Content: 3.1 Classification of air compressor - Construction and working of single stage and two stage reciprocating air compressors with P-V. diagram. Necessity of multistaging and inter cooling. Construction and working of rotary compressors i) Centrifugal compressor ii) Axial flow compressor iii) Screw compressor, Comparison of various compressors. 3.2 Air compressor terminology like i) Free air delivered, ii) Capacity of Compressor, iii) Piston displacement, iv) I. P., v) B. P., vi) Volumetric efficiency, vii) Isothermal efficiency, viii) Overall Isothermal or Compressor efficiency. (Only definitions), Factors affecting volumetric Efficiency of reciprocating air compressors. 3.3 Applications of compressed air: construction and working of i) Rock drill, ii) pneumatic torque wrench AIR COMPRESSOR : 1) It is a power absorbing or energy convergent device used to raise the pressure of air by reducing its volume. 2) It is of two types i) Reciprocating and ii) Rotary compressors. Normally Reciprocating compressor with inlet and exhaust valves, and rotary with inlet and exhaust port. 3) Following fig shows energy flow from prime-mover to compressor. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 1
Here Prime-mover like steam engine, I.C. engine, electric motor are needed to run compressor. CLASSIFICATION OF AIR COMPRESSOR : OR There are main two categories of air compressors i) Reciprocating ii) Rotary compressor. But according to Geometry and operational characteristics features followings are some types like- A) According to action : i) Single acting reciprocating air compressor. ii) Double acting reciprocating air compressor. B) According to stages : i) Single stage ii) Double stage iii) Multi stage C) According to position of cylinder : i) Vertical ii) Horizontal D) According to drive compressor : i) Motor driven ii) Engine driven iii) Steam/Gas turbine driven E) According to cooling medium : R.K.Yadav/Automobile Enggg Dept/New Polytechnic Kolhapur. Page 2
i) Air cooled ii) Oil cooled iii) Water cooled F) According to portability condition : i) Portable compressor ii) Stationery compressor G) According to type of displacement : i) Positive displacement e.g. Roots blower, vane type, mono screw; twin screw Reciprocating piston type & diaphragm type. ii) Non-positive displacement e.g. Centrifugal (or) radial flow & axial flow. AIR COMPRESSOR TERMINOLOGY : 1) Single acting compressor : Here suction, compression & delivery of air takes place on one side of piston only. Here we get one delivery stroke after one complete revolution of crank shaft. 2) Double acting compressor : Here suction, compression and delivery of air takes place on both sides of piston. Here we get two delivery stroke after one complete revolution of crank shaft. 3) Single stage compressor : In this compression of air from initial pressure to final pressure is carried out in one cylinder only. 4) Multi stage compressor : When compression of air from initial pressure to final pressure is carried out in more than one cylinder. 5) Compression ratio (or) Pressure ratio (P d /P i ) It is ratio of discharge pressure to the inlet pressure. Here discharge pressure is always more than inlet pressure. So value of C.R.is more than one. 6) Inlet pressure : It is the absolute pressure of air at the inlet of compressor. 7) Discharge pressure : It is the absolute pressure of air at the outlet of compressor. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 3
8) Swept volume:(or) (Displacement of compressor/ Piston) It is the volume of air sucked by compressor during its suction stroke which is expressed as- Vs = IIx D 2 4 X L Where : D = Diameter of cylinder bore. L = Length of piston stroke. 9) Capacity of compressor : It is the volume of air delivered by compressor and is expressed in m 3 /min. (or) m 3 /sec. 10) Free air delivery : (F.A.D.) It is the volume of air delivered under the condition of temperature and pressure existing at compressor intake. When free air conditions are not given, these are taken as S.T.P. condition i.e. pressure 1.0135 bar & temperature 15 o C. 11) Volumetric efficiency : It is the ratio of volume of free air delivered per stroke to the displacement of compressor. 12) Isothermal efficiency / compressor efficiency : It is the ratio of isothermal work to actual work to run the compressor. 13) Isothermal efficiency /compressor efficiency : It is the ratio of work (or) power required compressing the air isothermally to the actual work required to compress the air for the same pressure ratio. 14) Indicated power (I.P.) (or) Air power : It is the ratio of polytropic work into speed of compressor to 60. I.P. = W X N w Watts 60 15) Brake power (B.P.) (or) Shaft power : It is the power required to drive the compressor (or) power delivered to the shaft of compressor. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 4
