# High School Shop- Reverse Flow Build



## Gijoe985 (Apr 9, 2019)

Hello all,

I just signed up on here to stay this thread. I'm a high school shop teacher and I teach the big 3, auto, metals, and woods. We've had the idea for some time now to build a large smoker (that could potentially be used for burgers an dogs as well.)

I have a friend who owns a smoking business Cali (we are in Eastern Washington). He has sent me some pictures of his reverse flow offset smoker. His (unlike most I see) is NOT made from a barrel orcylinder, but is bent plate. We have a lot of sheets of metal to work with in a variety of thickness. We even have 1 that is near 1/2". 

Al that said, I dont even know what kind of questions I should be asking at this point. We either want to 1) go big or go home or 2) build a smaller guy to get our feet wet first. 

I'm not sure if we will build it on a trailer bed or not. It'll depend on how big we decide to go. 

We live in an orchard town, so I have an unlimited supply of apple and cherry wood. So, whatever design that would best accommodate that would be best. 

Anyway, where do we start?


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## kruizer (Apr 9, 2019)

I would recommend starting small. Take a look at the Oklahoma Joe smokers and maybe pattern on one of the smaller ones.


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## daveomak (Apr 9, 2019)

Follow this build tutorial....  It  will get you a great smoker.....

https://www.smokingmeatforums.com/t...eady-to-use-rev5-6-19-15.172425/#post_1264161


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## Gijoe985 (Apr 9, 2019)

Very nice, thanks.


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## radioguy (Apr 9, 2019)

Go bigger and put it on a trailer.  Experience talking here.

RG


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## motocrash (Apr 9, 2019)

.01 Use Dave's formula.
.02 This sounds like a "municipal smoker" of sorts,I say bigger-trailer mounted.
There's my two cents.


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## wbf610 (Apr 9, 2019)

My vote, go big.

I’m not a builder, but does the steel have to bent/rolled?  Wouldn't a square box be simpler?  Vertical offset?


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## JckDanls 07 (Apr 9, 2019)

An octagon one would be one way to go since you have plenty of plate...  1/4"-3/8" for everything ... pending the availability of a brake or a press a few of the corners can be bent instead of every one being welded ... 

or as mentioned above...  a cabinet smoker would work as well ... 

A "themed" smoker would be a good idea as well... If you look at my build (Antique tractor) you will see what I mean..  the link is in my signature line below ...


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## Gijoe985 (Apr 23, 2019)

Ok, so, by being such a novice at smokers, the above build calculations are a bit tough to follow. 

I have a 120 gallon LP tank. It is about 5'x2'. So from there I am trying to run my numbers. 120x231 is 17720 ci. 
That would make my firebox about 9150 ci? For just over 33%. Sound right? 

I was planning to build this firebox out of our thick plate. I might even insulate it and add an outer layer.

Anyway, I am going to keep reading and plugging at the above numbers, but extra direction would be great.


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## daveomak (Apr 23, 2019)

5' x 2' 
2' = 24"
5' = 60"
24x24x0.7854x60= 27,143.424 cu. in.
27,143 cu. in. x 0.004 = 108 sq. in. openings

Calculate each line and write down the number.... 
I did take some liberties where cu. in. x ?? = sq. in.  
It will work....


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## Gijoe985 (Apr 23, 2019)

Am I missing the math for how large the underside passage is supposed to be vs the cooking area? 

Anyone have suggestions on door(s) size(s)? 

I do need to get more exact measurements of the tank. We just got it today.


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## Gijoe985 (Apr 23, 2019)

Ok, I got 29,631ci. So 118si opening. Then the firebox would need to be 9,778ci. 

From what I'm understanding, the hole from the FB to the CC is the same size as the cross sectional area of the area under the RF and also the same as the passage where the air comes around to the top portion. 

As far as fire boxes are concerned. Is there any advantage to how you build them proportionately? I.e. a perfect cube vs a tall box vs a short and longer box. Etc...


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## Gijoe985 (Apr 23, 2019)

Another question (I hope it isn't blasphemy on here), if we wanted to "grill" on our smoker, would it be able to get hot enough/what would we need to do to make it dual purpose?


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## JWFokker (Apr 23, 2019)

Gotta have direct heat to grill, which is the opposite of what smokers are intended to do generally. You can put a top access door on your firebox and a grate to grill on. That's the easiest way to go. Grills and smokers are two different beasts. Tends to be easier to smoke on a grill than grill with a smoker. Most people have a dedicated grill if they need high heat/direct heat.


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## daveomak (Apr 24, 2019)

Sounds like you want a 4 WD and a Honda Accord all in one...  
Dual purpose makes for 2 units that don't do anything well....  IMO....


