Document rp5pNz3RDbnq1y5wvmZnkrrLJ

Heating Ventilating Air Conditioning Guide 1939 Chapter 13. Heating Boilers and return lines, but better practice, especially when there is much bar pipe, is to compute the emission from both bare and covered pipe surfar! in accordance with data in Chapter 39. A chart is shown in Fig. 3 indicat ing percentage allowances for piping and warming-up which are appljc" able to automatically-fired heating.plants using steam radiation. With direct radiation served by 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. r t i Fig. 3. Percentage Allowance for Piping and Warming-up 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. While in every case the estimated maximum load will exceed the design load if adequate heating response is to be achieved, there is however, no object in over-estimating the allowances, as the only effect would be to reduce the time of warming-up bV a few minutes. Otherwise, it might result in firing the boiler unduly and increasing the cost of operation. Performance Curves for Boiler Selection i 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. t 260 lection Based on Heating Surface and Grate Area ,, oerformance curves are not available, a good general rule for W tinnallv-designed boilers is to provide 1 sq ft of boiler heating convenu ^ ft 0f equivalent radiation (240 Btu per square foot) surface the design load consisting of connected radiation, piping f^>reSd domestic water heating load. As stated in the section on Boiler nftnut this is equivalent to allowing 10 sq ft of boiler heating surface per horsepower. In this case it is assumed that the maximum load 1 dine the warming-up allowance will be provided for by operating the hoiler in excess of the design load, that is, in excess of the 100 per cent raBrig on a boiler-horsepower basis. Due to the wide variation encountered in manufacturers' ratings for hoilers 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 = C X FX E (4) where G = grate area, square feet. B = required total heat output of the boiler, Btu per hour (see Selection of Boilers, p. 251). 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. P = calorific value of fuel, Btu per pound. E = efficiency of boiler, usually taken as 0.60. Example l. 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 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 257 and 258. 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 boiler manufacturer guarantees the boiler to be capable of operating at a 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 orthe installed radiation, the excess is to be considered as a part of the net load. 261