Downdraft Kiln Backfiring
Posted 21 September 2010 - 12:14 AM
Posted 21 September 2010 - 08:58 AM
I've just begun firing a small downdraft kiln I bought used. The flames go direct into the kiln fine when the air intake vents attached to them are closed. If opened more than 1/4 inch they start backfiring and I'm concerned about the flame being active outside the kiln. The initial solution is to not open the air intake valves more than 1/4", but then there is not enough heat combustion entering the kiln to fire it off in a reasonable length of time. I've tried lowering the gas while introducing more air, but this didn't help. Any suggestions?
What kind of burners are you running?
It sounds like the flame retention nozzles are maybe shot or at least deteriorated. The first step is an inspection of the nozzles and then likely a good cleaning with a soft wire brush.
That is assuming that the burners you have actually have flame retention nozzles. Some really cheapie burners have pretty poor integrated nozzles and are prone to this with any decent level of primary aeration. So you end up "stuck" with using secondary air to solve the issue..... not a great way to have to fire. Sometimes home-made pipe burners do not even have flame retention nozzles at all... and then you have to "play games" on every firng to keep the burner from backburning and/or fluffing out.
The front surface plane of the flame retention nozzle should be about 3/4" to 1" away from the plane of the outer surface of the kiln's exterior structure at the burner port. This allows a good flow of cooling air to keep the metal at a temperature at which it will not deteriorate too fast. Any closer and the radiant heat energy of the kiln starts to heat up the nozzle area excessively. In addition to "rusting" the nozzle out, this position reduces the flame quenching effect of the mass of (relatively) cold metal, and allows the flame to burn back toward the gas/air supply inside the burner when the flame speed of the particular gas/air mixture exceend the flow rate out of the burner tube. Higher aeration ratios heading up toward 100% primary typically increase the flamespeed.... so hence the problem you are seeing.
If the burners have (in use) been experiencing very high metal temperatures due to improper mounting and/or any backburning, then the metal of the nozzle will have deteriorated faster than "normal", and the restriction plate (slightly reduces the size of the mixing tube at the exit area) that is sized to help prevent the backburning will likely have probably eroded away somewhat. This also will make the burners tend do this. The worse the erosion there... the worse the problem.
You are right to be concerned. Heating up the mixing tube and whole burner assembly is not a good thing. That heat can conduct back into the manifold piping and get back to things like ball valves and electrical witing and such. And it will just make the backburning issue get worse and worse over time. You are getting heat energy in places that the system likely was not intended to have.
Possible solutions......... inspect and clean the retetntion nozzles for best operation............ maybe relocate the burner nozzles back more from the face of the burner ports ......... replace the retention nozzles ......... if the burners have integral retention nozzles (cheapie burners) then replace the burners ............ fire using reduced primary air to keep the flame stable at the nozzle and use secondary air to complete combustion.
Adjunct Professor of Ceramics; New Hampshire Insitute of Art
Former Guest Professor, Wuxi Institute of Arts and Science, Yixing, China
Former President and Past President; Potters Council
Posted 21 September 2010 - 11:20 PM
follow John's wonderful advice and if burners need to be replaced - ward burners are great and remont at correct distance from ports.
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