Development of Steam Generators

DEVELOPMENT OF STEAM GENERATORS When steam units were first used industrially the boiler was purchased, a brickmason employed to set the boiler in place and a labourer indiscriminately secured to shovel fuel under the boiler to generate enough steam for the process in question. Fuel, labour and material were cheap. It was not then economical to seek refinements in equipment or labour-saving devices; steam demands and pressures were low and in many cases, return tubular boilers answered the necessary requirements. The industrial development of the 20th century caused rapidly increasing de mands for power. These were first met by the improvement in the design of steam turbines, and the application of mechanical stokers to the existing boilers. This met the needs for a few years but about 1916 steam turbine units began to increase rapidly in size and in 1939 single turbines of 6o,000 kw. were in operation and compound units of 200,000 kw. turbines in course of construction. In 1939 power houses containing 770,000 kw. were in operation and power houses designed for i,000,000 kw. output have been projected. In 1920 there were a few stations operating above 25o lb. pressure. In 1939 there were several score of stations operating at over 1,200-1,400 lb. pressure and a few are now in operation at higher pressures.

With these changes steam boilermakers discarded rule of thumb methods and employed research engineers, metallurgists and various specialists, installed machine tools, and adopted scientific methods of manufacture. It was found at this time that the kind of water introduced into the boiler was of considerable importance ; that the superheater should be located in a position that would give a wide range of load with very little change of temperature ; that an evaporation of from 13 to 21 lb. of steam per square foot of heating surface per hour should be the mini mum; that the use of pulverized coal to secure these high ratings was desirable. Air-cooled refractory furnaces to burn pulverized coal at high ratings were tried but it was seen that, with water cooled furnaces, a unit could be designed that would combine economy and efficiency with low maintenance cost and maximum continuous hours of service. With new and larger plants, the question of the best cycle to be used became of paramount im portance affecting the boiler, superheater, feed water heater and economizer design. Means were needed to remove free oxygen from water to prevent corrosion. The water entering the econo mizer was of higher temperature than when exhaust steam alone was used and the temperature of the gases leaving the economizer were too high for the best efficiency. Consequently air preheaters

were developed to recover heat from the escaping gases, to provide hot air for drying the coal in the course of pulverization and to preheat the air for combustion before being introduced into the furnace. This cut down the time required for complete com bustion and raised the temperature of the furnace.

A

400,000 kw. plant will burn 200 tons of coal per hour and roughly one-tenth of this will be ash which must be removed. Ash, in a dry state, is removed by conveyors or by sluicing with water. A newer development is to melt the ash and to remove as slag. Safety consideration, abatement of smoke and noise, consideration of the design of the plant to fit in with surrounding buildings and landscape, were additional problems to be solved. It will be seen then that the problem of generation of steam to meet the requirements of modern industrial civilization has be come extremely complex. (J. B. C.) The principal fuels for the generation of steam are coal and oil and these are treated tinder the articles: FUEL; PULVERIZED FUEL ; PETROLEUM ; COAL AND COAL MINING. There are, how ever, many localities where there is a limited supply of certain other materials which are sometimes used for fuel. Coke breeze is the small size coke and dust which is burned successfully on chain grate stokers. Lignites (q.v.) are burned on chain grate stokers and in pulverized form. Wood is usually put through a hogger to reduce it to small sizes for greater ease in handling and better burning in the furnace. The furnace must be large to provide sufficient time for complete combustion. Bagasse is the residual sugar cane, left after the juice has been extracted. The furnace is especially constructed for admitting air around its sides. The silica in the bagasse runs on to the grate and prevents air coming in from underneath. Bonecoal is the name given to coal taken off the top of coal seams and contains a large amount of impurities. Formerly discarded, it is now being burned on stokers and in pulverized form. Anthracite coal is used for house hold use in the districts adjacent to the anthracite fields. There is, however, a large amount of small sizes and dust that is now being burnt on travelling grate stokers, especially built for this purpose. Natural gas (q.v.), blast furnace gas (q.v.), coke oven gas (q.v.) and producer gas (q.v.) are also frequent boiler fuels.