Document 5kyQyMGmVQG1D0yJpG6e1GM2R

690 CHAPTER 49 1960 Guide Table 1 .... Data for Determining Operating Characteristics of Snow-Melting Systems*-1 Period of No Snowfall Pmriod of Snowfall* Air Temperature, F* hoars of tnowfalP Below Wind car Over 32 to 32 freezing % of winter boon wttb no period,* mph Per- Hr Pr period* snow at above temperatures Albuquerque, N. M......... 74.7 Amarillo, Tex.................... 73.1 Boston, Mass.................... 64.6 24.7 26.0 31.4 Buffalo 1 ]lt -r Niagara Falls/*' *......... Burlington, Vt.................. Caribou \M limestone/ 46.5 39.0 21.4 46.9 54.5 70.6 Cheyenne, Wyo................. 46.4 Chicago, m....................... 45.4 Colorado Springs, Colo... 54.3 49.8 50.9 436 26.2 24.6 24.7 23.9 19.6 16.5 21.5 21.4 22.1 8.5 0.6 22 13.3 0.9 33 14.2 4.0 145 10.8 6.6 240 10.8 6.5 236 10.0 8.0 290 15.3 3.8 138 11.5 3.7 134 11.5 2.1 76 Columbus, Ohio............... 59.0 Detroit, Mich.................... 47.0 Duluth, Minn................... 12.6 38.1 49.3 80.5 24.5 24.1 14.5 10-0 2.9 105 10.6 3.7 134 12.0 6.9 250 Falmouth, Mass................ 68.5 Great Falls, Mont............... 49.0 Hartford, Conn................. 56.4 29-5 46.2 38.9 25.5 16.5 24.4 12.8 2.0 73 14.4 4.8 174 S.2 4.7 171 Lincoln, Neb..................... 45.0 Memphis, Tenn................. 87.2 )MmnMinneapolisL,-__ st.PiuT .............. 23.6 52.5 12.5 70.8 Mt. Home, Idaho............. 56.3 New York, N. Y.............. 55.7 Ogden, Utah..................... 50.0 42.6 42.2 45.6 20.8 27.0 16.9 24.9 24.2 24.3 10.1 2.5 91 11.5 0.3 11 11.1 5.6 203 9.5 1.1 40 11.8 2.1 76 9.4 4.4 160 Oklahoma City, Okla.... 79.0 Philadelphia, Pa.................. 75.8 Pittsburgh, Pa...................... 55.2 19.8 22.6 39.8 24.6 28.7 24.3 15.8 1.2 44 9.7 1.6 58 11.6 5.0 182 Portland, Ore.................... 92.9 Rapid City, S. D................. 45.2 Reno, Nev......................... 56.0 6.1 51.6 41.6 28.9 19.3 24.3 8.4 1.0 36 12.9 3.2 116 5.6 2.4 87 Required output, Btu per (sq ft) (hr)4 Maxi- atum 0 to 50 to 100 to 150 <0 200 to 250 to 300 to 350 to 400- output. Btu/ 49 99 149 199 249 299 349 399 (sq ft) (hr)' Frequency distribution of snowfall hours at above outputs, percent* 62.0 25.4 7.6 4.2 0.0 0.8 _ _ _ 259 94.1 5.9 82 33.7 35.4 15.4 10.7 3.0 1.8 -- -- -- 260 88.1 10.1 1.8 143 51.5 30.0 12.3 4.3 1.2 0.6 0.1 -- -- 320- 83.2 14.0 2.0 0.3 0.3 0.1 -- 0.2 -- 370* 50.7 32.6 11.2 3.7 1.4 0.2 0.2 _ 309 95.9 3.4 0.2 0.5 -- -- -- -- -- 192 53.7 29.9 13.2 2.5 0.6 0.1 -- -- -- 2S0 91.8 7.6 0.6 142 35.0 39.7 16.0 5.7 2.0 1.