RECIPROCATING AIR COMPRESSOR : We know that in reciprocating air compressor pressure of air is increased in its cylinder with the help of moving piston. Single Stage, Single acting Reciprocating Air compressor : Single stage reciprocating air compressor consist of the following main components. 1) Suction Valve, 2) Delivery Valve, 3) Piston Cylinder arrangement etc. In single stage reciprocating air compressor without clearance volume there are main three strokes-i) Suction, ii) Compression & iii) Delivery. While with clearance volume there are four strokes i) Expansion ii) suction iii) compression iv) Delivery. When piston moves from T.D.C. to B.D.C.,Pressure inside the cylinder falls below an atmospheric pressure. Due to this pressure difference suction valve gets opened and air is sucked into the cylinder. Now the piston moves upwards from B.D.C. to T.D.C. pressure inside the cylinder goes on increasing till it reaches the discharge pressure. At this stage delivery valve gets opened and air is delivered to container. When compression stroke is taking place both suction and delivery valve are closed. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 5
This process is repeated continuously. When pressure of air increases automatically volume of air decreases and temperature of air increases. In single stage, single acting, reciprocating air compressor, suction, compression and delivery of air takes place in two strokes of piston (or) one revolution of crankshaft. P-V diagram for working of single stage, single acting reciprocating air compressor without clearance volume are given below : 4-1 Represents suction of air at pressure p1.during this operation inlet valve remains open. 1-2 -Represents the compression of air polytropically. During this operation,inlet and delivering valves remain closed. 2-3 Represents the discharge of air to the receiver at a pressure p2.the outlet valves remain open during this period. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 6
P-V diagram for working of single stage, single acting reciprocating air compressor with clearance volume are given below : Here at the end of delivery stroke, the high pressure air is left in the clearance volume and the suction for the second cycle starts only when the air pressure falls to the atmospheric pressure. This is represented by expansion curve 3-4 Assuming the compression and expansion of the air follow the same law. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 7
Two Stage Reciprocating air compressor : In above fig. shows two stage reciprocating air compressor with water cooled intercooler. 1) First of all fresh air is sucked from atmosphere in low pressure (L.P.) cylinder during its suction stroke at inlet pressure p 1 and Temperature t1. 2) The air after compression in L.P. cylinder (1 st stage) from 1 to 2 is delivered to intercooler at pressure p 2 and Temperature t 2. 3) Now air is cooled in intercooler from 2 to 3 at constant pressure p 2 and from Temperature t 2 to t 3. 4) After that air is sucked in high pressure (H.P.) cylinder during its suct ion stroke. 5) Finally air after further compression in H.P. cylinder (i.e. second stage) from 3 to 4 is delivered by the compressor at pressure p 3 & Temperature t 4. 6) P-V diagram for two stages reciprocating compressor with intercooler is given below : R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 8
PV diagram for Two stage reciprocating air compressor. Necessity of Multi-staging Compression : We know, sometimes we required air at high pressure. In this case if we use single stage compression for producing high pressure air then it shows following drawbacks. 1) The size of cylinder should be too large. 2) Due to compression there is rise in temperature of air, so it is difficult to reject heat from air in small time available during compression. 3) At the end of compression temperature of air is too high. It may heat-up the cylinder head (or) burn the lubricating oil. 4) So, to overcome above difficulties two (or) more than two cylinders are provided in series with inter cooling arrangement between them. Advantages of Multistage Compressor : 1) It improves volumetric efficiency. 2) It reduces leakage losses considerably. 3) Work required per Kg. of air is reduced. 4) Size of two cylinders may be adjusted to suit volume and pressure of air. 5) It gives uniform torque so smaller size of flywheel is required. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 9
