I need some input on my reverse flow temps (UPDATE)

Discussion in 'Reverse Flow' started by sqwib, Aug 30, 2010.

  1. sqwib

    sqwib OTBS Member

    I need some input on my reverse flow temps

    This is my 3rd   cook on the RF and I am getting different temps along the smoke chamber, averaging 70 degrees, but on occasion has varied as much as 90 degrees.

    So I usual get 200 – 220 on the left and 270 – 290 on the right.

    My firebox is cut into the smoke chamber so approximately 6” of the top of the firebox is in the smoke chamber.

    [​IMG]

    [​IMG]

    The area outlined in red is the top of the firebox sitting in the smoke chamber.

    I have noticed many builds like this one but have not seen anyone complaining about temp variation this great.

    Here is what I Have tried so far to fix the problem:

    I Changed the position of fire from left (closest to smoke chamber), to right (furthest away from smoke chamber.

    I have tried various settings with the air intakes.

    I have tried various split sizes from 18" long 2" x 3"  and  8- 10" long 2" x 3"

    I believe the problem is convection, the top plate of the firebox is heating up causing some serious convection.

    I have 2 ideas I would like to run past you guys, hopefully the first will work
    1. place firebrick along the top hoping to reduce the convection
    1. Add a sloped baffle sloping upward to the chamber inlet from the right side of the firebox
    I was hoping to be able to use Rutland Hi Temp Stove and Gasket cement.

    [​IMG]

    Will this cement work with brick

     A smooth black silicate-ceramic type cement formulated specifically to adhere steel to steel and cast iron. Holds gasket in place without burning off. Withstands temperatures to up to 2000°F (1095°C). Bonds fiberglass replacement gaskets to cast iron and steel. Final cure with heat of stove. Offers superior adhesive performance. For best performance bonding metal to metal. This Rutland Hi-Temp Stove & Gasket Cement is one of many top quality items in our Stove Gaskets & Cements department.

    If the adhesive does not hold I may weld a plate underneath, sandwiching the fire brick between the 2 layers.

    [​IMG]
     
  2. I first learned to smoke on my brother-in-laws reverse flow (and still use it often), and its almost exactly the same size as yours.  One big difference we have, is a water trough.  There is a water trough (about 1.5 to 2 inches deep)welded in about 4 inches below the grates (the bottom of the trough is welded right were the top of the opening is for the firebox in the mainchamber. 

    The trough is welded flush to the edge of the end with the firebox, and the only opening is on the other end (left side of chamber, if you're facing it) and its about a 3 inch gap.  So, essentially, the smoke/heat must run from the box completely under the barrier, then up at the other end of the chamber, over the food and out the stack at the other end. 

    It keeps the temps from spiking on the side of the chamber where the firebox comes into play... we will have to fill it with water (or apple juice, whatever) 3 or 4 times throughout an all day smoke.  On our pit you can tell its a lifesaver because the water on the cooler end will be sitting there, while the water on the firebox end is boiling!  regulates it great though!
     
  3. Tuning plates should solve your problems with temperature variations.
     
  4. pineywoods

    pineywoods Staff Member Administrator Group Lead SMF Premier Member

    In my Lang I can adjust the tongue up and down to change temps from end to end not sure exactly how you'd do that with yours but maybe try a couple 2x4's under the tires and see if that changes anything
     
  5. sqwib

    sqwib OTBS Member

    will raising the tongue, cause the firebox side to run cooler, because I got about a 2" pitch(the firebox side being higher"
     
  6. sqwib

    sqwib OTBS Member

    Tuning plates in a offset maybe but not sure how I would go about it on a Reverse Flow, but was toying around with a 2 part baffle .

    [​IMG]

    The blue is the reverse flow plate, the green is the top of the firebox inside the smoke chamber and the red is a baffle to hopefully reduce the green and first 18" of the reverse flow plate from getting hammered by the heat
     
  7. sqwib

    sqwib OTBS Member

    Well yesterday after work I installed the double baffle as in the diagram.

    Here is a link of my Temp Test

    The thermo used in the center is a cheapo and is not accurate but gives an idea of temp variations, the baffle side and firebox side are Taylor probes and are accurate.

    This test was done with no food, unless you count the 2 little chicken tenders and chicken pot pie, I was hungry and didn't eat dinner, went straight out to the smoker to do the mod.

    https://spreadsheets.google.com/ccc...WpoRkZQdVdhYVpJdm5Lc3c&hl=en&authkey=CPDLgOUG

    I posted the whole link above if the one below doesn't work.

