Document NgD3D6DZ5175jwy0nYqGgpGb

176 CHAPTER 12 1962 Guide And Data Book following sections). Some engineers substitute degree days for the period, multiplied by 24, for the value of (t-ta)N in Equation 3. Since this amounts to the basis for degree-day calculations, assuming that <=65 F, some allowance is thus made for the fact that F will frequently be overestimated, when H is taken as the maviramn, ot design, heat load. Example t: A residence building is to be hrafod to 70 P from 6 a.m. to 10 p.m. and 65 F from 10 pan. to 6 ajn. The hourly heat loss is 120,000 Btu per hour based on 70 F inside at --10 F outdoors. If the building is to be heated by: metered steam, how many pounds would be required during an average heating season? Solution: The heating value of steam may be 4*Vfp as 1000 Btu per lb. and since it is purchased steam, the efficiency can be assumed as 100 percent. Assume average outdoor tempera ture as 36.4 F. The average inside temperature is: (16 X70)+(8 X65) --------- Substituting in Equation 1 120,000(68^ - 36.4)5088 F 1.00(70 - (-10)11000 " 243,000 lb- Example t: What would be the fuel cost to heat the building m Example 1 during an average heating season, using stokerfired bituminous corn at $14.00 per ton having a calorific value of 13,000 Btu per lb, assuming that the utilization efficiency of the system is 65 percent? Solution: Substituting in Equation 1 120,000(68.3 - 36.4)5088 0.65(70 - ( - 10)]13,000 lb. .The fuel cost is then (28,800 -s- 2000) (14) = $201.00 Example S: Determine the estimated fuel cost per year of heating a budding with gas, assuming that the calculated hourly heat loss is 60,000 Btu Dosed on --10 F. The design tempera tures are --10 F and 72 F. The normal heating aftaann ig 210 days, and the average outdoor temperature during the halting season is 35.1 F. The utilization efficiency will be 70 percent. The heating plant will be thermostatically controlled, and a tempera ture of 65 F will be maintained from 11 p.m. to 7. * m Assume that the price of gas is 7.5 cents per 100,000 Btu of fuel consump tion. Solution; The average hourly, temperature is (72 X 16) + (65 X 8) 24 r- The maximum hourly heat loss is H - 60,000 Btu. The seasonal heat loss is M = 60,000(69.7 - 35.1) X 24 X 210 100,000 X 0.70 X (72 - ( - 10)] = 1823 hundred thousand Btu. The estimated seasonal fuel cost will be 1823 X $0,075 - $136.73 It should be noted that savings from night setback may not result as calculated. Room temperature may not decrease and combustion efficiency may be poor during morning pickup.1* Several time-saving procedures have been devised for quickly estimating the hourly Bth loss of one- and two-story residences in order that fuel estimates can be predicted more quickly from Equation 1. A graphical method of calculating heat losses has been developed11 which' makes possible a quick solution if the gross wall, ceiling, or floor areas and re^ spective transmission coefficients are known. The graphical method11 when used within the limitations established, has been found to give reasonably accurate results for the average residence, but if precise estimates are required, the heat loss should be calculated as outlined in Chapter 25 of the 1961 Guide And Data Book. DEGREE-DAY METHOD This method is based on consumption data which have been taken from buildings in operation, and the results have been computed on a degree-day basis. While this method may not be os theoretically precise as the Calculated Heat Loss Method, it is considered by many to be of more value for practical use. The amount of heat required in a building depends upon the outdoor temperature, if other variables are eliminated. Theoretically it is proportional to the difference between the outdoor and indoor temperatures. The American Gas As sociation" determined from records in the hating of resi dences that the gas consumption varied directly as the de gree days, or as the difference between 65 F and the mean outdoor temperature. In other words, on a day when the mean temperature was 20 deg below 65 F, twice as much gas was consumed as on a day when the temperature was 10 deg below 65 F. For any one day, when the mean tempera ture is less than 65 F, there are as many degree days as there are degrees difference in temperature between the mean temperature for the day and 65 F. Degree days may be cal culated on other than the 65 F base for use mainly for ware house and other industrial spaces in which temperatures to be maintained are considerably below the 68 to 72 F range. They are listed in a later section of this chapter. Studies made by the National District Heating Associa tion" of the metered steam consumption of 163 buildings located in 22 different cities, and served with steam from a district heating company, substantiate the approximate cor rectness of the 65 F base chosen by the gas industry. Tabic 2 lists the average number of degree days that have occurred over a long period of years, by months, and the yearly totals for various cities in the United States and Canada. The number of degree days for United States cities were calculated by taking the difference between 65 F and the daily mean temperature computed as half the total of the daily maximum and the daily minimum temperatures. The monthly averages were obtained by adding daily degree days for each month each year and dividing by the number of days in the month; then totaling the respective calendar monthly averages for the number of years indicated and di viding by the number of years. The total or long term yearly average degree-day value is the summation of the 12 monthly averages. Degree days for Canadian cities were supplied by the Canadian Meteorological Division of the Department of Transport, and were computed from the mean temperature normals on record for the various stations. (See Note c. Table 2.) Any attempt to apply the degree-day method of estimating fuel consumption for less than one month would be of very little value. It should be noted that this method of calcula tion is based on a long term average and cannot be expected to coincide with any single year in calculating fuel require ment. Individual yearly degree-day calculations may vary as much as 20 percent above and below the long term aver age. If the degree days occurring each day are totaled for a rea sonably long period, the fuel consumption during that period as compared with another period may be assumed to be in direct proportion to the. number of degree days in the two periods. Consequently, for a given installation, the fuel con sumption can be calculated in terms of fuel used per degree Estimating Fuel Consumption or Energy for Space Heating 177 Table 3 .... Correction Factors for Outdoor Design Temperatures* Outdoor Design Temp F.... --20 -ro 0 + +20 Correction factor......... 