Applied Termodynamics 03

8/14/2019 Applied Termodynamics 03

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APPLIED THERMODYNAMICS

UNIT 3

STEAM BOILERS AND TURBINES


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STEAM BOILERS AND TURBINES

INTRODUCTION

A boiler is a closed vessel in which the heat produced by the combustion of fuel

is transferred to water in order to convert it into steam at desired temperature

and pressure, i.e., the function of a boiler is to evaporate water into steam at a

pressure higher than the atmospheric pressure.

The steam produced may be supplied to steam engines or turbines for power

generation, industrial process work, heating installation and hot water supply,

etc.

For safe operation of boiler, water free from salts, gases and non soluble solids

should be supplied and boilers are provided with several mountings for various

monitoring devices.

Classification of Boilers

1. Fire tube boilers

2. Water tube boilers.

FIRE TUBE BOILERS:

Hot flue gases from the furnace pass through the tubes which are surrounded by water.

These boilers are not able to produce steam at high pressure.

These boilers are used in industries because of their simplicity and fulfilling low

capacity requirements of industries.

Examples: Lancashire boiler, Cochran boiler, Cornish boiler and Locomotive boiler.

WATER TUBE BOILERS:

The water is circulated through a large number of smaller diameter tubes which are

surrounded by hot flue gases. This type of boiler can produce high pressure steam.

Examples: Babcock and Wilcox boiler, Stirling boiler, Lamont boiler and Benson boiler.


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COCHRAN BOILER:

It is a vertical fire tube boiler as shown in Fig.1. By burning fuel the hot gases go to the

combustion chamber.

The fire brick layer prevents the over heating of the boiler shell. The hot gases pass

through a large number of fire tubes and heat the surrounding water and convert it into

steam since the steam is lighter. It goes up to steam space.

The fire tubes normally have 62.5 mm external diameter and are 165 in number. The

crown of the boiler shell and grate are both hemispherical in shape.

The waste gases entering the smoke box are released through the chimney. The amount

of waste gases leaving the chimney is controlled by means of a damper manually.

When the damper is partly closed, amount of the waste gases leaving the chimney will

be reduced. Due to this action of the damper, the amount of air entering the grate will

also be reduced and obviously, only limited fuel can be burnt and the amount of steamgenerated also will be reduced. Thus, we find that the damper controls the rate of steam

generated.

Fig 1.Cochran Boiler

Through the manhole, the boiler attender can enter inside the boiler shell for cleaning.

By opening the door in the smoke box, the fire tubes and the smoke box can be cleaned

by a wire brush.

The diameter of the boilers varies from 1 to 3 m. The height of the boiler varies from 2

to 6 m. The evaporative capacity of the boiler ranges from 20 to 3000 kg/h.

LOCOMOTIVE BOILER


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It is horizontal fire tube boiler. It consists of a shell having 1 .5 m diameter and 4 m in

length. Fuel is fed into the fire box through the fuel door and air enters through the

damper and the slots in the grate plate.

The rate of combustion and the amount of steam generated is controlled by the

dampers. The fire brick arch deflects the hot gases and improves the combustion

efficiency.

Fig.2. Locomotive boiler

The hot gases pass through large number of fire tubes and enter the smoke box. The

circulation of air and hot gases is improved by means of induced draft produced in the

smoke box.

Waste steam from the engine enters the smoke box through the blast pipe and expands.

Due to the expansion, it produces a partial vacuum which improves the movement ofhot gases and air.

Waste gases go out through a short chimney. A door is provided in the smoke box for

inspection and cleaning.

To remove the moisture from the wet steam and to increase the temperature of steam, it

is superheated as shown in Fig.3.

The wet steam through the regulator enters the wet steam header and passes through

large number of superheated tubes and finally comes to the superheater header. Then

the superheated steam goes to the engine. To accommodate the superheater tubes, some

of the fire tubes are larger in diameter.

