Reverse flow theory

[ribwizzard]

Below is a simple drawing of my current build.  Generally, I don't "choke down" any of the letter designated areas, but totally rely on air intake and well sealed up chambers to control heat, ..but in this build I'm thinking of choking down "B" opening, trapping more heat under the reverse flow plate, and opening up "C" and "D" as to create very little resistance.   My thought is more cooking heat with less pressure in the cooking chamber to help create more of the "open pit" style of BBQ. Letting the smoke just freely linger by.

Any thoughts???....

 

[smokingribs]

I think it looks pretty cool. Let us know how it works. 

 

[woodcutter]

Maybe hinge a plate to hang for B and it could be flipped around and lay on top of the plate in case you have any drafting problems starting your fire. It could be flipped back to restrict again when you put the food on. I think you have a good idea.

 

[ribwizzard]

That is a good ideal,  these things do stumble a little until they start to warm up.

 

[sqwib]

I see you're at it again

LL1.png

• sqwib
• Jul 1, 2013

LL2.png

• sqwib
• Jul 1, 2013

adding thermal mass (green) this baffle type would take a long time to heat but in theory, you would trap radiant heat???

LL3.png

• sqwib
• Jul 1, 2013

Heating two plates to catch more heat???

Double reverse flow.

 

[ribwizzard]

Hey SQWIB

I was thinking opening up "A" and choking "B" would raise the temps in the lower section of the cooking chamber. and sort of trap it there, ( almost like an extention of the firebox)  it would be using more radiant heat vs. convection heat.  I would like to be able to get the temps high enough to do burgers and dogs.  The firebox lid will be capped and everything will be 3/16" . Cooking chamber 32 x 16 roundand firebox 16 x 16 round. So I will have a good size firebox/cooking chamber ratio.  I actually started it Saturday and got them mounted together and doors flanges and hinged. When I started making the reverse flow pan, I always hated the way it didnt go to the end of the cooking chamber, as I started thinking about a full length pan, thats what got me thinking about all of this.... Just not sure how much to choke it down.   Thinking just go with what the pit recommends for firebox/ cooking chamber size for "B" and going 150% for A. C, & D....

 

[sqwib]

Wouldn't chocking it down... so to speak, reduce air flow and in essence lower the temp, even if you opened up the firebox inlet???

Seems that there's no way around this.

What about the firebox directly under the chamber with an adjustable slide to increase or decrease firebox to cooking chamber inlet?

LL5.png

• sqwib
• Jul 1, 2013

I would imagine a larger than normal stack would be needed as well.

I'm just throwing stuff out there and I am not sure of any of it.

But I am curious to see what you come up with.

 

[ribwizzard]

Well....Im not really talking about shutting it off completely at point "B', .. according to Pit calculator, Recommended Firebox-to-Cooker
Opening Area is 25.7sq"4,  I was going with 3' x9" for point "B' and that puts me at 27sq", so that flow would still be good. Its just that point "A" will be about 36sq", and I'd go with 4" pipe on the exhaust. So, in theory, this thing should flow pretty good,.....I am thinking of putting in a dampner at point B though........

Maybe Im overthinking it, like I tend to do...

THEORY.png

• ribwizzard
• Jul 1, 2013

 

[dragons breath]

Most good ideas are worked out, not thought up finished like Mozart would do. Would you be thinking about an internal flue type valve? The beauty of it is it doesn't have to fit real well because you would never close it off anyway. It would just give you some control. Or maybe a slider from the RF plate. That would more tricky because of your grease lip. I thought about something along the flue design to control the heat smoke flow on my rig. But I would need 2, 1 at both ends. So I took the easy way out. Looking forward to how you handle this

Dave

 

[ribwizzard]

not sure yet.....I cut out the triangles for the drip pan tonight and cut the pieces for the "end cap".  Once I get the end cap fabricated and I can hold it in my hand, I'll fiqure out the valve....Im thinking butterfly valve maybe with the control lever positioned on the end cap......I'll get some pics tomorrow so yall can vision it better...

You know, ...sometimes when I have a hard time getting people to understand my logic.....Its a sign that its time to double check myself....I can get off in left field once in a while...

 

[smokejumper]

Here are my thoughts (worth every penny I charge for them)...The RF plate gets the majority of its heat from convection. Convection requires moving hot air across the plate. If you want it hotter, move more hot air across it, not less, or give it more surface area (fins?, tubes?, channels?) to extract more heat from the moving air.Think about how the second higher air inlet on the firebox works to control temps by regulating how much hot air we are moving through the cooker. There may be a limit to what percentage of the heat can be absorbed by the RF plate, but I have no idea what that limit would be.I say give it a try; you can always mod it later depending on how it performs.

 

[smokejumper]

Okay, now you got me overthinking it... A thinner RF plate would also conduct and radiate the heat better.

 

[ribwizzard]

Smokejumper,

I'm giving the reverse flow plate increased surface area by "dishing it deep"  , installing the top as high as possible and the center drain low with out restricting air flow. The plate will be 4 triangles with a center drain, hoping the "turbulance " will also aid in heat transfer. It will be 3/16" plate. I'm adding a heat sheild as you can see right inside opening A to keep from having a hot spot right at that end.  I'm not really looking to "Superheat" the plate, I just want to absorb as much heat as possible so I can open up the exhaust a bit more, point  "C' will be above grate level as in a Jambo style pit. I'm thinking recovery time and even heating will be better, and create a little lighter and cleaner smoke with less pressure in the actual cooking chamber. And ability to create higher temps when wanting to do burgers and dogs.

I got all the plates cut, but need to make the cart before I start welding them in, its going to start getting to heavy to lift now. I'll be working on that tonight.

 

[smokejumper]

With 27 sq" at B, it should breath just fine, and I like that C is above the grate, which we don't see in too many offsets. If you keep the grate close to the RF plate, you might be able to get grilling temps, but for general smoking you might want a higher set of rails. Looking forward to seeing how it turns out.

 

[ribwizzard]

Well, its going to be my last build for a little while, I want to take everything Ive learned over this past year playing with these reverse flow cookers and try to make me one I'll be happy with. Time to get back to cooking instead of building all the time.

 

[redwood carlos]

My 2 cents. Leave the opening the size you are comfortable with  , as long as it is larger than what you want to experiment with, and use baffle plates to tune the opening size to your liking using option 1 or 2.

reverse flow.png

• redwood carlos
• Jul 2, 2013

 

[ribwizzard]

Thats what I was thinking, I will not have access at area B but can easily do something at point C

 

[sqwib]

Can you make point c adjustable?

Smoke jumper you beat me to it I was thinking about your fin idea like in channels sort of a diagonal pattern. I think this would retain the heat more by adding thermal mass and friction??

Sort of like how a solar window box heater works.It may get a bit hotter with them (considering the friction), but at the least would hold the heat longer.

 

[jockaneezer]

Don't want to hijack the thread, but I'm starting my own small RF build and was taken with SQWIB's idea of the central firebox, but considered having openings at both ends of the RF plate and utilising a central chimney. Do you think this will have any advantage over an Offset type ? I like the idea, as I think the whole unit will look "balanced", but I agree it may need some sort of baffle over the firebox and I'm sure someone must have tried this already.

 

[jockaneezer]

Answered my own questions by doing a search from previous posts, should have thought about that first !