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"P" channel, please explain thoroughly.  RSS feed

 
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Hi All,

I've spent all morning so far reading RMH threads searching for information on the "P" channel.  Although I gained a bunch of great info while searching this topic, I could not find much info specifically on the "P" channel, despite it being mentioned quite often.

I do understand that it is a secondary air intake, and that it is usually placed against the barrel/manifold area at the end of the burn tube/before the heat riser, but for actual design considerations of/and dimensions/and materials, of the "P" channel, and which designs are benefited from the "P" channel, I find little to nothing in specifics.

Can someone please detail this detail? 
 
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There are several iterations of the p-channel, dependent on what type of firebox it's for. I.e. Ianto J-tube, PvdB batch-box standard version, then the side-winder version, and etc.

The first or original P-channel was for the Ianto style J-tube, details in this thread over at Donkey's site:

http://donkey32.proboards.com/thread/1058/core-tripwire-channel-kicktail-design

The batch box version P-channel info. can be found on Peter van den Berg's site:

http://batchrocket.eu/en/designs

 
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Please read this thread on Donkey's forum, about how it is working in an Evan's j-tube.
There's another, but similar device that is performing more or less the same thing, which is explained here.
 
Roberto pokachinni
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Thanks Much, gents.    I shall endeavor to view all and get back to ya if I need more info. 
 
Roberto pokachinni
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And great that this is leading me to the tripwire which was another thing I've been itching to ask about.
 
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Here's a view of mine in a J tube.  3/16th  plate spaced out with 1/4" bolts. Works flawlessly ! Extends apx 3/8" below the burn tunnel roof . Cooler fresh air is pulled in and creates a turbulence in the burn tunnel resulting in a cleaner , hotter burn. Secondary benefit, the metal is protecting face of the roof of the burn tunnel.
RMH-rebuild_144.JPG
[Thumbnail for RMH-rebuild_144.JPG]
P channel in a J tube
 
Roberto pokachinni
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Hi Thomas.  Thanks for posting that pic.  It looks like your plate of steel suspended by those bolt is sitting vertically in the feed tube; am I wrong?  Maybe my brain is twisting the image, but it doesn't look like you are showing the burn tunnel roof.  It's great that it also protects the roof.  Does having the burn tunnel roof cooler cause any potential problems?
 
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The p-channel plate protects the front face of the burn tunnel roof from sticks scraping or hitting it, and probably also reduces thermal shock from variations in airflow.

The bolts are simply spacers to keep the plate the right distance from the brick. The plate has wings that rest on the top of the feed tube to hold it at the right height so the bottom of the plate projects the right amount below the burn tunnel roof for proper suction and turbulence.
 
thomas rubino
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Here is another photo showing placement.
DSCN4014.JPG
[Thumbnail for DSCN4014.JPG]
 
Roberto pokachinni
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Ah, Thomas; So my eyes/brain were not deceiving me!  And Glenn's comments are accurate to your design?-it seems so, and it falls into place like a turd in the biochar/vermicompost pit! 

It seems that this version of the P channel is different from the one described here, in which the air is brought along the top of the burn tunnel roof down into the burn tunnel ever so slightly past the ceiling just before the heat riser  but similar to the one pictured here, where I think of it simply as a baffle in the burn tunnel side of the feed tube that extends slightly below the ceiling of the burn tunnel.

Why are some of the designs adding the 2nd air supply near the end of the burn tunnel and others at the beginning?  Are there differences?  Why one or the other? 

I noticed that Peter V Berg is quite an innovator (the P-channel name sake no less) who has tried both with success. 

I have been puzzling through these sites and also with other completely unrelated threads on this site and the web beyond, otherwise there would not be so many and long gaps in my thought process as this has shown.  My weekend is only so long, too, so I needed to go for a long bush walk and climb my favorite tree to stretch my bones.  I can only hack so much computer time.

I am beginning to understand the P channel now.  I'm curious about doing a p channel with tiles instead of steel.  What are you fellows thinking in regards to tile feed tubes, or is that too hard to insulate effectively?/cause a chimney effect?

I'm going to start a similar thread about the trip wires,,,, be warned.              
 
Roberto pokachinni
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Is that fan necessary for your draught, Thomas?
 
Roberto pokachinni
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It was always on my mind that if I was going to have a secondary air supply that I would
1.)want it at a different location to my primary air supply (thus probably nearer the heat riser than the feed tube.)
2.)want the air to be preheated so as not to cool the fire it was feeding. My thoughts on this were to have the tube fed by a copper pipe that was up against the barrel.  This might go against thermodynamics by pulling hot air downwards... so I'm not sure if it would work at all. 
3.)want it lower in the burn tunnel so as to cause turbulence in the system where it flows in

Upon reading and looking at pictures today, I have been led to think of other things... including...

In regards to 1, I have noticed that this is going to be more of a struggle to implement in comparison to the plate in the feed tube, especially if I'm innovating with clay tile.

In regards to 2, I have noticed that there is need for the cooling of the steel plate (so hot air is not desired) so that the steel is not reacting so much to the heat/oxygen combo... and that furthers my desire to try to design the secondary air intake with tile instead of steel, but I'm not sure, since it has more thermal mass than steel and thus could hold too much heat and cause a chimney effect in the feed tube.

In regards to 3, I have noticed that all P-channels have been raised in the system, not on or near the bottom/floor, and I'm curious as to why this is.  I'm thinking that if it came down below the feed tube and came up to it, but on the wall somewhere very near the heat riser port, that it would cause turbulence, not be prone to ash accumulation, would be hot-thus not cooling the fire at it's hottest point.     
 
Byron Campbell
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Roberto pokachinni wrote:

Why are some of the designs adding the 2nd air supply near the end of the burn tunnel and others at the beginning?  Are there differences?  Why one or the other? 



