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Lateral 8" Batch Box Rocket Argentina. Smoking P-channel Help!  RSS feed

 
Posts: 7
Location: Argentina or Canada
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Sidewinder (Lateral) Batch Box Rocket 8” in Argentina

Recently my husband and I converted our Rocket 8” for a Lateral Batch Box Rocket 8”and we’re running into some smoke problems from the p channel and now from the fire box.

Where did we go wrong? We need help Rocket experts please criticize!

Measurement breakdown:
Fire box: 33cm width/ 43cm height/ 71 cm depth
Portal: 7.5cm width/31cm height
P-channel: 5cm width/ height 3cm = 15cm2 (believe Peter’s is 16cm2)
Primary Air Channel: 36cm2 and 60cm2 (we haven’t decided which one to keep, see below for pictures)
Riser: 113cm height respecting 20cm octagonal diameter bottom to top. We left a space of 30cm above the top of riser without refractor brick, this elevated space is covered with a recycled cast iron side oven plate on top. (This is experimental in hopes of making a hot oven on top)  We’ve made what some people call a Japanese Bell on top of the riser.


Here’s a picture of our original Rocket, worked perfect for 4 years straight. We decided to convert to the BBR in order to heat up water in a recycled water tank in a barrel, which heats up water in less than one batch no problems in this part of the design.




The circuit of the BBR is unusual compared to most, firstly it has been built into a cob wall, where the fire box entrance is on the outside of the house. Note: We will eventually inclose the area where the door to the firebox is. Perhaps our system is suffering from outdoor temperature differences or southern wind inflow? It is winter now, fluctuating from -2C to 16.

How does the heated air flow in this system…(sorry I’m not Sketch Up savvy)
As you can see in this picture, the firebox entrance is currently outdoors, the firebox is in the wall, pooping out of the wall in the inside of the house.
The heat riser is inside the house. Heated air from the riser is guided back outside dropping into our water heater barrel and at the bottom of the barrel it is guided back into the house (crossing below the heat riser).
Heated air continues to circulate through the cob bench and out the chimney. This route runs approximately 16 meters in length ( measurement starts from the top of the heat riser to top of chimney. Q: Is this route too long for a BBR 8”?



We recycled an old cast iron stove frame instead of a custom made fitted door. As you can see the door is smaller than that of the brick fire box and thus giving us 2 optional primary air intakes. The oval size has a fitted door to control air intake 32cm2.
The opening underneath (60cm2) the iron frame is wide open, we will close this off only to access the ashes. When we seal this part, there is still smoke.

(Cut triangular bricks are set in place since this photo in order to respect the inner volume size of the fire box design)





Taking a look in the inside, we have put ceramic glass so we can watch the burn from inside.
You can see our p-channel intake at top left of the ceramic glass.

Why did we do the p-channel design like this?

This is the shortest p-channel length we could do from cool air inside the house directed over the portal (40cm length channel).We soon learned that smoke came out when lighting our fire but once roaring the air was sucked in fine. But when opening and closing the door to the house this caused a puff of smoke to come in. We did not for-see the air pressure change as a problem…?
After using this fire beast more and more we couldn’t reverse the small waves of smoke coming inside our home leaving a nasty stink and of course fear of Carbon Monoxide poisoning, we have our alarm on to let us know.

Another guess here for the p-channel smoke is that inflow of outside air from the firebox door + air intake is streaming in the direction that p-channel is lined up with inside the house.




Below you can see how we’ve placed the p-channel inside the firebox.
I wonder if the solution is so simple to just flip the p-channel around? We’ve done this backwards, without considering the physics of air flow…
Q:Is another solution to drop the p-channel from above the heat riser? Or will this give a chimney effect and still have problems with air pressure when opening and closing the door to the outside?





Thank you to all who have come this far and taking your time to assess where we went wrong!
 
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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15 meters of bench, plus a barrel of water to heat, way too much imho.

What is your chimney gases temperature?  If anything lower than 60C°, shorten the bench. Or get rid of the water heater.

You say the door is outside, but the P channel inside?

Look no further, primary air and secondary air should be taken at the same pressure. So in the same room, or both outside.

If your water heating barrel is outside, i would insulate the outside of the barrel. I'm not quite sure about what i see here.

How long has it been since you cleaned all the pipes in the bench?



 
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Location: South Australia
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Hi!

