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Chimney Sweeps have been legendary characters in English literature from Hans Christian Anderson to Charles Dickens. In the earlier days when houses had fireplaces, the Chimney Sweep did the function of cleaning the soot from the chimney. In the modern day boiler, the soot blower does the same function.
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In oil fired boilers, over a period of time the heat transfer tubes get covered by a layer of soot or fine carbon deposit. This reduces the heat transfer from the hot gases to the water and reduces the efficiency of the boiler.
In coal fired boilers, the furnace area gets covered by slag which is molten ash. The ash also sticks to the heat transfer surface in the other heat transfer areas. These ash accumulations reduce heat transfer and increase the tube metal temperatures leading to failure of the tubes.
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Tube cleaning is done periodically to remove the ash or soot deposits. Steam is the medium used for cleaning. The steam is taken from the boiler itself.
The soot blower consists of a lance tube with a nozzle at the end. When it is operated, the lance is extended into the boiler and steam is admitted through the lance. The steam comes out as a high velocity jet through the nozzles, which cleans the ash deposited on the surface. When the lance moves into the boiler it is also rotating so that it cleans the sweeping area covered by the circular travel of the nozzle. The lance is then retracted back.
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There are two types of soot blowers.
- One with a very long lance called the “long retractable soot blowers." This is normally used to clean the ash deposit from between the coils of superheaters and economisers.
- The other type is the shorter lance type called the “wall blowers." These are used to clean the furnace walls. The lance extends a short distance around 200 mm from the furnace wall. The nozzle direction is such that the steam impinges on the walls cleaning the surface. During operation, the lance rotates cleaning the radial area covered by the steam from the nozzle.
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The deposits on the walls are due to the chemical constituents of ash, and the amount of combustion air. If the ash contains more of Ferrous Sulphide, then the melting temperature of the ash is low which makes the ash melt and stick to the walls.
A large coal fired Thermal power plant will have around two hundred soot blowers of both types arranged to cover all the area of the boiler. This will be programmed to automatically operate to a required sequence.
Intelligent soot blower systems calculate the trends in the temperature increase in different sections of a boiler. The program then decides which soot blowers have to be operated and at what frequency.
High-pressure water lances are also used in some units where the slagging is very heavy.