Fire Tube Boiler
As is indicated by the name, the fire tube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed in water, in a closed vessel. Actually, in fire tube boiler one closed vessel or shell contains water, through which hot tubes are passed. These fire tubes or hot gas tubes heated up the water and convert the water into steam and the steam remains in the same vessel. As the water and steam both are in the same vessel a fire tube boiler cannot produce steam at very high pressure. Generally, it can produce maximum 17.5 kg/cm2 and with a capacity of 9 Metric Ton of steam per hour.
Types of Fire Tube Boiler
There are different types of fire tube boiler likewise-
Internal furnace fire tube boiler.
External furnace boiler can be again categorized into three different types-
Horizontal Return Tubular Boiler.
Short Fire Box Boiler.
Again, internal furnace fire tube boiler has also two main categories such as
Vertical tubular fire tube boiler.
The normally horizontal return fire tube boiler is used in the thermal power plant of low capacity.
It consists of a horizontal drum into which there are numbers of horizontal tubes. These tubes are submerged in water.
The fuel (normally coal) burnt below these horizontal drum and the combustible gasses move to the rear from where they enter into fire tubes and travel towards the front into the smoke box.
During this travel of gasses in tubes, they transfer their heat into the water and steam bubbles come up. As steam is produced, the pressure of the boiler developed, in that closed vessel.
Advantages of Fire Tube Boiler
It is quite compact in construction.
Fluctuation of steam demand can be met easily.
It is also quite cheap
Disadvantages of Fire Tube Boiler
As the water required for operation of the boiler is quite large, it requires a long time for rising steam at desired pressure.
As the water and steam are in the same vessel the very high pressure of steam is not possible.
The steam received from fire tube boiler is not very dry.
Ø Fire tube boiler consists of a vertical, cylindrical shell, equipped with a firebox in the bottom, water space in the middle portion and steam space in the upper portion. The fire grate is placed at the bottom of the firebox and coal is fired at the firebox. An ash pit is located at the bottom of the grate to collect the ash of burnt coal, which is periodically removed.
Ø One or more cross tubes are either flanged or riveted to the water space are located in the firebox to increase the heating surface area to improve the water circulation. A short chimney is connected at the top of the firebox to discharge the waste flue exhaust gases at some greater height. Manhole and hand holes are provided for cleaning the interior of the boiler shell and cross tubes.
Ø The boiler consists of a pressure gauge, water level indicator, safety valve, steam stop valve and a manhole as mountings to provide safety and ease of working.
Fuel burns on the grate in the firebox. The resulting hot flue gases are allowed to pass around the cross tubes. The water surrounding the cylindrical firebox also receives heat by convection and radiation. Thus steam is produced. The water circulation in the boiler depends on the density difference in water, created by the temperature difference in water.
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