Air to Water heat transfer

[Guest]

I'm still looking at heating my pool, and I've been trying to think outside
the square

Yesterday I had to get in our roof to install some downlights....and after I
got down and cooled off I got to thinking....
its is 18C outside and 35+ up there!

Did some googling, and some bright spark (actually a few) beat me to it, and
are marketing *MY* idea

So here is my question. I'm trying to work out if I can build an air - water
heat exchanger and if it would work.
Since they are being sold I assume they might work, but I'm a doubtful Tom
by nature..

My thinking has ranged from reuse of old Auto Radiators hooked together
(with the auto elec fans hooked up.)
through to aluminium plates & copper pipes etc...

Looking at the pics I've googled, looks like 'they' (the ones who stole *my*
idea!) are using a fan blowing air over
what appears to be an Auto intercooler.

Bottom line. Can someone provide , links or info so I can do some quick calc
to see if this concept of attic hot air => pool water
is feasible. As much as I like Math, most I've seen via google seems of-base
or a bit to detailed.

Any info appreciated.
Many thanks in advance

..

 

[Gymmy Bob]

Geeez. Go to your pool supply company and ask about their solar pool
heaters. You are trying to reinvent the wheel for more money.

 

[Guest]

Hi Gymmy
Have to love the consumer society!.
Couple of thoughts
1. Going to the shop instead of inventing, is no fun!
2. The stuff in the shop was invented by someone who had an idea and
took it further!
3. Current solar design looks crap on the roof!
4. Have you actually priced Solar heating? if you have& you think
it's reasonable, tell me where you live, 'cause here it's not reasonable!

$0.02

 

[Gymmy Bob]

The commercial stuff looks like crap? What will a couple of old car
radiators sitting on your roof look like?...LOL

I have never priced te pool panels but most of the pool owners here have
them and swear by them.

Canada, on the same parallel as Boston

 

[Guest]

The commercial stuff looks like crap? What will a couple of old car
radiators sitting on your roof look like?...LOL

Did you get up on the wrong side this am? Or out of caffine parhaps?

I never said "on", I'm talking "IN". The heat exchangers work using heat
INSIDE the attic space.
They also help cool the attic space, which saves on the A/C bills..

If you like poly panels to look at I'm please for you..
I'm also pleased that pool owners near you are happy.

 

[SQLit]

I'm still looking at heating my pool, and I've been trying to think outside
the square

Yesterday I had to get in our roof to install some downlights....and after I
got down and cooled off I got to thinking....
its is 18C outside and 35+ up there!

Did some googling, and some bright spark (actually a few) beat me to it, and
are marketing *MY* idea

So here is my question. I'm trying to work out if I can build an air - water
heat exchanger and if it would work.
Since they are being sold I assume they might work, but I'm a doubtful Tom
by nature..

My thinking has ranged from reuse of old Auto Radiators hooked together
(with the auto elec fans hooked up.)
through to aluminium plates & copper pipes etc...

Looking at the pics I've googled, looks like 'they' (the ones who stole *my*
idea!) are using a fan blowing air over
what appears to be an Auto intercooler.

Bottom line. Can someone provide , links or info so I can do some quick calc
to see if this concept of attic hot air => pool water
is feasible. As much as I like Math, most I've seen via google seems of-base
or a bit to detailed.

Any info appreciated.
Many thanks in advance

Me thinks,

Depending on where you live your not going to save an dime on a/c. In
Phoenix when the air is on there is an need to cool the pool.

Any kind of metal pipe is asking for trouble. Chlorine loves copper.
Stainless will even be eaten through. I had a friend that had an pool cooled
a/c condenser and he replaced the heat exchangers every couple of years. I
even went over and helped him install an heat exchanger from an boat that
only lasted 3 years.

I would worry about lots of water in the attic and leaks.

I have seen something like this years ago, it was disconnected in the attic.
The home owner knew nothing about it.

