Last Updated on June 5, 2020 by Amit Abhishek
Today air compressor is used in aerospace, chemical manufacturing, food processing, medical facilities, oil and gas applications, laboratories, dairy, heavy diesel engine, power machinery, panting, breaking system, pneumatic applications control and automation.
An air compressor takes atmospheric air, compresses it, and delivers at the desired pressure. Driven by a prime mover ( electric motor or diesel engine ); it delivers high-pressure air to storage tanks or air receiver.
Air compressors are one just critical auxiliary machine responsible for operating and starting major engine room applications.
Onboard ship, compressed air is used for control and service air applications, starting main and auxiliary engines, starting main and auxiliary engines, operating ship’s whistle, and operating pneumatic tools like lifting gear, drill machines, torque wrenches, etc.
Based on its design, construction, and working a compressor can be classified into the following five types; reciprocating, centrifugal, screw, rotary, and diaphragm. But only reciprocating and screw compressors are used widely for industrial applications.
Reciprocating Vs Screw Compressor
1. It uses piston arrangement to compress air by mechanical variation of cylinder volume.
1. It uses meshing helical rotors to trap and compress gas as they are forced through the screw casing.
2. Simple and cheap
2. Complex and expensive
3. Require internal suction and discharge valve
3. Do not require an internal suction and discharge valve.
4. Non-uniform and pulsating output.
4. Uniform and less pulsating.
5. More moving parts leading to a high risk of wear/tear or possible breakdown.
5. Less moving parts leading to less wear/tear or possible breakdown.
6. Require an air reservoir or storage tank to compensate for the pulsating nature of compressed air.
6. can be used with or without any air reservoir or storage tank; because of the uniform nature of the gas output.
7. Works faster but delivers less air compared to the screw compressor.
7. Works faster and delivers more air/gas in comparison with reciprocating compressor.
8. Higher working temp, can not be used continuously for 24 hrs.
8. Lower working temp can be used continuously uninterrupted ( 24hrs ).
9. Higher working pressure up to 200 bar.
9. Less working pressure limited to 14 bar.
10. More efficient at a higher pressure and less use.
10. More efficient at lower pressure and continues operation.
11. Used in refrigeration, air conditioning, and starting the heavy diesel engines.
11. For low-pressure air requirements such as advanced air conditioning and service air compressor.
Main Components of Modern Air Compressor
- Prime Mover: A prime mover is a critical component of the air compressor responsible for operating piston assembly. External source such as an electric motor or diesel engine is used to rotate the compressor flywheel.
- Piston Assembly: It is the main component that produces reciprocating piston movement compressing gas or air. A typical piston assembly consists of parts such as piston, connecting rod, big end bearings, piston rings, and gudgeon pin.
- Suction Valve: It is a spring-loaded valve fitted at the inlet side of the air compressor along with the inlet air filter. When the cylinder pressure becomes negative this valve opens with the fresh charge of air coming inside.
- Discharge Valve: It is a spring-loaded valve fitted on the discharge side of the air compressor. The discharge valve opens discharging air to the storage tank when the cylinder pressure reaches a preset limit.
- Inter Cooler: An intercooler is installed between the two stages of air compressor cooling output air from the first stage to reduce work done required in the second stage.
- Safety Device: There are various safety devices fitted on an air compressor to avoid any unwanted circumstances. Ex: relief valve, fusible plug, bursting disc, and different alarms ( High air temp, High cooling temp, Low lube oil pressure ).
Working Principle of Reciprocating Air Compressor
Now that you have understood what an air compressor is, its type and application; you can now understand how this device works.
A reciprocating air compressor works much like the engine fitted in your car, motorcycle, or any other machinery. It consists of parts such as the piston, connecting rod, crankshaft, valves, and cylinder.
Together they achieve a reciprocating piston assembly that compresses gas by decreasing cylinder volume. But since there is no combustion within the cylinder; prime mover is used to sustaining the piston movement.
Most air compressors are usually powered by electric motors as their prime mover with some powered by petrol or diesel engines.
With each piston stroke the air gets sucked into the cylinder, compressed and discharged. the sucked air gets into the cylinder via inlet valve under negative cylinder pressure.
When compressed the air went out from discharge valve when reached a certain pressure limit.
In its most general form a reciprocating air compressor works on the 2-stroke cycle. The processes that happens in one complete cycle includes:
- 1 to 2 ( Compression Stroke ): Piston moves from BDC to TDC compressing the air inside the cylinder.
- 2 to 3 ( Discharge Stroke ): Piston approaches TDC and discharge valve opens. this leads to the delivery of compressed air.
- 3 to 4 ( Expansion Stroke ): Air undelivered is trapped within the cylinder and further expands as the discharge valve shuts and the piston moves from TDC towards BDC.
- 4 to 1 ( Inlet or Suction Stroke ): The negative pressure inside the cylinder then open the inlet valves with the movement of fresh air inside the cylinder.
As the piston moves down from TDC to BDC air inside the cylinder expands to reach negative pressure thus opening the suction valve to admit fresh air.
At BDC the inlet valve closes and air inside the cylinder starts to get compressed as piston moves from BDC to TDC inside the cylinder.
Now as the piston approaches the TDC the pressure inside the cylinder becomes too high and the discharge valve opens releasing compressed air to the air bottle.
Here, Volumetric efficiency = Actual suction volume / Theoretical suction volume
For better compressor efficiency the compression should be isothermal but in actual compression in an air compressor is polytropic.
Difference Between Isothermal, Adiabatic And Polytropic Compression
1 ) Isothermal Compression
Isothermal compression is the idlest compression process that happens at a constant temp. If the air is to be compressed isothermally all the heat generated in the process had to be taken by cooling water, cylinder, and the piston itself to keep the gas temp constant.
From a thermodynamic point of view it would have been the idlest compression with minimum work done.
2 ) Adiabatic Compression
It is a thermodynamic process where the air is compressed instantaneously without any heat loss. This means all the heat generated within the compression process is kept within the air and not transferred.
Theoretically compressing air without any heat transfer would require maximum work done; but is not possible to achieve practically.
3 ) Polytropic Compression
It is one compression process that is used to refer actual compression process that is neither isothermal or adiabatic but comes somewhat in between these two compression processes.
Since compression requires fast piston action keeping the process closer to adiabatic compression. But since we get the least work done when it is compressed isothermally; so water cooler is used to reduce work done and thus we reach polytropic compression.
Why we require multistage compression and inter cooling?
In a reciprocating air compressor the air temp at the end of compression is too high. Further since the compression is a quick process; the heat produced is not properly dissipate increasing the risk for cylinder head burn or lubricating oil burning.
In order to overcome above mentioned difficulties we either need to use larger cylinders or implement multistage compression with inter coolers.
In a multistage compressor with inter cooler the work done per kg of air is much less in comparison with single stage compressors for the same delivery pressure.
It has been seen that if the air is compressed in stages the final compression graph is much nearer to that of isothermal compression. This further results into improved volumetric efficiency, reduced leakages, better lubrication and much less thermal distortion.
Other benefits of multistage compressor includes, increased valve life, reduced carbon buildup, lower cost of operation and smaller flywheel.
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