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Heat Exchanger in a stove

12/26/2009 7:06 PM

Hi

I have question I was hoping you guys could illuminate me on regarding the use of copper as a heat exchanger in a stove im working on. Im afraid I don't have any pictures but if needs be, im sure something can be worked out.

So, the plan is to build a stove, (just a one off DIY project) which is capable of many things, including heating a very large room very quickly. The way im thinking of building it is using two different size propane gas canisters, the small one, which acts as the burn chamber fits inside the big one leaving a space in between of 30 mil. The walls of the canisters are 3mil.

In this 30 mil gap I want to coil copper pipes. These pipes will have three functions; one will have water being heated in it, going to the tank and then radiators. Another will be taking air directly into the burnchamber, preheating it along the way for maximum efficiency. The remaining space will be used to coil as many pipes as I can which simply take in cold air, and with the aid of a fan, pass it through the pipes and blast it out into the room as hot air.
Once the pipes have been set in the gap, I will pour in a clay slurry mix, which should add to the thermal mass, and protect the copper.

Im not really sure of the temperatures, but I would imagine about 600 degrees at least inside the burn chamber, its going to be very big and with a high thermal mass.

The Question is will the copper pipes take the heat over time? I found the pip im using connecting two fans, (the refrigeration types) and they are 2 mil walled with a 26 mil diameter. Some people say they will oxidise up and wear away. I would use steel, but obviously copper is a lot easyer to work with and I don't have a hydraulic pipe bender.

So any thoughts, suggestions.. anything other than copper that is easy to manipulate or get fixings for. I really want this thing to last.


Thanks
theo

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#1

Re: Heat Exchanger in a stove

12/26/2009 10:17 PM

There are many things to consider here. I think there are various steam cleaners that have copper coils as their heat exchange surface, but they are filled with water/steam at temperatures much lower than the melting point of copper. Air-filled tubes might be vulnerable to melting, depending on the flame temperature where it impinges on the tubes.

Encasing the tubes decreases the area that can transmit thermal flux to the tubes. This will protect the tubes but will decrease the efficiency. Overcooling of flue gases will condense acids onto the tubes, risking corrosion. Be aware that there is quite a balancing act that must be done.

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#2
In reply to #1

Re: Heat Exchanger in a stove

12/27/2009 11:16 PM
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#3

Re: Heat Exchanger in a stove

12/27/2009 11:18 PM

Hi Theo,

Several thoughts come to mind for your water heating project. Combined Heat & Power is always my preference. I have several videos listed in my Favorites about Wood Gasification that apply here. As matter of fact 1 video referenced below is about building a wood gasifier from propane tanks , so you might like it. A Woodgas cleaner section would use coiled copper tubing to exchange the heat with water inside copper tubing so, I know it works & long term as in years. The cleaning process cleans the gas so you can run a motor with it - like a generator. Join my facebook group If you need any further refferences, its noted below too.

Hope some of that helps you !

1st link:

http://www.youtube.com/user/GeorgiaAdobe#p/f/17/-cXzDGscKt4

Joe Woodall, Managing Partner

Georgia Adobe Rammed Earth & Renewable Energy

http://www.georgiaadobe.com

http://www.dopestcontrolyourself.com

http://www.facebook.com/georgiaadobejoewoodall

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#4

Re: Heat Exchanger in a stove

12/28/2009 9:03 AM

You don't have to use copper in the stove. Steel would probably be as good and resist to higher temperature. If rusting is a problem, Stainless could be used.

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#5
In reply to #4

Re: Heat Exchanger in a stove

12/28/2009 9:50 AM

I suppose its finding the balence between copper and steel, bear in mind i dont have the facilities to bend steel thinker than 2 mil, so 1.2 mil is what i could get/could bend. Whereas with the copper i have, it is a 2.5 mil wall. Not sure what would be most durable, Stainless would be fantastic, but again i do not have equipment or know how to work with it.

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#6

Re: Heat Exchanger in a stove

12/28/2009 10:06 AM

Theo ,

SS would be a better animal but it is more expensive. Bending into coils is pretty easy with the copper , if its the thick walled you can add sand & pack it before bending to assure no kinks. Tap and wash it out after. Use a small dia log on some saw horses with a v notch & a younger strong back to turn the thing, with a pole bored through it, on one end.

Keep us updated !

Joe

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#7

Re: Heat Exchanger in a stove

12/28/2009 10:26 AM

I have several suggestions and ideas, but I need to know what type of fuel you are using and are your temperature; is it in deg. C or F?

The copper will work as long as you avoid condensation, direct flame and over heating.

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Guru

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#8

Re: Heat Exchanger in a stove

12/28/2009 10:48 AM

30 mil is about 0.75 mm. 3 mil is just 75 micrometers. Is this understanding correct? Nothing can be done with such dimensions.

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#9

Re: Heat Exchanger in a stove

12/28/2009 10:49 AM

Here's a drawing we use to discribe the heat transfer process to our clients. Maybe it will be of help to you too.

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