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DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANTS UNIT III

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POWER PLANT ENGINEERING - DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANT

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DIESEL, GAS TURBINE & COMBINED CYCLE POWER PLANTS UNIT III

  1. 1. POWER PLANT ENGINEERING S.BALAMURUGAN - M.E ASSISTANT PROFESSOR MECHANICAL ENGINEERING AAA COLLEGE OF ENGINEERING & TECHNOLOGY UNIT 2 – DIESEL, GAS TURBINE AND COMBINED CYCLE POWER PLANTS
  2. 2. • A Generating station in which diesel engine is used as prime mover for generation of electrical energy is known as diesel power station . • Diesel power plant produce power in the range of 2 – 50 MW . • Diesel Power stations are favored where demand of power is less sufficient quantity of coal and water is not available • They are used as standby set for continuity of supply such as hospitals, radio stations, cinema theatres, etc. DIESEL ENGINE POWER PLANT ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  3. 3. ADVANTAGES • The design and layout of the plant are quiet simple. • It occupies less space. • It can be located at any place. • It can be started quickly and can pick up load in a short time. • It requires less quantity of water for cooling. • The overall cost is much less than that of steam power plant of same capacity. • The thermal efficiency of the plant is higher. • It requires less operating staff. DISADVANTAGES • The plant has higher running costs as the fuel that is used is costly. • The plant does not work satisfactory under overload conditions for a longer period. • The plant can only generate small power. • The cost of lubrication is generally high. • The maintenance cost is also high. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  4. 4. APPLICATIONS • They are used as central stations for small power supplies. • They can be used as standby plants to hydrodynamic plants and steam power plants In case of an emergency. • They can be used as peak load plants in combination with thermal or hydro plants. • They are quiet suitable for mobile power generations and are widely used in transportation systems such as automobile, railways, ships. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  5. 5. GENERAL LAYOUT ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  6. 6. ESSENTIAL ELEMENTS OF DIESEL POWER PLANT • Starting system • Air Intake System • Fuel supply system • Exhaust system • Cooling system • Lubrication system • Governing system ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  7. 7. STARTING SYSTEM • 1. Starting by Auxiliary engine (petrol driven) • Clutch is disengaged, auxiliary engine started by hand or electric motor, once reach the optimal stage, clutch is engaged. 2. Use of Electric motors or Self Starters – Battery – Electric motor is coupled with flywheel 3. Compressed air system • The function of this system is to start the engine from cold by supplying compressed air at about 17 bar from an air tank that is admitted into few cylinders making them work like reciprocating air motors to run the engine shaft • Fuel is then injected into remaining cylinders and ignited in the normal way causing the engine to start ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  8. 8. ENGINE SYSTEM • Generally classified as Two stroke engines and 4 stroke engines. • In diesel engine air is admitted into the cylinder and is compressed. • At the end of the compression stroke fuel is injected. • The burning gases expand and does work on the piston. • The gases are then exhausted from the cylinder. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  9. 9. AIR INTAKE SYSTEM • The air intake system conveys fresh air through pipes or ducts to • (i) the air intake manifold • (ii) the supercharger inlet of a supercharged engine. • Air filter is used to remove the dust from air which is taken by the engine. • The super charger is used to increase the pressure of air supplied. • Air is first drawn through the filter to catch dirt particles that may cause excessive wear in cylinders. • Filters may be of two types • Dry type (paper, cloth etc.) • Wet type (oil impingement type where oil helps to catch particles ) ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  10. 