Chemical Engineering - Apllied Mechanics

Turbines et al

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TURBINES et. al. A. Vania Politecnico di Milano, Dept. of Mechanical Engineering , Milan, Italy APPLIED MECHANICS –Prof. Andrea VANIA 2 STEAM TURBINES A steam turbine is a type of continuous internal combustion engines . A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft . Because the turbine generates rotary motion, it is particularly suited to be used to drive an electric generator . The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process . These turbines receive steam from a boiler and exhaust it to a condenser . The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state, typically of a quality near 90 % . A boiler is a closed vessel in which water is heated . The fluid does not necessarily boil . The heated or vaporized fluid exits the boiler for use in various processes or heating applications like water heating, boiler -based power generation, sanitation, etc .. APPLIED MECHANICS –Prof. Andrea VANIA 3 STEAM TURBINES Machine train diagram steam inlet (High Pressure) HP -IP steam turbine LP steam turbine HP IP LP HP: High pressure IP: Intermediate pressure LP: Low pressure shaft end connected to the generator rotor generator rotor APPLIED MECHANICS –Prof. Andrea VANIA 4 GAS TURBINES A gas turbine is a type of continuous internal combustion engine . It has an upstream rotating compressor coupled to a downstream turbine , and a combustion chamber or area, called a combustor, in between . The basic operation of the gas turbine is similar to that of the steam power plant except that the working fluid is air instead of water . Fresh atmospheric air flows through a compressor that brings it to higher pressure . Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high -temperature flow . This high -temperature high -pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process . The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft . The energy that is not used for shaft work comes out in the exhaust gases, so these have either a high temperature or a high velocity . These exhaust gases can be used for many industrial processes . Gas turbines can be particularly efficient when waste heat from the turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration . The purpose of the gas turbine determines the design so that the most desirable energy form is maximized . Gas turbines are used to power aircraft, trains, ships, electrical generators, pumps, gas compressors, etc . APPLIED MECHANICS –Prof. Andrea VANIA 5 JET ENGINES Typical axial -flow gas turbine turbojet , sectioned for display . Flow is left to right, multistage compressor on left, combustion chambers center, two -stage turbine on right . Air -breathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines . Jet engines that produce thrust from the direct impulse of exhaust gases are often called turbojets , whereas those that generate thrust with the addition of a ducted fan are often called turbofans (in this case a ducted fan uses the mechanical energy from the gas turbine to accelerate air rearwards ). Schematic diagram of a high -bypass turbofan engine . APPLIED MECHANICS –Prof. Andrea VANIA 6 INDUSTRIAL GAS TURBINES turbine stages combustors compressor stages APPLIED MECHANICS –Prof. Andrea VANIA 7 HYDRAULIC TURBINES Hydraulic turbines are devices that convert water kinetic energy into mechanical energy and electric power . Pelton turbine Kaplan turbine APPLIED MECHANICS –Prof. Andrea VANIA 8 STIRLING ENGINE A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas (the working fluid ) at different temperatures, such that there is a net conversion of heat energy to mechanical work . More specifically, the Stirling engine is a closed -cycle regenerative heat engine with a permanently gaseous working fluid . Closed -cycle , in this context, means a thermodynamic system in which the working fluid is permanently contained within the system , and regenerative describes the use of a specific type of internal heat exchanger and thermal store, known as the regenerator . The inclusion of a regenerator differentiates the Stirling engine from other closed cycle hot air engines . Stirling engines have a high efficiency compared to steam engines, being able to reach 50 % efficiency . They are also capable of quiet operation and can use almost any heat source . The heat energy source is generated external to the Stirling engine rather than by internal combustion as with the Otto cycle or Diesel cycle engines . Because the Stirling engine is compatible with alternative and renewable energy sources it could become increasingly significant as the price of conventional fuels rises . APPLIED MECHANICS –Prof. Andrea VANIA 9 STIRLING ENGINE Beta configuration Alfa configuration A beta Stirling has a single power piston (A) arranged within the same cylinder on the same shaft as a displacer piston (B ).The displacer piston is a loose fit and does not extract any power from the expanding gas but only serves to shuttle the working gas between the hot and cold heat exchangers . When the working gas is pushed to the hot end of the cylinder it expands and pushes the power piston .When it is pushed to the cold end of the cylinder it contracts and the momentum of the machine, usually enhanced by a flywheel, pushes the power piston the other way to compress the gas . Unlike the alpha type, the beta type avoids the technical problems of hot moving seals, as the power piston is not in contact with the hot gas . A regenerator must be placed in the gas path around the displacer . An alpha Stirling contains two power pistons in separate cylinders, one hot and one cold .The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is situated inside the low temperature heat exchanger . This type of engine has a high power -to - volume ratio but has technical problems because of the usually high temperature of the hot piston and the durability of its seals .[41 ]In practice, this piston usually carries a large insulating head to move the seals away from the hot zone at the expense of some additional dead space . The crank angle has a major effect on efficiency and the best angle frequently has to be found experimentally .An angle of 90 °frequently locks . The figure does not show internal heat exchangers in the compression and expansion spaces, which are needed to produce power .A regenerator would be placed in the pipe connecting the two cylinders . A B cold end hot end cold cylinder hot cylinder APPLIED MECHANICS –Prof. Andrea VANIA 10 STIRLING ENGINE - Beta configuration 1 1. Power piston (dark grey) has compressed the gas, the displacer piston (light grey) has moved so that most of the gas is adjacent to the hot heat exchanger. 2. The heated gas increases in pressure and pushes the power piston to the farthest limit of the power stroke . 3. The displacer piston now moves, shunting the gas to the cold end of the cylinder. 4. The cooled gas is now compressed by the flywheel momentum. This takes less energy, since its pressure drops when it is cooled. 2 3 4 APPLIED MECHANICS –Prof. Andrea VANIA 11 WIND TURBINES A wind turbine is a device that converts the wind's kinetic energy into electrical power . Wind turbines are manufactured in a wide range of vertical and horizontal axis types . APPLIED MECHANICS –Prof. Andrea VANIA 12 WIND POWERED SYSTEMS Windmills, sailing boats (and some types of vessels) are wind -powered systems . compared to a conventional diesel powered vessel of the same size .These rotors were well tested back in the 1920 s but never gained wide use because of the cheap price of fuel . During wind flow, the rotors spin to create lift force (10 -14 times that of a trapezoid -shaped sail) through what is called the Magnus Effect . The designer said he expected with the addition of the four rotors to achieve fuel savings of 30 % – 40 % wind lift force lift force top view