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325 Gallon Reverse Flow Build

post #1 of 80
Thread Starter 

Hello Everyone. Been looking around the forum here for a few months getting ideas for my own smoker build. A buddy and I built our own cooker a few years back for cooking whole hogs, but I really cant use that cooker for true bbq smoking.


After looking through the forumn for countless hours taking notes, seeing what works, and the HUGE amount of knowledge being shared, I decided what I would like to do. I debated about starting a new thread (as many already exist), but relaized most of the threads that exist are for 250 gallon or 500 gallon pits, not 325. Even though alot of my build will be the same as a 250 of 500, maybe some things will be different.


Where I stand today: I picked up a neat looking trailer a few weeks back on Craiglist. It is solid, but needs a little TLC like most craiglist finds. Wheel bearings where replaced this past weekend. I also picked up a 325 gallon propane tank this past weekend for $150.00. I had to travel a ways to get it because small propane tanks (250-350 gallon) seem to be in short supply or extremely expensive (up to $600) here in Indiana.


My plan: I have drawn up a fairly detailed sketch of what I plan to do (see below). I hope to have one of the many experienced builders here take a look and check my theory. I used the BBQ calculator from Dave as well as the circle calculator for the FB - CC opening. I think I have the basic concept/design figired out, but need a little help with the details. Any advice or tips would be greatly appreciated. Thanks!


I have some pictures of the tank & trailer, but I need to figure out how to post them from my Ipad. Hopefully I can figure that out tonight.


325 gallon build.pdf 531k .pdf file
post #2 of 80
Thread Starter 
post #3 of 80

Welcome,  Looks like you will have a nice size smoker

post #4 of 80
Thread Starter 
post #5 of 80
Thread Starter 
post #6 of 80

You have a good start there Nightrod. I'm doing a 300 right now.

Keep the pictures coming.

Any questions, you know this is the place to go.



post #7 of 80

I just brought home a 300 gallon tank myself!  I'm planning to build a reverse flow, with propane burners installed for the option of going gas if needed.  Keep the pictures rolling; I'm anxious to see how this turns out for you.  Good luck!

post #8 of 80
Thread Starter 

Good Morning All,


Thank you for the welcomes and support. I realize this morning that I didnt really explain my plan very well in my original post (see PDF from original post).




325 gallon (water capacity) propane tank (.195 in wall thickness). Overall length is 9'-10" with a length of about 88" between welds (cant rememeber exactly).


2 CC doors approx 34" wide with 1.5"x1/8" flat stock flanges. 1/2" round bar hinges. (Plan to use ribwizards flange mounting idea)


2 or 3 expanded metal cooking grates. I plan to use 2 most of the time but have the option to add a third for cooking ribs when needed.


FB size of 24"x 32" x 34" made of 1/4" plate.


Exhaust stack size of either 6" or 8"


9 - 3" square FB intakes with sliding dampers.


With that being said, I have a few questions for you guys:


1. How wide of door flanges do you guys usually use? I have figured 1.5" wide flanges which I think should cover and seal up just fine.


2. FB size. Feldons calculator says I have 104% capacity with my current size. Is this ok? I know people say to have slightly larger than recommended but how much larger?


3. FB shape. I picked a 24" FB height in order to have max ground clearence for the FB. I plan to mount the CC/FB assemby on the trailer at a height to maintain 12" clearance from ground to bottom of FB. I picked a FB width of 32" (slightly larger than the tank diameter of 30"). Lastly I was forced to use a 34" FB length in order to get the correct FB volume. I guess my question is is there a magic number/ratio for these dimensions? I cant see how there could be one but maybe I am missing something?


4. FB Intakes. I plan to mount three 3" suare intakes on two sides (34" sides) of the fire box a few inches above my grate height. The last 3 intakes will be mounted on the charging door (2 toward the middle of the door and 1 centered slightly higher). Thoughts?


5. Chimney. I have not decided on a chimney height yet. I know most guys use 6" chimneys, but with a 6" chimney I would need more than 46" aboce the CC. This seems too high from a asthetic standpoint as well as function (overhead garage door issues). Have any of you made folding chimneys? I was thinking something that could fold down to a lower height during transport/storage? Ideas?


6. FB mounting to CC. I believe my FB to CC opening is correct, but I dont understand how far the FB is mounted INTO the end of the tank (Horizontal dimension). My plan is to cut the opening right at the vertical weld line. Thoughts?


These are just a few questions that I have. I ordered some materials yesterday and plan to mount the tank to the trailer this weekend. Any advice would be great. Thanks.

post #9 of 80
I'm new to this too, as I'll be starting up my first (sophisticated) build in the next week or two. The information you provided jives with all the research I've been doing. I'm in the same boat as you on chimney size. With 6" pipe, my calculations call for a length of 43.2". That just seems too long.

As for the flanges, I plan to use 1.5 - 2 inches as well (may not mean much coming from a newbie). There's a cooker here where I work, and that's what it has...this cooker works very well, so that seems OK to me.