For secondary combustion air to be most effective, the air is injected at a point in the fire box that best makes use of turbulence. That is key, since the turbulence promote mixing of the secondary air with combustible gases / smoke, and increase heat riser "afterburner" efficiency. By design, those ideal injection points differ between the Ianto Evan's J-tube and the Peter van den Berg Batch-Box and batch-box variations.

Building your first RMH? This book will be invaluable in saving you time, labor, and money:

https://permies.com/t/57365/rocket-mass-heaters/Rocket-Mass-Heater-Builder-Guide
 
Peter van den Berg
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Roberto,
Both p-channel implementations are meant for two separate rocket combustion cores.

The first one, the simple plate is meant for the j-tube Evans' style. It provides pre-heated fresh air in the spot where it is most needed, at the top of the burn tunnel. By hanging down a bit lower than the ceiling of the burn tunnel it'll also creates a slightly narrower pass-through in the spot where the majority of the wood gas is formed. This narrowing creates a Bernoulli effect, in that the gas velocity goes up temporarily accompanied by a pressure drop. So it will suck in air from behind the plate, cooling the plate and the first firebrick in one go. Bringing in air at the bottom of the feed or tunnel doesn't trigger the same effect, air feeding into the riser is too far downstream since all the mixing has been done in the tunnel already.

The trip wire is another implementation which creates little turbulent eddies along the burn tunnel's ceiling. The burn tunnel ceiling is ramping down slightly to end in a sharp corner or "bluff", breaking up the boundary layer along the ceiling. See this Crash Course in Aerodynamics and Turbulence. There's information about the trip wire and when you want to know more about this try Googling "Ludwig Prandtl", he's the grandfather of modern aerodynamics.

The second p-channel is a secundary air supply used in the batch box rocket heater core, which is quite a different animal. No feed tube, the tunnel is much wider and called a firebox here. Behind this box is a riser and between those is a quite narrow opening creating the same "pressure drop through an orifice" in a much stronger fashion as described above. The riser is acting as an afterburner here, creating a very turbulent environment. Rocket heaters are all about mixing, it is not difficult to add air, to mingle that air with the combustible gases is the magic trick and it helps a lot when that air is scalding hot before it is added.

Oh yes, please don't use ceramic tiles in the feed tube, they'll break up in little pieces because of the severe heat shock and temperature differences. Been there, done that and turned to something else.
 
thomas rubino
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Roberto;  That particular photo was taken on a first firing after a rebuild. So yes I use that little fan to push air with a cold very wet core. After a core is built and warmed up the biggest issue I have is not keeping a match lit while trying to light the feed tube... to much draw.
 
Roberto pokachinni
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The burn tunnel ceiling is ramping down slightly to end in a sharp corner or "bluff", breaking up the boundary layer along the ceiling.

  So, Peter, is this what you personally do with your batch box.  Does the burn tunnel ceiling slope for it's full length, or just near the end.  Is the bluff just a slight protrusion descending from the ceiling or is the bluff the exterior wall of the heat riser that the flow slams into and thus has to descend in order to enter the heat riser.  When I was reading about trip wires on the links I was given above in this thread, I read and visualized a groove cut into part of the burn tunnel.  So, in relation to that last thought, is the bluff a divet or groove in the ceiling that breaks the boundary layer, the sharp corner being an indent rather than a block of protruding substance?  I remember seeing on of your sketch ups that had multiple angles and slopes, but that doesn't seem to quite match what you are saying here... I will have to review those sites in light of the Thermodynamics Crash Course.  I can't say that I fully understand all of the course, by the way.  He mentions cD a few times, specifically near the end, but did not define it from what I could find.  Perhaps I need a big yellow no name brand: Thermodynamic Crash Course for Dummies.      
 
Roberto pokachinni
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Building your first RMH? This book will be invaluable in saving you time, labor, and money:

https://permies.com/t/57365/rocket-mass-heaters/Rocket-Mass-Heater-Builder-Guide

  Thanks for your help Byron, and for the link.
 
Roberto pokachinni
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that little fan to push air with a cold very wet core.

Thanks for Clarifying Thomas.
 
Glenn Herbert
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The tripwire is only used with a J-tube configuration. In a firebrick construction, one brick (usually the second from the feed tube) will be ground down at its ends so it drops 1/4" or so below the rest, and the bottom face ground to a slope that meets the first brick smoothly, ramps down from the ceiling, and leaves a sharp trailing edge where it meets the third brick.

For a cast J-tube, this shape is just molded into the ceiling.
 
Peter van den Berg
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Roberto pokachinni wrote:So, Peter, is this what you personally do with your batch box.  Does the burn tunnel ceiling slope for it's full length, or just near the end.


No I didn't, the batch box doesn't sport a burn tunnel as I mentioned. Only the J-tube benefits from the tripwire. Please read the first and second page of this thread, it will explain a lot.
 
Roberto pokachinni
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Please read the first and second page of this thread, it will explain a lot.

  this was a much better read the second time around, particularly after the crash course in Thermodynamics! 

The tripwire is only used with a J-tube configuration. In a firebrick construction, one brick (usually the second from the feed tube) will be ground down at its ends so it drops 1/4" or so below the rest, and the bottom face ground to a slope that meets the first brick smoothly, ramps down from the ceiling, and leaves a sharp trailing edge where it meets the third brick.

For a cast J-tube, this shape is just molded into the ceiling.

  Perfect explanation, particularly with all the images in Peter's Donkey thread.


By Jove, I think I've got it!  Phew!  That was a bit of a chore for my brain, and THANKS huge to Peter, Glenn, Thomas, Byron!  I really appreciate it, gents!
 
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