There are about 157 fire tubes of 47.5 mm diameter and 24 fire tubes of 130 mm

diameter. By superheating, the heat energy per unit mass of steam is increased and the

thermal efficiency of the steam plant is considerably increased.

BABCOCK AND WILCOX BOILER


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This is a water tube boiler. It consists of a steam water drum mounted on fire brick

work. Hot gases from the furnace pass through a zig zag path through the fire brick

baffles before going to the chimney through the damper.

The damper controls the rate of burning and thereby the steam generation. The damper

is operated by a chain passing through a set of pulleys.

Water from the steam water drum comes down to the down take header and then goes

to the uptake header through a large number of water tubes, inclined at about 14 for

better circulation as shown in Fig.3. It should be noted that there are many different

types of Babcock and Wilcox boilers.

Fig. 3 Babcock and Wilcox boiler

1. Downtake header 2. Mud drum 3. Uptake header 4. Anti priming pipe

5. Wet steam header 6. Superheater tubes, 7. Superheater steam header,

8 & 9. Fire brick baffles

The wet steam comes to the wet steam header through an antipriming pipe. The

antipriming pipe removes some moisture from the steam.

Then it passes through a large number of superheater tubes and reaches the superheater

header. From the superheater header, it goes to the main steam valve and finally to the

steam turbine.


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At the end of the downtake header, a mud drum is connected from where impurities can

be removed. Boiler is provided with two inspection doors and other mountings such as

the water gauge, the pressure gauge and the safety valve.

Normally, the furnace is provided with a moving grate or otherwise called as chain

grate. In a boiler provided with a moving grate, the rate of fuel burning can easily be

controlled by changing the thickness of the fuel bed and also by changing the speed of

the moving grate. Compared to a fire tube boiler, evaporative capacity, the pressure of

steam and the thermal efficiency of this boiler will be higher.

LAMONT BOILER

This is one of the high pressure water tube boilers working on forced circulation. Thecirculation is maintained by a centrifugal pump driven by a steam turbine using the

steam from the boiler.

Due to forced circulation, the rate of heat transfer and the evaporative capacity of the

boiler are increased. This boiler is highly suitable for a power plant and this has a high

thermal efficiency.

Normally, in high pressure boilers, either furnace oil or solid fuel in a pulverized form

is used in the furnace. A simple layout sketch is given in Fig. 4. Water is circulated

through the evaporator tubes. Hot gases from the furnace or the combustion chamber

heat the water and evaporate into steam. Wet steam will come to the steam space in the

steam-water drum.


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Fig.4 Lamont boilerBOILER MOUNTINGS

1. Water Level Indicator

This indicates the level of water inside a boiler.

For small capacity boilers, this is made with a thick glass tube with necessary

protection and safety devices.

By automatic control using float mechanism, the water level is kept constant with the

help of a feed pump or a water injector. According to boiler regulations two water

gauges should be fitted in each boiler.

In case the gauge glass breaks, the rush of water and steam will carry the all valves to

the position shown by dotted lines and prevent water or steam coming out of the boiler

shell.

2. Pressure Gauge

This is to indicate the pressure of steam inside the boiler.

At atmospheric pressure, the gauge will read zero. Periodically, the pressure gauge

should be tested with a standard gauge and calibrated if necessary.


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Flange will be fixed on the boiler shell or drum and connected to the steam pressure.

Depending upon the pressure, the spring tube will deflect.

This deflection will be magnified to the pointer through the link mechanism consisting

of rod, toothed sector and pinion. The U-tube will be filled with water.

3. Safety Valve

This valve is designed to open and let some steam out when the pressure exceeds the

safe designed value. In each boiler, there should be a minimum of two safety valves, as

per the boiler regulations.

There are two types of safety valves. One is called lever safety valve, other type is

spring safety valve.

In this valve, both the valves are kept closed by the spring. Pressure of the steam will

be acting on the valve through the valve chest.

If the pressure exceeds the designed value, the spring will be pushed up due to which

the valves open and letting the steam out.