0.778 0.875 1.000 1.167 1.400 - lua ________________ie 3. which ere hkh (or mild climate* eod low(or cold iiiimn *re sot in error u might smear. The unit figure* in Table 4 are per l5#iiYnot cf radiator or Btu heat to** per degree day. For aquiva- and heating Mm those in warm climate* have lower design fry. | | |P and radiator quantities than those in cold cities. Consa onestly. the waif figure in quantity ot fad per (square foot o( radiator) (degree day) i* larger for warm local!tie* than (or colder region*. Since the northern ifLl bive more radiatorsurface per given building ana a higherieeeonal degreo- tots) th" tiliea in the south, the total fuel per season.will be larger for the northern city. (jay for any sufficiently long period, and compared with simi tar ratios for other periods to determine the relative operating efficiencies with the outdoor temperature.variable eliminated. Such results should be used with some reservation since it is poiible to have wide variations, for example, as between early and late winter periods.14 Computation and Application The general equation for calculating the probable fuel con sumption by the degree-day method is: F - U X X D X Cf (6) where P = fuel consumption for the estimate period. U m unit fuel consumption, or quantity of fuel used per (degree day). Qntilding load tout). Nt a cumber of huSding load unite (when available, use calculated hourly heat loss instead of actual amount of radiation installed). D = number of degree days for the estimate period. Cf = temperature-correction factor from Table 3. Values of N* depend on the particular building for which the estimate is being prepared and must be found by survey ing plans, by observation, or by measurement of the build ing. Values of U for use in this equation are the unit fuel consumptions per degree day, obtained as a result of the col lection of operating information and listed in Table 4. Atten tion is directed to the nature of these units in the next follow ing sections. Unit Fuel Consumptions per Degree Day The quantity of fuel used per degree day in a given heat ing plant can be reduced to a unit basis in terms of quantity Table 4 .... Unit Fuel Consumption Constants taitd on 0 F outdoor tetnperatvre, 70 F indoor temperature Fort ond onrfi G*s in therms* Dfl in gallons* Coal in pounds0 Utilization Efficiency J60 | 70 80 Unit fuel consumptho per degree day per 1000 Shift design heat lose 0.00572 0.00405 0.0476 0.00490 0.00347 0.0408 0.00429 0.00304 0.0357 i* equal to 100,000 Btu. I oa * beating value of 141.000 Btu per gallon on a beating value ot 12,000 Btu per pound. of fuel or steam per degree day per thousand Btu hourly heat loss at design conditions. A less frequently used basis is quantity of fuel per (degree day) (square foot of floor area). In fact any convenient unit can be used to relate the con sumption to the degree day and to the building. The choice of these units requires explanation, and some discrimination and judgment. If the volume basis is used, the net heated space is preferable to the gross building cub age, since gross cubage includes outer wails and certain por tions of attic and basement space that are usually unheated. In the absence of data on net heated volume, a value of 80 percent of the gross volume may be used to obtain the esti mated net heated volume. The volume basis has been rather widely used primarily because it is simple to apply. In in dustrial buildings it is usually easier to obtain the correct volume of a given building than to measure and evaluate the heating capacity of its heating system, or calculate its maxi mum hourly Btu loss. The comparison of buildings on a straight volume basis does not allow for variation in expo sure, type of construction, ratio of exposed area to cubical contents, and type of occupancy. It is inaccurate for estimat ing purposes unless the buildings are of very similar nature. The calculated heat loss or the heating capacity of the in stalled radiation may be used as the unit. The use of the heat ing capacity of the installed radiation is of questionable value when referring to heat-transfer surfaces used in warm air furnace or central air-conditioning systems. Where steam or hot water radiation is already installed, care should be exercised when using the installed radiation as the basis for estimating, since actual installed radiation may differ con siderably from the exact radiation requirements. In view of all these considerations, it is believed that the unit based on thousand* of Btu of hourly calculated heat loss for the design hour is probably the most desirable. Estimating Gas Consumption Values of the Unit Fuel Consumption Constant U for various fuels are given in Table 4. They are based on an indoor design temperature of 70 F and an outdoor design tempera ture of 0 F, and apply only to these conditions. For other outdoor design conditions corrections must be made by applying factors given in Table 3. The factors for gas in Table 4, as corrected if necessary, are satisfactory for regions having 3500 to 6500 degree days per heating season. In regions with less than 3500 degree days the unit gas consumption is higher than given; where over 6500, the unit is less than given. Ten percent addition or deduction .in these cases is recommended by A.G.A. publications. This table cannot be used for making estimates for industrial buildings where low inside temperatures are maintained. For gas heat values other than those given in Table 4, simply interpolate or extrapolate. In order to use Table 4, the efficiency of utilization must be known. A range of values is suggested in Table I which may be used as a guide. It is suggested that the lower end of the range be used unless there is definite knowledge that the system involved is of high quality and properly controlled. Example 4- Estimate the gas required to heat a residence located in Chicago, HI , where the heating season has 6310 degree . days and the gas has a heating value of 800 Btu per cu ft. The heating system is a gas designed unit. The design heat loss ofThe house is 60,000 Btuh based on design temperatures of --10 F out door and 70 F indoor. Solution: Table 1 indicates that the expected efficiency of utili zation will be between 70 and 80 percent. The higher efficiency applies to a near perfect system and unless one is assured of this condition, it is safer to use the low to midrange value. In this problem an efficiency of utilization of 70 percent is assumed. From Table 4, the gas consumption for a design outdoor tempera- i! !*> ii *? f\ j 4 }. t! Hi