Author Topic: will this make a simple thermosiphon effect?  (Read 23972 times)

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johnE

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will this make a simple thermosiphon effect?
« on: August 19, 2012, 10:55:44 AM »
Hello all,

Hope you can help me, thanks if you can,

I have a very small sheet metal wood burning stove in which I've placed a copper coil.


The coil has cold water coming in from house mains and warm/hot water going out towards my
outdoor bathtub.  This takes forever due to the stove's small size and inefficiencies of my design, lack of knowledge, poor breeding  :P etc....

What I want to do is re-circulate the tub water (instead of heating direct from mains) without using a pump if possible. So hoping for a thermosiphon effect with this small wood stove boiler contraption.



Before I melt my hoses on either end of the coil I thought I'd pose the question here.

I can move the stove higher or lower than the tub if necc. but not too much maybe 1 meter.

[the coil and in-out  hose inner diameter are both 6mm, flow rate of water is 1-3 liters per minute filling the tub from mains pressure, 1 L/min being hot 40C and 3L/min being coldish 30C.  Mains water temp is 23C now.]

Thanks very much if you can help,

john E

ghurd

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Re: will this make a simple thermosiphon effect?
« Reply #1 on: August 22, 2012, 09:24:53 AM »
Inside the stove, the pipe must always go up from the cold side to the hot side.
Half of the coil is going 'down'.
The coil needs mounted vertically.

Also, the hot side traveling to the tub must go upward the entire way.  If there is any part of the pipe from the coil to the tub that can hold an air bubble, the flow will stop.

Have fun,
G-
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madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #2 on: August 22, 2012, 10:27:08 AM »
John,

As ghurd pointed out, the tub needs to be above the heater with no air bubbles.

I modified a wood burning stove(s) to assist heating a hot tub in the winter and used it for several years. It was a PITA but I finally got it working OK. I had other issues becuase the stove was inside and also used to warm the house.

Anyway, the coil won't last long hung as it is. If it goes dry it will warp very quickly but even wet it will droop fairly quickly. I don't know what volume of water and temp. you are shooting for, but that single coil won't make much hot water very quickly.

I've always found that unless it is done perfectly, thermo siphons can be tricky. Any chance of a small pump?

Jonathan

johnE

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Re: will this make a simple thermosiphon effect?
« Reply #3 on: August 25, 2012, 10:18:41 PM »
Thanks so much ghurd and madlabs for your detailed answers!

I had searched for layman termed info on the thermosiphon effect but couldn't find a detailed explantation
like the one you guys gave, they are greatly appreciated.

madlabs, thanks for info on the coil, I was thinking of putting it lower and changing shape to a large spiral instead of the long spring like shape now.  Was hoping to go without using any electric but that seems out of the question now.  I did get small drill-pump and will try that instead, seems the thermosiphon will not work for me.  If the drill-pump doesn't work, I'll try a small fish tank pump.

I'll post and update and let you all know how it goes,

johnE

madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #4 on: August 26, 2012, 11:03:53 AM »
John,

Thermosiphons do wrok and the theory is easy. Once you get 'em set up the work fine.

That aside, for heat extraction you need lots of surface area of coils. In my case I actually started pulling so much heat from my stove that I began to have creosote build up in my chimney. You won't have that problem, or at least it will be easy for you to clean.

You are going to need something steel to support all the copper and minimize sag. I found that I could do copper coils in the chimney but in the fire box the copper aged quickly. My best iteration was 1/2" steel pipes welded into the top of the fire box and copper coils in the chimney.

Might we ask what the purpose of the project is? Volume of water, desired temp, etc?

Jonathan

johnE

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Re: will this make a simple thermosiphon effect?
« Reply #5 on: August 26, 2012, 11:40:27 AM »
Thanks again madlabs,

Great advice on coils and wear on them.  I haven't seen stainless tubing in my shopping, except for the flexible type that would probably wear fast.  I was thinking of originally placing the coil in the chimney but went the easy route.  I can always remove the coil now and make a larger one if need be, as well as change the location of the coil.  As is when it is running hot and making about 2 liters per minute of hot water I don't mind waiting 2 hours that it takes to fill the tub with hot enough water.  Of course 4 liters per minute would be ideal and maybe take less than an hour to fill it.

