Unit 5
Unit 5
Unit 5
A lean air/fuel mixture is when there is more air than required to burn the fuel. A rich
mixture is when there is too little air for the given quantity of fuel.
Boiler mountings are a set of safety device installed for the safe operation of a
boiler. There are seven main mountings on a boiler shell; safety valve, steam
stop valve, vent valve, pressure gauge, water level indicator, feed check valve
and fusible plug.
Vent valve is installed on the boiler shell to vent air from steam drum during
starting of boiler. These vent valves also comes handy during boiler shut down
as it let fresh air to enter the boiler drum avoiding its collapse under pressure. A
vent valve can also be used to release / dump moist steam at start.
A steam stop valve is connected to the boiler to stop and regulate steam flow
from boiler to the distribution lines. Main steam stop valve on boiler is kept shut
to avoid back-flow of steam to the boiler. The flange of the steam stop valve is
bolted on top of the steam drum.
Boiler accessories are devices which are not compulsory to be attached to it but if we use them it
increases efficiency and overall performance.
Boiler accessories are those components which are installed either inside or outside the boiler to
increase the efficiency of the plant and to help in the proper working of the plant.
Fusible Plug is a fitting used in Steam Boilers to enhance the safety during operation. It's working
depends upon the water level in the Boiler shell. A Fusible Plug is a threaded metallic cylindrical object,
with a tapered hole in it which goes through its entire length.
Blow-Off Cock Function: The function of blow-off cock is to discharge mud and other sediments
deposited in the bottom most part of the water space in the boiler, while boiler is in operation. It can
also be used to drain-off boiler water. Hence it is mounted at the lowest part of the boiler
ECONOMIZER : A common application of economizers in steam power plants is to capture the waste
heat from boiler stack gases (flue gas) and transfer it to the boiler feedwater. ... SUPERHEATER : A
superheater is a device used to convert saturated steam or wet steam into superheated steam or dry
steam.
Boiler accessories are those components which are installed either inside or outside the boiler
to increase the efficiency of the plant and to help in the proper working of the plant.
Definition of equivalent evaporation. : the rate in pounds per hour at which water would be vaporized
in a given steam boiler if supplied and evaporated at the normal boiling point and normal atmospheric
pressure.
Boiler efficiency is a measure of how effectively chemical energy in fuel is converted into heat energy in
steam going to the turbines.
Boiler mountings are installed on the boiler to provide safe operation of the boiler.
Boilers cannot operate without boiler mountings. In other words mounting are necessary for the
boilers to operate.
Examples
Safety valves, steam stop valve, salinometer valve, water level indicator, water level alarms
cutout,feed check valve etc
Boiler mounting are very essential to run the boiler in safe manner. To control the Boiler also
mounting are required. That’s why IBR make the guidelines for mandatory use of Mountings is
Boiler
Boiler mountings are used to run a boiler in a safe way ex: safety valve, Water level indicators
etc. Boiler mountings are primarily intended for the safety of the boiler and for complete control
of steam generation process.
Boiler Mountings and Accessories both are the essential part of Boiler. One (Mounting) is for
safety purpose then another one (Accessories) are to improve efficiency of Boiler.
•It is fitted in front of the boiler and generally present two in number.
•It is used to indicate the water level inside the boiler. It shows the instantaneous level of water
that is present inside the steam boiler which is necessary for its proper working.
2.Pressure gauge
•It is used to measure the pressure of the steam inside the boiler.
•The pressure gauges generally used are of Bourden type
3.Safety Valves
•It is used to prevent explosion due to excessive internal pressure. When the internal pressure
inside the boiler exceeds its working pressures than the safety valves blow off the steam and
maintains the internal pressure.
•It is usually fitted on the highest part of the boiler with the help of a flange.
•The main function of the stop valve is to control the flow of steam from the boiler to the main
steam pipe.
•The functions of blow off cock is to empty the boiler whenever required.
•To discharge the scale, mud and sediments which gets collected at the bottom of the boiler.
•It is non-return valve and fitted to a screwed spindle to regulate the lift.
•It regulates the supply of water which is pumped into the boiler by feed pump.
