Do you know Classification of Air Compressor? Classification will help you to better understand the working of compressor. The principle of operation is same in each type but according to stages, the building of discharge pressure is different in each compressor.
The reciprocating type compressor consists of a piston which is enclosed within the cylinder & equipped with suction & discharge valves. The piston receives power from main shaft through crankshaft and connecting rod. A flywheel/belt wheel is fitted on the crankshaft which is driven by electric motor or diesel engine. It supplies uniform motion through out the cycle of operations. The compression of air is done by first drawing a volume of air into its cylinder through suction valves during suction stroke by the piston & then compressing & discharging it on the return stroke of the piston through delivery valves. This simple working is used in every Reciprocating Air Compressor. You will easily understand compressor engineering from below description.
How Single Stage Reciprocating Air Compressor Works?
In single stage reciprocating air compressor the entire compression is carried out in a single cylinder. If the compression is affected in one end of the piston & cylinder then it is known as single acting & if the compression is affected in both ends of piston & cylinder then it is known as double acting reciprocating air compressor.
The opening & closing of simple check valve (plate or spring valve) is depend upon difference in pressure, if mechanically operated valves are used for suction & discharge then their functioning is controlled by cams.
The weight of air in the cylinder will be zero when the piston is at top dead centre, if we neglect clearance volume. When piston starts moving downwards, the pressure inside the cylinder falls below atmospheric pressure& suction valve/inlet valve opens. The air is drawn into the cylinder through suction filter element. This operation is known as suction stroke.
When piston moves upwards, compresses the air in cylinder & inlet valve closes when pressure reaches to atmospheric pressure. Further compression follows as the piston moves towards the top of its stroke until, when the pressure in the cylinder exceeds that in the receiver. This is compression stroke of compressor. At the end of this stroke discharge/delivery valve opens & air is delivered to receiver.
When it is double acting reciprocating air compressor, suction stroke is in process at one end of piston while at same time discharge stroke is in process at other end of piston. In simple word we can say that suction & compression took place on both end of piston & cylinder in double acting reciprocating air compressor.
How Double Stage Reciprocating Air Compressor Works?
Double stage or two stage reciprocating air compressor consists of two cylinders. One is called low pressure cylinder and another is called high pressure cylinder.
When piston in low pressure cylinder is at it’s outer dead centre (ODC) the weight of air inside cylinder is zero (neglecting clearance volume), as piston moves towards inner dead centre (IDC) pressure falls below atmospheric pressure & suction valves opens due to pressure difference. The fresh air is drawn inside the low pressure cylinder through air suction filter. This air is further compressed by piston and pressure inside & outside the cylinder is equal, at this point suction valves closed.
As piston moves towards ODC compression of air took place and when the pressure of air is in range of 1.5 kg/centimeter square to 2.5 kg/centimeter square delivery valves opens & this compressed air is then entered into high pressure cylinder through inter cooler. This called as low pressure compression.
If suction & discharge stroke took place on both side of piston then it is called Double Acting Low pressure compression.
Suction valves of high pressure cylinder opens when air pressure in high pressure side is below to the receiver pressure & air from low pressure cylinder drawn into high pressure cylinder.
As piston moves towards the ODC, first stage air is further compressed. When air pressure from low pressure cylinder and inside the high pressure cylinder is equal, suction valves closed. Now air is further compressed by piston until the pressure in the High Pressure Cylinder exceeds that in the receiver & discharge valves opens. This desired high pressure air is then delivered to receiver.
Same procedure is repeated in every cycle of operation. If suction & discharge stroke took place on both side of piston then it is called double acting high pressure compression. In double stage reciprocating air compressor air pressure can be developed in range of 5.5 kg/centimeter square to 35 kg/centimeter square.
Normally where we required air pressure above 7.0 kg/centimeter square & delivery of air above 100 cubic feet/min. this double stage reciprocating air compressor is used. This is most common model used in various engineering plants. If we required air pressure above 35 kg/centimeter square, this double stage reciprocating air compressor is not useful. There we have to use three stage air compressors.
Working Principle of Multi Stage Air Compressor
Some industries required air pressure more than 35 kg/centimeter square to produce their product. For example mineral water bottle (pet blowing) requires air pressure more than 40 kg/centimeter square to produce desired shape of bottle at bottom side. Here Two Stage reciprocating air compressor is not useful, we required the air compressor, who generates air pressure above 35 kg/centimeter square. This discharge air pressure is achieved by multi stage air compressor. We normally use three stage reciprocating air compressors as multi stage air compressor. If we required discharge air pressure above 85 kg/centimeter square then four stage air compressors is useful.
The working principle of multi stage air compressor is same as double stage air compressor.
In three stage air compressor, fresh air from atmosphere enters in first stage (low pressure) cylinder through suction filter. This air is compressed by piston up to 4 kg/centimeter square & then delivered to second stage (middle pressure) cylinder through inter cooler for further compression. In this stage suction and compression took place on both side of piston.
In second stage cylinder low pressure air is compressed up to 14 kg/centimeter square & discharge to third stage (high pressure) cylinder through second inter cooler to achieve air pressure up to desired delivery pressure. In this stage suction and compression took place on one side of piston.
At high pressure cylinder the 14 kg/centimeter square air pressure is increased up to desired discharge range from 35 kg/centimeter square to 85 kg/centimeter square by the piston reciprocating inside the high pressure cylinder. In this stage suction and compression took place on both side of piston.
Multi stage reciprocating air compressors are used in few industries like Pet Blowing, CNG (Compressed Natural Gas) Center etc. All these high pressure compressors are manufactured in India by Chicago Pneumatics, Ingersoll Rand, and Atlas Copco.
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