500 gallon RF Build revised

Discussion in 'Reverse Flow' started by nwbhoss, Mar 2, 2016.

  1. nwbhoss

    nwbhoss Smoking Fanatic

    Well I decided to start a new thread on my build since my recent schooling from the calculator has completely changed my build. I removed the wood stove I thought I would use as a firebox after finding out it was less than half the size I needed and the FB to CC opening was no where near large enough either. I grabbed a 40 inch long price of 24 inch diameter 1/2 wall pipe that I originally had planned to use that puts in much better shape in the volume Dept. I had an old barrel stove and a Washington iron works stove kit that I am going to use the door and feet off of for my firebox. From here on out I will be following the calculator as close as possible here is a couple pictures of the new and improved build. [​IMG]
  2. appalachian q

    appalachian q Smoke Blower

    That's looking great
  3. nwbhoss

    nwbhoss Smoking Fanatic

    I am really having a problem with the math on this calculator? I have a couple corrections that I made to my original figures but even still if I have my numbers right this 24"dia x 40" long pipe is still half the size I need? Then figuring out the opening from the firebox to the cook chamber comes out to 430 square inches and I dont understand how to convert that into how big the opening in the cook chamber should be in inches?

    If anyone wants to help me crunch the numbers my cook chamber is 500 gallons it is 37" diameter and total length of 118 inches (I did the math both ways and it comes out fairly close)

    My firebox is 24" diameter and 40" long

    Any help in making sure I do this correctly the second time around would be greatly appreciated!  
  4. nwbhoss

    nwbhoss Smoking Fanatic

    Here is firebox version 2 the feet are welded up and and I took the cook chamber off the trailer it was on in order to get the height correct. I am waiting on a couple pieces of 1/2 plate for the ends of the firebox and when I figure out how big to cut the opening from the firebox to the cook chamber I will make that cut and put the reverse flow plate in.
  5. nwbhoss

    nwbhoss Smoking Fanatic

  6. nwbhoss

    nwbhoss Smoking Fanatic

    Well I finally stumbled across the actual Feldons BBQ Pit Builder and now that I am not having to make myself dizzy trying to figure out the numbers that come so easily to some folk. Up unit now the only thing I was using was the calculations that are listed in the revised calculator thread. I realize I am not going to be able to use this 24"x40" pipe for a firebox either as it is still 1/2 the size I need for a efficient smoker. So I scrapping this plan and getting enough 1/2 inch plate to build a 33"x33"x33" square firebox that will meet the minimum firebox size requirements. I guess trying to take the cheap way out and use what I have does not cut it when I am trying to build a quality smoker that will perform well. Now I thinking about how I can add this 24"x40" pipe to my smoker build as a cold smoker or warming box or something? Any ideas will be appreciated. 

    Using Feldons and knowing what I need for the FB to CC opening and the stack size and length are a huge relief to me I get my pleasure out of building this stuff not doing the math to figure out how it functions. I am sure glad there are people here who enjoy that stuff and are willing to give this info out so I can build a great smoker and not have to be a rocket scientist to build it. 

    It should be smooth sailing from here on out 
  7. nwbhoss

    nwbhoss Smoking Fanatic

    and upon further investigation I found Gary's Ratios for Reverse Flow Smokers calculator and ran those numbers as well. They are not too far off from each other and I have an easier time reading the Feldons calculator but I was able to compare the numbers of each and it looks like I am going to be fine with a 36"x36"x36" firebox. The one thing that I am going to look into further is the upper and lower inlets to the fire box vs just a single air inlet figure for the Feldons calculator. 
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member



    Mark, afternoon.....   First, when looking at the tutorial for building a RF smoker, ONLY use the first post......  IGNORE the other posts for the build...

    The first post is a step by step how to calculate each number for the build....

    Gallons x 231 = 115,000 cubic inches..

    115,000 X 0.004 =  460 cu. in.  FB/CC opening / area under the RF plate / area at end of RF plate...

    115000 X 0.33 =  38,000 cu. in.  for the FB.... (24 x 40 pipe is 18,000 ) too small  (36x36x36 is a great size)

    115,000 X 0.001 = 115 sq. in. for the FB inlets...

    115,000 X ( .017 to .022 ) = 2,000 to 2,500 cu. in..  I lean toward the larger volume to insure good air flow..

    Exhaust Stack Volume ...

    2500  / 0.7854 = 3180 cu. in. / 81 (9x9) = 39" of stack ABOVE the Cook Chamber...   That's what I would use....

    FB/CC opening...

    Inside diameter of your tank is approx. 36" ...  use 18" for the radius.....

    16" segment height works well...   but it can be safely lowered to give more room where the food is cooked....  Because the diameter of the CC is wide, there is reduced friction.....  Also, if you slide the FB into the CC, that will reduce surface area and friction loss under the RF plate...

    I would slide the FB into the CC until it has passed the welded bead so you are now into the straight section of the tank...   That will make this build soooo much easier...  the RF plate will now be a rectangle, the friction will be reduced and I'm going to say the Segment Height can be reduced to 14", giving you 21" above the RF plate for food racks...    The RF plate width will then be 35"...  It can then be welded directly to the top plate on the FB for easy fitment and the FB will be 1" wider than the CC for easy fitment...

    For the air inlets to the FB, I would put ~20 sq. in. of opening directly across from the FB/CC opening ...

