These compressors are not of reciprocating nature, therefore does not have any pistons and crankshaft. Instead, these compressors have screws, vanes, scrolls, and other devices which rotate and thus compress air. The rotary compressors are classified into screw type, vane type-lobe type, scroll type and other types.
The screw compressors are efficient in low air pressure requirements. Two screws rotate intermeshing with each other, thus trapping air between the screws and the compressor casing, forming pockets which progressively travel and gets squeezed and delivering it at a higher pressure which opens the delivery valve. The compressed air delivery is continuous and quiet in operation than a reciprocating compressor.
The Vane type Air Compressor is having a fixed casing and a rotary rotor disc which has slots for holding the sliding plates as shown in the figure. As the rotor rotates, the disc also rotates, thus allowing the sliding plates to slide as the inner surface of the casing is eccentric. Thus the sliding plates moves away from the center, huge quantities of air will be trapped in, thus as the plates converge, the air gets compressed and thus results in compressed air.
The Scroll type compressors are having scrolls driven by the prime mover. The scrolls outer edges trap air and then as they rotate, the air travel from outwards to inwards thus getting compressed due to the reduction in the area. Thus the compressed air is delivered through the central space of the scroll to the delivery air line.
The Lobe type air compressor is very simpler type with no complicated moving parts. There are single or twin lobes attached to the drive shaft driven by the prime mover. The lobes are displaced by 90 degrees. Thus if one of the lobes is in horizontal position, the other at that particular instant will be in vertical position. Thus the air gets trapped in between these lobes and as they rotate they get compressed and delivered to the delivery line.