10. FUEL SUPPLY SYSTEM • It includes storage tank, fuel pump, fuel transfer pump, strainers and heaters. • Pump draws diesel from storage tank through the filter to day tank. • The day tank is usually placed high so the diesel flows to engine under gravity. • Diesel is filtered before being injected into the engine by the fuel injection pump. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  11. 11. FUEL SUPPLY SYSTEM This System performs the following function • Filter the fuel. • Meter the correct quantity of the fuel to be injected. • Time the injection process. • Regulate the fuel supply. • Distribute the atomized fuel properly in the combustion chamber. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  12. 12. Classification of solid injection systems Common rail injection system: The system is named after the shared high- pressure (100 to 200 bars)reservoir (common rail) that supplies all the cylinders with fuel. With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection. •Individual pump injection system: Distributor system: ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  13. 13. CRDI ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  14. 14. Individual pump Injection System ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  15. 15. Individual pump Injection System The schematic is shown in fig. An individual pump or pump cylinder connects directly to each fuel nozzle. Metering and injection timing controlled by individual pumps. Nozzle contains a delivery valve actuated by the fuel pressure. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  16. 16. Distributor System The schematic is shown here. The fuel is metered at a central point. A pump meters, pressurizes and times the fuel injection. Fuel is distributed to cylinders in correct firing order by cam operated poppet valves which admit fuel to nozzles. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  17. 17. EXHAUST SYSTEM • This includes the silencers and connecting ducts. • The exhaust gases coming out of the engine is noisy. • Silencer is provided to reduce the noise. • Exhaust pipe leading out of the building in should be short in length with minimum number of bends to provide as low a pressure loss as possible. • Each engine should have its independent exhaust system. • Where possible exhaust heat recovery should be made to improve plant thermal efficiency. Eg. Air heating, steam generation in diesel steam power plant etc. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  18. 18. COOLING SYSTEM • The cooling system consists of a water source, pump and cooling towers. The pump circulates water through cylinder and head jacket. The water takes away heat from the engine and it becomes hot. The hot water is cooled by cooling towers and re circulated for cooling. • The temperature of the burning fuel inside the engine cylinder is in the order of 2750deg Celsius. In order to lower this temperature water is circulated around the engine. • Above 65°C, the lubricating oil will begin to evaporate. • The water envelopes(water jacket) the engine. The heat from the cylinder, piston, combustion chamber etc., is carried by the circulating water. • The hot water leaving the jacket is passed through the heat exchanger • The heat from the heat exchanger is carried away by the raw water circulated through the heat exchanger and is cooled in the cooling system. • Only 30% of heat – Useful Work • 40% of heat – exhaust gas • 1-1.5% of heat – Lubricating oil • Remaining 30 % heat ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  19. 19. COOLING SYSTEM Liquid cooling: • In this method, the cylinder walls and head are provided with jackets through which the cooling liquid can circulate. • The heat is transferred from the cylinder walls to the liquid by convection and conduction. • The liquid gets heated during its passage through the cooling jackets and is itself cooled by means of an air cooled radiator system. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  20. 20. COOLING SYSTEM ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  21. 21. COOLING SYSTEM There are two methods of cooling I.C. engines: 1.Air cooling. 2. Liquid cooling Air cooling: • In this method, heat is carried away by the air flowing over and around the cylinder. • Fins are added on the cylinder which provide additional mass of material for conduction as well as additional area for convection and radiation modes of heat transfer ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  22. 