Document DmXxY0x4Qa8G72XxB1Qpva1B

Heating Ventilating Air Conditioning Guide 1938 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 39. A chart is shown in Fig. 3 indicat ing percentage allowances for piping and warming-up which are applic 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. 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 by .a few minutes. Otherwise, it might result in firing the boiler unduly and increasing the cost of operation. 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 Fuef 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 Steani 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 frqm soot deposit,' poor fuel or inefficient attention.' " : 254 Chapter 13. Heating Boilers 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: G= CX FXE ^ where G = grate area, square feet. H -- 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. 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. G = 6 X 500,000 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 252 and 253. 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 cbmmercial 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. ; 1 i."' . ' ' : i\