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![WORK REQUIRED DURING
POLYTROPIC PROCESS
Work done on air during Polytropic compression is given by,
For Polytropic process,
𝑊 =
𝑛
𝑛− 1
×𝑚𝑅 𝑇1 ×[
𝑃2
𝑃1
𝑛− 1
𝑛
−1]](https://image.slidesharecdn.com/aircompressors-211109061833-250120034818-71d49760/75/aircompressors-mechanicakl211109061833-pptx-12-2048.jpg)
![WORK REQUIRED DURING
ADIABATIC (ISENTROPIC) PROCESS
Work done on air during Isentropic compression is given by,
For Polytropic process,
𝑊 =
𝛾
𝛾 −1
×𝑚𝑅 𝑇1 ×[
𝑃2
𝑃1
𝛾 − 1
𝛾
−1 ]](https://image.slidesharecdn.com/aircompressors-211109061833-250120034818-71d49760/75/aircompressors-mechanicakl211109061833-pptx-13-2048.jpg)
aircompressors- mechanicakl211109061833.pptx
- 1. DEPARTMENT OF MECHANICALENGINEERING SUB: POWER ENGINEERING AND REFRIGERATION (PER) 22562 TOPIC:AIR COMPRESSORS
- 2. INTRODCUTION Compressors are mechanicaldevices used for increasing the pressure of a gas. A machine which takes in air or gas during suction stroke at low pressure and then compresses it to high pressure in a piston cylinder arrangement is known as Compressor. External work must be supplied to the compressor to achieve required compression. Fig. Energy flow from prime mover to compressor
- 3.
- 4. USES OF COMPRESSEDAIR • To drive a compressed air engine • For producing an air blast for a workshop • For spraying the fuel (atomizing) into a boiler furnace • For operating pneumatic drills and tools • For operating pneumatic brakes for locomotives and rolling stock • Pneumatic conveying and for pumping of water by compressed air • For working compressed air engines especially in mines • In gas turbine power plants and air-conditioning plants The several uses of compressed air are as follows:
- 5. COMPRESSOR TERMINOLOGY Single-acting compressor:If the air admission from the atmosphere is on only one side of the cylinder. Double-acting compressor: When, the air from the atmosphere is drawn on both sides of the piston. Single-stage compressor: If the total compression is done fully in one cylinder. Multi-stage compressor: If the compression is carried out in more than one cylinder, and every cylinder carries out a part of the compression.
- 6. COMPRESSOR TERMINOLOGY Free AirDelivered (FAD): The actual volume of air delivered by the compressor when reduced to normal temperature and pressure condition. Capacity of compressor is generally given in terms of free air delivery. Its SI Unit is Displacement or Swept Volume of the Compressor: The volume displaced by piston movement between two dead centers is called displacement or swept volume. For a single acting compressor, swept volume, Compressor capacity:-It is the volume of air delivered by the compressor. It is express in
- 7. COMPRESSOR TERMINOLOGY Intake Pressure:The absolute pressure of air entering the compressor at inlet. Discharge Pressure: The absolute pressure of air leaving the compressor at outlet. Compression ratio or Pressure ratio: The ratio of discharge pressure to the intake pressure. Piston Speed: The distance travelled by piston in one minute. It is given as . It is measured in
- 8. RECIPROCATING COMPRESSOR If compressionis done in a conventional cylinder with a closely fitted piston making reciprocating motion, then the compressor is called a reciprocating compressor
- 9. SINGLE STAGE RECIPROCATING COMPRESSOR Fig.Single Stage Reciprocating Compressor
- 10. WORK REQUIRED FORSINGLE ACTING SINGLE STAGE RECIPROCATING COMPRESSOR ❖ There are two possibilities of work done: 1. Work done without clearance volume 2. Work done with clearance volume ❖ Amount of work required depends on nature of compression ❖ Three compression process 1. Polytropic compression ( 2. Isothermal compression 3. Isentropic Compression )
- 11. WORK REQUIRED DURING ISOTHERMALPROCESS Work done on air during Isothermal compression is given by, For Isothermal process, = 𝑊 = 𝑃 1 𝑉 1 log𝑒 𝑃 2 𝑃 1 𝑊 =𝑚𝑅𝑇 1 log𝑒 𝑃 2 𝑃 1
- 12. WORK REQUIRED DURING POLYTROPICPROCESS Work done on air during Polytropic compression is given by, For Polytropic process, 𝑊 = 𝑛 𝑛− 1 ×𝑚𝑅 𝑇1 ×[ 𝑃2 𝑃1 𝑛− 1 𝑛 −1]
- 13. WORK REQUIRED DURING ADIABATIC(ISENTROPIC) PROCESS Work done on air during Isentropic compression is given by, For Polytropic process, 𝑊 = 𝛾 𝛾 −1 ×𝑚𝑅 𝑇1 ×[ 𝑃2 𝑃1 𝛾 − 1 𝛾 −1 ]