Obviously this varies from Peter's design a lot. I recall he told me to stay within 5% of specs. One big variation I see is the ISA. 16 meters of 8" pipe is 10.1sqm of area, plus the barrel plus what the water is removing.
An 8" (200mm) system needs around 9.4sqm total.

Primary air should be 63cm2
Riser height is minimum 115 so you're pretty close there.

I think Satamax is correct. Shorten the bench. I assume the water heater an essential.

I hope you solve the problem and would love to hear about the solution when you find it!
 
Jane Ashworth
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Thanks guys!

Yes it is a long run. We were actually planning to put in a divider so we can switch between heating water in the barrel or have it go straight to the bench. We should be able to do this where the pipes meet the barrel.

For those ever thinking about heating water it's a good idea to have this option so you never over heat your water system and avoid pipes busting. We do have a thermometer to install to keep check of the water temperature and this system does has an emergency escape pressure pipe that reaches the height of our water storage tank, not sure of the correct terminology for this.

We'll check the temperature at the exit of the chimney, that's good to know. The draft seems to be pulling well and we cleaned all of our ducts when we made the change so there shouldn't be any blockage.

Thanks for confirming the pressure difference from inside and outside. It was quite obvious when we had a burn going that this was a huge problem, so we'll have to dismantle part of the fire box to have it redirected outside. It's hard to see the whole system from the pictures and my description, I'll see if I can get a video clip posted so that's more clear.
To clarify we're still building the house, so when I say the firebox door is outside it is at the moment but this part of the house will be enclosed this coming year...but as the system is in two separate rooms there will still be a pressure difference so we'll have to get that p-channel on the same side of the firebox door.

Thanks again and I'll keep posting updates
 
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It's not easy to see how it's been built. But Max is right, that bench on it's own is probably already too much. When it's also a piped bench with bends in it you are running into much friction in the smoke path which is something batchrockets are very picky about.
But I do see another blooper that's often made by a lot of people. The specification of the p-channel is: as wide or wider than the port. So the narrowest spot in the port is where the p-channel is. Which means the greatest velocity and lowest pressure is exactly where it need to be.

What you have done is just a flap hanging over the port but it's smaller than the port and is open on both sides. So the highest velocity is deeper in the port and under pressure in the channel isn't as strong.

And yes, exhaust temperature in the heart of the exhaust need to be equal or higher than 60º C in order for the heater core to work properly.
 
Jane Ashworth
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Here's a short video of our BBR 8" that should help clarify how it's laid out.

Thanks again to all!
 
Satamax Antone
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Your fire seems very lazy.
 
Jane Ashworth
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Thanks again for all your comments, I hope that the video showed in more detail how the system is running. We're going to have the p-channel switched around sometime this week so we resolve the pressure problem and post the temperature exiting the chimney.

In regards to the p-channel Peter, to make sure we don't do another "BLOOPER"!!! 
Would it be better to position the channel within the portal with the air ejecting from the top? And do you recommend having a flap here?

(After reading so many forums on p-channels I thought I understood that above and in front of the portal is the most effective for adding turbulence... but now that we will switch the direction of the channel I don't believe it's a good idea to have a longer length of channel running in the firebox that will most likely effect the air temperature injecting into the portal. Would love to know more thoughts on this part of the design)

Satamax, good observation, the fire was very slow in that moment to get started as the whole system was completely cold and humid. Usually after the first lot it's much more active, also probably due to the water tank getting nice and hot.

Next we have to decide where we'll put in the bypass to avoid the water tank we'll post an update how that goes.



You are all so wonderfully helpful! Much appreciated.

 
Peter van den Berg
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Jane Ashworth wrote:Would it be better to position the channel within the portal with the air ejecting from the top? And do you recommend having a flap here?



Your questions show you didn't get the principles (yet). So I'll try explain it again, although it would be better to stop thinking and just do what is recommended. Air ejecting from the top inside the port itself wouldn't work any better, rather the other way around. Just because the incoming air wouldn't be heated up as much.

The basics are on the website but here we go again: the port act as a temporarily restriction in the smoke path. As such, it is a venturi wherein the gas speed increases and the pressure decreases. It's a law of physics called the Bernoulli theorem, part of the law of conservation of energy. It doesn't matter whether or not you understand how it works, even believing otherwise won't change laws of physics.

Now how to utilize this phenomenon in order to get a sufficient quantity of fresh, hot air into the port. In order to understand this, it is imperative to realize that the lowest pressure in the port happens to be just a fraction of an inch beyond the narrowest point, seen from the firebox' side.

