Document B5DdxNEwKQ3ZG19G3zJ58Kywo

American Society of Heating and Ventilating Engineers Guide, 1937 ' and return lines, but better practice, especially when there is much bare pipe, is to compute the emission from both bare and covered pipe surface in accordance with data in Chapter 36. With direct radiation served bv bare supply and return piping the percentages may be higher than those stated, while in the case of unit heaters where the output is concentrated in a few locations, the piping tax may be 10 per cent or less. Warming-Up Allowance The warming-up allowance represents the load due to heating the boiler and contents to operating temperature and heating up cold radiation and piping. (See Item 4.) 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. Performance Curves for Boiler Selection In the selection of a boiler to meet the estimated load, the A.S.H.V.E. Standard Code for Rating Steam Heating Solid Fuel Hand-Fired Boilers recommends the use of performance curves based on actual tests con ducted in accordance with the A.S.H.V.E. Performance Test Code for Steam Heating Solid Fuel Boilers (Code No. 3), similar to the typical curves shown in Fig. 2. It should be understood that performance data apply to test conditions and that a reasonable allowance should be made for decreased output resulting from soot deposit, poor fuel or inefficient attention. Selection Based on Heating Surface and Grate Area Where performance curves are not 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 consisting of connected radiation, piping tax and domestic water heating load. As stated in the section on Boiler Output, 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. Due to the wide variation 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: where G= H CX FXE (4) G = grate area, square feet. H = required total heat output of the boiler, Btu per hour (see Selection of Boilers, p. 451). C = combustion rate in pounds of dry coal per square foot of grate area per hour, depending on the kind of fuel and size of boiler as given in 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 454 Chapter 25--Boilers 500,000 Btu per hours a combustion rate of 6 lb per hour, a calorific value of 13,000 Btu per pound, and an efficiency of 60 per cent. enn ruv\ The boiler selected should have a grate area not less than that deter mined by Formula 4. 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 Steel Heating Boilers Boiler ratings previously described under the Steel Heating Boiler Institute's Boiler Rating Code are intended to correspond with the esti mated design load based on the sum of items 1, 2 and 3 outlined on pages 451 to 453. Insulated residence type boilers for oil or gas 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 . -i------ o-narantees the boiler to be capable of operating at a Table 4. Boiler Ratings Based on Net Load3 Hand Fibed Ratings Steam Radiation Sq Ft Net Loads Steam Radiation Sq Ft MECHANICAU.T Fibed Ratings Steam Radiation Sq Ft Net Loads SteamTtadiation Sq Ft 1,800 2,200 2,600 3.000 3.500 4.000 4.500 5.000 6.000 7,000 8.500 10,000 12.500 15.000 17.500 20.000 25.000 30.000 35.000 1 J&9 1,702 2,020 2,335 2,732 3,135 3,540 3,945 4,770 5,608 6,885 8,197 10,417 12,500 14,584 16,667 20,834 25,000 29,167 2,190 2,680 3,160 3,650 4.250 4,860 5,470 6,080 7,290 8.500 10,330 12,150 15,180 18,220 21.250 24,290 30,360 36,430 42.500 1,695 2,089:. 2,461 2,853 3,335 3,830 4,330 4,834 5,850 6,885 8,490 10,125 12,650 15,183 17,708 20,242 25,300 30,359 35,417 Adopted by the Steel Heating Boiler Institute in cooperation with the Bureau ofStandards, United States Department of Commerce Simplified Practice Recommendation R 167-85. bThe net load is made up by the sum of the estimated design load, items 1 and 2 (pages 451 to 453). AU net loads are expressed in 70 F. For hand fired boiler ratings less than 1800 sq ft of steam or 2880 sq ft of water and mechanically fired boiler ratings of 2190 sq ft of steam or 3500 sq ft of water, apply the factor 1-3 to the net load to determine the boiler size. For water boilers use the equivalent net load for steam boilers of similar physical size. maximum output of not less than 150 per cent of net load rating with over all efficiency of not less than 75 per cent with at least two different makes of each type of standard commercial burner recommended by the boiler manufacturer. 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. 455