Document 7OeMwVZoqv9wMyeneKz7Dyvyo

384 CHAPTER 20 1946 Guide Values given in Table 3 assume the use of oil with a heating value of 140,000 Btu per gallon. For other heating values, multiply the values in Table 3 by the ratio of 140,000 divided by the heating value per gallon of fuel being used. Example 6. Estimate the seasonal oil consumption of an oil-fired boiler in a building located in Minneapolis having a calculated heat loss of 192,000 Btu per hour, burning 144,000 Btu per gallon oil and operating at a seasonal efficiency of 60 per cent. The out side design temperature for Minneapolis is --20 F, and the inside design temperature is 70 F. Solution. From Table 3, under 60 per cent efficiency and opposite the bottom column, the value of U is found to be 0.00476 gal per 1000 Btu hourly heat loss for 0 F outside temperature. The correction factor for -- 20 .F outside design temperature from Table 5 is 0.778. Solving, 0.778 X 0.00476 = 0.00370. Making a further correction for the heating value: 0.0037 X 144 00(1 = 0.0036 gal per 1000 Btu per hour calculated heat loss per degreeday. From Table 1, the normal degree-days for Minneapolis is 7989. Since U is expressed in 1000 Btu, N is equal to 192. Substituting in Equation 4: F = 0.0036 X 7989 X 192 = 5525 gal. Estimating Coal or Coke Consumption / Coal or coke consumption estimates are made in exactly the same procedure as for oil. Values of U are given in Table 4 which only apply to inside design temperatures of 70 F and an outside design temperature of 0 F. A correction must be made for other conditions by use of the multi plying factors in Table 5. Data in Table 4 are based on 12,000 Btu per pound coal and for other heating values of coal they must be multiplied by the ratio of 12,000 divided by the heating value of fuel used. Example 7. A building in Marquette, Mich., has an hourly heat loss at design con ditions of 240,000 Btu per hour. Based on an inside design temperature of 70 F and an outside design temperature of --20 F, what will be the estimated norpial seasonal coal consumption for heating if 12,000 Btu per pound fuel is burned at a 50 per cent seasonal efficiency, and what part of the total will be used during November, December, and January? Solution. From Table 4, U is 0.0666 lb of coal per 1000 Btu per hour heat loss. Cor recting for the outside design temperature of -- 20. F from Table 5, the value of V is 0.778 X 0.0666 - 0.0518. From Table 1, D is 8786 and from the problem, is 240. ' Substituting in Equation 4: F = 0.0518 X 240 X 8786 = 109,200 lb. Fuel used over any period is, according to the theory of the degree-day, proportional to the number of degree-days during the period. From Table 1, the average numbers of degree-days for November, December, and January in Marquette are 927, 1306, and 1471, a total of 3704. The yearly total is 8786, so that during these three months the estimated consumption is: 3704 S=mr X 109,200 = 46,200 lb. Estimating Steam Consumption In estimating steam consumption the efficiency is generally assumed at 100 per cent. If for low pressure steam an average heating value of 1000 Btu per pound of steam is used no correction is necessary. In com paring values from different cities, correction should be made for design temperature (see Table 5) when the unit figures are in terms of square feet of radiation but not when the values are in terms of building volume or floor space. Estimating Fuel Consumption for Space Heating 385 Where the heat loss is calculated in Btu per (hour) (degree difference in temperature) the simple Equation 5 may be used: 11 X 24 X D 1000 (5) where F -- pounds of steam required for estimate period. H = calculated heat loss, Btu per (hour) (degree difference). D -- number of degree days for the period of estimation. 1000 = Btu delivered per pound of steam condensed. In this method the number of degree-days automatically takes care of average inside and outside temperature difference. When degree days are taken from Table 1, an average inside temperature of approximately 65 F is assumed throughout the period. If an average inside temperature other than approximately 65 F is to be used, the number of degree-days should be obtained for the new base. Example 8. An eight story building in Pittsburgh maintains daytime temperatures of 70 F but allows night temperature to drop to not lower than 60 F. Its calculated heat loss is 10,500 Btu per (hour) (degree temperature difference). .What is the estimated average yearly steam consumption for building heating? Solution. Since the average inside temperature is approximately 65 F, the degreedays from Table 1,- based on 65 F may be used. Therefore, from Table 1, Pittsburgh has 5,466 degree-days per normal season. Inserting in Equation 5 10,509 X 24 X 5466 1,000 1,377,432 lb of steam. Consideration has been given to the difference in steam utilization of different types of buildings and Table 6 shows actual average units for these various types. These figures were obtained from operating results in 896 buildings located in all sections of. the United States. Being averages, and for small groups in each type, the figures may need con siderable modification to allow for local variations. It should be especially noted that the steam used for heating hot water is not included in the values given in Table 6. Example 9. A store in Philadelphia with a heating system designed to maintain 70 F inside in 0 F weather has 250,000 cu ft of heated space. What would be the estimated average yearly steam consumption of purchased steam for heating? Solution. According to Table 6, a store-would use 0.624 lb of steam per degree-day per 1000 cu ft heated space. From Table 1, Philadelphia has 4749 degree-days per normal year. Inserting in Equation 4: F = 0.624 X 250 X 4749 = 740,000 lb of steam. Degree-Day as an Operating Unit The degree day is also widely used as a means of comparing the efficiency of the fuel consumption of one period with another for the same building..- Since, the fuel, consumption is proportional to the weather (degree-days) and since the periods to be compared may not have the same weather conditions, the comparison can be made only after the fuel consumptions have been computed on a comparable weather basis, that is, upon the actual number of degree days occurring for a given month and year in the city under consideration. Since fuel consumption is proportional to the number of degree-days, plant operators frequently compute each month the fuel burned per degree-day by the heating plant. The resulting unit figure, by eliminating the outside temperature