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Using dampers for correct smoke retention/residence time

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Don't kill me for something that I know must have been discussed ad nauseum, but I cannot find the answer to my question.  I have gotten pretty good at regulating temperature in the Egg.  That is not my problem.  All posts I see about regulating deals with using the dampers evenly.  I have been under the impression that when smoking meat, you want the smoke to pass through the egg, not be trapped therefore causing an astringent taste.  My method is to regulate to my target temp using the bottom damper as much as possible, leaving the top open so the smoke has the opportunity to leave as soon as it can without getting trapped.  If I need more regulation, I then start using the daisywheel.  I always have airflow, so the bottom isn't shut, but pretty close.  I usually end up with a credit card slit on the bottom and the holes on the daisy wheel either all the way open or just barely shut.  I can't complain about my results, but I am not opposed to doing better when possible.

Best Answer

  • Mark_B_Good
    Mark_B_Good Posts: 1,550
    Answer ✓
    Options
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    Napoleon Prestige Pro 665, XL BGE, Lots of time for BBQ!
«1

Answers

  • lousubcap
    lousubcap Posts: 32,949
    Options
    Welcome to the forum.  From what you describe above regarding temperature control, if you have it dialed in then I wouldn't mess with it. As you note, you can regulate the intake or discharge or some combination.  (Think of a garden hose and the faucet or nozzle.) Regarding the smoke presence, once the VOC's have burned off and the smoke smells good it's just fine.
    You can always play around with other ways to control temperature.  Full open on the bottom and throttle the discharge or throttle the intake and full open on the DFMT to get the two extremes.  FWIW-

    Louisville; Rolling smoke in the neighbourhood. # 38 for the win.  Life is too short for light/lite beer!  Seems I'm livin in a transitional period.
  • toadshade
    toadshade Posts: 4
    Options
    Yes.  The question was not about temperature control.  It was about smoke retention.
  • Legume
    Legume Posts: 14,761
    Options
    When you limit the exhaust (top damper) you're also limiting the intake - so I'm not sure you can really setup a longer dwell time for the smoke.

    That being said, I use the bottom damper to control temp and leave the top off until I get into long cooks and want the temp to sit at 250 or so.
    Not a felon
  • WeberWho
    WeberWho Posts: 11,137
    Options
    The only time I use the daisy wheel is for cooks under 300 degrees. Otherwise I run the BGE without anything on top and use the lower vent for temperature control. 
    "The pig is an amazing animal. You feed a pig an apple and it makes bacon. Let's see Michael Phelps do that" - Jim Gaffigan

    Minnesota
  • toadshade
    toadshade Posts: 4
    Options
    Then is it correct to say that getting rid of the smoke by leaving the top open is better than closing it to keep the smoke inside?  So using the top is only a good idea if you are trying to maintain a lower temperature if not possible with just the base damper (as I do)?  I just see a lot of posts that describe adjusting temperature control by using both dampers simultaneously instead of just bottom first 
  • WeberWho
    WeberWho Posts: 11,137
    Options
    toadshade said:
    Then is it correct to say that getting rid of the smoke by leaving the top open is better than closing it to keep the smoke inside?  So using the top is only a good idea if you are trying to maintain a lower temperature if not possible with just the base damper (as I do)?  I just see a lot of posts that describe adjusting temperature control by using both dampers simultaneously instead of just bottom first 

    The original BGE from the 70's never came with a top daisy wheel. The top daisy wheel, DFMT, or whatever they call it nowadays came years later. It was originally only controlled from the lower vent. They eventually added the daisy wheel for fine temperature tuning on top. 

