Boilers can be classified as follows
- According to the flow of water and hot gases – fire tube (or smoke tube) and water tube boilers.
In fire tube boilers, hot gases pass through tubes which are surrounded with water. Examples: Vertical, Cochran, Lancashire and Locomotive boilers. There may be single tube as in case of Lancashire boiler or there may be a bank of tubes as in a locomotive boiler.
In water tube boilers, water circulates through a large number of tubes and hot gases pass around them. Eg., bobcock & Wilcox boiler.
2. According to the axis of the shell – vertical and horizontal boilers.
3. According to location or position of the furnace. Externally and internally fired boilers.
In internally fired boilers, the furnace forms an integral part of the boilers structure. The vertical tubular, locomotive and the scotch marine boilers are well known examples.
Externally fired boilers have a separate furnace built outside the boiler shell and usually below it. The horizontal return tube (HRT) boiler is probably the most widely known example of this type.
4. According to the application – stationery and mobile boilers. A stationary boilers is one of which is installed permanently on a land installation.
A marine boiler is a mobile boiler meant for ocean cargo and passenger ships with an inherent fast steaming capacity.
5. According to steam pressure – low, medium and high pressure boilers.
The image shows the simplest form of an internally fired vertical fire-tube boiler. It does not require heavy foundation and requires very small floor area.
Cylindrical shell:
The shell is vertical and it attached to the bottom of the furnace. Greater portion of the shell is full of water which surrounds the furnace also. Remaining portion is steam space. The shell may be of about 1.25 metres diameter and 2.0 meters height.
Cross-tubes:
One or more cross tubes are either riveted or flanged to the furnace to increase the heating surface and to improve the water circulation.
Furnace (or fire box):
Combustion of coal takes place in the furnace (fire box).
Grate:
It is placed at the bottom of fire box and coal is fed on it for burning.
Fire door:
Coal is fed to the grate through the fire door.
Chimney (or stack):
The chimney (stack) passes from the top of the firebox through the top of the shell.
Manhole:
It is provided on the top of the shell to enable a man to enter into it and inspect and repair the boiler from inside it. It is also, meant for cleaning the interior of the boiler shell and exterior of the compbustion chamber and stack (chimney).
Hand holes:
These are provided in the shell opposite to the ends of each cross tube for cleaning the cross tube.
Ashipt:
It is provide for collecting the ash deposit, which can be removed away at intervals.
Working:
The fuel (coal) is fed into the grate through the fire hole and is burnt. The ashpit placed below the grate collect the ashes of the burning fuel.
The combustion gas flows from the furnace, passes around the cross tubes and escapes to the atmosphere through the chimney.
Water goes by natural circulation due to convection currents, from the lower end of the cross tube and comes out from the higher end.
The working pressure of the simple vertical boiler does not exceed 70 N/cm^2.
The following mountings are fitted in the boiler:
Pressure gauge: it indicates the pressure of the steam inside the boiler.
Water gauge (water level indicator): this indicates the water level in the boiler.
Safety valve: it prevents an increase of steam pressure in theboiler above its design pressure.
Steam stop valve: it regulates the flow of steam supply to requirements.
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A boiler which generates steam at a pressure of 85 kgf/sq.cm or above is termed as a“high pressure boiler”. The present tendency is towards the use of high pressure boilers in power plants. The modern high pressure boilers used for power generation have capacities of 40 to 1600 tonnes/hr of superheated steam with a pressure upto 210 kgf/sq.cm and a temperature of about 650’C. One of the largest modern steam power plants in the world is in japan with a steam capacity of 1600 Tonnes/hr. In India, the trombay power plant has a steam generating capacity of 550 tonnes/hr, Ramagundampower power plant with 320 tonnes/hr and bokaro plant with 160 tonnes/hr.
Water tube boilers are generally preferred for high pressure and high output whereas fire tube boilers for low pressure and low output.
Advantages of high pressure boilers:
Method of water circulation
Water circulation through the boiler may be either natural circulation due to density difference or by forced circulation. In high pressure boilers, water circulation is made with the help of a centrifugal pump which forces water through the boiler tubes. This is called “forced circulation of water”. The use of natural circulation is limited upto 120 kgf/sq.cm. Steam pressure and forced circulation upto 210 kgf/sq.cm. Forced circulation increases the rate of heat transfer and hence increases the steam generating capacity of boilers.
Size of drums
The high pressure boilers are characterized by the use of very small steam separating drums or by the complete absence of any drum.
Type and arrangement of tubes
The heat of combustion is utilized more efficiently by the use of small diameter and light weight tubes in large numbers. To avoid large resistance to the flow of water , the high pressure boilers have a parallel set of arrangement of tubes.
Compactness
The boiler components can be arranged horizontally, giving greater accessibility and operational convenience as high head required for natural circulation is eliminated by using forced circulation. The space required is hence less and arrangement is compact.
Foundation cost
Due to the light weight tubes and small size drums required and the arrangement being compact, the cost of foundation is reduced.
Efficiency
The efficiency of the power plant is increased upto 40%, by using high pressure superheated steam. Also steam can be raised quickly after the boiler is fired.
Cost of electricity
Since efficiency of the plant is increased by using high pressure boilers, the cost of electricity production is reduced.
Overheating
All the parts are uniformly heated; therefore the danger of overheating is reduced. Also thermal stress probelm is avoided.
Scale formation
The tendency of scale formation is eliminated due to the high velocity of water through the boiler tubes.
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