A four stage compressor compresses air in four stages to efficiently increase the pressure from atmospheric to 7 barg. It comprises four compressor elements, four motors, and pistons arranged in series with the outlet of one stage connecting to the inlet of the next. Compressing air in multiple stages with cooling between allows reaching the final pressure more efficiently than a single stage compressor.

1. CompressorMULTI STAGE COMPRESSORAir compressor is a compressor that takes in air at atmospheric pressure and delivers it at a higher pressure. An air compressor is not very efficient if you attempt to compress the air from atmospheric pressure to 7 barg in one go. It is better to compress the air in four goes or stages, cooling the hot air between the stages. A compressor which reaches the final working pressure using four stages of compression is called a four-stage compressor. Four stage compression is most efficient.

2. Four stage compressor unit comprises four different compressor elements and four separate electric motors with an adjustable speed. It also comprises pistons in series. The outlet of a compressor element of one lower stage is connected to the inlet of a successive compressor element of a successive higher stage. The mass flow rate is constant in each of the stage. The optimum efficiency of the compressor unit is obtained by optimizing the speed of each stage and, therefore, the pressure ratio over each stage.

3. Features of Four Stage Air CompressorOil free operation.Air cooled.Low and easy maintenance.Less wear due to no moving parts in contact.Long service life.No contamination of gas with ambient air.Neutral room between compression room and mechanical drive.Compact and lightweight design.

4. Parts of Compressor

6. Operation:To understand the operation of an air compressor, let us assume the cycle and indicator diagram for a simple single stage reciprocating air compressor, as shown below.

7. The simple reciprocating air compressor has a piston which reciprocates inside the cylinder wall and cylinder head. The piston is attached to the crankshaft with the help of a connecting rod and thus the rotation of the crankshaft causes the piston to move up and down inside the cylinder. The crankshaft is mounted on the crank case. The cylinder head contains valve pockets where the suction and delivery valve are fixed.These suction and delivery valves are of simple pressure differential types. They open and close, due to the pressure difference on either side of the valve plates.

8. 1. When the compressor stops or idles for some time, it is always assumed that there is some residual compressed air left in the cylinder space. This residual air expands when the piston moves down. The pressure drops in the cylinder space at a particular point as the piston moves down, where the pressure inside the cylinder becomes lesser than the atmospheric pressure. Thus this difference in pressure makes the suction or inlet valve open.

10. 2. This opening of inlet valve allows fresh air to be drawn inside the cylinder space as the piston still continues to move in the downward direction. The inlet valve will remain open till there is pressure difference between the atmosphere and inside of the cylinder space. As the pressure difference starts to reduce, the inlet valve starts slowly closing.The inlet valve closes completely when there is no pressure difference and then the piston reaches bottom dead center (BDC), and it starts to travel in an upward direction. At this position, both the inlet and delivery valve remains closed. Thus as the piston moves up, the pressure starts to build inside the cylinder space.

12. 3. The delivery valve starts to open when there is a pressure difference between the cylinder space and air receiver. Let us assume the air receiver is at a pressure of 7 bar. The delivery valve will not open until the pressure inside the cylinder space is slightly above 7 bar. As the piston moves in upward direction, the pressure increases and at some point the pressure grows beyond 7 bar making the delivery valve open. Thus the compressed air is delivered into the air receiver.

14. 4. As the piston reaches top, the pressure starts to fall and the delivery valve starts to close. The residual compressed air remaining in the space again starts to expand as the piston moves down continuing the next cycle.

16. Types Of Compressor Centrifugal compressors

17. Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. Adiffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 horsepower (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).Many large snowmaking operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers andturbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. Sometimes the capacity of the compressors is written in NM3/hr. Here 'N' stands for normal temperature pressure (20°C and 1 atm ) for example 5500 NM3/hr.

18. Diagonal or mixed-flow compressorsMain article: Diagonal or mixed-flow compressorDiagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to an axial rather than radial direction.

19. Axial-flow compressors

20. Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like airfoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design.The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary. The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.

21. Axial compressors can have high efficiencies; around 90% polytropic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants.

22. Reciprocating compressorsA motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders

23. Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW) are commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>18000 psi or 180 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, and more costly than comparable rotary units. Another type of reciprocating compressor is the swash plate compressor, which uses pistons which are moved by a swash plate mounted on a shaft - see Axial Piston Pump.Household, home workshop, and smaller job site compressors are typically reciprocating compressors 1½ hp or less with an attached receiver tank..