22. LUBRICATION SYSTEM The following are the important functions of a lubrication system: • LUBRICATION: To keep parts sliding freely past each other, reducing friction and wear. • COOLING: To keep surfaces cool by taking away part of the heat caused by friction. • CLEANING: To keep the bearings and piston rings clean. • SEALING: To form a good seal B/W the piston rings and cylinder walls. • REDUCING NOISE: to reduce the noise of the engine by absorbing vibration. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  23. 23. LUBRICATION SYSTEM • It includes the oil pumps, oil tanks, filters, coolers and connecting pipes. • The purpose of the lubrication system is to reduce the wear of the engine moving parts, cool the engine • Part of the cylinder such as piston , shafts , valves must be lubricated. The lubricant is cooled before recirculation. • Lubrication oil starts evaporating when the temperature inside the cylinder exceeds 70deg ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  24. 24. GOVERNING SYSTEM • The function of the governing system is to maintain the speed of the engine • This is done generally by varying fuel supply to the engine according to load. • It is achieved with use of governors. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  25. 25. SUPERCHARGING The apparatus used to increase the air density is called supercharger. It is similar to a compressor(centrifugal type), which provides greater mass of charge with same piston displacement. • The purpose of supercharging is to raise the volumetric efficiency above that value which can be obtained by normal aspiration. • The engine is an air pump, increasing the air consumption permits greater quantity of fuel to be added, and results in greater potential output. • The power output is almost directly proportional to the air consumption. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  26. 26. SUPERCHARGING Three methods to increase the air consumption are 1. Increasing the piston displacement: leads to more size and weight, cooling problems 2. Running the engine at higher speeds leads to mechanical wear and tear. 3. Increasing the density of the charge, so that greater mass of charge is introduced in same volume. The supercharger produces following effects: 1. Provides better mixing of air fuel mixture due to turbulent effect of supercharger. 2. The temperature of charge is raised as it is compressed, resulting in higher temperature within the cylinder, so better vaporization of fuel. 3. Power required to run the supercharger is obtained from engine ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  27. 27. DIESEL CYCLE ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  28. 28. A diesel engine has a compression ratio of 15 and heat addition at constant pressure takes place at 6% of stroke. Find the air standard efficiency of the engine. Take γ of air as 1.4. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  29. 29. The stroke and cylinder diameter of a compression ignition engine are 250 mm and 150 mm respectively. If the clearance volume is 0.0004 m3 and fuel injection takes place at constant pressure for 5% of the stroke. Determine the efficiency of the engine. Assume the engine working on the diesel cycle. Length of stroke L = 250 mm = 0.25 m Molecules of neon, a monatomic gas, have only one atom. Molecules of diatomic gases, such as hydrogen, have two atoms ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  30. 30. TURBINES: Machines to extract fluid power from flowing fluids Steam Turbine Water Turbines Gas TurbinesWind Turbines Aircraft Engines Power Generation •High Pressure, High Temperature gas •Generated inside the engine •Expands through a specially designed TURBINE Wind turbine VS Gas Turbine ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  31. 31. GAS TURBINES • Invented in 1930 by Frank Whittle, Patented in 1934 • First used for aircraft propulsion in 1942 on Me262 by Germans during second world war • Currently most of the aircrafts and ships use GT engines • Used for power generation • Manufacturers: General Electric, Pratt &Whitney, SNECMA, Rolls Royce, Honeywell, Siemens – Westinghouse, Alstom • Indian take: Kaveri Engine by GTRE, Bangalore(gas turbine research establishment) (DRDO) • A Gas turbine is a Turbo-machine and basically similar to steam turbine regarding its working principle • The first turbine to produce useful work was probably a wind mill, where no compression and no combustion exist • Now a days gas turbine include a compression process and combustion process. