Need Help With A New Reverse Flow Build 275 gallon

Discussion in 'Reverse Flow' started by blacked out, Oct 22, 2014.

  1. blacked out

    blacked out Fire Starter

    Hello, I'm new to the forum, but have helped roast pigs for a few years on borrowed and rented roasters.  I love the idea, process, and challenge of smoking and recently bought a stalled project from someone else.  Its a 275 gallon oil tank with a good deal of the metal work already done.  The builder put a thin water or ash lower tray with a slightly elevated charcoal tray above it, with the pan being about 16" from the bottom of the tank.  About 7.5" above the lower tray is the primary cooking grate, which makes it approximately 24" from the bottom of the tank.  The builder said he planned to put a smoker box "off of the back".  He seemed like a great guy, but I don't know how much he knew about building a smoker.  I want to add a firebox on the right side as you are facing the grates and hope to make it out of 1/4" metal.  I plan to add a piece of 1/4" metal below the water / ash pan and cut the pan down to allow smoke to flow around the left side.  I believe this is best as it will allow me to open the access door above the firebox to add water to the pan and remove it when done for easy cleaning.  I have looked at the Feldon calculator, but am having difficulty deciding on the below listed questions.  Thanks in advance and I will update photos as I work to finish this during the winter months.  I anticipate a variety of opinions and hope the discussion will help me decide what is best for my purposes.  I do plan to add wheels and a hitch assembly at some point. 

    1.  What size should my firebox be?

    2.  What size should my opening between the firebox and the tank be?

    3.  How much area should I allow for flow between the water pan / baffle and the cooking area?

    4.  What size exhaust opening should I use?  

    Last edited: Oct 22, 2014
  2. smokingunny

    smokingunny Meat Mopper

    I'll save Dave the trouble and post the calculations. They should answer all of your questions once you plug in the numbers.

  3. lendecatural

    lendecatural Smoking Fanatic

  4. You have a lot of space in the bottom  a 275 gal = 63,525 cu in  fire box should be a minimum of 1/3 of that 21,154 cu in  I like to go at least 10 to 20 % larger doesn't hurt a thing  A 6" stack would need to be approx 38"    FB to CC opening should be   254 sq in

    How wide is the opening where that rack pulls out on the end ?

  5. blacked out

    blacked out Fire Starter

    Thanks for the responses.  I have printed the drawing from SmokinGunny and will work on the numbers.  As gary s pointed out, the initial builder built the lower chamber larger than I would have done.  The only advantage I can see to the larger bottom chamber is that it may help to regulate and even out the temperature above the water tray.  Do you think I need to lower the water tray or will it work where it is?  If it will work, I would prefer to leave it at its current height and shorten it to allow smoke to flow around the left side of it.  I did not measure the water tray, but think it is 3-4" narrower than the overall width of the tank.   
  6. blacked out

    blacked out Fire Starter

    Again, thanks for the information so far.  My thoughts are to add a 1/4" plate under the current water pan after I shorten it.  Unless someone can offer a reason why not to do so, I think I'll continue it out the right side of the tank and use it to form the top of the firebox.  Given the large area below the water pan, I will likely flush the left side of the firebox to the right outside of the tank and cut an opening large enough to satisfy the needed air flow.  I understand this will add some length to the overall smoker, but it should allow more even flow under the water pan.  I'm new to building a smoker, so please let me know if I'm missing something.  I picked up a cheap trailer someone made out of a 1964 Ford pickup that I'm going to use as the frame for the smoker.  Sorry, but the Christmas themed bed has got to go.  

  7. blacked out

    blacked out Fire Starter

    I'm slowly progressing on this smoker.  I picked up a 1/4" thick round steel tank that is 30" wide and 33" long at its shortest point, to use as the firebox.  The two ends are rounded, so the volume is greater, but will obviously be decreased when cut to attach to the cooking chamber and when I flatten the top.  The RF plate will be 1/4" steel and will form the top of the firebox to allow for a flat top surface.  From what I've read, Feldon's calculator needs some adjusting for a reverse flow smoker and "DaveOmak and friends" have put together a successful calculating process.  I've run the numbers, but want to make sure I'm not missing anything before I start cutting. Also, it seems the design is largely dependent on the firebox size, but is calculated based on the cooking chamber size, which has me a little confused.  Looking back at my initial post and the previous owner's design, I will likely move the water pan down to whatever the appropriate location for the RF plate is.  I will use the old water pan location for a second rack that is pictured sitting in the water pan above. 

    275 gallon tank = 63525 cubic inches

    Firebox to cooking chamber opening = 254.1 square inches (63525 x 0.004)

    Area under RF plate =  254.1 square inches (63525 x 0.004) I'm not sure how to apply this number because the space under the RF plate is a measure of volume, not area.

