AIR COMPRESSORS

Elements Of Mechincal EngineeringElectrical

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INDEX1.1 Introduction1.2 Uses of Compressed air1.3 Reciprocating compressors1.4 Operation of a compressors1.5 Work of compression1.6 Power required 1.7 Reciprocating compressor efficiencies1.8 Multistage reciprocating compressors1.9 Rotary compressors

Introduction• The machine which takes in air or any other

gas at low pressure and compresses it to high pressure are called compressors.

• They are power consuming machines in which mechanical work is converted into the pressure head of air or gas.

• They are also considered as reversed heat engine.

• Generally, the compressors are driven by electric motors, I.C. engine or gas turbines.

• A compressor is used for increasing the pressure of air is called air compressor.

Classification of compressors(1) Method of compression• Reciprocating compressors:.• Rotary compressors:• Centrifugal compressors(2) Delivery pressure• Low pressure • Medium pressure• High pressure• Very high pressure

(3) Principal of operation Positive of displacement Rotodynamic or steady flow compressor(4) The number of stages Single stage compressor - pressure up to 5

bar Multistage compressor - pressure above 5

bar(5) The number of cylinder Single cylinder Multi cylinder

(6) Volume of air delivered• Low capacity• Medium capacity• High capacity(7) Fluid to be compressed • Air compressor• Gas compressor• Vapour compressor

Uses of compressed air In refrigeration cycle Operation tools like drill hammers etc. Filling the air in automobile tyres Spray painting Increasing inlet pressure of I.C. engine To operate air motor I mines where fire risk are

more Pumping water Gas turbine power plant Conveying the materials like sand and concrete

along a pipe line For sand blasting Operating blast furnace Operating air brakes used in buses truck trains etc.

Reciprocating compressor

A single stage reciprocating compressor. It used for compressing air and it is satisfactory for all ranges of pressure it consist of the cylinder in which a piston reciprocates the piston is driven by crank through connecting rod .the crank is mounted in a crank case. the value are generally pressure differential type.

Work for compression(1) Compression without clearanceW = p1 V1 (p2/p1) = p2 V2 loge(p2/p1) = pV

loge (p2/P1)

(2) Compression with clearance

W = n/n-1 mad RT1 p2/p1 n-1/n – 1 J/cycle

Power required

(1) Indicated Power: I.P. = pmLAn KW 60000

(2) Brake power: B.P.= 2NT 60,000

Reciprocating compressor efficiencies

(1) Mechanical efficiency ῃ= I.P/B.P (2) Isothermal efficiency ῃiso = p1 V1 loge (p2/P1) [ (n/n-1) p1V1 (p2/p1)n-1/n -1}]

(3) Volumetric efficiencyῃ = 1- C [(p2/p1)1/n -1)]

Multistage reciprocating compressor There are several disadvantages to

compress the air at a high pressure in a single cylinder the air is compressed by more than one cylinder in series in a single stage compressor if the pressure ratio is increased the volumetric efficiency decrease .by the equation when the pressure ratio is p2/p1=[1+1/c]n

Advantages of multistage compressor• Without inter cooling the curve of

compression will follow the path hence the saving work input due to inter cooling .

• Volumetric efficiency is increased due to the smaller pressure range as the effect of expansion of air in the clearance volume is less

• 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 smoother torque – angel diagram is obtained

In multistage compressor the low pressure cylinder is lighter

There is less leakage problems due to less pressure difference for each stage

Rotary compressor

(1) Centrifugal compressor

• A general arrangement of this compressor is similar to a centrifugal pump Fig. shows main components of a centrifugal compressor It consist of a rotating elements called impeller and a volute casing impeller rotates at a very high speed may be up to 30,000 RPM The air enter into the compressor through the suction eye of the impeller. due to the rotation of the impeller at a high velocity.

• Then air with high velocity enter into a diffuser ring. the diffuser blade of the diffuser ring are so shaped that these provided an increased area of passage to the air which is passing outward due to which the velocity of air leaving the impeller is reduced and its pressure is increased the high pressure air then flows to the divergent passage of volute casing the velocity air is further reduced due to increased cross sectional area of volute casing causing very small rise in pressure from the casing the compressor air leads to exit pipe and finally comes out of the compressor.

(2) Axial Flow compressor

• It consists of a casing fitted with 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 function of the fixed blades is to receive the high velocity air from the moving blades.

• Axial flow compressor is also a high speed machine and speed may even vary from 10,000 to 30,000 RPM. Pressure ratio of 10:1 can be achieved.

Positive Displacement Compressors

(1) Roots Blowers• The roots blower consists of two lobe For

higher pressure ratio three and four lobes may be used here the pressure of air delivered is slightly above the atmosphere.

• The machine consists of a fixed casing two shafts fitted with two lobe rotors one of the rotor is driven by electric motor or other prime movers and other is driven through the gears from first.

• The air is drawn through the inlet pipe due to rotation of due rotors. the volume of air is entrapped between one rotor and casing for a very short interval due to rotation of lobe trapped air is carried to the discharge side continued rotation of rotors open the trapped space to the discharge port. the air is pushed to the receiver due to the continued rotation of rotors.

(2) Vane type compressor or Blower

• This type of compressor is shown inn fig. The compressor is consists of rotor drum mounted eccentrically in the cylindrical casing. The rotor is provided with vanes in the slots. These vanes or blades are made from non-metallic material usually fiber or carbon.

• The vanes can slide in and out in the slots. The volume between two vanes keeps on changing due to eccentric motion of the rotor. The rotation of the rotor causes space to be created between the vanes,the rotor and casing.The space is connected to suction pipe so that air enters into the created space and filled.

• A volume V1 of air trapped between the vanes and casing. With the rotation of rotor of rotor, air gets compressed due to reduction of space towards delivery side.

• The fluid volume reduces to V2,when it communicates with the 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.

• Due to rotation of vanes air is delivered to the receiver. In this type of compressor. the total pressure rise is partially due to internal compression between the vanes and partially due to back flow of air from receiver.

• This type of compressor can produce a pressure ratio up to 6 per stage