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  32. 32. GAS TURBINE - PRINCIPLE OF OPERATION • Intake – Slow down incoming air – Remove distortions • Compressor – Dynamically Compress air • Combustor – Heat addition through chemical reaction • Turbine – Run the compressor • Nozzle/ Free Turbine – Generation of thrust power/shaft power Limitations – Not self starting – Higher rotor speeds – low plant efficiency – Low efficiency at part loads – Thermal efficiency = 20 – 30%, (steam PP 40%), combined plants 45% Aviation & Marine - Light weight, not requiring cooling water & fit into the overall shape of the structure Power Generation – Simplicity, Lack of cooling water, needs quick installation, quick starting. Oil & Gas industry – Cheap Fuel & low installation cost. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  33. 33. Advantages and Disadvantages • Great power-to-weight ratio compared to reciprocating engines. • Smaller than their reciprocating counterparts of the same power. • Lower installations & maintenance costs. • Less vibrations due to perfect balancing • Poor quality fuels can be used • The ignition & Lubrication systems are simpler. • Special metals & alloys are required for different components of the plants. • Use of Nickel Chromium alloy, the manufacture of the blades is difficult & costly. • Special cooling methods are required for cooling the turbine blades. • High temperature in Blades (1000K) & Combustion chamber (2500K) & Centrifugal force leads to short life of this components • For the same output, gas turbine produces five times exhaust gases than I.C engines • Poor thermal efficiency at Part loads, as the quantity of air remains same irrespective of load, & output is reduced by reducing the quantity of fuel supplied. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  34. 34. BRAYTON CYCLE: IDEAL CYCLE FOR GAS-TURBINE ENGINES Gas turbines usually operate on an open cycle Air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. • The high-temperature gases then enter the turbine where they expand to atmospheric pressure while producing power output. • Some of the output power is used to drive the compressor. • The exhaust gases leaving the turbine are thrown out (not re-circulated), causing the cycle to be classified as an open cycle. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  35. 35. • The open gas-turbine cycle can be modelled as a closed cycle, using the air-standard assumptions. • The compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat addition process from an external source. • The exhaust process is replaced by a constant-pressure heat rejection process to the ambient air. Closed Cycle Model AIR-STANDARD ASSUMPTIONS • Air is the working fluid, it obeys the perfect gas law, pV = mRT • The engine operates in a closed cycle. The cylinder is filled with constant amount of working medium & the same fluid is used repeatedly, mass remains constant. • The compression & expansion processes are assumed to be adiabatic • The values of specific heat (Cp & Cv) of the working fluid remains constant • Combustion replaced by Constant pressure heat addition, Exhaust replaced by Heat rejection process. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  36. 36. 37 COMPARE OPEN CYCLE AND CLOSED CYCLE GAS TURBINES OPEN CYCLE: 1. Warm-up time. Once the turbine is brought up to the rated speed by the starting motor and the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up time. 2. Low weight and size. The weight in kg per kW developed is less. 3. Open cycle plants occupy comparatively little space. 4. Open-cycle gas turbine power plant, except those having an intercooler, does not require cooling water. 5. The part load efficiency of the open cycle plant decreases rapidly as the considerable percentage of power developed by the turbine is used to drive the compressor. 6. The open-cycle gas turbine plant has high air rate compared to the other cycles, therefore, it results in increased loss of heat in the exhaust gases. