Turbines 1 Fudamental

7/28/2019 Turbines 1 Fudamental

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Turbo Mach ine

is defined as a

device that extracts energy from a

continuously

flowing fluid by the dynamic action ofone or more rotating elements .

The prefix turbo is a Latin word

meaning spin orwhirl implying that turbo

machines rotate in some way.

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TypesofTurbines

1. Steam Turbines

2. Gas Turbines (Combustion Turbines)

3. Water (Hydraulic) Turbines

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Steam Turbines

A steam turbine is mainly used as an ideal prime mover in

which heat energy is transformed into mechanical energy in

the form of rotary motion.

A steam turbineis used in

1. Electric power generation in thermal power plants.

2. Steam power plants.

3. To propel the ships, submarines.

In steam turbines, the heat energy of the steam is first

converted into kinetic (velocity) energy which in turn is

transformed into mechanical energy of rotation and then

drives the generator for the power generation.

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Based on action of steam or type of

expansion:1. Impulse or velocity orDe Laval turbine

2. Reaction or pressure orParsons turbine

3. Combination turbineBased on number of stages:

1. Single stage turbine 2. Multi-stage turbine

Based on type of steam flow:1. Axial flow turbine 2. Radial flow turbine

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. The steam is made to

fall in its pressure by

expanding in a nozzle.Due to this fall in

pressure, a certain

amount of heat energy is

converted into kinetic

energy, which sets the

steam to flow with a

greater velocity.

The rapidly moving particles of the steam enter therotating part of the turbine, where it undergoes a

change in the direction of motion, which gives rise

to a change of momentum and therefore a force.

This constitutes the driving force of the turbine. 8


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Impulse Turbines (De Laval

Turbine)

In this type of turbine, steam is

ini t ially expanded in a nozzlefrom highpressure to low pressure. High velocity

jet of steam coming out of the nozzle is

made to glide over a curved vane, calledBlade.

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The jet of steam gliding over the blade gets

deflected very closely to surface. This

causes the particles of steam to suffer achange in the direct ion o f mo t ion, which gives

rise to a change of momentumand therefore a

force, which will be centr i fugalin nature.

Resultantof all these centrifugal forces

acting on the entire curved surface of the

blade causes it to move.

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Principle of working -

In this type of turbine, thehigh pressure steam does

not initially expand in the

nozzle as in the case of

impulse turbine, but

instead directly passes

onto the moving blades.

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Blade shapes of reaction turbines aredesigned in such a way that the steamflowing between the blades will be

subjected to thenozzle effect

. Hence,the pressure of the steam dropscontinuously as it flows over theblades causing, simultaneous increasein the velocity of the steam.

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Reaction force:

is due to the change inmomentum relative velocity

of the steam while passing

over the blade passage.

Centr i fugal forc e:is the force acting on the

blade due to change in

radius of steam entering

and leaving the turbine.

Resu ltant forc e:

is the resultant of Reaction

force and Centrifugal force.

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Fixed BladeMoving Blade


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Impulse Turbine Reaction Turbine

The steam expands

(pressure drops) completely

in nozzles or in the fixed

blades

The steam expands both in

the fixed and moving blades

continuously as it flows over

them

The blades havesymmetrical profile of

uniform section

The blades have converging(aerofoil) profile

The steam pressure while

passing over the bladesremains constant

The steam pressure while

passing over the bladesgradually drops

Because of large initial

pressure drop, the steam

and turbine speeds are very

Because of gradual pressure

drop, the steam and turbine

speeds are low 17

Difference between Impulse & Reaction Turbines


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Impulse Turbine Reaction Turbine

Power is obtained only

due to the impulsive force

of the incoming steam

Power is obtained due to

impulsive force of

incoming steam as well as

reaction of exit steam

Suitable for small capacityof power generation &

occupies less space per

unit power

Suitable for medium &high capacity power

generation and occupies

more space per unit power

Efficiency is lesser Efficiency is higher

Compounding is

necessary to reduce

speed

Compounding is not

necessary

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Compounding of Impulse TurbinesAs the complete expansion of steam takes in one stage

(i.e., the entire pressure drop from high pressure to lowpressure takes place in only one set of nozzles), the turbinerotor rotates at very high speed of about 30,000 rpm(K.E. is fully absorbed).

High speed poses number of technical difficulties likedestruction of machine by the large centrifugal forcesdeveloped, increase in vibrations, quick overheating of

blades, impossibility of direct coupling to othermachines, etc.

To overcome the above difficulties, the expansion ofsteam is performed in several stages.

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Utilization of the high pressure energy of

the steam by expanding it in successivestages is calledCompound ing .

Methods ofCompound ing:

Veloc i ty compound ing(Curt is Impulse Turbine)

Pressure compounding

Pressu re-veloc i ty compound ing

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Velocity compounding

Comprise of nozzles and two or more

rows of moving blades arranged in

series. In between two rows ofmoving blades, one set of guide (fixed)

blades are suitably arranged.

Guide (fixed) blades are fixed tocasing and are stationary.

