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Batch box p channel question  RSS feed

 
Jon McLain
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Building a 6"bb. With the secondary air port (think I want to do floor channel but either way) i understand the sizing, however is the consistent size through the tube the important factor or if I have a slightly oversized tube internal to the box but the proper inlet and outlet size will it work properly?
 
Glenn Herbert
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My understanding is that it is good to have the tube somewhat larger than the actual injector. I'm sure that it wouldn't hurt.
 
Peter van den Berg
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For a floor channel: the outlet size according to specifications, the inlet and horizontal duct 50% to 100% larger. That's a quite generous tolerance, you should get away with it. The reason for to have a larger csa for inlet is that the air is staying in longer and as a consequence will heat up further.

For a p-channel the whole of the duct the same, tolerance between 5% and 6.5% of riser csa. The situation is different here, only the vertical part is inside the firebox. But because of the port being lower than the ceiling that short piece of steel is getting awfully hot, as is the ceiling and upper wall part. In full burn the temperature output of that channel is around 450º C, equivalent to 842º F. This is well above the self combustion temperature of wood and wood gas, thus helping the secondary combustion.
 
Jon McLain
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Peter, thank you so much for your help and research and being a part of this community. I was searching the batch rocket site and having a hard time finding the dimensions or ratio for floor channel...or is it essentially the same as a top mount p channel ? From what I understand once I get the magic number for outlet, the inlet and tubing is much less specific? I apologize if you've weny over this, it is very possible I somehow over read past it not realizing it happened. I'm planning on a 6" core if it matters
 
Peter van den Berg
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Jon, there's a separate article about the floor channel with dimensions, on the batchrocket site. See http://batchrocket.eu/en/designs#floorchannel The dimensions given there assume a 50% larger inlet as compared to the outlet. The ratio mentioned there is a minimum, this could be as much as 100%. Any larger feed channel isn't tested so I can't tell you how it will behave.
 
Russell Dinning
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Hi Peter is the floor channel delivering the entire air supply to the fire or is there still some primary air coming in the door if so what are the proportions primary air and secondary air via the floor channel
 
Peter van den Berg
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No, the floor channel isn't the sole air supply. In fact, all the air is coming in low through the door, exactly at the same level as the floor channel but at a distance of an inch or so. Imagine that the incoming air is icy cold as compared to the interior of the firebox, so this cold air has the tendency to form a puddle in front of the floor channel and the steel threshold. So whatever the door opening is, the floor channel is always served first. The rest of the incoming air is going up and over the threshold.

The primary air and secondary air can't be defined by an opening since the air velocity is radically different in the floor channel as opposed to the main firebox. On top of that, the respective velocities are changing during the burn so the inlet arrangement should be tailored to that phenomenon. My own heater, the red bell, is operated like that. The whole of the air inlet is 22% of the riser csa, at the start the door is open a crack too. When the chimney temperature is rising to 60º C (140º F) I close the door. Rising to 90º C (195º F) I restrict the air inlet to 10% of riser csa, flames in the firebox will get quite lazy but the afterburner flame keep on raging like nothing else. The air velocity in the floor channel is very high, in the firebox just a trickle. As such, the air supply could be implemented as an automated system,  steered by the temperature of the chimney as the main parameter.

As you can see, this isn't just a matter of how wide the respective openings are. Based on how much air is supplied by which source and at what air speed. In fact, it's all about volume of air streaming in, where and at which stage of the burn. Level of sophistication is just a step up from the simpler p-channel arrangement. The floor channel is very easy to replace as an added bonus, the very reason why I decided to give it a go.
 
Russell Dinning
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Thank you Peter that explained it nicely. So it's best to experiment with proportions between floor and primary air sizing but total around 22% csa. Mine is a 4 inch batch. I'm thinking about the floor channel as it looks way simpler to replace. With the floor channel it's the opening in the port that you are calculating as part of the system csa if I read that right and you want it's inlet to be approx 100% bigger than it's outlet correct? So I was thinking go with 10% floor and work up and down from there. Oh any what model test do you use.
 
Kelly Pridgen
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Peter,
Thank you so much for your work and sharing.
I'm planning an 8" batchbox using your plans. I've been studying your site and the section on the floor channel. Very well done, except I can't seem to find specified what the height of the vertical part of the channel should be?
 
Peter van den Berg
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That's easy, total height of the vertical part, measured from the firebox floor (i.e. the top of the horizontal part) is half that of the port. Port is 2.2Base, so floor channel is 1.1 to 1.2Base. For easier reference: the sharp pointed ends, without the 45 degree "roof" piece is just 1Base. Height of this part isn't that critical so just 1/8" (3 mil) plus or minus isn't important, especially for the larger sizes. Base figure is 72.34% of the riser's diameter, in your case 5.8" or 144.7 mm.
And before you ask, distance between the port's edges and the vertical part is half of the port width on both sides, measured in any horizontal plane.
 
Kelly Pridgen
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Great! Thanks!!
 
Kelly Pridgen
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Peter,
One more question (hopefully last).
What should the height of the steel threshold at the front of the box be?
 
Peter van den Berg
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To keep things simple, as high as the sloped sides left and right.
 
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