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  37. 37. 38 COMPARE OPEN CYCLE AND CLOSED CYCLE GAS TURBINES CLOSED CYCLE: 1. The machine can be smaller and cheaper than the machine used to develop the same power using open cycle plant. 2. The closed cycle avoids erosion of the turbine blades due to the contaminated gases and fouling of compressor blades due to dust. Therefore, it is practically free from deterioration of efficiency in service. 3. The need for filtration of the incoming air which is a severe problem in open cycle plant is completely eliminated. 4. The maintenance cost is low and reliability is high due to longer useful life. 5. The system is dependent on external means as considerable quantity of cooling water is required in the pre-cooler. 6. The response to the load variations is poor compared to the open-cycle plant ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  38. 38. Emission in Gas Turbines •Lower emission compared to all conventional methods (except nuclear) •Regulations require further reduction in emission levels ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  39. 39. The ideal cycle that the working fluid undergoes in the closed loop is the Brayton cycle. It is made up of four internally reversible processes: 1-2 Isentropic compression; 2-3 Constant-pressure heat addition; 3-4 Isentropic expansion; 4-1 Constant-pressure heat rejection. Compressor isentropic Efficiency ηcomp.= 𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛 𝑖𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝐴𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 𝑇2 −𝑇1 𝑇2 ′−𝑇1 Turbine isentropic Efficiency ηturbine.= 𝐴𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 𝐼𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 = 𝑇3 −𝑇4 ′ 𝑇3 −𝑇4 THE BRAYTON CYCLE ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  40. 40. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  41. 41. BRAYTON CYCLE Heat Addition (Combustion Chamber) = m Cp (T3 – T2) Heat Rejected (Cooler) = m Cp (T4 – T1) Efficiency η = 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝐻𝑒𝑎𝑡 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛 = 𝐻𝑒𝑎𝑡𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛−𝐻𝑒𝑎𝑡 𝑅𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝐻𝑒𝑎𝑡 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛 = m Cp (T3 – T2) −m Cp (T4 – T1) m Cp (T3 – T2) = 1 - (T4 – T1) (T3 – T2) Isentropic process (1-2 & 3-4) 𝑇2 𝑇1 = 𝑃2 𝑃1 𝛾 −1 𝛾 𝑇3 𝑇4 = 𝑃3 𝑃4 𝛾 −1 𝛾 = 𝑃2 𝑃1 𝛾 −1 𝛾 T2 = T1 𝑟 𝑝 𝛾 −1 𝛾 T3 = T4 𝑟 𝑝 𝛾 −1 𝛾 Air Standard Efficiency η = 1 - (T4 – T1) (T4 𝑟 𝑝 𝛾 −1 𝛾 –T1 𝑟 𝑝 𝛾 −1 𝛾 ) = 1 - 1 𝒓 𝒑 𝜸 −𝟏 𝜸 Work ratio = 𝑊 𝑛𝑒𝑡 𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒 = 𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒 −𝑊𝑐𝑜𝑚𝑝𝑟𝑒 𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒 = 1 - (T2 – T1) (T3 – T4) = 1 - 𝑻 𝟏 𝑻 𝟑 𝑟 𝑝 𝛾 −1 𝛾 rp = P2 / P1 ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  42. 42. The air enters the compressor of an open cycle constant pressure gas turbine at a pressure of 1 bar and temperature of 20°C. the pressure of the air after compression is 4 bar. The isentropic efficiencies of compressor & turbines are 80% & 85% respectively. The air-fuel ratio used is 90:1. If flow rate of air is 3 kg/s, Find (i) Power Developed (ii) Thermal Efficiency of the cycle. Assume Cp = 1.0 kJ/kg K and Ƴ = 1.4 for air & gases. Calorific value of fuel = 41800 kJ/kg. GIVEN – P1 = 1 bar, T1 = 20°C = 20+273 = 293 K, P2 = 4 bar, ηcomp = 80%, ηcomp = 85% Air –fuel ratio = 90:1 , Air flow rate ma = 3 kg/s, C.V = 41800 kJ/kg Power Developed, P = Net work, Wnet = Wturbine - Wcompressor 𝑇2 𝑇1 = 𝑃2 𝑃1 𝛾 −1 𝛾 = 4 1 1.4 −1 1.4 = 1.486 , T2 = 293 × 1.486 = 435.4 K ηcomp = 𝑇2 −𝑇1 𝑇2 ′−𝑇1 , 0.8 = 435.4 −239 𝑇2 ′−293 , 𝑻 𝟐 ′= 471 K Heat supplied by fuel = Heat taken by burning gases mf × C.V = (ma + mf ) Cp (𝑻 𝟑 − 𝑻 𝟐′) C.V = ( ma mf + 1) Cp (𝑻 𝟑 − 𝟒𝟕𝟏), T3 = 41800 91 + 471 = 930 K = T3 ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  43. 43. Problem continued…. 𝑻 𝟑 𝑻 𝟒 = 𝑷 𝟑 𝑷 𝟒 𝜸 −𝟏 𝜸 = 𝑷 𝟐 𝑷 𝟏 𝜸 −𝟏 𝜸 = 𝟒 𝟏 𝟏.𝟒 −𝟏 𝟏.𝟒 = 1.486, T4 = 𝟗𝟑𝟎 𝟏.𝟒𝟖𝟔 = 625.8 K ηturbine = 𝑻 𝟑 −𝑻 𝟒 ′ 𝑻 𝟑 −𝑻 𝟒 , 0.85 = 𝟗𝟑𝟎 −𝑻 𝟒 ′ 𝟗𝟑𝟎−𝟔𝟐𝟓.