Document 856rqYrMMqVZG88BLz2wGDKVk

362: ' CHAPTER 15 Other items to be considered in boiler selection are: 1950; Guide a. Efficiency with hard or soft coal, gas, or oil firing, as the case may be. b. Grate area with hand fired coal, or fuel burning rate with stokers, oil, or gas. e. Combustion space in the furnace. of bdo. tThy.pe of heat liberation, whe' ther continuous or intermittent, or a combination e. Convenience in firing and cleaning. /. Adaptability to changes in fuel and kind of attention. g. Height of water line. h. Miscellaneous items Buch as draft available, possibility of future extension', possibility of break-down, and head room in the boiler room. . i. The most economical size of boiler is usually one that is just the.right.size for the load. Either larger or smaller boilers may be less economical. Cast-Iron Boilers Net load ratings ,of . cast-iron boilers are usually available from- manu facturers' catalogs. They may also be obtained conveniently from pub lished tables of- I =. B = R ratings,* or from recommendations of the Heating, Piping and Air. . Conditioning Contractors National Association7-1. and can be used- in; selection, of boilers, unless the heating system contains'; an unusual amount of ;barh pipe, or the nature of the connected load issuch that the normal allowances for pipe loss and pickup do not apply' In such a case, the selection must be based on the gross output. ; Steel Heating Boilers V ' ^ ` SBI catalog ratings, in accordance with the previously mentioned Steel, Boiler Institute, Inc. code, are intended .to correspond with the estimated' . Table 5. Practical Combustion Rates fob Coal-Fibed Heating Boilers Operating at'Maximum Load on Natural Draft of from t in. to } in. Water* Kind op Coal No. 1 Buckwheat Anthracite SqFt Grate Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 Lb of Coal per Sq Ft " Grate per Hour 3 :. 4 Anthracite Pea Up to 9 10 to 19 20 to 25 m5 6 Anthracite Nut and Larger Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 8 9 10 11 13 Bituminous In IStowf" U"aUy h"* Up to 4 5 to 14 15 and above 9.5 12 16.5 combustion rata lor cate area exceeding IS sq ft than those Indicated Heating'Boilers, Furnaces, Space Heaters - 363 design load. When the heat emission of . the piping is not known, the net load to be considered for the boiler may be determined from Tables 1 and 2. The difference between design load and net load represents an amount which is considered normal for piping loss of the ordinary heating system. Boilers with less than 177 sq ft of heating surface, and having SBI net ratings (steam) of not more than 3,000 sq ft if mechanically-fired and 2,480 sq ft if hand-fired, are classified as residence size. An insulated residence boiler for oil, gas, or stoker firing may carry a net load expressed in square' feet of steam radiation of not more than 17 times the square feet of heating surface in the boiler, provided the.boiler has been tested in accordance with the SBI Code for Testing Oil-Fired. Steel Boilers at output rates of 125, 150, and 175 per cent of the SBI Net Rating. The SBI Net Rating (square feet steam) for hand-fired residence boilers is not greater than 14 times the heating surface. If the heat loss from the piping system exceeds 20 per cent of the installed radiation, the excess is to be considered as a part of the net load. Heating Surface and Grate Area Basis Where neither the net load nor gross output ratings based upon per formance tests are available, a good general rule for conventionally de signed boilers is to provide 1 sq ft of boiler heating surface for each 14 sq ft of equivalent radiation (240 Btu per square foot) represented by the design load. This is equivalent to allowing 10 sq-ft of boiler heating surface per boiler horsepower.. In this case it is assumed that the maximum load including the warming-up allowance will be provided, for by operating the boiler in .excess of the design load, that is, in excess of the 100 per cent rating on a boiler-horsepower basis. SBI ratings for hand firing are based on 10 sq ft of heating surface per boiler-horsepower. Due to the wide variation which may be encountered in manufacturers' ratings for boilers of approximately the same capacity, it is advisable to check the grate area required for heating boilers burning solid fuel by means of the following formula: G~ CXFXE (1) where . O = grate area, square feet. //= required gross oufptif of the boiler, Btu per hour (see Selection of Boilers). C = desirable combustion rate for fuel selected, pounds of dry coal per square foot of grate per hour (see Table 5). F = calorific value of fuel, Btu per pound. ;. E = efficiency of boiler,, usually taken as 0.60. . ' Example 1. Determine the grate area for a required groee oulput of the boiler of 500,000 Btu per hour, a combustion rate of 6 lb per hour, a calorific value of 13,000 Btu per pound, and an efficiency of 60 per cent. 500,000 10.7 sq ft " 6 X 13,000 X 0.60 The boiler selected should have a grate area not less than that determined ^ by Equation 1. With small boilers,'where it k desired to provide sufficient coal capacity for approximately an eight-hbiir firing period plus a 20 per