Thermodynamics heating water from a copper tube

[only1db]

Ok, first off I need to explain my project first.

I have a wood pellet stove that I want to use to heat hot water for a radiant heating system. I will have to do a custom job because nobody makes a kit for my particular stove.

My question is howong of a colper tube is needed to heat the well water from 55 deg f to about 180degrees f?

Il probably be using .5in copper tubing. The surface temp of the top of the burn chamber, where I wou l d like to run the pipe, has a temp range of 250 to 450 deg f. Some kits have the copper tubing directly into the burn chamber. My particular model will not allow me to do this...so capturing the radiant heat is my only option.

Are there other variables that I am missing?

 

[Puma]

There isn't enough information to form an answer. It depends on the flow rate of the water, the volume of hot air you have in the chamber and the input rate of heat into the system and so on.

 

[sophiecentaur]

Are there other variables that I am missing?

Quite a lot needs to be known. What is the temperature of the inlet and exit water for the heat exchanger? What is the flow rate, what is the estimated Power output of the burner? (You can only expect a small proportion of this to get delivered to the water - most of it will go up the flue - as usual in low tech burners).

 

[only1db]

The pipe is going to be coiled up (probably like a snake but flat) into a space of about 18x3x12inches. Flow rate is determined by the thermal efficiency of the particular room (have not don't those calcs just yet...but from everything i have read its approx 10 to 15gpm.

Inlet is going to be variable. When it first starts it will be 55 degrees (might run a small electric water heater just has storage and have the temp set low...so possibly a start of 90 to 100 degrees. the temp once set out sound be somewhere around thw 100 to 110deg f.

There is no flu for the heat to go up. I am not wrapping it around the pipes but rather laying the pipe directly on the metal of the burn chamber, which gets up to temps of 250 to 450 deg f.

 

[sophiecentaur]

There is no flu

How can that be? The room would fill with smoke.
The main issue here, as I see it, will be contact area. Either the pipes need to be in the hot flu gases or the pipes need to be stuck (brazed) against the wall of the stove -. Heat transfer rate is proportional to temperature difference, area of contact and inversely proportional to thickness of the thermal path etc. etc. Steam engines tend to use boilers with water in pipes, passing through the hot gases or with gas in pipes, going through the water - intimate mixing!

 

[only1db]

Would i be better off trying to get the pipe oin the fire chamber itself as opposed to trying to get the transfer of energy from above the chamber?

 

[sophiecentaur]

Would i be better off trying to get the pipe oin the fire chamber itself as opposed to trying to get the transfer of energy from above the chamber?

Yes. Of course., in principle. The greater the temperature difference across your pipes, the faster the heat transfer. It's the system used in Central Heating Boilers. You could have problems with local boiling, doing it that way and that could pose a serious danger. Wood is a harder fuel to meter than gas. Also, the pipes could get eroded by the hot gases too.
I was talking to my neighbour yesterday, as it happens, He has had a wood burner with a back boiler installed. It's a serious Eco House, he's having built.

 

[only1db]

Are there any formulas that i might be able to use to even getg in the ball park?

 

[sophiecentaur]

Are there any formulas that i might be able to use to even getg in the ball park?

Not for the squeamish, I'm afraid but you may find something in this link. It all depends on you level of competence with doing this sort of thing. There will almost certainly not be a simple formula which does what you want directly. There are so many variables and you would need to specify the construction and how the tubes are put in contact with the heat source. You really would need to get to know more about this stuff in general - O level or A level Physics would be a start.
You could always put an upper limit on what you could expect if you know the heat rating of the burner itself. You could then say that you could expect perhaps 10% of that (absolute tops).
Otoh, you could always just suck it and see, if building things is your thing. I think you would need to be careful about wrapping the stove in any sort of insulation, though as it could overheat, locally and warp / crack when hot. Also, you need to be careful to allow the flue to get nice and hot or it will not draw correctly.

 

[only1db]

This is a pellet stove, not a wood stove. there is an exhaust fan that pulls out the exhaust. there is no butterfly flu to change the temp. it simply burns and heats a ceramic coated tgemp sensor and when t hits a certain tgemp the blower fan comes on and blows heat. when the termastat reaches the temp then it burns out and turns off.

Im simply trying to harhness some of the wasted heat from outside the chamber.

 

[sophiecentaur]

OK, so the flue could be kept coolish (as with a condensing boiler) without compromising the burning process. Thinking along those lines, perhaps a long coil of copper tube, wrapped around the flue would give a worthwhile amount of hot water (cold water inlet at the far end and hot water outlet just next to the stove). You might need to deal with condensate, though.
There's still serious problem with doing the sums, though.

 

[Puma]

Never mind the calculations. You need a pump which is regulated by a thermostat kind to stop the water boiling and to get the water out at an even temperature, a relief valve, a large immersion tank to feed your heating or hot water, and another thermostatically controlled device to pump the water through your radiators / heating system when the whole lot nears boiling point. If the heat from where ever you are taking it is completely wasted just use as much copper piping as will fit around the flue and insulate it to maximise extraction.

 

[only1db]

Here is a picture. I am not wrapping the copper around the exhaust pipe. I am simply placing it into an "oven" the triangular portion to the right is the burn chamber and the area sloping down to the back (top triangle) is the pellet holding chamber. that empty space inbetween is where i want to put the pipe in. hell once the water warms up and the cycle starts the imcoming temps will be much higher then the original 55 deg inlet from the well.