- 14. WORK REQUIRED FORSINGLE ACTING SINGLE STAGE RECIPROCATING COMPRESSOR WITH CLEARANCE VOLUME Clearance volume is the volume left in the cylinder when piston reaches TDC Let = Clearance volume = Swept volume = = Actual volume = Let the compression and expansion processes follow the same law, Work done per cycle, W = area (1–2–3–4–1) Fig. p-V diagram with clearance
- 15. WORK REQUIRED FORSINGLE ACTING SINGLE STAGE RECIPROCATING COMPRESSOR WITH CLEARANCE VOLUME Work done per cycle, Now,
- 16. POWER REQUIRED TODRIVE SINGLE STAGE RECIPROCATIG AIR COMPRESSOR Power required in driving the compressor is given as N=N for Single Acting N=2N for Double Acting
- 17. MULTISTAGE COMPRESSION Multistagecompression is carried out in more than one cylinder. Disadvantages of Single Stage Compression • Handling of very high pressure range in one cylinder resulting in leakage past the piston, • Ineffective cooling of the gas, • Necessitating robust construction of the cylinder to withstand the high delivery pressure. • The volumetric efficiency of a single-stage compressor with fixed clearance decreases with an • Increase in pressure ratio and thus reduces the capacity.
- 18. MULTISTAGE COMPRESSION • Theair can be cooled perfectly at pressures intermediate between the suction and delivery pressures resulting in less power required as compared to a single-stage compressor for the same pressure limits and quantity of free air delivered. • The mechanical balance of the machine is better due to phasing of the operations. • The pressure range and hence the temperature range in each stage can be kept within desirable limits. • Work required for single stage compression increases due to increase in pressure ratio, which can be reduced with multistage compression. NECESSITY OF MULTISTAGE COMPRESSION
- 19. TWO STAGE RECIPROCATINGAIR COMPRESSOR WITH INTERCOOLING Two stage compression with intercooling Schematic arrangement p-V diagram
- 20. TYPES OF INTERCOOLING Perfect IntercoolingImperfect Intercooling
- 21. PERFECT INTERCOOLING When thetemperature of the air leaving the intercooler (i.e. T3) is equal to the original atmospheric air temperature (i.e. T1) then the intercooling is known as complete or perfect intercooling. In this case, the point 3 lies on the isothermal curve as shown in below figures:
- 22. IMPERFECT INTERCOOLING When thetemperature of the air leaves the intercooler (i.e. T3) is more than the original atmospheric air temperature (i.e. T1), then the intercooling is known as incomplete or imperfect intercooling. In this case, the point 3 lies on the right side of the isothermal curve as shown in below figure:
- 23. WORK REQUIRED FORTWO STAGE RC WITH INTERCOOLER When Intercooling is imperfect When intercooling is perfect
- 24. CONDITION FOR MAXIUMEFFICIENCY OF TWO STAGE RECIPROCATING COMPRESSOR Efficiency is maximum when work required for compression is minimum. Perfect intercoolin g Same pressure ratio Same am ount of work
- 25. CONDITION FOR MAXIUMEFFICIENCY OF TWO STAGE RECIPROCATING COMPRESSOR Condition for Maximum Efficiency Same pressure ratio in each stage Work required for each stage is same
- 26. VOLUMETRIC EFFICIENCY The volumetricefficiency of a reciprocating compressor is defined as the ratio of the actual free air delivered to the swept volume of the compressor. The free air delivered is (whereas the sweptvolume is (. ηV = V a V s = V 1 −V 4 V 1 −V C • Very high speed • Leakage through piston seals • Too large a clearance volume • Obstruction at inlet valves • Overheating of air by contact with cylinder walls • Inertia effect of air Factors Affecting Volumetric Efficiency
- 27. ISOTHERMAL EFFICIENCY Itis defined as the ratio of isothermal work to that of actual indicated work. • Cooling cylinder by spraying water during compression stroke • Circulation of water surrounding to cylinder by providing jackets • Installing inter cooler between two cylinders • Providing greater fins on cylinder • By selecting suitable material for cylinder Methods to improve Isothermal Efficiency
- 28. MECHANICAL EFFICIENCY It isdefined as the ratio of indicated power to the shaft power.
