66
CHAPTER SIX
6 STEAM GENERATOR'S COMPONENTS
6.1 Components
A complete unit of a steam generator consists of
1. The pressure parts, (that is boiler and superheater tubes), which is supported by adequate steel
structure.
2. Enclosure or setting- this is a wall, or casing, suitably shaped, to enclose and direct the flow of
combustion gases, to support other components, and to insulate against flow of heat from the
interior to the boiler room atmosphere.
3. Combustion equipment - consists of the furnace, burners, stokers etc. Auxiliary - water pumps,
fans, regulators, water treatment plants etc.
6.2 Design Criteria
A steam power plant should
1. give high thermal efficiency achieved by use of auxiliary heat transfer surfaces, well-
insulated casings and effective baffling,
2. have high availability to service (should run for along time without frequent steps) and
3. have the ability to deliver clean steam and ability to adjust with varying loads.
To achieve the above mentioned requirements, the following design factors apply:
1. Physical factors such as headroom and floorspace requirements as well as hydraulic factors such
as operating pressure and type of circulation. Boilers operating at high pressure use forced
circulation. They also have high steaming rate because of low latent heat thus the need to supply
water more rapidly to avoid tube starvation and consequently overheating.
2. Heat transfer factors. These include baffling, discharge rate, economisers, air preheaters.
3. Thermal factors. To avoid thermal stresses, feed water should be discharged into the boiler at as
near the saturation temperature as possible. Cold water discharged against the boiler leading to
contraction stresses. Joints and seams should be well protected from the direct action of flames
or hot gas. During burning the flame should not play directly upon the tube surfaces.
4. Safety factors: - heads, gauges, safety and control devices
5. Maintenance factors - provision of access to the boiler (manhole etc) replacement of parts,
cleaning of scale and soot.
MPE 571E: POWER PLANT ENGINEERING
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6.3 Types of Boilers
Boilers may be classified on the basis of usage, tube contents, tube shape and position, furnace
position, heat source and circulation. An example is shown here below
boiler classification
Tube Heat
source Circulation
Usage contents
Stationary Fire tube Fossil e.g. coal Natural
Hot waste gas
Mobile Water tube Electrical, nuclear Forcedl
Fig.6.1. Classification of boilers
The stationary boilers are normally larger than mobile. Locomotive boilers are examples of the later
case. In Fire tube boilers, the hot flue gas passes through the tubes and gives their heat to the
surrounding water. Their advantages include compactness in construction, portability and cheap. It
has a relatively large ratio of steaming capacity hence can meet varying load without complicated
automation. In the case of water tube boiler, the water flows through the tubes and received heat
from the surrounding hot gases. They are normally applicable for large scale steam power plants.
Various fuels can be used as source of thermal energy in the boiler. As such we have coal, oil and
gas fired boilers as well as wood, bagasse, waste oil, waste gas fired boilers. Nuclear energy is also a
source of heat for steam generation. The circulation of the water steam system can be achieved
either naturally or by mechanical means. The tubes of the boilers can be straight of bent. The straight
tubes have the advantage of easy replacements, ease of cleaning, ease of flow and stocking. On the
other hand, the bent tube exhibits eases of expansion and contraction and provide more lengths and
thus increased heat transfer surface. Some boilers can be fired externally whereas others internally.
The boiler is said to be externally fired when combustion takes place outside the region of boiling
water. The boiler is said to be internally fired if the furnace region is completely surrounded by
water cooled surface.
6.3 Water - walls
Water walls are vertical tubes connected into the general circulation of the boiler. They have three
purposes namely to increase the evaporative capacity of a water tube boiler, to provide protection
against high temperature for the furnace walls by partial shielding them and to reduce the furnace
temperature. Water walls, although originally associated with pulverised coal, now are also used
with stoker and oil fired boilers. When pulverised coal was introduced, refractory walls were
standard, but pulverised coal could be burned by 10% to 20% excess air, whereas 50% was
considered good in the existing stoker - fired furnaces. The results of the higher furnace
temperatures were destruction of refractories, walls and tubes.
