The basic design of a typical water tube boiler consists of two drums, the larger steam drum being placed above a smaller water drum, with a water-cooled furnace placed to one side in a manner similar to that of the two drums, bent tube boilers.
The furnace has water walls consisting of close-pitched approx. 50 mm diameter tubes at the roof, side, and rear. The lower headers for these water walls are rectangular sections and are supplied with water from the water drum by means of underfloor tubes. External downcomers, which are bigger in diameter, supply the water drum.
The gases leaving the furnace pass over rows of 50 mm diameter screen tubes before reaching a multi-loop type superheater placed in the lower part of the uptake.
This superheater consists of mild steel elements, expanded and bell-mouthed into the headers. Up to a temperature above this temperature, alloy steel tubes welded to alloy steel headers are used. These superheater elements are suspended from beams cast from heat-resistant steel and supported at their inboard ends by water-cooled support tubes.
The gas flow is directed to the bottom of the superheater by baffle plates. After leaving the superheater the gases enter the economizer section so heating the incoming feed water. Refractory material is used on the furnace floor and front burner wall. It is also used behind the water walls, etc.
The temperature of the superheated steam is controlled by an air-cooled attemperator. The steam from the primary superheater coils flows through the attemperator to the secondary superheater coils.