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The science behind smokestack extensions

Discussion in 'Side Fire Box' started by cobble, Oct 26, 2010.

  1. cobble

    cobble Smoke Blower

    So I've been doing a lot of mod research here, and while I'm not trying to refute proven methods, I'm still trying to understand the logic behind extending the smokestack into the cooking chamber. As I found in my first failed smoking experiment, excessive smoke is the enemy. Just to make sure I'm correct in my assumptions I'm just going to list out my observations. Please feel free to correct/educate me if you feel differently.

    1. Creosote is about the nastiest thing you can eat, and should be avoided at all costs.

    2. Creosote is caused by excessive smoke in the cooking chamber

    3. Extending the smokestack into the cooking chamber interrupts the flow of air, trapping more air/smoke in the cooking chamber.

    It just seems to me that by extending the smokestack, you're trapping all that smoke in the chamber for longer periods of time, and promoting creosote buildup. Obviously, I must be missing some benefit, or reasoning behind this, but I cant see what it is. Thoughts?
  2. forluvofsmoke

    forluvofsmoke Smoking Guru OTBS Member

    To answer your question pertaining to stack extensions (on horizontal sfb smokers), it provides a means for better control of the smoke chamber gas flows. It allows for better flow of gases closer to the cooking grate which can in turn allow more evenly heated smoke chamber space. Without a stack extension, the gases will more readily travel directly to the upper most point in the smoke chamber and this condition can result in widely varied temperatures between the sfb end and vent end of the smoke chamber.

    Creosote can be caused by a few things including use of unseasoned (green) or coniferous smoke wood, as well as stale or excessive smoke in the chamber of your smoker.

    While the methods for avoiding the first two are obvious, the last two may not be. A few causes which come to mind are using the vent damper to control chamber temps, which should be accomplished with the intake draft control, instead. Another cause would be using too much smoke wood or the smoke wood being exposed to too much heat/air in the fire box.

    What we refer to as thin blue smoke is the sweet spot we try to achieve. You will notice that when wood first begins to smoke, it will give off a white smoke. That is caused by water vapor and some volitiles contained in the wood which are released when it begins heating. This is normal and expected when you add smoke wood, etc. To get the thin blue smoke we want to have, you must balance the amount of heat and air which the smoke wood is exposed to. More heat/air will result in a faster, heavier and shorter lived smoke.

    Ultimately, if you can smell smoke exiting your chamber vent, or if stings your eyes if you put your face in the path of the vented gases, your smoking. Less is more, IMHO. You can smoke your foods for the entire time they are in the smoker, but this is really not necessary. The longer it is exposed to smoke, the heavier the flavor, but going overboard is very tempting.

    EDIT: You will find that the smoke flavor of your foods may not seem very strong to you, as you have been exposed to it numerous times during a long smoke of brisket, pork butt or a turkey, as examples. The causes for this are that your sensory perception of the smoke will be deadened somewhat from the constant exposure to the aromas of smoke. It's in your hair, the fabrics of your clothing and on all your exposed skin. So, you carry it with you everywhere throughout the day and into the night (or vice versa) on a long smoke, until you remove your clothing and get a fresh shower. The day of the smoke, your food will not taste like it has much smoke flavor to you, but to those not exposed to the smoke as you have been will feel differently on the subject. The day after, it will taste just fine, as your normal sensory perceptions will have somewhat recovered by then. Taste and smell will have returned to normal.

    One last note: whether you use a smoke box/pan or some type of container for the smoke wood, there should be a black/charred piece left over from your smoke wood after several hours. If ashes are present, the wood began burning at some point during the smoke...you want to avoid this condition.

    Hope this explains things better for you.

    Enjoy you addiction to smoked foods!

    Last edited: Oct 26, 2010
    jjwdiver likes this.
  3. cobble

    cobble Smoke Blower

    I was with you right up to this part:
     Using the minion method, with a deep basket, I cant see how a fair amount of ash can be avoided.
  4. forluvofsmoke

    forluvofsmoke Smoking Guru OTBS Member

    Yea, I guess I would have gotten confused there myself...I was referring mainly to a vertical smoker with that, I guess.

    If placing the larger chunk smoke wood into the briquettes, you get what you get...no control over that, and it's still OK. I think having the smoke wood exposed to such close proximity to the heat source, and being much larger pieces helps in keeping the smoke lighter. You just use much larger pieces than you would with a vertical smoker with a smoke wood pan/box, or if using a semi-closed container for the smoke wood in either type of fire box. It's a trade-off...you can use much smaller quantities of wood if you can control the heat/air getting to it.

    I have actually used an elevated container to place smoke wood into in my sfb to control smoke using a full hot bed of coals, but I never had much success using the minion method (incidentally, I no longer use my SNP, as it is a major fuel hog...approx 20-30lbs in 12-15 hrs).

    I think the smoke from the charcoal slowly starting up can get too heavy, anyway. I have noticed a very strong flavor from adding cold briquettes to the fire, but can't compare this to the minion method, as I had to abort and go to a full fledged hot bed of coals to get my rig up to temp. I'm not saying the minion method of fire management serves no purpose or has no merit, as many do use the minion method with great results, and I aplaud them for mastering this skill. It just doesn't seem to suit my needs very well in my SNP, as I had much difficulty reaching target chamber temps. This was likely due to the full tuning plate mod I added about 18 months ago...too much heat was carried accross the bottom and out the vent in order to reach even grate temps throughout the smoke chamber...huge trade-off involved just to be able to use the entire width/depth of the cooking grates.

