Fire box to cook chamber opening

[bhille42]

Hello all! I just stumbled across this forum and I'm sure glad I did. I'm planning on building a RF flow smoker out of a 100 gallon hot water heater tank. I have figured up the size of the fire box I need to use and the area for the opening using the pit calculator. I'm having a hard time converting the firebox to cooker opening area into dimensions on the tank. My tank is 24 inches in diameter with rounded ends and I'm going with a square firebox that is 20 x 20 x 20. Any help would be greatly appreciated!!

[Gizmo]

Welcome bhille42!
You came to the right place to get your calcs done. The proceeds from that help keep this site online and there's nothing worse than building a great smoker that doesn't work - take it from one who has...

[The Czar]

[k.a.m.]

bhille42, going by your 100 gall. tank size and the 20" square fire box you need a 64.00 Sq.in opening into the firebox. A half moon of 13" wide x 6.5" tall will give you an opening of 66 Sq.in. This is slightly larger but when you add a grease bridge about 1.5" tall at the bottom of the opening you will be back in your numbers. Cut your elliptical end 1" to 5" below your desired grate height. I make make my opening at the top of my firebox so it is a continuation of the R/F plate. Then you slide the firebox into place. There are plenty of pics of this process through the builds here that will guide you through this along with questions and answers you will be going in no time .
I hope this helps.

[bhille42]

Thank u all for the help!! I have one more question though. Does the baffle plate need to be exactly at the top of the firebox to cook chamber opening or is it ok if it's a little higher? It works out well ( in my mind) that I put the rf baffle an inch and a half above the opening and the cooking grate 4 inches above that. That puts my cooing grate directly in the center of the cook chamber.

[k.a.m.]

bhille42, I prefer that my R/F plate be even with the top of my firebox it makes for a smoother transition of the heat/smoke entering the cooking chamber. I shoot for anywhere from 1" to 5" clearance between the R/F plate and the top of the grate.
Your idea will work I just prefer the smoother transition.
I hope this helps.

[bhille42]

Thanks k.a.m.! What are the negatives if the cooking grate is higher than 5 inches above the rf plate? Or should I just lower the cooking grate?

[k.a.m.]

bhille42, a distance much more than 5" will cause you to lose some of your efficiency because the cooker relies on the radiant heat from the plate for most of its cooking needs. The tuning plates on my Hybrid are set at 5.5" I did this because I do like to run the cooker with tuning plates and I prefer a distance of about 4.5" to 6" for those. When I run in R/F mode I do burn a bit more fuel than my R/F build for the Smokin' Ace's BBQ Team. The amount is not that much but the difference is there.
At what position do you have your grate set at now?

[bhille42]

If I put the rf baffle even with the top of the firebox and the cooking grates in the center of the cook chamber I will have 5.5" between the two. I would really like to be as dialed in as I can. I am going to use a 100 gallon 24" diameter tank 20" x 20" x 20" firebox with 13" wide by 6.5" tall firebox opening. Would it be better to lower the cooking grate or leave it in the middle? If I do lower the grate then I will have to make my doors larger. Is this acceptable?

[TuscaloosaQ]

making your doors larger is acceptable, they will not be much bigger and your gain {radiant heat from the baffel plate) will be the big payoff, with less fuel consumption.
I hope that helps, I am sure Kam does not mind one of his students (me) chiming in, usually I am flying under the radar..... but hoop there it is!!!!!

[k.a.m.]

bhille42, I am not sure what you have in mind for grate placement. When I build off a 24" diam. I make my door cut at 12:00 and 1" below 3:00. The 1" below 3:00 allows me to set my rails and with 1" grate material be back at 3:00 with the top of my grate height. Basically you have roughly 12" of height to play with on your firebox opening. Your opening I figured is 6.5" tall which is a bit larger than needed to allow for a grease bridge.
You can decrease the opening to 12.75" wide x 6" 3/8" tall this will put you in your numbers almost exactly. You can then adjust the firebox position up on your elliptical end to 4" below 3:00 creating a natural 1" 5/8" bridge for the grease and getting you closer to the grate.
Place a 1" fall on the plate from the firebox end to allow for the grease to run to your drain. I would also add a secondary drain for cleaning out the R/F tunnel.
I hope this helps.
Something like this.

[bhille42]

Well I'm an idiot! I understand now. I guess I was assuming the hole for the fb/cc had to be at the bottom of the tank. Thank u for clearing that up k.a.m.!!

[k.a.m.]

bhille42, your very welcome. If you need some more help feel free to ask.

[TrailerBuilder]

bhille42, you follow k.a.m.'s info to the letter and your build will be money guaranteed. Between the pit calculator he uses and his personal wealth of knowledge, he has pulled me out of the fire a few times on my builds.

As always Kevin, spot on info

[bhille42]

I got another quick question? If I build my smoker using the specs from the pit calculator, how hot of temps should I be able to consistently keep? I'd like to be able to maintain 400 degrees? Is this possible? Thanks for all the help!!

[SoloQue]

I have never hit 400 in my RF but that's not to say it can't be done. More I think about it I have never even stoked mine up to attempt such a temp but I'm guessing it would take me more than 2 hours to get there and that might be a push. I'm sure others on here will have more experienced feedback for ya.

[Frank_Cox]

I have hit 400 in my cook chamber a couple times. For best performance you would want a bigger BP gap.

Sent from my SAMSUNG-SGH-I747 using Tapatalk 2

[lostbuckaroo]

Ok was geeking out on my design and reading through the threads... I've run my dimensions and spec through the Feldon's Calculator several times and it's says that my CC to FB opening (halfmoon) should be 12.59" wide .. I get that but how tall should it be? Thanks, sorry for resurrecting an old thread..

[The Czar]

....we have our own calculator.... ......

[lostbuckaroo]

Oops my bad sorry if I offended anyone..

[The Czar]

Oops my bad sorry if I offended anyone..

Not at all.....its just the Feldon Calc. in IMO is like shooting a shotgun.....vs our calc. which is like using a sniper rifle...but I digress.....somebody should be able to help you on here.....

[Frank_Cox]

No problems on the calc. don't matter which one you use, just don't mix them. not a huge difference between them but there are some mild differences. we still have work to do to ours before it is considered finished.
anyway, I really cant answer you without knowing more info about your tank. granted I looked at the diagram above but I didn't see any volume info. just diameter. do you know the total required area of your CC to FB opening (throat)? if you know the radius and the width of the half moon then you should be able to find the height.

[lostbuckaroo]

Thanks Frank! Here's all the info:

[lostbuckaroo]

Anybody??

[Tom_Heath]

I gotta pass on this one. Sorry.