4. Main Steam Valve

This is to regulate or stop the flow of steam going out of the boiler to the turbine,

engine or process work.

5. Blow of Valve

This valve is fitted at the lowest level of water. This helps to remove the salt deposits

and other impurities accumulated in the bottom operation of the boiler shell or drum.

This valve should be periodically opened keeping the steam under low pressure of

about 2 bar.

6. Fusible Plug

This is one of the safety devices in many boilers. This prevents overheating of the

firebox and other parts of the boiler, in case the water level becomes too low due to the

failure of the automatic control.


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The plug melts and creates an opening through which water and steam are allowed to

put out the fire in the grate.

The plug is made of a special alloy which has a comparatively low melting point.

BOILER ACCESSORIES

The applications installed to increase the efficiency of the boiler are known as boiler

accessories. The important accessories used in boilers are discussed below.

1. Economizer

A economizer or feed water heater is an appliance in which the feed water is heated

before it is supplied to the boiler. It is mounted between the boiler and chimney.

It consists of a large number of vertical tubes connected between the two horizontal

tubes. The water is pumped to the lower horizontal tube and passes to the upper

horizontal tube through the vertical tubes.

The water becomes hot by absorbing heat from the flue gases which passes over the

tubes. By this accessory overall efficiency of the plant is increased and the evaporative

capacity is increased.

2. Superheater

The function of a superheater is to increase the temperature of steam above the

saturated temperature without increasing its pressure.

It is located in the path of hot flue gases where the temperature is not less than 500C.

It consists of two headers which act as supplier and receiver of steam. Headers are

connected by a number of steel U-tubes. The wet steam from the supplier header passes

through the tubes and reaches the receive header as superheated steam.

The over heating of steam here can be controlled by damper, if the superheater

increases the efficiency of the plant.


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3. Air Pre Heater

Air pre heater is an appliance which recovers heat from the flue gases and transfers the

same to the air before it passes into the furnace for combustion process.

It is placed between the economizer and chimney.

The overall efficiency of the plant may be increased by 10 % b its use.

The two types of air pre heaters arerecuperative type and regenerative type.

In recuperative air pre heater, the heat from the flue gases is transferred to air through a

metallic medium, i.e., the air which passes through tubes absorbs heat from hot gases

surrounding the tubes.

In regenerative air pre heater, air and flue gases are made to pass alternatively through

the matrix and it receives heat from the hot gases and transfers it to the cold air. The pre

heating of air helps the burning of low grade fuel.

STEAM TURBINES

A Steam turbine is a prime mover in which rotary motion is obtained by the gradual

change of momentum of the steam. This is used to run alternators or generators in

thermal power plants. It is also used to rotate the propeller of ships.

The main parts of steam turbines are nozzles, rotor, blades and casing.

The nozzle guides the steam in the proper direction to strike the blades. The nozzles are

kept very close to the blades to minimize the losses.

The rotor or runner consists of a circular disc fixed to a horizontal shaft. On the

pheriphery of the rotor a large number of blades are fixed.

The steam jet from the nozzle impinges on the blades due to which the rotor rotates.

The surface of the blade is made smooth to reduce frictional losses.

A steel casing encloses the rotor, blades, etc. The casing helps the flow of steam andalso protects the inner parts.


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WORK DEVELOPED

The high pressure steam is expanded in the turbine during which the rotor blades of the

turbine rotate and thus giving work output. The work is developed at the expense of steam

enthalpy.

Where h1 is enthalpy of steam before expansion, h2 is enthalpy of steam after expansion.

TYPES OF TURBINES

Steam turbines are classified as impulse turbines and reaction turbines. Difference

between the terms impulse and reaction are explained below.

Impulse is the force obtained on an object when a jet of fluid strikes the object with a

velocity.

Reaction is the force obtained on an object when fluid leaves the object with a higher

relative velocity.