𝟖 , 𝑻 𝟒 ′ = 670.6 K Wturbine = mg × Cp × (T3 – T4’) mg – mass of hot gases formed Wturbine = ( 𝟗𝟎+𝟏 𝟗𝟎 )× 1.0 × (930 – 670.6) = 262.28 kJ/kg of air Wcompressor = ma × Cp × (T2’– T1) = 1 × 1.0 × (471 – 293) = 178 kJ/kg of air Wnet = Wturbine – Wcompressor = 262.28 – 178 = 84.28 kJ/kg of air Thermal Efficiency = ηthermal = 𝑾𝒐𝒓𝒌 𝒐𝒖𝒕𝒑𝒖𝒕 𝑯𝒆𝒂𝒕 𝑺𝒖𝒑𝒑𝒍𝒊𝒆𝒅 = 𝟖𝟒.𝟐𝟖 * 𝟏 𝟗𝟎 𝟒𝟏𝟖𝟎𝟎+ = 0.1814 = 18.14% Power Developed = P = 84.28 × 3 = 252.84 kW ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  44. 44. 45 The thermal efficiency of an ideal Brayton cycle depends on the pressure ratio, rp of the gas turbine and the specific heat ratio, γ of the working fluid. The thermal efficiency increases with both of these parameters, which is also the case for actual gas turbines. A plot of thermal efficiency versus the pressure ratio is shown in fig. for the case of γ =1.4. Pressure ratio – It is the ratio of cycle’s highest to its lowest pressure, (Compressor outlet to compressor inlet) PARAMETERS AFFECTING THERMAL EFFICIENCY ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  45. 45. 46 The early gas turbines (1940s to 1959s) found only limited use despite their versatility and their ability to burn a variety of fuels, because its thermal efficiency was only about 17%. Efforts to improve the cycle efficiency are concentrated in three areas: 1. Increasing the turbine inlet (or firing) temperatures. The turbine inlet temperatures have increased steadily from about 540°C (1000°F) in the 1940s to 1425°C (2600°F) and even higher today. 2. Increasing the efficiencies of turbo-machinery components (turbines, compressors). The advent of computers and advanced techniques for computer-aided design made it possible to design these components aerodynamically with minimal losses. 3. Adding modifications to the basic cycle (intercooling, regeneration or recuperation, and reheating). The simple-cycle efficiencies of early gas turbines were practically doubled by incorporating intercooling, regeneration (or recuperation), and reheating. IMPROVEMENTS OF GAS TURBINE’S PERFORMANCE ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  46. 46. METHODS TO IMPROVE THERMAL EFFICIENCY - INTERCOOLING • Compressor utilize major percentage of power developed by turbine • The work required by compressor can be reduced by compressing the air in two stages & incorporate intercooler between the compressor. • Work input (with intercooling) = Cp (T2’ – T1) + Cp (T4’ – T3) • Work input (with out intercooling) = Cp (TL’ – T1) = Cp (T2’ – T1) + Cp (TL’ – T2’) • Work input with intercooling < Work input without Intercooling • Work ratio = Net work output / Gross work output = 𝑾𝒐𝒓𝒌 𝒐𝒇 𝑬𝒙𝒑𝒂𝒏𝒔𝒊𝒐𝒏 −𝑾𝒐𝒓𝒌 𝒐𝒇 𝑪𝒐𝒎𝒑𝒓𝒆𝒔𝒔𝒊𝒐𝒏 𝑾𝒐𝒓𝒌 𝒐𝒇 𝑬𝒙𝒑𝒂𝒏𝒔𝒊𝒐𝒏 ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  47. 47. METHODS TO IMPROVE THERMAL EFFICIENCY - REHEATING • The output of a gas turbine can be increased by expanding the gases in two stages with a reheater between the two turbine • High pressure turbine output = Work input required for compressor • Net work output of L.P. Turbine ( With Reheating) = Cp(T5 – T6’) • Net work output of L.P. Turbine ( Without Reheating) = Cp(T4’ – TL’) • Increases the Net work output Net work output of L.P. Turbine ( With Reheating) > Net work output of L.P. Turbine ( Without Reheating) • Reduces the Thermal Efficiency, Heat supplied = Cp(T3 – T2’) + Cp(T5 – T4’) ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  48. 48. METHODS TO IMPROVE THERMAL EFFICIENCY - REGENERATION • Exhaust gases from a gas turbine carry a large quantity of heat . Higher than the ambient temperature. • This can be used to heat the air coming from the compressor, thereby reducing the mass of fuel supplied in combustion chamber. • 2’ – 3 = Heat flow into the compressed air from Heat exchanger • 3 – 4 = Heat taken from Combustion chamber, 6 – Temperature of Exhaust gas • Effectiveness = 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑒𝑛𝑡𝑕𝑎𝑙𝑝𝑦 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑎𝑖𝑟 𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑖𝑛𝑐𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑒𝑛𝑡𝑕𝑎𝑙𝑝𝑦 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑎𝑖𝑟 = 𝑇3 −𝑇2 ′ 𝑇5 ′−𝑇2 ′ ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  49. 