+ Reply to Thread
Page 2 of 2 FirstFirst 12
Results 11 to 20 of 20

Thread: Ramp=draft

  1. #11
    Join Date
    Jan 2012
    Location
    MN
    Posts
    400

    Default

    Velocity. Wouldn't volume be increased because of velocity?

  2. #12
    Join Date
    Jan 2020
    Location
    Stirling ontario
    Posts
    136

    Default

    the volume will be determined by the size of the chimney.If the cross sectional area is less than the
    chimney in any part of the arch, the velocity will increase there.This is why your draft should be small
    with narrow openings under the grate.You want high velocity air blowing into the fire.So if you have no
    ramp you will force the flame directly up into your pan for excellent heat transfer.If the area under your
    pan is small the rest of the way, the flame will race back at high velocity along the bottom with low transfer
    to your pan.I don't think one inch of airspace under your pan is a benefit.
    As mentioned earlier a ramp will keep the flame hitting a larger portion of your pan straight on
    before it heads back and exits. It you have high flue temperatures your inefficient and wasting wood.
    Last edited by ir3333; 01-16-2021 at 12:32 PM.

  3. #13
    Join Date
    Jan 2012
    Location
    MN
    Posts
    400

    Default

    I have a 22" stack on a 4x14. What exsactly do you mean when you say "If the cross sectional area is less than the
    chimney in any part of the arch, the velocity will increase there."?

  4. #14
    Join Date
    Jan 2020
    Location
    Stirling ontario
    Posts
    136

    Default

    Use the area of your stack in square inches.
    Think of a wide lazy river that comes to a narrow section.
    To move the same amount of water through the narrow section it will have to move much faster
    The venturi principal..if you decrease the size of a tube you will increase velocity..or vice versa.

  5. #15
    Join Date
    Jan 2020
    Location
    Stirling ontario
    Posts
    136

    Default

    a 22" diameter chimney would be about 380 square inches.If your arch is 48 inches wide and you had 6 inches under your pan on the way to the chimney
    that would be 48 x 6 or 288 inches.The fire would move about 33% quicker in the arch until it got to the chimney where the area is wider and then the
    velocity would slow down going up the pipe.An arch that size would require a bit of experimenting / calculating to find the sweet spot!

  6. #16
    Join Date
    Jan 2012
    Location
    MN
    Posts
    400

    Default

    I've thought that since all my flues are 2 inch tubing they restrict/slow down the draft. Since my stack is almost 100 sq inches larger then all the 2 inch pipe added up, 282 sq in, could that be the cause for me not getting the ideal draft or velocity being created to pull more heat to the back of my big pan?
    Last edited by tonka; 01-16-2021 at 11:10 PM.

  7. #17
    Join Date
    Jan 2020
    Location
    Stirling ontario
    Posts
    136

    Default

    well yes...restricting it will offer some resistance but the volume will still have to match the chimney.If you aren't
    getting the heat at the back of the pan your flame may be transferring it's heat to the sap as it should and be to
    cool by the time it reaches the back...what is your stack temperature?
    It may be as simple as increasing chimney length or making a hotter fire.I have been adjusting my small unit for 3
    years now and hope this years' changes will produce hotter and more complete boil.But it may not either?

  8. #18
    Join Date
    Jan 2012
    Location
    MN
    Posts
    400

    Default

    The hottest I've seen my stack is 600 to 650 degrees. My fire is about as hot as I'm going to get it since I have auf and aof, from my understanding stack height dosent matter when using forced air right?

  9. #19
    Join Date
    Jan 2020
    Location
    Stirling ontario
    Posts
    136

    Default

    with forced air everything changes.At 650 degrees you may be optimum for your arch.There are so many variables.

  10. #20
    Join Date
    Jan 2012
    Location
    MN
    Posts
    400

    Default

    You got that right about so many variables. I also realize it's hard to understand how my cooker functions without being there and what kind of tweaks I could make to improve it if theres room to improve. I'm still hopeful that I can get that stack temp up to 800 or so degrees I could do something to create more draft to pull the heat further into my tube pan I feel like I can achieve the increase in stack temp.

    Ive noticed on some of the stack bases theres an air intake to aid in draft, could that be an idea to do something like that to encourage more draft and improve the velocity? I found some fans that mount on the stack of wood stoves to improve draft but they're only rated for 8 to 10" pipe. https://www.homedepot.com/p/Auto-Dra...AD-1/302203167

+ Reply to Thread
Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts