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In need of input to RF build design

post #1 of 4
Thread Starter 

So I just came into a source that will hook me up with about all the scrap iron I want for dirt cheap, maybe even free.  I figure that I might be committing a crime if I didn't exploit that to build a RF smoker, right?  He's keeping an eye out for me, and it's only a matter of time before we've got what I need to get started.  I'll likely be using a 30" cast iron pipe, about 1/2" thick, so this is gonna be one heavy S.O.B., and I'll have no choice but to mount it on a trailer.  I have no welding skills, but I want to learn, and fortunately I know a guy who is an extremely gifted fabricator with extensive trailer build experience who would love to help out, esp on the trailer side so I don't get someone killed.  Right now I'm thinking of cutting the pipe to length and welding plate steel to the ends to make my cooking chamber, so it will be flat on the ends and not rounded like you get with propane tanks.


Appx dimensions of my cooking chamber will be 30" diameter x 54" length (subject to change, depending on what materials I'm able to get).  That yields cooking chamber volume of 38,151 cubic inches.  So here are a few questions I've had as I think through initial design concerns.


1. All info I've seen says you want your firebox to be 1/3 the volume of your cooking chamber.  That would leave me with a SFB of about 24" cubed, to be on the safe side.  Is figuring SFB size that simple, or is there something more to it than that?  Also, is that total volume, or do you need to add additional space to allow for an ash pan?  Last, how much area do you need for ventilation holes?  Is there a formula for that, or do I just need to make sure it's plenty big to allow lots of airflow when I'm firing it up, and that I've got a good mechanism to control airflow once I'm to temp?

2. Assuming my SFB winds up being constructed out of 1/2" cast iron, would there be any benefit to insulating it?  It seems to me that iron that thick should do just fine at holding heat on its own even in cold temps once it's to temp, but if there would be any benefit to insulating it, I'm open to that.  If it's worth doing, it's worth overdoing, after all.


3. How do I figure out how big the hole from the SFB to the cooking chamber needs to be?  I'm sure there are calculators or formulas to optimize this, but I haven't seen them.  Same question re: vertical placement of the hole.


4. Same questions, but about the exhaust pipe?  Also, how do I figure how long the exhaust pipe will need to be?


5. Same question, but about the gap at the non-SFB side of the RF plate?


6. How about vertical placement for the RF plate?  I'm guessing that if you get it too low or too high in the cooking chamber, it could make maintaining consistent cooking temps difficult.  That will also influence the size of the plates I need also, since they're going in a cylinder.


7. I'm leaning toward a split RF plate design with a piece of angle iron running length-wise down the middle, leading to the drip valve (or whatever that's called).  What is the optimal angle for the plates relative to the rail to allow drippings to make it to the rail without sending too much heat to the outside of the cooking chamber?  Or is there one?  Also, how steep of an angle do you want to direct drippings to the valve?  And is it better to have the valve on the SFB side, or other side of the RF plate?  Or does that matter?  (I've been known to over-think things).


8. I'm also planning on incorporating a propane burning rail to help get it to temp.  That much iron will take a lot of energy to heat up, I'll bet.  Plus sleeping during longer cooks is very appealing.  That would need to be mounted near the bottom of the cooking chamber, well under the RF plate, correct?  I think I'll also incorporate some kind of log lighter in the SFB too.


I also welcome any input or suggestions on design.  I've read a lot of the build threads on here and I'll be flagrantly stealing a lot of ideas from all y'all in my design.  But since I'm very new to this whole thing, I will happily listen to anyone's input.

post #2 of 4

Here ya go. http://www.feldoncentral.com/bbqcalculator.html


That will answer most of your questions.

post #3 of 4

if the pipe is cast hit it with a grinder , lots of sparks and it will weld little to no sparks and it wont weld

post #4 of 4

Cast iron is difficult to work with. It's the only metal I hesitate to weld. It can be done. Must be preheated usually to around 600 to 800 degrees before welding and then cooled down slowly to prevent cracking and requires expensive nickle welding rod.  You probably have steel but check it out like Clyde suggested.

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