Document 8VD50w3jE7NXKGe32xjKNae35

352 CHAPTER 18 1948 Guide published tables of I=B=R ratings8, or from recommendations of the Healing, Piping and Air Conditioning Contractors National Association7 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. Selection of Steel Heating Boilers SBI catalog ratings in accordance with the previously mentioned Steel Boiler Institute, Inc. code are intended to correspond with the estimated 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 iii 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. Selection Based on Heating Surface and Crate Area Where neither the net load nor gross output ratings based upon per formance tests are available, a good general rule for conventionally designed 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. maxi mum load including the warming-up allowance will be provided for.by Table 4.' Wahming-up Allowances for Hand-Fired Low-Pressure Steam and Hot Water Heating Boilers a-b-c DESIGN Load Inns 2,(Rzprssbntxng Suxhation or l, and 3) Bta per Hour Equivalent Square Feet of Radiationd Up to 100,000 100,000 to 200,000 200,000 to 600,000 600,000 to 1,200,000 1,200,000 to 1,800,000 Above 1,800,000 Up to 420 420 to 840 840 to 2500 2500 to 5000 5000 to 7500 Above 7500 Pbbckntagb Capacitt-'to Add roa Warmxng-Upo 65 60 55 50 45 ' 40 aThis table is taken from the A.S.H.V.E. Code of Minimum Requirements for the Heating and Venti lation of Buildings, except that the second column has been added for convenience in interpreting the design load in terms of equivalent square feet of radiation. bSee also Time Analysis in Starting Heating Apparatus, by Ralph C. Taggart (A.S.H.V.E. Transac tions, Vol. 19.1913. p. 292); Report of A.S.H.V.E. Continuing Committee on Codes for Testing and Rating Steam Heating Solid Fuel Boilers (A.S.H.V.E. Transactions, Vol. 36, 1930, p. .35); Selecting the Right Size Heating Boiler, by Sabin Crocker {Heating, Piping and Air Conditioning, March. 1932). eThis table refers to hand-fired, solid fuel boilers. A factor of 20 per cent over design load is adequate when automatically-fired fuels are used. 4240 Btu per square foot. Heating Boilers and Furnaces 353 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: C= H C X F XE (1) where C = 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. U 500,000 6 X 13,000 X 0:60 10.7 sq ft The boiler selected should have a grate area not less than that deter mined 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 cent reserve for igniting a new charge," more grate area may be required depending upon the depth of the fuel pot. yTable 5. Practical Combustion Rates for Coal-Fired Heating Boilers Oper ating at Maximum Load on Natural Draft of from % in. to 14 in. Water Kind of Coal No. 1 Buckwheat Anthracite Sq Ft Gratb .. Lb of Coal per Sq Ft Gratb per Hour Up to 4. 5 to 9 10 to 14 15 to 19 20 to 25 3. m 4 5 . r... ' 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 IS sd ft than those indicated in this table.