20gal build Finially DONE

Discussion in 'Reverse Flow' started by regularjoe, Jan 2, 2014.

  1. regularjoe

    regularjoe Smoke Blower

    I have got a ton of great info on this sight and have been enjoying smoking on my Webber kettle for a few years now. I do not post much but really try to research.  I have made some great food and have been proud of my last two briskets. The first two were leather.  I could not of figured them out without the posts on here. So let me say thanks for that. 

    I started to build a 120 gal smoker in July but it is bigger than what I need right now and more work than what I can commit to. I have a 20gal that would work perfect for my needs and I should be able to get it done much quicker. Once I figure this one out I will finish the bigger unit.

    This is the info from the calculator.

    Link to BBQ Pit Calculator

    I need help understanding the Firebox to cooker number. It is giving a 6.41 for half moon. When I looked at the tank which is 14in round a 6.41 in wide cut would only be a inch or so off the bottom. I was planning on cutting a 10in wide moon/slot into the tank 3in deep. This would take me through the rounded section of the tank to here it turns flat, stops curving in. It would make the Reverse Flow plate around 2.25 in off the bottom of the tank and 10in wide. So the moon would be 10in wide and 2.25 high.

    The tank is a estimate 14 inches round and 31inches wide. The straight section is 25 with about 3 inches on each side that rounds to the tip. Thanks for any help or info you can all give.

    Here is a picture of the tank. Edit: something keeps telling me the plate should be 1/3 of the height of the tank. So if the tank is 14 in high the plate should be 4.5 in off the bottom. Is this correct?

    Last edited: Jan 2, 2014
  2. dog1234

    dog1234 Smoke Blower

    If I am understanding you correctly you are saying the hole in the cook chamber is going to be larger than the calculator states. I see no problem in that as long as you make the fire box hole as stated in the calculator. 6.41 square inches. Any size will be fine as long as it 6.41 sq in total.

  3. regularjoe

    regularjoe Smoke Blower

    That is where I am confused. How do I determine the size of the hole in the firebox. Is there a calculator to tell me if a opening  shaped like a moon that is 10 inches wide and curves down 2.5 inches is 6.41 square inches. What I do not get is if I have a 14in circle/tank how far up from the bottom would I make my slice and what width would that be to give ma a 6.41 sq in opening. I was unsure if the 6.41 number just means I measure 6.41 in across but now I understand it does not and it actually means I need a 6.41sq inch opening in the firebox.
  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Tank = 4620 ci
    FB = 1525 ci +++++MINIMUM
    FB air inlet 6 Sq in .... additional inlet across from the FB/CC opening about 3 sq in....
    FB/CC opening = 20 sq in (24)
    Area under the RF plate = 20 sq in (24)
    Exh stack vol = 80 cu in. = 2.5" ID x 24" above the CC

    Some of the above numbers allow for a correction in friction loss....

    FB/CC opening

    7" radius semi circle 4" high will give you 24 sq in under the RF plate and FB CC opening... which is good...

    That also translates to a RF plate 11.5" wide and it should be welded to the top of the FB and stop short of the weld on the other end of the CC to allow for adequate air/smoke flow around the RF plate....
    Cut into the CC at the height of the RF plate when you put it inside to check fitment... I would slide the FB into the CC to just past the weld so the RF plate is in the straight section of the CC....
    An easy way to get the FB to fit to the CC is... set the RF plate into the CC.... project a line from the RF plate to the conical end of the tank... cut the end on that line.... slide the FB into the newly cut opening.... scribe the bottom of the CC onto the FB and cut out that part circle... while all this is happening, check the RF plate fitment to the top of the FB... If the FB is too high, no problem.. scribe the RF plate to the FB for height and scribe the CC to the FB for the cut out....
    Cut out the part circle on the FB and check all the fits.... install drains etc. and go for it...

    The above is what I might do to your tank etc..... after listening to all the folks here and tweaking numbers so the smoker works better...

