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Post by mwalimu on Dec 27, 2014 0:09:15 GMT -8
The copper coil has VERY little volume. If I connected it to the coil, it would NOT thermosiphon, it would just sit there getting hotter and hotter until it finally boils, flashing to steam and blowing up my pipes. The way it is currently, it will heat up the buffer water very nicely and safely. Hmm, in Africa we had only one meter of a 1/2" or 3/4" of steel pipe (a horizontal U-Pipe attached to the baking oven wall) in the fire chamber of a kitchen stove as a heat exchanger. It thermosiphoned reliably with a plastic barrel and we had never a fail. It was an open system. The barrel was standing upright, higher than the stove, one pipe was connected to the bottom, the other to about the center of the barrel. |
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Post by Donkey on Dec 30, 2014 11:04:22 GMT -8
Hmm, in Africa we had only one meter of a 1/2" or 3/4" of steel pipe (a horizontal U-Pipe attached to the baking oven wall) in the fire chamber of a kitchen stove as a heat exchanger. It thermosiphoned reliably with a plastic barrel and we had never a fail. It was an open system. The barrel was standing upright, higher than the stove, one pipe was connected to the bottom, the other to about the center of the barrel The temperature difference between the water in the coil and the fire itself are so great that transfer efficiency would be rather high. You are describing a totally different kind of system than the one posted here.. More of the classic, coil in the fire thermosyphon system. That system is likely quite safe (being an open system), though putting the fire in direct contact with a (relatively) cold coil of water will reduce efficiency of the fire, which isn't typically very high in wood fired ovens. The beauty there is simplicity. Not a lot to go wrong there. |
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stil
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Post by stil on Mar 29, 2015 21:58:28 GMT -8
I've done a little testing of this idea. This coil is made with about 49 feet of 3/8 inch, soft copper pipe.  Here, I set it in a pot of water on a propane stove and let it boil for some time. No problems.  May I ask if someone has used a car radiator as heat exchanger, would that work? |
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Post by Donkey on Mar 30, 2015 20:26:52 GMT -8
I don't know that anyone has used a car radiator, I've wondered if it would work myself but I've not heard of it being done yet. I called the manufacturer of a different heat exchanger (hot air to water) and after a long conversation, learned that that type of heat exchanger probably won't work. I think the thing to do is estimate how much water to water surface area is present in your exchanger. You want as much as possible. As a comparison, 60 feet of 1/2 inch copper pipe (as a coil) has around 7.8 square feet of surface area. My small experience suggests that twice as much may be about right. |
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Post by edwardb on Mar 31, 2015 0:22:16 GMT -8
I think you might be in danger of reinventing the wheel...
In the UK, until relatively recently, all hot water systems were by law open-vented so the pressure at the tap was limited by the height of the header tank in the loft. Mains-pressure systems are now permitted but are heavily regulated so lots of open-vented systems are still being installed. Although solid-fuel water heaters are obsolescent, a lot of the design principles derived from managing their not-very-controllable heat output are still embodied in current plumbing practice.
Most households now have gas-fired condensing boilers. The heat-exchanger in a gas boiler transfers heat from hot combustion gases to water, and does so efficiently. The combustion gases in a clean-firing rocket should not be significantly different in composition to those from a gas-fired boiler, essentially C02 and H20 plus some NOx, so perhaps repurpose an old gas-fired boiler? This is really the only part of the system that's different from an old coal-fired one, which had boilers extracting heat from the body of the fire (undesirable in a wood fire).
If you have the height differences, a primary hot water circuit will thermosyphon effectively. Systems were designed not to need electrical power so you still had heat and hot water after an outage. Modern systems are less resilient. The primary circuit contains the boiler heat-exchanger and the primary coil which sits inside the lower half of a secondary storage cylinder. It is open-vented with a header tank fed by a float-valve from the cold mains; an expansion pipe vents any over-flow back into the header tank. For a coal-fired system, the header tank would be galvanised steel but most are now plastic. The fluid in the primary circuit is water, treated with a corrosion inhibitor to prevent electrolysis between the iron and copper components; the only losses are by evaporation from the open vent. Boil conditions can occur but are alarming rather than dangerous if the header tank is steel. For most effective thermosyphoning, it is best to use 28mm/1in copper, with a circulating pump you can use 22mm/(3/4 in).