SINGLE-STAGE IMPULSE TURBINE (DE-LAVAL TURBINE)

First the pressure energy is converted into velocity energy or kinetic energy by the

expansion of steam through a set of nozzles.

Normally, in steam turbines, we make use of convergent-divergent nozzles.The kinetic energy is converted into mechanical energy with the help of moving blades

fixed on a rotor.

The rotor is connected to the output shaft.

All the above mentioned parts are enclosed in a casing as shown in Fig.5(a).

The pressure-velocity diagram, given in Fig.5 (b), is for a single stage impulse turbine.

A simple or single stage impulse turbine is only suitable for low pressure steam.

In case the steam pressure is high, when it expands in one set of nozzles, the outlet

velocity of the steam from the end of the nozzle is too high due to the high velocity of

the steam; the rotor will rotate at a very high speed.

Such a high speed is not suitable for practical purposes. So, in practice, we make use of

the multi-stage impulse turbines or compound impulse turbines.


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Fig. 5(a) Single-stage impulse turbine Fig. 5(b) Pressure - velocity diagram

COMPOUNDING OF IMPULSE STEAM TURBINES

In multi-stage impulse turbines, there are three types of compounding namely pressure

compounding, velocity compounding and pressure velocity compounding. In this section, the

three types are discussed.

1. Pressure Compounding

The pressure drop or expansion of steam is done in more than one set of nozzle and

each set of nozzle is followed by a set of moving blades, the turbine is known as

pressure compounded impulse turbine.

A two stage pressure compounding is shown in Fig.6 (a)

Fig.6(a) Two-stage pressure compounding Fig.6(b) Two-stage velocity compounding


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Fig.6(c) Two-stage pressure velocity compounding

2. Velocity Compounding

Here the entire expansion of steam occurs in one set of nozzles resulting in a very high

velocity at the outlet.

The steam is then passed through several sets of moving blades followed by fitted

blades.

Moving blades are fitted on the rotor while the fixed blades are fixed on the casing.

The function of the fixed blades is to change the direction of steam and guide the steam

in the proper angle to the next set of moving blades.

A two stage velocity compounding is shown in Fig.6 (b).

3. Pressure Velocity Compounding

In power plants, pressure velocity compounding is more common.

In this arrangement, for each pressure stage, there is a velocity staging a two stage

pressure velocity compounding is shown in Fig.6(c).

In practice, there will be more than 20 stages in a power station.

PARSONS REACTION TURBINE

In this turbine the power is obtained mainly by an impulse force of the incoming steam

and small reactive force of the outgoing steam.

As shown in Fig.7 (a), this turbine consists of rotor of a varying diameter.

Moving blades are attached to the casing. Steam is admitted to the first set of moving

blades through nozzles.

The blades receive the impulsive force of the incoming steam. Then it goes to fixed

blades which act as nozzles.


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Fig.7 (b) pressure drop in a reaction turbine Fig.7 (a) Parson s reaction turbine

VELOCITY DIAGRAM OF MOVING BLADES (IMPULSE TURBINE)

The velocity of steam relative of the blades, work done on the blades, etc., can be veryeasily found out from the velocity diagrams. Referring to Fig.8 (a) which shows the velocity

diagrams of a single stage impulse turbine.


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Fig.8 (a) Velocity diagram of a Single-stage impulse turbine

Fig.8 (b) Combined velocities or vector diagram

Assume that there is no frictional loss. The steam jet leaves the nozzle with a velocity

1.

The high velocity steam as may be seen from Fig.8 (a) is directed at some angle , with

the plane of the wheel and it is moving with an absolute velocity 1.

The tangential speed of the blades is b. Choosing some convenient scale, layout the

vector 1 at an angle with the direction of motion of the blades.

Now subtract the vector b and r1 = ac which is the velocity relative to the moving

blade at inlet and makes an angle with the wheel tangent.

The vector 1 can be resolved into two components one in the direction parallel to the

direction of moving blades known as velocity of whirl w1 which is represented by the

vector bd and the other is parallel to the turbine known as velocity of flow f1 which is

represented by the vector ad in the diagram.

