Steam Boiler


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steam boiler

[′stēm ‚bȯi·lər]
(mechanical engineering)
A pressurized system in which water is vaporized to steam by heat transferred from a source of higher temperature, usually the products of combustion from burning fuels. Also known as steam generator.

Steam boiler

A boiler in which water is raised to or above saturation temperature at a desired pressure, and the resultant steam is drawn off for use in the heating system.

Steam Boiler

 

a device that has a firebox and is heated by the gaseous products of organic fuel burned in the firebox; it is designed to produce steam at a pressure above atmospheric for use outside the device. The working medium in most cases is water. Electrically heated steam generators (electric boilers), which are rarely used, are also called steam boilers.

References to steam boilers as steam generators separated from the firebox are encountered in scientific papers by G. della Porta of Italy (1601), S. de Caus of France (1615), and E. S. Worcester of England (1663). However, the industrial use of steam boilers began at the turn of the 18th century in connection with the vigorous development of metallurgy and coal mining.

The early steam boilers resembled a sphere or kettles for cooking food and were made at first of copper and later of cast iron. One of the first real boilers was that of D. Papin, which he proposed in 1680.

The design of modern steam boilers evolved with changes in the structural types of the simplest cylindrical steam boilers, which were manufactured until the second half of the 19th century and had a capacity of 0.4 ton of steam per hour. The heating surface of these boilers did not exceed 25 sq m, the maximum steam pressure was 1 meganewton per sq m (MN/m2), or 10 kilograms-force per sq cm (kgf/cm2), and the efficiency was no less than 30 percent. The development of the steam boiler proceeded in two directions: an increase in the number of streams of gases (fire-tube boilers) and an increase in the number of streams of water and steam (water-tube boilers). The first fire-tube boilers were cylindrical vessels that initially had one, two, or three large-diameter tubes (fire tubes), and subsequently dozens of tubes of smaller diameter (flue tubes), to carry the gases.

The heating surface of fire-tube steam boilers was increased not only in cylindrical boilers of the original size but also even in smaller boilers. The result was a certain increase in boiler capacity, with an insignificant increase in total weight, and also better heat transfer from the flue gases to the heating surface, which led to a reduction in the temperature of the gases at the outlet from the boiler (that is, higher efficiency).

Fire-tube boilers were distinguished from cylindrical boilers by their smaller dimensions and higher efficiency (60 percent), but their capacity, which was limited by the dimensions, did not exceed several tons per hour, and their structural characteristics restricted the steam pressure to 1.5–1.8 MN/m2. Therefore, fire-tube boilers are now used only in transportation power plants, such as steam locomotives and steamships, and have been completely supplanted in stationary plants by water-tube boilers.

Water-tube boilers were developed by increasing the number of cylinders making up the boiler, initially up to three to nine cylinders of relatively large diameter (boiler banks) and then tens and hundreds of small-diameter cylinders, which became generating tubes and later waterwalls.

The increase in the heating surface of water-tube boilers was accompanied by an increase in dimensions, primarily in height, but there was also a manyfold increase in capacity, a decrease in the specific consumption of metal, and a continuous improvement in the parameters of the steam and in efficiency.

Natural-circulation compartmented and sectional horizontal water-tube boilers in which the generating tubes were set at a slope of 10°-12° to the horizontal were manufactured in the second half of the 19th century. The compartmented boilers consisted of one or more drums, the headers connected to them, and banks of generating tubes that were pressed into the headers. With a heating surface of 350 sq m, the capacity was 10 tons/hr at a pressure of 1.5 MN/m2. The replacement of flat compartments by separate sections, into each of which one bank of tubes was pressed, made possible an increase in steam pressure, and as the number of sections making up the boiler was increased, the heating surface reached 1,400 sq m.

In 1893 the Russian engineer V. G. Shukhov designed a water-tube boiler consisting of a longitudinal drum and tube banks made up of two clusters of tubes pressed into the flat walls of short cylindrical chambers. The heating surface of the boiler could vary from 62 to 310 sq m and the capacity from 1 to 7 tons/hr at a steam pressure of up 1.3 MN/m2, depending on the number of banks (one to five). The design of the Shukhov boiler solved the problem of standardization of the individual elements and their dimensions.

Vertical water-tube boilers appeared in the early 1900’s and in a very short time reached a highly advanced state. In 1913 the capacity of such boilers did not exceed 15 tons/hr at a steam pressure of 1.8 MN/m2; by 1974, Soviet boilers had capacities of up to 2,500 tons/hr at a pressure of 24 MN/m2, and American boilers had a capacity of 4,400 tons/hr at the same pressure. At first the vertical water-tube boilers consisted of one upper and one lower drum connected by a set of straight tubes. However, as early as the 1920’s they had been entirely supplanted by more reliable boilers with bent tubes. The standard design in this group of steam boilers was the three-drum model of the Leningrad Metalworking Plant that was manufactured in the 1930’s. The heating surfaces of these boilers ranged from 650 to 2,500 sq m, and the capacity from 50 to 180 tons/hr. The boilers were equipped with a chamber furnace for the combustion of pulverized coal. The pulverized-coal fireboxes, which were introduced at the same time, very quickly gained wide acceptance. On the one hand, they had an important influence on the development of steam-boiler designs by substantially increasing their capacity, and on the other they made possible the use of any low-grade locally available coals with high efficiency. The introduction of chamber furnaces led to the development of waterwalls that consisted of evaporating tubes in the walls of the firebox chamber. At first the waterwalls covered only part of the walls and were intended to protect the lining from the direct action of the flames, which leads to formation of slag and destruction of the lining; however, they gradually covered an increasing proportion of the firebox walls, and modern steam boilers have fireboxes that are completely water-cooled. The waterwalls that absorb the heat radiated by the flames and the hot flue gases (the radiant heating surfaces) operate at a greater intensity than the generating tubes located in the lower-temperature zones (the convection heater surfaces). Consequently, the heating surface of waterwall boilers is substantially less than that of nonwater-cooled boilers of the same capacity; in boilers that have a completely water-walled firebox, which are called radiation boilers, the generating bank of tubes is very small. In the 1930’s, L. K. Ramzin of the USSR designed water-tube boilers with forced circulation.

All steam boilers in the USSR that operate at pressures exceeding 0.17 MN/m2 must be fabricated, assembled, put into operation, and operated according to the regulations issued by the Boiler Inspection. Power boilers must also be operated with observance of the regulations for the operation of electric power plants.

REFERENCES

Maksimov, V. M. Kotel’nye agregaty bol’shoi paroproizvoditel’nosti. Moscow, 1961.
Parogeneratory. Edited by A. P. Kovalev. Moscow-Leningrad, 1966.
Zakh, R. G. Kotel’nye ustanovki. Moscow, 1968.
Shchegolev, M. M., Iu. L. Gusev, and M.S. Ivanova. Kotel’nye ustanovki, 2nd ed. Moscow, 1972.
Gusev, Iu. L. Osnovy proektirovaniia kotel’nykh ustanovok, 2nd ed. Moscow, 1973.

G. E. KHOLODOVSKII

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