7. Fusible Plug
•Its function is to extinguish fire in the furnace when the water level in the boiler falls to an
unsafe limit. This avoids the explosion that may takes place because of the overheating of the
furnace plate.
Theoretical Air and Air-Fuel Ratio -The minimum amount of air which
will allow the complete combustion of the fuel is called the Theoretical Air (also
referred to as Stoichiometric Air). In this case the products do not contain any
oxygen. If we supply less than theoretical air then the products could include carbon
monoxide (CO), thus it is normal practice to supply more than theoretical air to
prevent this occurrence. This Excess Air will result in oxygen appearing in the
products.
The standard measure of the amount of air used in a combustion process is the Air-
Fuel Ratio (AF), defined as follows:
Thus considering only the reactants of the methane combustion with theoretical air
presented above, we obtain:
Fig. 30.5.
Preheaters
(ii) Plate type: In this preheater,
alternative gas and air passages are
formed between closely spaced parallel
plates. The air flows through the
alternate spaces of these parallel plates
and flue gas passes through the
remaining passages as shown in Fig.
30.6.
Fig. 30.6.
(iii) Regenerative type: The hot flue gas and air are made to
flow alternatively through the same path consisting wire mesh as
shown in Fig. 30.7. The hot flue gasses are made to pass through
wire mesh by opening valves 1 & 4 and closing valves 2 & 3.
While passing through wire mesh, the hot flue gasses reject and
store their heat into wire mesh. In the alternate pass, when the
atmosphere air is passed through wire mesh by opening valves 2
& 3 and closing valves 1 & 4, it receives heat from wire mesh
and as a result the air gets heated.
Fig. 30.7. Regenerative-preheater
Advantages of preheating of air:
(1) Waste heat from the flue gases is recovered for heating air
and causes a fuel saving of about 1.5% for each 100°C drop in
gas temperature.
(2) Inferior grades of coal can be burnt efficiently with
preheated air.
(3) Less excessive air is required to burn fuel and thus cost of
producing draught will be less.
(4) Combustion can be more efficient and an intense flame can
be achieved in the furnace. This increases the evaporation rate of
the boiler.
Disadvantages:
(i) Increase in the capital and running cost of the preheater as
induced draft fan for removing gases and forced fan for forcing
cold air through the air preheater are used.
Problem 30.2: The fuel used in a boiler is having a calorific
value of 25120 kJ/kg. The temperature of the gas leaving the
economizer chamber is 260°C and is reduced to 150°C while
leaving the pre-heater chamber. If 18 kg of air is supplied per kg
of coal burnt and the efficiency of pre-heater is 80% determine
(a) the percentage saving of heat of coal in the pre-heater, (b) the
temperature of air leaving the pre-heater if the initial
temperature of air is 27°C. Take Cp of air and flue gases equal to
1.05 kJ/kg.
Solution:
Given: Calorific value of fuel used in a boiler = 25120 kJ/kg
Temperature of the gas leaving the economiser and
entering pre-heater, tg,i= 260°C
Temperature of the gas leaving pre-heater, tg,e = 150°C
Temperature of the air entering pre-heater, ta,i = 27°C
Efficiency of pre-heater, ηcp = 80%
Mass of air supplied to boiler furnace per kg of coal
burnt = 18 kg
Cp of air and flue gases = 1.05 kJ/kg.
(a) Determine the percentage saving of heat of coal in the
pre-heater:
Formula: Percentage of saving of heat of coal in the pre-heater
=
Finding unknown,
Heat transferred to air in the pre-heater per kg of
coal burnt = mf .Cp .(te-ti). ηcp
Finding unknown, mf ;
Mass of flue gases/kg of coal, mf = mass of
coal + mass of air
=18 + 1 = 19 kg
Therefore, heat transferred to air in the pre-heater
= mf .Cp .(tg,e-tg,i). ηcp
Loeffler Boiler is a forced circulation, high pressure, and water tube boiler with internally fired
furnace. In this boiler, the 2/3 of superheated steam is used to evaporate the water in the
evaporating drum and remaining 1/3 of the steam from the superheater is used by the turbine. A
steam circulating pump is used to circulate the steam into the boiler
Working Principle
Its main working principle is to evaporate the feed water by the use of superheated steam from
the superheater. 2/3 of the total steam generated by the superheater is made to flow into the
evaporator drum. The superheated steam changes the feed water into saturated steam. And 1/3
part of the superheated steam is used by the turbine to do work.
