The usage of compressed air is not limited only to industries, but they are also used in manufacturing, welding, constructions, power plants, ships, automobile plants, painting shops, and for filling breathing apparatus too. Thus there are so many types of air compressors used specifically for the above purposes. Let us discuss on various types of air compressors.
Types of Air Compressors:
Compressors are classified in many ways out of which the common one is the classification based on the principle of operation.
1. Positive Displacement and
2. Roto-Dynamic Compressors.
Positive displacement compressors cab be further divided into Reciprocating and rotary compressors.
Under the classification of reciprocating compressors, we have
- In-line compressors,
- “V"-shaped compressors,
- Tandem Piston compressors.
- Single-acting compressors,
- Double-acting compressors,
- Diaphragm compressors.
The rotary compressors are divided into
- Screw compressors,
- Vane type compressors,
- Lobe and scroll compressors and other types.
Under the Roto-dynamic compressors, we have
- Centrifugal compressors, and the
- Axial flow compressors.
The compressors are also classified based on other aspects like
- Number of stages (single-stage, 2-stage and multi-stage),
- Cooling method and medium (Air cooled, water cooled and oil-cooled),
- Drive types ( Engine driven, Motor driven, Turbine driven, Belt, chain, gear or direct coupling drives),
- Lubrication method (Splash lubricated or forced lubrication or oil-free compressors).
- Service Pressure (Low, Medium, High)
The Attached picture shows the clear classification of different types of compressors.
Reciprocating In-line Compressors:
These are most commonly used compressors with varying pressure ranges. These are simple in design with almost very little automation. The cylinders of various stages are found in a straight line when seen from top. These compressors are commonly direct driven by electric motors or diesel engines. Refer the attached diagram of the reciprocating in-line compressors.
These are usually air cooled compressors with concentric valves mounted on each cylinder head unit. The compressor has different units displaced usually by 90 degrees, may or may not be connected to same crank pin on the crank shaft. Higher capacity compressors are water cooled. Better torque and balancing is achieved by displacing the units by certain angle.
These are usually reciprocating compressors, which has piston working on air only in one direction. The other end of the piston is often free or open which does not perform any work. The air is compressed only on the top part of the piston. The bottom of the piston is open to crankcase and not utilized for the compression of air.
These compressors are having two sets of suction/intake and delivery valves on both sides of the piston. As the piston moves up and down, both sides of the piston is utilized in compressing the air. The intake and delivery valves operate corresponding to the stroke of the compressor. The compressed air delivery is comparatively continuous when compared to a single-acting air compressor. Thus both sides of the pistons are effectively used in compressing the air.
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.
In my next article, the other types of compressors involving centrifugal and axial compressors will be discussed along with the other types of compressors classified on the basis of cooling mehtods, number of stages etc.