creosote

[gyrfalcon]

The flue gases exiting my stove into my single-wall pipe begin cooling more rapidly than yours going into your double-wall pipe. The benefit I gain (at least in my imagination) is a bit more heat radiating into the living space from the stovepipe. The price I pay is the fact that creosote formation occurs when the moisture in the flue gas stream that's carrying along with it nasty other combustion products cools to the point of condensation. That probably happens lower in my system than in yours...but I can't say for sure, because my system has but 2 45-degree direction changes, no 90's. Your intuition is correct...gook deposition is a complicated phenomenon, but among the factors that are signifianct in exacerbating it are decreasing temperature and decreasing mass flow rate...this is why we're always harping on folks not to dump a 6" flue into a gaping 8" x 8" chimney...everything s l o w s w a a a y d o w n when you do that. Rick

Ah. Gotcha. One last fine point occurs to me-- the diff between the hot air/gasses from the stove and the temp of the flue wall and chimney wall are what causes the condensate, but now I'm suddenly questioning why the exterior flue is a such a big no-no. Flue gasses are roughly what, 600 or 700 degrees, no? The interior temp of a room is 65-70, and on a bad day, the outside air is, say, 0. IOW, I'm wondering why that 65-degree diff in comparative air temp (and most of the time, it'll be a even less than that, maybe 40 or 50 degrees difference) appears to loom so large.

I promise that's my last question for the day, but since I've just gotten my mind wrapped around the concept of mortgage-based securities and the like, I figure I'm on an intellectual roll and ought to take advantage of it while it lasts.

 

[begreen]

At issue is the point where the flue gases cool enough to condensate. I think this is at ~250 degrees. So if the stove flue gas is at 400 degrees when exiting the pipe, no problem. But if it's at 400 when exiting the stove, but cools down to 220 by the time it reaches the last section of pipe, problem. Here's a couple points of view on the problem(s) with exterior chimneys:

http://www.woodheat.org/chimneys/evilchim.htm
http://www.woodstove.com/pages/good_chimney.html

and one on creosote:

http://www.mastersweep.com/CREO.HTM

 

[fossil]

The rate of heat transfer through a material is most significantly affected by two things: the thermal properties of the material itself (double-wall pipe, for example, will transfer heat more slowly than single-wall), and the temperature differential across the thickness of the material. This is why any exterior chimney should be insulated, and it certainly doesn't hurt to use insulated connector pipe, either. The flue gases may exit the stove at the flue collar at 700 degrees, but then they've moved away from the heat source, and so begin immediately to cool...when the temperature of the flue gas stream approaches 212 (lower as you increase in altitude), condensation will be be happening. The longer it takes that gas stream to get to daylight, the cooler it becomes. The gas stream will cool more rapidly if it's moving more slowly, if the temperature differential across the pipe wall is greater, and if the thermal resistance to heat flow of the pipewall is lower. And yes, I have a Master's Degree in Gookology. Oh wait, no, it's Geekology, sorry. Rick

 

[gyrfalcon]

At issue is the point where the flue gases cool enough to condensate. I think this is at ~250 degrees. So if the stove flue gas is at 400 degrees when exiting the pipe, no problem. But if it's at 400 when exiting the stove, but cools down to 220 by the time it reaches the last section of pipe, problem. Here's a couple points of view on the problem(s) with exterior chimneys:

http://www.woodheat.org/chimneys/evilchim.htm
http://www.woodstove.com/pages/good_chimney.html

and one on creosote:

http://www.mastersweep.com/CREO.HTM

(smacking self upside of head) Of course! Thank you. As I said, physics aint my strong suit. Appreciate your patience.

 

[gyrfalcon]

The rate of heat transfer through a material is most significantly affected by two things: the thermal properties of the material itself (double-wall pipe, for example, will transfer heat more slowly than single-wall), and the temperature differential across the thickness of the material. This is why any exterior chimney should be insulated, and it certainly doesn't hurt to use insulated connector pipe, either. The flue gases may exit the stove at the flue collar at 700 degrees, but then they've moved away from the heat source, and so begin immediately to cool...when the temperature of the flue gas stream approaches 212 (lower as you increase in altitude), condensation will be be happening. The longer it takes that gas stream to get to daylight, the cooler it becomes. The gas stream will cool more rapidly if it's moving more slowly, if the temperature differential across the pipe wall is greater, and if the thermal resistance to heat flow of the pipewall is lower. And yes, I have a Master's Degree in Gookology. Oh wait, no, it's Geekology, sorry. Rick

So I figure with the eddication you two are giving me, I can maybe graduate 8th grade next year.

Thanks, Rick. Elementary stuff, and I really appreciate the patient explanation. Too easy to lose track of the fact that these are dynamic processes, not static ones.

And I should know better because I've spent a lot of hours over the years explaining how hawks migrate through thermal elevators and under what conditions you get the best ones, and this is the same set of general principles at work. Duh.

 

[begreen]

Rick knows a lot more about high altitude physics. He's been balloon surfing the upper atmosphere. Me, I'm a flatlander.

 

[fossil]

Heat transfer will never be a static process. It will continue until the entire universe reaches the same temperature, then heat transfer will cease to exist. But I think we have some time. Rick

 

[gyrfalcon]

Rick knows a lot more about high altitude physics. He's been balloon surfing the upper atmosphere. Me, I'm a flatlander.

I noticed that. I think it's exposure to all that burning weasel condensate over the years.

