Evaporative cooling
nick pine
John Canivan <canivan@...> wrote:
>Air blowing across the surface of the water will definitely increase the rate of evaporation.
I. S. Bowen's 13-page 1926 PhD thesis
http://journals.ametsoc.org/doi/abs/10.1175/1520-0477%281995%29076%3C2433%3ATSBTBR%3E2.0.CO%3B2
says the ratio of heat loss by evaporation to heat loss by
convection over a pond is 100(Pw-Pa)/(Tw-Ta), regardless of windspeed,
with "Hg vapor pressures and F temperatures. An A ft^2 smallish pond
in a V mph light wind with a U = 1.5+V/2 Btu/h-F-ft^2 airfilm
conductance would lose (Tw-Ta)UA Btu/h by convection.
Bowen's ratio is -1 at the wet bulb temp, by definition, which makes
Twb = 9621/(22.47-ln(Ta-Twb+100Pa)) in absolute Rankine degrees. For
instance, 70 F (460+70 = 530 R) air at 50% RH has Pa = 0.374 "Hg, so
Twb = 9621/(22.47-ln(567.4-Twb)). Plugging in Twb = 520 R (60 F) on
the right makes Twb = 516.9 on the left. Repeating, Twb = 518.7,
517.7, 518.3, 517.9, then 518.1, so Twb = 518 R (58 F),
approximately.
> Now that you are doing this experiment it would be interesting if you could cover the shallow pond with a sheet of plastic...
Or inflate a large tarp or opaque piece of black plastic film with the
edges below the water level with an air pump during the day.
Nick
CJE
As I may have mentioned; last year I installed a Cool/R metal roof with a selective coating in a color called: Solar White. I picked the color because it has a solar reflectivity of 0.68 and an emissivity of 0.85 - which are the highest.
I think that works out to about 90% of the sunlight's heat not getting past the roof surface or getting into the attic.
So far I have been astonished to find that my attic temperatures trending only about 2º warmer than the conditioned spaces below the attic. The under side of the roof decking runs about 1º warmer than the attic air temperature.
On a recent 94º day the second floor of the house was 74º (maintained with mechanical cooling) - the attic air temperature was 76º - and the underside of the wooden roof decking was 77º.
stephen
-----------
Gary BIs
That's pretty impressive.
Do you recall what the temperatures were like before the new roofing? A good
deal higher I take it?
What part of the country are you in?
How do you like the roof all around? Aesthetics? Noise? Shows dirt?
If you are up for it, we could do a page on it with a few pictures and your
measurements?
Best,
Gary
CJE
I'm in southern NJ.
Everyone asks me about the noise. <g> So far as I can tell there is no difference in noise levels. I think people picture what a plain metal roof unsupported over purlins sounds like. Like a shed or industrial building. With this metal being screwed down solidly to wood decking it's not like that.
I was ready to be impressed but I am really astonished. When I first went up into the attic this summer, on a 90+ degree day, I thought the air ducts had come apart and that I was air conditioning the attic. <g>
Luckily it was very pleasant up there crawling around searching air duct leaks. When I didn't find any (and I couldn't imagine how Allen Funt could be hiding anywhere) I laid there a minute to ponder it. Then I thought to reach up and touch the roof decking - and I found it cool to the touch. <g>
That's what made me go up next time with a thermocouple to actually measure.
My attic temps used to trend about 30-50º over ambient in the summer so it was common to find the attic 130-140º on a hot day.
Another problem I found with asphalt shingles is their thermal mass. I don't know their specific heat but they weigh about 30 lbs. per square foot per roof layer. And many roofs are double layered - as mine was. Triple layered in some places.
The metal roofing weighs about 1 lb. per square foot I think. Although mine weighs less as I used aluminum rather than aluminized steel. With shingles they are a huge hot mass of stored heat for much of the night too. After midnight my shingled roof was very warm to the touch.
