Air is used in cement industry for operation of various equipment like, pneumatic
cylinders, aeration systems, dense phase or lean phase material conveying systems
etc. Pressure required for the air depends upon the applications. For low pressure
i.e. below 1 kg/cm2 air requirement, the twin lobe blower is used, where as for the
high pressure i.e. 7 to 10 kg/cm2 air requirement, the air compressors are used. In
air compressors there are two types of compressors generally used in cement
industry i.e. Reciprocating type and Rotary Screw type. Reciprocating compressors
were widely used in yester years. In the latest development these compressors are
fading out slowly and their place is taken by rotary screw compressors, which are
widely used at present in industry.
Following are the descriptive details of these compressors.
10.1 Rotary Screw compressors
Now a day, the lubricated rotary-screw air compressor is the most widely used
design for cement Industries applications. It’s characterized by low vibration, a simple
installation with minimal maintenance in broad ranges of capacity and pressure. A
rotary-screw air end consists of two close-clearance helical rotors turning in
synchronous mesh. The male rotor has four helical lobes and the female rotor has
five or six mating grooves. In a lubricated rotary-screw compressor, the male rotor
drives the female rotor.
Ambient air is drawn through a suction port into a space between the spinning
rotors, which then force the air into the decreasing inter-lobe cavity until it reaches
the discharge port at the opposite end of the rotor. Oil injected into the rotor
housing lubricates the moving parts, removes heat and seals the clearances to
prevent back slippage of the compressed air. The air/lubricant mixture discharges
into the lubricant reservoir, which also serves as a separator.
The lubricant separated from the air stream circulates through a cooler and filter
before being injected back into the air end. The temperature of the lubricant at
the injection port needs to be high enough to prevent condensation from forming
in the lubricant. Air-cooled units use a thermostatic valve to maintain an injection
tem perature of 65 Deg.C to 75 Deg.C. Water-cooled packages use a water flow
regulating valve or thermostatic valve, or both.
To remove condensate, an after cooler is used to reduce the discharge air temperature
and a moisture separator with an automatic drain. Most industrial applications
use air-cooled heat exchangers for the lubricant and compressed air. Water-cooled
models use shell-and-tube exchangers.
Since the cooling takes place right inside the compressor, the working parts never
experience extreme operating temperatures. The rotary screw compressor, there
fore, is a continuous duty, air cooled or water cooled compressor package.
The Fig.1 and 2 shows the working of a screw compressor.