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## indaswamp (Apr 24, 2019)

First question I would ask is how many people do you want to feed? Will you be doing a lot of big cooks? Or a lot of small cooks just for the class?  Hard to cook small on a big unit......

If you go big, you can use it for school fund raiser cooks....


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## Gijoe985 (Apr 24, 2019)

indaswamp said:


> First question I would ask is how many people do you want to feed? Will you be doing a lot of big cooks? Or a lot of small cooks just for the class?  Hard to cook small on a big unit......
> 
> If you go big, you can use it for school fund raiser cooks....



Can you describe the difficulties of cooking small on a big unit? Again, I'm learning here. I've only ever smoked on my home grill, bit thought that this would be a fun and appropriate shop project. 

I have a friend who smokes as a profession and he told me that (though you won't get direct hear) you could still do some simple burgers and hot dogs on the smoker if you got it hot enough. We do have grills for grilling, so I dont need to make this perfectly multi purpose. I just figured since it was so big, it'd be nice to cook everything in one spot. Hence if we weren't smoking, we could grill. Just an idea. But again, I'm open to redirecting. 

The tank we got is .140". So not super thick. 

Maybe we will make 2 smokers. A large one and a smaller one. I still have tons of sheets of 1/4" and 3/8" metal that I could use.


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## indaswamp (Apr 24, 2019)

Cooking small on a big unit is different than cooking large. For one, you'd be wasting a lot of fuel. The damper settings will be different...Large cookers are designed to run with a certain amount of meat which acts as a heat sink (think room temperature meat in a 250* smoker). The meat also slows down air flow which is taken into account when the unit is designed. A large cooker will not run optimally mostly empty. You can close off the intake vent to slow the flow down, but now you are choking off the fire. Choke it too much and you sacrifice a clean burn. I'm not saying you CAN'T cook small on a big unit, just that the design parameters won't be met, the vent settings will be different, and fire management will be very different.


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## indaswamp (Apr 24, 2019)

120lb. LP tank is a nice sized unit. I would not call that "large". But I guess size is subjective....

When I think Large cooker, I'm thinking of a 1,000 gallon LP tank, or a pit that can cook 100 Boston Butts at 1 time....

You could cook hamburgers and hot dogs on a 120lb. LP tank fairly easily. Super hot smoke them....275*~350*. 

If you want to kick up high temps on it, I would oversize the firebox by 20% to accommodate a slightly larger fire to produce those temps more efficiently.


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## Gijoe985 (Apr 24, 2019)

So, maybe a 12,000ci firebox?

Also, does the proportions of the FB matter much? Is taller vs longer any better?

After doing the circle math, my RF should be about 7-7.5" above the bottom of the tank. Chord AB=22" Arc AB =28.375.

Have you guys done math to account for a slight angle of the RF so that a drain can be installed for easy cleaning? I've seen designs with a slight angle leading to a small trough with a tube to drain out. Sounds like a good idea. 

Sound good?


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## Bearcarver (Apr 24, 2019)

I personally would recommend going with a Small unit first.
That way if they wanted to, the students could each build their own personal smoker.
Then after most of them learn a bit about using them, they can all get together with you (Teacher), and build a big one that could be used for some type of School Shindig, as a celebration of completion. Naturally the big one would need to be trailer mounted to move it, but being also the Auto Teacher should make that easy enough for some more instructive classes along the way.

Bear


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## Gijoe985 (Apr 25, 2019)

I think we are going to stick with the 120 gallon unit for now. We've got the tank. It'd actually take less metal to build then trying to fab up one from scratch. Though a smaller unit would be great for the future.


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## Gijoe985 (May 1, 2019)

Anyone have good pictures of their firebox? I'm just trying to make sure we include all the needed features.

Also, what are the specs on the expanded steel you guys usually use for the grating?


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## JckDanls 07 (May 1, 2019)

I made my FB the same width as the tank where it was going to connect to it...  pretty sure it was 22" ... my tank is also a 120 gallon....   I believe my FB ended up being 22" cubed ... Here's a couple of pics...  they don't really show where it's connected/welded to the tank though ...













Tractor Smoker 013.JPG



__ JckDanls 07
__ Dec 18, 2016


















Tractor Smoker 018.JPG



__ JckDanls 07
__ Dec 18, 2016






As for the grates..  luckily I had some heavy duty 1" sq stainless screen that I used...  but most use the #9 (3/4") flat expanded metal for the grates..  For my FB grate I used 3/4" catwalk expanded metal (I'm sure that's not the proper name for it) ... but ended up laying a piece of the #9 flat expanded on top of it...