    It all comes down to personal preference and how you want to use your BGE. There are multiple ways to control the temp and how picky you want to get with it. 
    "The pig is an amazing animal. You feed a pig an apple and it makes bacon. Let's see Michael Phelps do that" - Jim Gaffigan

    Minnesota
  • toadshade
    toadshade Posts: 4
    Options
    THANK YOU MARK B GOOD!  That is what I have been trying to find out.  That makes a lot of sense.  That also makes more sense now in relation to using both dampers together instead of just top or bottom.  Thanks again for the detailed response.
  • Legume
    Legume Posts: 14,761
    Options
    @Mark_B_Good are you saying once you close down the top vent on an established fire, the exhaust velocity increases, the smoke and airflow generates more internal convection AND that this stays this way vs just being temporary until the air in, the fire and the exhaust settle into a newer equilibrium?
    Not a felon
  • fishlessman
    fishlessman Posts: 33,006
    Options
    toadshade said:
    Don't kill me for something that I know must have been discussed ad nauseum, but I cannot find the answer to my question.  I have gotten pretty good at regulating temperature in the Egg.  That is not my problem.  All posts I see about regulating deals with using the dampers evenly.  I have been under the impression that when smoking meat, you want the smoke to pass through the egg, not be trapped therefore causing an astringent taste.  My method is to regulate to my target temp using the bottom damper as much as possible, leaving the top open so the smoke has the opportunity to leave as soon as it can without getting trapped.  If I need more regulation, I then start using the daisywheel.  I always have airflow, so the bottom isn't shut, but pretty close.  I usually end up with a credit card slit on the bottom and the holes on the daisy wheel either all the way open or just barely shut.  I can't complain about my results, but I am not opposed to doing better when possible.

    i do mostly what you are doing. longer time in the smoke is what i do, dont fall for the smoke penetration only happens under a certain temp though, meat takes on smoke the whole time its in the smoker. lengthen that time by putting the meat on when its cold from the fridge for low and slows. ill put extra smoking wood mixed into the lump down deeper for more smoke. ill add pellets towards the end of the cook for a more hickory finish flavor (turkey), ill even add a small amount of mesquite for the right crowd and add it after the stall (butts).
    fukahwee maine

    you can lead a fish to water but you can not make him drink it
  • dbCooper
    dbCooper Posts: 2,202
    edited July 8
    Options
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    This is not accurate.  If true it means air flow is constant regardless of vent opening(s).  How then does the temperature decrease when the vent(s) openings are reduced?
    *edit to add - I'd really like to see your data/videos showing the convection patterns inside the Egg, that is something I've always been curious about.
    LBGE, LBGE-PTR, 22" Weber, Coleman 413G
    Great Plains, USA
  • JohnInCarolina
    JohnInCarolina Posts: 31,373
    Options
    dbCooper said:
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    This is not accurate.  If true it means air flow is constant regardless of vent opening(s).  How then does the temperature decrease when the vent(s) openings are reduced?
    *edit to add - I'd really like to see your data/videos showing the convection patterns inside the Egg, that is something I've always been curious about.
    It took me a bit to understand what @Mark_B_Good was suggesting, but I think I have.

    Most of us appreciate that you can control the flow of air by either the top or the bottom vent.  Essentially, the smallest opening has a pretty big influence on what the flow of air is.

    Imagine for a moment that you are contolling the flow and temperature through the bottom vent.  You've got a tiny slit there, say set to what you'd want to use to hit 225F in the dome.  

    Now the question is, what changes as you adjust the top vent?   Start with the one extreme of when it's wide open.  The flow out the top is whatever it needs to be, governed by what is coming in through the bottom plus combustion gases. This is just the volume that has to come out.  

    Under these assumptions, as you start to close the vent at the top, the velocity of the air will increase (while the flow remains the same).  
    "I've made a note never to piss you two off." - Stike
  • fishlessman
    fishlessman Posts: 33,006
    Options
    There's more smoke flowing thru a metal smoker than a ceramic at low temps. That's the problem, metal verse ceramic and air flow verse temps. Don't treat the two the same......
    fukahwee maine

    you can lead a fish to water but you can not make him drink it
  • dbCooper
    dbCooper Posts: 2,202
    Options
    First I can't get a grasp on butt or shoulder for pork steak, now this! 
    If flow rate (oxygen introduction) remains constant (due to increased velocity), why do temps drop when vent size is reduced?  Isn't the whole idea stable temps in the Egg based on adjusting vents for a consistent amount of oxygen for a given temp?
    LBGE, LBGE-PTR, 22" Weber, Coleman 413G
    Great Plains, USA
  • JohnInCarolina
    JohnInCarolina Posts: 31,373
    Options
    dbCooper said:
    First I can't get a grasp on butt or shoulder for pork steak, now this! 
    If flow rate (oxygen introduction) remains constant (due to increased velocity), why do temps drop when vent size is reduced?  Isn't the whole idea stable temps in the Egg based on adjusting vents for a consistent amount of oxygen for a given temp?
    They do not always drop when vent size is reduced.  

