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re: condensation on a/c ducts.
1 aug 2005
stretch wrote:
>the formula for the outside surface temp of a duct is:
>
>t(surface)= oat-(oat-iat)*osf/(isf+di+osf)
>t(surface) is the outside surface temperature of the duct
>oat is the outside air temperature...
>iat is the inside air temperature...
>osf is the outside surface film (r-factor)... about 1.67 for shiny new duct
>isf is the inside surface film (r-factor... about r-0.2
>di is the duct insulation r-factor, (about r-8 for your ducts)
oat = 90 f and iat = 40 f makes
t(surface) = 90-(90-40)*1.67/(0.2+8+1.67) = 81.54 f...
like this, viewed in a fixed font:
t(surface)
|
isf di | osf
iat ---www---www------www--- oat
<---------------------
i
heatflow i = (oat-iat)/(isf+di+osf) = (90-40)/(0.2+8+1.67) = 5.066 btu/h,
and t(surface) = oat-i*osf = 90-5.066x1.67 = 81.54 f.
wrapping it all with r19 fiberglass insulation and poly film duct might
decrease i to 1.732 and raise t(surface) to 90-0.67x1.963 = 88.87, while
reducing the rate of condensation (if any) and saving energy. exposing
ducts to more attic air ~~~wastes~~~ energy.
t(surface)
|
0.2 8 1.67 19 | 0.67
40 ---www---www---www---www------www--- 90
<---------------------------------
i = (90-40)/(0.2+8+1.67+19+0.67) = 1.693 btu/h
>the dew point of the attic air will be the same as for the outdoor air,
>which you can get from the weather man or a sling psychrometer.
or td = (460+oat)/(1+(460+oat)ln(rh/100)/9621)-460. for instance, 90 f air
with rh = 50% has td = 550/(1+550ln(0.5)/9621)-460 = 69 f.
nick
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