Richard Gralnik on fri 10 sep 99
Everyone,
John Baymore sent me this email about the first firing of my kiln last weekend.
There is a lot of great information about how venturi burners work and how to
address the different events that happened over the course of the
firing. I hope
the information is as useful to everyone on the list as it is to me.
Richard
>Date: Thu, 9 Sep 1999 22:51:16 -0400
>From: John Baymore
>Subject: First firing
>Sender: John Baymore
>To: Richard Gralnick
>X-MIME-Autoconverted: from quoted-printable to 8bit by branco.desktalk.com
>id TAA00184
>
>Richard,
>
>Hi. Congrats on getting the thing finished. And in breaking the ice on
>your first firing. Forget the nervous thing.... and just learn from each
>firing. What is the worst thing that is likely to happen...... you might
>screw up some pots. No big deal. It's only dirt . Gas kiln
>explosions are pretty rare.
>
>(clip)
> I fabricated brackets to hold the thermocouple/pilot assembly VERTICAL
>instead of my original horizontal and clamped them to my manifold.
>(snip)
>
>If by this you mean that the pilot is firing parallel to the axis of the
>burner mixing tube and nozzle and the thermocouple is oriented the
>same...... that is the way it should be. Just make sure that the tip of
>the thermocouple is close enough to the flame to "lock" the BASO valve on
>but not so near hot refractories that the radiant heat will keep it locked
>on and it won't drop out if the flame goes out. If it was horizontal and
>the burners are vertical it would have been located across the face of the
>retention nozzle..... a sure problem generator.
>
>
>(snip)
>At 1467degF the burners suddenly made a "FoomP" sound and got quiet.
>Too quiet. I looked under the kiln expecting to see a series of
>extinguished
>burners. Instead I saw 4 blue streams of flame being inhaled by the kiln
>where moments before the flames had been yellow, nervous, and kind of
>loud.
>(clip)
>
>Since I have had some input into your kiln situation in the past, I will
>venture to add a little info here that otherwise I would not venture to
>share unbidden. It might help you understand the mystery "froomp"
>situation a bit more. Feel free to tell me to shove it . If you think
>it is valuable to the others on the CLAYART list and you are comfortable
>with it, I'll leave it to YOU to clip and forward the comments to the whole
>list..... otherwise it'll be private to you. That is why I sent this
>privately.....your decision.
>
>
>Hard to be a doctor from afar with no real first hand or visual
>information..... but......... I think I have a pretty good explanation for
>your "Foomp" and sudden flame change...and it relates to the basic burner /
>kiln combination.
>
>(snip)
>Those of you who have followed the saga know my precision engineered 18 cu
>ft kiln has 6 MR750's being fed from a 1 1/2" diameter gas line.
>(clip)
>
>That's a lot of "firepower" for an 18 cubic foot kiln. That is why you get
>such dramatic temp rise with no pressure to the burners, and each burner
>added makes such a jump. It is also the actual root cause of the "froomp".
>
>Hate to say this but...... the burners you mention you have are not really
>very good when compared to more industrial burners like say a Maxon or
>North American or Pyronics mixing unit. The main feature the MR750's have
>for potters is that they are cheap. Fraction of the cost of say... a
>Pyronics mixing unit with a retention nozzle. End of story. The mixing
>tube/nozzle casts are not by any means precision so the amount of primary
>air thay can entrain is very low, and the integral "flame retention
>nozzles" are close to useless....and particularly so at very low gas
>pressure settings. That is the crux of the "froomp" and the flame
>character change, I think.
>
>The job of a flame retention nozzle is to keep the flame seated burning on
>the TIP of the burner...where it is supposed to be. I think this is not
>happening in your case until kiln induced draft is coming into play well
>into the firing, at which point the burners suddenly get enough flow
>throught them (due to additional air entrainment caused by increasing
>draft) and slightly increased inlet gas pressure to suddenly "seat" the
>flames on the retention tips. The sound and the flame character change you
>describe fit this scenario to a "T".
>
>Every mixture of air and gas has what is called a "flame speed". That is
>the rate at which a flame front burns through the mixture. From this you
>can extrapolate that without a flame retention system of some sort, a
>burner will have only a few possible settings at which the flame front will
>be burning back toward the source of gas and air at the same rate that it
>is exiting from the burner nozzle. In all other cases it is either burning
>toward the burner too quickly or not quickly enough.
