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re: aluminum foil heat exchanger
18 dec 1997
don lancaster wrote:
>nick pine wrote:
>> cometx wrote re:
>> >...a heat exchanger made of... regular kitchen alumunum foil... a box
>> >the size of an 8ft high clothes closet... about 20 rows of the foil
>> >hanging down... 2'wide x 8'high x 20layers=320sq. ft. of surface...
>seems to me most of the delta-t would be across the foil, rather than
>between foil and medium.
well, the slowly-moving air film resistance on each side of the foil
might be about us r3, ie a thermal conductance of 1.9 w/m^2-c, and
aluminum has a thermal conductance 211 w/mc, so 0.001" of foil has a
conductance of 211/(0.001/0.0254) = 5360 w/m^2-c, about 3,000 times
more conductive than the air films. i think polyethylene film would
work better than foil here, since it would have an air film resistance
closer to us r2/3, and a lower thermal conductance, but still much
higher than that, so the overall series thermal resistance would be
almost all due to the air films. poly film is also 3-4x cheaper, and
it comes in larger widths.
william l. bahn wrote:
>...the aluminum foil is a physical barrier between two air streams,
>so that there is air at one temperature on one side and air at another
>temperature on the other side.
sounds good to me.
>the way i envision this is to take a bunch of sheets of foil...
or poly film...
>and space them parallel to each other. then through half of the spaces
>you pass your high temperature air (say from left to right)
or bottom to top...
>and you pass your low temperature air (probably from right to left...
or top to bottom...
>through the other half of the spaces.
sounds good to me. if warmer air moves upwards and cooler air moves
downwards, this may not even need a fan.
>...i'm curious how the efficiency of a heat exchanger is defined.
>i can imagine that, in theory, the outlet temperature of each stream
>could approach the inlet temperature of the other...
a counterflow heat exchanger with 100% efficiency might have equal
volumes of 30 f outdoor cold air emerging at 70 f into a room and
70 f room air flowing outdoors at 30 f. this is close to achievable
with a large heat transfer area and a small airflow rate.
nick
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