Document e3BVnMrn6d87qZ8NDX92XvG4

676 CHAPTER 49 1959 Guide Snow Melting 677 the footnotes to Table 3. Briefly, snow-melting systems are classified as to the urgency for melting as follows: Class I (minimum): Residential walks or driveways nH interplant areaways. Class 11 (moderate): Commercial (stores and offices) side walks and driveways, and steps of hospitals. Class III (maximum): Toll plazas of highways and bridges, and aprons and loading areas of airports. These classifications depend upon the allowable rate of snow melting. For example, a residential system does not have to melt snow as rapidly as a commercial system. In fact, a depth of snow of an inch, for an hour or so during a heavy storm might not be objectionable with a residential system. On the other hand, a store manager would consider tiie system inadequate if half an inch of snow accumulated on the sidewalk in front of the store. The difference, then, between a Class I system and a Class U system is in the required ability of each system to melt snow. The one feature that is common to' all classes is that the systems must be adequate for some combination of weather factors. The this classification. Design rates may be altered by the de signer if he feels that a particular job should have different design criteria from those given in the footnotes of Table 3. Any change in design conditions used should bebased on the frequency distribution given in Table 1. Use of Tables 1, 3, and 4 is illustrated by Example I. Example t: An engineer has been retained to design snowmelting systems for the service areas of a turnpike running from the eastern edge of the Wisconsin-Illinois border north west to the Wisconsin-Minnesota border just east of St. Paul. He decides that Chicago data wifi be adequate for the southern terminus and that Minneapolis-St. Paul data will be adequate for the northern terminus. His problem is to determine the heat and hydraulic requirements of the systems for service areas between Chicago and St. Paul. Solution: Assume, for this example, that the city in question is Madison, Wis. Weather bureau records indicate that the annual average number of days with snow cover of an inch or more would be 100, and that the engineer assume an aver age snowfall of 40 inches. In addition, he can estimate about II days per year with a snowfall of an inch or more (see Refer ence 3). quate for A, - 1 for 99.4 percent of the time. Similarly, 275 would be adequate 99.4 percent of the time in St. Paul. There fore, 275 Btun per sq It seems sufficient for the emergency areas. Table 3 in Class III column lists 350 Btuh per eq ft for Chicago and 254 for St. Paul but for uses similar to the areas in this example, 275 should be adequate. Hydraulic Requirement After determining the heating requirements, it is necessary to determine the hydraulic requirements of the system. This can be done by means of the procedures explained in Chapter 4, Fluid flow, but it is necessary to use the proper physical properties of the antifreeze solution. A complete discussion of tiie hydraulic problem is given in Reference 4. The main consideration is the proper allowance for vis cosity. Table 5 gives viscosities for typical fluids used as Table 6 .... Conversion of Kinematic Viscosity Units* Cen&tofcc* IFF/Sec) X 10* ssu>> (FF/Sec) X1 CentfxtoJtes 10* 1 SSU** antifreezes for snow-melting systems. Viscosities are given in feet squared per second. Table 6 can be used in conversion of viscosity units. A large increase in viscosity will be noted for glycols and oils--about 20 times--as the fluid temperature changes from 160 F to 0 F. This viscosity change has two effects, first, an increase in viscosity mil increase the fluid friction in the piping circuit. Second, an increase in viscosity will decrease the pump capacity--in both volume and head. The effect of viscosity on fluid friction in the piping circuit is illustrated in Fig. 2. For large installations, the friction losses should be cal culated by the Fanning equation where hf = the loss in head of the fluid under conditions of flow, in feet. I -- the length of the pipe, in feet. designer may select equipment having capacity to melt snow whenever conditions are milder than some critical values, but be willing to have an inadequate system for a given fraction of the time. In other words, the designer will take a calculated For the walkways to the restaurant from the parking area a Class I design rate could be used. This rate could be taken as 90 Btuh per sq ft. This is in good