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Heating Ventilating Air Conditioning Guide 1939 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 4. Selection of Gas-Fired Boilers Gas-heating appliances should be selected in accordance with the percentage allowances given in Fig. 3. 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 Table 4. Boiler, Ratings Based on Net Load3 '\ Hand Ferid Ratings Steam Radiation Sq Ft Net Loadb Steam Radiation 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 . 1,389 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 Mechanically Fired Ratings Steam Radiation Sq Ft Net Loadb Steam Radiation Sq Ft 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, 12,650V 15,183 17,708 20,242 25,300 30,359 35,417 Adopted by the Steel Heating Boiler Institute in cooperation with-the Bureau of Standards, United States Department of Commerce Simplified Practice Recommendation R 157S5. bThe net load is made up by the sum of the estimated design load, items Iand2 (pages 257 and 258). AH 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. \ < 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. 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; 262 Chapter 13. Heating Boilers L rpfnre it IS a matter Ul sciuus-a uuiaui uco.. wiuu. ttnereiv ' ajlable SUrface. Assuming a combustion efficiency of 75 per with tne ^ conversjon installation the b'oiler output would be 2 X 0.75 cen.t `0.r.es the connected load, which allows 50 per cent for piping tax s 'ckuo '-tIn. converting lloarrgiroe Vbio/Silielorrsc, fthlioe rdlAetteArrmmiinnaaftlinonn noff ttVhieA rrPe--, gtu input should not be done by an arbitrary figure or factor but Lp hased on a detailed consideration of the requirements and characteristics of the connected load. An efficient conversion installation depends upon the proper size of fl connection. Often the original smoke breeching between the boiler d chimney is too large for gas firing, and in this case, flue orifices can be They are discs provided with an opening of the size for the gas H j. Uged in this boiler. The size should be based on 1 sq in. of flue area for each 7500 hourly Btu input. , jf 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 shutoff 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 of the boiler may be influenced by other con siderations, some of which are : 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 light. The space in front of the boilers should be sufficient for firing; stoking, ash removal and cleaning or renewal of flues, and should be at least 3 ft greater than the length of the boiler firebox. A space of at least 3 ft should be allowed on at least one side of every boiler for convenience of erection and for accessibility to the various 263