  5. cobble

    cobble Smoke Blower

    I have the same concern with the minion method. The whole purpose is to have lots of unlit coal and wood in for basket, but as you mentioned, the unlit charcoal produces a lot of nasty white smoke when it burns, so I guess my question here is:

     How do you use the minion method, but at the same time, keeping that nasty creosote taste out of your meat?
  6. diesel

    diesel Smoking Fanatic

    This is a good thread.  I have a RF smoker and have been modding my air intake to get the right setting.  I have used it probably 20 times and have finally got it.

    When I add wood to the fire the smoke get heavy at first but doesn't take long to achieve the TBS. 

    I used a side firebox for several years and I remember that the food would get "too smokey" very easily.  I had to use charcoal and just add small amounts of wood throughout the cooking time.  With the RF I use all wood and even chicken and turkey are not getting too smokey.  I hope this is a result of the correct air flow.  And I didn't put the stack lower in the chamber.  It is pretty flush. 

    Hope this ramble added some value to this conversation. 
  7. cobble

    cobble Smoke Blower

    I saw a video on youtube where a guy had a SFB smoker and he attached a computer case fan to the firebox damper and a 12 volt battery, and used that to help regulate temperature. I thought that was a good idea. You could even use it on the smokestack damper to pull the air through, I guess. This is all just guesswork on my part though. I've yet to produce anything edible from my smoker.
  8. eman

    eman Smoking Guru Staff Member Moderator OTBS Member

    When i had my SFB i did use the minion method and i had very little problem w/ heavy smoke after the initial light up.

     i believe this was due to the charcoal being heated by the allready burning coals till it ignited.

    I also added small splits or chunks of wood  over the whole smoking time.

    Diesel, You should not have to extend the stack on your RF as the RF plate does the same thing as the exstention .It alters the flow of heat / smoke from the fire box .Just much more efficiantly than the stack mod.
  9. sqwib

    sqwib Smoking Guru OTBS Member

    You are correct on the creosote, avoid it at all cost.

    I believe that this is specific to the smoker.

    What I mean by that is if you have a smoker like my RF  its 20" in diameter and the smoke stack is about 2' below the top and the grates are at center 10", so when I lay a pork but on the grates its about 4" - 6" from the top and about 2 - 4"' below the smoke stack.

    So in my case extending to the grate level is not necessary, however in the case of someone using a tall oval shape or large circumference where there is a huge dead space, it may benefit lowering the stack by evening out the temps a bit and reducing the possibility of a cold spot. As far as stale smoke goes if you have decent draft pulling the smoke through, you shouldn't have a problem. IMHO

    You can see what I mean in my picture below.

  10. forluvofsmoke

    forluvofsmoke Smoking Guru OTBS Member

    You know, I had thought about doing that very thing about a year ago. I have since then purchased a brinkmann gourmet charcoal smoker, and my experiences with it has led me to believe that using a forced air intake system would create an environment which would suspend any ashes which were disturbed and carry them far into the smoke chamber. The realization of that condition quickly dispensed any further thoughts of trying this mod.

    With my gourmet, if I need to add charcoal, I remove the entire barrel from the fire pan and set it aside before attempting to do anything which might stir up ashes. If I don't follow this precaution, my meats end up wearing a nice gritty/grey coating of ash...yea, it's just that touchy feely...I don't even bump my vertical charcoal smoker when it's hot, and keep a wide stance from it when moving around in my outdoor kitchen. If I remove or slightly raise the lid to insert a meat thermo probe, open the side door to add smoke wood, or do anything else which may cause vibrations or shocks to the smoker, I do so with extreme care.  

    Using a fan on the exhaust vent would help to reduce disturbances in the fire box, however, the fan and motor would now be exposed to much higher heat...something in the range of 250-350* depending on chamber temps and the amount of draft. I don't think very many small electric motors will live very long in that environment. Then, there's the issue of the smoke getting into the non-enclosed motor, leaving the eventual build-up behind. This will likely seize the motor in time, as the rotor would be bound-up, unable to rotate. If it were a brushed motor instead of brushless, the brushes and armature would be very dirty in a rather short period of time, causing unreliable operation of the fan motor.

    I know that a moderately sized horizontal like my SNP 40", especially with a full tuning plate, would not be as suseptable to suspended ashes as a vertical charcoal fired rig, but the possibility of the ashes being carried accross the meat still exist, and I think the risk could increase with a reverse flow. Well, the velocity of the smoke chamber gases on a wide and tall horizontal path would likely be reduced to the point which might allow any suspended ash to fall out. I guess this would bring up a question in my mind: if ashes are falling out inside the smoke chamber, how long would it take for the ashes to build up to a point which required the smoker to be vacuumed out under the baffle/tuning plate? Depending on the design and construction of the smoker, ash clean-out from the this area could be a difficult undertaking.

  11. diesel

    diesel Smoking Fanatic


    Thinks for the response.  I have a question for you about your toggle clamps.  I just picked up a couple.  What did you do to replace the rubber end on the clamp?  I have been contemplating what to put in place of it.

    OH and that is one fine display you have there on that smoker!
  12. sqwib

    sqwib Smoking Guru OTBS Member

    I used felt padding then wrapped in duct tape.

    and Thankyou