Document 1QwrOpj9agzY3Dr8Kx1pLerdZ

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944 The warming-up allowance represents the load due to heating the boiler and contents 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 and should be applied to the estimated design load as determined by Items 1, 2, and 3. 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. (A) Miscellaneous items such as draft available, possibility of future extension, pos sibility of break-down, and head room in the boiler room. Table 3. Warming-up Allowances for Low Pressure Steam and Hot Water Heating Boilers3' c Design Load (Representing Summation or Items 1,2, and 3 Btu per Hour Equivalent Square Feet of Radiation^ Percentage Capacity to Add for Warminq-Upo . 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 -______________________________ s______________ 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 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). This table refers to hand-fired, solid fuel boilers. A factor of yo per cent-over.design load is adequate: when automatically-fired fuels are used. 6240 Btu per square foot. Selection of Cast-Iron Boilers The net load is the sum of the radiation load and' hot water-supplyload. Net load ratings of cast-iron boilers are usually available from manufacturers' catalogs. They may also be obtained conveniently from published tables of I-B-R ratings8, 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 riot apply. In such a case, the selection must be based on the gross, output which is the sum. of Items 1, 2, 3, and 4 in. the section Factors Involved in Boiler Selection. I-B-R Ratings for Cast-Iron Boilers (Institute of Boiler and Radiator Manufacturers). 'Engineering Standards/Part II, Net Square Feet Radiation Loads in 70 Deg Fahr., Recommended for Low Pressure Heating Boilers. 1943 {Heating, Piping andAir Conditioning ContractorsNtUional Association): ' 244> CHAPTER 12. HEATINC BOILERS 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 conventionallydesigned 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 consisting of connected radiation, piping tax and domestic water heating load. As stated in the section on Boiler Output and Efficiency, this is equivalent to allowing 10 sq ft of boiler heating surface per boiler horeepower. 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. 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= H, CX FXE (1) where G = grate area, square feet. H -- required total heat 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 1). 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 heat 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 Formula 1. With small boilers where it is desired to providesufficient 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 Steel. Heating Boilers Ratings obtained from the previously mentioned Steel Heating Boiler Institute code are intended to correspond with the estimated design load based on the sum of Items T, 2, and 3 listed previously under Factors Involved in Boiler Selection. When Item 3, the heat emission of the piping, is not known, the net load, consisting of Items 1 arid 2, to be considered for the boiler may be determined from Table 2. The dif ference 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 128 sq ft of heating surface are classified as residence size.. An insulated residence boiler for oil or gas, not convertible, 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, pro- 245