STEAM BOILERS AND TURBINES

UNIT 3

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Steam

Vapor of water. It does not obey laws of perfect gases, until it is perfect dry.

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Tem

p (t

)

Enthalpy (h) Solid

Melting Liqu

id S

tate

Vaporization Gaseo

us Sta

te

Temperature – Enthalpy diagram for formation of steam at constant pressure

Latent heat of iceSensible heat

of water Latent heat of steam Heat of super heat

Formation of steam

A

F D

C

E

B

LkG

(Critical Point)N M

S.H L.H of vaporisation H.S

p2

p1

p

Water region

Superheated Region

Liqu

id li

neD

ry Steam

line

Temperature Vs Total Heat Graph in steam formation

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Wet steam: moisture or water particles Dry steam: does not contain any water particles Superheated steam: dry steam is further heated at constant

pressure it is said to be superheated steam Dryness fraction (quality of wet steam): Mass of actual dry steam

/ mass of same quantity of wet steam X= mg / (mg + mf)

Enthalpy = S.H. + L.H For wet steam h = hf + x hfg

For dry steam h = hf + hfg

For super heated steam h = hf + hfg + Cp (tsup – t)

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Important terms for steam

Specific volume For wet steam v = x vg

For dry steam h = vg

For super heated steam vsup / Tsup = vg / T

Important terms for steam

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Pressurized steam is accelerated through a nozzle and then directed (almost) tangentially onto blades attached to a rotating wheel.

Steam Turbines

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By details of stage design Impulse Reaction

By direction of steam flow Axial flow Radial flow Tangential

By no of stages Single Multi-stage

Classifications of Turbines

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• Advantages Can give very high powers Has good steam economy Is very reliable Has long life Is small in size for its power

• Disadvantages Is non reversible Has poor starting torque

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The action of the jet of steam, impinging on the blades, is said to be an impulse and the rotation of the rotor is due to the impulse force of the steam jets

steam passes through stationary converging nozzles reduce its pressure (and its temperature) and increase its

velocity converting its "heat energy" (enthalpy) into kinetic energy Runs by the impulse of steam.

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The steam enters the wheel under pressure and flows over the blades.

The steam while gliding propels the blades and make them to move

So the runner is rotated by the reactive forces of the steam jet.

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High pr and high temp Boiler pr to condenser pr (1 to 125 bar) pr drop carried out

only in one stage, so velocity of entering steam is extremely high

To control the extreme entering velocity of steam jet compounding is used.

Types: Velocity compounding Pressure compounding Velocity – Pressure compounding

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Fixed nozzle and Moving Blades

In nozzle: Pressure Decrease; Velocity Increase

Moving blades: Pressure constant; Velocity decrease

Total pressure drop occurs in different fixed nozzle, not in single nozzle

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Fixed nozzle, Moving Blades and Guiding Blade

In nozzle: Pressure Decrease; Velocity Increase

Moving blades: Pressure constant; Velocity decrease

Guiding blades: Pressure constant; Velocity slightly decrease

Total velocity drop occurs in two or more number of moving and guiding blades.

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Fixed nozzle, Moving Blades and Guiding Blade

In nozzle: Pressure Decrease; Velocity Increase

Moving blades: Pressure constant; Velocity decrease

Guiding blades: Pressure constant; Velocity slightly decrease

In this method large amount of pressure drop is allowed, so less number of stage is enough

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In impulse turbines The entire pressure drop takes place in fixed nozzles, The pressure across the moving blades remains constant. The velocity decreases through the moving blades and

remains fairly constant in the fixed blades.

In reaction turbines Specially shaped fixed and moving blades replace the nozzles The drop in pressure takes place equally across both fixed

and moving blades, Falling progressively throughout the turbine. Absolute velocity decreases in the moving blades, but

increases in the fixed blades.

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1. The steam jet impinges the turbine blades

2. Steam may or may not be admitted over the whole circumference

3. Steam pressure constant in moving blade

4. Blades are symmetrical 5. No of stages required is less

1. Steam glides over the moving vanes with pressure and K.E

2. The steam must be admitted over the whole circumference

3. Steam pressure reducing in moving blade

4. Blades are asymmetrical5. No of stages required is more

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MountingsFittings mounted on the boiler for its proper and safe functioning.

AccessoriesThe devices which are used as integral part of a boiler and help in

running efficiently.

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• Measure the pressure of steam inside the boiler

• Placed in front of boiler• Bourdon pressure gauge

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• To indicate the level of water inside the boiler

• Placed in front of boiler

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• To prevent the explosions due to excessive internal pressure

• Two safety valve• Placed on boiler• It is used to maintain

constant safe pressure inside the boiler

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• Advantage is readily tempered.

• Disadvantage is heavy load which these valves carry

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• Loaded with spring instead of weights

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• It is the largest valve• To control the flow of

steam from the boiler to the main steam pipe

• To shut off the steam completely when required

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• Placed at bottom of boiler• To empty the boiler when

required• To discharge the mud,

scale or sediments which are accumulated at the bottom of the boiler

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• Non return valve• It is located slightly below

the normal water level of the boiler

• To regulate the supply of water

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• It is fitted to the crown of the furnace

• To put off the fire when the water level falls to an unsafe level

• It avoids explosion by over heating of boiler

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Accessories

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• To increase the temperature of saturated steam without raising its pressure.

• Placed in the path of hot flue gas from the furnace.

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• To heat the feed water • Utilizing the exhaust flue

gas• Before leaving to chimney• Green economizer• 15 to 20 % of coal saving• Prevent scale formation• Scraper

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• To recover heat from exhaust gas

• Placed between the economizer and chimney

• Air temperature is raised• Increase the evaporative

capacity• Less soot

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