04_thermal

Styrofoam 3 winter 2008 4 1 thermal considerations

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Unformatted text preview: 0.03 can be better than air! Styrofoam 3 Winter 2008 4 1 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Conduction: Heated Box A Cold Finger • A 1 m 1 m 2.5 m ice-fishing hut stands in the 10° 2.5 10 C cold with 2.5 cm walls of wood • Imagine a plug of aluminum connecting the inside to the outside – how much will it change the story? – cylindrical shape, length t, radius R – = 205 W/m/K – just based on conduction alone, since difference in thermal conductivity is a factor of 7000, the cold finger is as important as the whole box if it’s area is as big as 1/7000 the area of the box. – A = 12 m 2 – t = 0.025 m – 1 W /m/K • To keep this hut at 20° C would require would P = · T ·A/t = (1.0)(30)(12)/(0.025) = 14,400 W – Outrageous! – Replace wood with insulation: = 0.02; t = 0 .025 P = · T ·A/t = (0.02)(30)(12)/(0.025) = 288 W – This, we can do for less than $40 at Target • this corresponds to a radius of 2 mm !!! • So a cold finger can “short-circuit” the deliberate short-circuit” attempts at insulation – provided that heat can couple to it effectively enough: this will often limit the damage • First example unfair – air won’t carry heat away this fast: more on this later Winter 2008 5 Winter 2008 6 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 R-value of insulation Wikipedia on R-values: • • In a hardware store, you’ll find insulation tagged with you’ an “R-value” R-value” Note that these examples use the non-SI definition and are per inch. Vacuum insulated panel has the highest R-value of (approximately 45 in English units) for flat...
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