BP to firebox throat relationship

[bswisher]

I am at the point I am ready to build my firebox and BP for my 60 gal air tank RF smoker and am looking for clarification on something. I have used the calculators and determined that for my CC 20x48, I can do a firebox 15.5 wide x 18 high x 18 deep. (loading wood from end not front). For my throat, I have an opening 15.5x3.75 high. What confuses me is, I have read some posts that recommend placing the firebox/BP 1" or so below the center line of the tank. Others say anywhere between 2-6" below the lower grate is fine.

The way I have measured my CC for the FB cutout is to run a level along the bottom end of the tank, came up 3.75" and that gave me my 15.5" that just happens to be right at the point where the tank and bell seam meet. To me that seems right. That would make the are under my BP a consistent 15.5x3.75 the entire length of the CC. If I were to mount my FB 1" below the center line with the 15.5x3.75 cutout, the are under the BP to the bottom of the tank would obviously be greater.

So my question is, should the area under the BP be the same as the throat area or does that matter as long as by BP gap at the other end is 75-100% of the throat area?

Thanks in advance,

Bart

[Big T]

The area under the BP is usually a little bigger than the throat but never smaller than the throat or it becomes a choke point. I have never saw a BP mounted 1'' below the center of the tank so I wouldn't have it that high. I try to mount the FB as low as I can with in reason and have a shadow plate that extends 12''-18'' into the CC with a 1'' air gap between the SP and BP. 2''-6'' is the range from the BP to the bottom rack , I prefer 3.5''-4.5'' above the BP.

[bswisher]

Thank you for your response, that makes sense to me. From your experience, do my calculations look correct for the throat opening? I have seen some posts that suggest making the throat 50% larger than the feldon and pit calculators suggest. Right now mine is set up to be around 40 sq in just as the pit calc recommended.

[Big T]

Your FB to CC ratio is right on the money at 108% and I would stick with the 40 sq in that the calculator recommends. The 15.5x3.75 throat comes up to 58 sq in if my math is correct so I would drop that down some and the shape of the throat doesn't matter as long as the size is correct. I have built several pits using the smoker builder pit calculator and I always stick really close to the dimensions that it recommends and the pits all perform great.

[Pete Mazz]

BigT has you on the right path, as usual. Only difference I see is your cut-out seems to be correct according to the calc.

[bswisher]

BigT and Pete, thanks for the advice. I am thinking about what you said about the shadow plate. The way I wanted to do my BP is to make it one piece so it is also top of the firebox. What do you guys think about me making the shadow plate as a slide in piece from the firebox side. That way I could use different length pieces for tuning purposes. It would have a lip on the plate in order for it to seal the gap between it and the bp. I realize this would essentially reduce the throat for those first 12 inches or so.

Also, I have access to a free sheet of 3/16 that would be large enough to do both my BP and firebox. I know 1/4 is the preferred thickness but, since this is not a large smoker and won't be used more than maybe once a month, would the 3/16 be ok? I know it won't last as long or retain heat as well. Just trying to use what I have if I can. Thanks again for the advise.

[Big T]

BigT has you on the right path, as usual. Only difference I see is your cut-out seems to be correct according to the calc.

Screenshot-2017-12-17 Online calculator Throat Calculators.png

You are correct sir. I didn't realize that he was doing a half moon because I use a rectangular/trapezoid throat.

[Big T]

I'm not sure about the adjustable shadow plate as I have never seen it done so I'm not sure how it would work. I wouldn't choke the throat down or you'll have performance issues. As for the 3/16'' it will work but you'll need to brace the FB door and door opening so that it doesn't warp. The CC doesn't get that hot so you shouldn't have any issues there but the FB is a different story, I've seen them warp when they were built with 1/4'.

[bswisher]

Good enough for me. If I have enough of the 3/16 maybe I will do an insulated box. If not, I will just go ahead and get 1/4 for the FB and still brace the door just in case.

[Rodcrafter]

BigT knows his stuff.

[bswisher]

Ok fellas... Don't be shy and give me your honest opinion on something. I got a free Logwood 2421 cast iron wood stove given to me. My plan is to use it to replace the one I have in my pole barn with it. BUT... I came across the picture below and it is the exact same stove. Will this work or is it a waste off my time to try to use it as my firebox?

[Big T]

If it was within the pit calculators recommended size I would say go for it but if not I would build a FB the correct size. You can make anything work with some effort, modifications, trial and error but if you ever build one using the calculator and see how well it performs you'll never do it any other way.

[Squiggle]

Man, BigT & RC have you well covered & Pete is the math guru here so you're in good hands.

[bswisher]

Well I got the measurements off of the stove, 31x18x16 so, way too deep for what I need. That's ok, it heats my barn just fine. It's like i tell my 20yr old son, "You have a choice, you can build it right the first time or the second time" LOL, I am going to go ahead and use 1/4 plate and build my FB right the first time.

Thanks for all of the responses guys, it is appreciated.

[Rodcrafter]

[ajfoxy]

Don't think you'll be unhappy with following the pit calculator as opposed to bodging the stove to fit the task. And you've now got a stove to heat your barn.... win-win.

[Rodcrafter]

Don't think you'll be unhappy with following the pit calculator as opposed to bodging the stove to fit the task. And you've now got a stove to heat your barn.... win-win.

So AJ what does “bodging” mean? You guys down under have some cool terms.


Making memories!

[Squiggle]

Bodging is an Aussie term to fix something in a slightly dodgy way, like "just bodge it up & she'll be fine"

[Rodcrafter]

So now what does dodgy mean? you know you can't use an Aussie term to explain an Aussie term right? Sqiggle you are a handful.

[The Czar]

Dodgy is what Frank and Tom are RC


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[bswisher]

Well, I went with the 1/4 plate. I did use the cast iron door off of my old wood burner. Just have to finish welding in my drain and the expanded metal for my fire basket.

[Big T]

[Rodcrafter]

Dodgy is what Frank and Tom are RC


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Oh that explains it!!!

[bswisher]

I just wanted to thank everyone that helped me out with their experience for my build. I am just about finished and was able to do my burn in and seasoning. It did amazing!!! I never had more than about a 10 degree difference between the FB end and the BP gap end. It stayed pretty much about 2-3 degrees apart and at times was exactly the same.I still have to paint it and add the FB vent but i am very very happy with the way it performs.

Here is a quick video of the seasoning and temps. I apologize for the poor video, it was shot in the dark and it was cold and windy.
https://www.youtube.com/watch?v=fc5eM5M4ErA

Thanks again

[Big T]

You've built yourself a fine smoker I'm not sure why you're seasoning it while it's 17 degrees, down here in L.A. we don't go outside when it's that cold!