Document 4J7X10zQVgdZ2JEMgwz3pgVQN

346 CHAPTER 18 1946 Guide to operating temperature, and heating up cold radiation and piping.. The factors to be used for determining the allowance to be made should be selected from Table 3. Other items to be considered in boiler selection are: () Efficiency with hard or'soft coal, gas, or oil firing, as the case may be. () 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. (A) Miscellaneous items such as draft available, possibility of future extension, pos sibility 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. Selection of Cast-Iron Boilers Net load ratings of cast-iron boilers are usually available from manu facturers' catalogs. They may also be obtained conveniently from published tables of I=B = R ratings6, or from recommendations of the Heating, 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 pick-up 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, tjie 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 Table 3. Warming-up Allowances for Hand-Fired Low-Pressure Steam . and Hot Water Heating Boilers a-b-c Design Load (Hepresenting Summation or Items i. 2. and 3) Bto per Hour Equivalent Square Feet of Radistiond - Percentage Capacity to Aod roa Warming-Up 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 65 60 55 50 45 40 This 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 RatingSteam 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). Tliis table refers to hand-fired, solid fuel boilers. A factor of 20 per cent over design load is adequate when automatically-bred fuels are used. ` d240 Btu per square foot. Tfmtine Boilers and Furnaces_____________ ______________.' : ____________ 347 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 squarefeet 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 sis a part of the net load. Selection Based on Heating Surface and Grate 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 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 f _________ CX FXE y' where 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 4). F = calorific value of fuel, Btu per pound. 1 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 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. Selection of Gas-Fired Boilers After determining the net load for the installation, gas designed boilers can usually be selected from manufacturers' tables of net load ratings which are based on piping and pickup allowances varying from 56 per cent for boilers of 200 sq ft and less to 35 per cent for boilers of 4000 sq ft and larger. If the '.piping and pickup load or other factors create an unusual load, a boiler should be selected which has an A.G.A.