Design and Proportions of Economizers and Air Preheaters

Abstract

Thirty years ago 200 lb. working pressure and 40 lb. of coal burned per square foot of grate surface were the maximum. Super-heat had not come into use. Since that time there has been a rapid development or evolution in the design of steam-generating and fuel-burning equipment. Today the rate of evaporation per square foot of boiler heating surface and the rate of fuel burning per foot width of furnace have increased 1000 per cent, and 1400 lb. steam pressure and 800 deg. steam temperature have passed the experimental stage, with indications that both pressures and temperatures may go higher where conditions of service and of fuel cost warrant. Modern high pressures and temperatures and high rates of fuel combustion have not only required much greater care in the selection, fabrication, design, and construction of pressure parts and furnaces and fuel-burning equipment, but also an entire rearrangement and distribution of heat-absorbing equipment. The temperature of the gases leaving the stack is an approximate measure of the overall efficiency. Recovery of the heat contained or carried in the gases leaving the steam generator proper may be accomplished by an economizer or an air preheater or both. Economizers of vertical cast-iron tubes decidedly antedate the increase in working pressures which required the use of steel tubes, now mostly horizontal and of the return-bend type, and because of the greater susceptibility to interior corrosion, may be fed only by water practically oxygen free. The introduction of pulverized coal requiring preheated air for proper combustion has greatly extended the use of air preheaters, of which three types are in use: tubular, plate, and regenerative. While the additional efficiency of a generating equipment runs from 3 to in some cases 7 per cent with economizers, it runs from 5 to 10 and sometimes more where air preheaters are used. Service and operating conditions may warrant the use of either or both, each plant being an individual problem of station economics.