Steam turbinesSteam turbines

Definition:Definition: Steam turbine is prime mover in which work is Steam turbine is prime mover in which work is

obtained by gradual change in momentum of obtained by gradual change in momentum of steam.steam.

Working:Working: Steam turbines consist of essentially two parts:Steam turbines consist of essentially two parts:

(a) Fixed Nozzles (b) Moving Curved Blades(a) Fixed Nozzles (b) Moving Curved Blades High velocity jet coming out of steam nozzle High velocity jet coming out of steam nozzle

impinges on curved blades.impinges on curved blades. Blades are so curved that steam enters without any Blades are so curved that steam enters without any

shockshock Steam glides tangentially on the curved blades, fixed Steam glides tangentially on the curved blades, fixed

to periphery of moving wheel.to periphery of moving wheel. Blades start to move due to change in direction of Blades start to move due to change in direction of

flowing steam.flowing steam.

Thus, a steam turbine:Thus, a steam turbine: Converts high pressure steam (from steam Converts high pressure steam (from steam

boiler) into high velocity steam jet (in steam boiler) into high velocity steam jet (in steam nozzles)nozzles)

Converts high kinetic energy of steam into Converts high kinetic energy of steam into mechanical work by changing direction of mechanical work by changing direction of steam flow (in moving blades)steam flow (in moving blades)

Steam turbines are used in power plants to drive Steam turbines are used in power plants to drive electric generators.electric generators.

They are also used to propel large ships, They are also used to propel large ships, submarines etc.submarines etc.

Small steam turbines are used to drive fans, Small steam turbines are used to drive fans, compressors and pumps.compressors and pumps.

Advantages of steam turbines over Advantages of steam turbines over steam enginesteam engine

1.1. Steam turbines develop high speedSteam turbines develop high speed2.2. Less steam consumptionLess steam consumption3.3. Hence high efficiencyHence high efficiency4.4. More uniform torqueMore uniform torque5.5. Less frictional losses due to less moving partsLess frictional losses due to less moving parts6.6. Enclosed in casing, hence quite safeEnclosed in casing, hence quite safe7.7. Less space and light foundationLess space and light foundation

Classification:Classification: According to mode of actionAccording to mode of action(a) Impulse turbine (b) Reaction turbine(a) Impulse turbine (b) Reaction turbine According to direction of steam flowAccording to direction of steam flow(a) Axial flow turbine (b) Radial flow turbine(a) Axial flow turbine (b) Radial flow turbine According to exhaust condition of steamAccording to exhaust condition of steam(a) Condensing turbine (b) Non-condensing turbine(a) Condensing turbine (b) Non-condensing turbine According to pressure of steamAccording to pressure of steam(a) High pressure (b) Medium Pressure (c) Low Pressure(a) High pressure (b) Medium Pressure (c) Low Pressure According to number of stagesAccording to number of stages(a) Single stage turbines (b) Multi-stage turbines(a) Single stage turbines (b) Multi-stage turbines

Impulse TurbineImpulse Turbine Impulse turbines work on basis of Newton's second Impulse turbines work on basis of Newton's second

law of motion i.e. change in momentum produces forcelaw of motion i.e. change in momentum produces force In this turbine, steam coming out of boiler expands in In this turbine, steam coming out of boiler expands in

stationary nozzle and its kinetic energy increases at cost stationary nozzle and its kinetic energy increases at cost of enthalpy.of enthalpy.

This steam, then impinges on curved blades mounted This steam, then impinges on curved blades mounted on periphery of rotor (rotating wheel)on periphery of rotor (rotating wheel)

As the steam glides tangentially over the blades, its As the steam glides tangentially over the blades, its direction is changed, changing its momentum.direction is changed, changing its momentum.

This produces impulsive force on the curved blades This produces impulsive force on the curved blades which makes the rotor to spin rapidly.which makes the rotor to spin rapidly.

Impulse TurbineImpulse Turbine

It may be noted that for an impulse turbine:It may be noted that for an impulse turbine:(a)(a) Total expansion takes place in the steam Total expansion takes place in the steam

nozzles only. Hence pressure remains almost nozzles only. Hence pressure remains almost constant while moving through bladesconstant while moving through blades

(b)(b) Velocity of steam rises in nozzles and falls Velocity of steam rises in nozzles and falls while passing through turbine bladeswhile passing through turbine blades

Reaction turbines:Reaction turbines:

Reaction turbines work on principle of Reaction turbines work on principle of Newton’s third law of motion i.e. reaction force Newton’s third law of motion i.e. reaction force developed is equal and opposite to action force.developed is equal and opposite to action force.

In this turbine, the rotation is caused by reaction In this turbine, the rotation is caused by reaction force generated by momentum change of fluid.force generated by momentum change of fluid.

However, pure reaction turbines are not in use.However, pure reaction turbines are not in use.

Impulse-Reaction TurbinesImpulse-Reaction Turbines In this type of turbines drop in pressure takes In this type of turbines drop in pressure takes

place in fixed nozzles as well as moving blades.place in fixed nozzles as well as moving blades. Pressure drop in moving blades further increases Pressure drop in moving blades further increases

kinetic energy. This gives rise to reactive force kinetic energy. This gives rise to reactive force on moving blades.on moving blades.

It may be noted that blade passage is converging It may be noted that blade passage is converging in this type of turbines.in this type of turbines.

Most commonly used turbine of this type is Most commonly used turbine of this type is PARSONS TURBINE.PARSONS TURBINE.

Blade structureBlade structure

Parsons TurbineParsons Turbine