I'm just trying to make a simple Japanese style outdoor bath as cheaply as possible for relaxation and for a backup for my electric hot water if we have future problems with the grid or skyrocketing electric rates here.

Here is a simple but expensive boiler fired unit http://www.auvelcraft.co.jp/rotenburo/
I'm not interested in the tub, but the boiler unit, they cost 4-500$ or so. So I was hoping to do something like it for much less money.  I'm up to a bout 110$ now.   The above linked unit  is non-electric and must work on thermosiphon effect.  As you can see the two hoses are rather wide.  Also I don't like having the boiler so close to the tub, seems smoky and messy.

The blue tub I have is 120liters but needs about 100liters if you count the displacement of the bather.
I also have an extra  full sized tub of about 200liters if things become successful.

Ideally I want to have 40C/100F water temps give or take.

I tried out my small drill powered pump that cost about 10 dollars today.  All was going well till I realized the impeller is packed with a kind of petroleum jelly...ugg, greasy water.

I'll have to go with the fish tank water pump. Something like one of these cheapies,  http://www.marinedepot.com/powerheads_pumps_taam_rio-ap.html Unfortunately the cheapest types are fully underwater pumps.  Don't like the idea of sitting in a tub with a 100Volt appliance plugged in and  beside me IN the water, yikes.  Should be safe if I don't chew on the wire or something  ???

Does an in-water pump sound crazy or dangerous to you guys?

johnE


madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #6 on: August 26, 2012, 12:16:10 PM »
John,

Ahh! A fellow hot tubber...

In my experience, cheap and hot tub don't go together well. The learning curve of building your first set up will likely be more than just buying the stove, but for me at least bulding is a major source of fun. It keeps me out of the bars and newspapers. YMMV. I kept a tub hot for almost 10 years using a combination of solar, wood and propane. Mind you, mine was much larger than yours and kept hot instead of heating for use.

No freaking way would I use a submersible AC pump. Nor would I bother with a cheap pump. I can tell you from experience that things can get ugly if the pump quits and the water in your copper coils steams off. And you used solder for the joints. That sucked.

I would use a magnetic drive pump rated for the temperature. Locate the pump before the heater, not after. Use flare fittings on the copper, don't solder. If in the firebox, use a piece of sheet metal to protect the coils from direct flame.

Do you weld? For long term low maintainence I found that steel pipes across the fire box worked pretty well and were easy if labor intensive. The types of heater you linked, at least the two similar ones I have seen in operation, have a water jacket instead of coils. I am thinking of trying to build one for my new tub. I moved offf grid three years ago and am finally getting close to setting up a tub again.

Jonathan

bob g

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Re: will this make a simple thermosiphon effect?
« Reply #7 on: August 26, 2012, 04:32:54 PM »
you might consider using a 12vdc pump, such as the bosch auxiliary heater pump used on german autombiles. they are about bullet proof, safe and made for hot water use, not bad priced if new on ebay, and very cheap from a wrecking yard used... they are made for 5/8" heater hose, which works nearly perfect for connection to half inch copper.

that is what i would use, if electrocution hazard is a concern

btw, they only draw about 50-60watts

bob g
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madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #8 on: August 26, 2012, 04:52:11 PM »
Bob,

Got some makes/models that use that pump? Id like to check that out.

Jonathan

Simen

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Re: will this make a simple thermosiphon effect?
« Reply #9 on: August 27, 2012, 01:53:58 AM »
I found one in an old Mercedes; don't remember model though. It wasn't Bosch, but Johnson; a CM10;

http://www.johnson-pump.com/JPMarine/products/circulation/cmco.html
As Bob said; they're about bullet proof. The CM10 i found use around 10-15W...
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

johnE

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Re: will this make a simple thermosiphon effect?
« Reply #10 on: August 27, 2012, 10:37:43 AM »
Hi all,

Thanks for all the tips on type, placement of pump, safety and well just about everything.