agreement with'data in Tables 3 and 4 which give the rate at 89 (design rate) for Chicago and 95 (idling rate) for.Minneapolis. risk providing he knows the odds of that risk. For a residential The lanes leading from the turnpike to the gasoline pumps system, where initial cost must be at a minimum, the designs- and parking areas should be rated as Class II areas. A check must accept more frequent snow accumulations. of Tables 1 and 3 would indicate that 160 Btuh per sq ft would Table 3 contains the design heat requirements for the 3 be adequate. classes of snow-melting systems. Under Class I systems, the If an emergency area were included, for a wrecking truck, values in parentheses are idling rales and, since they exceed ambulance, or police garage, it would be wise to consider a Class III rate for such areas. An inspection of Table 1 for i, the Class I design rates, should be taken as design output for Chicago shows that a rate of 275 Btuh per sq ft would be ade- Table 5 .... Physical Properties of Antifreeze Solutions* 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10 2.15 2.69 3.23 3.77 4.30 4.84 5.38 6-46 7.53 8.61 9.68 10.8 32.6 36.0 36.0 37.6 39.1 40.8 42.4 45.6 48.8 52.1 55.5 58.9 31 32 33 34 35 36 37 38 39 40 33.4 34.4 35.5 36.6 37.7 145.7 150.2 154.7 159.2 163.7 38.7 39.8 40.9 42.0 43.0 168.2 172.7 177.3 181-8 186.3 V " the velocity, in feet per second. g *- the acceleration due to gravity = 32.174 ft per (second) (second). D " the internal diameter of the pipe in feet. / = a dimensionless friction coefficient which can be de termined from Ftg. 4, Chapter 4. The Reynolds num ber can be computed from data in Table 5 of this chap ter. Solutions for the pipe friction should be plotted for tem peratures at the starting condition (probably 0 F) and at the operating condition (use either 120 or 160 F). Then on the same graph, the operating curve of the pump should be plotted (see Reference 4 for such a graph). The intersection of the fluid friction curve and pump operating curve will give Temp, F -20 0 20 40 120 140 160 200 the operating point for the system. Table 7 can be used to 11 11.8 62.4 41 44.1 190.8 allow for the viscosity effect on the pump. 12 12.9 66.0 42 45.2 195,3- The designer must decide on the tolerable viscosity limit. 13 14.0 69.8 43 46.3 199.8 Generally it is between 300 and 500 SSU, although for 14 15.1 73.6 44 47-3 204.4 Ethylene Glycol 15.3%byvol. +20 V X 10* _ -- _ 2.64 0.840 0.687 0.577 0.45S -- -- -- 0.935 0.956 0.960 0.962 0.969 to -- -- -- 64.0 62.8 62.5 62.1 58.8 15 16.1 77.4 45 48.4 209.1- 16 17.2 81.3 46 49.5 213.7 17 18.3 85.3 47 50.6 218.3 Ethylene Glycol 31.4% by vol.- 0 * X 10* __ 6.86 4.18 1.19 0.955 0.784 0.609 c -- -- 0.833 0.850 0.895 0.905 0.910 0.923 to -- -- 65.7 65.4 64.1 63.7 63.3 60.0 18 19.4- 89.4 48 51-6 222.9 19 20.4 93.6 49 52.7 227.5 20 21.5 97.8 " 50 53.8 232.1 Ethylene Glycol 42.7% by vol. Ethylene Glycol 51.2% by vol. Heat Transfer Oil -20 w X 10* _ 16.1 9.47 5.75 1.46 1.20 0.950 0.748 c -- 0.764 0.775 0.788 0.832 0.845 0.856 0.884 tr " 66.8 66.7 66.3 65.3 64.9 64.4 60.6 -40 w X 10* 46.3 21.3 12.13 7.54 1.77 1.46 1.14 0.870 c 0.682 0.717 0.726 0.745 0.809 0.823 0.835 0.854 to 68.2 67.6 67.4 67.2 65.9 65.4 65.0 61.1 -40 y X 10* 105 43.1 29.7 14.0 3.13 2.51 . 2.06 1.48 c 0.382 0.390 - 0.400 0.408 0.444 0.452 0.462 0.480 w 62.9 62.5 62.0 61.6 59.1 58.6 57.6 55.3 21 22.6 102.0 55 59.2 255.2 . 22 23.7 106.4 .60 64.6 278.3 23 24.7 110.7 65 69.9 301.4 24 25.8 115.0 70 75.3 324.4 25 26.9 119.3 26 28.0 123.7 27 29.1 128.1 28 30.1 132.5 29 31.2 136.9 30 32.3 141.3 Water +32 r X 10* -- _ 1.71 0.603 0.494 0.413 0.328 c -- -- -- 1.005 0.999 0.999 1.001 1.005 to -- -- -- 62.4 61.7 61.4 61.0 60.1 * Baaed on data fivea in Reference S. * -- Kinematic vfeeceity, (feet squared per second) (*. for ofl at 10 F, - 0X00140 ft* per tee.}. e - apedfie best, Btu per (pound) (Fahrenheit <fe*ree). w.- apedOo weight, pounds per cubic foot. Over 70X ccstfetokei, SSU - 4.835 X eeaUrtokee. * CMnath TMautjr ia ftVtec 1X76 X 10 1 X MtblAka. b Values listed far 8SU (Seybolt Secauda--Universal) are for fluid tempera* tores of 100 F. To obtain tbs Saybolt Universal viscosity equivalent to a kine matic visaoeity determiiMd at a Fahrenheit temperature (, multiply the equiva lent Saybolt Universal visoeeity at 100 ? by.l + (1 - 100) 0X00064; for example, 10 eentistekas at210 Pareequivalent to 68-9 X 1XC7D or 59Jsoc Saybolt Universal t 110 F. (Taken from ASTU D 448 - 61).. (For I-in. Fipe) Rg. 2 .... Effect of Viscosity on Friction Loss pcerasaowsss