    Only one of them is actually controlling the flow, and it tends to be the one with the smallest aperture.  This is why adjusting both the top and bottom vent at the same time isn’t necessary.  
    "I've made a note never to piss you two off." - Stike
  • fishlessman
    fishlessman Posts: 33,006
    Options
    dbCooper said:
    First I can't get a grasp on butt or shoulder for pork steak, now this! 
    If flow rate (oxygen introduction) remains constant (due to increased velocity), why do temps drop when vent size is reduced?  Isn't the whole idea stable temps in the Egg based on adjusting vents for a consistent amount of oxygen for a given temp?
    They do not always drop when vent size is reduced.  

    Only one of them is actually controlling the flow, and it tends to be the one with the smallest aperture.  This is why adjusting both the top and bottom vent at the same time isn’t necessary.  
    This
    fukahwee maine

    you can lead a fish to water but you can not make him drink it
  • Elijah
    Elijah Posts: 750
    Options
    1+3 =4, 2+2=4, 3+1=4. Numbers represent vent up top and bottom. I've never had my egg pressurize or come under vacuum  =) there are going to be temporary fluctuations and the chimney effect comes into play but is also temporary as far as temps / smoke goes. It'll all equal out
  • Canugghead
    Canugghead Posts: 11,745
    Options
    If I may digress... 0-400 wangs is all the rage in the Traeger world. Set temp to 400, place wings on cold grate and switch on, cook 30 min, flip, another 30 min.  I tried that today, but since my baby Traeger runs hotter than full sized ones, I went 0-380, 30 min, flip, 10 min. It was really good and smokier but not too much compared to usual hot and fast. 

    I dunno, may be it worked well because meat was cold and pellet pooper was smokiest at start up.
    canuckland
  • Legume
    Legume Posts: 14,761
    Options
    If I may digress... 0-400 wangs is all the rage in the Traeger world. Set temp to 400, place wings on cold grate and switch on, cook 30 min, flip, another 30 min.  I tried that today, but since my baby Traeger runs hotter than full sized ones, I went 0-380, 30 min, flip, 10 min. It was really good and smokier but not too much compared to usual hot and fast. 

    I dunno, may be it worked well because meat was cold and pellet pooper was smokiest at start up.
    It's because the convection flow in a square pellet smoker has right angles which hit the wings perpendicular to the feather follicles causing micro-coriolis effects which obviously makes the smoke stick more better.
    Not a felon
  • Botch
    Botch Posts: 15,691
    Options
    Legume said:
    If I may digress... 0-400 wangs is all the rage in the Traeger world. Set temp to 400, place wings on cold grate and switch on, cook 30 min, flip, another 30 min.  I tried that today, but since my baby Traeger runs hotter than full sized ones, I went 0-380, 30 min, flip, 10 min. It was really good and smokier but not too much compared to usual hot and fast. 

    I dunno, may be it worked well because meat was cold and pellet pooper was smokiest at start up.
    It's because the convection flow in a square pellet smoker has right angles which hit the wings perpendicular to the feather follicles causing micro-coriolis effects which obviously makes the smoke stick more better.
    Sorry, that's only below 3,800 ft altitude, and even less if the humidity hits 57%.  
    And if the wings are facing backwards (thin/rear edge towards the convection flow) then all bets are off.  
    ___________
     
    Most of us have an above-average number of eyes.  


  • Canugghead
    Canugghead Posts: 11,745
    Options
  • TheFishWhisperer
    Options
    There's more smoke flowing thru a metal smoker than a ceramic at low temps. That's the problem, metal verse ceramic and air flow verse temps. Don't treat the two the same......
    Don’t cross the streams!
  • Mark_B_Good
    Mark_B_Good Posts: 1,550
    edited July 9
    Options
    dbCooper said:
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    This is not accurate.  If true it means air flow is constant regardless of vent opening(s).  How then does the temperature decrease when the vent(s) openings are reduced?
    *edit to add - I'd really like to see your data/videos showing the convection patterns inside the Egg, that is something I've always been curious about.
    I was trying to keep it simple for everyone to understand.

