Reverse flow FLOP!!!

Discussion in 'Reverse Flow' started by uga fan, Aug 24, 2012.

  1. boykjo

    boykjo Sausage maker Staff Member Moderator Group Lead OTBS Member

    Thanks everyone for the input on the smoker and we will look at every ones Ideas.... The first thing we would like to do is try to increase the air flow from the fire box to the smoke stack and try not to hack this thing to death.. i agree the design is poor but we would like to make whats there work first..... We plan on removing the stack and replacing it with a 6 inch stack and get rid of the round  dampers and replace it with a larger square damper that will run across the lower part of the fire box....... Just need to make small steps before we start to walk......

    Keep the ideas coming........

  2. darbinco

    darbinco Fire Starter

    I have nothing to add except Joes offer to correct the situation with the smoker is worth some more praise. Joe, great job... Im sure it will come back around for you!

    I will be watching this thread.

    Please include before and after pictures of the mods you guys do.
    Last edited: Aug 27, 2012
  3. boykjo

    boykjo Sausage maker Staff Member Moderator Group Lead OTBS Member

    A buddy of mine has some 6" pipe so were good for material there.........
  4. uga fan

    uga fan Smoke Blower

    Ok sounds good!! I had a friend looking for some, but I will tell him nevermind. I will try and locate some 1/4" and have just in case we need it for the firebox!!
  5. rabbithutch

    rabbithutch Master of the Pit OTBS Member SMF Premier Member

    Hey Joe!

    Good on ya, man! That kind of generosity is not all that easy to find nowadays (anywhere but SMF, that is!). As a native of Raleigh, I can tell you that it is perfectly in the spirit of the old city!

    One small suggestion FWIW: Keep a digicam handy and take pics of all the changes you make and sell the ideas for improvement back to the guys who built it. The work should be valuable to them, especially if before and after results and pics accompany the changes.

    Oh, don't forget to post pics here, too.
  6. [​IMG][​IMG]   cant wait to see what your solution is to an obvious bad day. good luck guys
  7. First, the size of the wood, how it is split and its moisture content is important. I have seen a couple of these  in operation and it is a design flaw from the get go. Just like in heating and air conditioning, there is a rule for circulating flow in a Reverse Flow and it works best if you calculate it like this. 


    If you have cumbustion air vent (outside air damper into firebox)  = 24 total square inches of opening

    You need an opening from firebox into cooking chamber to be 25% larger = 30 square inches

    Now you need a chimney or an up-draft that equals or is bigger than those two numbers added together 24 + 30 = 54 square inches.

    An 8 inch pipe will give you 50.28 square inches leaving a difference of about 4 inches. Now you need to make your chimney stack at least 4 feet long from where it leaves the top of the pit. 5 feet is even better because heat rises and  it will draw on the firbox better.

    If you use a 9 inch pipe it will give you a little over 63 square inches and the stack only needs to be 1 foot high

    Here is a link to help calculate what you have and then you will know what you need to make it work everytime. Good luck
  8. boykjo

    boykjo Sausage maker Staff Member Moderator Group Lead OTBS Member

    Thanks for the calculation link BM... We will be using a 6 inch smoke stack and try to open/air damper the fire box as much as possible... something like this that will be adjustable

    Last edited: Sep 2, 2012
  9. You're welcome Joe, There are thousands of ideas on smoker designs out there and only a few really work well. You can also go with a twin 6 inch stack design. Just make sure to put a damper in both and put them at the firebox end of the pit. I know you will be smilin' when you make these adjustments and I look forward to seeing some before and after pics.

    Here is another link that will help with calculating cubic feet to gallons and more. They are very helpful for any smoker build.
  10. uga fan

    uga fan Smoke Blower

    Ok guys, just a quick update on things... Yesterday I fired the smoker up and cooked some chicken breast, ribs and 2 8 pound butts.. After getting a good fire going in the box, the temp in the body was only showing 200 degrees.. I put the meat on anyway, thinking I had all day if it took a little longer. Much to my surprise, the chicken reached 180 in about 2 1/2 hours and the ribs were perfectly cooked in about 3 1/2 hours.. The butts took around 10 hours give or take about 15 minutes.. All this tells me that the temp of 200 isn't accurate at all!! There is no way I could get those cook times at 200 degrees... Anyway, today is clean the smoker day and tomorrow it makes the trip north to see Joe!! Hopefully after we get the dampers/exhaust fixed and I can spend some time "mapping" the hotspots and learning how to cook on this thing.. Thanks for all u guys that suggested ideas and commented!! I can't wait to post how good this thing is cooking soon!!
  11. BBQ Man,

    I'm having a hard time with how the lenght of the pipe can make a difference. The diameter I understand. Its not that I doubt you, its that I just cant understand the concept.

    Can you explain it to me? Are we talking about velocity here?
  12. uga fan

    uga fan Smoke Blower

    RW, I am certainly no expert.. But the way I understand the explanations, look at it as a vacuum.. the longer the pipe the more smoke going through it. The more smoke/hot air going up=more cool air the "vacuum" pulls in through the damper. If what I explain isn't correct, I apologize and hope someone with more knowledge will clarify things for us both!!
  13. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    RW, evening...  The length and diameter of the stack perform 2 functions....  More diameter equals less friction loss on the pipe walls vs. the amount of air/smoke/heat traveling through it....  second, the taller the stack the more pressure from the bottom  will move more air....  Warm air is less dense than cold air and that aids is the draw......  I know that sounds stupid but, the air pressure changes for every foot in elevation....  Our forefathers knew that when they designed hay barns.... they put a "cupola" on top of the barn (if that is the correct term) and had floors that were slotted to allow air flow....  the slotted floors were built several feet above the ground for good air flow....    A chimney or a pipe will flow air, without assistance, if it is tall enough....  