By hanging down a small piece of the channel over the opening, that said narrowest part of the port is the p-channel itself. So the lowest pressure and highest velocity is generated where the air from the channel comes in. That way, air is literally sucked in.

What has been done wrong in your implementation is just a flap hanging over the port, with generous openings left and right. This is so much cross section area together that the narrowest spot in the port isn't where it should be but further downstream instead. That flap should be a U-shape, and the channel should be at least as wide as the port. Entering from the side doesn't help either, none of my drawings shows that and for a reason. The part of the channel  inside the firebox shouldn't be horizontally but vertically instead for best performance.

To conclude: please see that the recommended dimensions, shapes, sizes, proportions, end temperatures and whatnot are adhered to. Just copy the whole of the core design and it'll work, right out of the box. For reference and pictures: see http://batchrocket.eu/en/designs#pchannel and http://batchrocket.eu/en/workings#turbulence. Both of the embedded pictures show how the p-channel should look like. If at all possible, build just the core outside using bricks and clay/sand to seal the crevices and fire the thing up.

P.S.: the bench together with the water barrel is way too much. As long as it's there the thing won't work as desired so measure the temperature inside the exhaust pipe to the roof, not the outside.
 
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Jane Ashworth wrote:


For those ever thinking about heating water it's a good idea to have this option so you never over heat your water system and avoid pipes busting. We do have a thermometer to install to keep check of the water temperature and this system does has an emergency escape pressure pipe that reaches the height of our water storage tank, not sure of the correct terminology for this.

Scott in Iowa -   I would say this statement, falls short on the advice required for heating water with a "uncontrolled fire"   That being said, if you have a closed system or even one with a blow off (mandatory) and your fire is going great guns and power goes out. There is little a person could do even if your right there. And chances are you might not be.    With a two part system-  i.e.  an open system for heating the water, and second system for circulation through the first heated water generally solves the worries.    The open system, allows for total expansion, even if steam would develop.  No pressure could develop other than steam blasting through the pipes to a completely open tank (or half full of water)

This is just advice, have plumbed a few and seen a few scary ones.  

You can all look up the expansion rate of steam, but what many don't read, is just how fast and to what degree this can happen. (more than a blow off valve can handle on a closed system, if things are really cooking)

 
Jane Ashworth
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Thank you Peter for clarifying in such detail, it's greatly appreciated and we keep learning better how your system works.

You commented here "What has been done wrong in your implementation is just a flap hanging over the port, with generous openings left and right." (I'd like to clarify on what you mean by "right")

When looking at the (not so clear) photo of our existing p-channel I would like to point out that there is only one opening, directly above where the flap is (this makes the opening on the bottom of the horizontally placed channel). The right side we had soldered, so this is closed and no air is moving directly from right to left (if this is what you mean?). The flap you see hanging is the size of the cut we made for injecting air above the portal and so after we made this cut we bent the metal out to make the flap.

Thank you!
 
Peter van den Berg
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I know the flap is bent down from the opening in the horizontal channel. But this shouldn't be just a flap, but a U-shape with open sides down and to the riser instead. Again, please have a close look at the picture where it's clearly visable at the lower end of the vertical part.



And pay attention to the text that's directly under the picture which says:

"The downward hanging end stops a little lower than the top of the port, providing a small overhang. To keep things simple, this overhang is the same as the depth of the channel. At the back which is facing the riser a small piece is cut out to promote the suction of air. The same principle that causes the p-channel to work, Bernoulli's principle, means the pressure within the system is lower than that outside. Hence no smoke will escape the p-channel, indeed any other small cracks or the main air inlet itself. Conversely, if smoke does come out of the p-channel or main air inlet, then it means there is something wrong in the system."

That "something wrong in the system" need to be resolved as well before the heater will run properly.
 
Peter van den Berg
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I took the time to draw a p-channel that would fit a sidewinder batchrocket. The horizontal part should be outside the firebox in its entirety. The vertical part should be in the firebox, directly above the port and hanging over that U-shaped piece.  Proportions should be fitting for an 8" version.

 
Jane Ashworth
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THANK YOU PETER!

 
Jane Ashworth
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Thank you for your comments Scott, good to know these finer details of water heating systems. We did rush into changing our system to try and get ready for the cold winter so there's still much to experiment with...
 
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