>
>Lets say that the flame speed of a given mixture is 100 fpm. If the burner
>is set so that the mixture is coming out at 100 fpm, the flame will stay
>ignited at the point it was initially lit.....this usually at the nozzle
>end. If the mixture is coming out at 120 fpm....... in a short while the
>flame is moving away from the front of the burner until the gas/air mixture
>is disbursing into the surrounding air to a point that it can't sustain
>combustion due to mixture change and also disapating heat energy. This
>condition is usually called "fluffing out".
>
>If the flame speed is 100 fpm but the mixture is only coming out at 70 fpm,
>the flame burns back toward the orfice of the burner. In a short time the
>flame is burning back down inside the burner mixing tube right to the
>orifice. The large mass of (relatively) cold metal of the pipe around the
>orfice and the tiny opening serve to quench the flame from burning back
>into the gas pipe itself.... along with the lack of oxygen in there. This
>is usually called "back burning". Backburning is accompanied by a throaty
>rumbly rattle sound in the burner...... sort of like someone perpetually
>clearing their throat .
>
>Because the mixture is burning back inside the mixing tube..... the gas and
>air are not mixing before combustion is starting to take place. So the
>dynamics of the flame are not the same as they would be if the gas and air
>were mixed before combustion began..... you get some "cracking" of the
>carbon chains and so on.... and get a more "luminous" flame....... the
>fluffy, disturbed, and "yellow" character you mention. The air coming in
>through the primary air shutter in this case is actually acting more like
>secondary air than primary air.
>
>Every burner has what is called a turndown ratio. This is a design range
>factor designed into the burner by the folks that made the original design.
> It is a ratio of the highest output of the burner to the lowest output
>that remains stable. It is important to note that it is easier to
>stabilize the high limit flame than the low limit. The better the
>precision of the casting of the venturi mixing tube, and the better the
>quality of the cast of the retention nozzle, the higher this turndown ratio
>is. This is one thing you are paying for in an expensive burner (another
>is the percent of primary air they will entrain). The turndown ratio for
>the MR's (and the air entrainment percentage) is not so good.
>
>With a situation where you have a lot of firepower for such a small
>kiln...... you have to run them very low. You are probably trying to run
>them WELL below their minimum stable low fire rating. You are exceeding
>thier turndown ratio. So they tend to back burn.
>
>At the low settings you get a yellow, poorly formed flame and the burners
>sound throaty. When the flow through them gets high enough the flame
>suddenly seats onto the tip of the retention nozzle and the primary air
>acts more like primary air, the flame gets blue and the throaty sound goes
>away. The sudden change from burning inside the tubes to burning on the
>retention tips gives the "foomp' sound.
>
>The flame burning back inside the burner is heating the burner excessively,
>and is not good for it's life expectancy. It can put a lot of heat where
>you never intended it to be when you were designing the kiln or kiln room.
>It also can condust heat to places that will cause other components to get
>excessively hot....depending on the design of the manifolding and the
>layout of components...... for example guages and BASO valves have parts
>that don't usually like high heat. Watch this situation closely.
>
>Some possible solutions..........
>
>1.) Get different burners with a closer matching of required input to toal
>BTU uotput at high limit for the total burners. This is the expensive
>solution but by far the best one.
>
>2.) Fire only using 4 of the 6 burners. This will work OK if you can get
>heat distribution into the chamber OK. Plug the additional burner ports
>with ITC 100HT covered fiber until you can afford new burners. (all the
>usual fiber handling warnings apply .)
>
>3.) Put smaller orifices in all the burners. This will allow you to run
>more gas pressure yet still not have excessive BTU input. This additional
>kinetic energy of the gas coming out of the orifice MIGHT aleviate some of
>the low pressure setting problems. While with more precision burners there
>is a minimum orifice size that they will take without really screwing up
>the tangental flow of gas at the throat of the mixing unit...(and hence the
>amount, as a percentage, of teh primary air entrained, the MR's are so bad
>a cast that this is probably not a significant concern in this particular
>case.
>
>Hope this is of help in figuring out what is going on.
>
>BEst,
>
>.....................john
>
>
>John Baymore
>River Bend Pottery
>22 Riverbend Way
>Wilton, NH 03086 USA
>
>603-654-2752
>JBaymore@compuserve.com
>John.Baymore@GSD-CO.COM
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