UNIT IV AIR COMPRESSORS 9

Classification and working principle, Work of compression with and with-out clearance, volumetric, iso-thermal and isentopic efficiencies of reciprocating air-compressors, Multi-stage compression and intercooling, Work of Multi-stage compressor.

Rotary compressors, Concept of positive displacement, Roots blower, Vane type blower, Screw compressor, Axial flow and centrifugal compressors (Description only)

INTRODUCTIONCompressors are work absorbing devices which are used for increasing pressure of fluid at the expense of work done on fluid. The compressors used for compressing air are called air compressors. Compressors are invariably used for all applications requiring high pressure air. Some of popular applications of compressor are, for driving pneumatic tools and air operated equipments, spray painting, compressed air engine, supercharging in internal combustion engines, material handling (for transfer of material), surface cleaning, refrigeration and air conditioning, chemical industry etc. Compressors are supplied with low pressure air (or any fluid) at inlet which comes out as high pressure air (or any fluid) at outlet, Fig. 16.1. Work required for increasing pressure of air is available from the prime mover driving the compressor. Generally, electric motor, internal combustion engine or steam engine, turbine etc. are used as prime movers. Compressors are similar to fans and blowers but differ in terms of pressure ratios. Fan is said to have pressure ratio up to 1.1 and blowers have pressure ratio between 1.1 and 4 while compressors have pressure ratiosmore than 4.

Compressors can be classified in the following different ways.(a) Based on principle of operation: Based on the principle of operation compressors can beclassified as,(i) Positive displacement compressors(ii) Non-positive displacement compressorsIn positive displacement compressors the compression is realized by displacement of solid boundary and preventing fluid by solid boundary from flowing back in the direction of pressure gradient. Due to solid wall displacement these are capable of providing quite large pressure ratios. Positive displacement compressors can be further classified based on the type of mechanism used for compression. These can be (i) Reciprocating type positive displacement compressors (ii) Rotary type positive displacement compressors

Reciprocating compressors generally, employ piston-cylinder arrangement where displacement of piston in cylinder causes rise in pressure. Reciprocating compressors are capable of giving large pressure ratios but the mass handling capacity is limited or small. Reciprocating compressors may also be single acting compressor or double acting compressor. Single acting compressor has one delivery stroke per revolution while in double acting there are two delivery strokes per revolution of crank shaft. Rotary compressors employing positive displacement have a rotary part whose boundary causes positive displacement of fluid and thereby compression. Rotary compressors of this type are available in the names as given below;(i) Roots blower(ii) Vaned type compressorsRotary compressors of above type are capable of running at higher speed and can handle large mass flow rate than reciprocating compressors of positive displacement type. Non-positive displacement compressors, also called as steady flow compressors use dynamic action of solid boundary for realizing pressure rise. Here fluid is not contained in definite volume and subsequent volume reduction does not occur as in case of positive displacement compressors. Nonpositive displacement compressor may be of ‘axial flow type’ or ‘centrifugal type’ depending upon type of flow in compressor.(b) Based on number of stages: Compressors may also be classified on the basis of numberof stages. Generally, the number of stages depend upon the maximum delivery pressure. Compressors can be single stage or multistage. Normally maximum compression ratio of 5is realized in single stage compressors. For compression ratio more than 5 the multi-stagecompressors are used. Typical values of maximum delivery pressures generally available from different types of compressor are,(i) Single stage compressor, for delivery pressure up to 5 bar(ii) Two stage compressor, for delivery pressure between 5 and 35 bar(iii) Three stage compressor, for delivery pressure between 35 and 85 bar(iv) Four stage compressor, for delivery pressure more than 85 bar(c) Based on capacity of compressors: Compressors can also be classified depending upon the capacity of compressor or air delivered per unit time. Typical values of capacity for different compressors are given as;(i) Low capacity compressors, having air delivery capacity of 0.15 m3/s or less(ii) Medium capacity compressors, having air delivery capacity between 0.15 and 5 m3/s.(iii) High capacity compressors, having air delivery capacity more than 5 m3/s.(d) Based on highest pressure developed: Depending upon the maximum pressure available from compressor they can be classified as low pressure, medium pressure, high pressure and super high pressure compressors. Typical values of maximum pressure developed for different compressors are as under;(i) Low pressure compressor, having maximum pressure up to 1 bar(ii) Medium pressure compressor, having maximum pressure from 1 to 8 bar(iii) High pressure compressor, having maximum pressure from 8 to 10 bar(iv) Super high pressure compressor, having maximum pressure more than 10 bar.

