[NA]Fluids & thermodynamics[NA]

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
Let's say you have a 1/2" ID pipe full of 55°F tapwater, laying on the sidewalk with the sun beating on it (it's 85°F here now). You want to move that water into a vessel in such a way as to extract the most heat possible from it. (Hose holds one gallon)
How fast to move the water?

If you move the water too fast, seems like you will get little heat from the system.
If the water does not flow, the tube gets a lot of heat but there is no benefit; it's all in the tube.
So, it would seem that somewhere in the middle is optimal.

Should the water start to flow only after the temp in the pipe has come within 95% of its maximum? That would mean pulsing the fluid. How would you decide on the best flow rate if you wanted to avoid pulsing?
Haha, get your slide rule out!
(Hoping to do this theoretically without a lot of trial and error.)
 
closed system? put the tank above the pipe and let it circulate naturally...
 
Hi Dave,

we fight that very issue when designing engine cooling systems. There are rules of thumb that have been developed for specific engines but in the end, for each installation we test and often end up adding throttling plates into some water pipes to adjust the flow rate to optimize performance. IOW it's still a lot of trial and error.
 
JHW said:
Hi Dave,

we fight that very issue when designing engine cooling systems. There are rules of thumb that have been developed for specific engines but in the end, for each installation we test and often end up adding throttling plates into some water pipes to adjust the flow rate to optimize performance. IOW it's still a lot of trial and error.
Click to expand...

Maybe one of the reactor guys can help. I'd have to consult a heat transfer guy or dig out the books.
 
OK, well I am thinking one 'hose-full', or one gallon per minute.
That is the circ rate of a lot of small pumps anyway; ie aquarium pumps.

Open system. Garden hose leading to 600gal 'swimming pool'.
 
Let'sgoflying! said:
If you move the water too fast, seems like you will get little heat from the system.
Click to expand...
You will get less heat that way per unit of time, but you will get it faster and over time you will accumulate more heat.

But is this a closed. recirculating system where you can store the heat, or is it an open system that requires the water to contain the maximum amount of heat as it is expelled?
 
JOhnH said:
But is this a closed. recirculating system where you can store the heat, or is it an open system that requires the water to contain the maximum amount of heat as it is expelled?
Click to expand...

Open as in unpressurized
Closed as in recirculating
 
Let'sgoflying! said:
Open as in unpressurized
Closed as in recirculating
Click to expand...
The lower the temperature of the water in the hose, the more heat transfer you get from yea olde sun and the less heat loss you get to the air. The disadvantage to pumping faster is the cost of pumping. Pump, baby, pump.
 
OK! So more heat transfer with a larger delta.
I will pump!

I found more hose; now the capacity is 2.5 gallons.

I cooled the hose with water and left it in the sun for 5 minutes after filling it.
The water out of the spigot is 70°F and out the end of the hose after 5mins is 80°F
So I am getting a 10° rise in 2.5 gallons after 5minutes and that is at 7:00pm!

I think this is going to work famously (the "pool" is only 639 gallons; an Intex - it was great last year after a hard day at work to sip beer and read the news while we floated around....just a bit cool.)

I will probably go with the 1gal/min aquarium pump; dirt cheap and readily available plus I think it will be a flow that will work.
 
Capt. Geoffrey Thorpe said:
The lower the temperature of the water in the hose, the more heat transfer you get from yea olde sun and the less heat loss you get to the air. The disadvantage to pumping faster is the cost of pumping. Pump, baby, pump.
Click to expand...

Yabut in order to raise the temperature in the pool ya gotta get the temperature up. In the end I doubt that it will particularly sensitive to rate once above some particular value. I liked the first guess of 1 minute exposure.
 
Someone is going to trip on the sidewalk, and you savings will be consumed by a homeowners claim:D

Coil black plastic hose (or go copper if the cost/reward delta is worth it) and throw it in a lexan covered box with black interior or place it on the roof of your home. Dark colors are your friend.
 
You need a hose that effectively transfers heat. Copper for instance. Aluminum block with ports bored in it (that might fatigue after too many thermal cycles though). Rubber hose is a poor conductor.

