|
Post by pyrophile on Jun 4, 2017 9:03:39 GMT -8
Hi! I would like to share a few questions and thoughts about the threshold in the firebox wich seems to change more things than it seems to (for a better combustion, apparently!).What is your opinion?
In this new configuration with the "simple" and elegant solution of the threshold, are the slopping bricks in the bottom of the firebox still necessary? As the embers are not anymore strongly activated by primary air, couldn't there be a flat sole (and a simpler building)? In the threshold mode, could we say that there is hardly no primary air anymore as there is no much air directly activating the embers and make the wood to gasify? Instead, we have two secondary air supplies : 1) one arriving in the firebox much above the embers, at the level of the pyrolysing wood (by the action of the threshold leting some air going up) , this air slowly burning the wood gases and 2) another secondary air arriving directly in the riser and burning the main part of woog gases. Both airs burn wood gases, no one -or very few- activating embers, that is both having the rôle of secondary air? It would be a bit like in german-austrian mode where there is no more primary air coming below embers. In this cas, the heat necessary for wood gasification is -hardly- only created by the burning of gases and the hot firebox's walls.
One could suppose that the use of the threshold, without air strongly activating the embers, should slower the speed of gases entering the port, no? And also the speed of gasifing, especially below the wood which was, before, very much exposed to highly activated embers?
This configuration wouldn't nned to clean the firebox each day as there is no more air flow at the bottom of the firebox!
Maybe the use of the threshold configuration might partly change the batch box mode but it makes it always as fascinating!
Thanks Peter, thanks all!
Benoit
|
|
|
Post by ericvw on Jun 4, 2017 20:05:15 GMT -8
Interesting... I have been away for a bit pyrophile - is this a new development? Link?
|
|
|
Post by peterberg on Jun 5, 2017 0:09:07 GMT -8
Interesting... I have been away for a bit pyrophile - is this a new development? Link? The threshold is a feature that goes with the floor channel. It's all to do with distribution of air in two streams in a dynamic way. The higher the velocity in the port, the more air will be drawn through the floor channel at the cost of the primary air. The emphasize is shifting to secondary during the burn, all done with the temperature difference of incoming air on the one hand and that of the firebox on the other. Please see the recent discussion with Klemen, there's also a link to a more comprehensive explanation. Benoit, I'll answer your questions later. And yes, the floor channel and threshold is a very sophisticated but stll simple to implement combo.
|
|
|
Post by peterberg on Jun 5, 2017 2:57:59 GMT -8
In this new configuration with the "simple" and elegant solution of the threshold, are the sloping bricks in the bottom of the firebox still necessary? As the embers are not anymore strongly activated by primary air, couldn't there be a flat sole (and a simpler building)? Good question. When loaded full, part of the fuel is behind the threshold and won't be strongly activated by primary air. When the sloped sides aren't there, there's proportionally more fuel behind the threshold. Also, the proportions of the firebox' volume will change significantly. Together, this will be a deviation from the tested design so I think you are in uncharted territory then. In the threshold mode, could we say that there is hardly no primary air anymore as there is no much air directly activating the embers and make the wood to gasify? Instead, we have two secondary air supplies : 1) one arriving in the firebox much above the embers, at the level of the pyrolysing wood (by the action of the threshold leting some air going up) , this air slowly burning the wood gases and 2) another secondary air arriving directly in the riser and burning the main part of woog gases. Both airs burn wood gases, no one -or very few- activating embers, that is both having the rôle of secondary air? I don't think so, since the proportion between primary and secondary air is changing during the burn. At the height of the burn the air velocity through the floor channel is high. Less so in the lighting stage and the embers stage. The overall effect is that a possible fuel overload is less likely to occur. One could suppose that the use of the threshold, without air strongly activating the embers, should slower the speed of gases entering the port, no? And also the speed of gasifing, especially below the wood which was, before, very much exposed to highly activated embers? The rate of gasifying is lowered during a critical stage, that's true. But velocity in the port isn't slowed at all, rather the contrary in my experience. This configuration wouldn't nned to clean the firebox each day as there is no more air flow at the bottom of the firebox! That's true, most of the time I'd leave a layer of ash on the bottom. The charcoal residue is burned the next time so the ash layer isn't growing as much as you would expect. At first it will grow to a certain volume, after that the growth will more or less turn to a trickle. Especially when there are lots of nails in the fuel is my experience. Maybe the use of the threshold configuration might partly change the batch box mode but it makes it always as fascinating! It's altering the batch box workings, yes. And in my humble opinion for the better.
|
|
|
Post by drooster on Jun 5, 2017 9:05:41 GMT -8
... are the sloping bricks in the bottom of the firebox still necessary? As the embers are not anymore strongly activated by primary air, couldn't there be a flat sole Correct me if I'm wrong, but aren't the sloping bottom sides there to try to align the ignited wood more centrally with the portal and get a better double vortex than from a flat woodpile?
|
|
|
Post by pyrophile on Jun 5, 2017 12:48:48 GMT -8
Thanks Peter!
|
|
|
Post by peterberg on Jun 6, 2017 23:52:12 GMT -8
Correct me if I'm wrong, but aren't the sloping bottom sides there to try to align the ignited wood more centrally with the portal and get a better double vortex than from a flat woodpile? Hmmm... Not exactly I'm inclined to say, not meant that way in the development stage. The intial reason was to concentrate the coals in the middle during the last stage of the burn. The threshold construction causes a stretched out coal phase as a consequence, since the coals aren't directly in the line of air stream to the port anymore. A small price to pay for a more stable burn in my opinion.
|
|
|
Post by pyrophile on Jun 8, 2017 3:23:21 GMT -8
I add this aspect in this thread to stay in the subject
I may be wrong but I have the feeling that Dvawolk and Peter's inlet system (in the door) injects air rather downward, mainly towards the secondary air inlet (floor channel). Another aspect is that Peter's outer skin of its stove is rather thin, helping to directentering air rather downward. It is possible that a "straight" inlet, not injecting downward would have a different action.
I say that because I build a batch box stove in july and my outer skin's walls will be about 12 cm thick. Such a big space beetween door and floor channel may help the entering air to go more easely upwards (slower flow, less draw downward?).
Maybe a fear for nothing!
Benoit
|
|
|
Post by peterberg on Jun 8, 2017 5:29:20 GMT -8
It's probably true that the smaller space between the door and the threshold helps to restrict the primary air stream. Will the depth between the door and floor channel be really 12 cm? Looks rather large for a steel door frame, mine is 60 mm. Do the outer skin of your heater consist of normal brick?
|
|
|
Post by pyrophile on Jun 8, 2017 12:16:15 GMT -8
Yes, I use normal bricks, flat. I use another way to attach my doors. I still don't know how I will inject air through the door, with your system (but I should help the air to go surely down with my 12 cm between door and floor channel)or with two separate inlets.
|
|