RECIPROCATING COMPRESSORSReciprocating compressor has piston cylinder arrangement as shown in Fig. 16.2.

Reciprocating compressor has piston, cylinder, inlet valve, exit valve, connecting rod, crank, piston pin, crank pin and crank shaft. Inlet valve and exit valves may be of spring loaded type which get opened and closed due to pressure differential across them. Let us consider piston to be at top dead centre (TDC) and move towards bottom dead centre (BDC). Due to this piston movement from TDC to BDC suction pressure is created causing opening of inlet valve. With this opening of inlet valve and suction pressure the atmospheric air enters the cylinder. Air gets into cylinder during this stroke and is subsequently compressed in next stroke with both inlet valve and exit valve closed. Both inlet valve and exit valves are of plate type and spring loaded so as to operate automatically as and when sufficient pressure difference is available to cause deflection in spring of valve plates to open them. After piston reaching BDC it reverses its motion and compresses the air inducted in previous stroke. Compression is continued till the pressure of air inside becomes sufficient to cause deflection in exit valve. At the moment when exit valve plate gets lifted the exhaust of compressed air takes place. This piston again reaches TDC from where downward piston movement is again accompanied by suction. This is how reciprocating compressor keeps on working as flow device. In order to counter for the heating of piston-cylinder arrangement during compression the provision of cooling the cylinder is there in the form of cooling jackets in the body. Reciprocating compressor described above has suction, compression and discharge as three prominent processes getting completed in two strokes of piston or one revolution of crank shaft.

The isothermal efficiency of a compressor should be close to 100% which means that actual compression should occur following a process close to isothermal process. For this the mechanism be derived to maintain constant temperature during compression process. Different arrangements which can be used are:(i) Faster heat dissipation from inside of compressor to outside by use of fins over cylinder. Fins facilitate quick heat transfer from air being compressed to atmosphere so that temperature rise during compression can be minimized.(ii) Water jacket may be provided around compressor cylinder so that heat can be picked bycooling water circulating through water jacket. Cooling water circulation around compressorregulates rise in temperature to great extent.(iii) The water may also be injected at the end of compression process in order to cool the air being compressed. This water injection near the end of compression process requires special arrangement in compressor and also the air gets mixed with water and needs to be separated out before being used. Water injection also contaminates the lubricant film on inner surface of cylinder and may initiate corrosion etc. The water injection is not popularly used.(iv) In case of multistage compression in different compressors operating serially, the air leaving one compressor may be cooled up to ambient state or somewhat high temperature before being injected into subsequent compressor. This cooling of fluid being compressed between two consecutive compressors is called intercooling and is frequently used in case of multistage compressors.Considering clearance volume: With clearance volume the cycle is represented on Fig. 16.3 (b)The work done for compression of air polytropically can be given by the area enclosed in cycle 1–2–3–4. Clearance volume in compressors varies from 1.5% to 35% depending upon type of compressor.

ROTARY COMPRESSORSRotary compressors are those compressors in which rotating action is used for compression of fluid. Rotary air compressors have capability of running at high speeds up to 40,000 rpm and can be directly coupled to any prime mover such as electric motor, turbine etc. due to compact design, no balancing problem and less no. of sliding parts. Comparative study of rotary compressor with reciprocating compressor shows that rotary compressors can be used for delivering large quantity of air but the maximum pressure at delivery is less compared to reciprocating compressors. Generally, rotary compressors can yield delivery pressure up to 10 bar and free air delivery of 3000 m3/min. Rotary compressors are less bulky, and offer uniform discharge compared to reciprocating compressor even in the absence of big size receiver. Lubrication requirement and wear and tear is less due to rotary motion of parts in rotary compressors compared to reciprocating compressors. Rotary compressors may work on the principle of positive displacement and dynamic action both. Rotary compressors having positive displacement may be of following types:(i) Roots blower(ii) Screw type or Helical type compressor(iii) Vane type compressor

