a shaft to a
motor (or engine). Figure 2.7 is a diagram of an open
drive twin-screw compressor used for larger applications.
Figure 2.8 is a complete chiller package with a
centrifugal compressor that is typically used in large
commercial and industrial applications. Performance
data for these larger machines will be provided in chapter
5.
The impact of operating characteristics on the
actual performance of a compressor can be summarized
in curves or tables. Table 2.3 is a typical presentation
that includes the gross compressor refrigeration capacity,
the required input power, and the refrigerant mass
flow rate for a hermetic scroll compressor with R-134a
refrigerant.
Table 2.3 provides the performance information for
various combinations of evaporating and condensing
temperatures at conditions that are typically encountered
in air-conditioning applications. The temperature of the
vapor entering the compressor is 20°F (superheated)
above the evaporating temperatures in the table. Com-
Figure 2.7
Rotary twin-screw compressor.
Figure 2.8
Chiller with centrifugal compressor.
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