Constant flow, not pulsed. Keep it simple.

Run the fluid you are transferring heat to in a direction opposite of the flow of the fluid that you are taking the heat from. This maximizes the relative delta T all along the exchanger and makes for a more efficient system.
 
And, a cover over the pool to limit evaporation when it is not being used will be way more effective than using a hose as a solar heater.
 
Subsea said:
Someone is going to trip on the sidewalk, and you savings will be consumed by a homeowners claim:D
Click to expand...

Yeah, like a javalina or (perhaps) a rattlesnake! :D
 
For melting bees wax we use a solar melter. A black box with glass on top angled directly towards the sun runs around 170F.

Install a 50' roll of 3/8" od copper tubing in there and run approx 1 GPM.
 
Subsea said:
or place it on the roof of your home. Dark colors are your friend.
Click to expand...

I would think black tubing serpentined along your roof (if it faces the sun) would work very well.
 
When I bought my first home with a pool in SoCal many decades ago, I found some used 4x10' solar panels and put them on the roof, with all the needed hardware, and thought it would make a huge difference. I had 5 panels up there, and after a few months of use, I could barely tell the difference in water temp. I ran the pump at the hottest part of the day, and did manage to collect some heat, but I was completely disappointed with the results. 200 sq ft of panel barely made a few deg difference in the water. If used during the early and late seasons it may extend your pool use by a week or so. If you just like steaming hot pool water in July and Aug I guess that will help. I also had a cover on a roller that I would put on at night and I think that helped far more than the solar.
 
Let'sgoflying! said:
Let's say you have a 1/2" ID pipe full of 55°F tapwater, laying on the sidewalk with the sun beating on it (it's 85°F here now). You want to move that water into a vessel in such a way as to extract the most heat possible from it. (Hose holds one gallon)
How fast to move the water?

If you move the water too fast, seems like you will get little heat from the system.
If the water does not flow, the tube gets a lot of heat but there is no benefit; it's all in the tube.
So, it would seem that somewhere in the middle is optimal.

Should the water start to flow only after the temp in the pipe has come within 95% of its maximum? That would mean pulsing the fluid. How would you decide on the best flow rate if you wanted to avoid pulsing?
Haha, get your slide rule out!
(Hoping to do this theoretically without a lot of trial and error.)
Click to expand...

Don't confuse heat with temperature. You will transfer more heat to the pool with more circulation. It's the reverse of an engine cooling system. Engine thermostats restrict the circulation of the water to transfer less heat from the engine, so it can maintain a minimum temperature in cold ambient temperatures.
 
Serendipitously, I stumbled across an array of aquarium pumps at Harbor Freight, right at the entrance last week. 7$ and up. I bought a 158gph one (<3gpm) for $13.
I used some adapters to upsize the outflow fitting to my garden hose and started it at 1:00pm. There is about 120' of 5/8" dark green/black garden hose laying in 4-5 parallels on the brick sidewalk.

Today was not a hot day; 75°F and mostly sunny.
The pool started at 68°F and the water coming out of the hose was at 78-80°F most of the afternoon. The pool is now 6° higher, at 74°! (7:30pm).
{~6hrs at 2.5gph x 6 hours resulted in a 6° rise}

I will leave it running til the inflow temp is close to the outflow as the sun nears the horizon, then shut it down. It will probably lose a few degrees tonight but I have it covered.
I am pretty impressed and think this will drastically increase the number of usable days for the pool! Esp if we get some 90° days which are more usual.
 
Let'sgoflying! said:
Serendipitously, I stumbled across an array of aquarium pumps at Harbor Freight, right at the entrance last week. 7$ and up. I bought a 158gph one (<3gpm) for $13.

Today was not a hot day; 75°F and mostly sunny.
The pool started at 68°F and the water coming out of the hose was at 78-80°F most of the afternoon. The pool is now 6° higher, at 74°! (7:30pm).
{~6hrs at 2.5gph x 6 hours resulted in a 6° rise}

I will leave it running til the inflow temp is close to the outflow as the sun nears the horizon, then shut it down. It will probably lose a few degrees tonight but I have it covered.
Click to expand...