    I would put ~80-90 sq. in. about 4-5" above the floor of the FB and have the wood grate about 6-7" above the floor of the FB....
    charswifterie and nwbhoss like this.
  9. nwbhoss

    nwbhoss Smoking Fanatic

    Thank you very much for the very informative reply! You answered all my questions and then some! One thing I could not understand was the FB to CC opening. I was getting ready to post and ask somebody to please post a picture of a FB installed with the hole cutso I could visualize it. Your telling me I can locate the FB and basically use the top of the FB as the beginning of my RF plate tells me exactly what I was hoping for! I pictured the FB located and the entire area inside the CC cut away and the RF plate above that opening. You made sense of something I could not get. Thank you!
  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    ...[​IMG]....    You can slide the FB into the CC as far as you want...  The farther, the more you can lower the RF plate because there will be less friction as stated above....
  11. nwbhoss

    nwbhoss Smoking Fanatic

    I will definitely install the firebox a couple inches past the seam so that it will be into the smooth portion of the cook chamber. That will make everything else so much easier as you said. I am looking forward to not having to make that radius cut on my FR plate. 

    I have a dimension of 17 inches from the bottom of the cook chamber to the bottom of my doors so if I go with the 14" segment height and put my angle for the first food rack an inch above the bottom of the door opening that would give me  a total of 4 inches of space between the FR plate and the food rack. 

    What is the rule of thumb on this space? I have not seen it mentioned anywhere how high the first food rack should be above the FR plate? 

    For the reverse flow plate I plan to take two pieces of plate and weld them in the center lengthwise and add a 1" slope towards the center and add a drain for cleaning. 

    Should I put this drain at the far end or right in front of the fire box? I am thinking a 2" diameter pipe with a ball valve under the cook chamber. 

    again Thank you very much for the help! This is all much easier to understand now 
  12. nwbhoss

    nwbhoss Smoking Fanatic

    One major huddle for me is that I am going to be in the future putting this smoker inside a "smokehouse" I am converting a outbuilding into a outdoor kitchen with a meat prep room with stainless counters and sinks and a fridge and my crab cooker and smokers and will be building a small smokehouse for smoking salmon. With all that said I will need a stack that will vent either out thru a wall or up thru the roof so I am wondering how this will effect the exhaust flow? I will be using the smoker outside for a short time until the building is ready. 
  13. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    When and if you do those changes, you'll figure out what has to be torn out so your smoker works again....
  14. nwbhoss

    nwbhoss Smoking Fanatic

    Well I spent a couple hours making measurements and remeasuring and measuring again and again.
    My cook chambers inside diameter is 37 inches and my plan is to put my 36x36x36 firebox into the cook chamber 5 1/2 inches past the weld seam and that will leave 12 inches of firebox extending out from the end of the cook chamber and with a segment height of 14 inches I will have a measurement of 35 inches across the cut in the cook chamber so I will have just a 1/2 inch of the firebox on each side on the cook chamber. If I put my first rack 1 inch above the bottom of the door opening that will leave about 6 inches between the reverse flow plate and the top of the bottom rack. I plan to leave 10 inches between the bottom rack and the top rack. Now I just need to get more steel and get my firebox built.
  15. bmudd14474

    bmudd14474 Smoking Guru Staff Member Administrator Group Lead OTBS Member SMF Premier Member

    My firebox is 32x32x32 and the RF plate is welded to the top of the firebox. If you want a picture I can get you some if that helps.
  16. jckdanls 07

    jckdanls 07 Master of the Pit Group Lead OTBS Member

    here's another little note... on the corner of the firebox that will have the RF plate welded to... do not weld this corner when building the firebox.... when it comes time to cut out the opening (I did this after box was mounted to chamber).. you'll only have to cut the radius... leaving a nice factory edge to start off the RF plate ...

    If I also could say... If you bring the FB back out of the CC to only an inch or so past the weld... this leaves more surface on top of the FB to set splits on for pre heating ...
    nwbhoss likes this.
  17. nwbhoss

    nwbhoss Smoking Fanatic

    I would love to see the pictures! Any visual input I can get to help me have an idea how the inside of this thing should look is welcome! 

    During the many searches I have done of various builds one thing I have yet to see is the actual firebox opening cut and how it relates to the reverse flow plate location. 

    The idea of having a square firebox and locating it so that its part of the the reverse flow plate makes a ton of sense to me! 
    Last edited: Mar 7, 2016
  18. nwbhoss

    nwbhoss Smoking Fanatic

    That is a good point, my thought was with a 36"x36"x36" firebox even with the Firebox 5 1/2" past the weld seam I still have 12" from the outside end of the bell to the end of the firebox and 36" width since DaveOmak said the farther the Firebox is into the cook chamber the lower the reverse flow plate could be due to less friction I figured I would hedge my bet. I am sure I can find a happy medium. I have decided to not make that cut until I have the box complete and ready to install. 

    Not making the weld across the top of the firebox where the reverse flow plate will be welded is a GREAT idea!! 
  19. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Since you have cut the door opening, it is important to follow, "I don't want the CC to warp" welding techniques...  

    Welding the FB to the CC can cause the tank to warp...  making the door a poor fit up....  short welds..  alternate opposite side welds.... let the welds cool thoroughly before moving to the next welds....  

    Poorly fitting doors is a royal PITA.....   You want the smoker to be air tight when finished....
    nwbhoss likes this.
  20. nwbhoss

    nwbhoss Smoking Fanatic

    Thank you for another very important bit of info! I will follow your advise. When I am done with this smoker I am going to write a "building a smoker for dummies" post and go thru all the mistakes I made and all the great advise I got to save myself from screwing up a good piece of steel. 

    It seems I am not the only person who goes into this blindly and has to have his hand held through much of it. 

    I felt I did a lot of research before I started but my mistake was stopping for almost 2 years and jumping back in like I knew what I was doing when I had forgotten everything I had learned. 

    I am not an egotistical guy and dont mind admitting my mistakes when I make them. I try very hard to learn from those and not repeat them.

    Thanks again for the help!  
    charswifterie likes this.

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