    The reality is ... the pressure in the egg DOES go up marginally due to back pressure from the vent.  This increase in pressure does cause a restriction in flow intake to the vent, and hence less oxygen flow in, so burn rate goes down. This is the reality of your experience, WHEN you close the vent.

    BUT - you missed something VERY important in my message ... it's critical (and I did it on purpose to help everyone understand what is going on). I said at "A GIVEN TEMPERATURE"!  To get to the same temperature, whether the vent is open more or less ... you need to be burning coal at the same rate, which means SAME oxygen flow in, and same vent gas flow out!
    Napoleon Prestige Pro 665, XL BGE, Lots of time for BBQ!
  • Foghorn
    Foghorn Posts: 9,936
    Options
    dbCooper said:
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    This is not accurate.  If true it means air flow is constant regardless of vent opening(s).  How then does the temperature decrease when the vent(s) openings are reduced?
    *edit to add - I'd really like to see your data/videos showing the convection patterns inside the Egg, that is something I've always been curious about.
    It took me a bit to understand what @Mark_B_Good was suggesting, but I think I have.

    Most of us appreciate that you can control the flow of air by either the top or the bottom vent.  Essentially, the smallest opening has a pretty big influence on what the flow of air is.

    Imagine for a moment that you are contolling the flow and temperature through the bottom vent.  You've got a tiny slit there, say set to what you'd want to use to hit 225F in the dome.  

    Now the question is, what changes as you adjust the top vent?   Start with the one extreme of when it's wide open.  The flow out the top is whatever it needs to be, governed by what is coming in through the bottom plus combustion gases. This is just the volume that has to come out.  

    Under these assumptions, as you start to close the vent at the top, the velocity of the air will increase (while the flow remains the same).  
    It's just a guess, but I wouldn't expect the velocity of the airflow through the very short channel of the top vent to affect anything.

    However, my question is this:  On my XL or XXL if I take the controller off the top vent and put the bottom vent at 1/8" the egg will run about 300 degrees.  In order to get to 250 one has to put on the top lid and restrict airflow further.

    Does this mean that some oxygen is making it's way in from the top of the egg to the fire - or is the oxygen flow unidirectional (bottom to top) but restriction of flow only at the bottom does not create enough resistance to flow to allow one to get the temp down to the 250 range? Resistance to flow is proportional to the length of the restricting channel... 

    XXL BGE, Karebecue, Klose BYC, Chargiller Akorn Kamado, Weber Smokey Mountain, Grand Turbo gasser, Weber Smoky Joe, and the wheelbarrow that my grandfather used to cook steaks from his cattle

    San Antonio, TX

  • Botch
    Botch Posts: 15,691
    Options
    Foghorn said:
    However, my question is this:  On my XL or XXL if I take the controller off the top vent and put the bottom vent at 1/8" the egg will run about 300 degrees.  In order to get to 250 one has to put on the top lid and restrict airflow further.

    Does this mean that some oxygen is making it's way in from the top of the egg to the fire - or is the oxygen flow unidirectional (bottom to top) but restriction of flow only at the bottom does not create enough resistance to flow to allow one to get the temp down to the 250 range? Resistance to flow is proportional to the length of the restricting channel... 
    I have always suspected this; and add the top cap for any low-n-slow I do; otherwise I leave the top cap off.  
    I think it could be reasonably tested by someone with two same-size Eggs: clean both out, add an identical amount of lump to each, light and bring them both up to, say, 300º, and then shut both bottom vents completely, and cap off the top of one, leave the other one open.  I'll bet the topless one takes a lot longer for the fire to go out.  
    ___________
     
    Most of us have an above-average number of eyes.  


  • JohnInCarolina
    JohnInCarolina Posts: 31,373
    Options
    Foghorn said:
    dbCooper said:
    A couple of things, and some of these are stated above.
    - once the volatiles (VOCs) have burned off ... the smoke is what you actually want to penetrate the meat and add flavor.  It's really only during the initial heat up, just don't put the meat on too early. But once you've heated up and it's stable ... the smoke is what you want.

