BASICS OF MECHANICAL ENGINEERING

1. BASICS OF MECHANICAL ENGINEERING

2. TURBINES • Steam turbines • Classification • Principle of operation of impulse and reaction turbines • Delaval’s turbine • Parsons’s turbine • Compunding of impulse turbine • Gas turbines • Classification • Working principles and operations of open cycle and closed cycle gas turbine • Water turbines • Classification • Principle of operations of Pelton wheel • Francis turbine • Kaplan turbine

3. STEAM TURBINES • Prime mover – Is a self moving device which converts available natural source of energy into mechanical energy of motion to drive the other machines. • Steam turbine is a prime mover • Converts heat energy of steam into mechanical energy in the form of rotary motion.

4. STEAM TURBINES • Heat energy of steam --> Kinetic energy --> mechanical energy of rotation (Nozzle) (Turbine) • Steam turbines are mainly used in Thermal power plants Ships Gas compressors Textile and sugar industries

5. STEAM TURBINES • Propelling force in the steam turbine • Steam is caused to fall in its pressure by expanding in a nozzle • Due to fall in pressure a certain amount of heat energy is converted into KE • The rapidly moving particles of steam enter the rotating part of the turbine • Where it undergoes change in direction of motion • Which gives rise to a change of momentum and therefore a force. • This constitutes the driving force of the turbine

6. STEAM TURBINES • Expansion of steam in the nozzle • A high velocity jet of steam is produced by expanding a high pressure steam in a convergent divergent nozzle as shown in fig. • As steam passes between the entry and throat, it expands to a low pressure. • Due to this expansion in this portion of the nozzle the enthalpy of the steam is reduced • This loss in enthalpy of the steam must therefore be equal to the increase in the velocity of the steam. • The divergent portion is provided to complete any remaining expansion.

7. STEAM TURBINES • Classification of steam turbines • Depending on the drop in pressure due to the expansion of the steam that take place either before it is passed on to the turbine wheel or on the turbine wheel itself and the nature of the resulting propelling force • The steam turbines are classified into • De Laval Turbine (Impulse turbine) • Parson’s Turbine (Reaction turbine)

8. STEAM TURBINES • De Laval Turbine (Impulse turbine) • In this type of turbine the steam is initially expanded in a nozzle • The high velocity jet of steam coming out of the nozzle is made to glide over a curved vane called blade. • From fig we found that the jet of steam gliding over the blade gets deflected • This causes the particle of steam to suffer a change in direction of motion which gives rise to change in momentum and therefore a force. • The resultant of these forces acting on entire curved surface of blade causes it to move. • When a number of such blades are fitted on the circumference of a revolving wheel, called rotor as shown in fig. they will be moved by the action of steam.

9. STEAM TURBINES • Impulse turbine • Pressure velocity changes in impulse turbine

10. STEAM TURBINES • Parson’s turbine (Reaction turbine) • In this type of turbine the high pressure steam does not initially expand in the nozzle as in the case of impulse turbine, but instead directly passes onto the moving blades. • The blades are designed in such a way that the steam flowing between the blades will be subjected to the nozzle effect • Fig • The increase in velocity of the steam flowing over the blades develops a force

11. STEAM TURBINES • Reaction turbine • Pressure – velocity changes in reaction turbine • The actual reaction turbine consists of a number of rows of moving blades fitted on the different rotors keyed to the turbine shaft with alternate rings of fixed blades rigidly fixed to the casing of the turbine. • Both fixed and moving blades are designed in the shape of the nozzles • Therefore the expansion of the steam takes place both in fixed and moving blades

12. STEAM TURBINES • Comparison between impulse and reaction steam turbine

13. GAS TURBINES • Steam turbine: • Heat obtained from combustion of fuel --> to generate steam-->steam turbine • Gas turbine • Hot gases of combustion --> to turbine • A gas turbine essentially consists of a combustion chamber in which a liquid fuel is burnt in presence of air supplied by a compressor. • The air compressor sucks the air from the atmosphere and compresses it, there by increasing its pressure • In combustion chamber compressed air combines with fuel and the resulting mixture is burnt. • The burning gases at very high pressures expands rapidly and made to pass over the rings of moving blades mounted on the turbine shaft • (kinetic energy is absorbed by the moving blades imparting rotary motion)

15. GAS TURBINES • Principle of operation of closed cycle gas turbine • It consists of a compressor, a heater, a cooler and the gas turbine. • Fig--- • Both compressor and turbine are mounted on same shaft

16. GAS TURBINES • Principle of operation of open cycle gas turbine • It consists of a compressor, combustion chamber and the gas turbine. • Fig --- • The gas turbine and compressor are mounted on same shaft

17. GAS TURBINES Differences between closed and open cycle gas turbine

18. WATER TURBINES • Water turbines are the machines that convert the kinetic and potential energies possessed by water into mechanical rotary motion • Water turbines are the prime movers which when coupled to an electric generator produces electric power. • Hydro electric power can be developed whenever continuously flowing high pressure water is available • Water is carried out from water reservoirs to turbine stations through large pipes called penstocks and in the turbines its hydraulic energy is converted into mechanical energy

19. WATER TURBINES • Classification of water turbines • Based on the type of hydraulic action water turbines are classified as • Impulse turbine Ex: Pelton wheel • Reaction turbine Ex: Francis turbine, Kaplan turbine • Impulse water turbine • In an impulse water turbine the whole pressure energy of the water is converted into the kinetic energy in nozzles before it is passed on to the turbine wheel • An impulse turbine requires high head and low discharge at the inlet of the turbine

20. WATER TURBINES • Impulse water turbine • PELTON WHEEL

21. WATER TURBINES • Reaction water turbine • The water supplied to the reaction turbine posses both pressure as well as kinetic energies • First the water passes to the guided blades which guide or deflect the water to enter moving blades • When water flows over moving blades, part of the pressure energy is converted into the kinetic energy which will be absorbed by the turbine wheel. • The water leaving the moving blades will be at a low pressure. • The difference in pressure between entrance and exit of moving blades is called the reaction pressure. • A reaction turbine requires low head with high rate of flow.

22. Difference between impulse and reaction water turbines

23. WATER TURBINES • FRANCIS TURBINE • Is the medium head reaction turbine in which water flows radially inwards • Fig --

24. WATER TURBINES • KAPLAN TURBINE • The Kaplan turbine is a low head reaction turbine in which water flows axially