APCW Case Study

5. ROTARY SCREW COMPRESSOR

The compressors used in APCW, Ultra Tech Cements Limited are single stage, oil-flooded, water cooled rotary screw compressors.
5.1. Screw compression Process
The rotary screw compressor draws air in between the lobes and then inter lobular space is sealed and volume is reduced and then sent to outlet.
Position 1:
As the lobe-flute pair unmeshes, the air inlet is uncovered and air at atmospheric pressure is drawn into the space between lobe-flute pair.
Position 2:
As the rotation continues, the inter-lobular space is sealed off from air inlet and compression continues.
Position 3:
The compression continues as lobe-flute pair moves from one end to another end by reducing the volume of air.
Position 4:
As the lobe-flute

Rotor set

The rotor set has a male rotor and a female rotor. The male rotor has four helical lobes which are 90 degrees apart and female rotor has six helical flutes 60 degrees apart. The male rotor is driven by the motor and female is meshed with male. For meshing the male has to rotate 1.5 times faster than female. The asymmetrical rotor profile is used for rotors.
In symmetric profile, the lobe of male rotor perfectly fits flute freely but shallowly. Asymmetric profile fits male lobe more deeply into female flute than symmetric profiles. The advantages of asymmetric profile are
1. Smaller blow-hole between rotors leads to lower internal losses resulting in higher efficiency.
2. Higher capacity for same size rotors and same rpm as profile is cut more deeply.
3. Ability of rotors to withstand rotor contact without problems for oil injected screws, due to small relative motion between the rotors where contact occurs. This means we do not need timing gears.

5.2.2. Rotor speed

As the compressor works on principle of positive displacement, it is obvious that higher rpm the compressor has, higher the quantity of air it delivers. As we increase the speed of screws the losses will be increasing dragging the efficiency down. The losses have to be considered while choosing the compressor. The major losses which are to be considered