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HI- 622 CHAPTER 23 1957 Guide ^ Step 10. Total Panel Output Follow the procedure for slab-on-grade construction. Step 10, substituting heat flow downward (qs) for the apportioned downward and edgewise heat flow Step 11. Fluid Circuit - Follow the procedure for slab-on-grade construction, Step 11. Step 12. Boiler Size Follow the procedure for slab-ori-grade construction, Step 12. Installation Details, Accessories and Controls The design of the two-pipe, direct return forced circulation system which is used in a panel heating system is described generally in Chapter 21 Hot Water Heating Systems. In addition to the usual valves, a balancing valve should be installed in the supply or return connection of ea,ch panel in order to permit the heat output of the panel to be balanced with other panels in the same zone. While all piping should be designed to minimize trapping of air or water, few panel heating systems can be installed to be completely free of air traps. For that reason, panel heating system's cannot usually be filled by flooding nor drained by gravity alone. The system can be filled and the air removed if the system is flushed with a sufficient flow of water. To empty the system a flow of compressed gas is used to assist gravity drainage of the water. Provisions for both flushing and blowing should be included in the piping. Devices for the collection and venting of the gases that will appear during operation should also be provided. The control of panel heating systems is discussed in Chapter 38 Auto matic Control. Warm Air Panels The first three steps in the design of warm air panels are the same as those outlined for warm water panels and the same performance curves can be used. The balance of the design can be determined from the data in the Chapter on Forced Warm Air Systems and Manual 7-A of the National Warm Air Heating and Air Conditioning Association. Electric Panels Electric pane! heating systems can be designed by using part of the pro cedure for warm water panels as a guide. See also Chapter 41 Electric Heating. Step 1. Heat Loss Follow the procedure for warm water panels, Step 1. Step 2. Required Panel Output Follow the procedure for warm water panels, Step 2. Step S. Panel Surface Temperature Follow the procedure for warm water panels, Step 3. Step 4. Panel Heat Loss Determine the heat loss from the panel, using the panel surface temperature found in Step 3 and the factors in Chapter 9 Heat Transmission Coefficients of Buildmg Materials. Panel Heating 623 Step 6. Panel Rating Select a panel which has a rating at least equal to the required panel output, plus i the panel heat loss (3413 Btu = 1 kilowatt hour). If the selected panel has a rating that is appreciably higher than the required panel output plus the panel loss, the panel heat loss should be redetermined (Steps 3 and 4). If the required panel output plus the panel heat loss is greater than the ratings of available panels, a larger panel should be used, the heat loss of the room should be reduced, and in addition or alternately supplementary heating should be pro vided. Step 6. Installation and Wiring See Chapter 41 Electric Heating and manufacturers recommendations and in structions. SNOW MELTING The practicability of melting snow by means of heated coils has been demonstrated in a large number of installations in sidewalks, roadways, ramps, and runways. In addition to eliminating the need for snow removal, other advantages gained are greater safety to pedestrians and vehicles, and reduction of labor in removal of slush from floors. The design of a snow-melting system involves primarily (1) a determina tion of the heat requirement which depends on snow fall and atmospheric conditions, (2) the coil and piping design which depends on heat transfer from a panel and on friction loss in piping due to the circulating medium used, and (3) the selection or design of a heat exchanger for heating the cir culating medium. DESIGN Heating Requirements The heating requirements for snow melting are affected by four atmos pheric factors: (1) rate of snowfall, (2) air temperature, (3) wind velocity, and (4) humidity. The effects of these factors can be evaluated by con sideration of the action of snow falling on a warmed surface. The first flakes fall on a dry, warm surface, and are then warmed to 32 F and melted. The water from the melted snow soon forms a film over the entire area and starts to evaporate. The evaporation of the ffitn is a mass transfer from the surface to the atmosphere. In addition, there is a heat transfer from the film to the ambient air and surfaces. Both the mass and heat transfers attain a maximum only when the is. entirely free of snow. Theoretically this is impossible because some time will elapse while the snow is warmed to 32 F. For all practical Purposes, however, it is permissible to assume the snow to be melted as fast it falls so that the surface may be considered entirely free of snow. Hie ratio of free area to total area is known .as the free area ratio, RA. WheniJA = l, there is no snow on the surface and the mass and heat trans*er are at a maximum. When = 0, the snow covers the entire area and there is no mass or heat transfer. Research on the insulating effects of snow indicate that there are just three practical values for the free area ratio, RA; they are 1, 0.5, and 0. A general rule to follow is that emergency areas should be designed with yr = 1; commercial areas (e.g., sidewalks around stores, etc.) may be unsigned with RA = 0.5; and private driveways, or areas where low instalntion costs are imperative, may be designed with RA = 0. For a more