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re: heated sidewalk btu calculation
17 jul 1997
wrote:
>we are considering putting a heating device under a concrete sidewalk to
>melt ice build-up, and am wondering how many btus will be necessary...
that depends on the amount of ice. it takes about 144 btu to melt a pound,
and water weighs about 64 lb/ft^3, and snow might weigh a tenth of that, so
melting 1" of ice or 10" of snow on a square foot of sidewalk might take
144x64/12 = 768 btu, if you were not trying to heat the rest of the planet.
>here are the details: the concrete is four inches thick, the square
>footage of sidewalk is twelve...
a twelve square foot sidewalk? with a heat capacity of about 12x25x4/12 =
100 btu/f, and a thermal conductance of about (2+10/2)x12ft^2 = 84 btu/h-f,
in an average 10 mph wind, and an rc time constant of 100/84 = 1.2 hours,
not counting the soil below. the average windspeed in phila in january is
about 10 mph, and the average temperature is about 30 f.
>our idea is to keep the heating device running while our area goes through
>its daily freeze-thaw cycles for about 1 month per year. we hope to keep
>the sidewalk ice-free...
how about just turning on the melter when ice starts to form, since this thing
would only have a 1 hour time constant, with some insulation underneath? or put
a white wall to the north and paint the concrete dark to reduce wind and snow
accumulation and help collect more sun, about 1,500 btu/ft^2/day where i live,
in january. this might melt about 1500/768 = 2" of ice per day...
nick
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