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Velocity Compounding (Curtis Impulse Turbine)

N Nozzle

M Moving Blade

F Fixed Blade


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Pressure compounding

Consists oftwo stage of nozzles

followed by two rows of moving blades.

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Pressure Compounding


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Pressure-Velocity Compounding

(Combined Impulse Turbine)

Total pressure drop is divided into two stages & the total

velocity obtained in each stage is also compounded

A Axial clearance, N Nozzle, M Moving Blade, F Fixed Blade

Pi and Pe Pressure at inlet & exit, Vi and Ve - Velocity at inlet & exit


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A Gas turbineuses the hot gases ofcombustion directly to produce the

mechanical power.

Fuels used - Kerosene, coal, coal gas,

bunker oil, gasoline, producer gas, etc.,

Classification:1. Open cycle gas turbine

2. Closed cycle gas turbine

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ApplicationsGas turbines are used in:

Electric power generation plants

Steel, oil and chemical industries

Aircrafts, Ship propulsion

Turbo jet and turbo-propeller engines likerockets, missiles, space ships etc.,

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Open cycle gas turbine:

The entire flow of the working substance

comes from atmosphere and is returnedto the atmosphere back in each cycle.

Closed cycle gas turbine:

The flow of the working substance ofspecified mass is confined within the cyclic

path. ( Air or Helium is the workingsubstance)

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COMPRESSOR:

draws in air and compress it before it is fed

into combustion chamber COMBUSTOR:

fuel is added to the compressed air and

burnt to produce high velocity exhaust gas TURBINE:

extracts energy from exhaust gas

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Open cycle gas turbine


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Open cycle Closed cycle

Lesser thermal efficiency Higher

Loss of working fluid No loss of working

fluid

Bigger in size SmallerBig compressor is needed Smaller one is

sufficient

Possibility of corrosion of blades and

rotor

Free from corrosion

Economical Not economical

Exhaust gases from turbine exit to

atmosphere

Fed back into the

cycle

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Difference between open & closed cycle turbine


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PharmaceuticalPharmaceuticalPharmaceuticalPharmaceutical

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HospitalsHospitalsHospitalsHospitals

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Pulp and PaperPulp and PaperPulp and PaperPulp and Paper

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It is a prime mover, which converts hydro

power (energy of water)into mechanical

energy and further into hydro-electric

power.

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Classification of Water TurbinesBased on action of water:

1. Impulse turbine

pelton wheel.2. Reaction turbine francis and kaplan.

Based on name of originator:1. Pelton turbine or Pelton wheel

2. Francis turbine3. Kaplan turbine

Based on head of water:1. Low head turbine

2. Medium head turbine

3. High head turbine

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Pelton Turbine(Pelton Wheel or Free Jet Turbine)

High head, tangential flow, horizontal

shaft, impulse turbine

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PELTON TURBINE


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47Pelton Turbine Runner


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Only a part of the pressure energy of

the water is converted into K.E. and

the rest remains as pressure head.

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Fi t th t t th


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First, the water passes to the guide

vaneswhich guide or deflect the water

to enter the blades, calledmovingblades, mounted on the turbine wheel,

without shock.

The water from thegu ide bladesare

def lected on to themoving blades,

where i ts part of thepressu re energyis

converted intoK.E., which w i ll be

abso rbed by the turbine wheel . Thewater leaving the moving blades will

be at a low pressure.

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The difference in pressure between the

entrance and the exit of the moving bladesis called React ion pressu re, which acts on

moving blades of the turbine wheel and

sets up the turbine wheel into rotation inthe opposite direction.

Examples:Francis turbine, Kaplan turbine,Propeller turbine, Thompson turbine, Bulb

turbine.

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Francis TurbineMixed flow, medium head reaction turbine.

Consists of a spiral casingenclosing anumber of stationary gu ide bladesfixed allround the circumference of an inner ring of

moving blades (vanes)forming the runner,which is keyed to the turbine shaft.

Radial entry of water along the periphery of

the runnerand discharge at the center of therunner at low pressure through the divergingconical tube called draft tube.

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FRANCIS TURBINE


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Francis Inlet Scroll, Grand Coulee Dam
http://en.wikipedia.org/wiki/Image:Francis_Turbine_inlet_scroll_Grand_Coulee_Dam.jpg


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Francis Runner,

Grand Coulee Dam
http://en.wikipedia.org/wiki/Image:Francis_Runner_grandcoulee.jpg


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FRANCIS TURBINE & GENERATOR


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Kaplan TurbineAxial flow, low head.

Similar to Francis turbine except the runner

and draft tube.

The runner(Boss orHub) resembles with the

propeller of the ship, hence some times it is

called as Propel ler tu rbine.

Water flows parallel to the axis of the shaft.

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59KAPLAN TURBINE

(SCROLL CASING)

(GUIDE VANE)

(RUNNER VANE)


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Vertical Kaplan Turbine(Courtesy: VERBUND-Austrian Hydro Power)

Propeller Turbine Runner


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Propeller Turbine Runner


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Turbines 1 Fudamental