| nicksanspam@ece.villanova.edu 2005-06-26, 6:25 pm |
| Stretch <sixfoot7@sccoast.net> wrote:
quote:
>
>Nonono. Don't deliberately store mosture. Dry out a slab in wintertime
>and let it absorb mosture in summer months to avoid energy-inefficient
>compressive dehumidification, in a fairly airtight house, instead of
>keeping the RH constant all year. Use the mosture storage capacity of
>a house for good vs evil. But as you say, this won't work everywhere.
>
>
>Which device? Wilmington NC has w = 0.0048 in January and 0.0168 in July.
>If we dry out a 4"x2400ft^2 slab in a house at 65 F in January, how much
>mosture can the slab absorb in July before the RH hits 55% at 80 F?
It weighs about 4/12x2400x150 = 120K pounds. In 55% air, its long-term
moisture content is about 5% of 55%, ie 2.75%, so it stores 0.0275x120K
= 3300 pints of water. Air with w = 0.0048 has Pa = 29.921/(0.62198/w+1)
= 0.229 "Hg, vs 65 F air at 100% RH with Pw = e^(17.63-9621/(460+65))
= 0.630 "Hg. RH = 100Pa/Pw = 36% makes the moisture content 0.05x36%
= 1.8%, so it stores 0.018x120K = 2160 pints. The difference is 1140.
If the house begins a humid spell with a 1.8% slab moisture content and
leaks 30 cfm of outdoor air with w = 0.0168, how long before it reaches
55% at 80 F indoors, after the slab stores 1140 pints of water?
At 36% RH, 80 F air has Pa = 0.36e^(17.863-9621/(460+80)) = 0.377 "Hg and
wid = 0.612198/(29.921/w-1) = 0.00794, and 55% 80 F air has wiw = 0.0122;
wiavg = 0.0101, and the slab might absorb 24hx60x30x0.075(0.0168-0.0101)
= 21.8 lb/day of water, filling up in 1140/21.8 = 52 days, approximately.
With a more accurate differential equation, if the slab stores
P = 60C0.075(0.0168-wi) net lb/h of water and wc = 0.018+Pt/120K
and 100% 80 F air has w80 = 0.02256 and RH = 100wi/w80 = 20x100wc,
wc = 0.4513wi, so wc = 0.018+dt-cwit, where c = 5.64x10^-4 and
d = 1.89x10^-5 and d/c = 0.0372 (the equilibrium moisture content
at 80 F with w = 0.0169 and 74% RH), so wc = 0.018 +(0.018-0.0372)e^-ct
= 0.0275 after 55 days, with an RC time constant with C proportional
to the slab mass in series with R = 1/cfm.
Summer sun might keep a concrete wall or a roof or a LiCl roofpond
under a greenhouse dry...
Nick
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