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840 CHAPTER 76 1962 Guide And Data Book '' between the snow-melting system and the drinking water supply. Ethylene glycol is not considered flammable. In fact, aqueous solutions of less titan 60 percent glyool are used for fire sprinkler systems. These solutions do not freeze, and they are effective fire extinguishing agents. Petroleum distillates suitable for fluids in snow-melting systems are classified as non-flammable, but have fire points between 300 and 350 F. When using fluids of this type, care should be taken to collect any oil dripping from the seals on {For Nfi, > 3000) (6ow<f <> date gimo in Rsforooc* 6) Fig. 3 .... Friction Correction Foctor for Viscosity the pump. It is good practice also to provide some barrier between the oil lines and the boiler so that in the event of a leak, a flash back from the boiler will not ignite the oil. There are other non-flammable fluids, such as those used in some transformers, that ran be used as the antifreeze. These fluids are three or four times as expensive as the glycols or oils, but combine the fire protection of the glycols with the corrosion protection of the oils. Interna! Corrosion Ethylene glycol solutions tend to become corrosive in service; therefore, rust inhibitors are generally included. Even with an inhibitor, the solution should be tested annually to determine any change in acidity. If the test indicates that the inhibitor has been exhausted, the entire system should be drained and a fresh solution installed. To increase the life of the inhibitor, the beat pTr.hAnger surfaces should be kept below 285 F which corresponds to about 40 psig steam. Temperatures above 300 F accelerate the deterioration of the inhibitors. Slab Construction It has been found satisfactory to use %-in pipe or tube on 12-in. centers as a standard oil. If pumping loads re- quire a reduction in friction the pipe size may be increased tn 1-in., but the slab depth must be increased accordingly. * The piping should be supported in such a mftnner ^ there is a minimum of 2 in. of concrete above And below the pipe. This requires a 5-in. slab for n. pipe and fo 1-in. pipe. If an insulating material is used beneath the structural slab, it is generally good practice to provide a moisture barrier between the insulation and the fill. A roofing material (such as a 55 lb felt) is often used as a moisture barrier. The joints in the barrier should be mopped, and the fill smooth enough so that there will be no holes or gaps for moisture transfer. Also, the edges of the barrier should be flashed to the surface of the slab so that the ends are If the pipe must pass through an expansion joint, a pro tective coating should be applied to the pipe for a foot or two on both sides of the joint. If the pipe is kept dry at all times in both summer and winter, no external corrosion problems will occur. Thermal Stresses This problem is discussed in Reference 7. In general, there will be no ill effects from the thermal stresses if these nils of installation and operation are followed: 1. Keep the temperature difference between the fluid and the slab surface to a minimum by: a. Close pipe spacing (see Fig. 1). b. Low temperature drop in fluid, at < 20 F deg. c. Continuous operation (if economically feasible). 2. Keep pipe near surface to obtain about 2 in. cover. 3. Use reinforcing steel designed for thermal stress if high structural loads are expected (such as on highways). Control Normally snow-melting systems are manually controlled. They are started by an operator when snow is anticipated or is falling, and they are shut off when the snow has stopped falling and the area is clear. Thb b strictly two-position control either 0 or 100 percent operation. One version, with some degree of control, provides a surface-temperature control. The system b started manually, as before, but a thermostat controls the heat input to maintain a surface temperature of approximately 33 F. As stated previously, a low-limit thermostat should be set to prevent the antifreeze solution from cooling to a temperature where the kinematic viscosity would be excessive. .There are several fully automatic control systems, how ever. In general, these systems provide some means of detecting precipitation and simultaneously sensing air temperature. One method b to set a thermostat for a 35 F air temperature and arrange to have the signal from this thermostat activate a heating element on a receptacle. Thb warm receptacle collects the precipitation, and melts it if it b frozen. The water then runs through a tube and activates another circuit. Thb second circuit can be used to start the snow-melting system. Another thermostat can be used to control tiie surface temperature of the snow-melting slab. -For basic information on control equipment, such as thermostats, and for fundamentals of control circuits, see Chapter 13, Automatic Control, of the 1961 Guide And Data Boos. Testing After installation and before pouring concrete,* all piping should be tested to about 100 prig. Thb pressure should be SSnijW Melting - maintained until all welds and connections have been checked 'i^for If an oil b used as an antifreeze, the test should be nerformed with air or some gas (only 50 pri required with an - `&ir test) but not with water. The danger of using water b that the pip Qt tie thoroughly dried when the oil b intro duced. The water will tend to collect, and when the tempera- ff>H below 32 F the collected water may freeze and Draining proper drainage b necessary for both the