(1 0.5 0.1 -- 378 92.0 7.5 0.5 138 10.5 94.3 45.8 91.5 26.8 98.4 65.8 97.7 60.4 95.9 23.7 94.8 50.0 91.5 26.2 94.6 48.4 80.4 32.7 97.2 48.4 85.0 28.4 96.5 74.2 98.1 53-1 87.6 64.6 88.8 27.8 95.7 62.3 84.3 53.6 93.3 78.0 91.5 29.7 97.6 82.6 90.2 26.2 5.4 37.4 8.1 36.3 1.6 19.4 0.3 11.4 0.3 19.0 -- 13.1 3.1 0.1 7.5 -- 8.6 1.4 -- 4.4 -- 22.4 8.0 1.7 2.3 27.7 9.8 1.6 3.5 0.6 32.9 20.6 13.7 4.7 0.0 0.3 1.7 0.8 4.3 0.2 33.9 7.4 27.6 4.8 34.6 16.7 14.2 1.1 16.7 0.6 11.2 2.2 1.6 -- 16.4 -- 4.3 0.6 0.3 -- 7.5 -- 0.8 -- 26.2 20.0 13.9 2.6 0.0 0.0 28.3 6.7 13.3 8.3 6.7 -- 31.4 21.7 14.1 3.1 0.3 0.1 5.7 0.2 3.3 -- 3.5 21.9 3.9 _ _ 1.9 31.8 9.4 2.2 1.5 9.6 15 0.7 0.3 29.2 6.8 0.3 0.1 9.4 1.4 0.3 0.1 18.7 4.3 23.6 14.0 30.8 5.9 17.0 10.4 1.1 8.4 0.7 12.6 23 0.2 4.6 0.1 14.3 0.9 0.4 1.9 -- 16.9 5.1 8.5 -- -- -- 29.0 16.0 8.4 6.3 2.2 0.2 -- -- 15.4 1.8 0.2 -- 8.0 1.6 0.2 -- 4.7 4.2 4.7 2.6 499 129 0.(3 0.2 0.1 -- 368 -- -- -- -- 165 5.5 0-5 -- -- 311 -- -- -- -- 63 0.4 -- _ _ o.a -- -- -- 2.5 1.7 0.6 -- -------- 261 72 278 140 382 206 _ _ _ 204 -- -- -- -- 144 4.6 0.3 0.5 0.2 451 -- -- -- -- 138 0.7 -- 0.1 -- 396 0.1 -- 0.1 -- 383 1.5 _ _ _ 293 -- -- -- -- 202 -- -- -- -- 227 -- -- -- -- 144 O.f 0.3 -- -- 313 155 __ __ 1.7 -- 0.1 -- ---- ---- 0.3 -- ---- ---- -- 5.9 2.7 1.0 _ 0.5 -- -- -- -------- 0.7 -- -- -- -------- 143 90 385 298 216 216* 394 81 296 229 282 157 125 -- -- -- -- 97 3.6 l. 2.C 3.1 581 -- -- -- -- 102 -- -- -- -- 152 ---- - 154* Snow Melting 691 Table 1 .... Data for determining Operating Characteristics of Snow-Melting Systems*- 1 {Concluded) Period of No SnowfaO Period of Snowfall* Air Temperature, F* Hour* of snowfall* Required output, Btu per (sq ft) (hr)4 Maxi- to to to toCity Below Over 32 or equal to 32 % of winter hours with no freezing period* Wind speed freezing period,* mph Per cent Hr per year < 2 a O to 49 50 to 99 100 149 150 199 200 249 250 to 299 300 to 349 350 399 400- Bto7* up (sq ft) Frequency distribution of snowfall boars at snow at above temperature* e above outputs, percent* Sault Sbe. Marie, Mich.. 68.7 60.0 21.3 30.4 69.2 25.0 18.6 11.5 0.9 33 n 42.9 31.4 16.7 852 11.6 2.6 7.1 0.6 1.9 - - - - 225 152 10.9 1.5 54 44.9 31.9 12.7 7.6 2.2 0.7 - - - 286 93.5 6.2 0.3 120 9.4 9.5 345 h \o 45.7 97.9 32.8 14.3 2.0 0.1 5.7 1.4 0.1 - - 262 144 Seattle \ Wa#sh............... 1 acomaj Washington, D. C.......... 88.0 48.5 77.9 21.2 26.8 5.9 1.2 44 /1 86.3 91.0 12.3 8.1 1.4 0.9 137 128 10.7 5.4 196 h 62.6 92.0 28.7 .7.4 7.8 0.2 1.1 2.0 -- -- -- - 205 127 9.6 0.9 33 a \o 59.0 85.7 29.8 10.6 11.8 2.5 0.6 154 121 * From Air Conditioning, Heating and Ventilating, August, 1957, p. 87. > The period covered by this table is from No-1 to Hatch >1, tad. with February taken aa a Z&M day month. Total hoot* in period -- S630. * The pereeatace to Cotumna S and 3 plus the percent under bouts of snowfall total 100 potent. Note that Boor* /SttovfaB does not include idling time, and b not actual operating time. 8eeteact. * Snowfalls ol trace amouutaarenot included; heaoe, Hmn ofBnxneJaU indudee only those hours of0.01 inches water equivalent per hour snowfalL < Out put does not include allowance for beck or edge losses date these depend on dab construction- Percentages total 100 percent of the numbo* of boon of snowfall. * Frsejuv Period is that during No Snowfall when tile air temperature b 33 F or below. * When heat output for A, -- 0 equals or exceeds the heat output for A, " 1, the beat transfer ft b from the air to the dab. This occurs when snowfall b at temperatures above S3 F. The sensible heat, q,, to raise the temperature of the snow to 32 F is ~~ g. = 2.6* (32- U :...