  This project is just about as high tech as my industrial arts training gets me. I don't weld or even solder, haha, but I think I can swing a flange or two.

About the stainless steel bars in the firebox there madlabs do you mean for support or protection of the copper coil, or for water heating in the stainless pipes?  Well done on running a full sized hot tub (and home) on alternative means, I'm having trouble with 100liters hahaha.  As you said it's the learning curve. 

I'm trying to locate an automotive type DC pump, but the best I can find right now on the net are like this, judging by shape and price they may be substantial enough for my tiny flow.  http://item.rakuten.co.jp/auc-is8syoji/12v3w-pump/#12v3w-pump

I'm afraid one of the larger Bosch type pumps would be overkill even if I doubled my coil size.  About 4 liters per minute seems about all I could heat.

If I were to run one of these pumps would it be best to power it from  an AC to DC transformer/adapter that exactly  matched the specs on the pump ie: DC12V0.72A(9W) or slightly higher or lower?  Or something else?

Thanks,

johnE



madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #11 on: August 27, 2012, 11:12:28 AM »
John,

I was talking about regular steel pipe, not stainless, just the stuff they have at the hardware store, and I was using it instead of copper pipes. I cut holes in the sides of the stove, stuffed the pipes through and welded 'em.

Since you are sticking with copper, perhaps you could get some all thread and drill some small holes in the side of the stove and use that to support the tube. And I really do reccomend that you use some sheet metal for a heat shield for the tube. And you are going to need more tube to make that tub heat up faster than forever. I did some coil inside a coil ones which were pretty good for using up space. Use flare fittings/tees to connect them all together. You can also use compression fittings but you will have to tighten them a few times a first to stop them leaking but then they settle down and hold water.

Don't worry about pump speed VS coil size. I did some pretty extensive testing, both with my wood stove and thermal panels and what I found is that with a smaller pump you get a smaller volume of hotter water and with a bigger pump you got more less hot water but the BTU's remained the same. In my large tub the bigger pump helped with thermocline effects overnight, but that won't apply to you. However, being able to handle the heat is important or the pump won't last.

As to the power supply/pump question, make sure the volts is 12VDC and the current is 1 amp or greater. While the pump only needs .7 of and amp, leaving some overhead in the power supply will help it run cooler and last longer.

Simen, the link wouldn't work for me. :-(

Jonathan


Simen

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Re: will this make a simple thermosiphon effect?
« Reply #12 on: August 28, 2012, 02:57:06 AM »
Quote
Simen, the link wouldn't work for me. :-(

That's strange; it does for me... :)
Maybe it's a site one needs to navigate yourself?: http://www.johnson-pump.com/ I chose 'Marine, circulation pumps' from the top menu, and then 'CM10'...

John;
I also have an outdoor hot-tub, which started with an older 200 litres tub and a submersible pond pump circulating water through a large (7.2sqm) solar-heater. It worked, but it was irritating sitting in the tub with a 230V pump between your legs... ;)
Now i've upgraded to a 350 litres tub, and a small sandfilter pool-pump. :)

Instead of the pond pump you linked to, you could use a small inline freshwater pump like this one: http://biltema.no/no/Bat/Pumpe/Vannpumpe/Inline-pumpe-25981/ They won't last forever, but should get you going. :)
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

johnE

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Re: will this make a simple thermosiphon effect?
« Reply #13 on: September 09, 2012, 08:07:42 AM »
Hi guys,

The best pump I can locate so far is the cheesy   DC12V0.21A(3W) http://item.rakuten.co.jp/auc-is8syoji/12v3w-pump/#12v3w-pump  all the other, higher power pumps are sold out.

