Document KJpnv678M6bJkaMDoMYev7YR6

American Society of Heating and Ventilating Engineers Guide, 1934 Chapter 25--Heating Boilers preferable to ignoring entirely the load due to heat loss from the supply 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 35. 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 2 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. 1. 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 tp check the grate area required for heating boilers burning solid fuel by means of the following formula: 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 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 Gas-heating appliances should be selected in accordance with factors given in Table 1, Chapter 28, which include an allowance for heating-up cold radiation, and for the piping tax. These factors are for thermo statically-controlled systems; in case manual operation is desired, a warming-up allowance of 100 per cent is recommended by the A.G.A. A gas boiler selected by the use of the A.G.A. factors will be the minimum size boiler which can carry the load. From a fuel economy standpoint, it may be advisable to select a somewhat larger boiler and then throttle the gas and air adjustments as required. This will tend to give a low stack temperature with high efficiency and at the same time provide reserve capacity in case the load is underestimated or more is added in the future. Conversions In the case of a solid fuel boiler converted to gas burning, the heat units supplied in the gas should be approximately twice the connected heating load. A combustion efficiency of 75 per cent for a conversion installation would provide a boiler output of 2 X 0.75 = 1.5 times the connected load, which allows 50 per cent for piping tax and pick-up. The presumption for a conversion job is that the boiler already is installed and probably will not be made larger; therefore, it is a matter of setting a gas-burning rate to obtain best results with the available surface. The conversion of a coal or oil boiler to gas burning is accomplished much more rapidly than the reverse since but little furnace volume need be provided for the proper combustion of gas. Other Considerations in Selection of Boilers As it will usually be found that several boilers will meet the speci fications, the final selection of the boiler may be influenced by other con siderations, some of which are: where G -- grate area, square feet. G= H CX PXE m K' 1. Dimensions of boiler. 2. Durability under service. 3. Convenience in firing and cleaning. 4. Adaptability to changes in fuel and kind of attention. 5. Height of water line. H = required total heat output of the boiler, Btu per hour (see Selection of Boilers, p. 337). 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. In large installations, the use of several smaller boiler units instead of A one larger one will obtain greater flexibility and economy by permitting the operation, at the best efficiency, of the-required number of units according to the heat requirements. Boiler rooms should, if possible, be situated at a central point with 340 341 N/