Boiler

I

Introduction

Boiler, device for heating water or generating steam above atmospheric pressure. All boilers consist of a separate compartment where the fuel is burned, and a compartment where water can be evaporated into steam.

II

Early History

The first evidence of the idea of using steam energy to produce power appeared in the Pneumatica of the Greek inventor and mathematician Hero of Alexandria in the 1st century ad. In it he described an aeolipile, a steam turbine consisting of a boiler connected by two hollow tubes to the poles of a freely spinning hollow sphere. The sphere was equipped with two canted nozzles that issued steam, causing the sphere to rotate. Other references are found in works from the Middle Ages and Renaissance, but no practical devices seem to have been built until the Italian architect and inventor Giovanni Branca designed a boiler emitting steam that struck blades projecting from a wheel, causing it to rotate.

The first practical steam engine, built by the English engineer Thomas Savery in 1698, used two copper vessels alternately filled with steam from a boiler. Savery's engine was used for pumping water out of mines, as was the one developed in 1705 by the British inventor Thomas Newcomen.

The Scottish inventor James Watt improved upon Newcomen's steam-engine design and introduced the first significant boiler advance, the spherical or cylindrical vessels heated from below by an open fire. Watt's boiler, built in 1785, consisted of a horizontal shell encased in brick, with flues to circulate the hot combustion gases over the boiler. Watt, who was one of the first engineers to arrive at the approximate thermodynamic properties of steam, used the lever safety valve, pressure gauges, and water cocks to control the flow of water and steam in his boilers.

III

Fire-Tube Boiler

Savery, Watt, and Newcomen engines all operated at pressures only slightly above atmospheric pressure. In 1800 the American inventor Oliver Evans built a high-pressure steam engine utilizing a forerunner of the fire-tube boiler. Evans's boiler consisted of two cylindrical shells, one inside the other; water occupied the region between them. The fire grate and flue were housed inside the inner cylinder, permitting a rapid increase in steam pressure. Simultaneously but independently, the British engineer Richard Trevithick developed a similar “Cornish” boiler. The first major improvement over Evans's and Trevithick's boilers was the fire-tube “Lancashire Boiler” patented in 1845 by the British engineer Sir William Fairbairn, in which hot combustion gases were passed through tubes inserted into the water container, increasing the surface area through which heat could be transferred. Fire-tube boilers were limited in capacity and pressure, and were also, sometimes, dangerously explosive.

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 5,000 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 per cent 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.