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Post by patamos on Dec 31, 2017 17:07:06 GMT -8
Thanks for the pics and specific information Karl
Still, if you put another riser on it and fire the system up we can see how the core performs
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Post by Deleted on Dec 31, 2017 18:43:30 GMT -8
Less tan 50 degree C are not enough regardless the height.
I could not even prime it with gas torch.
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Post by coastalrocketeer on Jan 1, 2018 12:10:43 GMT -8
Less tan 50 degree C are not enough regardless the height. I could not even prime it with gas torch. I'm not quite understanding... Are you saying that the material is TOO insulative and thus cannot develop a draft? |
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Post by Deleted on Jan 1, 2018 12:32:56 GMT -8
Less tan 50 degree C are not enough regardless the height. I could not even prime it with gas torch. Are you saying that the material is TOO insulative and thus cannot develop a draft? Yes. The whole core has just about 2 Kg and most of the volume is air. You have reported the high reflection as well. |
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Post by branja on Jan 3, 2018 10:11:23 GMT -8
What about adding a "thermal buffer"? A mass with a small "C" so it heats up quick to high temp. A simple coating on the inner side might work.?
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Post by patamos on Jan 3, 2018 11:32:41 GMT -8
Karl, so long as you have adequate draft the fire should build up regardless of how insulating its surroundings are.
That looks like a pretty long burn tunnel, so adding a heat riser 2x its length or more and carefully sealing it to the riser base should take care of things.
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Post by Deleted on Jan 3, 2018 12:52:21 GMT -8
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Post by wiscojames on Jan 3, 2018 17:22:34 GMT -8
The natural chimney effect will occur regardless of insulation or lack thereof, right?
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Post by Deleted on Jan 3, 2018 18:08:23 GMT -8
No.
The pressure in the riser needs to be lower than in the feed and at the outside.
A stable system requires an high chimney or a thermal buffer.
It is pretty simple to calculate with the gas laws.
Temperatures must be given as absolute temperatures in Kelvin.
For connected vertical tubes the outside air exerts more pressure at the opening of the shorter.
To double the volume of a gas with 273.15 K the temperature must be raised by 273.15, to triple it by 2*273.15.
I hope this is understandable enough.
If the riser has double the high of the feed and the feed has 900 K how much is required in the riser ?
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Post by pigbuttons on Jan 3, 2018 19:17:51 GMT -8
Just want to add some experience with highly insulated, short riser, j-tube priming. Even with paper and dry pine kindling it is hard to get my 4" j-tube to burn in the right direction. What I've taken to doing is to fully saturate 5 or 6 charcoal briquets with BBQ lighting fluid in a paint can. There should be enough fluid put into the can that after shaking the briquets in the can for one minute there is still some fluid that is unabsorbed by the briquets, hence "fully saturated". I then stack these briquets in the bottom of the feed tube and put in some oak splits about 14mm in dia. to half fill the feed tube. I then light the briquets and there is quite a flame coming out of the feed tube, but a strong puff of air blown down the feed tube will change the direction of the flame to the riser where is will remain because of the draft effect with this hot flame in the burn chamber. It works every time. The briquets will burn at a high rate for about three minutes and by then the oak is lit, the draft established, and heavier splits added. My riser is only 2.5 times the feed tube height.
I hope this helps.
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Post by patamos on Jan 3, 2018 20:19:57 GMT -8
Interesting... I'm always learning something in this forum...
I've been building 4.5" J-systems lately with fire brick splits in the feed and burn tunnels, then CFB risers. So i am not speaking from the same experience.
Usually to light any J system i twist a sheet of news paper to about 10" long and light the end, move it around til it has fully caught then stick the lit end way into the burn tunnel. Once i see or feel the draft begin to draw up the riser i pull the twist back til flames are showing in the feed tube and add a bunch of scrunched up paper. Only once this wad of tinder has fully caught do i add kindling.
May or may not help in this situation...
And leaves me wondering when the buoyancy of hot gasses trumps the relative atmospheric pressures ??
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Post by Deleted on Jan 4, 2018 6:18:49 GMT -8
And leaves me wondering when the buoyancy of hot gasses trumps the relative atmospheric pressures ?? For a J-Tube there is one gas column in the riser and two gas columns for the feed, composed of the gas column in the feed and the gas column in the height difference. Once the composed gas collums outweighs the gas column in the riser the "draw" force which is actually a pushing force kicks in. The opposite connected gas colums form an usually instable system, which can be stabilized by a high chimney or a thermal buffer. The material of my small core cannot provide the required thermal buffer. |
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Post by drooster on Jan 4, 2018 10:53:43 GMT -8
 "Lost Form" removed. The small core stands on two about half an inch thick formed 3/4 rings for better insulation to the ground. Are you going to put the riser on soon? |
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Post by Deleted on Jan 4, 2018 12:09:24 GMT -8
At the moment I have no time and success is very unlikely anyway.
Anyone who wants to calculate it with the formula and information given above ?
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Post by peterberg on Jan 6, 2018 1:53:12 GMT -8
Since by far, most of the heat is reflected the extreme insulation of the small core makes it practically impossible to get the short riser hot enough to sustain draft. At the start the rather soft wood I have got does not deliver enough heat energy to get it going. Trying to get a j-tube to work with a riser that's marginally higher than the feed and a relatively long tunnel is doomed to fail. Also, it might be that the material isn't as insulative and/or dry as one thinks. |
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