That's pretty good. I would say most of your gain was from the cover, but certainly some of it was from the circulation too. Did you pick up a pump for me too? I could use a 3-4GPM pump if they are $7 or so.
 
Are you picking your water up from the bottom drain? That would be the best because it starts the coldest.
 
docmirror said:
That's pretty good. I would say most of your gain was from the cover, but certainly some of it was from the circulation too. Did you pick up a pump for me too? I could use a 3-4GPM pump if they are $7 or so.
Click to expand...

I didn't get any from the cover doc; I have the pool sitting in the shade of a carport.
 
Let'sgoflying! said:
The pump is submersible and I have it sitting on the floor of the pool.
Click to expand...

Ah, I guess small pool in your carport? I was thinking something like 20x40x8' in-ground.

The sub pump at that price is a heck of a deal. Would like to have a couple.
 
So this is working OK, not ideal but acceptable.
Downsides are:
I have to move the 200' of hose 2X per day because there is no clear space next to the pool that gets sun all day.
If the hose falls out of the pool or develops a leak, I think it will drain the pool pretty quickly.
I can't bring myself to leave the pump plugged in every time I use it so I have to readjust the timer to 9am:8pm a lot.
If it rains, the temp here drops below pool temp so I have to get back to shut it off or it will pump the heat out instead of in.
Overall though, I am happy with the 86°F it makes the pool. ~10$ pump plus electricity
 
No pump is needed.

The hose needs to be full, completely from end to end.

One end of the hose at the bottom ( and coldest point) of the reservoir.

the hose leads up to an area that receives the most sunlight. You can paint it black if you want.

the return end needs to be just below the surface of the water in the pool.

The result is the 'Thermo-syphon' effect. As water is heated, it becomes less dense ( like that you recall when swimming in a lake) and will rise in the pipe; displacing water into the pool.

The motion is so slow that very little pressure drop is noted.

Early engines used the thermo-syphon effect to move water in engine jacket.
 
44Gort said:
No pump is needed.

The hose needs to be full, completely from end to end.

One end of the hose at the bottom ( and coldest point) of the reservoir.

the hose leads up to an area that receives the most sunlight. You can paint it black if you want.

the return end needs to be just below the surface of the water in the pool.

The result is the 'Thermo-syphon' effect. As water is heated, it becomes less dense ( like that you recall when swimming in a lake) and will rise in the pipe; displacing water into the pool.

The motion is so slow that very little pressure drop is noted.

Early engines used the thermo-syphon effect to move water in engine jacket.
Click to expand...

But he wants to maximize heat transfer to the pool, more flow = better for that purpose.
 
Let'sgoflying! said:
So this is working OK, not ideal but acceptable.

Downsides are:

I have to move the 200' of hose 2X per day because there is no clear space next to the pool that gets sun all day.

If the hose falls out of the pool or develops a leak, I think it will drain the pool pretty quickly.

I can't bring myself to leave the pump plugged in every time I use it so I have to readjust the timer to 9am:8pm a lot.

If it rains, the temp here drops below pool temp so I have to get back to shut it off or it will pump the heat out instead of in.

Overall though, I am happy with the 86°F it makes the pool. ~10$ pump plus electricity
Click to expand...


Seems like a simple thermostat based AC switch would automate the power cycle thing pretty well without breaking the frugality bank on that one?

How about leaving two sections of hose connected all the time and a timer based valve to switch heating loops?

Obviously you'd want cheap and reliable stuff to make it worth it, and I haven't hunted to see what's out there, but both seem like they'd be such common goals that there must be some economies of scale for small reliable components that do those jobs.

A water level sensor to kill the pump if the pool gets below a certain depth, also seems like it'd be a simple cheap and reliable solution to one of the problems mentioned.
 
He used cheap pump, dark hose. Works great.
 
SCCutler said:
He used cheap pump, dark hose. Works great.
Click to expand...


Right. But he said he needs to move it twice a day. Anything that repetitive is begging for automation. He already invested in a timer. Just add a valve and a second length of hose.

He also said he's worried about a hole in it draining the pool. A water level sensor to kill the pump takes care of that worry nearly perfectly.
 
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