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Se prod thermo_chapter_2_compressor

  1. 1. <ul><li>Compressors </li></ul>ME0223 SEM-IV Applied Thermodynamics & Heat Engines Applied Thermodynamics & Heat Engines S.Y. B. Tech. ME0223 SEM - IV Production Engineering
  2. 2. Outline <ul><li>Use of compressed air, Classification. </li></ul><ul><li>Single – stage reciprocating compressor with and without clearance. </li></ul><ul><li>Work and Power Calculations. </li></ul><ul><li>Two – stage air compressor with Perfect Intercooling. </li></ul><ul><li>FAD and volumetric Efficiency. </li></ul>ME0223 SEM-IV Applied Thermodynamics & Heat Engines
  3. 3. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Air Compressors COMPRESSOR – A device which takes a definite quantity of fluid ( usually gas, and most often air ) and deliver it at a required pressure. Air Compressor – 1) Takes in atmospheric air, 2) Compresses it, and 3) Delivers it to a storage vessel ( i.e. Reservoir ). Compression requires Work to be done on the gas , Compressor must be driven by some sort of Prime Mover ( i.e. Engine )
  4. 4. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Air Compressors Energy received by Prime Mover 1) Fraction absorbed for Frictional work . 2) Fraction Lost to Radiation . 3) Fraction absorbed by Coolant . 4) Rest is stored as Useful Work in air. Prime Mover Compressor Source Heat Heat Rejected Work LP Air In HP Air Out Heat to Coolant Heat to Radiation Frictional Work
  5. 5. Uses of Compressed Air Compressed Air ME0223 SEM-IV Applied Thermodynamics & Heat Engines Powering portable small Engines Drills and Hammers in road building Excavating Tunneling and Mining Starting the Diesel engines Operating Brakes for buses, trucks and trains
  6. 6. Compressed Air Uses of Compressed Air Blowing Converters ME0223 SEM-IV Applied Thermodynamics & Heat Engines Smelting of Metals CUPOLA Furnaces Air Conditioning Drying Ventilation
  7. 7. Classification Air Compressors Single – acting Double - Acting ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Rotary No. of Sides of Piston in operation No. of Stages for Compression Centrifugal Single – stage Multi - stage
  8. 8. Classification ME0223 SEM-IV Applied Thermodynamics & Heat Engines Air Compressors Nature of Work ( i.e. Use ) To Produce Vacuum . i.e. to remove air from a system to create low pressure zone. Air Pumps / Exhausters Blowers / Superchargers Pr. rise is small . i.e. 0.7 – 1.05 bar Boosters Pr. rise for HP gas i.e. already compressed gas.
  9. 9. Reciprocating Compressor - Working ME0223 SEM-IV Applied Thermodynamics & Heat Engines
  10. 10. Reciprocating Compressor - Working ME0223 SEM-IV Applied Thermodynamics & Heat Engines
  11. 11. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work Operations : 4 – 1 : Volume V 1 of air aspirated into Compressor, at P 1 and T 1 . 1 – 2 : Air compressed according to PV n = Const . from P 1 to P 2 . -> Temp increase from T 1 to T 2 . 2 – 3 : Compressed air at P 2 and V 2 with temperature T 2 is delivered. Volume Pressure P 1 P 2 V 1 V 2 3 2 2” 2’ 4 1 ( Polytropic ) ( Adiabatic ) ( Isothermal ) Entropy Temperature V 1 V 2 P 1 P 2 2 2” 2’ 1
  12. 12. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work During Compression , due to the excess temperature above surrounding, the air will exchange the heat to the surrounding . As Compressor is a work consuming device , every effort is desired to reduce the work. Work done = Area under P-V curve Isothermal Compression : Not possible in practice . Every attempt made to bring the actual process as close as possible to Isothermal. e.g. Addition of cooling fins / water jacket to the cylinder. Compression Index, n is always less than γ , the adiabatic index. 1 – 2” : Adiabatic Compression = Max. Work . 1 – 2 : Polytropic Compression 1 – 2’ : Isothermal Compression = Min. Work .
