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re: aluminum foil heat exchanger
18 dec 1997
i wrote:
>...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.
whoops. that's 211/(0.001x0.0254) = 8,307,086 w/m^2-c, 4 million 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...
the thermal resistance through the heat transfer surface might be about
1.2x10^-7 + 2(0.53) = 1.06 w/m^2-c for shiny aluminum foil,
1.2x10^-7 + 2(0.117) = 0.23400012 for black aluminum foil, and
1.8x10^-4 + 2(0.117) = 0.23418 for polyethylene film...
(lower thermal resistance makes better heat exchangers.)
>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.
this might be built into a closet on an outside wall, with a couple of
50 cfm fans or dampers that open on a sunny day, when the humidity in
an airtight house is too high... it might look like this:
4' 4'
---------------------- |cold | 1" | |cold |
i rising house air--> o |air | foam | |air |
n ---------------------- u |enters|spacer| |leaves|
d <--falling outdoor air t |house | ? | . |house |
o ---------------------- d | | | . | | 8'
o --> o | | | . | |
r ---------------------- o |warm | | |warm |
s <-- r |air | | |air |
---------------------- s |leaves| | |enters|
--> |house | | |house |
---------------------- | | | | |
...
[from the top] [from indoors]
nick
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