6) It provides effective lubrication due to lower temperature range. 7) Reduces power required to drive the compressor 8) It reduces cost of compressor. 9) Lesser vibration and less maintenance. Necessity of Inter cooling : When we use multistage compression there is necessity of intercooler between stages. We know, air is sucked in first cylinder then compress it after compression temperature of air increases. If it does not pass through intercooler and directly send to second stage because of increase in temperature volume of air increases. So amount of air taken in cylinder decreases so atomically pressure is decreased, so volumetric efficiency decreases. If Intercooler is placed between cylinders then temperature of air decreases and it is maintained at temperature of atmospheric temperature without decreasing pressure of air. So volumetric efficiency increases and input of compressor decreases (i.e. electricity used to drive compressor). PV diagram for multi stage air compressor. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 10
Factors Affecting Volumetric Efficiency of reciprocating air compressor : 1) Clearance volume increases, volumetric efficiency decreases. 2) Piston ring leakage. 3) If the speed of rotation is high, then charge of air taken in is less which decreases efficiency. 4) Restricted passage and leakage of inlet valves affects on volumetric efficiency. 5) Obstruction at inlet valves. 6) If fresh air comes in contact with hot valves it gets expended; which decreases the charge taken in therefore volumetric efficiency decreases. 7) Overheating of air by contact with hot cylinder wall. 8) Inertia effect of air in suction pipe. Rotary air compressor : In rotary air compressor the air is entrapped between two sets of engaging surface and pressure of air is increased by squeezing action. Centrifugal air Compressor : In above figure shows simplest form of centrifugal compressor which consist of rotor (ie. Impeller),eye or main shaft, diffuser, casing. To impeller number of curved vanes are fitted symmetrically. The rotor rotates in an air tight volute casing with inlet port (ie. through impeller eye) and outlet port. The casing of compressor is made by cast iron and designed in such away that, kinetic energy of air is converted into pressure energy Before it leaves the casing. The mechanical energy is provided to rotor from external source. As the rotor rotates, it sucks air through its eye and increase its pressure due to centrifugal force and forces the air to flow over diffuser (not shown in fig). The pressure of air is further increased during its flow over the diffuser. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 11
Finally, the air at a high pressure is delivered to receiver. The air enters the impeller axially and leaves vanes radially. Advantages of Centrifugal Compressor : 1) Speed of compressor is high. 2) It handles large volume of air. 3) Free from vibrations and noise. 4) It can be directly coupled to motor (or) prime mover. 5) Efficiency is high. 6) Only few parts required lubrication. Disadvantages of Centrifugal Compressor : 1) Multi stating is difficult. 2) Pressure ratio is smaller. Applications : 1) Used as air blower in smithy operation. 2) In Turbocharger on vehicles. 3) In furnace for combustion process. 4) In saloon shop for hair drier. Axial Flow Compressor : Construction and working : It consist of number of rotating blade rows fixed to rotating drum (i.e. rotor). The drum rotates inside in air tight casing to which fixed (stator) blade rows are shown in fig. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 12
The blades are made of aerofoil section to reduce the loss caused by turbulence and boundary separation. The mechanical energy is provided to rotating shaft which rotates the drum (i.e. rotor). The air enters from left side of compressor. As the rotor rotates air flows through alternately arranged fixed and moving blade. As air flows from one set of fixed and moving blade to another it gets compressed thus successive compression of air in all sets of fixed blade and moving blade; the air is delivered at high pressure at outlet. Here total number of stages from : 5 to 15. It handles air upto 30,00 m 3 /min. Pressure rise in each stage is 10-15%. It is used in gas turbine power plant, in turbo jet, turbo prop engines. Screw Compressor : Construction and working : It consist of two mutually engaged helical grooved rotors which are closed in a casing. Out of two rotors male rotor is driver and female rotor is driven. Male rotor has four lobes and female rotor has six flutes. During rotation both rotors are rotates exact opposite to each other (i.e. here driver in clockwise direction and driven rotor in anticlockwise rotation) and air enters and takes place between male and female rotor. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 13
