Compressor by Patait S.B

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AIR COMPRESSORS Elements OfMechincal Engineering Electrical

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INDEX 1. Introduction 2. Usesof Compressed air 3. Reciprocating compressors 4. Operation of a compressors 5. Work of compression 6. Power required 7. Reciprocating compressor efficiencies 8. Multistage reciprocating compressors 9. Rotary compressors

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Introduction • • • • • The machine whichtakes 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.

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Uses of compresed 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.

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Positive displacement compressor Inthe positive-displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge. At constant speed, the air flow remains essentially constant with variations in discharge pressure. Ex: Reciprocating compressors, screw compressors, vane compressors & so on. Prepare by :- Shivkumar 7

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Dynamic compressors:  Dynamic compressorsimpart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds. The velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers. In the centrifugal-type dynamic compressors, the shape of the impeller blades determines the relationship between air flow and the pressure (or head) generate. Ex: centrifugal compressors, axial compressors. Prepare by :- Shivkumar 8

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Classification Air Compressors Reciprocating Rotary Single– acting Double - Acting No. of Sides of Piston in operation No. of Stages for Compression Centrifugal Single – stage Multi - stage

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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

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(3) Principal ofoperation  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

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(6) Volume ofair delivered • • • Low capacity Medium capacity High capacity (7) Fluid to be compres ed • • • Air compressor Gas compressor Vapour compressor

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Block diagram ofreciprocating compressor It is a piston and cylinder device with (automatic) spring controlled inlet and exhaust valves There is a clearance between the piston crown and the top of the cylinder.

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Reciprocating compres or 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.

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Reciprocating compressors r usually has tw Principleof Operation  Fig shows single-acting piston actions in the cylinder of a reciprocating compressor.  The piston is driven by a crank shaft via a connecting rod.  At the top of the cylinder are a suction valve and a discharge valve.  A reciprocating compresso o, three, four, or six cylinders in it. 9

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Working principle ofreciprocating air compressor

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Working principle ofreciprocating air compressor

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Working principle ofdouble acting reciprocating air compressor

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polytropic compression 1 – 2 ( 2 2– 3 ( 3 3 – 4 ( 4 4 – 1 ( 1 delivery valve opens) delivery at constant pressure and temperature delivery valve closes) polytropic expansion suction valve opens) suction at constant pressure and temperature suction valve closes) p V 1 23 4

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p V 1 23 4 polytropic compression 1 – 2 ( 2 2– 3 ( 3 3 – 4 ( 4 4 – 1 ( 1 delivery valve opens) delivery at constant pressure and temperature delivery valve closes) polytropic expansion suction valve opens) suction at constant pressure and temperature suction valve closes)

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Reciprocating Compressor: Ideal IndicatorDiagram 1 – 2 ( 2 2 – 3 ( 3 3 – 4 ( 4 4 – 1 ( 1 polytropic compression delivery valve opens) delivery at constant pressure and temperature delivery valve closes) polytrophic expansion suction valve opens) suction at constant pressure and temperature suction valve p V 1 23 4

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Reciprocating Compressor -Working

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Reciprocating Compressor –Equation for Work Pressure P1 P2 3 2’ 2 2” 4 1 (Polytropic) PV n C PV  C (Isothermal) PV  C (Adiabatic) V2 V1 Volume Operations : 4 – 1 : Volume V1 of air aspirated into Compressor, at P1 and T1. 1 – 2 : Air compressed according to PVn = Const. from P1 to P2. → Temp increase from T1 to T2. 2 – 3 : Compressed air at P2 and V2 with temperature T2 is delivered.

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Reciprocating Compressor –Equation for Work During Compression, due to the excess temperature above surrounding, the air will exchange the heat to the surrounding.  Compression Index, n is always less than γ, the adiabatic index. As Compressor is a work consuming device, every effort is desired to reduce the work. Work done = Area under P-V curve  1 – 2” : Adiabatic Compression = Max. Work.  1 – 2 : Polytropic Compression  1 – 2’ : Isothermal Compression = Min. Work.