    TEMP TEST BAFFLE MOD
     
  8. Looking at your diagrahm, I would say you definately need to get that firebox away from your RF plate.  The way you have it set up, you are esentially turning the right side of your RF plate into a 400 degree burner.  You need to get that fire away.

    I played with your sketch a little and don't know if you can do this at this stage.  I proposed two options.  Both involve separating the firebox from the RF plate.

    The red line is one option and or you can just put a new plate in the top of the firebox and run it into the tank a little ways. 

    [​IMG]

    I don't think using firebricks will do much.  They work the same as steel and will store and conduct the heat, not insulate or deflect it.
     
  9. jirodriguez

    jirodriguez OTBS Member SMF Premier Member

    Yeah I think the baffle is the way to go, but like MattS said, don't attach it to the RF plate at all. Attach it as far back in the firebox as you can to help prevent the thermal transfer from getting to the RF plate.
     
  10. sqwib

    sqwib OTBS Member

    Yeah I did run the baffles under the reverse flow plate 18" past the firebox, then I installed a  second baffle under that sloping slightly downward to the right side.

    [​IMG]

    Did a test burn last night did anyone read the Temp Chart?

    Ended up averaging 23 - 24 degree difference from end to end, the baffle definitely did the trick.

    Seemed like I got a better burn too.

    I probably can get it even better, but I like the fact that I can get 225 on 1 side and 250 on the other...did anyone say Ribs and Butts
     
  11. how far below the RF plate is that red plate in your sketch?  The further you can get the better.  Also another thing I just thought of, how big is your opening between the FB and tank.  If it is a lot bigger than it needs to be then that will let too much heat under there as well.

    We should be able to tune this thing down closer. 
     
    Last edited: Sep 1, 2010
  12. sqwib

    sqwib OTBS Member

    1/4 to 1/2 inch.

    I sort of like the idea of a 25 degree difference, but the option to run even temps and 25 degree difference is appealing.

    I guess I could use the piece that was cut out of the firebox as a damper that would swing up into the smoke chamber.

    I posted my pit calculations below.

    Before I do this mod did you look at my Temp Test.

    The opposite end of the firebox ran hotter the first hour then the firebox started to climb the next 2 hours settling down at 3 hours, The temps maintained a 23-24 degree difference, using smaller splits and not opening the smoke chamber lid for the rest of the test about 100 minutes.

    [​IMG][​IMG][​IMG]

    I needed 51.84 and have 61.418, but probably slightly less now that I baffled about 16"wide by 1/2" high of the firebox to smoke chamber inlet.

    Her a post from my build blog

    My concern is, "will it be enough of an opening to the smoke chamber from the firebox?"
    The pit calculator has no way to accept odd shapes, but the Firebox to smoke chamber inlet will be exactly 1/4 of the tank diameter
    The board in the picture is a tad lower than the cooking grates will actually be, the cooking grate will be 6" from bottom of the grate to top of thermal plate.
    Hopefully this formula is correct

    20" DIAMETER TANK 10" X 10" X 3.1358024 divided by 4 = 78.395, actually that does not apply to sectioning the tank in this matter, but thanks to a fellow builder and his CAD program he came up with my numbers actually being, 61.418, so I am good to go.
    Below is an explanation of this from JIRodriguez at SMF.

    Quote:
    Figured out where the difference was coming from the formula 3.14xRxR gives you the area of a wedge shaped opening exactly 1/4 of a 20" dia. pipe (like a pizza cut top to bottom & side to side).

    But the shape profile is different when you split the pipe with parallel lines every 5". The two areas closest to the center point have a larger area than the two areas on the outside. So you end up with the two middle sections measuring 95.661 in. sq., and the two outside sections measuring 61.418 in. sq.
     
    Last edited: Sep 3, 2010
  13. you might want to try welding a heat shield in like the circle in your pic....

    [​IMG]

    a side view on what it will look like with the blue circle as a vent to allow heat to escape out....

    [​IMG]

    you will just need a vent hole in the top of the chamber you closed off to allow heat to escape from it [​IMG]  ............bob

    ....
     
    Last edited: Sep 5, 2010
  14. sqwib

    sqwib OTBS Member

    That is actually a pretty good idea, maybe even put a warmer on that side.
     

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