slab surface and the coilThe slab must be sloped so that the water from the melted enow can run off. Puddles are objectionable on sidewalks or drives. They cause splashing, and they retard heat flow from the embedded pipes. The pipes must be placed so that they may be drained. If the antifreeze becomes corrosive it must be drained. It b not good practice to rely entirely on blowing of air through the system. Drifting Snow It b quite likely that some drifting will occur on every system that b adjacent to a wall or vertical surface. Tbe deigner should try to anticipate thb condition and add extra piping in these areas. If possible, coils should be added in the vertical surface. Another expedient b to carry the drainage to the area expected to be drifted. The drainage will tend to wash away some of the snow. Design Example Example S: Assume that there are three independent hy draulic systems to be used in the snow-melting application described in Example 1. One system will be for the Class I area, another for the Class.II, and the third for the emergency area, or Class III area.. Determine tbe hydraulic requirement for the Class II sys tem, assuming an area of 100,000 sq ft. Solution: The heating requirement has been determined as 160 Btuh per sq ft. Since an area of thb type has a large per imeter, imiiim the edge and back losses add about 40 percent or 64 Btu per sq ft. The total heat requirement, then, will be taken as 224 Btuh per sq ft or 22.4 X 10* Btuh for the 100,000 eqft area. The fluid temperature is determined by using Equation 7: U = 0.5 q. + If q. * 160 Btuh per sq ft, and tf = 33 F tm - 0.5 X 160 + 33 = 113 F. If an oil antifreeze is used, the specific beat. Cm , tbe kine matic viscosity , and the specific weight to* , at 113 F can be found by interpolation in Table 6 as Cm -- 0.440 Btu per (lb)(F deg) = 4.30 X 10~* sq ft per sec. . An approximate determination of viscosity can be by interpolation in Table 6, but an accurate value requires the Uae of ASTM Standard Ktsctmly Chart B. uw = 59.4 lb per cu ft Then from Equation 12 r Apq, 10* X 224 ~ 160wj*c_ " 160 X 59.4 X 0.440 " 0,10 Equation 12 is based on a temperature drop of 20 F deg, therefwe, the fluid must enter the slab at 123 F and leave at 103 F in order to have the desired mean temperature of 113 .F. 84! Assume that the coil with the longest equivalent length is 300 ft and has a flow of 12 gpm. Such an arrangement would suggest the use of a coil of 1 in. IPS. From Table 1, Chapter 50, olthe 1961 Goide And Data Boos, the ID " 1.049 in. " 0.0875 ft, which gives an internal cross-sectional area of 0.00G00 sq ft. For 12 gpm the flow is 0.0268 cu ft per sec and the velocity is 0.0268 0.00600 4.47 fps and Reynolds number, Nb*, is VD 4.47 X 0.0875 0m 4.30 X 10- From Fig. 19, Chapter 7 of the 1961 Guide And Data Book, using a roughness factor e = 0.00015, whence e/D ~ 0.00172, the motion rector / is found to be 0.034. Substituting in Equation 10 yields LV7 300 4.47* -- - 0.034 X * D2g 0.0875 X 2 X 32.16 15.1 psia The friction loss could have been approximated two ways: (1) by using Fig. 2, and (2) by using Fig. 3 of thb chapter in conjunc tion with Fig. 5 of Chapter 8. In order to use Fig. 2, the viscosity of the oil must be known in SSU. As given above the kinematic viscosity of the oil at 113 F is 4.30 X 10"* sq.ft per sec. From Table 8, 4.30 X 10"* sq ft per sec b equal to 39.1 SSU. Entering Fig. 2 at 39.1 SSU and reading friction loss for 12 gpm gives 12.8 ft per 100 ft. For a 300-ft coil thb b 38.4 of fluid. Thb compares fairly well with the more exact figure of 36.6 ft as found by using the Fanning equation. If the friction loss were approximated by Fig. 3 and Fig. 5 of Chapter 8, the result would have been 40.5 ft which b deter mined as follows: From Fig. 5 of Chapter 8, the loss for water flowing at 12 gpm in a one-inch pipe b 1200 milinches per foot or 10 ft per 100 ft. In thin case, then, the loss for water would be 30 ft. The correction factor for oil as compared to water is given in Fig. 3 as 1.35. The friction loss then would be 1.35 X 30 or 40.5 it. Both of the approximation methods are conservative, but for large systems, thb conservative approach may be costly. The pump must deliver 5380 gpm against a 14.$ psia head. The heat exchanger must deliver 22.5 million Btun with an inlet temperature of 103 F and an outlet temperature of 123 F. Actually some allowance should be made for the temperature drop in the piping between the slab and the heat exchanger. Both the pump and heat exchanger manufacturers should be advised that the fluid is an oil and will operate over a wide temperature and viscosity range. If a low-limit thermostat b placed in tbe fluid line so that the'minimum fluid temperature is 5 F, it can be seen from Tables 6 and 7 that the viscosity can reach about 300 SSU. From Table 8, it can be found that such a viscosity can reduce the pump head 5 percent and the capacity 3 percent. This means that the pump should be rated 14.8 5380 at ------- 15.6 psia and------ --- 5550 gpm. 0.95 0.97 The expansion tank should be designed in accordance with the procedure outlined In Chapter 8, Hot Water Heating Systems. The amount of expansion should be based on the rihn-ngn in volume from the low temperature limit to tbe high temperature limit. Assuming the temperature range to be 62.4 - 59.1 5 to + 123 F, the expansion would be------------------- 5.6 59.1 percent of the total volume in the system. REFERENCES 1 W. P. Chapman and S. Katunich: Heat requirements of snow melting systems (ASHAE Transactions, VoI. 62, 1956, p. 359).