The tank I have has flat ends, where your's are rounded. I plan to just slam my firebox right up against the flat ends, and not cut into the tank. I'm not sure if that is wrong or not. Also, I have all the calcs for FB-to-CC opening, FB intake opening, etc. One calc that I'm missing is how high off the bottom of the tank the reverse flow plate should be. Do you know a good solution for that?
post #10 of 80
Thread Starter 

The tank I have has flat ends, where your's are rounded. I plan to just slam my firebox right up against the flat ends, and not cut into the tank. I'm not sure if that is wrong or not. Also, I have all the calcs for FB-to-CC opening, FB intake opening, etc. One calc that I'm missing is how high off the bottom of the tank the reverse flow plate should be. Do you know a good solution for that?


Michaelwise, I will let a expert comment on your question because I believe my calcualtion for RF plate height is incorrect. I have been assuming the RF plate will be at the same elevation as the top of the FB. After reading more, I have seen where people have been saying the the area below the RF plate needs to be 1.5X the FB CC opening. I still am unsure about this so hopefully someone will come along and educate both of us on the subject.

post #11 of 80

Nightrod, morning and welcome to the forum......  We will need the diameter of the tank.......


Please take a moment and stop into " /Roll Call/  " and introduce yourself and get a proper welcome from our members....    

We're glad you stopped in and joined our group...    Enjoy the long smokey ride....     Dave

post #12 of 80

MichaelWise..... Morning......  Please start a different thread for your build...   Having to answer two sets of questions, on one thread, can get me confused ......   Thanks for your understanding......



post #13 of 80
Thread Starter 



The tank diameter is 30".


Below is a better picture of my sketch & calculations. Sorry it is not to scale and a litle cluttered.

post #14 of 80

Nightrod, morning again.......   A long narrow tank poses a problem with air/heat/temp flow and control......   Because of the long tank, the area under the RF plate increases the friction loss.....   10.5" (ED) segment height, is not enough for good air flow due to the friction loss....  A 12" ED would be my choice for minimum and 13" for a 50% increase in area would be what I would shoot for.....   The problem is, the RF plate is then approx 29" wide and taking up half of the Cook Chamber....  And you have no room for cooking racks....  And the volume/surface area is subject to a lot of heat loss..


Do you need a smoker that long ???   Consider cutting it down into 2 smokers if it is doable....   Short and Fat smokers lend themselves to more room for racks and less heat loss.....   (Volume vs surface area thing..)


Personally, I would not build a RF smoker with those dimensions because of the above....    Dave

post #15 of 80

Another thought cut the tank at the welds take the two ends , weld some legs on them and you have a fire pit or a charcoal grill. That way you will wind up with a 7 ft tank instead of an almost 10 ft tank. should make a difference with the RF plate and air flow.



post #16 of 80

Cutting off the 2 ends leaves a ED of 11.5" and RF width of 29.2"  ...... It's a good start and selling the fire pits will improve the cash flow..... 

post #17 of 80

One more thought, instead of cutting at the welds go another 6"  make the fire pits a little bigger and better and decrease the tank by another foot, no you are down to a 6' tank should work.



post #18 of 80
Thread Starter 

Thanks for the feed back guys. This is the kind of stuff I want to find out before I start.


I understand what Dave is saying. My original plan was to use a 250 gallon tank (same diameter as the 325) but I had trouble finding any within a 150 mile radius of my house. To answer your question Dave, I guess the answer is "no" i dont need anything that long but since it was the only thing I could find I was hoping the length would not be a problem.


I like your idea of cutting the end off on the tank. What do you think about cutting off only one end? I like the look of the rounded ends which is why I was so determined to find a propane tank (everyone I talked to in my area suggested I use a fuel tank with flat ends). If you where to cut off the end near the FB how much would you cut off? Enough to make the CC size equal to 250 gallons? If you think it would still work, I would like to shoot for a little more than 250 if possible. Either way I would rather have a smoker that works good even if I have to sacrifice size.


I will rework my sketch/calculations today and see what I can come up with. Thanks again for your help.

post #19 of 80

Tank length depend on how many racks you want....   A short tank with the RF plate low enough to add 2-3 racks will cook as much as a long skinny smoker, use less fuel, weigh less...   etc.....  

If you cut it in half to 160 gallon, the  (ED) seg. ht. 7.5" for the RF plate leaving 22" for rack height and chord AB is 26" for the RF plate width...   You could get 2 racks with about 80% of the long smoker in half the space....   run the RF plate to the weld at the round end....   Dave

post #20 of 80

You could cut one end to maintain the rounded look on the other end, or you could cut 3 - 4 feet out of the middle. More work and welding but if you have the time I would consider that.

One of our builds I wanted an oblong smoke chamber so we took a 20" piece of pipe split it and added in 6" plate. Lot of extra cutting and welding but turned out exactly what I wanted. Neat thing about a torch, plasma cutter cut off wheel and a welding machine you can do what ever you want.



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