    In terms of dwell time ... you're not really changing much by opening or closing the top vent.  Let me explain.  Air in + combustion gases = air out ... the flow cannot be changed. It is directly linked to your burn temperature ... pretty much, at a given temperature, you will generate a certain vent gas flow. It does not change, unless you change the burn temperature (higher or lower temperature).  The volume of the egg is fixed (the dome volume) ... the time the vent gases spend in the egg is simple math:

    time (in seconds) = freeboard volume of egg (cubic ft) / vent gas flow (cubic ft/second) ... if both freeboard volume and vent gas flow are fixed (at a given burn temperature) ... then you can see, irrespective of what you do with the vent opening ... the dwell time is unchanged.  

    What happens is the velocity of the gas exiting the vent increases as you close the vent ... flow remains same, but that flow = vent gas velocity x the area of the vent opening ... so as you decrease vent opening, the gas velocity increases to compensate ... so it can release whatever vent gas flow is generated inside.

    THIS is what does happen ... there is more recirculation of gases inside the egg, as you close the vent (more of a convection oven ... and the concentration of smoke increases somewhat).  So the smoke actually hits the meat in circular motion many more times before it exits, as you close the top vent.
    This is not accurate.  If true it means air flow is constant regardless of vent opening(s).  How then does the temperature decrease when the vent(s) openings are reduced?
    *edit to add - I'd really like to see your data/videos showing the convection patterns inside the Egg, that is something I've always been curious about.
    It took me a bit to understand what @Mark_B_Good was suggesting, but I think I have.

    Most of us appreciate that you can control the flow of air by either the top or the bottom vent.  Essentially, the smallest opening has a pretty big influence on what the flow of air is.

    Imagine for a moment that you are contolling the flow and temperature through the bottom vent.  You've got a tiny slit there, say set to what you'd want to use to hit 225F in the dome.  

    Now the question is, what changes as you adjust the top vent?   Start with the one extreme of when it's wide open.  The flow out the top is whatever it needs to be, governed by what is coming in through the bottom plus combustion gases. This is just the volume that has to come out.  

    Under these assumptions, as you start to close the vent at the top, the velocity of the air will increase (while the flow remains the same).  
    It's just a guess, but I wouldn't expect the velocity of the airflow through the very short channel of the top vent to affect anything.

    However, my question is this:  On my XL or XXL if I take the controller off the top vent and put the bottom vent at 1/8" the egg will run about 300 degrees.  In order to get to 250 one has to put on the top lid and restrict airflow further.

    Does this mean that some oxygen is making it's way in from the top of the egg to the fire - or is the oxygen flow unidirectional (bottom to top) but restriction of flow only at the bottom does not create enough resistance to flow to allow one to get the temp down to the 250 range? Resistance to flow is proportional to the length of the restricting channel... 
    Agree on the velocity of the flow affecting much.  I think maybe the original argument was that for a low velocity you might get more swirling of the smoke, but I'm skeptical of this personally.

    The "one vent only" concept is imperfect as an explanation of what's going on in the Egg.  The reality is that the bottom vent and top vent do work together to impact how easy it is for air to flow through the BGE.  So I suspect what's going on in your example is that its about the bottom not quite creating enough of a restriction on its own, as you had guessed towards the end of your response above.
    "I've made a note never to piss you two off." - Stike
  • Legume
    Legume Posts: 14,761
    Options
    Can you address this the fracture mechanics of the egg now?
    Not a felon
  • JohnInCarolina
    JohnInCarolina Posts: 31,373
    Options
    Legume said:
    Can you address this the fracture mechanics of the egg now?
    You want me to explain why it cracks?  Or you want me to design one that doesn't ?  
    "I've made a note never to piss you two off." - Stike
  • Legume
    Legume Posts: 14,761
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    Just grade the design 
    Not a felon
  • JohnInCarolina
    JohnInCarolina Posts: 31,373
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    Legume said:
    Just grade the design 
    D minus
    "I've made a note never to piss you two off." - Stike