The steam then glides over the blades without any shock and discharges at a relative

velocity ofro at an angle with the tangent of the blade.


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The problem is now to find the absolute velocity of steam as it leaves the blades. If

there is no frictional loss through the blades the velocity of steam relative to the blades

does not change; that isr1 = ro, layout ch = ad in length.

We now have the relative velocity of the steam and the velocity of the blades. The sum

of the vectors of velocities ro and b shall be equal to the absolute velocity o of steam

at exit. Having obtained the vector ofo its tangential component or velocity of whirl

wo can be drawn and also its axial component or velocity of flow fo.

Since the velocity of whirl is tangent to the turbine wheel it is responsible for rotating

the wheel and the velocity of flow is responsible for axial thrust.

PROPERTIES OF GAS MIXTURES - DALTONS LAW OF PARTIAL PRESSURES

If there is a mixture of gases which does not react with one another, then according to

Gibbs-Dalton law, the internal energy, pressure and the entropy of the mixture of gases

is equal to the sum of the respective constituent gases when each gas has the volume

and temperature the same as that of the mixture.

In the study of gas mixtures, a mole is equal to the molecular weight of the substance.

Therefore a kg mole of hydrogen is 2 kg.

Mole fraction is defined as the ratio of the moles of a component in a given volume of

the mixture to the total moles of the mixture having the same volume.

Gas mixtures are also described by the use of the gravimetric analysis.

Let us imagine a homogeneous mixture of ideal gases a, b, c, etc., According to Gibbs-Dalton

law

p = pa+ pb+ pc . .

Where pa, pb, and pc are the partial pressures of the respective gases, and is the total pressure of

the gas mixture, similarly


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where Rm is the gas constant for the mixture

Comparing the above equations we have

The sum of all the gravimetric fractions for the mixture is unity.

Suppose gas alone occupies the volume occupied by the mixture. Then the temperature

and pressure of the gas will fall. If the gas is to retain the temperature and pressure of the

mixture, the volume of the gas is to be reduced to Va. If all the constituent gases are handled in

this manner, the sum of all the volumes Va , Vb and Vc will be again

V which is the volume of the mixture.


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PROPERTIES OF STEAM:

INTRODUCTION

Conservation of heat energy into mechanical work needs a working medium, i.e., which

can receive the heat released from a fuel and used to drive an engine to produce the required

work. Steam is an excellent working medium used in the operation of team turbines and steam

engines because of its following unique properties

1) Ability to carry large quantity of heat

2) Source of steam is water which is readily available and cheap

Fig. 9. Formation of supersaturated steam from ice at constant pressure

STEAM FORMATION

At constant pressure the transformation mass of ice into unit mass of steam is in Fig. 9.

Consider one kg of ice under pressure bar and temperature -10C and heat is radically

applied keeping pressure constant, temperature increases till the melting temperature of

0C is reached.

This process shown by curve 1-2. Further heat is added, ice begins to melt into water

till all the ice becomes water at constant temperature which shown by curve 2-3.

The heat required to convert all the ice into water is called latent heat of fusion.

DRYNESS FRACTION

The dryness fraction is defined as the ratio of the weight of actual dry steam to the

weight of the wet steam containing it. The dryness fraction is denoted by x.


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USE OF STEAM TABLES

The various properties of steam like specific volume, saturation temperature, sensible

heat, latent heat, total heat and entropy corresponding to various pressures are tabulated in

steam tables by performing experiments.

1. Sensible Heat, hf

The amount of heat required in kJ to increase the temperature of 1 kg of water from0C to the boiling point temperature with respect to the given pressure, is called as sensible

heat of water. This is denoted by hfand also known as enthalpy or total heat of water.

2. Latent Heat, hfg,

The amount of heat required in kJ to form 1 kg of dry saturated steam from 1kg of

water at its saturation temperature for a given pressure is called as latent heat of vaporization.

