I ran into a masonry stove guy named Dale at this year's Pyromania workshop. (Pyromania is Ianto's annual workshop with rocket stove students & researchers: http://www.cobcottage.com)
Dale talked about two rules of thumb from the traditional masonry stove world:
1) 90 degrees F. If your flue gas is below that temperature, anticipate problems. They're working with a vertical chimney column that is both masonry mass and exit chimney. The only horizontals are in the baffled area. So it tends to gum things up if the flu gas gets too cold to rise up the chimney.
2) Dale does step down the baffles, putting in 2 in the first half and 3 in the second (downstream) half, reducing the effective cross-sectional area, like you suggest. With the width of the bricks, he thinks the channel goes from about 8" to 7" or 6." He does this gradually, not all at once. He says he does this to produce an even heat across the breadth of the masonry column, because if you push the cooling gas through a bigger space (quickly) it doesn't transfer enough heat. The extra "turn" through the extra baffle balances out the heat pretty well.
It would also compensate for (or take advantage of) the diminishing volume with temperature.
From experience, we did the opposite on an outdoor rocket stove in Portland (stepped up from 7" to 8" when we ran out of the main pipe size, figuring it wouldn't matter as much at the end). We also accidentally used a 6" instead of 7" heat riser. So the whole system was stepped bass-ackwards, with the hottest parts having a narrower diameter.
The stove was very sluggish. While the cob mass was still wet, the exhaust was definitely flowing down. An external chimney (8 feet of stovepipe) made the stove smoke back, but when it was removed (the flue gas poured out onto the ground from an ankle-height pipe) the stove worked somewhat better.
We've also been experimenting (since then) with bringing the exhaust back past the heat source on stoves with a vertical exhaust. We hope this will warm up the flue gasses again on their way out, and help with the vertical draft. Slight waste of heat, but in a few situations (windward side of house, so vertical stack needed to clear the ridgebeam) it seems worth it.
IDEAL GAS LAW: From my physics textbook: pV=nRT (ideal gas law)
p=pressure (absolute, not gauge). In most of our systems this is approximately atmospheric pressure. Yet we depend on the slight pressure differences between burn tunnel and barrel to push flue gas onward instead of using a constantly-rising "hot stack." Barometric pressure and wind affect our stoves, and on marginal stoves the weather makes a big difference in the performance.
n= number of gas molecules/moles present (variable in our case, but assume we want to know about a given "amount" or mass of gas as it moves through the system)
R= the gas constant, 8.31 J/K*mol (or convert to units of your choice)
Ernie says he read in a book called something like "Burning Wood" that at about 1900 degrees F, the last big carbon-based molecules burn away, and you get odorless "smoke" (exhaust or flue gas). If your exhaust still smells faintly like wet charcoal (9-methyl ketone), you're not quite at this temperature yet. 1900 F = 1038C = 1311 K volume = 4.5 L (True complete combustion)
At the heat riser throat, experimental stoves may get hot enough to melt iron, but we try to avoid going above red-hot on household models. (The barrels last longer that way. ;-} )
1500 C = 2730 F = 1773 K, volume = 6 liters (steel melts at 1777 K)
My barrel outside the heat riser gets hot enough to fry an egg. Clean burning woodstoves are above 350 C according to "Dr Brian" (http://www.betterwellsboro.org/cleanair/woodburningchem.html) So I imagine the streams of air, wood gas, flame, and flue gas mingling at temperatures like these:
426 C = 800 F = 700 K volume = 2.4 liters (Our guess as to a typical burn) 350 C = 662 F = 623 K Volume = 2.1 liters ("clean burn” for a woodstove; low for us) 232 C = 450 F = 500 K volume = 1.7 liters (books burn) 100 C = 212 F = 373 K volume = 1.27 L (water boils) 71 C = 160 F = 344 K volume = 1.17 L (eggs fry)
In the flues, the gas quickly cools from "cooking" temperatures (above) to the low 100's F by the exit vent. 100's F = 40-50 C = 313K to 323K, volume = 1.1 L (very hot day in Australia)
As discussed, under 90 F you begin to have denser-than-air problems with the flue gas. 90 degrees F = 30 C = 303K volume = 1.03 L (baby-bottle temperature)
and of course, if it gets back down to room temperature, it theoretically goes back down to 20 degrees C = 68 degrees F = 293 K, volume =1 liter (room temperature)
If you're dealing with outside air being at a colder temperature, then you could have air coming in colder. (Some people pipe in outside air for their wood-burning stove; I don't). Dense cold air is also circulating around your outdoor stack (It usually drafts well by contrast, but could also chill your chimney and make a cold plug)..... well, I'll give you our "cold day" and a polar day just to round out your ranges. There is a rocket stove bench in Antarctica, sez Ernie. So I'm not leaving anybody out.
