Document GmQVZqkqQ7xV3BJdXEryxLe1q

HEATINC VENTILATING AIR CONDITIONING ClIIDE 1942 where C = grate area, square feet. H -- required total heat output of the boiler, Btu per hour (see Selection of Boilers, p. 242). 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. B -- 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. ^500,000in_ G 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 Ratings obtained from the previously mentioned Steel Healing Boiler Institute code are intended to correspond with the estimated design load based on the sum of items 1, 2 and 3 outlined on page 242. 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, provided the boiler manufacturer guarantees it to be capable of operating at a maxi mum output of not less than 150 per cent of net load rating with overall 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. When the estimated heat emission of the piping (connecting radiation, and other apparatus to the boiler) is not known the net load to be con sidered for the boiler may be determined from Table 2. Selection of Cas-Fired Boilers Gas-heating appliances should be selected in accordance with the percentage allowances given in Fig. 1. 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 under-estimated or more is added in the future. 246 CHAPTER 12. HEATING BOILERS Conversions The conversion of a coal or oil boiler to gas burning is simpler than the reverse since little furnace volume need be provided for the proper com bustion of gas. When a solid fuel boiler of 500 sq ft (or less) capacity is converted to gas burning, the necessary gas heat units should be approxi mately double the connected load. The presumption for a conversion job is that the boiler 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. Assuming a combustion efficiency of 75 per cent for a conversion installation the boiler output would be 2 X 0.75 = 1.5 times the connected load, which allows 50 per cent for piping tax and pickup. In converting large boilers, the determination of the re quired Btu input should not be done by an arbitrary figure or factor but should be based on a detailed consideration of the requirements and characteristics of the connected load. An efficient conversion installation depends upon the proper size of flue connection. Often the original smoke breeching between the boiler and chimney is too large for gas firing, and in this case, flue orifices can be used. They are discs provided with an opening of the size for the gas input used in this boiler. The size should be based on 1 sq in. of flue area for each 7500 hourly Btu input. If dampers are found in the breeching they should be locked in position so that they will not interfere with the normal operation of the gas burners at maximum flow. In the case of large boiler conversions, automatic damper regulators proportion the position of the flue dampers to the amount of gas flowing and may be substituted for existing dampers. Generally in residence conversions automatic dampers are not of the proportioning type but close the flue during the off periods of the gas burners. Automatic shut-off dampers should be located between the back draft diverter and the chimney flue. Automatic dampers are usually designed to operate with electric contact mechanism, but frequently an arrangement is utilized which functions with mechanical fluid or gas pressure. Physical Limitations As it will usually be found that several boilers will meet the speci fications, the final selection may be influenced by other considerations, as: 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. In large installations, the use of several smaller boiler units instead of 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. Space Limitations Boiler rooms should, if possible, be situated at a central point with respect to the building and should be designed for a maximum of natural 247