Rotary compressors employing dynamic action may be of centrifugal type or axial type epending upon the direction of flow. These centrifugal type or axial compressors may also be termed as nonpositive displacement type steady flow compressors.(i) Roots blower: Roots blower is a positive displacement type rotary compressor. It has tworotors having two or three lobes having epicycloid and hypocycloid or involute profiles such that they remain in proper contact. Figure 16.13 shows two lobe rotors in a roots blower. To prevent wear and tear two rotors have clearance in between. Out of two rotors one is driven by prime mover while other one is driven by first rotor. When two rotors rotate then their typical geometry divides the region inside casing into two regions i.e. high pressure region and low pressure region. Although there occurs slight leakage across the mating parts which can only be minimised not eliminated completely.

Figure 16.13 b shows the general arrangement in roots blower. It has inlet at section 1–1 and exit at 2–2. Air at atmospheric pressure enters the casing and is trapped between rotor A and the casing. When the rotor rotate then air trapped in volume space V is displaced towards high pressure region due to rotation of rotor. Exit end is connected to receiver in which air is gradually transferred and the pressure inside receiver increases due to cumulative effect of air being transferred from atmospheric pressure region to receiver region. In one revolution this positive displacement of air trapped between rotor and casing from inlet end to receiver end shall occur four times in case of two lobe rotor as shown. While in case of three lobes rotor this transfer shall occur six times. Every time when V volume of air is displaced without being compressed to the receiver side high pressure region, then the high pressure air rushes back from receiver and mixes irreversibly with this air until the pressure gets equalized. Thus, gradually air pressure builds up and say this pressure becomes p2. For inlet air pressure being p1.(ii) Screw type or Helical type compressor: Screw type compressor is very much similar to roots blower.

These may have two spiral lobed rotors, out of which one may be called male rotor having 3–4 lobes and other female rotor having 4–6 lobes which intermesh with small clearance. Meshing is such that lobes jutting out of male rotor get placed in matching hollow portion in female rotors. Initially, before this intermeshing the hollows remain filled with gaseous fluid at inlet port. As rotation begins the surface in contact move parallel to the axis of rotors toward the outlet end gradually compressing the fluid till the trapped volume reaches up to outlet port for getting discharged out at designed pressure. Since the number of lobes are different so the rotors operate at different speed. The material of casing may be cast iron or cast steel while rotors may be of steel and generally internally cooled by circulation of lubrication oil. Surface of lobes are smooth and the shaft is sealed by carbon rings at oil pressure. Two rotors are brought into synchronization by the screw gears. Thrust upon rotors is taken care of by oil lubricated thrust bearings. These compressors are capable of

handling gas flows ranging from 200 to 20000 m3/h under discharge pressures of 3 bar gauge in single stage and up to 13 bar gauge in two stages. Even with increase in number of stages pressures up to 100 bar absolute have been obtained with stage pressure ratio of 2. Mechanical efficiency of these compressors is quite high and their isothermal efficiencies are even more than vane blowers and may be compared with centrifugal and axial compressors. But these are very noisy, sensitive to dust and fragile due to small clearances.(iii) Vane type compressor: Schematic of vane type compressor is shown in Fig. 16.15. It hascylindrical casing having an eccentrically mounted rotor inside it. The rotor has number of slots in it with rectangular vanes of spring loaded type mounted in slots. These vanes are generally non metallic and made of fibre or carbon composites or any other wear resistant material. These vanes remain in continuous contact with casing such that leakage across the vane-casing interface is minimum or absent. It has one end as inlet end and other as the delivery end connected to receiver. Upon rotation the eccentric rotor has the vanes having differential projection out of rotor depending upon their position. Air is trapped between each set of two consecutive blades in front of inlet passage and is positively displaced to the delivery end after compressing the volume V1 initially to V2, V3 and V4. When compressed volume comes in front of delivery passage and further rotation results in the situation when partly compressed air is forced to enter the receiver as their is no other way out. This cumulative transfer of partly compressed air in receiver causes irreversible compression resulting in gradual pressure rise. The p-V representation shown in Fig. 16.15 (b) indicates that the total pressure rise is due to the combined effect of reversible pressure rise inside casing and irreversible pressure rise inside receiver. Generally, the contribution of reversible pressure rise and irreversible pressure rise is in proportion of 50 : 50.