(if uhere t = rate of snowfall, inches of water equivalent per hour. t, = air temperature, Fahrenheit. The heat of fusion, qm , to melt the snow is qm - 746g (4) The heat of evaporation, 9, ',' (mass transfer) is tcAere 9. " AM0.0201b + 0.055)(0.185 - p,,) (5) A/( = heat of evaporation at the film temperature, Btu per pound. v ** wind speed, miles per hour. pn = vapor pressure of moist air, inches of mercury. The heat transfer, 9* , (convection and radiation) is where 9 - 11.4(0.0201* + 0.055)(</ - O (6) t/ " water film temperature, Fahrenheit. In addition to determining the four heating requirements, it is necessary to make allowance for back and edge losses. These losses vary from 30 to 50 percent, depending upon the slab construction and fluid temperature. The equation for the required fluid temperature to provide an output 9 has been derived in Reference 2. For construc tion similar to Fig. 1, the equation is t- - O.59. + (, (7) where tm =* mean fluid temperature (antifreeze solution), Fahrenheit. Equation 7 will apply to 1 in. as well as ^ in. IPS pipe-- see Fig. 1. Equations 2 to 7 permit the designer to determine the heating requirement of a snow-melting system. The solutions of these equations, however, require the simultaneous con sideration of the four climatic factors: (1) wind speed, (2) air temperature, (3) relative humidity, and (4) rate of snowfall. It is not satisfactory to use annual averages or marimnma for the climatic factors. If averages or marimnms are used there is no assurance that they will ever occur simultaneously. It is necessary, therefore, to make a frequency analysis of the solutions to Equation 2 for all the occurrences of snowfall for a period of several years. Such an analysis for 33 cities appears in Table 1 which contains the operating information for a snow-melting system. For freezing temperatures (32 F and below) without snowfall the system may be idling which means that some heat is supplied to the slab so that there wtll.be immediate melting when snow starts to fall. Column 4 of Table 1 gives the average temperature during freezing periods. This temperature is used in calculating the idling load. The other term needed to calculate idling load is the wind speed during the period of freezing temperatures. The column, Hours of Snowfall, indicates the number of hours that snow is falling at rates equal to or greater than 0.01 in. of water equivalent per hour. There are snowfalls of