MPE 571E: POWER PLANT ENGINEERING
CHAPTER SIX
6 STEAM GENERATOR'S COMPONENTS
6.1 Components
A complete unit of a steam generator consists of
1. The pressure parts, (that is boiler and superheater tubes), which is supported by adequate steel
structure.
2. Enclosure or setting- this is a wall, or casing, suitably shaped, to enclose and direct the flow of
combustion gases, to support other components, and to insulate against flow of heat from the
interior to the boiler room atmosphere.
3. Combustion equipment - consists of the furnace, burners, stokers etc. Auxiliary - water pumps,
fans, regulators, water treatment plants etc.
6.2 Design Criteria
A steam power plant should
1. give high thermal efficiency achieved by use of auxiliary heat transfer surfaces, well-
insulated casings and effective baffling,
2. have high availability to service (should run for along time without frequent steps) and
3. have the ability to deliver clean steam and ability to adjust with varying loads.
To achieve the above mentioned requirements, the following design factors apply:
1. Physical factors such as headroom and floorspace requirements as well as hydraulic factors such
as operating pressure and type of circulation. Boilers operating at high pressure use forced
circulation. They also have high steaming rate because of low latent heat thus the need to supply
water more rapidly to avoid tube starvation and consequently overheating.
2. Heat transfer factors. These include baffling, discharge rate, economisers, air preheaters.
3. Thermal factors. To avoid thermal stresses, feed water should be discharged into the boiler at as
near the saturation temperature as possible. Cold water discharged against the boiler leading to
contraction stresses. Joints and seams should be well protected from the direct action of flames
or hot gas. During burning the flame should not play directly upon the tube surfaces.
4. Safety factors: - heads, gauges, safety and control devices
5. Maintenance factors - provision of access to the boiler (manhole etc) replacement of parts,
cleaning of scale and soot.
MPE 571E: POWER PLANT ENGINEERING
, 67
6.3 Types of Boilers
Boilers may be classified on the basis of usage, tube contents, tube shape and position, furnace
position, heat source and circulation. An example is shown here below
boiler classification
Tube Heat
source Circulation
Usage contents
Stationary Fire tube Fossil e.g. coal Natural
Hot waste gas
Mobile Water tube Electrical, nuclear Forcedl
Fig.6.1. Classification of boilers
The stationary boilers are normally larger than mobile. Locomotive boilers are examples of the later
case. In Fire tube boilers, the hot flue gas passes through the tubes and gives their heat to the
surrounding water. Their advantages include compactness in construction, portability and cheap. It
has a relatively large ratio of steaming capacity hence can meet varying load without complicated
automation. In the case of water tube boiler, the water flows through the tubes and received heat
from the surrounding hot gases. They are normally applicable for large scale steam power plants.
Various fuels can be used as source of thermal energy in the boiler. As such we have coal, oil and
gas fired boilers as well as wood, bagasse, waste oil, waste gas fired boilers. Nuclear energy is also a
source of heat for steam generation. The circulation of the water steam system can be achieved
either naturally or by mechanical means. The tubes of the boilers can be straight of bent. The straight
tubes have the advantage of easy replacements, ease of cleaning, ease of flow and stocking. On the
other hand, the bent tube exhibits eases of expansion and contraction and provide more lengths and
thus increased heat transfer surface. Some boilers can be fired externally whereas others internally.
The boiler is said to be externally fired when combustion takes place outside the region of boiling
water. The boiler is said to be internally fired if the furnace region is completely surrounded by
water cooled surface.
6.3 Water - walls
Water walls are vertical tubes connected into the general circulation of the boiler. They have three
purposes namely to increase the evaporative capacity of a water tube boiler, to provide protection
against high temperature for the furnace walls by partial shielding them and to reduce the furnace
temperature. Water walls, although originally associated with pulverised coal, now are also used
with stoker and oil fired boilers. When pulverised coal was introduced, refractory walls were
standard, but pulverised coal could be burned by 10% to 20% excess air, whereas 50% was
considered good in the existing stoker - fired furnaces. The results of the higher furnace
temperatures were destruction of refractories, walls and tubes.
MPE 571E: POWER PLANT ENGINEERING