    Opening at end of RF plate opposite firebox =  254.1 cubic inches (63525 x 0.004)

    Firebox air inlet = 63.525 cubic inches (63525 x 0.001) Using an upper and lower air inlet, each should be 31.7625 cubic inches (63.525 / 2)

    Exhaust Stack volume = 1079.925 (63525 x 0.017)

    Exhaust Stack length using 6" diameter exhaust = 38.194" (1079.925 / {0.7854 x 6 x 6})

    Fire box to cooking chamber opening in shape of half circle = 12.375 inches above tank floor at 26.75" inside tank width (circle calculator at 12.375" high & 13.375 radius = 254.28 cubic inches

    Of course, I'll double check all my physical measurements tomorrow, but I wanted to confirm I am on the right track to start cutting and welding.
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

  9. blacked out

    blacked out Fire Starter

    Thanks DaveOmak. I'll do the lower intake around 58.76 cubic inches (63.525 x 0.925) and upper around 4.76 cubic inches (63.525 x 0.075).  Obviously, these are the ideal numbers and final number will not be that exact. 

    My firebox is not much larger than required.  The ends are convex, so when I cut out the half circle for the firebox to cooking chamber opening, the lower portion of the firebox will extend under the cooking chamber.  Is there any problem with cutting the bottom of the cooking chamber out where it is inside the firebox?  I've seen it done a few ways, but removing it should add more usable area in the firebox.  However, I didn't know if having too large of an opening between the firebox and cooking chamber could be a problem.  Again, thanks for all the input.  
  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    You can insert the FB into the CC as far as you want... It may cause some hot spot on the RF plate... BUT.... that can be handled with a plate..... Suspend the plate with about 1/4" air gap... The new oversized opening FB/CC is not a problem...

  11. blacked out

    blacked out Fire Starter

    I'm slowly making progress and yes, I know I need to post photos.  I could use some suggestions about how to set up the plenum and chimney.  As the photos show, the first building cut the top at an unusual angle and used 5/8" x 3" bar stock to line the upper portion of the smoker where the lid meets.  Both racks now slide out the front and a water pan slides under the lower rack to the right side of the smoker, on top of the RF plate.  I have not closed in the area to above the water pan and to the right of the lower rack yet as I'm trying to sort out the chimney first.  If I center the plenum and chimney, the top of the plenum will be an inch or so above the top rack.  I'm concerned that such an arrangement will not flow well and will not get smoke over the top of the hog or other taller items on the top rack.   

  12. smokejumper

    smokejumper Smoking Fanatic

    Hey Blacked Out,

    Glad you're back. I had not seen your build before and I like it.

    Your exhaust should not be blocked by a rack as you have described.

    I would recommend placing the bottom of your plenum just above your top rack. You can move it to the rear a bit to get room to mount it on the side.

    It will flow just fine there and you will get plenty of smoke throughout your CC.
  13. blacked out

    blacked out Fire Starter

    Thanks SmokeJumper.  I'm still concerned that moving the chimney too much toward the rear will affect air flow.  However, in order to keep it more toward the center of the cooking chamber, I have to lower it because of the angle of the top.  That concerns me as I want to move smoke around the top of the smoker for when I have a full hog inside. A few poorly made draws to give idea of options.

  14. smokejumper

    smokejumper Smoking Fanatic

    Hey Blacked Out,

    Of the three options you listed, either of the last two with the angled plenums would be OK. Your first options with the centered plenum does not appear to have enough opening area for good draft.

    The chimney to the rear will not adversely affect air flow; the smoke will always find the exit no matter where you put it. Smoke will reach and circulate in every square inch of your cook chamber regardless of how high the chimney is, or whether it is to the back of the front. I used to be of the opinion that the exhaust opening should be down at grate height, but have since changed my mind. I have seen too many good working smokers with the exhaust directly out of the top, and realize that the velocity of the smoke, the turns and obstacles will generate enough turbulence to distribute the smoke throughout the cook chamber. By the same token, I do not believe in the "stale smoke" theory, the environment in the CC is just too dynamic. JMTCW
  15. blacked out

    blacked out Fire Starter

    Thanks again for the information and opinion.  I think the last option with the chimney as high as possible and to the rear is the best choice I have.  I think the larger plenum inlet, though it will be angled in a few directions, should allow for more draft than the first two options.  I'll put the bottom of the plenum about an inch above the upper rack and angle it up to the exhaust stack.   
  16. smokejumper

    smokejumper Smoking Fanatic

    That should work well.
    Looks like you are pretty close to firing it up.
  17. blacked out

    blacked out Fire Starter

    Man, talk about stacked!

    I have some grinding to do and need to fill in a few areas near the bottom corners, but the plenum and 6" stack are in.  I pushed it as high as I could, which meant moving it toward the back of the smoker, so that I could get as much air flow as possible.  The previous owner's design limited my options.  The bottom of the plenum is about an inch above the upper rack.  One step nearly done, many more to go.  
  18. You can get it out the door ?

  19. blacked out

    blacked out Fire Starter

    Yes, it'll fit out the door. The photos may be a deceptive, but the stack is only about 39" long. It was that length when I pulled it out of the scrap bin and will need to be cut down about an inch to get the 38.194".  Since its near the top of an already tall 275 gallon tank, I won't be worried about smoke blowing in my face from the chimney.      
  20. smokejumper

    smokejumper Smoking Fanatic

    Holy Smokes!

    That is a good looking stack and plenum. Nice job with the multiple angles.

    As long as it will fit out the door, no need to cut the stack down any.

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