I am fond of the look of the new roof. Here are some pics taken as we were installing it. I think I have some pics showing it now but I'll have to track them down.
I'll get a remote reading thermometer and install it with the display down in the hallway. I use them all the time on my hot water solar system anyway.
BTW: I also reduced my A/C capacity from 6 tons to 3 tons and the units still cycle off on 100º days - so they are still oversized for the cooling load. I am going to replace the first floor furnace as soon as the weather is cooler so I think I'll try a 1 ton compressor in the downstairs condensing unit.
stephen
-----------
jim
| If one can get an air wash under the white metal ... even better --- On Tue, 7/26/11, CJE <cje...@verizon.net> wrote: |
|
CJE
Let's talk about that. I talked to Nick (I think it was Nick <g>) about it beforehand and also gave it a lot of thought.
But my eventual conclusion was that having the air space requires the assumption that the underside of the metal will be above the outdoor temperature.
And that didn't seem likely to be the case - if I was going to be reflecting/emitting over 90% of the sunlight away from the metal.
Venting the underside of the metal would keep the roof decking at about outdoor ambient temperature - which is exactly what I was trying to avoid.
What do you picture the gain to underside venting to be?
stephen
-----------
jim
| quicker cooling when the sun goes down ? better night time cooling ? |
jim
nick pine
> Another problem I found with asphalt shingles is their thermal mass. I don't know their specific heat but they weigh about 30 lbs. per square foot per roof layer.
http://www.usinspect.com/resources-for-you/house-facts/basic-components-and-systems-home/roofs/materials
How do we get the black plastic film to deflate reliably, with no big
bubbles?
It might have a central air supply/return pipe above the waterline...
Nick
CJE
Nick of course you are right - 3 lbs is what I meant to write.
90 lbs per bundle - four bundles per square - plus tarpaper and nails. But the 300 lbs per square number I used came from measured dumpster weights from stripping various roofs.
Oh wait - I'll have to go check - maybe I am wrong about the metal weight too. <g>
stephen
----------
Gary BIs
I'll put together a draft page from the material you sent, and you can have a look and see what you think and make changes/corrections.
Should have something to look at tomorrow.
Thanks,
Gary
. On Jul 26, 2011, at 10:54 AM, Gary BIs wrote: Hi Stephen, That's pretty impressive. Do you recall what the temperatures were like before the new roofing? A good deal higher I take it? What part of the country are you in? How do you like the roof all around? Aesthetics? Noise? Shows dirt? If you are up for it, we could do a page on it with a few pictures and your measurements? Best, Gary On 7/26/2011 7:58 AM, CJE wrote:
CJE
Thanks.
I bought the remote / indoor temperature stat today but I didn't get it installed in the attic here yet. I used it all day to see the difference between my truck's interior and the air coming out of the vents. <g>
I e-mailed Sheffield to ask how much the metal weighs too. Although my concern for thermal mass was pretty much for nothing anyway - because if no heat comes through the Cool/R metal, then the asphalt shingles won't store any heat anyway. The installers told me that 99% of the time they just go right over the asphalt shingles. That seemed crazy to me but now that I can measure the results - it doesn't seem that crazy anymore. <g>
Still; I didn't want all the old roofing up there anyway - so I stripped it all down to the wood decking.
stephen
----------
nick pine
July day
in Phila with an 86.1 and 67.2 high and low and average 81.4 and 72
day and
night temps and w = 0.0133 and an 8 mph windspeed.
An 80 F house with a 200 Btu/h-F thermal conductance that uses 300 kWh/
month of
electricity indoors would need 24h(76.7-80)200+34.1K = 18.3K Btu/day
of cooling.
With 100 cfm of natural air leakage and 2 gallons per day of
evaporation from
occupants and w = 0.0120 indoors (inside ASHRAE's 55-2004 standard
comfort
zone), we need to remove 24hx60x100x0.075(0.0133-0.0120)+16.6 = 30.6
pounds of
water vapor per day.
