Automatic Split-Damper temperature control

Discussion in 'Charcoal Smokers' started by geek with fire, Dec 18, 2007.

  1. geek with fire

    geek with fire Master of the Pit OTBS Member

    Warning: Science Content.....

    I'm going to experiment with adding some forced air to my temperature control system. Several weeks ago, I wrestled with a large wind storm and even with the plywood shelter, I still had problems. I placed a box fan in front of the firebox, and the issue calmed a bit.

    This got me thinking about adding a dc fan to control system. However, I still like the idea of the rotating damper, and wanted to add the fan only for rapid increases. For example, if I don't monitor my fuel well, and have to add fresh lump to the box, it takes some time to heat up again. The fan should resolve this.

    To aid in the control (and this is where the "science content" warning applies....turn away if your brain gets sore easily[​IMG] ) I added the ability to "overlap" the percentage of use between a primary and secondary damper. In my case, I'm going to have the primary control operate the rotating damper, and the secondary control operate the fan. From the illustration below you can see that I am going to start with about 75% overlap. This means that the secondary damper overlaps the primary damper on the last 75%. So the primary damper can operate up to about 40-50% will little or no help from the fan. As the control gets higher, the fan RPMs start to increase to the point of.......well, possibly melting the fire box (let's hope not).

    I'm going to start extensive tests during the Christmas break, but issues I'm sure I will be dealing with are:
    1.) fan will I'm sure blow ash into the smoking chamber. That will need to be dealt with.
    2.) ????.....ideas from you guys....what do you think?

  2. fatback joe

    fatback joe Master of the Pit OTBS Member

    What size fan were you planning on using?
  3. geek with fire

    geek with fire Master of the Pit OTBS Member

    Well, I'm starting out with a fan made for cooling computers. I'm sure it's way overkill, but it works well with the motor controller that I can operate from my computer. It's about a 150 CFM fan, but I hopefully won't ever need it to operate at it's full power.
  4. fatback joe

    fatback joe Master of the Pit OTBS Member

    Hmmm......150......yeah, that might blow some ash running wide open, but like you said it should not have to. I was thinking that I have never heard complaints of the guys running the gurus and similiar gadgets, but most of them aren't going over 25 cfm that I have seen/heard. So it might be tough to draw much comparision between the two.

    I guess there is one sure way to find out. [​IMG] LOL

    I would think that the angle of the fan in relation to the opening from the box to the cook chamber could play a factor as well...........hmmmm. Can't wait to see your results.

    Good luck.
  5. ron50

    ron50 Master of the Pit OTBS Member SMF Premier Member


    I'd like to help ya, but my brain is already hurting. Good luck with your testing.
  6. richtee

    richtee Smoking Guru OTBS Member

    A> Suppose you add the fan to the exhaust stack. You'd need a metal blade, maybe even a 'squirrel cage" design, but it would greatly increase airflow, and not be blowing directly on the ash.


    B> contemplate a reverse flow design, perhaps with interior baffles thereby dumping any blown ash into the recirc cavity.
  7. geek with fire

    geek with fire Master of the Pit OTBS Member

    yea, I was thinkin' I wanted to try the reverse flow concept at some point anyhow, so I may do that. I'll have to move the stack, so it won't happen for a while. For now, I've got some thick stainless mesh that I'll stand up in the smoke chamber during my testing, just to see how much stuff I collect. Really though, testing should prove that I don't need to move that much air, and I can calibrate the fan not to run that high.
  8. smokebuzz

    smokebuzz Master of the Pit OTBS Member

    Why don't you just "drive" your fan motor down to a manageable/controllable speed?
  9. bbq bubba

    bbq bubba Master of the Pit OTBS Member

    May be a lil overkill Josh, i think the Guru run's a 10 cfm blower, unless you can get it slowed down?
  10. geek with fire

    geek with fire Master of the Pit OTBS Member

    Yea, at full power, it is definately overkill. However, as I understand how the Guru and Stoker work, at regular intervals full power "puffs" engage; basically just turning it on and off as needed to increase oxygen, fueling the fire. I'm, however, using an electronic motor controller to set a specific RPM based on what is needed to fuel the fire to the desired temperature.

    So, to your point (BBQ Bubba), normal operation wouldn't require the fan to operate at full power. But, I will have to plan around things like opening either the smoking chamber lid, or firebox lid. Both will cause a temperature drop the the control system will try to offset, maxing out the damper and fan until the desired temperature is met. Now there are a dozen ways I can deal with that, but there's no sense doing anything until I see what damage the full power fan does in a controlled test. Then scale it back until I get the results I want.

    This week, I'm finishing up an enclosure that will house the fan and servo-damper, that will be connected to the firebox by a 4.5" diameter metal tube. Once I get that done, I will post some pictures; possibly a video.
  11. richtee

    richtee Smoking Guru OTBS Member

    Well thought out, Sir!
  12. wavector

    wavector Smoking Fanatic

    I like the fan idea, and I use one sometimes on my vertical from a distance. Originally, I had the fan in the proximity for use on me.

    I still believe in the concept of having a stack on the firebox too. This would allow the billowing smoke to escape and give time for a fresh log to be brought up to temp. By closing the damper during this process, the ash would then have time to escape with the billowing smoke and give time for the TBS to be achieved before opening the damper to the smoking chamber. Monitoring temperatures inside the firebox and visually observing the TBS and corresponding temperatures would give a base with which to work. I also believe in a heat transfer through the metals used for both the smoking chamber and the firebox. I believe in a thicker steel used for the fire box which would seem, in theory, to conduct to the smoking chamber steel thus allowing for less fuel to maintian stable temperatures.
  13. peculiarmike

    peculiarmike Master of the Pit OTBS Member

    Ya know Geek, it's getting closer and closer to the day I make a run to Sedalia to check out your rig. When the weather warms a bit keep an eye out for an old gray haired fat guy on a lava red Harley. [​IMG]
  14. geek with fire

    geek with fire Master of the Pit OTBS Member

    I like that idea.....a lot! The only issue with that is the temperatures in the firebox typically exceed the usable range of the common thermistor probe. It would work for me, because I use thermocouples and have one that will go up to 1200 degrees. But they are very expensive and I'm trying to keep the "common man" in mind when I design this incase I go public at some point (I'm not more well off than the common man, but I'm willing to spend the extra dollar for the geek factor!).

    No problem, but make sure you've got yer leathers....I'm out in the boonies!
  15. billybones

    billybones Smoke Blower

    Yes but clearly you've ignored, like most people, the increased fluctuations in heat output due to the self contained vector modulations in the coal sub-molecular data stream.
    So then, assuming X is constant:

    .47f(x)/yx will be roughly = f(y)/x^3+(y^2*.47)= smoke some food, drink some beer, rinse, repeat!

    ...and I think we all know what that means![​IMG]
  16. fatback joe

    fatback joe Master of the Pit OTBS Member


    I think you got your 2 and your 3 reversed in the equation. LOL [​IMG]
  17. geek with fire

    geek with fire Master of the Pit OTBS Member

    dude, you had me at "vector modulations" had me at "vector modulations"

  18. billybones

    billybones Smoke Blower

    My are so right. I completely ignored Saddlebaum's phony forum math principle which states clearly: "When creating a phony math formula on an online forum, always use sequential numbers to further enhance the illusion that you know what the hell you are talking about!"
  19. billybones

    billybones Smoke Blower

    You complete my vector modulations....[​IMG]
  20. I definately would encourage the reverse flow design. I built mine a couple months ago and thoroughly enjoy it. I think there are pictures in roll call. It's a little crude but it works.

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