New reverse flow build

Discussion in 'Reverse Flow' started by mmmmeaty, Oct 7, 2015.

  1. Hey everybody, it's been a while since I last logged on (a little over3 years actually...) But I haven't stopped checking in on the new builds occasionally. Anyways I am working towards finishing my first reverse flow on a trailer and would like some input on some of the finishing touches. Well here goes.

    This tank was one I got about 3 years ago and has been waiting for me to find a trailer like this to mate it to.


    I thought about what routes I could take with this tank and I decided on a reverse flow with my calculations being 5/16" thick fb and cc 64"x24" cooking chamber 28" x24" fire box with a 1/4" reverse flow plate.

    Reverse flow plate don't mind that rust :biggrin:

    I wanted It to be a little unique so I cut a hole in the trailer and sat the fire box down in it. It also helped with the overall height because I have to stand on the ground to cook.

    Getting the smoker parts onto the trailer was difficult and would have been impossible with the tools I have if I put it together on the ground and then tried to get it up there. But me and a buddy managed. I also got the legs on and it and attached it together

    Got the chimney on and it still needs lowered possibly, I went with a 6" and at this point it's around 40" I made it long to cut down, it does draw smoke and heat great the way it is though

    I guess that's the most of the build and here it is in "working " order.

    Now for the questions...

    1. With the fb down in the trailer I still need to come up with a plan for air intakes, thought about holes underneath and a lever of some kind to open And close and also indicate postion. Does that make sense or should I cut them in a little higher?

    2. The smoker still turned out taller than I wanted it and the temporary handle I have works but I plan on building a hinged table so I can't put a permanent handle their, I fought about welding log chain links that wrapped around the tank, one on each end of the door but that's only because I can't think of anything better. Any advice?

    3. If my chimney is drawing good is there any need to shorten it other than the way it looks?

    4. I don't have any air intakes as mentioned but when I open and close The fb door as air is needed the temperature fluctuates a lot like 50 degrees either way in as quick as a minute, is that normal? I only open the door maybe a half inch at a time.

    5. Last question, my heat is close at Times across the chamber, within 10 degrees from one end to the other. At other Times it isn't as much as 45 degrees hotter on the fb end, is this due to the amount of air I'm letting in? Are there any tips to keeping as close to the same most of the time?

    Thanks for going throught this post, I have cooked a little last weekend on it and everybody said it was good, just a loin but it was moist and had plenty of smoke in it.

  2. What size cook chamber is that ?

  3. The grill is 23 1/2" by 60" with an overall of 24x64"
  4. 18" to 20" stack length 

  5. I'll cut it down to 24" and see what that does, what are you thoughts on the air intakes on the fb? Or should I just use the door for ventilation
  6. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Below is an excerpt from Feldon's smoker building calculator....

    • A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
    • Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
  7. You want to make sure its tall enough to keep the smoke from blowing in your face or clouding around too much, especially at start up when its really chugging white smoke.
  8. It draws good at the height that it is I'm just not sure if it could draw better if it was shorter?

    I know the door is open so it's not a great example but you can see what I mean
    Last edited: Oct 9, 2015
  9. If it works good and your happy I'd leave it,  The 18 to 20 inches I suggested are recommended stack volume based on a 6" pipe   Dave's pit calculator is probably pretty close to that as well.

  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Gary, morning..... I think you misinterpreted my calculator.... It recommends a stack height of 36" +/-.... below is copied from my calculator...

    Exhaust Calculation..

    ESV in cubic inches_____________________________ ... = Stack Length in inches (36" +/-)
    0.7854 X Stack Diameter X Stack Diameter

    Adjust the diameter of the stack, until the proper length is achieved... be sure to measure the actual internal diameter of the pipe used..
    this is for round stacks only..
  11. smokejumper

    smokejumper Smoking Fanatic

    No, it will not draw better if it was shorter.

