IV.

Water-Tube Boiler

Boiler pressures, however, remained limited until the first successful design of a water-tube boiler, patented in 1867 by the American inventors George Herman Babcock and Stephen Wilcox. In the water-tube boiler, water flowed through tubes heated externally by combustion gases, and steam was collected above in a drum. This arrangement used both the convection heat of the gases and the radiant heat from the fire and the boiler walls. Wide application of the water-tube boiler became possible in the 20th century with such developments as high-temperature steel alloys and modern welding techniques, which made the water-tube boiler the standard type for all large boilers.

Modern water-tube boilers can operate at pressure in excess of 5000 psig (lb/sq in gauge) and generate more than 9 million lb of steam per hour. Because combustion temperatures may exceed 1650° C (3000° F), the water flow is controlled by natural or forced circulation. By using so-called superheaters, modern boilers can achieve almost 90 percent fuel efficiency. Air preheaters heat the incoming air with combustion gases that are discharged to the stack; water preheaters use the flue gases to heat the feedwater before it enters the boiler. Draft control and chemical treatment of the water to avoid scale deposits and corrosion also contribute to efficient operation.