Custom Fab Insulated RF Smoker Build

[bjake2001]

Hello all!

I have had enough with tiny smokers and am in the beginning stages of planning an insulated RF build.  I've been lurking for a bit, and thought I should ask some advice before I spend too much more time going round in circles trying to figure this out.  I'll be building an insulated box out of 12 gauge inside and out, separated by 1 inch tubing to fit the insulation all the way around.  I've got so far as drawing up a model minus the details (doors, shelves, smokestack, etc.) and wanted to make sure I was on the right track before I get them figured out.

Cook Box.JPG

• bjake2001
• Jan 29, 2014

I used a calculator I found on this forum to calculate sizes, and here is what I came up with....

Cook Chamber - 60" wide, 30" tall, 24" deep

Fire Box - 24" wide, 30" tall, 24" deep

Firebox Opening - 6" x 22"

Smoke Channel? (area under cook box connecting to fire box) - 8" tall x 24" wide

I'm hoping to get a little input on anything I am doing that is readily apparent as stupid.  :)

Also, advice on the smokestack would be great.  I need to look at the numbers again, but I believe I was right between a 5" and 6".  However, I am curious on how to best position it.  I'd like to lower it into the cook chamber a bit, but would like to try and minimize the interference with the racks.  So two main questions.  How low should it go?  And if it is low, would it be best to run inside the cook chamber along the right side side of the box (shortening my racks from ~60" to ~54", or would it be ok to pull out at a right angle on the outside, right side of the box, shooting up over the top of the firebox?  I would essentially cut a hole in the right side of the box, head out an inch or so, and then right angle vertical.

Thanks so much for any and all help!  Looking forward to starting the build and sharing some real pictures!

 

[bjake2001]

So I've been doing some research (looking at smokers online) and it looks like putting the chimney on the side of the box will work ok.  I'm thinking of putting it near the midline of the cook box.  It is 30" tall, so about 15" down.  I'm hoping that will help give me more consistent temperatures across the cook chamber.

 

[lendecatural]

http://www.smokingmeatforums.com/t/153772/post-pic-of-pit

Posts number 3 and 4 are both good ways to add a stack on the side of the cook chamber. I would have less friction loss than a 90 degree.

 

[bjake2001]

Very nice!  I like that idea a lot, thanks for posting it.  Any reason you couldn't use a square tube instead of round for a chimney?

 

[lendecatural]

No problem at all, just make sure you have the correct volume and a length of at least 30 inches from the calculator.

Good luck!

Len

 

[smokejumper]

 
....Any reason you couldn't use a square tube instead of round for a chimney?

When using square tube for the chimney, the size of the tube will be the same as a round tube of the same diameter.

(4" square tube is equivalent to 4" diameter round tube)

If using a rectangular tube, you need to use the Equivalent Diameter (ED), also known as the Hydraulic Diameter.

This will compensate for the added friction from a square or rectangular tube.

Hydraulic diameter (circular) = 4 * Area/wetted perimeter

ED = 2 x (L x W) / (L + W)

 

[ribwizzard]

The problem I've found with extending the stack low inside the CC is that the food on the upper racks will get a darker color to them than the lower racks, especially true with chicken. I actually went back and trimmed the stacks higher on #19 due to that reason. From then on, I've only extended them as low as the top off the food would be on the top shelf, and I'm much happier with the results.

 

[bjake2001]

Thanks everyone for the comments.  I'm pumped to get this guy wrapped up and start cutting metal.  Here is what I have come up with so far, sorry the picture is a bit grainy, at a higher quality it takes forever to render....

smoker picture.JPG

• bjake2001
• Feb 13, 2014

I'd love to hear any thoughts before I detail out any further.  The walls will be 1" thick, with 12 gauge material on either side of 1" tubing, rock wool insulation in between the two sheets.  I hid the right door to make it easier to see inside, and the final product will only have 4 racks instead of 5.  Cook chamber is 58" wide, 26" tall, and 24" deep.  Firebox is 24" cube, with a 5"x12" vent on the bottom and a 3"x12" vent on the top of the door.  Both will have an adjustable slide.  It is a reverse flow design, and the firebox to cook chamber opening is 5"x24".  The chimney is 5.5" square, 30" tall after the 45 degree bend.

