Could The Airflow Reverse?

RhumAndJerk · January 2000

I am still contemplating how the base got so hot and the dome above the kiln shelves was relatively cool. The radiant heat is probably the best explanation so far, but could that account for 1000 plus degree difference between the dome and the base? [p]Is it possible that once the fire got to a certain temperature that the airflow reversed, instead of the normal bottom vent to dome vent, the other way around? It just seems to me that if there was enough airflow to get the fire that hot, then the heat should have also raised into the dome?[p]I do have good news to report; KennyG’s Magic Coal Grate passed the ultra high temperature with out a scratch. [p]Thank you for your input,
RhumAndJerk[p]

KennyG · January 2000

RhumAndJerk,[p]I'm not sure but seriously doubt that you might have achieved reverse airflow. When I first started experimenting with firebricks, the first batch I picked up were the real thing - heavy and 1 1/2 inches thick. I covered practically the entire grilling surface with them and experienced a similiar situation, extreme heat in the bottom at wide open intake and dome temps of only 400* even with no chimney cap on. I suspect that once the ceramic mass approaches some percentage of the grill surface (maybe 60%??) the restriction in airflow to the chimney begins to degrade the Egg's efficiency and cooking ability exponentially. Just a guess. [p]If you really want to know how hot the bottom of your Egg got, I have a thermometer (reference standard thermocouple) that can handle up to 1800*[p]K~G

RhumAndJerk · January 2000

KennyG,
Those are some very good points. Something is definitely happening around the 60%, but what?[p]On the question of efficiency, I am not sure degrade is the term that I would use. If you think about it, we are dealing with a fuel-air mix. For the fire to generate the very high temperatures, more oxygen is needed to support combustion. If the efficiency of the egg were reduced, the temperatures produced by the fire would at some point peak and hold there as long as the fuel-air mix remained constant.[p]The efficiency question may lie with the heat transfer of ceramic mass and its qualities to do so. One such quality could be the thickness of the ceramic mass. Maybe that in combination with the radiant heat is the answer. But then where does all of that heat go? Hot air rises quickly.[p]Do not misunderstand, I am not arguing, I am just trying to resolve it in my mind.[p]Maybe I should just go nuke the final Jamaican Meat Patties and enjoy. [p]Boy, I wish I had a Red Stripe.[p]RhumAndJerk[p]

Tim M · January 2000

RhumAndJerk,
I had to go back and find your earlier post to see what you were talking about with regards to a reverse airflow in the Egg. That just won't happen unless a headwind blows down the chimney (know as "back-puffing") but it will only do so if it can over come the physics of hot air rising and so it only lasts for brief moments on the Egg. So that is not the problem. I have to figure out what you ment by "the base" I assumed you meant the base or bottom of the Egg - and yes it does get hot sometimes. I now assume you mean the lower half of the Egg along the walls got hot. Now I can eggsplain it to you if that is the part you refer to. [p]I did some measurents at low temps in my Egg some many months ago. I had 5-6 probes stuck all over the Egg (2 inside, chimney, door seal and dome. My conclusion is that the insides of the Egg are where the majority of heat travels in the Egg. In other words, the heat doesn't go up in the middle as much as it swirls to the sides of the Egg and then up them to the dome and out the top. Close up the top a little and it restricts the top and heat builds up in the dome. At a point it will start to restrict airflow in the vent and lower temps will occure in time. If you add a baffle (pizza stone, bricks, setter, kiln shelves) you now force even more hot gases towards the sides. This will be even more noticable in a med Egg since the ratio of diameter to the baffle size is less thus forcing the gasses into a smaller area and forcing the kiln shelf to act as a restriction to hold the heat at that level and below - thus heating the sides more and more. That ceramic will just get hotter and hotter. I hope this helps you understand it better.[p]Tim

RhumAndJerk · January 2000

Tim M,
Your assumptions were correct. I also think that you are right on with your observations with the swirling heat and baffling. Controlling the intensity of the fire may actually create a hotter cooking environment.[p]I just assumed that I needed to run my small egg at full tilt to overcome any airflow issues. I wish that I had purchased a slid top for my small when I had a chance.[p]Between you, Spin and KennyG, I am going to get this right in spite of myself. But once I do …[p]Thanks,
RhumAndJerk

Tim M · January 2000

RhumAndJerk,
I forgot you had a small - I thought you had a medium. I wish I had a small so I could better help you out, but I am sure that's what is causing the sides to get hotter than normal. Question is does it hurt anything. My gut feeling is no.[p]Tim

Randy · January 2000

RhumAndJerk, Ive basically quit using fire bricks. I found that they dramaticaly increased the lump consumption and didnt really improve anything. I also noticed it was much harder to maintain higher temp. I had the same experience as Kenny G was speaking of, not being able to get to over 400. I agree with his assesment of the heat radiating back down. I only use a drip pan now.[p]Randy

KennyG · January 2000

Randy,[p]IMHO, Char-Woody had this one figured out a long time ago. Firebricks/ceramic mass, etc. can be beneficial as long as you cover no more of the grill surface than the "footprint" of the meat you are cooking. Anything larger than a standard pizza stone is probably too big. a 14" diameter stone represents 60% of the Big Egg's cooking area. I use thin 1/4 inch ceramic tiles and no longer the thick bricks.[p]K~G

King-O-Coals · January 2000

Tim M, we firefighters call it "backdraft". That occurs when the fuel is starved for o2. The heat and fuel is there but there is not enough oxygen to support combustion. The heated fuel source will seek oxygen from any opening available. If your bottom vent is open and your top one is not, it will have to inhale through the bottom. If you have developed a fire in the firebox, and you close the bottom vent, it will inhale through the top one. Trust me on this one. This is probably my only area of expertise except for my understanding of the universe and women. Hee! Hee!. When firefighters encounter a heavily heat charged building that seems to be puffing smoke, then clearing up with no smoke showing, we usually make our first opening high up in the building so as not to feed too much fresh air directly into the starving pre-heated fuel source. Open up the lower regions and fresh air will travel directly to the heated fuel. Venting the upper regions allowes the majority of unburned fuel to escape without combusting. If we,, or even the heat, breaks a glass in the lower regions of a super-heated building, Oxygen will be drawn directly to the heated fuel,, the results will be an explosion due to the oxygen supporting the combustion of the smoke (which is really unburned, vaporized fuel. Have you ever opened the egg dome shortly after closing it and got a quick flash. Same thing.. Sorry to be so elaborate, but someone finally talked about something I know everything about. :]

Could The Airflow Reverse?

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