    Here is the plan I start with then make adjustments depending on the size of the smoker.... adjustment are made based on friction loss estimates.....
    ... click on pic to enlarge ...
    Last edited: Jan 3, 2014
  5. regularjoe

    regularjoe Smoke Blower

    Last edited: Jan 2, 2014
  6. alaska

    alaska Meat Mopper

    Welcome to the forum...This is going to be good to watch.
    regularjoe likes this.
  7. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member


    I do believe you have it..... Dave
  8. After you do all of your math, look at all the other builds and see what you have draw out ( before you cut anything ) looks to be in proportion to the others. Regardless of size, they all should have the same " look ". A poor designed smoker will look " off ".
    regularjoe likes this.
  9. And you may not want to extend the firebox so far in on that small cooker, it may run a bit hot.
    regularjoe likes this.
  10. regularjoe

    regularjoe Smoke Blower

    Thanks for the help Dave. I will be sure to take plenty of pics and keep you all updated. 
  11. Not trying to say your wrong with the size of that tank but have you checked the length and diameter of it to double check the capacity?
    It looks to be an old air tank and I have found many to be inaccurate when measured compared to what the
    manufacturer says.
    I based my build
    off of the measurements as the tank stated 20gal but the math worked out to be 21gal.
    I just dont want you to spend all this time calculating things based on the wrong size tank.
    Your tank appears bigger than mine.
  12. regularjoe

    regularjoe Smoke Blower

    Thanks for the heads up. I was mainly going off the tag on the side. I will double check the tank to see what I have.
  13. regularjoe

    regularjoe Smoke Blower

    Dave I took some more time and studied your guide and have a better understanding of it. I see you figures always reference the CC to come up with the solution.

    My question for everyone is I have planned a Firebox of 14x12x12 which gives me 2016 cu in when I only need 1540. I made it a little bigger for three reasons.

    1. I wanted to have room to build a fire and pile some charcoal without struggling in a small space.

    2. I was going to build a rack around 3in off the bottom to stack wood and charcoal and it would give some room for the ashes to fall and I knew as it burned and filled with ash I would loose cu.

    3. I thought I read on here to shoot a little bigger. 

    Should I shrink the height up some or will it not hurt to be a little big? 

  14. This one is a 30lb propane tank, looks close to your size. The firebox is damn near as big as the cooking chamber.  The smaller you go, the more you need to oversize the firebox ( in my opinion)

    regularjoe likes this.
  15. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    As RW notes.... bigger is better for a FB...... The size of the FB does not affect how the smoker cooks... It doesn't affect the exhaust... or the amount of heat you cook with.... A larger FB just makes life easier like ash storage, adding wood, uniform temps, more thermal mass, etc....
  16. I would tend to agree with RW & Dave in that being big wouldn't hurt anything, aside from the additional weight.
    In this 21gal the firebox was 14x14x? I forgot the height.
    the only problem I found while testing it is the fire is constantly beating the top of the firebox and being sucked into the cook chamber, if not careful when adding more wood,knowing what I know now I'd probably add 2-4" to the height to minimize hotspots on the reverse flow plate.
    You'll notice in my build of it that I lost the bottom 3-4" by adding the ash clean out drawer. Just my 2 cents while your in the planning stages anyway.
    Last edited: Jan 3, 2014
  17. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    JW, morning.... Have you installed an additional air inlet across from the FB/CC opening...
  18. Right on jwsimpleville,   so many builders dont realize how much height the fire really needs, and that the flames will actually be higher  ( tall and thin )with the door shut and vents adjusted correctly .
  19. Dave,
    I didn't install the additional vent on that one due to the buyer wanted it ASAP. However given the opportunity to do it over I would have.
    The build I'm helping with now,


    is going to have 2 pie vents on the firebox door and one on each side to help with "pushing" the heat and smoke into the cook chamber.
    Last edited: Jan 3, 2014
  20. regularjoe

    regularjoe Smoke Blower

    JW that smoker is real close to what I had in mind.

    Well you all put to rest any concerns I had on the Firebox being too big. I will be taking the tank to the fire hall tomorrow to make sure it is a 20 gal. tank. It is too cold here, 9 degrees, to turn on the outside water valve. If everything checks out I hope to be starting it this weekend.  Thanks.

Share This Page