Here air traps and moves axially and radially with rotation of rotors and gets compressed due to volume reduction. Then this air though outside from upward direction speed of rotor is different due to different number of lobes and flutes. It handles 3.5 to 300 m 3 /min. and Maximum pressure ratio of 20 This system requires lubrication. This compressor is noisy in operation used in refrigeration industry. Advantages of screw compressor : 1) Uniform and continuous air flow is obtained. 2) There is absence of suction and discharge valve eliminates pressure drop. 3) Directly coupled to prime mover. 4) It is perfectly balanced. 5) It is simple; durable. 6) High volumetric efficiency is obtained. Comparison of Reciprocating & Rotary Air Compressor : Reciprocating Compressor Rotary Compressor 1 Delivery pressure is high as 1000 bar. 1 Maximum delivery pressure is low i.e. 10 bar. 2 Maximum free air discharge is about 300 m 3 /min. 2 Maximum free air discharge is high as 3000 m 3 /min. 3 They are suitable for low discharge 3 They are suitable for high and high pressure. discharge and low pressure. 4 The speed of air compressor is low. 4 The speed of air compressor is high. 5 The size of compressor is large for given discharge. 5 The size of air compressor is small for same discharge. 6 The air supply is intermittent. 6 The air supply is continuous. 7 The balancing is major problem. 7 There is no balancing problem. 8 Air delivered is less clean as it comes in contact with lubricating oil. 8 Air delivered is more clean as it does not come in contact with lubricating oil. 9 e.g. Piston cylinder arrangement. 9 e.g. Centrifugal, Roots blower, screw Axial compressor type. 10 The lubricating system is 10 Lubricating system is simple. complicated. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 14
Centrifugal Compressor & Axial flow Compressor : Centrifugal Compressor Axial flow Compressor 1 The flow of air is perpendicular to axis of compressor. 1 The flow of air is parallel to axis of compressor. 2 It has low manufacturing and running cost. 2 It has high manufacturing and running cost. 3 It required low starting torque. 3 It required high starting torque. 4 It is not suitable for multi stating. 4 It is suitable for multi stating. 5 Eg. Air blower in furnace, In shops for smithy operation, In turbocharger, in hair dryer etc. 5 Eg. In gas turbine power plant, In turbojet,turbo prop engine. Single Stage Compressor & Multi Stage Compressor : Single Stage Compressor 1 Compressor in which single cylinder is used is called as single stage compressor. Multi Stage Compressor 1 Compressor in which more than two cylinders are used then called as multistage compressor. 2 No use of intercooler. 2 Intercooler is used in bet n L.P. and H.P. stage. 3 More power is required to drive 3 Less power is required to drive compressor. compressor. 4 Required heavy and bigger cylinder. 4 Required small cylinder. 5 Difficult in lubrication due to higher working temperature. 5 Lubrication is simple due to lower working temperature. 6 More leakage losses. 6 Leakage losses are reduced. 7 Suitable for small capacity and low delivery pressure. 7 Suitable for large capacity and high delivery pressure. 8 Air is compressed from intake to delivery pressure in one cylinder. Applications of Compressed air : 8 Two (or) more than two cylinders are used to compress air from intake to delivery pressure. We know in day to day life compressed air is used for many purposes like Industrial, Commercial equipments. Some of applications are given below; 1) For operating power tools like pneumatic drills, riveters, Torque wrench, Hammer, rock drills. 2) In CNC-VMC shops for cleaning table, slots, jobs etc. 3) For clamping & de-clamping jobs. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 15
