EME (Air compressors)


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What is Air Compressors? :

What is Air Compressors ? COMPRESSOR : “The machine which takes in air or any other gas at low pressure and compresses it to high pressure are called compressors.” The compressor used for increasing the pressure is called “air compressor.”

Uses of compressed air:

Uses of compressed air In refrigeration cycle. Operating tools like drill, hammers etc. Filling the air in automobile tires. Spray painting. Pumping of water. Gas turbine power plant. Conveying the materials like sand and concrete along the pipe line. Sand blasting Operating blast furnace To operate air motor in mines where is high fire risk.

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Reciprocating Compressor

Reciprocating compressors :

Reciprocating compressors This compressors are widely used for compressing air for all range of pressure . Construction : It consists if the cylinder in which a piston reciprocates . Piston is driven by crank connecting rod. The crank is mounted in crankcase. The valve are generally pressure differential type . Thus they operate automatically by the difference of the pressure across the valve.

Operations of a compressor :

Operations of a compressor Operation without clearance : It is assumed that in an ideal compressor there is no clearance volume at the end of the stroke. In this type compressor, when piston is moving away from TDS, pressure inside cylinder will decrease and volume will increase. Hence pressure difference across the valve is created. The spring operated inlet valve will be opened automatically. Therefore atmospheric air enters into cylinder at constant pressure p1 with increase in volume. The piston moves towards the TDS from BDC during the second stroke.

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P P 2 3 2” 2’ 2 P 1 4 1 V 1 V Adiabatic compression Polytrophic compression Isothermal Compression Suction valve opens Swept volume Piston on BDS

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The air is compressed adiabatically with increase pressure (as shown in curve {1-2}.) When the pressure of compressed air becomes equal to the pressure of receiver in which the air is delivered, the spring operated delivery valve opens automatically and the air is forced into the receiver at constant pressure P2 from the cylinder. This process is shown by the horizontal line (2-3). Again the piston moves away from TDS. Thus, the cycle is completed and the same cycle will be repeated. The area of the diagram (1-2-3-4) represents the work required to compress the air from pressure P1 to P2 for adiabatic compression.

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Operation with clearance : Clearance Volume required because of Preventing striking of piston at cylinder head Thermal expansion due to high temperature at the end of compression Maintaining machine tolerance. In the graph clearance volume is denoted by V3 or Vc . The residual compressed air at pressure p2=p3 is filled in clearance volume at the end of upward stroke of piston. During the suction stroke this compressed air expands and it is denoted by the curve (3-4) on p-V diagram. This expansion will reduce the pressure from p2=p3 to intake pressure p4=p1 and inlet valve will begin to open and permit the intake of a fresh air.

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The volume (v1-V4) is known as suction volume or effective swept volume or free air delivery at suction condition. The effect of clearance is to reduce the volume of air actually sucked in per working cycle. P P 2 =P 3 P 1 =P 4 Effective swept volume Delivery process Expansion process Compression process Suction process 2 3 4 5 1 Swept volume At point 2: delivery valve opens. At point 4: suction valve opens Clearance volume V 3 V 1

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Indicate power: The power required to compress the air is called the indicated power or cycle power or air power. Brake Power : The power required to drive the compressor is called the brake power or shaft power of compressor. B.P = 2 π NT / 60000 Where N= Compressor speed, RPM T= Torque, Nm For compressor the B.P is greater than I.P .

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Mean effective pressure ( p m ): For compressor it is calculated by work done per cycle. Work done/ cycle = p m LA, Where LA= swept volume m 3 /cycle. P m = {n/n-1x p 1 (v 1 -v 4 )x[(p 2 /p 1 ) n-1/n – 1]} /V s Mechanical efficiency : The ratio of indicated power to the brake power of compressor is called mechanical efficiency. n m =

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Isothermal efficiency: It is defined as the ratio of isothermal work to indicate work. n iso = Isothermal work / Indicated work n iso = p 1 V 1 log e ( p 2 / p 1 ) (n/n-1) p 1 V 1 [( p 2 / p 1 ) n-1/n -1 ] Volumetric efficiency(n v ): It is the ratio of effective swept volume to swept volume of cylinder. n v = 1-C[( p 2 / p 1 ) 1/n – 1] From above eq n , volumetric efficiency reduces with the increase in pressure ratio and clearance ratio C.

Multistage reciprocating compressor:

Multistage reciprocating compressor Process:

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ADVANTAGES: Without intercooling, the curve of compression will follow the path (1-4”),hence the saving in work input due to inter cooling. Less shaft power is required for a given pressure ratio due to the saving in work input. Due to smaller working temperature, better lubricating effect is provided. Better mechanical balance and smooth torque-angle diagram is obtained. Less leakage problems due to less pressure difference for each stage.

Rotary compressors:

Rotary compressors

Centrifugal compressors:

Centrifugal compressors

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It is consist of a impeller and a volute casting. Due to rotation of the impeller at high speed produces the centrifugal force which cause the air to move out of the impeller at a highe velocity. The air is entered in a diffuser ring and paasing out at a high pressure. The high pressure air then flows to passage of volute casting and velocity is reduced due to increased cross sectional area and then finally air comes out of a compressor.

Axial flow compressor:

Axial flow compressor

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In this compressor air flows parallel to the axis of compressor. It consisting of several rows of fixed blades and rotor attached with several rows of moving blades. The fixed and moving blades are placed on alternate rows. The fixed air receive high velocity air and they decrease the velocity and increase the pressure of air and direct to next row of moving blades. The basic working principle is same as the centrifugal compressor.

Roots Blower:

Roots Blower

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For high pressure ratio, three or four lobes may be used. It is consist of fixed casting which contains two shafts fitted with two lobe rotors. One is driven by motor or prime mover and other one through gears from first. The air is drawn through the inlet pipe due to rotation of the rotors. Due to rotation of lobes trapped air is carried to discharge side. Continued rotation of rotors, opens the trapped space to the discharge port. The air is pushed to the receiver due to the continued rotation of rotors.

Vane type compressor:

Vane type compressor

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It is consist of rotor drum mounted eccentrically in the cylinder casting. The rotor is provided with vanes and they remain in contact with the wall of casing due to centrifugal force . The vane can slide in and out in the slots and volume between two vanes keeps changing due to motion of the rotor. With the rotation of rotor, trapped air between the vanes and casing gets compressed due to reduction of space towards delivery side. Since the receiver pressure is higher than the compressed air between the vanes, the back flow of air from the receiver will take place.

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This is cause increase in the pressure. Due to rotation of vanes air is delivered to receiver. This type of compressor can produce a pressure ratio up to 6 per stage.

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