LG RCWW Operation & Maintenance Manual page 15

3. STRUCTURE OF SCREW CHILLER
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- Compression process
As shown in Figure 6 below, during the rotation of the rotors, the absorbed gas is pushed to the discharge side. In other
words, the V-shaped space formed between the male and female rotors is continuously moving to the discharge side.
The gas sealed in the lobe space which is reduced by the discharge side is compressed, and the highly compressed gas at
the end part is discharged to the oil separator. The size and geometry of the outlet port determine the so-called "internal
volume ratio (Vi)" of the compressor. In actual operation, if the compression ratio is either bigger or smaller than this value,
the efficiency will be lowered.
(C) Discharge (B) Compression (A) Suction and sealing
Figure 6 Compression process
- Capacity control system
The screw series compressors are equipped with either 3-step/4-step capacity control system or continuous capacity
control system.
The capacity control system consists of slide valve, piston rod, cylinder, piston and piston rings while slide valve and
the piston are connected by the piston rod. The principle of operation is using the oil pressure to drive the piston in
the cylinder as shown in Figure 6, the lubrication oil flows from the oil tank through the oil filter and capillary, then is
charged into the cylinder due to the positive oil pressure which is bigger than the opposite side of spring force plus
the high pressure of the gas.
This pressure difference causes the piston to move toward the right side in the cylinder and the slide valve to move
toward the right side, increasing the amount of compressor outlet, thereby increasing the refrigeration capacity.
Also, when a solenoid valve (in 3-step/4-step capacity control system) is opened, the high pressure oil in the cylinder
is bypassed, which causes the piston and the slide valve to move toward the left side, and some of the refrigerant
gas returns to the suction side. As a result, the discharge amount is reduced and the refrigeration capacity decreases.
When the compressor is stopped, the piston spring pushes the piston back to the original position, thereby reducing
the starting current for the next startup.
If the compressor started at full load capacity it may result in over-current start.
The capillary inhibits the oil flow and makes the piston move smoothly under stepless control or controlled by micom
or temperature switch.
If the oil filter, capillary, or solenoid valves are not working properly, the capacity control system will be operated ab-
normally and make the system useless.