Vane compressors are available for capacity up to 150 m3/min and pressure ratios up to 8 and efficiency up to 75%. For higher pressure ratios the efficiency of vane compressors is more than that of roots blower but the vane compressors have maximum speed up to 2500 rpm as compared to 7500 rpm in case of roots blower. Vane compressors have large power requirement as compared to roots blower

CENTRIFUGAL COMPRESSORSCentrifugal compressor is a radial flow machine compressing the fluid due to the dynamic action of impeller. Centrifugal compressors have impeller mounted on driving shaft, diffuser and volute casing as shown in Fig. 16.16. Centrifugal compressors have air inlet at the centre of impeller. The portion of impeller in front of inlet passage is called impeller eye.

Impeller is a type of disc having radial blades mounted upon it. Compressor casing has a diffuser ring surrounding impeller and the air enters the impeller eye and leaves from impeller tip to enter diffuser ring. Volute casing surrounds the diffuser ring. Volute casing has cross section area increasing gradually up to the exit of compressor. These impellers of centrifugal compressors may also be of double sided type such that air can enter from two sides (both) of impeller. Thus double sided impeller shall have double impeller eye compared to single impeller eye as shown in Fig. 16.17.

Air enters the impeller eye axially and flows radially outwards after having entered compressor. Radial flow of air inside compressor is due to impeller (blades) rotating about its axis. These impeller blades impart momentum to the air entering, thereby rising its pressure and temperature. Subsequently the high pressure fluid leaving impeller enters the diffuser ring where the velocity of air is lowered with further increase in pressure of air. Thus in diffuser ring the kinetic energy of air is transformed into pressure head. High pressure air leaving diffuser is carried by volute casing to the exit of compressor. Due to increased cross section area of volute casing some velocity is further reduced causing rise in its pressure, although this is very small. Total pressure rise in compressor may be due to ‘impeller action’ and ‘diffuser action’ both. Generally, about half of total pressure rise is available in impeller and remaining half in diffuser. Pressure and velocity variation in centrifugal compressor is shown in Fig. 16.18.

Centrifugal compressors are used in aircrafts, blowers, superchargers, etc. where large quantity of air is to be supplied at smaller pressure ratios. Generally, pressure ratio up to 4 is achieved in single stage centrifugal compressors while in multistage compressors the pressure ratio up to 12 can be achieved. These compressors run at speed of 20,000–30,000 rpm.

AXIAL FLOW COMPRESSORSAxial flow compressors have the fixed blades and moving blades mounted along the axis of compressor. Air enters axially and leaves axially. It has primarily two components i.e. rotor and casing. The rotor has blades mounted on it constituting moving blade ring. Blades are also mounted on the inner side of casing thereby constituting stages as fixed blade ring followed by moving blade ring followed by fixed blade ring, moving blade ring and so on. Due to the reduction in volume the volume space for compressed air may be gradually reduced. Gradual reduction in volume can be done by flaring the rotor while keeping stator diameter uniform or by flaring the stator while keeping rotor diameter constant as shown in fig. 16.21. The pressure of fluid entering the axial flow compressor increases upon passing through the fixed and moving blades. This flow of fluid over moving blades is accompanied by enthalpy rise while the fixed blades merely deflect the fluid so as to facilitate smooth entry into moving blades. Absolute velocity of air increases along axis of rotor due to work input from the prime mover. Relative velocity of air decreases during its flow through rotor. Blades have aerofoil section so as to have minimum losses due to turbulence, boundary layer formation and separation, eddy formation etc.