radiation of heat from stoves and pipe

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RustyShackleford

Minister of Fire
Jan 6, 2009
1,334
NC
I started talking about this in another recent thread and realized I was probably hijacking the thread and this probably deserves its own thread. We were talking about possible efficiency loss by using double-wall connector pipe, so I wanted to try to calculate what this would be. That is, how much heat does a stovepipe at various temps radiate into a room ...

It might be interesting, if we tried to get an idea of how much heat is lost with double-wall. I guess we'd need folks with IR thermometers to try to report the outer-surface pipe temps they see with both types of pipe under similar conditions, then use some physics to figure the difference in BTUs/hr radiated.

The Stefan-Boltzmann law describes energy radiation by black bodies:

watts = 5.67x10^-8 * e * area * (Tr^4 -Tc^4)

... where ^ means exponent, area is in square meters, T’s are in temperature Kelvin (Centigrade + 273), Tr and Tc are temperatures of radiator and surroundings, respectively, and e is emissivity (probably pretty close to 1 for a black stovepipe).

So let’s say Tc = 27 degrees C, or 300 K and stovepipe is 100 degrees C hotter. Area of a 6†diameter pipe 6ft high is roughly 1 square meter. So the radiation is about 1 kilowatt which is about 3500 btu/hr. If the stovepipe is 200 degrees C hotter than the room this figure triples (about 10,000 btu/hour). If it is only 50 degrees hotter, it’s only radiating about 1000 btu/hr. It’s pretty non-linear, not surprising with exponents of 4 in the equation. I’ve never measured my stovepipe temps, so I’m not quite sure where on the curve we are operating when talking about typical outer temperatures of single- versus double-wall pipe.

Of course, this ignores convective effects - air being heated by flowing over the hot metal.

Anyone wants to check my math, PLEASE feel free !
 
LOL I would check your math (if I could). That being said....your math is correct.
 
Couple of comments:
The pipe is (I hope obviously) not at a fixed temp along its length, with the net result that IR emission declines rapidly as you leave the vicinity of the stove.
When the temp gets low enough, any nasties in the exhaust gas start to condense in the pipe, adding internal insulation. (And fire hazard.)

My conclusions some time back: Keep the fire burning clean, and give it extra air until the pipe is well up to temp. To reduce availability of nasties and the likelihood of their condensing. Once the firebox is up to temp, just keep it clean and extract what heat is practical. (This can be a lot for the ~8' of pipe connected to my stove, from IR thermometer readings of pipe surface.) Forget going for 100%; be happy with 80-85%. Pipe cleaning will be much simpler, too.
 
Holy algebra batman!
Someone has way to much time on their hands ..lol.
 
HotCoals said:
Someone has way to much time on their hands ..lol.
My Nancy is off on a healing session with John Redcorn ...
 
I did a little spreadsheet showing some typical heat radiation losses from pipes of different lengths for different surface temps.
All results are in btu/hour.

Remember, this makes the following assumptions:

1. Emissivity of the stove pipe is unity. It's probably a little less than this, but not much.
2. The stovepipe is the same surface temperature all over.
3. The room is at 75-degrees F.
4. The stovepipe is 6" OD. Of course the whole point of this is to look at single- versus double-wall
pipe, and the latter has a slightly higher diameter. Just multiply the numbers by the diameter ratio
squared, for double-wall pipe.
 

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My double-wall pipe has holes in the outer wall at the bottom and top of each section. I've always wondered if these are vented as part of meeting clearance temperatures, or an attempt to sneak out a little convective heat.
 
madrone said:
My double-wall pipe has holes in the outer wall at the bottom and top of each section. I've always wondered if these are vented as part of meeting clearance temperatures, or an attempt to sneak out a little convective heat.

I don't have double wall pipe and know NOTHING about how it's installed, but those holes almost sound like screw holes for connecting the sections.... no?
 
RustyShackleford said:
HotCoals said:
Someone has way to much time on their hands ..lol.
My Nancy is off on a healing session with John Redcorn ...

:lol: Well done! I hope your Nancy comes back glowing with, erm, uhm, "health".

As someone else who has too much time on his hands (the man with the yellow hat is on travel), you have, unfortunately, caught my interest. Oh well, sucks to be you! :)

As much as I respect math as a tool, I'll use a different tool, and try to present a useful alternative viewpoint--what sort of pipe the stove is designed for, and the overall performance of the stove-chimney-operator-wood system, not just the pipe. On the plus side is the possibility of more heat, on the minus side is the possibility of too-cool exhaust causing creosote build up and insufficient draft.

What I'm suggesting is that harvesting extra heat from the pipe is a bad thing if it causes the system to preform poorly, and that will depend on all four factors.

A classic example is the endless Magic Heat debate, where some say it's the best thing since sliced bread, while others snicker into their coffee. The question is, is the stove *and* wood *and* chimney *and* burning technique causing more heat to go up the chimney than is needed for sufficient draft and a clean chimney? If so, a Magic Heat could be a good thing, though a different stove or burning technique could produce more heat without it.

I'm suggesting that the same considerations apply to getting more heat out of a single wall pipe, versus keeping the gases warmer with a double wall--and your own stove is an excellent example. The promotional video on the BK site shows the operator proudly sticking his hand *inside* the flue, because the gases are below 200 F--the point being that the stove has already extracted the optimal amount of heat. Likewise, BK specifies an 8" pipe and double-wall stovepipe for the King, and has a FAQ on how to reload the stove--because that cool exhaust increases the risk of smoke spillage.

So in that particular case it may be that sucking more heat out of the stovepipe would be a bad thing, though perhaps a very skilled operator with a baseball cap and aviator sunglasses could make it work somehow.

To summarize all that rambling, I'm suggesting that--although I am sincerely interested in your numbers and analysis, because I've wondered the same thing--an important consideration is the overall stove-pipe-wood-operator system.

"That is all." ;)

EDIT: No it isn't. :) I just saw the "Getting Maximum Heat Out of a BK" thread, and that you covered all my points in about three sentences, lol.
 
Hugo said:
madrone said:
My double-wall pipe has holes in the outer wall at the bottom and top of each section. I've always wondered if these are vented as part of meeting clearance temperatures, or an attempt to sneak out a little convective heat.

I don't have double wall pipe and know NOTHING about how it's installed, but those holes almost sound like screw holes for connecting the sections.... no?

No, they're 2" long vertical slots about 1/4" wide and 1/2" apart going all the way around the pipe. They're deliberate venting.
 
madrone said:
Hugo said:
madrone said:
My double-wall pipe has holes in the outer wall at the bottom and top of each section. I've always wondered if these are vented as part of meeting clearance temperatures, or an attempt to sneak out a little convective heat.

I don't have double wall pipe and know NOTHING about how it's installed, but those holes almost sound like screw holes for connecting the sections.... no?

No, they're 2" long vertical slots about 1/4" wide and 1/2" apart going all the way around the pipe. They're deliberate venting.

Interesting. I plan on going to double wall on my 7' foot run of stove pipe, after reading much info here, in a year or so. I hate to do it now since I just did the install in late November... Figure I should get a little wear from the pipe I spent good money on. Thanks for the info.
 
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