Q11Basic Mechanical Engineering

Construction and Working: It fundamentally consists of a large, heavy-duty horizontal steel drum (containing water and steam) suspended by steel girders. A massive array of straight, solid-drawn steel water tubes are connected beneath this drum via headers (uptake header at the front, downtake header at the rear). Crucially, these tubes are inclined at an angle of roughly $15^{\circ }$ to the horizontal. This inclination is vital to promote the natural thermosyphon circulation of water.

Combustion occurs on a grate beneath the elevated front end of the tubes. The intensely hot combustion gases rise and are deliberately forced by firebrick baffles to travel in a winding, zig-zag path across the tube bank, ensuring maximum heat transfer before exiting to the chimney. As the water inside the inclined tubes absorbs this fierce heat, it begins to boil and its density drops. The lighter steam-water mixture naturally flows upward along the incline into the uptake header and discharges into the main drum. The heavier, cooler water from the bottom of the drum simultaneously flows down the downtake header to replace it, establishing a continuous natural circulation loop. Inside the drum, anti-priming pipes separate the dry steam, sending it to a superheater before it exits the boiler to drive a turbine.