    Up to a point, the taller the stack the better it draws. That point where it stops drawing better is the point where it is so long that the smoke cools before it can exit. You are no where near that point.
  12. 2 days, 3 hours ago

    • 8 Posts. Joined 5/2012
    • Location: South Kansas City
    • Points: 10
    The grill is 23 1/2" by 60" with an overall of 24x64"     26,024 cu. in

    My recommended stack vol  520.4     Volume in cu. in. x 0.020

    Daves recommended stack vol. 572

    Volume in cubic inches X 0.022  0.017  = Exhaust Stack Volume in cubic inches, above the CC.... (ESV)
    (The increase in volume ~30% shows a dramatic improvement in equalized temperatures across the cooking surface... edit 6/19/15 )

    A    4.5 id would need to be 36”  if 36” is what you are looking for

    A  5” id would need to be 29”

    A  6” id would need to be 20.25”

    All of these are the same volume 
  13. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Gary.... Bold letters don't indicate an understanding of the calculations.... An increased volume of 22% is just what is says..... Increase the volume of the stack.... Shortening the height and increasing the diameter does not increase the volume....
  14. Bold letters or not, you like to argue, You are always right and everyone else is wrong.   Volume is volume      If you capped off the bottom of each stack, filled them with water, they will hold the same amount.

    No if you want everybody to use a 36" stack you should so,   I have probably built way more smokers than you, and I don't see that a 36" stack is an absolute rule I think sometimes it works and sometimes it doesn't.

    My 2 cents worth

  15. To further expand, I have a friend who has a PHD in Physics and is a smoking fanatic  I have asked him many question over they years and apply them to our builds.

    This is from about 3 - 4 years ago................

            ,  good morning

    I wrote you a few weeks back about insulating fireboxes, I have another set of questions. As I told you my son and I try to build a smoker or two every year and always want the next one to be better than the last.

    There has been much discussion on the size and length of the smoke stack on various BBQ sites. Using Feldon’s pit calculator you can size your chimney.  I have been kicking around stack size/length I am thinking that I had much rather have a shorter, larger dia. stack rather than a smaller, longer one. Here is my reasoning, Let’s just say you have a CC that is 20" dia and 36" long over 11,000 cu in, if I am going to try to get that amount of air/smoke out as quickly as possible a 4 or 5 inch pipe would evacuate quicker than let’s say a 3 or 3.1/2" pipe, Like trying to push a fat man through a narrow door vs. a wide door. I get that, if you have a 3" pipe that's 26.5" long and a 4" pipe that's 15" long they both have 188.4 cu in vol. But the 4" will move more vol. quicker. What do you think ? I am starting to think the smallest dia. on a smoker should be 4" and go up from there, and unless you are building a really small smoker then 3" would be alright. I asked this question to several different people but really never got an answer that made complete sense to me. Please shed some light on this dilemma.

    Thank you

    Gary Spriggs


        This is a bit tricky because the vent is used to control combustion as well as adjust the temperature. But the pit calculator is incorrect and your intuition closer to the mark.

        Hot air is less dense than cold air. So the combusted air in the smoker wants to rise. This motion is impeded by the friction of the smoker and mostly the chimney. Curiously, friction in a pipe depends on its length and diameter and the air velocity itself. Making calculations difficult. But you can make a few generalizations.

        As you double the length of the pipe the air velocity reduces by the square root of the length, or 0.7x. When you double the diameter, the velocity increases by the square root. or by a factor of 1.7x

        In a fireplace or smoker, you care less about the air velocity than the flow rate- e.g the volume of air per second exhausted. The flow rate “F” is the air velocity times the area of the pipe. So F is proportional to (D^5/L)^.5 , where D is the pipe diameter and L is the pipe length. Yikes. Not simply the volume.

        For example, if you start with a 3” diameter, 6 foot long chimney and compare to a 4” diameter, 3.4 foot long chimney (both have the same volume), the flow rate is actually 2.75x higher in the larger diameter chimney. Even though the volumes are identical.

        For a large diameter pipe (5” and above say), the chimney volume has no effect on the flow rate- the damper completely controls the flow rate. For a small diameter pipe, its diameter and air friction matters. Some people like to use a chimney with enough friction that it acts as a damper and self-limits the flow and combustion. Other prefer to fiddle with the damper (and firebox vent).

        A 4” pipe is a good compromise.
  16. Alright well I think I answered my own question here with the help of the stack is actually 6 5/8" id with a length of 45" and even though it is too long, it still works in my opinion better than any other smoker I've used, at some point I will shorten it to see if it draws better but for now I'm satisfied.

    Now, air intakes... If I don't build a fire grate my air intakes will be about 4" above the lowest point of the fire, but realistically will be centered in the fire after building it up, now the lower the better makes sense to me but a lever system for air intakes below trailer level, might be a little complex for my skills. Any thoughts?

    And thank you guys for all your advice so far.
  17. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member on pics to enlarge....


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