Document ZJG0gGwXBkDbQ73m2noMODgYL

624 CHAPTER 23 1957 Guide. complete discussion of free area ratio and its effect on system performance, see Reference 14.. The equations for the heating requirements of a snow-melting system have been derived and thoroughly explained in Reference 15. The general equation for slab output, g0, is - 2o = 2 + ?i + Ra(?. + 2i>) (15) where 2, = sensible heat transferred to snow, Btu per (hour) (square foot). gm = heat of fusion, Btu per (hour) (square foot). Ra -- ratio of snow-free area to total area, dimensionless. g,, = heat of vaporization, Btu per (hour) (square foot). gh = heat transfer by convection and radiation, Btu per (hour) (square foot). The sensible heat, gs,to raisethe temperature of the snow to 32 F is q, = 2.6s (32 - <.) (16) where s = rate of snowfall, see Table 5, inches of water equivalent per hour. U = air temperature, Fahrenheit. The heat of fusion, gm, to melt the snow is g,, = 746s. The heat of vaporization, ge, (mass transfer) is (17) g. = 1074(0.0201 + 0.055)(0.185 - p,,)RA (18) where p,, = vapor pressure of moist air, inches of mercury. The heat transfer, gh, (convection and radiation) is gh = 11.4 (0.0201c + 0.055) (ff - U)Rk (19) where = wind'velocity, miles per hour. Ji = water film temperature, Fahrenheit. In addition to determining the four heating requirements, it is neces sary to make allowance for back and edge losses. These losses vary from 30 to 50 percent, depending upon the slab construction. The equation for the required fluid temperature to provide an output g has been derived in Reference 15. For. construction. similar to Fig. 19, the equation is fe, = 0-5go + t, (20) where tm -- mean fluid temperature (antifreeze solution), Fahrenheit. Equation 20 will suffice for 1 in. IPS as well as % in. -- see Fig. 19. Values for s for certain cities, are given in Table 5. Using the values for s given in Table 5, the appropriate values for and v, the values for g0 may be found in Table 6. In preparing Table 6, only values of p,,v fr relative humidities of 80 percent, were used. .The variation in g,, and tm Panel Heating 625 Table 5. Snowfall Data for Various Cities* ClTT Col. 1 Albany, N. Y............... Asheville, N. C.................. Number op Headings with ' Maximum Temperature in 6-Hour Period bblow-Freezing at Various Snowfall Rates . Snowfall Rate in Equivalent Inches of Water per Six Hours. . Total Readings Taken 'Assumed Design i Rate of Snowfall 0.00 to 0.24 Col. 2 0.25 to 0.49 Col. 3 0.50 to 0.74 Col. 4 0.75 to 0.99 Col. 5 Col. 6 3 Col. 7 ' 2062 463 29 1300 3720 Buffalo, N. Y......................................... 1323 1871 2390 11 23 1 3720 0 13 3720... 1363 19 1 1498 1 Cincinnati. Ohio................... Cleveland. Ohio Colnmhns, Ohio 1351 3 3720 3720 1830 Evansville, Ind............ Hartford, Conn ........ Madison. Wise..... Minneapolis, Mjnn 916 2370 2703 44 12 1. 1 0 T" Oklahoma City, Obljj Omaha, Neb. Philadelphia, Pa. Pittsburgh. Pla........... Portland. Mnin 1795 3720 3720 alia 10 2 1 n in 1365 2 2054 33 1 3720 0 10 St. Louis. Mo.. Salt Lake City, mh Spokane. Wash. Washington. D. O. New York. N. Y * -- 1482 1545 533 5 11 ,1 0 1 3348 0.10 U ?' WuMxrBureau. Based on readings taken 1:30a.m., 7:30a.m., 1:30 p.m., and 7:30 p.m. Deriir November 15 to February 15 from 1940 to 1949. (Where the total readings are less than 3720 the 5 " rf?ord 18 1888 than 10 years.) The difference between Col. 0 and the sum of readings in Cols. 2, 3, , MBS IS the number of readings with a maximum temperature (in the 6-hr period) above freezing. col,,rr^,h!Ld?i^ rate is found as follows: Proceed to left (on line from any city) from Column 5 until the rolumr! the tenth reading is found. Assume that the larger value in the heading of the selected "Oimimreji maximum value, and should be multiplied by 2 to obtain the maximum rate for a 9-hour Gn_r U.i,,7? "t? divided by 6 is the design rate per hour. This is erjuivalent to dividing the 8 vnoe in the heading of the selected column by 3. b i^rrS^TMple,: For Albany, N. Y. Columns 5 and 4 total six readings, and consequently the tenth reading WateriSin 3'. , Sh, i1?8 fhe U*r*r v8j.ucf ,w n the column heading. Dividing 0.49 by 3, the design e Qulvttlent of 0.16 inches per hour is found, as listed in Column 7. ParahlA^L^WL City record,was not used in this tabulation sinoe the reoords for that station were not comF^*oie with those of the other stations. humidities of 75, 80, 85 and 90 percent are given in Reference 14, and may be calculated by solving Equations 21 and 22.