Handbook of air conditioning and refrigeration / Shan K

11.44 CHAPTER ELEVEN
Scroll Compressors
Condenser
coil
Scroll
compressors
Figure 11.25 shows an air-cooled, scroll DX cooler. It consists of two scroll compressors, an aircooled
condenser, two DX coils, and two thermostatic expansion valves (TXVs). A scroll refrigeration
system has almost the same refrigerant piping and safety controls as the reciprocating system
except there is no crankcase heater.
An air-source scroll heat pump has nearly the same system structure as an air-cooled, scroll DX
cooler except that a four-way reversing valve is added in the heat pump. Heat pumps are discussed
in the next chapter.
In 1987, the first U.S. manufacturer using a computer-aided manufacturing process produced a
scroll compressor for residential air conditioning with tolerances up to 0.0001 in. (0.0025 mm). A
scroll compressor consists of two identical spiral scrolls mated face to face, as shown in Fig. 11.26.
Each scroll is bound on one side to a flat plate. The upper scroll is stationary and contains the discharge
port. The lower one moves in an orbit around the shaft center of the motor at an amplitude
equal to the orbit radius. The two scrolls are fitted together to form vapor pockets between their respective
baseplates and lines of contacts of their vane walls. During suction, vapor refrigerant enters
the space between the two scrolls through the lateral openings. The openings are then sealed off to
form trapped vapor pockets, thus completing the intake process.
During the successive revolutions of the motor shaft, the volume of the trapped vapor pockets is
reduced, and the compression process is complete when the gaseous refrigerant is compressed to its
maximum pressure and just before the compressed gaseous pocket is opened to the discharge port.
During discharge, the trapped gas pockets opens to the discharge port. All compressed hot gas is
discharged through the discharge port to the discharge line. The volume of the gas pockets is
reduced to zero. The sequence of intake, compression, and discharge occurs simultaneously in the
two trapped gas pockets.
Radial and Axial Compliance
Air-cooled
condenser
FIGURE 11.25 An air-cooled scroll DX cooler.
Charging
valve
Filter
dryer
Solenoid
valve
Sight
glass
DX coils
(evaporators)
In scroll compressors, the scrolls touch one another with force sufficient to create a seal but not
sufficient to cause wear. This is the result of radial and axial compliance. Continuous flank contact
because of the centrifugal force minimizes the gas leakage.
TXV