madlabs wrote
Quote
As to the power supply/pump question, make sure the volts is 12VDC and the current is 1 amp or greater. While the pump only needs .7 of and amp, leaving some overhead in the power supply will help it run cooler and last longer.

my question:
Won't running the 0.21A pump at a higher amperage burn it out faster?



madlabs wrote
Quote
Don't worry about pump speed VS coil size. I did some pretty extensive testing, both with my wood stove and thermal panels and what I found is that with a smaller pump you get a smaller volume of hotter water and with a bigger pump you got more less hot water but the BTU's remained the same. In my large tub the bigger pump helped with thermocline effects overnight, but that won't apply to you. However, being able to handle the heat is important or the pump won't last.

Thanks so much for this!  If not for your explanation I would never figure out about the BTU's staying constant.

Hope to get something fired up next weekend,

johnE



madlabs

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Re: will this make a simple thermosiphon effect?
« Reply #14 on: September 09, 2012, 04:48:40 PM »
John,

The power supply current rating being higher than the pump's current rating DOESN'T mean that the pump will burn out faster. The pump will only pull what it can pull regardless. Think of it this way: If you had a small 12V motor and a car battery, it will run for X hours before the battery is dead. Hook more batteries up in parallel and it won't cause the pump to work harder, just longer.

So, overhead in a power supply is a good thing. If you have a supply rated for 1 amp and you are drawing .75 of an amp, the power supply will only be running at 75% of it's capacity and will run cool. If your supply was capable of .75 amps, then it would be running hot at 100% capacity. Now if we are getting nit picky, running a power supply at 50% capacity, especially swithing supplies will be less effecient.

Jonathan

bob g

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Re: will this make a simple thermosiphon effect?
« Reply #15 on: September 09, 2012, 08:15:32 PM »
here is an example of a bosch auxiliary heater pump

http://www.ebay.com/itm/water-Pump-auxiliary-heater-control-300SD-W126-Mercedes-/380461999001?pt=Motors_Car_Truck_Parts_Accessories&hash=item58954d6399&vxp=mtr

they are very common on ebay, and run in price from very inexpensive to insanely so.

the big clunky black bracket is actually a hard rubber that isolates the noise from being transmitted into the body of the car, it can easily be removed if you like.

btw, i would not be afraid of a used unit, i have bought several, some 30 years old and looking like hell, all worked just fine, none of them leaked.  in a system where you have no pressure i would expect they would never leak, at least not much more than a drip once in a blue moon.

they do a very good job in my opinion for the price, very easy to connect to with 5/8 and 3/4 barb fittings, (i don't remember which for sure, but i am thinking they are 3/4"barbs.

if you have a pick a part yard, they sell them for around 5 to 7 dollars if you remove them yourself,  if you go take a pair of side cutters, a 10mm wrench or socket and ratchet and a utility knife to cut the heater hoses, (much easier to remove the hose remains once  out of the car.

if you find several, i would strongly suggest buying more than one, they come in real handy for a variety of projects and are really easy to resell in the diy market... at a nice profit (to pay you for your trouble)

fwiw
bob g
research and development of a S195 changfa based trigenerator, modified
large frame automotive alternators for high output/high efficiency project X alternator for 24, 48 and higher voltages, and related cogen components.
www.microcogen.info and a SOMRAD member

johnE

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Re: will this make a simple thermosiphon effect?
« Reply #16 on: September 15, 2012, 03:11:21 AM »
Thanks again guys for your help,

I've got the stove fired up and the cheesy little (12V- .2A [3W]) pump running.  I had two AC-DC adapters laying around and hooked them up to the pump.  One adapter is 9V- 600mA the other is 7V 1200mA.

The best they could make was 0.5 Liters/minute which is not satisfactory to heat the water in the tub quickly enough.  I need at least 2 liters per minute of flow from the pump.  So I guess I need an adapter more closely matched or slightly above the rating of the pump as madlabs told me before.