  13. 13. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work Thus, comparison between the Isothermal Work and the Actual Work is important. Thus, more the Isothermal Efficiency, more the actual compression approaches to the Isothermal Compression . Actual Work = W act = Area 4-1-2-3-4 W act = Area (4-1) – Area (1-2) – Area (2-3) Isothermal Efficiency, η iso = Isothermal Work Actual Work P 1 P 2 V 1 V 2 3 2 2” 2’ 4 1 ( Polytropic ) ( Adiabatic ) ( Isothermal )
  14. 14. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work Now, P 1 P 2 V 1 V 2 3 2 2” 2’ 4 1 ( Polytropic ) ( Adiabatic ) ( Isothermal )
  15. 15. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work The solution of this equation is always negative . This shows that Work is done ON the Compressor. P 1 P 2 V 1 V 2 3 2 2” 2’ 4 1 ( Polytropic ) ( Adiabatic ) ( Isothermal )
  16. 16. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work Clearance Volume : Volume that remains inside the cylinder after the piston reaches the end of its inward stroke. Thus, Effective Stroke Volume = V 1 – V 4 Actual Work = W act = Area 1-2-3-4-1 W act = Area (5-1-2-6-51) – Area (5-4-3-6-5) P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 3 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  17. 17. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Equation for Work But, P 4 = P 1 and P 3 = P 2 P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 3 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  18. 18. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Volumetric Efficiency Volumetric Efficiency : Ratio of free air delivered to the displacement of the compressor. Ratio of Effective Swept Volume to Swept Volume . Presence of Clearance Volume Volumetric Efficiency less than 1 . ( 60 – 85 % ) ( 4 – 10 % ) Volumetric Efficiency = Effective Swept Volume Swept Volume V 1 – V 4 V 1 – V 3 = V c V s = = γ Clearance Volume Swept Volume Clearance Ratio = P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 3 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  19. 19. Reciprocating Compressor – Volumetric Efficiency ME0223 SEM-IV Applied Thermodynamics & Heat Engines ↑ Pr. Ratio ↑ Effect of Clearance Volume … .Clearance air expansion through greater volume before intake Cylinder bore and stroke is fixed. Effective Swept Volume (V1 – V4) ↓ with ↑ Pr. Ratio ↓ Volumetric Efficiency P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 3 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  20. 20. Reciprocating Compressor – Volumetric Efficiency <ul><li>Volumetric Efficiency is lowered due to : </li></ul><ul><li>Very high speed </li></ul><ul><li>Leakage past the piston </li></ul><ul><li>Too large Clearance Volume </li></ul><ul><li>Obstructions at inlet valves </li></ul><ul><li>Overheating of air due to contact with </li></ul><ul><li>hot cylinder walls </li></ul><ul><li>6. Inertia effect of air in suction pipe </li></ul>ME0223 SEM-IV Applied Thermodynamics & Heat Engines P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 3 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  21. 21. Reciprocating Compressor – Actual P-V Diagram 1-2-3-4-1 : Theoretical P-V Diagram. <ul><li>At 4 , inlet valve does not open due to : </li></ul><ul><li>There must be a pressure difference across the valve to open. </li></ul><ul><li>Inlet valve inertia . </li></ul>Pr. Drop continues till sufficient level for valve to force its seat. Some valve bounce is set ( wavy line ). Eventually, the pressure sets down at a level lower than atmospheric pressure. This negative pressure difference is known as Intake Depression. Similar situation appears at 2 , i.e. at the start of the delivery . Pressure rise, followed by valve bounce and then pressure settles at a level higher than the delivery pressure level. Air delivery to a tank / receiver, hence, generally known as Receiver Pressure . ME0223 SEM-IV Applied Thermodynamics & Heat Engines P 1 P 2 2 1 3 4 Valve Bounce Intake Depression Atmospheric Pressure Receiver Pressure
  22. 22. Reciprocating Compressor – Multistage Cycle 1-2-3-4-1 : Clearance air expansion = 3-4 Swept Volume = (V 1 – V 4 ) Cycle 1-5-6-7-1 : Clearance air expansion = 6-7 Swept Volume = (V 1 – V 7 )…..< (V 1 – V 4 ) Limiting Condition : Cycle 1-8-1 : Clearance air expansion = 8-1 Swept Volume = (V 1 – V 1 )…..= 0….No Delivery..!! ME0223 SEM-IV Applied Thermodynamics & Heat Engines 2 1 3 4 Clearance Volume 5 6 8 Volume Pressure P V n = Const 7 2. T 2 < T 5 < T 8 Energy Loss Conclusions : 1. ↑ Delivery Pressure ↓ Mass Flow
  23. 23. Reciprocating Compressor – Multistage ME0223 SEM-IV Applied Thermodynamics & Heat Engines High Pressure required by Single – Stage : This demands for MULTI – STAGING…!! <ul><li>Requires heavy working parts. </li></ul><ul><li>Has to accommodate high pressure ratios. </li></ul><ul><li>Increased balancing problems. </li></ul><ul><li>High Torque fluctuations. </li></ul><ul><li>Requires heavy Flywheel installations. </li></ul>
  24. 24. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Multistage 1. Air cooling possible between intermediate pressure levels. 2. Energy loss can be overcome. 2. Power required is less than that required for a single – stage compressor. 4. Better mechanical balance . 5. Temperature could be maintained within limits . 6. Reduced losses due to air leakage . 7. Improved lubrication . 8. Improved volumetric efficiency. 9. Lighter construction possible for low pressure cylinders. Advantages : Limitations : Expensive….!!!
