Document zoY9R0Q5pEKenp2886rg5rkGa

590 CHAPTER 24 1956 Guide qt = total heat requirement (slab output, q0, plus back and edge losses) Btu per (hour) (square foot). Co = specific heat at temperature to, Btu per (pound) (Fahrenheit degree). A, = temperature drop through circuit, Fahrenheit degrees. For a temperature drop of A( = 20 F, Equation 15 becomes G Apgt 160woCo (16) where G = gallons per minute. ton -- mean specific weight at temperature (o, pounds per cubic foot. INSTALLATION There are certain precautions that must be taken during installation. They concern internal corrosion, flammability, toxicity, cleaning, joints, Tabu: 6. Viscosity Effect on Centrifugal Pump Characteristics ssu*Kinematic Viscosity Pump Head Correction Factors Pump Capacity Pump Efficiency 0 to 30 50 100 200 300 400 500 600 700 800 900 1000 1.00 1.00 0.98 0.96 0.95 0.93 0.92 0.91 0.90 0.89 0.88 0.87 1.00 1.00 1.00 0.98 0.97 0.96 0.96 0.95 0.94 0.94 0.93 0.92 1.00 0.94 0.88 0.79 0.73 0.68 0.65 0.62 0.59 0.57 0.55 0.53 ,*J38U = Universal Sayfoolt second. and hookup. A comprehensive discussion of these precautions may be found in Reference 9. Safety Since ethylene glycol and petroleum distillates, are slightly toxic, the system should be installed ana maintained independently. There should be no permanent connection between the snow melting system and the drinking water supply. Ethylene glycol is not considered flammable. In fact, aqueous solu tions of less than 60 percent glycol 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 drip ping from the seals on 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. Panel Heating 591 There are other non-flammable fluids, such as those .used in some trans formers, that can 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. Internal Corrosion Ethylene glycol solutions tend to become corrosive in service-, therefore, rust inhibitors are generally included. Even with an inhibitor, the solu tion 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 heat exchanger surfaces should be kept below 285 F which corresponds to about 40 psig steam. Tempera tures above 300 F accelerate the deterioration of the inhibitors. Slab Construction It has been found satisfactory to use % pipe or tubing on 12-in. centers as a standard coil. If pumping loads require a reduction in friction the pipe! size may be increased to 1-in., but the slab depth must be increased ac cordingly. - - ___ The piping should be supported in such a manner that there is a minimum of 2 in. of concrete above and below the pipe. This requires a 5-in. slab for %-in. pipe and 5M-in. for 1-in, pipe. If an insulating material is used, 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 should be mopped, and the fill-made, 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 tiie ends are sealed. See Fig. 16. If the pipe must pass through an expansion joint, a protective 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 ex ternal corrosion problems will occur. > " Thermal Stresses This problem is discussed in Reference 11. In general, there will be no ill effects from the thermal stresses: if these rules of installation and opera tion are followed: 1. Keep the temperature difference between the fluid and the slab surface to a minimum by: . a. Close pipe spacing (see Fig. 16). . .- b. Low temperature drop in fluid, Af < 20 F deg. c. Continuous operation (if economically feasible). 2. Keep pipe near surface--about 2 in. cover. ! 3. Use reinforcing steel designed for thermal stress if high structural1 loads are expected (such as on highways).. . ..... Testing After installation and before pouring concrete, all piping should be tested to about-100 psi. This pressure should be maintained until all welds and connections have been checked for leaks. If an oil is used as an anti freeze, the test should be performed with air or some gas (only 50 psi re quired with an air test) but not with water. The danger of using water is that the pipe may not be thoroughly dried when the oil is introduced.