Im also aware of the two aqua states and associated blow off valves need. simply just trying ti figure out how long of a coil

 

[only1db]

Some reason i can't upload my pic

 

[Lok]

This is a pellet stove, not a wood stove. there is an exhaust fan that pulls out the exhaust. there is no butterfly flu to change the temp. it simply burns and heats a ceramic coated tgemp sensor and when t hits a certain tgemp the blower fan comes on and blows heat. when the termastat reaches the temp then it burns out and turns off.

So are there two fans? One for exhaust and one for hot air dispersion?

 

[only1db]

Yes. The exhaust turns on right away when the stove turns on. Then after the ceramic element is up to temp the second blower fan kicks on and hot air is blown into the room. Where I want to put the piping doe not interfer with operation of the stove.

 

[only1db]

*crickets chirping*

Nothing else uh? no suggestions? trial and error it is then.

 

[sophiecentaur]

*crickets chirping*

Nothing else uh? no suggestions? trial and error it is then.

Well - if you don't want to affect the operation of the stove in any way, that sort of implies no significant heat output from any bolt on system. If you want a useful amount of heat then you need 'intimate contact' somewhere. That will either reduce the surface temperature of the stove, require getting the heat from the flue gases or putting more fuel in in any given time. The beauty of using the flue gases is that their heat would be just wasted. I realize that involves a lot more engineering.
You could do some interesting ball-park measurements of Power output by putting a pan of water on top of the stove and seeing how fast a given mass is given a rise in temperature.
Power (=Watts = Joules per second) = mass (kg) X rise in temperature(°C) X 4,200 / time in seconds

 

[CWatters]

You need a pump which is regulated by a thermostat kind to stop the water boiling..

I'm not an expert on the worlds regulations but in some countries the use of a pump is banned on safety grounds. Instead they require a gravity loop. Predicting flow rates for that must be tricky. I think some countries also require a dump rad if a large thermal store isn't fitted? A pressure relief valve to the outside would be essential on any system.

 

[Puma]

Well all heating systems in the UK have a pump in them so I doubt having a pump would be illegal in anyway. He's aware of the relief valve and I mentioned the need for a large tank.

 

[sophiecentaur]

I am familiar with old fashioned back boilers producing bumping / boiling noises. The main problem is that it makes the hot water all brown!

 

[Puma]

The bumping noise is due to a blocked heat exchanger most commonly or insufficient water flow. There should be an automatic shut off but they don't work so well once the system gets blocked. They should also have an expansion bladder which usually takes care of the bumping noises in the heating side. You can get cleaning compounds to dissolve the iron deposits into solution so they can be drained out, similarly there are oxidation suppressants which you can put in afterwards to keep the heating water clear. If you have brown water out of the tap you have a very old system!

 

[sophiecentaur]

you have a very old system

Oh yes. The house was built in the early 1930s and I was using the back boiler in the late 1970s.
What 'expansion bladder'? haha. Just two fat pipes going vertically to a (galvanised steel) tank in the bedroom. It never exploded - at least not while we lived there!
You can't beat a bath in water that's already brown, before you even get in it.

 

[SamaritanElad]

I would get a heating baseboard section for hot water home heating, as you would want a lot of area (to capture/transfer the heat from its fins) in the stove if you have the internal room area. Might be easier to clean the dust/film ash of those pipes instead of a tightly pack coil near the internal wall. Think ahead!
Putting it in the flame box - Not Me, that force air on the pellets is like a forge furnace. An empty pipe will probably burn through. Too hot there. You would need a reliable water circluation pump and if you don't use the water, your tank is going to boil. I wouldn't do it unless you have an indoor pool to use all that heat up

I too was thinking of doing the same (heat exchange) for my pellet stove (Harmond p68 - 68000 btu unit). My exhaust is very touchable as there is not a lot of heat going up out of the flue as the fan reuses most of that exhaust heat to combine with fresh air for the burn pot). A good pellet stove is very efficient
Any extra water heat you can is savings, so go for it. Be sure your hot water tank's pop-off valve works (I'd put 2 in just for safety)...
Good luck and let me know how it working.
O, just install a 25'-30' pipe straight up (open at top/filter) on your hot water line as you won't have the normal house pressure to raise water that high to be your buffer, and that will be a cheap permanent replacement for a bladder tank that usually goes out often
Your brown water is from sediments being disturbed from the popping/boiling water... Sounds like your tank needs a good flush...

 

[sophiecentaur]

Sounds like your tank needs a good flush.

That tank (and whole system) was consigned to the scrap heap years ago. But I am only the ex-old back boiler user and not the OP.

 

[sophiecentaur]

there is not a lot of heat going up out of the flue

I have a condensing gas boiler and there is very little nasty stuff in the condensate. It seems to be drained away quite well and there is no build up in the flu (CH₄ is a very clean burner). I was wondering what the situation is with a pellet burner. Does the flue need to be cleaned often and is there any accumulation of condensate?

 

[Lok]

*crickets chirping*

Nothing else uh? no suggestions? trial and error it is then.

Ahh, been a bit busy, sorry for the hyper-ultra-late reply. What I would like to point out is that you already have an engineered object that should function quite well within the working parameters that you now wish to change.
If there are already two fans there, then it is highly unlikely that you will get a lot of heat from some copper tubing with water and so on. As while some components tend to heat up to boiling temperature, their power level is so low that heating water above a few degrees is not really feasible in a reasonable time frame.
You can try this by placing a water container near your pipe and measuring how much time it takes to reach it's maximum temp (which will not be a lot IMO). If it takes more that an hour to get from 20 to 60 C', let it as is.