Document 4v3wNLNg8Vd7o2Z7G1KJrvwme

American Society of Heating and Ventilating Engineers Guide, 1930 Table 1. Practical Combustion Rates for Relatively Small Coal-Fired Heat %ing Boilers Operating on Natural Draft of from In. to In. Water So. Ft. Gbate Up to 4 5 to 9 10 " 14 15 " 19 20 " 25 Up to 9 10 to 19 20 " 25 Up to 4 5 to 9 10 " 14 15 " 19 20 " 25 Up to 4 5 to 14 15 and above. Kind or Coal . Lb. or Coal So.per Ft. Gbatb per Hour 3 3 U> 4 5 5 5)4 6 5 6 7 s 9 4 7 10 surface and grate area) corresponding by heating surface to the estimated design load as defined in Section V of the Code of Minimum Require ments, being the sum of items (A), (B) and (C) in Paragraph 1. (This is the same (1), (2) and (3) on page 264 of this chapter.) BOILER RATINGS The use of boilers the output of which has been determined in accor dance with the boiler testing code No. 3 of the American Society of Heating and Ventilating Engineers is recommended. A method sometimes used for estimating the approximate reasonable output of a boiler where more accurate and reliable test data are not available, is as follows: The output in B.t.u. is obtained by multiplying together the square feet of grate surface by an assumed rate of combustion (in pounds of coal per square foot of grate per hour) by the B.t.u. value of the fuel (per pound) by the efficiency of the boiler (at the load and rate of combustion assumed). ' This gives an approximate rating. For data on rates of combustion, see Table 1, also Chapter 10, Heating by Coal, and Chapter 14, Chimneys. Ordinary heating boiler efficiencies for normal rate of combustion usu ally range between 55 per cent and 75 per cent. Gas-Fired Boiler Ratings The rating of gas boilers, in contrast to that of coal boilers is a com paratively simple matter. The following conditions of rating established by the American Gas Association.have been generally accepted: 1. The boiler must deliver rated output without chimney connection. 2. There must be no carbon monoxide formed when the gas pressure in service line is normal, 50 per cent below normal, or 50 per cent above normal. 3. The boiler must have an efficiency of at least 75 per cent at rated output. 4. Gas pressure drop in all valves and fittings must not exceed 1 in. of water. 266 Chapter 15--Boilers for Steam and Hot Water Heating The maximum B.t.u. input or gas that can be burned may be deter mined by the manufacturer but is checked by the American Gas Asso ciation laboratory before being approved. An arbitrary efficiency of 80 per cent is assumed in computing the output rating from the B.t.u. input. The actual B.t.u. delivered by any gas boiler when burning gas at the rated B.t.u. input will vary from the A.G.A. output rating in the direct proportion of the actual efficiency with the gas being burned to the assumed efficiency of 80 per cent. Oil-Fired Boiler Ratings The output of a boiler with oil fuel will depend upon the combined efficiency of the boiler and burner and the quantity of oil that can be burned properly within the space provided in the boiler. Oil burner ratings in use at present have been obtained either from tests with typical burners or have been obtained by multiplying the coal rating by a constant. Coal boilers converted to oil may have as high or a higher rating and as high or higher efficiency than with coal. To inaugurate unproved terminology: Size means physical dimensions, in contradistinction to rating or output. Rating means the output of the boiler; the work which it performs. TYPES OF BOILERS Broadly speaking, three types of boilers are commonly used--sectional, fire-tube and water-tube. Sectional boilers may be of the rectangular pattern with vertical sec tions, or of the round pattern with horizontal sections. They are usually constructed of cast iron; and on account of their sectional construction and the usual practice of being shipped knocked down and assembled on the job, they are easily handled through small openings at any stage of construction. Their design also facilitates the removal and replacing of darqaged parts as well as an increase of decrease in capacity by the addi tion of removal of sections. Cast iron boilers usually are limited to operating pressures of 15 lb. steam or 30 lb. water, excepting when used for hot water supply, in which case the boilers are built for operation at city pressure and are tested at pressures two and one-half times the operating pressure. Fire-tube boilers may be of the horizontal tubular brickset pattern, the horizontal tubular firebox pattern (with or without supplementary brick setting), the vertical tubular brickset pattern or the vertical tubular firebox pattern. Fire-tube steel boilers may be built for high or low pressure, but when used for heating with consequent usual low-pressure operation, they generally are built according to the 4.5. M. E. Steel Plate Heating Boiler Code which limits working pressure to 15 lb. per square inch for steam boilers, or 250 deg. fahr. or 160 lb. per square inch for water boilers.. In construction they may be riveted or weldedr Where higher operating pressures are required, boilers of the-same dimensions as low-pressure boilers can be obtained at a comparatively small increase in cost. 267