This is where the vents like to be to run at 225`








This is my preheating method..  I let this burn down to coals..  that way my chamber is about 350`..  I then spray water all around the inside of the chamber for a steam clean...  it also brings temps back down for smoking


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## Bigmark1127 (May 3, 2019)

I am following the guide and had all the numbers I am getting tripped up on the stack or stacks if you will I came up with1718.5 ESV and to run 4 inch I'd pipe and I come up with 108 inches above the c.f. can I cut this half if I put 2 4 inch stacks on?
Thanks for any help


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## Gijoe985 (May 6, 2019)

What's the optimal distance from the RF plate to the lower cooking rack? I'm seeing 4-5" from what I've found via searching, but others have said closer. 

On a 5ft long smoker, 1 big door, or two smaller doors? Pros/Cons?


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## motocrash (May 6, 2019)

Gijoe985 said:


> On a 5ft long smoker, 1 big door, or two smaller doors? Pros/Cons?


To whole hog,or not to whole hog?


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## Gijoe985 (May 13, 2019)

4-5" up sound good for the lower grate? I need to figure this out before we cut the door. 

Also, is there any reason why would wouldn't want the bottom of the door to be about even with the bottom of the grate? That way the grate can slide out. I figure the door doesn't need to be any lower than that.


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## Gijoe985 (Jun 3, 2019)

daveomak said:


> Follow this build tutorial....  It  will get you a great smoker.....
> 
> https://www.smokingmeatforums.com/t...eady-to-use-rev5-6-19-15.172425/#post_1264161



If I make the stack drop down into the smoker like in the "alternative" stack picture in the link, do I calculate the total height of the metal, or just what it above the smoker? For a 4x4 smoker stack it told me that I need a 40.75" stack. Sound right?


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## JckDanls 07 (Jun 3, 2019)

Measurement is from the TOP of the tank/smoker...  does not include what protrudes down into the tank/smoker or a plenum out the end/side ...


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## Gijoe985 (Jun 4, 2019)

Can anyone elaborate on the benefits to having the stack protrude down into the tank more or less? My only thought is that more smoke would linger in the smoker if the stack went further down into the smoker.


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## Gijoe985 (Jun 4, 2019)

We're trying to use materials that we have on hand. We have 2.5" exhaust pipe that we could use for the stack(s). From the math provided, we'd need 132" of pipe. What I thought would look cool (and give us that volume [~650cu in]) would be cutting 4 _different_ lengths that add up to the 132. Then bunch them together, but they'd be unequal at the top. I thought that'd look cool, but maybe the different lengths would be less efficient.

Thoughts?


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## Gijoe985 (Jun 4, 2019)

Ok, and not after doing more reading, it seems that more stacks = more friction. We just don't have anything else that size. Our best option B is to weld some 4" wide flat stock to make a 4" square exhaust.


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## Bigmark1127 (Jun 5, 2019)

Gijoe985 said:


> We're trying to use materials that we have on hand. We have 2.5" exhaust pipe that we could use for the stack(s). From the math provided, we'd need 132" of pipe. What I thought would look cool (and give us that volume [~650cu in]) would be cutting 4 _different_ lengths that add up to the 132. Then bunch them together, but they'd be unequal at the top. I thought that'd look cool, but maybe the different lengths would be less efficient.
> 
> Thoughts?


I have 1079 escape I want to run two 4” pipes because that’s what I have coming up with 42” tall above CC does seem right?


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## daveomak (Jun 5, 2019)

Something to remember, or learn.... 
2.5" = 4.9 sq. in. area...  X 4 = 19.6 sq. in.  X  34" height = 666 cu. in... 
However, *2.5" dia. X 3.14 = 7.85 circumference*...  X 4 = 31.3 in. of circumference total.. 

A 5" D pipe = 19.6 sq. in. area X 34" = 667 cu. in.
*5" D X 3.14 = 15.7 circumference*

The thing to note here is...   FRICTION from the pipe interior...

31.3 cir. inches compared to 15.7 cir. inches is twice the friction...
Friction is a killer in a gravity smoker....

Now, make a 5" square exhaust...


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## Jonok (Jun 5, 2019)

What would happen if you took some of your 2” exhaust pipe, ripped it in half lengthwise, and welded it back together like a 5 or 6 leaf clover? Seems to me you’d be able to achieve more than enough cross sectional area, and you’d have a great math and fabrication project to boot.


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## Gijoe985 (Jun 5, 2019)

That may be better than what I had been dreaming up. We are low on 1/8"x 4" flat... lots of 1/4", but that'd be a very heavy stack!


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## Jonok (Jun 5, 2019)

If you've got any budding tig welders, "walking the cup" in a groove like that over a 5' stack 6 times would be a dream.


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## Gijoe985 (Jun 13, 2019)

Ok, we did a quick test burn (just using 2x4 wood we had at the school shop.). 

We have two thermometers. The one closer to the firebox quickly climbed to 300 while the further one slowly raised to 200-225. 

Suggestions?


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