I wonder if this idea was so successful there is not more of them or better
kits being sold. No offence to your idea. I just do not think that the idea
would work very well.
Lets say you have 150F in the attic. You have an fan blowing across the
radiator. The radiator has 5 gallons in it. Pool water is 70F at 10k
gallons. When the air is blown across the radiator the temp is going to go
down some. Even if you pumped at 2 gallons an hour you could never raise the
pool up much before the sun went down. So if you increase the tank/radiator
to 200 gallons,,,,,,,,,,, gee will the structure take 1600 pounds more?
Try this link for solar stuff. When I had my solar system I was considering
increasing the collectors so I would not used electricity at all. (domestic
hot water).
http://www.heatwithsolar.com/parts.do
The pricing was good when I was checking around.
Why did I not do it. I sold the home.

 

[daestrom]

I'm still looking at heating my pool, and I've been trying to think
outside the square

Yesterday I had to get in our roof to install some downlights....and after
I got down and cooled off I got to thinking....
its is 18C outside and 35+ up there!

Did some googling, and some bright spark (actually a few) beat me to it,
and are marketing *MY* idea

So here is my question. I'm trying to work out if I can build an air -
water heat exchanger and if it would work.
Since they are being sold I assume they might work, but I'm a doubtful Tom
by nature..

My thinking has ranged from reuse of old Auto Radiators hooked together
(with the auto elec fans hooked up.)
through to aluminium plates & copper pipes etc...

Looking at the pics I've googled, looks like 'they' (the ones who stole
*my* idea!) are using a fan blowing air over
what appears to be an Auto intercooler.

Bottom line. Can someone provide , links or info so I can do some quick
calc to see if this concept of attic hot air => pool water
is feasible. As much as I like Math, most I've seen via google seems
of-base or a bit to detailed.

Just a 'random thought'...

How about those HW baseboard pipes? Take the finned copper pipe from one or
two baseboards, and plumb them horizontally up near the peak inside the
attic. No, it would probably not get as much heat as a forced-air
heat-exchanger, but it would be cheaper to operate and simple to install.

Of course, the amount of heat you can gain from the attic isn't so much
determined by the temperature you saw while up there (+35C) as the amount of
solar collected by the roof. Put a really large, forced-air heat-exchanger
and you might get the attic temperature down to very close to the pool water
temperature. Good for the A/C, but somewhat overkill on the sizing as far
as warming the pool water. A smaller unit wouldn't cool the attic down as
much, but could gather almost as much heat.

daestrom

 

[Gymmy Bob]

Here is what your post said

"3. Current solar design looks crap on the roof!"

I guess you changed your mind about the whole thing. Maybe get what you want
to ask down clear and then post it.

 

[m II]

Just a 'random thought'...

How about those HW baseboard pipes? Take the finned copper pipe from one or
two baseboards, and plumb them horizontally up near the peak inside the
attic. No, it would probably not get as much heat as a forced-air
heat-exchanger, but it would be cheaper to operate and simple to install.

I don't know if condensation would be a problem or not. It would be a
shame to do all that work and then find that the fins are dripping water
on the insulation below. The setup gets hard to do if you have to run an
angled plastic trough under the length of each pipe.

On the plus side, you'd have a lifetime supply of distilled water for free.




mike

 

[[email protected]]

I'm still looking at heating my pool... Yesterday I had to get in our
roof... its 18C outside and 35+ up there!... I'm trying to work out
if I can build an air - water heat exchanger...

My thinking has ranged from reuse of old Auto Radiators hooked together
(with the auto elec fans hooked up.) through to aluminium plates
& copper pipes etc...

Plates and pipes sound expensive. Used auto radiators are nice, but
their electric fans are inefficient and require a 12 V supply.
I'd use 120 or 240V fans.

...Can someone provide , links or info so I can do some quick calc
to see if this concept of attic hot air => pool water is feasible.

There may be easier ways to heat a pool. Paint it black. Then add
a clear cover, when not in use. Then paint the skirt black, and
flood it whenever it reaches 120 F. Or use two layers of
poly film on a flat platform.

Have you actually priced Solar heating?... here it's not reasonable!

