Document rBO1e51B5pw1mXokypQQg6Znv

382 CHAPTER 18 1949 Guide Other items to be considered in boiler selection are : 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. c. Combustion space in the furnace. d. Type of heat liberation, whether continuous or intermittent, or a combination of both. e. Convenience in firing and cleaning. /. Adaptability to changes in fuel and kind of attention. g. Height of water line. h. Miscellaneous itemssuch as draft available, possibility of future extension possibility of break-down, and head room in the boiler room. '' t. 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 castriron 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 Heading, Piping and Air Conditioning Contractors National Association1 and can be used in selection of boilers, unless the heating system, contains an unusual amount of bare pipe, or the nature of the connected load is such 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 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-Fired Heating Boilers Operating at Maximum Load on Natural Draft of from I in. to i iN. Water* Kind of Coal . No. 1 Buckwheat Anthracite Sq Ft Gratb Up to 4 5 to 9 10,to 14 15 to 19 20 to 25 ' Lb of Coal per Sq Ft Gratb per Hour 3 3J* 4 4M 5 Anthracite Pea Up to 9 10 to 19 20 to 25 5 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 Up to 4 5 to 14 15 and above 9.5 12 16.5 * Steel boilers usually have higher combustion rates for grate areas exceeding 15 sq ft than those indicated in this table- Heating Boilers, Furnaces, Space Heaters 383 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 ordinaiy 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: H G= CXFXE (1) where l G = grate area, square feet. H = required gross output 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 gross output 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' G 6 X 13,000 X 0.60 10.7 sq ft The boiler selected should have a grate area not less than that determined by Equation 1. With small boilers, where it is desired to provide sufficient coal capacity for approximately an eight-hour firing period plus a 20. per