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Discussion in 'Reverse Flow' started by nick v, Nov 22, 2013.
If I understand correctly, it should be the same size as the FF->CC opening.
Thanks jweller. I'm sure if I research enough I could figure it out but what you say makes sense so I'll just go with that.
The opening from the RF plate to CC should be 1.5 times the area of the FB-to CC opening, or a bit larger.
There is an on-line calculator that is used often in this forum. You can find a post by Dave Omak there is a link in his signature line. Or you can search "Feldon's BBQ Pit Builder Calculator" on your favorite search engine. The important thing is the calculator bases everything on the size of the FB and not the size of the Cook chamber. So long as your FB is as large (or larger) than the recommended 100% size on the calculator you should be good to go. Then plug in numbers to get the FB size close to 100% for your CC. The calculator will give you recommended sizes for the rest of the openings, vents and so on.
This site has a lot of information that will help you through your build. I'm sure some of the more experienced guys will chime in.
Thanks man. I appreciate the feed back. I've been to Feldons calculator before but I didn't notice that calculation on there. I'll take another look since that's what I've based my numbers on so far.
That number is not actually on the derived by the calculator. There is some fine print saying for reverse flow that opening should be equal or larger than your CC to FB opening. The actual 1.5 times the FB-CC opening is what I got from reading builds and asking questions.
Here is a link to my questions / answers.
Just understand the calculator is a guideline. Most of what I have read is that the openings get progressively larger as the smoke cools through a RF cooker.
Your build is looking great so far, just keep with it. You'll be making great Q in no time. Probably before me.
nick, morning...... smoker looks good..... the opening at the end of the RF plate should be at least the area under the RF plate.... Considering the circle shape under the RF plate, an equal distance to the end of the CC will add enough area, because that area is a rectangle.... If you have 6' under the RF plate, 6" at the end is perfect... that allows the heat and smoke to take a 180 deg turn... the extra area reduces the friction loss.....
I would set the RF plate 1 1/2" above the FB/CC opening.... you can set the RF plate on angle iron that has been tacked in place and have it removable..... If you need to make any changes, it becomes real easy...
Thanks Dave. I welded in the 1" angle already to help reduce warping when I burned it. The RF plate will be 1" above the opening. Hopefully that's enough. Thanks for the words of wisdom.
When you do a "burn out".... keep the fire in moderation... maybe 500 ish degrees.... try to keep it from warping.... Straightening a warped pit is the PITS... pun intended... If you can get the door built, hinges and door seal, bind or turn buckle it all with chain so if it does move, it will come back to somewhere near normal... let it cool naturally.... The heating of the pit should relax the steel also... when done, the door should fit close to perfect.....
Someone recently burned out a propane tank and it "egg shaped" on him.... I think the fire was REALLY hot.... don't know.... wasn't there...
Good to know. Fortunately the only thing that really warped on me was the top lip that my door attaches to. I welded some flat stock down to tie my hinges to but I didn't lift the door and tack it on the inside so it got a little wavy on me under that flat stock.
Hey Dave. Will I need a air inlet in the upper portion of my firebox door or do I need to give her a test run to determine that? If I do need one, any idea on size? My firebox is approx. 25.5 X 23.75 X 22
Nick, morning.... It would be a good idea... The upper air inlets seem to make several improvements in SFB smokers... As far as size goes, I'd put 3 holes, similar to the lower air intakes and some sort of slide damper, or tilt damper.... may as well make it look like you wanted it there.. and planned for it......
Thanks Dave. Will do.
Is there a rule of thumb for how much air in relation to the lower vent. 50% as large? Obviously with a slider, I can choke it down if need be, but I'd like to get it close to correct.
As for now, I just SWAG it..... If you are having to have the lower air openings wide open to get heat to the CC, and the FB is 600 deg,, there is probably an air flow issue through the CC... So, an upper air inlet will move that 600 deg heat into the CC.... closing down the lower inlet and opening the upper inlet will provide higher temps in the CC and consume less wood... I generally suggest up to 25% of the lower inlets and be adjustable...
Since every smoker flows air differently, length of the smoker, size of the stack etc.... I wouldn't know how to calculate a perfect set of air inlets...
This whole smoker build/calculations is pretty much seat of the pants stuff..
The only thing I know for sure..... make the air flow pattern as least restrictive as possible to move the heat out of the FB and into the food grate area and out the stack....
Got some more done on the smoker.
Above was the door stop. Next is the start of my stack.
Put some angle in the bottom for a shelf.
What do you guys think so far?
Sweet, great skills!
I like the stack brace, the curved plenum and the splayed legs... Very nice touches.... Dave