4) In automobiles service stations for washing & drying vehicles. 5) In spray painting. 6) In starting & supercharging of I.C. engine. 7) In operation of lift, rams, pump. 8) Used for producing blast of air in blast furnace. 9) In chemical industries in fertilizer plants. 10) Refrigeration industry depends on compressor only. 11) It is used in gas turbine power plants. 12) It is used to run air motors. 13) Operating brakes on buses, trucks and trains. 14) In tyre tubes of vehicles. 15) For drying wet products. 16) Excavating. 17) For automation purpose in pneumatic system, for material handling in shops. Application of Compressed air in power tools like; i) Pneumatic rock drill. ii) Pneumatic torque wrench. i) Pneumatic rock drill:- Generally in all pneumatic rotary tools, rotary vane motor is used which is nothing but a rotary actuator, with the help of compressed air it provide rotary motion to spindle. (Speed up to 25000 rpm) R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 16
Fig. Pneumatic Drill R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 17
Construction :- In fig shows Pneumatic drill. It is having cast iron body in which air motor (vane type) is used. The motor shaft is attached to 3 gear train and gear housing, this arrangement is similar to epicyclic gear train used in automobiles. Drill chuck is attached to spindle of gear train which transfer power developed by air to drill. There is a air supply pipe and flow of air is controlled by air flow control valves. Working :- When flow control valves similar to trigger of pistol is pressed the pressurized air will pass over the vanes of air motor, as shown by arrows near each vane and rotor will rotate is clockwise direction. This rotary motion will transfer to 3 gear train and thus drill will rotate. ii) Pneumatic torque wrench :- The working principle of pneumatic torque wrench is similar to that of pneumatic drill. Only difference is that instead of drill, here we use spindle on which we can fit box spanner set. according to need we replace it very quickly. This power tool generally used for fitting (or) removing nuts from body. for assemble (or) dismantle the parts. in shops, Garages etc. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 18
Construction and working :- Write Construction and working as Similar to that of pneumatic drill. (refer previous article) Now, For rotate the spindle in a clockwise (or) anticlockwise direction. here spool type direction control valve is used. As well as for controlling torque there is provision of control valves. Generally such type of power tools are connected to air compressor by flexible pipe (or) hose pipe along with F.R.L Unit which are shown below. Setup for power tools R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 19
Question Bank 3. Air compressor 18 Marks 1 Define term free air delivery (F.A.D.) 2 m. 2 Define capacity of Compressor. 2 m. 3 Define Isothermal efficiency of air Compressor. 2 m. 4 5 6 State necessity of inter cooling in air compressor. Why reciprocating compressor is called as positive displacement compressor. Briefly explain the different methods adopted for increasing isothermal efficiency of reciprocating air compressor. 7 Define i) Volumetric efficiency, ii) Isothermal efficiency of an air compressor, iii) Overall efficiency, iv) Capacity of compressor, v) Indicated Power 2 m. 2 m. 2 m. 4 m. 8 Define air Compressor terminology. 4 m. i ) Free air delivery ii) Capacity of compressor iii) Piston displacement iv) Volumetric efficiency. v) Compressor efficiency vi) Mechanical efficiency 9 10 State various uses of compressed air Write classification of air compressor (any 4 pts ) 4 m. 4 m. 11 Classify compressor according to i) capacity of compressor ii) Air 4 m. motion 12 Explain working of single stage reciprocating air compressor with help 4 m. of P-V diagram. 13 Explain construction and working of two stage reciprocating air compressor with help of P-V diagram. 4 m. 14 Explain with neat sketch working of axial flow compressor (rotary) Compare Centrifugal and Axial flow compressor 8 m. 15 Explain necessity of multi staging & inter cooling of compressor using 4 m. diagram. 16 Explain in brief factors affecting volumetric efficiency of reciprocating 4 m. air compressor. 17 Compare merits & demerits of reciprocating compressor with rotary compressor. 8 m. 18 Differentiate between Reciprocating and rotary air compressor. 8 m. 19 Sketch and explain working of Screw compressor 4 m. 20 Explain working principle of Centrifugal compressor with neat sketch. State its merit over reciprocating compressor. 8 m. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 20