Am I on the right track?  Or doing anything too crazy?  I added the upper vent on the firebox door, and the reverse flow plate will be slightly elevated over the level of the bottom sheet of the top of the firebox, creating a bit of separation there.  Reading through a few other threads seemed to recommend both of those pretty highly.

If nothing else, I'm getting much better at Solidworks. :)

 

[bjake2001]

Got the better rendering to finish...

hq.JPG

• bjake2001
• Feb 13, 2014

 

[bjake2001]

After some more research, I think I am going to lower the top vent on the firebox to closer to the middle of the door.  I'll probably keep it separate from the bottom one by 2-3 inches. 

 

[bjake2001]

Any thoughts?

 

[lendecatural]

Looks great! You are on track by moving the firebox upper intake lower. 12 gauge will be a little tougher to weld up since it will want to warp on you. You will need to tack/stitch it all together before you attempt to weld it up solid. It will work just fine, you just need to be careful not to get any one place too hot. Are you going to get the sheet sheared? You may also want to consider going with a thicker plate for the interior of the firebox since that is a caustic environment and you will be fighting rust. You could use 1/4 inch plate on the inside and still insulate and cover with 12 gauge.

 

[bjake2001]

I'm a little spoiled and will be using a CNC laser to cut the plate/skin and brake press to handle the corners/bends.  I do a lot of work for a stamping/fabrication house and am calling in a favor.

I've seen some similar builds with ~12 gauge inside the cook/fire box, but then again, they are always brand new, not after a couple dozen smokes and a year or two outside.  I was hoping the interior tubing would give it the strength it would need.  I'm a little anxious to add all that weight to the box, but I definitely don't want it rusting out....  Not sure what to do.  It actually almost makes sense to use stainless on the interior if you can get away with a thinner material.  It should be much more resistant to rust and warping, and much lighter if I can get away with a smaller thickness.  It's 3-4 times as expensive, but if I am using half the amount (half the thickness), it's a small premium for the reduced weight.

 

[lendecatural]

Sounds like you have the hookup! With all the time you will be saving on the fab, spending a bit more on stainless sheet for the firebox should not be that big of a hit. I'm gathering stainless to build mine and I make monthly rounds to all the metal yards to gather materials and there are some good buys out there. Just use 309 wire to weld to the carbon and you will be fine.

 

[navarre]

12 ga is pretty thin for the inside. It would be ok for the outside probably. What you need for a smoker is thermal mass to minimize temp swings. The firebox needs to be thicker metal, 3/16in minimum for durability. The framework will have to be very strong in all dimensions to not warp and lose seal. Look at some of the smokers here for ideas. I am building one out of a propane tank and a piece of 24 inch dia 1/4in steel pipe for the firebox. The firebox will be 300+ degrees for a long period of time. 

If you want to use that metal, look at building an Ugly Drum Smoker (UDS). You can adapt those plans for what you want possibly. 

Mike

 

[gary s]

We built a vertical, Insulated Reverse Flow, used lighter stuff on the outside and inside of the cook chamber.  Inside of the Fire Box we use 1/4 " all the way 

Gary

 

[bjake2001]

 
We built a vertical, Insulated Reverse Flow, used lighter stuff on the outside and inside of the cook chamber.  Inside of the Fire Box we use 1/4 " all the way 

Gary

That seems like it would be a good compromise.  12 gauge in and out of the cook chamber, and outside of the firebox.  1/4" internal to firebox.  Both the cook chamber and firebox will be framed with 1" tubing and well insulated, so hopefully that would take care of the strength and the thermal mass.