This is denoted by hfg.

3. Total Heat, h

The amount of heat required to convert 1 kg of water at 0C into steam. This is denoted

by H. The total heat for the various steam conditions are given below.


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Total Heat of Wet Steam,

The amount of heat required to convert 1kg of water at 0C into wet steam at constant

pressure is known as total heat (enthalpy) of wet steam.

hwet = hf + x.hfg

Where hf= sensible heat in kJ/kg, h = latent heat in kJ/kg and x = dryness fraction

Total Heat of Dry Saturated Steam, hs,

The amount of heat required to convert 1kg of water at 0C into dry saturated steam at

constant pressure is called as total heat of dry saturated steam.

hs = hf + hfg

Total Heat of Super Heated Steam,

The amount of heat required to convert 1 kg of water at O into superheated steam is

called as total heat of superheated steam.

hsup = hf+ hfg+ Cp(Tsup Ts)

Where hf= sensible heat in kJ/kg, hfg = latent heat in kJ/kg, Cp = specific heat in kJ/kg (average

value is 2.09 kJ/kg K), Tsup = temperature of superheated steam in K and T s = saturation

temperature with respect to pressure in K.

MOLLIER CHART

The h-s chart, i.e., enthalpy - entropy chart is called as Mollier diagram which is

commonly used by engineers to obtain some of the properties of steam.

The chart is divided into two regions by the dry saturation line or dry steam line. The

regions above and below this line represent the super heated and wet conditions

respectively.

The lines of constant dryness fraction are shown in the wet steam region while the lines

of constant temperature are shown in the region of super heat. The lines of constant

pressure are straight in the wet steam region but curved in the superheat region.

An adiabatic process is represented by vertical line while the expansion at constant heat

is represented by horizontal line in this chart.

From this chart the drop mi the total heat during an adiabatic expansion can be directly

read. In order to felicitate taking readings, horizontal and vertical lines are drawn at

small and uniform intervals.


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JAYAM COLLEGE OF ENGINEERING AND TECHNOLOGY

DHARMAPURI

DEPARTMENT : EEE

YEAR / SEM : SECOND/ THIRD

SUBJECT : ME1211 / APPLIED THERMODYNAMICS

ASSIGNMENT NO 3

PART-A

1. Define latent heat of evaporization.

2. Define the term boiling point and melting point .

3. Define dryness fraction of steam.

4. Define speed ratio and stage efficiency as applied to steam turbine.

5. Name any two boiler accessories and state their functions

6. What is the necessity of compounding of steam turbine?7. Give the example of impulse and reaction turbines?

8. Mention the improvements made to increase the ideal efficiency of ranking cycle?

9. What is pressure compounding?

10. different between impulse and reaction turbines.

PART-B

1. A boiler working at a pressure of 14 bar evaporates 8.6 kg of water per kg of coal fired

from feed water entering at 39C. The steam at the boiler stop value is 0.92 dry.

Determine the equivalent evaporation from and at 100 C. Also determine the thermal

efficiency of the boiler if the calorific value of the coal is 30,200 kJ/kg.

2. A coal fired boiler plant consumes 400 kg of coal per hour. The boiler evaporates 3200

kg of water at 45 C into superheated steam at a pressure of 12 bar and 275 C. if the

calorific value of fuel is 32,760 kJ/kg of coal, determine

(i) Equivalent evaporation from and 100 C

(ii) Thermal efficiency of the boiler

Specific heat of super heated steam is 2.1 KJ / kgK

3. With neat sketch explain the principle of operation of impulse and reaction turbine.

4. Steam is contained in a closed vessel of 30 liters capacity at a pressure of 10 bar with

dryness fraction 0.95. Calculate its energy. Due to loss by radiation, the pressure of

steam falls to 7 bar. Calculate the total loss of heat and final quality of steam.

5. Explain about Steam Power Plant with neat sketch.

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Applied Termodynamics 03