Snow Melting - Area Free Ratio

The Ar, or the Snow Free Area Ratio, is of importance to client expectation. The quantity of sensible and latent heat flux will determine the condition of the surface based on the following Ar criteria.

When Ar=1, there is to be no accumulation of snow.  Projects where this is critical include medivac applications, steep (sloped) access to parking facilities and bridge decks where loss of control over vehicles can lead to severe property damage or casualities. When Ar=0, there can be 100 per cent accumulation of sufficient thickness to prevent heat and evaporation losses. Projects where this is tolerable might be patios, decks, non-pedestrian walks and or level driveways in noncommercial or non-critical applications.

When Ar=0.5, there can be some accumulation in the form of slush but not ice. Projects where this is tolerable might be driveways, walks, steps and ramps.

The Snow Free Area Ratio is defined by:

Ar = Af/At, where

        Af = equivalent snow-free area, ft²
        As = equivalent snow-covered area, ft²
        At = Af + As = total area, ft²

Frequency Percentiles

Once the designer has determined what coverage is acceptable based on applications and the client’s needs, there must then be some discussion as to practical expectations of the system. Designers should then ask if it is fine to have some slush in the most severe conditions, which may only occur five per cent of the year. This magnitude of severity is described by frequency percentiles (Class I, II and III Methodology is considered obsolete). ASHRAE offers guidance for 75, 90, 95, 98, 99 and 100 per cent. By definition, the frequency indicates the percentage of time that the required snow melting surface heat flux does not exceed the design value for a given snow free area ratio. It is used to design a snow melting system for a given level of customer satisfaction depending on criticality of  function. For example, “...a heliport at the rooftop of a hospital may require almost 100 per cent satisfactory operation at a snow-free ratio of one, a residential driveway may be considered satisfactory at 90 per cent and Ar = 0.5 design conditions.”¹ The load calculations, equipment and controls designed for 100 per cent satisfaction will be different for the same application with 90 or 75 percent expectations. For example, a system designed for 100 per cent Frequency Percentiles may require 170F or 140F for 75 per cent. A non-condensing boiler with return temperature protection might be required in the 170F scenario while a condensing boiler would be used in the 140F scenario without worrying about return temperatures.

¹ ASHRAE, 2003 Applications Handbook, Chpt. 50, Snow Melting and Freeze Protection.   

Click here for part I, Introduction
Click here for part II, Manual Controls     
Click here for part III, Idle/On Controls & System Performance 
Click here for part V, Conclusion

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