BOILERS AND STEAM GENERATORS Boilers are of the following designs : return tubular, water tube, electric, steam generator.
boilers consist of a shell 6o in. to io8 in. in diameter. The shells are made of several plates riveted to gether with the tubes acting as stay bolts for the heads. The gases pass through the tubes. The boilers are limited in size to about 3,00o square feet. This type of boiler is used where small quantities of steam are required, at pressures not exceeding 25o lb. per square inch. It is also used extensively for waste heat in the manufacture of open hearth steel and other processes, iri marine work and for locomotives.
are boilers in which the water passes through the tubes and the gases are on the outside of the tubes. One of the main advantages of this type of boiler is safety. The latent heat in hot water under pressure has a particularly de structive effect in case the pressure in the vessel is suddenly released.
By confining the water to tubes, the quantity of water in any particular part of the boiler is kept to a minimum and if a tube bursts, the resulting damage will be small. Another advantage is that the size of the unit is limited only by the size of plates available.
This boiler is made in several different classes; straight tubes with box headers and both longitudinal and horizontal drums, straight tubes with sectional headers, and both longitudinal and horizontal drums, bent tubes with vertical and semi-vertical tubes and drums.
For box header boilers the headers are formed by riveting to gether two plates, spaced apart to form a boxlike structure. This construction is usually limited to Io,o0O sq.ft. on account of the difficulty of shipping larger plates and to pressures of about 400 lb. per square inch.
The straight tube sectional header boiler is substantially of the same general cross-section as the box header except that in some cases there is a greater inclination of the tubes. The headers are made of forged steel and contain one or more vertical rows of tubes. This type of boiler has been built in sizes up to 35,000 sq.ft.
and can be built larger and for pressures up to 1,800 lb. per square inch.
The electric boiler is used to generate steam or hot water where a small quantity of steam is required or in larger sizes where electricity can be secured at reasonable rates to improve the load factor of plants.
The advent of pulverized coal (see PUL VERIZED FUEL) crystallized the design of complete steam gener ating units giving consideration to the proper furnace volume and co-ordination of radiant and convection heating surface, super heater, economizer and air preheater. Such construction can be fired by oil, gas or pulverized fuel. It can readily be built for pressures up to 2,000 lb. per square inch. The fuel may be intro duced at several points and sometimes the burners are located at the top of the furnace with the gases passing downward. In other designs the fuel is introduced at the front wall or at the four corners with the gases passing upward through the furnace. The economizer and fan may be placed directly back of the con vection surface and the fans directly above the air preheater so that the duct work, with its chances for radiation and leakage, is reduced to a minimum. Some steam generator units give effi ciencies of 89%.
When built for pressure of ioo to 35o lb. all boiler drums were constructed of rolled plates formed on bending ma chines and riveted together. As the pressure increases the plate is necessarily thicker, making it more difficult to form plates and drive the rivets. It is generally accepted in 1939 that riveted drums should be limited to not more than 11 inches. Up to 1928 a few boiler drums less than 2 in. thick have been constructed by forge welding. In Europe, but not in the United States up to 1929, drums have been constructed of drawn seamless tubes. In 1931 the A.S.M.E. boiler code made permissible the use of autog enous or electric welding for the construction of boiler drums. Prior to 1931 it was customary to construct drums 21 in. thick and over of forgings but welded drums are now made up to five inches in thickness.