Space heating with sunspaces or thermosyphoning air heaters can be.
Water heating can pay off year-round.

How about those HW baseboard pipes?

About $4 and 5 Btu/h-F per linear foot.

Take the finned copper pipe from one or two baseboards, and plumb them
horizontally up near the peak inside the attic.

A 35 C (95 F) attic might give (95-75)5x16 = 1600 Btu/h to 16' of fin
tube with 80 F pool water inside.

...it would probably not get as much heat as a forced-air heat-exchanger,
but it would be cheaper to operate and simple to install.

A $35 used auto radiator or a $150 SHW2347 MagicAire heat exchanger might
collect (95-75)800 = 16K Btu/h for 80 F water or (95-55)800 = 32K Btu/h
for 55 F well water. Being more compact than fin-tube, it's easier to
freeze-protect with a pump at night, and collecting condensation is easier.

Another alternative: collect heat in a 4' wide 10 cent/ft^2 drain-down
poly film duct on a horizontal platform (eg 10 cent/ft^2 welded-wire
fence or PV panels under a reflective north wall) under the ridge.

Of course, the amount of heat you can gain from the attic isn't so much
determined by the temperature you saw while up there (+35C) as the amount of
solar collected by the roof...

With A ft^2 of dark opaque R1 roof in full sun (250 Btu/h-ft^2) on a 70 F
still day and another A ft^2 on the shady side, collecting heat from T (F)
attic air into 80 F water with an 800 Btu/h-F heat exchanger, we might have
something like this, viewed in a fixed font like Courier:

south roof north roof
T
0.5/A X 1/A | 1/A 0.5/A
70---www-------www---*---www---www---70
--- | |
|---|-->|-- w
--- w 1/800 X is the outside surface of the south roof
250A w
Btu/h |
--- 80
---
|
-

If A = 32'x20' = 640 ft^2, we have

T
0.5/640 1/640 | 1/640 0.5/640
70---www-------www---*---www---www---70
--- | |
|---|-->|-- w
--- w 1/800
160K w
Btu/h |
--- 80
---
|
-

which simplifies to this

T
0.5/640 1/640 | 1/427 1/640+0.5/640 = 1/427
70---www-------www---*---www---70
--- | |
|---|-->|-- w
--- w 1/800
160K w
Btu/h |
--- 80
---
|
-
and this

T
0.5/640 1/640 | 1/427
--www-------www---*---www---70
| |
--- 195 w 70+160Kx0.5/640 = 195 F
--- w 1/800
| w
- |
--- 80
---
|
-
and this
T
1/427 | 1/427
--------www-------*---www---70
| -----> Y
--- 195 I w 0.5/640+1/640 = 1/427
--- w 1/800
| w Disconnecting the heat exchanger
- | at Y, I = (195-70)/(1/427+1/427)
--- 80 = 26.7K Btu/h.
---
|
-
and this

1/853
--------www-------
| |
--- 132.5 w 70+26.7K/427 = 132.5
--- w 1/800
| w So (132.5-80)(1/853+1/800)
- | = 21.7K Btu/h (6.35 kW) flows
--- 80 into water, if I did that right.
---
|
-

Insulating the north roof and replacing the south roof with transparent
polycarbonate would raise the (14%) efficiency, especially in wintertime.

Nick

 

[[email protected]]

MOst of the sun's heat is carried on the Infrared wavelength which
only penetrates about 3 feet into the water. So, if your pool is
deeper than 2 feet, painting the bottom black won't help.

It appears you have erred again, my good man.

HC Bryant and Ian Colbeck's paper, "A Solar Pond for London,"
(Solar Energy, v. 19, p 321-322), says the fraction of incident
energy remaining after passing through x meters of water (x varies
from 1 cm to 10 m) is 0.73-0.08ln(x), eg 0.73 after 1 meter.

Nick

 

[daestrom]

[email protected] wrote

MOst of the sun's heat is carried on the Infrared wavelength which
only penetrates about 3 feet into the water. So, if your pool is
deeper than 2 feet, painting the bottom black won't help.