 

[fossil]

Ferrets. Ferrets go in the stove. I brake for Weasels...possibly because I feel some sort of kinship, dunno. Rick

 

[KeithO]

Wood is basically a hydrocarbon fuel with some water added and some inorganic materials (ash). The hydrocarbon chains are not as long as crude or synthetic oils. When any hydrocarbons are burnt, particularly at temperatures below 600 degrees C, soot particles can act as condensation nuclii for hydrocarbons that are in the vapor phase. One gets something very similar with atmospheric clouds, in that small particles of dust are the catalyst for condensation forming in clouds. No dust and the onset of condensation is delayed until the temperature falls much lower.

The temperature of 600 C is listed because that is the temperature at which pure carbon will begin combusting. But in a wood fire scenario the nature of the fuel itself changes through the burn. To begin with, the volatile fractions (resins) are the first to burn, and after a certain time only carbon remains. By contrast, when burning anthracite coal, there are virtually no volatiles and the combustion process is consistant throughout. Bituminous coal will burn more like wood, except for the high sulphur content and much larger ash content. The sulphur and ash problems are what makes coal fired power generation a "dirty" process. The ash can be removed electrostatically, but removing SO2 is very problematic.

So from a practical perspective, unless one is burning wet wood, the majority of creosote condensation will occur when one re-fuels the stove and then dampers it down too severely. Combined with external factors (outside cold chimney, low stack temp at the end of the burn, sharp bends in the chimney etc) this is the most problematic period. Because of the large amount of volatiles released at the start of the burn, it is also a problematic time to maintain a stable burn temperature. There is a strong tendency to overfire, if one does not keep an eye on the primary air setting in the first 5 minutes. Of course it is safer to simply damper down more, but there is a risk that the burn will stall out and turn into a smouldering fire which is a nice creosote generator (and a disaster for air quality in the neighborhood).

To avoid creosote, the best recipe is to get the stack temperature back up, which makes for a good strong draft and gets the secondary burn going strong to burn off smoke. Rather a bit on the hot side than cold and if dampering down afterward there is less likelihood of the burn stalling out. If burning wet wood, it will be nearly impossible to get started and keep the fire going with all stages in between depending on just how wet it is.

 

[mranum]

Great discussion here been enjoying the explanations.

In my case, first year burning, I have an EPA fireplace with stainless insulated flue pipe running straight up through the ceiling from the burner. 15' total with 5' above my roof. Burning good seasoned fuel and running decent temps it sounds like I should have very little buildup of creosote in the pipe. Is this correct?

 

[gyrfalcon]

There is a strong tendency to overfire, if one does not keep an eye on the primary air setting in the first 5 minutes.

Nice explainer. Re coal-- I had a very vivid demonstration of how dirty it could be when i was in Eastern Europe one winter in the early '60s. The coal gunk in the air was so bad that you got black smudges around your nostrils in a matter of a few hours, and by the end of the day, half the people you saw on the street had a dark patina of coal dust covering their faces.

But more to the point-- I'm not sure what you mean by "overfire" in this context, ie, at the beginning of a burn. With my tiny firebox and last year's less than perfectly dry wood, I sure never had a problem of the stove getting too hot, quite the opposite. On the other hand, with a soapstone stove, I wouldn't know if the start-up was too hot before I start closing off the air gradually because of the lag time for the surface to register the firebox temp.

So I'm not entirely sure what you're saying. Could you elaborate a bit on those first 5 minutes and how they should optimally go?

 

[KeithO]

If you read the definition of dry wood (according to the stove manufacturers) it goes something like this:

"Add a large piece of wood to the stove when it has a good large bed of coals. It is dry if it is burning on more than one side within one minute. It is damp if it turns black and lights within three minutes. If it sizzles, hisses and blackens without igniting in five minutes it is soaked and should not be burnt."

So basically a split should be burning on more than 1 side in 1 minute. If the load burns that fast and you don't keep control of the primary air by gradually shutting it down, the stove can get hot enough to have the stove connector and stove pipe glowing red hot and burn away the secondary air tubes and the baffle. One sees overfired stoves on ebay every year with all the paint burnt off and the characteristic white appearance after it has cooled down.

If you have not yet experienced an overfire, you can get an idea by opening the ash door. The draft will increase dramatically with ash being carried up and out the chimney. In my installation, it only takes a few seconds until an audible resonance is set up that sounds like a rocket taking off. There is no mistaking that sound... And it is pretty scary to think that something like this could happen when the stove is unattended, if there were to be some form of mechanical failure.

 

[jpl1nh]

Interestingly enough, in my set up which is ok but not optimum, my creosote buildup is greatest in the single wall pipe from my stove to the chimney. The Keystone uses a 7" pipe and I vent into an interior tile lined masonary chimney that has a 6 x 10" flue. My stove is a cat too. Typically, the magnetic thermometer on my single wall pipe about 12" above the stove reads 250, though it may climb up to 300 once in a while if the cat is really glowing. I would speculate that in my case, the rectangular flue, the slightly oversize flue, and the lower temps of the exiting flue gasses, create a slow enough flow that it allows most of the condensation to occur within the single wall pipe. I typically clean it out several times a season and get several cups of flaky stuff just from that 5' pipe run. When I sweep the tile lined flue, which is about 15' from the thimble to the top, I get only a couple of cups. Perhaps, because the stove runs 24/7, the tile may stay more consistently warm than the interior single wall pipe. All speculation, but that's my personal experience with this stove set up.