  25. 25. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Multistage 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 ). Intercooler : Compressed air is cooled between cylinders. Internal Energy ↓ . Input Work ↓ . Final Delivery Temperature ↓ . 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
  26. 26. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Multistage Overall Pr. Range : P 1 – P 3 Single – stage cycle : 8-1-5-6-8 Without Intercooling : L.P. : 8-1-4-7-8 H.P. : 7-4-5-6-7 With Intercooling : L.P. : 8-1-4-7-8 H.P. : 7-2-3-6-7 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. T 2 = T 1 OR Points 1 and 2 are on SAME Isothermal line . Thus, Perfect Intercooling approaches more closely to Ideal Isothermal Compression. Intake Pr. P 1 or P s Delivery Pr. P 3 or P d 3 2 9 5 4 1 8 7 6 Intermediate Pr. P 2 Without Intercooling Perfect Intercooling L.P. H.P. Volume
  27. 27. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Multistage Ideal Conditions for Multi – Stage Compressors : Single – stage cycle : 8-1-5-6-8 A. Single – Stage Compressor : 3 2 9 5 4 1 8 7 6 L.P. H.P. Delivery Temperature,
  28. 28. Reciprocating Compressor – Multistage Without Intercooling : L.P. : 8-1-4-7-8 H.P. : 7-4-5-6-7 ME0223 SEM-IV Applied Thermodynamics & Heat Engines 3 2 9 5 4 1 8 7 6 L.P. H.P. B. Two – Stage Compressor (Without Intercooling) : This is SAME as that of Work done in Single – Stage . Delivery Temperature also remains SAME . Without Intercooling
  29. 29. Reciprocating Compressor – Multistage With Intercooling : L.P. : 8-1-4-7-8 H.P. : 7-2-3-6-7 ME0223 SEM-IV Applied Thermodynamics & Heat Engines 3 2 9 5 4 1 8 7 6 L.P. H.P. C. Two – Stage Compressor (With Perfect Intercooling) : Delivery Temperature,
  30. 30. Reciprocating Compressor – Multistage Now, T 2 = T 1 P 2 V 2 = P 1 V 1 Also P 4 = P 2 Shaded Area 2-4-5-3-2 : Work Saving due to Intercooler …!! ME0223 SEM-IV Applied Thermodynamics & Heat Engines 3 2 9 5 4 1 8 7 6 L.P. H.P. C. Two – Stage Compressor (With Perfect Intercooling) : With Intercooling : L.P. : 8-1-4-7-8 H.P. : 7-2-3-6-7
  31. 31. Reciprocating Compressor – Multistage Intermediate Pr. P 2 -> P 1 : Area 2-4-5-3-2 -> 0 Intermediate Pr. P 2 -> P 3 : Area 2-4-5-3-2 -> 0 ME0223 SEM-IV Applied Thermodynamics & Heat Engines Condition for Min. Work : 3 2 9 5 4 1 8 7 6 L.P. H.P. There is an Optimum P 2 for which Area 2-4-5-3-2 is maximum, i.e. Work is minimum…!! For min. Work,
  32. 32. Reciprocating Compressor – Multistage Condition for Min. Work : 3 2 9 5 4 1 8 7 6 L.P. H.P.
  33. 33. Reciprocating Compressor – Multistage P 2 obtained with this condition ( Pr. Ratio per stage is equal ) is the Ideal Intermediate Pr. Which, with Perfect Intercooling , gives Minimum Work, W min . ME0223 SEM-IV Applied Thermodynamics & Heat Engines Equal Work per cylinder…!!