I did however find another adapter from a laptop computer that is long gone.  This adapter is rated at 19V - 2.4A output.  I have it hooked up now and it is humming along putting out just under 2 liters per minute.  This is satisfactory.  However I realize this will probably kill the pump.  It has jumped the tub temperature greatly.

 I can't return the pump anyway and probably need a better one.  So I don't really mind soo much if it croaks, only if I'm in the tub at the time I guess. Just kidding, I'll turn it off or switch back to the other adapter if I need to run the pump and be in the tub at the same time for safety.

The pump is rated at 3.3Liters per minute and 1.45meters of head, it will raise water to that height supposedly.  Now it's only raising water about half of that and yet the flow was greatly reduced with either the underpowed adapter or the overpowered adapter.  So I guess the ratings on the pump are overstated.

That said.  Question Just how can I get the 19V down to 12V or thereabouts, if possible?
Or forget it and buy a used 12V adapter?

I do have two 12V / 105A  RV batteries wired in parallel, being charged by three 50Watt solar panels.
 Question Should I just run a wire from the batteries to the pump instead? , it's only about 30 feet distance.

Thanks again for taking the time to read this and my silly questions,

johnE



« Last Edit: September 15, 2012, 03:25:09 AM by johnE »

Simen

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Re: will this make a simple thermosiphon effect?
« Reply #17 on: September 15, 2012, 05:41:46 AM »
Quote
I do have two 12V / 105A  RV batteries wired in parallel, being charged by three 50Watt solar panels.
 Question Should I just run a wire from the batteries to the pump instead? , it's only about 30 feet distance.

In my opinion, that would be the optimal solution! :)

Regarding lifting height and liters/min, those two are connected, and i suspect that 3.3L/min are at no height at all, and at 1.45m, there's only a trickle. ;)

Also remember that restrictions in the hose (thinner hose inner diameter than the outlet of the pump) reduces flow and lifting height. Longer hose will also slow things down.
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

GaryGary

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Re: will this make a simple thermosiphon effect?
« Reply #18 on: October 21, 2012, 10:27:41 PM »
Thanks again guys for your help,

I've got the stove fired up and the cheesy little (12V- .2A [3W]) pump running.  I had two AC-DC adapters laying around and hooked them up to the pump.  One adapter is 9V- 600mA the other is 7V 1200mA.

The best they could make was 0.5 Liters/minute which is not satisfactory to heat the water in the tub quickly enough.  I need at least 2 liters per minute of flow from the pump.  So I guess I need an adapter more closely matched or slightly above the rating of the pump as madlabs told me before.

I did however find another adapter from a laptop computer that is long gone.  This adapter is rated at 19V - 2.4A output.  I have it hooked up now and it is humming along putting out just under 2 liters per minute.  This is satisfactory.  However I realize this will probably kill the pump.  It has jumped the tub temperature greatly.

 I can't return the pump anyway and probably need a better one.  So I don't really mind soo much if it croaks, only if I'm in the tub at the time I guess. Just kidding, I'll turn it off or switch back to the other adapter if I need to run the pump and be in the tub at the same time for safety.

The pump is rated at 3.3Liters per minute and 1.45meters of head, it will raise water to that height supposedly.  Now it's only raising water about half of that and yet the flow was greatly reduced with either the underpowed adapter or the overpowered adapter.  So I guess the ratings on the pump are overstated.

That said.  Question Just how can I get the 19V down to 12V or thereabouts, if possible?
Or forget it and buy a used 12V adapter?

I do have two 12V / 105A  RV batteries wired in parallel, being charged by three 50Watt solar panels.
 Question Should I just run a wire from the batteries to the pump instead? , it's only about 30 feet distance.

Thanks again for taking the time to read this and my silly questions,

johnE

Hi,
You might think about this pump: http://www.sun-pump.com/pumps.htm

It will run direct from a nominal 12 volt PV panel, or your setup with the batteries would be fine.
I used one of these all last summer and it seems like a well made pump -- its very efficient.
Mine runs fine on a 15 watt PV panel in good sun.

Gary