13F = -11C = 263 K volume = 0.9 L (very cold for Portland OR; like New England winters) -18 F = -28C = 245K volume = 0.83 L (Arctic residential temperatures)
Note that this is physics - the joke about curing "spherical chickens in a vacuum" is funny because it's true.
We don't actually have ideal gases in a stove. We have a plasma of ions and chemical compounds, which combine, react, and condense into simpler molecules, including solid ash and liquid water. Since "n" in the original equation is changing, it's going to affect the volume from fire to flue. I don't know the chemistry of wood well enough to predict these effects.
But about 70% of the original air is N2 (nitrogen), which doesn't much react. We hope. So it's this 70% of the gas that we can reasonably predict will be the same number of molecules on the way out that it was going in. That means you can use the ideal gas law, plus or minus 30%, which is not too shabby given our average masonry skills at a typical workshop ;-)
The O2, wood volatiles, CO2, and steam are all doing crazy things that affect volume. A lot of our flue gas is water. It condenses onto our pipes, improving heat transfer, and also increasing our problems with corrosion. Or it exits the pipes as fog, making the flue gas into a suspension of dense fluids.
Steam condensation: that's the reverse of the boiler-explosion process, to give you a visual for the volume changes involved. Of course it's slower and gentler, and smaller quantities and concentrations are involved. But I would expect it to be significant.
Hydrogen is a tiny fraction of the _weight_ of a carbohydrate like wood cellulose, but in terms of "C(H2O)" converting into CO2 and H2O, the H is roughly half the molecular composition. Not to mention there's a fair amount of water in the wood to begin with. And resins, and other flammables of varying compositions. There's less water in the air at drier temperatures... I wonder if this keeps the air at about the same density over any particular temperature range?
A good (reliable, clean-burning, "whooshing" burn sound) rocket stove will put out clear gas at the exit flue, which immediately spreads out in colder air and makes clouds of visible "steam" (*fog*) until it spreads out enough to evaporate again. Most smokestacks and chimneys do this to some extent; look for the hollow-cored "cone" or "jet" shape within the streaming fog.
A good (efficient, wood-miser) rocket mass heater will suck more heat from the flue gas before letting it escape, and will put out an already-condensing visible fog (and the liquid water within its pipes) that drifts sluggishly down from a lower exit point at "blood heat." This looks like that "witches' cauldron" (dry ice fog) from science fair.
If you're really curious and want a practical answer, you could try collecting some flue gas in a mylar balloon and heating to various temperatures (maybe submerged in water/oil of that temperature) to compare the results. Then if you got condensation inside the balloon from your flue gas, it would create the actual volume change, and you could see how it differs from the ideal as a result.
The space below the barrel, where exhaust is gathered and directed into the bench needs to be LARGER THAN YOU THINK!
(Peterberg replied:) Very true, Donkey. I've attended a rocket mass heater workshop in the Netherlands last week and we've had a heated discussion about this very same point. I'd cut out the opening at a little bit larger than system size, and the leader of the workshop insisted it was far too small.
This is what I've learned this week: There's a difference between a stream opening and a stream profile. The main culprit is the gap between insulation canister and barrel. When you're only looking at the opening in the side of the barrel, it's easy to think you have to make the opening the same as system size. This is utterly wrong, because the gases are not streaming straight out of the opening. Instead, it is coming from left and right, and from the top. Moreover, in the corners two streams can't pass there at the same time, so you have to compensate for that.
Maths can help here. Start with system size area, divide it by the gap between inner and outer barrel, add twice the size of the gap for the top corners and you've got the length of the stream profile. The profile length consists of the top rim and sides of the opening.
For example: system size of 8" equals a little bit more than 50" square. The gap is, say, 2" wide, which will get us at 25", adding the gap twice will give us 29". Presuming the opening is one foot wide, the height need to be half of 15", which is 7.5 inches. The open area in this instance will be about 90" square.
The resulting opening in the side of the barrel will look enormous, nevertheless this is absolutely the correct method. The recommended gap is smaller than in my example, so the profile length will be even larger.
P.S. The top rim is rounded, so you have to measure it along the contour, not as a straight line. ---------------------------------------------------------------------------------------------------------------------
For better understanding of the stream profile phenomenon. The imaginairy ring on top of the heat riser is exactly such a profile. It's very handy to calculate it like this: divide the cross sectional area of the heat riser by its circumference. Pi R2 : Pi D= height of ring.
For example, an 8 inch system via Pi R2 will give you 50 square inch, the circumference is Pi D, a little bit more than 25 inches. Fifty inches divided by 25 inches is exactly 2 inches for the gap between end riser and the top of the barrel. Mark however, this is the minimum size for such a system according to the book, page 36.
The same method goes for a square heat riser, by the way.
Last Edit: Sept 25, 2013 3:22:40 GMT -8 by peterberg
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