  34. 34. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Efficiency Isothermal work done / cycle = Area of P – V Diagram = P 1 V 1 log e (P 2 /P 1 ) Indicated Power : Power obtained from the actual indicator card taken during a test on the compressor. NOTE : Shaft Power = Brake Power required to drive the Compressor. Isothermal Power = P 1 V 1 log e (P 2 /P 1 ) N 60 X 1000 kW Compressor Efficiency = Isothermal Power Indicated Power Isothermal Efficiency = Isothermal Power Shaft Power
  35. 35. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Efficiency Adiabatic Efficiency : Ratio of Power required to drive the Compressor; compared with the area of the hypothetical Indicator Diagram; assuming Adiabatic Compression . Mechanical Efficiency : Ratio of mechanical output to mechanical input. Mechanical Efficiency, η mech = Indicated Power Shaft Power
  36. 36. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Efficiency How to Increase Isothermal Efficiency ? <ul><li>Spray Injection : Assimilation of water into the compressor cylinder towards the </li></ul><ul><li>compression stroke. </li></ul><ul><ul><ul><ul><ul><li>Object is to cool the air for next operation. </li></ul></ul></ul></ul></ul>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. <ul><li>Water Jacketing : Circulating water around the cylinder to help for cooling the </li></ul><ul><li>air during compression. </li></ul>
  37. 37. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Efficiency How to Increase Isothermal Efficiency ? <ul><li>Inter – Cooling : For high speed and high Pr. Ratio compressors. </li></ul><ul><ul><ul><ul><ul><li>Compressed air from earlier stage is cooled to its original </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>temperature before passing it to the next stage. </li></ul></ul></ul></ul></ul>D. External Fins : For small capacity compressors, fins on external surfaces are useful. <ul><li>Cylinder Proportions : Short stroke and large bore provides much greater surface </li></ul><ul><li>for cooling. </li></ul><ul><ul><ul><ul><ul><li>Cylinder head surface is far more effective than barrel surface. </li></ul></ul></ul></ul></ul>
  38. 38. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Efficiency Clearance Volume : Consists of two spaces. 1. Space between cylinder end & the piston to allow for wear. 2. Space for reception of valves. High – class H.P. compressors : Clearance Vol. = 3 % of Swept Vol. : Lead (Pb) fuse wire used to measure the gap between cylinder end and piston. Low – grade L.P. compressors : Clearance Vol. = 6 % of Swept Vol. : Flattened ball of putty used to measure the gap between cylinder end and piston. Effect of Clearance Vol. : Vol. taken in per stroke < Swept Vol. ↑ Size of compressor ↑ Power to drive compressor .
  39. 39. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Reciprocating Compressor – Work Done Assumption : Compression and Expansion follow same Law. Work / cycle = Area 1-2-3-4-1 P 3 = P 2 and P 4 = P 1 P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 4 =V s Total Volume, V 1 Clearance Volume, V 3 =V c
  40. 40. Reciprocating Compressor – Work Done P 1 P 2 V 1 V 4 6 2 5 1 3 4 V 3 Effective Swept Volume, V 1 -V 4 Swept Volume, V 1 -V 4 =V s Total Volume, V 1 Clearance Volume, V 3 =V c m 1 is the actual mass of air delivered. Work done / kg of air delivered :
  41. 41. Reciprocating Compressor – F.A.D. Free Air Delivery (F.A.D.) : Actual Vol. delivered at the stated pressure, reduced to intake temperature and pressure. (m 3 /min) Displacement : Actual Vol. in swept out per minute by the L.P. cylinder. (m3/min) F.A.D. < Displacement , due to : 1. Fluid Resistance through air intake and valves. This prevents the cylinder being fully charged with atmospheric air. 3. High – Pressure air trapped in the Clearance Vol. has to expand below atm. Pr. level for suction to start. So, a portion of the suction stroke is wasted for this expansion. 4. Losses caused by leakages. 2. Entering the cylinder, air expands. Thus, mass of air reduces. Abs. Atm Temp Abs. Temp. of the air in cylinder. < 1
  42. 42. <ul><li>Thank You ! </li></ul>ME0223 SEM-IV Applied Thermodynamics & Heat Engines

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