Q12Basic Mechanical Engineering
Question
Q.2. Discuss the classification of the Steam Boilers. Explain the working of any boiler with the neat sketch.
Answer
Steam boilers are broadly classified into Fire-Tube and Water-Tube types. A Babcock & Wilcox boiler is a prominent water-tube boiler designed for high pressure.
Classification of Steam Boilers: Boilers are enclosed pressure vessels used to generate steam from water. They are fundamentally classified based on the contents of the heating tubes:
- 1. Fire-Tube Boilers: The hot combustion gases from the furnace pass through the inside of the tubes, while the water meant to be vaporized surrounds the outside of the tubes. They are generally limited to lower pressures (up to 25 bar) and lower capacities. Examples: Cochran Boiler, Locomotive Boiler, Lancashire Boiler.
- 2. Water-Tube Boilers: The water flows continuously through the inside of the tubes, while the extremely hot combustion gases circulate around the outside of the tubes. This design can safely withstand immense pressures (over 100 bar) and generate massive quantities of steam rapidly. Examples: Babcock & Wilcox Boiler, Stirling Boiler.
Working of a Babcock & Wilcox Boiler (Water-Tube): This is a horizontal, externally fired, water-tube boiler used extensively in thermal power plants. Construction & Operation: It consists of a large main cylindrical steel drum positioned horizontally at the top, which contains both water and steam. Below the drum, a series of straight, inclined water tubes are connected via headers (uptake and downtake headers). The tubes are inclined at an angle of about to promote natural convective water circulation. Coal is burnt on the grate in the furnace below. The resulting hot gases rise and are forced by baffle plates to travel in a serpentine, zig-zag path over the inclined water tubes, maximizing heat transfer before escaping through the chimney. As the water inside the inclined tubes absorbs this intense heat, it partially boils, becoming less dense. This lighter water-steam mixture naturally flows upward through the uptake header into the main steam drum. Inside the drum, the steam separates and accumulates at the top, while the heavier, cooler water flows down the downtake header to re-enter the tubes, completing the continuous natural circulation loop.