Difference between Monotube Boiler, Water Tube & Fire Tube Boiler
A boiler is the most important component in a thermal power plant. Boilers are used to generate heat in thermal power plants so that it helps in generation of steam for the steam turbine. A boiler is basically a heat exchanger. According to the flow of water and hot gases, a boiler is classified in to fire tube boiler and water tube boiler.
Fire Tube Boiler Vs Water Tube Boiler
From its name itself, fire tube boilers are those boilers in which hot gases flow inside the tube and water outside the tube. Water tube boilers are just reverse. For a fire tube boiler, there is a limitation with the maximum operating pressure which can be achieved and also with its maximum size which is represented by the diameter of the shell. This is because hoop stress of the boiler shell increases with pressure and shell diameter. This limitation in maximum pressure and size is ultimately associated with an additional cost of material of the boiler shell which arises from an increase in thickness of the shell to accommodate large pressure but maintaining the hoop stress constant (hoop stress = pd/2t). Moreover, due to a high temperature of flue gases, fire tube may fail due to creep. So we use water tube boiler in large power plants as it can handle high pressure and more economical compared to fire tube boilers of the same capacity. This is because in water tube boiler we are accommodating the pressure rise in water tubes rather than in the shells. So we need relatively small diameter drum compared to fire tube boiler of the same capacity. The efficiency of water tube boiler is more compared to fire tube boiler. Water tube boiler is preferred for high pressure and high output.
However, water tube boilers have a high risk of explosion or bursting compared to fire tube boilers and require more skilled supervision. Treatment of feed water is necessary for water tube boilers as contaminated water can foul the water tubes. The combustion chamber in water tube boiler is located outside the boiler circuit, unlike fire tube boiler where combustion takes place internally.
However, water tube boilers have a high risk of explosion or bursting compared to fire tube boilers and require more skilled supervision. Treatment of feed water is necessary for water tube boilers as contaminated water can foul the water tubes. The combustion chamber in water tube boiler is located outside the boiler circuit, unlike fire tube boiler where combustion takes place internally.
If we look into the chronological order of development of these boilers, fire tube boilers are developed first, followed by water tube boilers. After that, a new class of water tube boiler emerged which is known as Once Through Boilers. Benson boiler, Monotube boiler etc are the examples of this category. They are also known as drumless boilers.
Monotube Boiler Vs Other Boiler
Monotube boilers is essentially a class of spiral water tube boiler. It is a type of "once through boiler". It contains only a single tube, unlike other water tube boilers which contain multi tubes. In the case of a fire tube boiler, Cornish boiler and simple vertical tube boilers are single tube boilers. Since monotube boiler is a type of "once through boiler", it possesses all the advantages like a drumless boiler, no evaporator required, flexibility in operation reduced weight etc. However, because of "once through" mode of operation, it requires high purity feed water.
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