BASICS OF MECHANICAL ENGINEERING

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

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.

STEAM TURBINES Steam turbines are mainly used in Thermal power plants 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

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

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.

De Laval Turbine (Impulse turbine) Parson’s Turbine (Reaction turbine) 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)

In this type of turbine the steam is initially expanded in a nozzle 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.

Pressure velocity changes in impulse turbine STEAM TURBINES Impulse turbine Pressure velocity changes in impulse turbine

Parson’s turbine (Reaction turbine) 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

Pressure – velocity changes in reaction turbine 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

Comparison between impulse and reaction steam turbine STEAM TURBINES Comparison between impulse and reaction steam turbine Impulse turbine Reaction turbine 1 The steam completely expands from high pressure to low pressure in the nozzle before it enters the moving blades The high pressure steam continuously expands successively in both the fixed and moving blades 2 Because of the large pressure drop in the nozzle, the steam speed and as well as the rotor speeds are high Due to smaller pressure drop over both fixed and moving blades, both the steam speed and rotor speed are low 3 Occupies less space per unit power Occupies more space for the unit power 4 Suitable for small power generation prime movers Suitable for medium and high power generation prime movers 5 Due to high rotor speeds compounding is required to reduce the speed The speeds are relatively less and hence no compounding is required

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)

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

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

Differences between closed and open cycle gas turbine GAS TURBINES Differences between closed and open cycle gas turbine Closed cycle Open cycle 1 Working substance is continuously re- circulated Working substance is continuously replaced in every cycle 2 Any fluid may be used as the working substance The working substance comprises of the mixture of air and the products of combustion of fuel 3 There is only heat and work transfer takes place between the system and surrounding There is mass transfer taking place in addition to heat and work transfer between system and surrounding 4 There is no loss of working substance In every cycle fresh air is drawn

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

Based on the type of hydraulic action water turbines are classified as 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

WATER TURBINES Impulse water turbine PELTON WHEEL

Reaction water turbine 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.

Difference between impulse and reaction water turbines Impulse water turbine Reaction water turbine 1 The whole of the pressure energy is converted into kinetic energy before it is passed on to the turbine wheel The water flows with both pressure and kinetic energies over the moving blades 2 The impulse force of the jet sets up the rotation of the turbine wheel The reaction pressure sets up the rotation of the turbine wheel 3 The water may be admitted over a portion of the circumference of the wheel The water must be admitted over the whole of the circumference of the wheel 4 The water discharges from the turbine wheel to the tail race Water discharges from the turbine into a draft tube from which it discharges finally into tail race.

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

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