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MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 1
Steam Boilers Definition of a Boiler
Boiler is defined as a closed metallic vessel in which the water is converted into steam above the atmospheric pressure by the application of heat generated by the combustion of fuels.
Function of a Boiler
The function of a boiler is to generate & supply the steam at the required constant pressure with its quality either dry, wet or superheated.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 2
Classification of Boiler 1. According to the axis of the shell:
a. Horizontal boiler b. Vertical boiler c. Inclined boiler
2. According to the application a. Stationary boiler b. Mobile boiler
3. According to the location of the furnace a. Internally fired boiler b. Externally fired boiler
SHELL Furnace SHELL
Furnace
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 3
4. According to the type of fuel used a. Solid fueled boiler
b. Liquid fueled boiler
c. Gaseous fueled boiler
5. According to the method of circulation of water a. Natural circulation boiler (circulation of water due to
difference in temperature)
b. Forced circulation boiler (pumps are used)
6. According to the flow of water and flue gases a. Fire tube boilers
b. Water tube boilers
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 4
Fire Tube Boiler:
The hot flue gases are led
through a tube or a nest of
tubes around which the
water circulates.
Example:
Lancashire boiler, Locomotive
boiler, Cochran boiler.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 5
Water Tube Boiler:
The water circulates inside
the tubes while the hot
gases are passed around
them externally.
Example:
Babcock and Wilcox boiler
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 6
1. Safety valve
2. Man hole 3. Steam pipe
4. Steam stop valve
5. Safety valve 6. Pressure gauge
7. Feed check valve 8. Water gauge
9. Blow off valve
10.Fusible plug
Lancashire boiler
B: Bottom central channel D: Dampers S: Side channels
C: Chimney E: Fire grate F :Flue tube K: Main channel
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 7
It is a horizontal, stationary, internally fired, natural
circulation, fire tube boiler.
By using this boiler, we can get steam up to a pressure of
15 bar.
Application:
It is widely used in sugar mills and chemical industries.
Lancashire boiler (Fire tube boiler)
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 8
Construction:
It consists of a horizontal cylindrical shell placed on a
brick setting.
2 large fire tubes of diameter 0.4 times that of boiler
shell are fitted inside the boiler shell & throughout the
boiler length.
In each of these fire tube 2 furnace grates are provided
at their front end.
The closed space below the grate is Ash pit.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 9
The brick setting is designed to provide an enclosed
chamber for each of the flue tubes at the rear end of the
boiler shell, which are connected to the bottom central
channel which in turn connected to the side channels 1 &
2 at their front end.
2 side channels are connected at their rear ends to a
common rear passage which is connected to chimney.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 10
The boiler shell is filled with water ¾ th of its volume
which will submerges both the flue tubes.
The remaining space above the surface of water in the
boiler shell is called steam space.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 11
In the 1st run, the heat transfer takes place from the hot
gases to water through the walls of the flue tubes.
In the 2nd run, the heat transfer takes place from the hot
gases to water through the bottom portion of the boiler
shell.
In the 3rd run, the heat transfer takes place from the hot
gases to water through the portion of the boiler shell
exposed to side channel.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 12
Babcock and Wilcox boiler A: Water tubes B: Uptake header C: Down take header D: Boiler drum E: Steel Caps F: Mud Box G: Dampers H: Doors J: Chain grate stoker K: Furnace door L: M: Lower junction box N: Upper junction box. O: Stop valve connection
P: Stop valve, Q: Safety valve, R: Water-level indicator, S: Super heater,
T: Pressure gauge, V: Feed check valve, W: Baffles, X: Blow off valve
R D
E
F
G H J K
L
N
M
O
P Q
s
T
V
W
X
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 13
Babcock & Wilcox boiler (Water tube boiler)
It is a horizontal, stationary, externally fired, natural
circulation, water tube boiler.
By using this boiler, we can get steam up to a pressure of 75 80 bar.
Application:
It is widely used in thermal power stations.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 14
Construction:
It mainly consists of 4 parts
Boiler drum
Water tubes
Chain grate stoker
Super heater
Boiler drum rests on iron channels.