- 29. ROTARY AIR COMPRESSORS Ina rotary compressor, the compression of air is achieved due to the rotating blades fitted in a rotor. It requires less starting torque as compared to reciprocating compressors because of direct coupling with the prime over. Usually, rotary compressors operate at high speed and supplies higher quantity of air than reciprocating compressors
- 30. TYPES OF ROTARYCOMPRESSORS ROTARY COMPRESSORS LOBE TYPE CENTRIFUGAL SCREW COMPRESSOR VANE TYPE
- 31. LOBE TYPE Theroots blower is used to supply air from 0.15 to 1500 with pressure ratio up to 3.6 per blower. Rotational speeds up to 12,500 rpm are used It can be directly coupled to a steam turbine or a gas turbine shaft without any intermediate gearing
- 32. VANE TYPE The vaneblowers require less power than the root blowers for the same capacity and pressure rise. They are used to deliver up to 150 of air at pressure ratio up to 8.5. The speed is limited to 3000 rpm.
- 33. SCREW COMPRESSOR Ascrew compressor is a type of rotary compressor which compresses air due to screw action. The main advantage of using this compressor is that it can supply compresses air continuously with minimum fluctuation in delivery pressure. It is usually applied for low pressure applications up to 8 bars
- 34. AXIAL FLOW COMPRESSOR In an axial flow compressor, the air flows throughout the compressor parallel to its axis. The axial compressor is generally driven by an internal combustion engine or a turbine. The work input to the rotor shaft is transferred by the rotor moving blades to the air, thus accelerating it. The space between the moving blades form diffuser passages so that the velocity of air relative to the blades is decreased as the air passes through them and consequently, the pressure rises.
- 35. DIFFERENCE BETWEEN Parameters ReciprocatingCompressor Rotary Compressor Suitability Suitable for low discharge at high pressure Suitable for high discharge at low pressure Working principle Compression is done by reciprocating motion of piston inside the cylinder Compression is done by rotary motion of blades Air Supply Intermittent or discontinuous Continuous Size For same discharge the size of air compressor is large For same discharge the size of air compressor is small Delivery pressure Upto 1000 bar Upto 10 bar Pressure ratio High Small
- 36. DIFFERENCE BETWEEN Parameters ReciprocatingCompressor Rotary Compressor Work required for compression High Less Maintenance cost More Less Lubrication Complicated Simple No. of moving parts More Less Wear and tear More Less Speed Low High
- 37. DIFFERENCE BETWEEN Parameters ReciprocatingCompressor Rotary Compressor Cleanliness of air Less clean will be delivered More clean air will be delivered Mechanical efficiency Low High Flow rate of air 3000 300 Application Pneumatic tools, spray painting, coal mines, auto garages etc. Oil refinery, refrigeration and air conditioning plants, turbo jet engine etc.
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