Fig. 5 of
http://www.agmcontainer.com/desiccantcity/pdfs/Desiccant%20performance.pdf
says
the maximum moisture content of bentonite clay (eg clumping kitty
litter) is 15%
at 113 F and 5% at 77 F, ie MC = 0.364-0.00278T(F), eg 0.142 at 80 F.
If the
clay is solar heated and dried at 113 F during the day and cooled to
80 F at
night, the house needs 30.6/(0.142-0.05) = 334 pounds (9.1 cubic feet)
of clay
(heh heh.) If air from a 1000 Btu/h-F car radiator cools the solar-
heated clay
bed at night, the house needs (18.3K+30.6K)/24h = 2038 Btu/h of
cooling, with an
80-2038/1000 = 78 F temp water supply.
The average vapor pressure of the July air Pa = 29.921/(1+0.62198/w) =
0.626 "Hg
and the dew point Tdp = 9621/(17.863-ln(Pa))-460 = 64.9 F and the wet
bulb
Twb(R)= 9621/(22.47-ln(76.7+460+100Pa-Twb)) = 528.3 R (68.4 F.) Phil
Niles says
a 78 F shaded pond in 76.7 F air with a 64.9 dew point and a 68.4 F
wet bulb
temp loses Qr = 1.63x10^-9((78+460)^4-a(76.7+460)^4) = 18.7 Btu/h-ft^2
by
radiation, where a = 0.002056x64.9+0.7378. Qc = (0.74+0.3x8mph)
(78-76.7) = 4.1
Btu/h-ft^2 by convection, and Qe = b(78-68.4)-Qc = 109.5 by
evaporation, where b
= 3.01(0.74+0.3x8)((78+68.4)/65-1), totaling 132.3, so it looks like a
2038/132.3 = 15.4 ft^2 pond could provide the 78 F water.
If A ft^2 of R1 glazing with 90% solar transmission evaporates 30.6
pounds of
water at 113 F and 0.9x1890A = 30.6K + 8h(113-81.4)A, 21 ft^2 of pond
glazing
would work, eg a 4'x8' clay bed with 3.4" of cat litter. We might use
more cat
litter to store more dryness for non-average days and use more glazing
in an
equilateral A-frame with an insulated north wall for winter air
heating.
Nick
nick pine
> Are you saying you could fill an attic with kitty litter and circulate hosehold humid air through a dry cold attic at night and allow the wet kitty litter to dry out during a sunny day?
Maybe. The clay has to be heated to about 113 F during the day, and
the water vapor needs to leave the attic during the day, so the attic
needs to be vented during the day. And the clay needs to be about 80 F
at night, hence the car radiator inside the house and the outdoor
shaded pond.
In my house, the small pond and clay bed would go on a flat roof, and
the radiator might take the form of water in poly film ducts on some
shallow trays under the ceiling in the room below the roof, as in a
Zomeworks architectural Cool Cell.
Bentonite has a specific heat of about 0.24 Btu/lb-F, so 334 pounds
with C = 80 Btu/F under a 4'x8' R1 cover and RC = R1/4'/8'x80 = 2.5
hours would cool from 113 to 80 F with 72 F outdoor air in -2.5/
ln(80-72)/(113-72)) = 1.5 hours.
That's not too long to wait, in a 12 hour night, but if the clay
adsorbs 30.6 pounds of water vapor in the next 10.5 hours making 30.6K/
10.5 = 2914 Btu/h of heatflow, it will warm to 72+2914xR1/4'x8' = 163
F, with a very low moisture content capacity. Hence the need for
cooling the clay.
Nick
John Canivan
greatly improve the heat exchange ability. How much cold water would have to
be stored to make this practical on a hot day? very open ended question...
John
----- Original Message -----
From: "nick pine" <ni...@early.com>
To: "sunspace" <suns...@googlegroups.com>
Sent: Sunday, August 07, 2011 7:45 AM
Subject: Re: anti solar - preemptive cooling - 2