A number of inclined water tubes at a very low inclination
are connected at right angles to the end boxes called
headers.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 15
The water tubes are arranged in a number of vertical rows, each row consisting of 4 to 5 tubes.
There will be a number of such vertical rows one behind the other.
Each row of such inclined water tubes are connected to one set of 2 headers.
Header shown at the right end of the water tube is called down take header and the other shown at the left end of the water tubes is called uptake header. Each set of header is in turn connected to the boiler drum, with one set of inclined tubes.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 16
A mud box is provided just below the down take header.
Any sediments or impurities collected in the mud box are blown off from time to time through the blow off pipe.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 17
Function of chain grate stoker:
If we use low quality coal, then the speed can be adjusted and hence the time for coal to reach from front end to rear end can be increased. Hence there will be effective burning of the fuel.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 18
Water circuit in the boiler
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 19
Path of flue gases
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 20
Circulation of Water
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 21
Superheating of the steam
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 22
Advantages of Water Tube Boilers over Fire Tube Boilers
1. Steam can be raised more quickly
2. Steam at higher pressures can be produced
3. Higher rate of evaporation
4. Sediment deposition is less
5. Suitable for any type of fuel and method of firing
6. Failure of one of the water tube will not affect the working of boiler
7. Occupies less Space 8. Easy maintenance.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 23
Disadvantages of Water Tube Boilers over Fire Tube Boilers
1. Not suitable for ordinary / impure water
2. Not suitable for mobile application
3. High initial cost and hence not economical
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 24
Differences / Comparison
High Low Rate of steam production
High ( Max. up to 75 bar)
Low (Max. up to 15 bar)
Operating pressure
Externally fired Internally fired Mode of firing
Water inside the tubes and hot gases surround the tubes
Hot gases inside the tubes and water is poured from outside
Flow of water & hot gases
Water tube Fire tube Particulars
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 25
Differences / Comparison
Small for given capacity
Large for given capacity
Shell diameter
Essential Not essential Treatment of water
Not safe, due to high pressure
Safe, due to low pressure
Safety
Ideally suitable for power generation plants.
Suitable for chemical & sugar industries & not for power generation
Applications Water tube Fire tube Particulars
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 26
Boiler Mountings and Accessories
BOILER MOUNTINGS, are required
To have complete control over the steam generation,
Measurement of some of the important steam properties,
To provide safety to the boiler. Manhole 8
Fusible plug 7
Feed check valve 6
Blow-off valve 5
Steam stop valve 4
Safety valves 3
Pressure gauge 2 Water level indicators 1
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 27
BOILER ACCESSORIES are required to improve the efficiency of steam power plant.
Steam trap 6
Steam separator 5
Feed pump 4
Super heater 3
Air preheater 2
Economizer 1
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 28
Functions of Boiler Mountings Water Level Indicator:
It is used to indicate the level of water inside the boiler drum.
Pressure Gauge: It is used to indicate the pressure of steam inside the boiler
drum.
Safety Valves: It suddenly blows off the excess steam from the boiler drum.
Steam Stop Valve: It is used to regulate the flow of steam from the boiler drum.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 29
Feed Check Valve: It supplies the additional water, when the level of water
inside the boiler drum falls. Blow off Valve:
It is used to remove periodically the sediments & also used to empty water in the boiler when required.
Fusible Plug: It is used to extinguish the fire in the furnace of the boiler
when the water level falls below the normal level.
Manhole: It is an opening in the boiler shell through which a man can
go in for periodic inspection and cleaning.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 30
Functions of Boiler Accessories Economizer:
It is a device, in which the waste heat of the flue gases is utilized for preheating the feed water.
Air preheater: It is a device, in which the waste heat of the flue gases is
utilized for preheating the air supplied for the combustion of the fuel in the furnace.
Super heater: It is used to increase the temperature of the steam above
the saturation temperature.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 31
Steam Separator:
It separates the water particles from the steam flowing in
the pipe lines.
