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3. STRUCTURE OF SCREW CHILLER
ENGLISH
4-steps capacity control
Step-less capacity control
Figure 7 Capacity control system
Since the compressor should be started at the minimum load condition, the solenoid valve (capacity reducing sole-
noid valve in stepless control system, or minimum load solenoid valve in the stage control capacity control system)
should be kept open for 20-30 seconds before stopping or starting the compressor so that there is no oil left inside
the cylinder when starting the compressor.
- Lubrication
The main function of the lubrication oil inside the screw compressor is sealing of the inside body, cooling and capac-
ity control. The oil pressure inside the cylinder moves the piston and slide valve which are connected to piston rod in-
side the compression chamber. Since the screw compressor is designed to include the lubrication system which is
based upon the pressure difference between the high and the low pressure levels, a separate oil pump is not re-
quired inside the compressor(such as reciprocating compressor). However, for some special equipment, it is neces-
sary to install an oil pump in the compressor additionally for the safety purpose.
The bearings used in the compressor are of a rather small size, but it needs a definite amount of lubrication oil. The
purpose of feeding oil into the compression chamber is to increase the efficiency and to form oil sealing layer in order
to absorb a part of the compression heat. In order to separate the oil from the refrigerant in which oil is mixed to-
gether, the oil separator is designed to allow the oil flow into the system to the minimum. The oil temperature which
is a very important factor to the life of the compressor bearings should be watched very closely.
High oil temperature reduces the oil viscosity and worsens the lubricity and heat absorbability inside the compressor.
It is recommended to keep the oil viscosity above 10mm
2
/s at any temperature. The oil temperature should be kept
above the system condensation temperature in summer season in order to avoid the refrigerant mixed with oil enter-
ing into the system. When the outside temperature is low, the oil viscosity rises high, and the compressor which is
of the high durability should be started, thus reducing the oil pressure inside the oil piping which is needed to load oil
into the compressor. A better solution is preheating the oil in order to reach the condensing temperature in a short
period of time.
In the compressor, at least 2 oil level gages are equipped, one is high oil level gage, the other low oil level gage.
The normal amount of oil in the compressor oil tank when the compressor is running, should be in the state that
when the oil level is maintained in the middle range between the top of the low level oil gage and the top of high oil
level gage.
Summary of Contents for LCWW
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