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Work for compression isothemal 𝑊= 𝑃1𝑉1𝐿𝑜𝑔𝑒 𝑃2 𝑃1 𝑊 = 𝑅𝑇1𝐿𝑜𝑔𝑒 𝑃2 𝑃1

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Work for compression Polytropic 𝑊= 𝑛 𝑛−1 𝑃1 𝑉1{( 𝑃2 𝑃1 ) 𝑛−1 𝑛 -1} 𝑊 = 𝑛 𝑛 − 1 𝑅𝑇1{( 𝑃2 𝑃1 ) 𝑛−1 𝑛 −1}

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Work for compression ISentropic 𝑊=  −1 𝑃1 𝑉1{( 𝑃2 𝑃1 ) −1  -1} 𝑊 =   − 1 𝑅𝑇1{( 𝑃2 𝑃1 ) −1  −1}

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Reciprocating Compressor – Efficiency Isothermalwork done / cycle = Area of P – V Diagram = P1V1 loge(P2/P1) Isothermal Power = P1V1 loge(P2/P1) N kW 60 X 1000 Indicated Power : Power obtained from the actual indicator card taken during a test on the compressor. Compressor Efficiency = Isothermal Power Indicated Power Isothermal Efficiency = Isothermal Power Shaft Power NOTE : Shaft Power = Brake Power required to drive the Compressor.

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Reciprocating Compressor – Efficiency AdiabaticEfficiency : Ratio of Power required to drive the Compressor; compared with the area of the hypothetical Indicator Diagram; assuming Adiabatic Compression. Brake Power required to drive the Compressor adiabatic           1   1   P2   1   P   1 P1V1 Mechanical Efficiency : Ratio of mechanical output to mechanical input. Mechanical Efficiency, ηmech = Indicated Power Shaft Power

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Reciprocating Compressor – Efficiency VolumetricEfficiency : Volumetric Efficiency :. Efficiency, ηvol =1 - C [(p2/p1)1/-1)] c= 𝑉𝑐 𝑉𝑠

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Power required KW (1) IndicatedPower: I.P. = pmLAn 60000 (2) Brake power: B.P.= 2𝜋NT 60,000

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Reciprocating compres or 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}]

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Multistage Reciprocating Compressors The compressionof air in single stage has many disadvantages and its use is limited where low delivery pressure is required. A multistage compression is more efficient and mostly employed for high pressures.

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Multistage Reciprocating Compressors

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Multistage Reciprocating Compressors

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Cross Section ofa multi stage compressor

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Reciprocating Compressor – Multistage Intercooler: Compressed air is cooled between cylinders. Series arrangement of cylinders, in which the compressed air from earlier cylinder (i.e. discharge) becomes the intake air for the next cylinder (i.e. inlet). L.P. = Low Pressure I.P. = Intermediate Pressure H.P. = High Pressure L.P. Cylinder I.P. Cylinder H.P. Cylinder Intercooler Intercooler Air Intake Air Delivery

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Reciprocating Compressor – Multistage IntakePr. P1 or Ps P3 or Pd 2 1 PV n C 8 Delivery Pr. 6 9 3 5 Intermediate Pr. 7 4 P2 PVC Without Intercooling Perfect Intercooling L.P. H.P. Overall Pr. Range : P1 – P3 Single – stage cycle : 8-1-5-6 Without Intercooling : L.P. : 8-1-4-7 H.P. : 7-4-5-6 With Intercooling : L.P. : 8-1-4-7 H.P. : 7-2-3-6 Volume Perfect Intercooling : After initial compression in L.P. cylinder, air is cooled in the Intercooler to its original temperature, before entering H.P. cylinder i.e. T2 = T1 OR Points 1 and 2 are on SAME Isothermal line.

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Multistage reciprocating compressor  Thereare 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.

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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

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 Due tosmaller 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

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Rotary compressor

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Centrifugal compressor

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Centrifugal compressor

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(1) Centrifugal compressor • heair enter he 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 T into the compressor through the suction eye of the impeller. due to t rotation of the impeller at a high velocity.

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• Then airwith 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

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Axial Flow compressor

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(2) Axial Flowcompressor • • • 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.

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Axial Flow compressor

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Positive Displacement Compressors

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Roots Blowers

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Roots Blowers

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(1) Roots Blowers • • Theroots 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.

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• The airis 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.

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Vane type compressoror Blower

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(2) Vane typecompressor or Blower • • ein the slots. The volum ion the eated nd ed to into 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 between two vanes keeps on changing due to eccentric mot of the rotor. The rotation of rotor causes space to be cr between the vanes,the rotor a casing.The space is connect suction pipe so that air enters the created space and filled.

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• A volumeV1 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. • Th a pressure ratio up to 6 per sta is type of compressor can produce ge

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Reciprocating Compressor – EfficiencyHowto Increase Isothermal Efficiency ? A. Spray Injection : Assimilation of water into the compressor cylinder towards the compression stroke. Object is to cool the air for next operation. Demerits : 1. Requires special gear for injection. 2. Injected water interferes with the cylinder lubrication. 3. Damage to cylinder walls and valves. 4. Water must be separated before delivery of air. B. Water Jacketing : Circulating water around the cylinder to help for cooling the air during compression.

Compressor by Patait S.B

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