Also Read:
1. Economiser: It preheats the feed water before entering into the evaporating drum.
2. Evaporating drum: It is placed at lower portion of the boiler. It contains the mixing nozzle.
Here the feed water is converted into saturated steam with the help of the superheated steam
from the superheater.
3. Steam circulating pump: It is present in between the evaporating drum and radiant
superheater. It circulates the steam into the boiler.
4. Radiant superheater: It superheats the steam with the help of radiations produced by the
burnt fuel in the boiler.
6. Mixing nozzle: It is present inside the evaporating drum. It mixes the steam from the
superheater with feed water and evaporates them.
Also Read:
Locomotive Boiler – Construction Working and Application
Scotch Marine Boiler Construction, Working, Advantages with Diagram
Benson Boiler – Construction, Working Principle and Advantages with Diagram
Working
In loeffler boiler, the feed pump forces the water to enter into the economiser. The
economiser preheats the feed water and then it is passed to the evaporator drum.
The evaporator drum has nozzles. The 2/3 of the superheated steam from the
superheater enters into the nozzles of the evaporator drum and the nozzles mix this
superheated steam with the feed water. This changes the feed water into saturated
steam. This saturated steam is then drawn from the evaporating drum by a steam
circulating pump and allows it to passes through the radiant superheater.
The radiant superheater superheats the saturated steam with help of radiation energy
produce from the burning of the fuel. The radiant superheater tubes are placed in
furnace. The heat transfer to the water takes place through the radiation produced. After
the radiant superheater, the steam is passed to the convective superheater.
The convective superheater is placed in the path of hot flue gases. It superheats the
steam coming from the radiant superheater to a temperature of about 500 degree C.
This superheated steam is than flows to the turbine and evaporating drum. In this boiler,
the 2/3 part of the superheated steam is used to evaporate the feed water into the
evaporating drum and remaining 1/3 part flows to the turbine.
Loeffler boiler has the capacity to produce 100 tonnes/ h of steam at a temperature of
500 degree C and pressure of 140 bar.
Advantages
ME Mechanical Team
Last updated: Oct 27, 2016
A boiler is called a high-pressure boiler when it operates with a steam pressure above 80
bars. The high-pressure boilers are widely used for power generation in thermal power
plants.
In a high-pressure boiler, if the feed-water pressure increases, the saturation
temperature of water rises and the latent heat of vaporization decreases. The feed water
can be heated to saturation of temperature in the economizer with the help of waste
heat. Recovery from the exhaust gases is escaping to the chimney. Then the boiler
supplies only latent heat of vaporization and superheat. Thus, a boiler operation at high
pressure will require less heat addition for steam generation.
1. Forced circulation of water – in all modern high-pressure boilers, the force circulation of
water is maintained with the help of pumps. It increases the help of pumps. It raises the
mean temperature of heat addition and evaporation capacity if the boiler.
2. Large number of small diameter tubes – the surface-area-to-volume-ratio (area density)
increases with the use of small-diameter tubes. It helps in a high rate of the heat transfer
to water flowing inside the tubes. Therefore, a large number of small density tubes in a
zigzag manner are used for water circulation in forced circulation. Further, use of a short
tube reduces the pressure loss and gives the better control the quality of steam.
3. Higher steam pressure and temperature – the steam is generated at a pressure of 80 to
300 bars and temperature of 450°C to 585°C with two superheaters in series. The use of
such stream is very suitable for power generation. It increases the thermal efficiency of
the plant and reduces the moisture contents in low-pressure stages of expansion in the
turbine.
4. Improved mode of heat transfer – modern high-pressure boilers use the heat transfer by
radiation along with conduction and convection. The total heat-receiving equipment is
divided into several parts. So they can easily be located in various zones of the furnace
for most efficient heat transfer to the water circuit.
5. Improved mode of heating – the high-pressure boilers use the following methods for
improved heat transfer rate:
o Evaporative of water above the critical pressure of steam.
o Heating of water by mixing superheated steam for high heat transfer rate.
o Increasing the combustion of air velocity over the tube.