Actually, water is pretty transparent from 400 to 700 nm wavelengths. And
the sun radiates about 50% of its total energy in this energy band. So
quite a bit of the sun's energy penetrates pretty well.

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/watabs.html

But, who *really* wants a black pool to swim in??

daestrom

 

[Ian McDonald]

.....>

The following article 'LIGHT PENETRATION IN WATER'
(http://www.utoronto.ca/env/jah/lim/lim02f99.htm) says this:

"The absorption of water (in %) is very high in the infrared portion
(long wavelengths) and results in rapid heating of water by incident
light. Approximately 53% of total light energy is transformed into
heat in the first meter of water."

....

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.

 

[[email protected]]

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)...

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?

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?

Sounds good to me.

Nick

 

[daestrom]

Hi Daestrom,

Infrared light, the light I was talking about, has wavelengths ranging
from 1 mm - 750 nm, which is greater than 400-700nm you referenced.

I got my information, partically from:

The following article 'LIGHT PENETRATION IN WATER'
(http://www.utoronto.ca/env/jah/lim/lim02f99.htm) says this:

"The absorption of water (in %) is very high in the infrared portion
(long wavelengths) and results in rapid heating of water by incident
light. Approximately 53% of total light energy is transformed into
heat in the first meter of water."

You said "most of the sun's heat..." If you meant most of the solar energy
(as I took you to mean), then you didn't read what I wrote. Almost half of
the solar energy falling on a pool surface is in the 400-700 nm range. So
almost half of the solar energy is *not* absorbed in the first meter or so,
water is transparent to those wavelengths. Infrared and longer wavelengths
only carry about 52% of the sun's total radiant energy.

One reference, "Solar radiation Curves for Engineering Use", states that 7.2
percent of the solar energy is ultraviolet, 40.8 percent is in the visible
light range, and 52.0 percent is in the infrared and longer wavelengths ( >
800 nm).

So although 52% of the solar energy is absorbed in the top meter or so, a
fair amount of the sun's solar energy does penetrate to the bottom of the
pool. And what happens there is a function of the pool floor's
emissivity/reflectance. Painting the bottom of the pool so it absorbs most
of the visible spectrum (i.e. black), could result in some 40% more solar
energy to be retained instead of just reflecting back up.

But of course, depending on your lattitude and angle of incidence, you lose
quite a lot right at the surface in the form of reflected light that doesn't
even enter the water.

daestrom

 

[Ian McDonald]

...

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.

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.[/QUOTE]

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.

 

[Sylvan Butler]

For years I have observed that black objects get hot in the sun, and
metallic objects seem to get hotter.

"black" objects usually absorb instead of reflect more of the energy.

metallic objects "seem" to get hotter because they conduct heat better.
(for example, a black cloth in the sun will get hot, but when you touch
it you feel only the heat from the fibers in the area you touch,
wherease a sheet of metal will conduct heat from the surrounding areas
to your fingers.)

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...?

Exactly. A clear cover allows radiation into the pool which will help
heat it. But it also allows radiation out of the pool which cools it.
Radiation will transfer more heat out at night, especially on a
cloudless night, because of the greater temperature difference.

But radiation is only one heat transfer mechanism. Evaporation carries
away heat. Conduction carries away heat. A cover helps prevent or slow
evaporation. Insulation between pool and ground and an insulated cover
slow conduction.

A cover is a big win. A cover that is clear in the day when you want to
add heat to the pool, and opaque at night might help a little bit more.
An insulating cover might help a little bit more. And that is where the
bubblewrap comes in.

sdb

 

[James Baber]

"daestrom" wrote




I read what you wrote. I went to the website you referenced. I didn't
find it collaborating
snip...

Sorry, I believe you meant corroborating instead.

ok. That's good collaboration....

Sorry again, I believe you meant corroboration instead.
The root word is corroborate which in essence is to validate, whereas
collaborate is "in cooperation with".
For an example "he collaborated with our enemy Bin Lauden".

--
Jim Baber
Email [email protected]


1350 W Mesa Ave.
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