Page 457 - Mechanical Engineers' Handbook (Volume 4)

446 Refrigeration

piston compressors are usually found in household refrigerators and small air conditioners
up to inputs of 2 kW. For a fixed-vane, rolling piston rotary compressor, the shaft is located
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in the center of the housing while the roller is mounted on an eccentric. Suction gas enters
directly into the suction port. As the roller rotates, the refrigerant vapor is compressed and
is discharged into the compressor housing through the discharge valve.
One difference between a rotary and a reciprocating compressor is that the rotary is able
to obtain a better vacuum during suction. 18 It has low reexpansion losses because there is
no high-pressure discharge vapor present during suction as with a reciprocating compressor.
Because rotary vane compressors have a light weight for their displacement, they are
ideal for transportation applications. Rotary vane compressors can be used in applications
where temperatures drop down to 40 to 51 C, depending whether it is in a single- or
two-stage system. Refrigerants R-22, R-404a, and R-717 are currently used with rotary vane
compressors. 18

Scroll Compressors
The principle of the scroll compressor was first patented in 1905. However, the first com-
19
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mercial units were not built until the early 1980s. Scroll compressors are used in building
air-conditioning, heat pump, refrigeration, and automotive air-conditioning applications. They
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range in capacity from 3 to 50 kW. Scroll compressors have two spiral-shaped scroll mem-
bers that are assembled 180 out of phase (Fig. 10). One scroll is fixed while the other
‘‘orbits’’ the first. Vapor is compressed by sealing vapor off at the edge of the scrolls and
reducing the volume of the gas as it moves inward toward the discharge port. Figure 10a
shows the two scrolls at the instant that vapor enters the compressor and compression begins.
The orbiting motion of the second scroll forces the pocket of vapor toward the discharge
port while decreasing its volume (Fig. 10b–10h). In Fig. 10c and f, the two scrolls open at
the ends and allow new pockets of vapor to be admitted into the scrolls for compression.
Compression is a nearly continuous process in a scroll compressor.
Scroll compressors offer several advantages over reciprocating compressors. First, rel-
atively large suction and discharge ports can be used to reduce pressure losses. Second, the
separation of the suction and discharge processes reduces the heat transfer between the
discharge and suction processes. Third, with no valves and reexpansion losses, they have
higher volumetric efficiencies. Capacities of systems with scroll compressors can be varied
by using variable speed motors or by use of multiple suction ports at different locations
within the two spiral members. Fourth, with a smaller number of moving parts, they have
the potential to be more reliable and quieter than reciprocating compressors.

Screw Compressors
2
Screw compressors were first introduced in 1958. These are positive displacement machines
available in the capacity range from 15 to 1100 kW, overlapping reciprocating compressors
for lower capacities and centrifugal compressors for higher capacities. Both twin-screw and
single-screw compressors are used for refrigeration applications.
Fixed suction and discharge ports, used instead of valves in reciprocating compressors,
set the ‘‘built-in volume ratio’’ of the screw compressor. This is the ratio of the volume of
fluid space in the meshing rotors at the beginning of the compression process to the volume
in the rotors as the discharge port is first exposed. Associated with the built-in volume ratio
is a pressure ratio that depends on the properties of the refrigerant being compressed. Peak
efficiency is obtained if the discharge pressure imposed by the system matches the pressure
developed by the rotors when the discharge port is exposed. If the interlobe pressure is
greater or less than discharge pressure, energy losses occur but no harm is done to the
compressor.