Steam trap:
It is used to drain off the condensed water accumulating
in the steam pipe lines while at the same time the high
pressure steam does not escape out of it.
Feed Pump:
It is used to force the feed water at higher pressure into
the boilers.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 32
Differences between Boiler mountings & Accessories
Boiler mountings Boiler accessories It is mandatory (compulsion to use)
It is optional
It speaks about safety It speaks about efficiency & economy
These are fitted directly on the boiler drum
These are not fitted directly on the boiler drum
It is used to measure some of the important properties
It is used to improve some of the important properties
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 33
Simple Vertical Boiler
S Shell
C Chimney
M Manhole
H Hand hole
T Cross-tube
F Fire hole
B Fire box
G Grate
A Ashpan
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 34
It is a vertical, stationary, internally fired, natural circulation, fire tube boiler.
By using this boiler, we can get steam up to a pressure of 7 bar.
The fire box is placed at the bottom of shell
The grate is provided at the bottom of the fire box.
The ash pan is provided beneath the grate.
The fire box is fitted with two or more inclined cross tubes to increase the heating surface and to improve the circulation of water.
Hand holes H are provided in the shell opposite to the ends of each cross-tube for cleaning the tubes.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 35
The coal is introduced over the grate G, where the fuel is
burnt, through the fire hole F.
The manhole is provided on the upper portion of the shell
to enable a man to enter into the shell to attend the
inspection and repair works.
The boiler shell is filled with 3/4th of its volume with water
which will submerge the fire box completely.
The remaining space above the surface of water in the
boiler shell is steam space.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 36
The hot gases are directly passed through the fire box.
Hot gases transfer their heat to water through the entire
heated wall surface of the fire box.
The heat is also transferred from the hot gases through
the walls of the tubes to the water circulating in it.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 37
Cochran Boiler
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 38
It is a vertical, stationary, internally fired, natural
circulation, fire tube boiler.
By using this boiler, we can get steam up to a pressure of 15 bar.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 39
It consists of a cylindrical shell and firebox.
The shell and firebox are both hemispherical; the hemispherical crown of the boiler shell gives maximum space and strength to withstand the pressure of steam inside the boiler.
The hemispherical crown of the firebox is also advantageous for resisting intense heat. The firebox and the combustion chamber are connected through a short pipe.
The flue gases from the combustion chamber flow to the smoke box through a number of tubes.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 40
The gases from the smoke box escape to the atmosphere through the chimney.
The combustion chamber is lined with firebricks to withstand intense heat.
A manhole near the top of the crown on the shell is provided for cleaning.
At the bottom of the firebox, there is a grate and the coal is fed in through the fire hole, which is kept closed by a fire door.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 41
Locomotive Boiler
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 42
It is a horizontal, stationary or mobile, internally fired,
natural circulation, fire tube boiler.
By using this boiler, we can get steam up to a pressure of 25 bar.
Application:
It is generally used in railway engines and road rollers.
It consists of 3 main parts: fire box, boiler shell and smoke box.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 43
The firebox which forms the furnace is constructed by
extending the boiler shell downward from the sides at its
rear end.
The annular space left in between the furnace wall and
the downward extension of the boiler shell is also
connected with the water space in the barrel.
The hot gases rising from the grate are deflected by the
fire brick arch which ensures more uniform proper
heating because the deflected hot gases come in contact
with the entire heating surface of the fire box.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 44
Boiler shell accommodates a large number of flue tubes which connect the furnace with the smoke box.
The flue tubes are made of 2 sizes.
Superheater tubes are inserted into the large diameter flue tubes.
The steam dome located at the top of the cylindrical shell is meant to collect the dry steam.
The smoke box is connected to the boiler barrel at its front end.
A superheater header which distributes the steam to the superheater tubes.
MADHWESH N, Faculty, Dept. of Mech & Mfg. Engg. 45
The chimney located at the top of smoke box is made very shorter inorder to avoid its striking with the tunnels and underbridge when the locomotive passes through the tunnels and below the underbridge.
The smoke box door provided at its front end facilitates cleaning inside the smoke box.