49. Brayton Cycle With Regeneration Temperature of the exhaust gas leaving the turbine is higher than the temperature of the air leaving the compressor. The air leaving the compressor can be heated by the hot exhaust gases in a counter-flow heat exchanger (a regenerator or recuperator) – a process called regeneration. The thermal efficiency of the Brayton cycle increases due to regeneration since less fuel is used for the same work output. Note: The use of a regenerator is recommended only when the turbine exhaust temperature is higher than the compressor exit temperature. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  50. 50. Inlet system Collects and directs air into the gas turbine. Often, an air cleaner and silencer are part of the inlet system. It is designated for a minimum pressure drop while maximizing clean airflow into the gas turbine. Compressor Provides compression, and, thus, increases the air density for the combustion process. The higher the compression ratio, the higher the total gas turbine efficiency . Low compressor efficiencies result in high compressor discharge temperatures, therefore, lower gas turbine output power. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  51. 51. 52 GAS TURBINE COMPONENTS Combustor Adds heat energy to the airflow. The output power of the gas turbine is directly proportional to the combustor firing temperature; i.e., the combustor is designed to increase the air temperature up to the material limits of the gas turbine while maintaining a reasonable pressure drop. Gas Producer Turbine Expands the air and absorbs just enough energy from the flow to drive the compressor. The higher the gas producer discharge temperature and pressure, the more energy is available to drive the power turbine, therefore, creating shaft work. Power Turbine Converts the remaining flow energy from the gas producer into useful shaft output work. The higher the temperature difference across the power turbine, the more shaft output power is available. Exhaust System Directs exhaust flow away from the gas turbine inlet. Often a silencer is part of the exhaust system. Similar to the inlet system, the exhaust system is designed for minimum pressure losses. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  52. 52. Intercooling and reheating always decreases thermal efficiency unless are accompanied by regeneration. Therefore, in gas turbine power plants, intercooling and reheating are always used in conjunction with regeneration. The work input to a two-stage compressor is minimized when equal pressure ratios are maintained across each stage. This procedure also maximizes the turbine work output. The net work output of a gas-turbine cycle can be increased by either: a) decreasing the compressor work, or b) increasing the turbine work, or c) both. The work output of a turbine can be increased by expanding the gas in stages and reheating it in between, utilizing a multistage expansion with reheating. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  53. 53. COMBINED GAS TURBINES & STEAM POWER PLANTS The combination gas turbine – steam cycles aim at utilizing the heat of exhaust gases from the gas turbine, improves the overall efficiency. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  54. 54. COMBINED GAS TURBINES & DIESEL POWER PLANTS Improve Efficiency More suitable for Rapid start & shut down Less cooling water requirement It gives high ratio of power output to occupied ground space ADVANTAGESOF COMBINEDCYCLE ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET
  55. 55. STARTING OF A GAS TURBINE • Auxiliary power source required till the plants own compressor inducts air & compresses it to a pressure such that expansion from reasonable temperature will develop enough power to sustain operation. IC Engine, Steam turbine, Auxiliary Electric motor, Another Gas turbine PROCEDURE • Run the unit • Actuate the combustion ignition system & inject the fuel. The fuel flow is controlled to obtain the necessary warm-up • Adjust the speed & voltage & synchronize the alternator • Build up the load on the alternator by governor gear control SHUT DOWN OF A GAS TURBINE • It occurs very quickly after the fuel is cut off from the combustion chamber. • Rapid Shut down – Minimizing the Chilling effect – (Cold air through Hot Turbine) • To avoid Thermal Kinking of shaft - Starting device operated at reduced speed to ensure symmetrical cooling of rotor. ME 6701 POWER PLANT ENGG. S.BALAMURUGAN AP/MECH AAACET

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