The storage cylinders, available off-the-shelf in almost all plumbers merchants today, are made from copper. They are called "indirect" if heated by a separate primary coil. "Direct" cylinders are also available. They are also open-vented with a larger feed-expansion tank, also kept in the loft. (pressurised cylinders are made of stainless steel). All the pipework on the secondary side is copper to avoid electrolytic corrosion. The cylinder is sized for household hot water demand,
A thermostatic valve on the primary return will divert heat to a relief radiator if the water in the storage cylinder is too hot. Normally water is stored at between 60 and 65 Celsius; 60 in hard water areas and 65 in soft. The cold water supply to baths/showers is fed from the same feed/expansion tank in the loft so it is at the same pressure as the hot - substantially below mains pressure. (in most UK houses, only the cold tap in the kitchen is fed from a rising main).
Hot water is mixed to a safe temperature at the outlet, nowadays with thermostatic mixer taps although when I was growing up it was mixed in the bath-tub.
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Post by matthewwalker on Mar 31, 2015 8:32:01 GMT -8
Great post Edward, thank you for taking the time to outline that so clearly.
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stil
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Post by stil on Mar 31, 2015 11:18:35 GMT -8
I don't know that anyone has used a car radiator, I've wondered if it would work myself but I've not heard of it being done yet. I called the manufacturer of a different heat exchanger (hot air to water) and after a long conversation, learned that that type of heat exchanger probably won't work. I think the thing to do is estimate how much water to water surface area is present in your exchanger. You want as much as possible. As a comparison, 60 feet of 1/2 inch copper pipe (as a coil) has around 7.8 square feet of surface area. My small experience suggests that twice as much may be about right. I have a small copper radiator something like this link. Radiators have a lot more surface area compared to just a copper pipe and this why I wanted to use it. I just don't know what happens as water flows out of the water line which is bigger diameter and flows into the narrow radiator channels? Pressure of the water will be increased and flow decreased I think. It is appealing to use it that is why I am asking. |
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Post by Donkey on Apr 9, 2015 1:08:09 GMT -8
It IS appealing. Cheap and simple too. I think that definitive answers will only come from trying it.
It should be pretty easy to test, since you have the radiator already. You don't even need to build a stove to see if the radiator will work, just a pot large enough to accommodate.
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Post by wrekinwanderer on Oct 23, 2015 4:35:55 GMT -8
Hi Donkey, wanted to ask your advice as I'm changing my hot water system to emulate yours. The system I've been using is based on UK plumbing concepts of open copper cylinder and header tank. It uses a hot water jacket riser feeding into an insulated copper tank. It works brilliantly, with a heat up time of 10mins, but the shower pump has died and I would prefer to spend my money on 50m copper tube, rather than on a cheap plastic pump that might give up the ghost in a few years. The water for the shower/bath is taken out of the top of the cylinder, and the hot water from the riser goes into the top of the cylinder, so with the hot water all concentrated at the top of the cylinder winding a coil into the length of the cylinder I don't think would achieve very much....
So, will move the hot water riser into the house and wind some copper coil into the cylinder, with the insulated riser coming up beneath. The batch stove I use is 4", with 5" flue. I've got a few options with the spacing in between the water tank and bell and would be grateful of your thoughts.
Okay, a 35cms diameter water tank into a 50cms or 56cms diameter bell. Or alternately, a 50cms old steel and stone lined tank into a 56cms bell.
What would you reckon would give the greatest heat transfer to the water?
Thanks very much for your time,
Mike
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jody
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Post by jody on Jun 13, 2022 4:01:19 GMT -8
Hi Donkey, I saw on you tube that a few years back you needed to replace the tank for this because it developed holes, did you ever work out why that was? also it's now a few years again since you replaced the tank has the problem occurred gain with the new tank?
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