Design of a snow-melting system for sidewalks, roads, parking areas, etc., involves the determination of a design amount of snowfall, sizing and layout of piping, and selection of heat exchanger and circulating medium. The pipe is placed under the wearing surface, with enough cover to protect it against damage from traffic loads, and a heated fluid is circulated through it.
If friction is too high in extensive runs, use parallel loops (Art. 13.15). All precautions for drainage, fabrication, etc., hold for snow-melting panels as well as interior heating panels.
Table 13.13 gives a design rate of snowfall in inches of water equivalent per hour per square foot for various cities.
Table 13.14 gives the required slab output in Btu per hour per square foot at a given circulating-fluid temperature. This temperature may be obtained once we determine the rate of snowfall and assume a design outside air temperature and wind velocity. The table assumes a snow-melting panel as shown in Fig. 13.27.
Once the Btu per hour per square foot required is obtained from Table 13.14 and we know the area over which snow is to be melted, the total Btu per hour needed for snow melting can be computed as the product of the two. It is usual practice to add 40% for loss from bottom of slab.
The circulating-fluid temperature given in Table 13.14 is an average. For a 20F rise, the fluid temperature entering the panel will be 10F above that found in the table, and the leaving fluid temperature will be 10F below the average. The freezing point of the fluid should be a few degrees below the minimum temperature ever obtained in the locality.
Check manufacturers ratings for antifreeze solution properties to obtain the gallons per minute required and the friction loss to find the pumping head.
When ordering a heat exchanger for a given job, specify to the manufacturer the steam pressure available, fluid temperature to and from the heat exchanger,??gallons per minute circulated, and physical properties of the antifreeze solution.