6. Pressurized combustion – for increasing the combustion rate and thus heat-release rate,
pressurized air is used in the furnace. It gives a large amount of heat in a small space.
7. Compactness – the high rate of heat transfer inside the boiler reduces the overall size of
the boiler, and the boiler becomes compact.
8. High efficiency – High-pressure boilers have better firing methods, monitoring, furnace
conditions, control flue gases and water velocity.
9. Intensive heating – the furnace temperature in high-pressure boilers is high enough and
therefore, 70% of heat is transferred to water by radiation. This is a faster way of heat
transfer.
10. Once through construction – in high-pressure boiler operating at and above the critical
pressure, the water directly flashes into steam in the tube itself. It eliminates the need of
a boiler drum.
High-pressure boilers use the forced circulation of water which ensures the positive
circulation of water and increased evaporative capacity.
They require less heat of vaporization.
They are compact and thus require less floor space.
Due to the high velocity of water, the tendency of scale formation is minimized.
All parts are uniformly heated and the danger of overheating is minimized.
The steam can be raised quickly to meet the variable load requirements without the use
of complicated control devices.
The plant efficiency is increased.
With the use of high-pressure boilers, the steam generation is economical.
Cochran Boiler is a vertical drum axis, natural circulation, natural draft, low pressure,
multi-tubular, solid fuel fired, fire tube boiler with internally fired furnace. It is the
modified form of a simple vertical boiler. In this boiler, the fire tubes are placed
horizontally. The efficiency of this boiler is much better than the simple vertical boiler.
Main Parts and Construction
1. Shell: It has a vertical axis cylindrical drum with a hemispherical dome-type shell at the
top.
3. Combustion Chamber: The burning of fuel takes place in the combustion chamber.
4. Fire Tubes: Cochran boiler has multi-tubular fire tubes. The hot flue gases from the
combustion chamber travels to the smokebox through these fire tubes. The fire tubes helps
in the exchange of heat from the hot flue gases to the water.
5. Fire Hole: It is the hole provided to fire the fuel inside the furnace.
6. Furnace: It lies at the bottom of the boiler. Furnace is the place where all the fuel is
burnt. Without furnace, the working of this boiler is not possible.
8. Fire Brick Lining. The fire brick lining is present in the combustion chamber and
helps in the combustion of the fuel.
9. Manhole: A manhole is provided for the cleaning and inspecting of the boiler from
inside.
10. Flue Pipe: It is a small passage connecting the firebox and combustion chamber. The
hot gases enter into the combustion chamber through the flue pipe.
Other Boiler mountings and accessories attached to the this boiler are:
Also Read:
Working
In Cochran boiler first the fuel is inserted into the firebox and placed on the grate. The
fuel is ignited through the fire hole provided at the right bottom of the boiler.
The fuel is burnt in the firebox, and due to the burning of the fuel, smoke and hot flue
gases emerge out. The hot flue gases enter into the combustion chamber through flue
pipes.
From the combustion chamber, hot gases enter into the fire tubes. The fire tubes are
surrounded by water. The hot flue gases inside the tubes exchange the heat from the hot
gases to the water. Due to the exchange of heat, the temperature of the water starts
increasing and it gets converted into steam. The steam produced rises upward and
collected at top of the boiler in the hemispherical dome. An anti-priming pipe is installed
at top of the boiler which separates the water from the steam and makes it dry steam.
This dry steam is then transferred to the turbines through the steam stop valve.
The hot flue gases and smoke after exchanging heat moves to the smokebox. From the
smokebox, the burnt gases and smoke is discharged to the atmosphere through the
chimney.
Burnt fuel is transferred to the ash pit. Blow off Valve is preset at left bottom of the
boiler and is used to blow off the impurities, mud, and sediment from the boiler water.
A fusible plug is also provided at the top of the combustion chamber. When the
temperature of the combustion chamber crosses the permissible level, the fusible plug
melts and the water through the combustion chamber enters into the furnace of the
boiler and stops the fire. In this way, a big fire accident can be prevented to take place
and also protects the boiler from damage.
Various boiler mounting and accessories are attached to the boiler for its efficient
working.
Advantages
Disadvantages