Document jd4nOEQVmpNy6mVmBgMamXry

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 seasonal efficiency and is designed to maintain an average inside temperature of 65 F when it is 10 F outside in a city where the average outside temperature is 45 F and the heating season is 5088 hours long? Solution. The area of the round leaders is: 12 in., 113 sq in.; 10 in., 79 sq in.; and 8 in., 50 sq in. From Table 1 the total Btu transmitted is: First Floor: [(4 X 113) + (2 X 50)] X 111 = 61,272 Btu per hour. Second Floor: (6 X 79) X 167 = 79,158 Btu per hour. Total 140,430 Btu per hour. Allowing 10 per cent for duct and furnace losses, the gross output would be 154,500 Btu per hour. GROSS CALORIFIC VALUE CROSS OUTPUT - HONORED FEET STEAM RADIATION I-----1 | t | | ) | I I I |---- 1-----1 I I I I I I I-----1----------I-----1--T-----T-T I I I-- o 5 10 20 25 30 GROSS OUTPUT- HUNDRED FEET WATER RADIATION Fig. 3. Gas Fuel Burning. Rate Chart Enter Fig. 3 at 154.5 on the upper horizontal scale, move to the 70 per cent efficiency curve and thence to the 500 Btu per cubic foot vertical scale and find ^ ^ to bq ap proximately 440 cu ft per hour. Substituting in Equation 2: F = 440 X (65 - 45) 5088 (70 - 10) = 746,428 cu ft.! Maximum Rate of Fuel Burning The rate at which fuel is burned during the maximum, or design hour is frequently useful in setting, or adjusting, the fuel feed devices attached to stokers, oil-burners, and gds burners. This rate is and can be found from the charts of Figs. 1, 2 and 3 in the same way as outlined in the Examples 4 and 5. In using the charts for this purpose, however, it should be noted that the efficiency (E) is the overall efficiency of the boiler 228 CHAPTER 12. HEAT AND FUEL UTILIZATION or furnace at the time of peak load. This efficiency is generally consider ably greater than the value selected for E when the seasonal efficiency of utilization is used in making seasonal fuel estimates. Failure to dis tinguish between the two essentially different meanings attached to E may result in grossly inaccurate estimates. The correct fuel burning rate can be determined directly from the several charts for oil or gas burning installations, as these customarily operate on a strictly intermittent basis. These fuel burning devices usually introduce the fuel at a single fixed rate during the on periods and this rate should be sufficient to carry the gross or' maximum design load. In the case of coal stokers, which are usually capable of variable rates of firing, it is desirable to operate at as low a rate as weather conditions will permit, but the maximum firing rate of the stoker should be sufficient to carry the gross load. This rate may be determined by the same method as used for oil or gas. Table 1. Heat Carrying Capacity of Gravity Warm Air Furance Round Leader Pipes 180 F Register Temperature Leader Pipe Third floor........................................................... HrBtu per at Desion Conditions S orper q In. Leader Pipe in 167 200 Example 6. The estimated net load (including domestic hot water supply) as calcu lated for a residence is 1500 sq ft of hot water radiation. Determine the firing rates for various mechanically fired fuels assuming an overall boiler efficiency of 70 per cent; using coal with a calorific value of 12,500 Btu per pound; No. 3 fuel oil and natural gas having a gross heating value of 1000 Btu per cubic foot. _ Solution. Referring to Fig. 1, Chapter 13, a piping and pick-up factor for a net load of 1500 sq ft is found to be 43 per cent or the gross output is equivalent to 1500 X 1.43 = 2145 sq ft of hot water radiation. Using the charts in Figs. 1, 2 and 3 project vertically from the gross output value on the proper horizontal scale to the intersection of the 70 per cent efficiency line. From the intersection of this line proceed horizontally to the proper vertical scale where a direct value of the required fuel burning rate is given. These values are rates of burning while firing device is in operation and are not indicative of hourly fuel consumption. By use of the respective charts the firing rates for the various fuels will be found to be: coal 36.8 lb per hour, oil 3.2 gal per hour, and gas 460 cu ft per hour. DEGREE-DAY METHOD This method is based on consumption data which have been taken from buildings in operation, and the results computed on a degree-day basis. While this method may not be as theoretically correct as the Calculated Heat Loss Method, it is of more value for practical use. The amount of heat required by a building depends upon the outdoor temperature, if other variables are eliminated. Theoretically it is pro portional to the difference between the outdoor and indoor temperatures. Some years ago the American Gas Association2 determined from experi- See Industrial Gas Series. House Heating, (third edition) published by the American Gas Association. 229