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Re: Air to Water heat transfer - Page 2

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Posted by Ian McDonald on November 3, 2004, 4:26 am
 
....>

Does that mean that in the case of my pool which, averages about 1.5m in
depth, there's not a lot of point in putting some sort of heat absorber on
the surface (eg black painted metal that would "pass on" the heat to the
water below) because the water will absorb the heat anyway?
The pool is also a dark blue (very attractive!) so the light that gets that
far will be absorbed and re-radiated back into the water? (or heat the
subsoil below...).

Does this mean that the best way to heat the pool is to cover it with a
transparent cover to keep existing heat in and pass incoming IR/light/heat
through?

(Ignoring for the moment a separate, independent heater).

Regards
Ian.



Posted by nicksanspam on November 3, 2004, 11:01 am
 


Yes. Heat (vs light) travels downwards very slowly in water. Page 89 of
the 1998 Schaum's Outline on heat transfer says: if a pool has a uniform
temp Ti and the surface temp at x = 0 is suddenly changed to Ts for all
time t>0, (T(x,t)-Ts)/(Ti-Ts) = erf(x/sqr(4at)), where erf() is the
Gaussian error function. Page 352 says 68 F water has thermal diffusivity
a = 0.00554 ft^2/h, and erf(0.5) is about 0.5, and (T(x,t)-Ts)/(Ti-Ts) = 0.5
= 6.7x/sqr(t), or t = 180x^2, so it would take 180 hours for a layer of Ti
= 60 F water 1' below the Ts = 70 F surface to warm to 65 F. Well, maybe
longer than that, since warm water rises. How much longer?


Sounds good to me.

Nick


Posted by Toby Anderson on November 3, 2004, 6:15 pm
 Greetings Ian,

My comments at the bottom....


I've not heard of a black-painted metal heat absorber, so I won't
comment too much on it, except to say, it may help retard
evaporation.. depending on where you live.

I recommend the 'transparent cover', but it's effectiveness kind of
depends on your climate. I live in the desert, and I had a pool for 5
years and my friends have pools, so I can tell you that often the
water is not as warm as we want. It's crazy. It's like 105degF
outside, and the pool doesn't get as warm as we would like. So, what
we do out here in Nevada, is roll out this plastic, bubble-filled,
tarp pool cover which floats on the pool. IMO, it heats up the pool
water for 2 reasons:
1. it prevents evaporation (the main reason)
2. it acts as insulation.

Toby

Posted by Hamish on November 3, 2004, 7:36 pm
 
Most heat loss from a pool is by eveporation. Next by conduction (
through sides, bottom and cover )Some poodl are installed with
polystyren or such like to reduce this loss.
Best way to keep it warm ( pasivly ) is cover it with a clear plactic
sheet to stop the evaporation. With the reate water absorbs light and
a blue botom will still absorb a significant portion of the light,
going black will gain a little, but does not look as nice!!

If you use a single sheet supported clear of the water you will see a
lot of condnsation form on it and drip back into the pool, If the
cover was sloped so that all the condesation ran into a collector, the
flowrate would tell you how much you are losing through the sheet.

To improve the heat holding capicity you could use something like
bubble wrap. Even better would be 2 layers of bubble wrap, bubble
sides facine each other.

Posted by Ian McDonald on November 4, 2004, 4:19 am
 

IR/light/heat

Well, you learn something new every day all right.
For years I have observed that black objects get hot in the sun, and
metallic objects seem to get hotter.
I recently thought to myself that a floating black sheet of metal say 1.2 x
2.4m (4' x 8'), painted black on the top side, would absorb the sun's heat
and conduct it through to the water below. I realise that there would be
inefficiencies with some heat radiating back into the air, but the aim was a
real cheap, real easy and not necessarily highly efficient heater. Then I
thought that if it was tethered and placed near the water inlet to the
filter pump, the water immediately below the metal would get quite hot and
get sucked into the filter and get pumped to the far end. Thus the pool
would reach near boiling in a matter of hours.(Yeah, right...). And I would
retire on the proceeds of designing, making and selling such wonders.
But it seems that the same amount of heat is entering/being absorbed by the
water anyway. And the pool sits stubbornly at about 17-19C, and from past
experience will peak midsummer (Jan/Feb here in NZ) at about 24-25C.
So I guess that an uncovered swimming pool radiates only slightly less heat
than it absorbs? And more at night?
And a cover would be REALLY cost effective...?

Ian.



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