    Building an exhaust stack that is large enough to move warm air easily will pull more hot air from the firebox into the smoke chamber..... 

    It ain't magic..... it's physics.....     Dave
  14. So, with that being said, how would a large diameter shorter pipe effect the airflow in the cooking chamber verses a long length of smaller diameter pipe? The airflow through the smaller pipe would have to have higher velocity in order to move the same amount of air correct?    How do you think that would effect the area of the cooking chamber nearest the pipe?  I would think the more air movement in that area would cook the food faster just like a convection oven does.   Im just trying to perfect the design, and probably being a little to anal about it huh?
  15. bruno994

    bruno994 Master of the Pit

    I like where this discussion is taking us.  Not to hijack UGA Fans thread, but these are the little facts I really enjoy digging into.  The larger diameter pipe is what I went with as well, 6" diameter to be exact, the calculator called for 24" I think, but I went with 32"+ (what I had) and man can I get some draft chugging away through the pit.  I like going to comps and seeing the cabinet style smokers with their little 4" stack on top kind of spitting and puffing smoke out and mine sitting there sending a steady cyclone of TBS skyward without any hiccups.  Looks cool, ya know?  Like I actually know what I'm doing.

    Keep up the science Dave, we're enjoying.
  16. That was one thing I noticed on my current build, switching over from those silly cabinet style smokers.....the cyclone the exhaust pipe makes!   It is cool.
  17. Ya know buddy... I feel bad for you. That welding shop tried to re-invent the wheel with this wacky new design of theirs. I would tell them you want your money back and get a Lang 60. 
  18. uga fan

    uga fan Smoke Blower

    Ok I am back home from Joe's. The modification went well! We(Joe) cut the 3" square exhaust out and we replaced it with a 6" round pipe!! I feel like Tim the Tool Man now with this mammoth sized pipe!! No mods were done on the firebox at this time, our intent was to see how much of a difference the exhaust makes before we go any farther. We are hoping that the new exhaust will draw so much on the firebox additional dampers won't be needed. I was able to find a site that sells the prefab dampers matching the existing ones. So I'm gonna order a couple to have on hand just in case!! A huge "THANK YOU" goes out to Joe, who was willing to help a complete stranger.. You have no idea how much I appreciate your help. For all of u wanting pictures, Joe did take pictures as we went along, I will work on uploading some "before" pictures!!
  19. boykjo

    boykjo Sausage maker Staff Member Moderator Group Lead OTBS Member

    Well Bart rolled in with his smoker around 11:30 today.....When he asked me what he should bring last week i told him some cooler weather and he brought was a beautiful day..... after jabbering for a while about the smoker we commenced removing the stack and cutting out the hole for the new one.... I didnt  measure the size of the square stack but it was small.. you see that flat part on the far right  of the cut out... thats part was the smoke stack.. in the picture it laying there looks so much bigger

    We checked Barts reach and that decided the length of the smoke stack...LOL.. had to make several cuts to get through the 6 inch pipe .....

    Here it is welded in place....

    We fabricated the vent cap

    I'm a little rusty but paint is a wonderful thing

    After we got done with the stack were a little unprepared to make the damper so we decided to hit a couple of places to find some round dampers to install but we were unsuccsesful in finding them.....When we got back we talked more about the dampers and air flow and decided to not go any further and see if it will work the way it is... I lit up a cigarett and placed it outside the dampers and was getting pretty good airflow...... I them lit the amnps and placed it in the firebox and the smoke found its way out to the smoke stack in no time with some good flow without coming out of the top damper on the firebox.......


    It was around 5ish and we talked for a while and decided to call it quits... Bart is going to fire it up when he gets back and see if it needs more dampers installed but I am thinking its going to be ok.... we talked about re doing the firebox grate that the wood sits on to make it a pull out so he can load a chimney and be able to clean it out easier.....

    The bad thing is we didnt get to have an afternoon snack so I sent him off with some jalapeno cheddar kielbasa and some hot dogs.......It was good to meet ya bart and I hopefully we'll see each other at a gathering...... [​IMG]............or if you need for more mods on the smoker..............

    Thanks for looking

    Last edited: Sep 12, 2012
    rowdyrawhide likes this.
  20. uga fan

    uga fan Smoke Blower

    Ok everyone, I am pleased to announce the problem has been solved!! After arriving home and eating a bite, I couldn't wait any longer. I started about half a chimney(not nearly enough I know, but I didnt wanna waste a bunch on a trial run) and threw a couple of small logs on. It took a while to see the temps start climbing, but I guess the is a lot of volume to heat up. I finally got the temp up to a steady 230 which is perfect for me. I played a little with closing the dampers some, closing the exhaust some to see how it affected the temp. Heck even raising and lowering the tongue foe the trailer would swing the temp 5-8 degrees.. I played with everything for about 3 1/2 hours and decided to call it a night. In terms of efficiency, I will say, aside from the initial logs I put on, I never had to add anymore. Maintaining temps from 230-270(after playing) 4 hrs of minimal maintenance makes this boy smile!! The two dampers allowed enough air to keep the fire burning with no problem. Adding more dampers would definitely allow me to manipulate where the fresh air enters the firebox and would maybe allow me to run the dampers choked back more. But the 6" pipe was really pulling the air in. You could see the flames dancing almost as if a small fan was blowing them. I'm excited and appreciative beyond words, and can't wait to throw some meat on the grates!! Im sure I will visit the local butcher in the morning!! Thanks to everyone for their concern and input and a special thanks to Joe, I owe u big time buddy!!

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