Reduction ?'s

[Biglou13]

i understand the concept of reduction..... in a  nut shell  fire where not enough oxygen is present to burn efficiently  / cleanly.

 

from a venturi burner  as the fuel leaves  and ignites  say within visual distance of flame its clean neutral or oxidation,  and within kiln is that flame in reduction if you dammpen exhaust?

 

same with forced air burner for maximum amount of heat flame should be neutral or oxidation either oxygen rich or oxygen neutral.

 

with wood fire, venturi gas, and forced air kiln atmosphere is usually controlled by exhaust  eg damping chimney...   causing some back pressure (?)  and slowing the draw of oxygen into kiln and with forced air some of that is controlled by forced air......

 

with forced air burner  im assuming its neutral to ox at flame source......does damping exhaust /chimney thus slowing oxygen through put make kiln atmosphere reduction?

 

my mind says yes, but got into discussion today and could not explain why?

 

argument was flame at burner is relatively neutral or ox./ relatively blue flame  at burner, why does it then go into a oxygen deficit as heat continue into kiln aka reduction.

 

to this response i had no intelligent answer please help.  

 

i know kiln will go into redux but cant explain it....

 

 

[Tyler Miller]

I think the answer to your question lies in the presence of the secondary air intake through the kiln's burner ports themselves, "powered" by the natural draft through the kiln.  At the burner, the flame is actually in reduction for a neutral/reducing kiln atmosphere and even an oxidizing atmosphere.  You could have an oxidizing flame at the burner--fully oxidized by the primary air intake only, but I don't imagine the flame would be terribly stable.  Jed (jrgpots) had a recent post about his burners backfiring--this would be an example of too oxidizing a flame and its instability.  Oxy-gas welders have to watch their flames because too oxidizing a flame will travel back up into the torch handle and even into the hoses and burn there instead of at the torch tip.

 

Does that make any sense? I feel like I confused my self a little with that explanation.

[Mark C.]

(does damping exhaust /chimney thus slowing oxygen through put make kiln atmosphere reduction?)

 

This is what I do with my natural draft burner system. I never mess with anything except the damper on 3 kilns one is an updraft.

Mark

[Benzine]

I got reduction in my homemade Raku kiln, the first time I fired, by accident. I had the burner too close, to the port, the burner cranked too high, and both ports too small. I ended up with a bit of soot on the kiln interior.

[Colby Charpentier]

It may help you to research the Venturi effect, and Bernoulli's principle. As Tyler mentions, the air intake through burner ports or any other intake orifices, are effected by the damper system. Commonly this is referred to as the draw or draft of the kiln. Bernoulli's principle addresses how the moving gas/air creates a low pressure stream that draws additional air into the kiln. The damper effects the flow rate at the rear of the kiln, which in turn effects how the draft behaves throughout the entire kiln.

 

 

It might be worthwhile to look at the chemistry that's going on in the load as well. Fire is oxidation. It's an exothermic reaction that requires fuel, oxygen, and energy. One has to consider that the flame path in a kiln can be rather long, as the reaction is taking place over a relatively long period of time and space. As Tyler also mentioned, the flame is not completely oxidizing at the source. This means that the oxidation atmosphere we speak of means that all of the fuel is completely combusted, where as the reduction atmosphere is partially combusted fuel source, with oxygen being the limiting reagent. Take note how a Bunsen burner does not give off much heat at the source of the flame. The highest heat is given off at the very tip of the inner blue flame, where more complete combustion has been allowed to occur.

 

Just some thoughts... Maybe someone can add a little more... 

[neilestrick]

Not to over-simplify this but:

 

- With venturi burners, secondary air (the air being drawn in by the draft of the chimney) plays a big role in the atmosphere of the kiln, so you can control the reduction by just adjusting the damper. This is a good thing because most kilns that use venturi burners use several of them, often mounted underneath the kiln. If you had to adjust each burner every time you put the kiln into reduction it would be a huge pain in the you-know-what.

 

- With wood burning kilns (at least all of those that I have fired), the intake air is usually decreased along with adjusting the damper (active or passive). In wood burning kilns, more so than boxy gas kilns, you have to be aware of the length, speed and location of the flame in addition to the atmosphere. Closing the damper alone can mess things up.

 

- With power burners, the chimney draft plays a minor role. It's more about maintaining pressure in the kiln and keeping the air moving through, rather than pulling air in. The blowers provide enough air on their own, which is why you can fire kilns with power burners that are sealed to the kiln. In order to put the kiln into reduction you need to reduce the amount of air going through the burner by either closing up intake on the blower or reducing the speed of the blower. When you do this the pressure in the kiln changes, and the chimney damper must be adjusted to maintain the proper pressure in the kiln.

 

Pressure in the kiln is important because without it you will not get even distribution of the heat and atmosphere. You'll get hot spots and cold spots and areas that aren't reduced. As the kiln heats the pressure increases it causes less draft since the pressure is pushing outward, and therefore causes the kiln to go into reduction.

 

I know John will have a more scientific explanation.

[Guest JBaymore]

No time to go into this fully now.  Sorry.

 

The BEST aspirating venturi burner casts (expensive units) can entrain a maximum of about 75% primary air.  Even if that flame looks "blue" when it is coming out.... it is running about a 25%  deficiency in primary air.  So if the kiln is not entraining that other 25% volume of air (about 10 cubic feet of air to 1000 BTUs of fuel value)........the chamber will have reducing conditions in it at least in SOME places.  As to how those "places" of reduction reatate to the wares themselves...... THAT is about what is known as "mixing" in the chamber.

 

best,

 

......................john

[Wyndham]

The damper allows the back pressure of the incoming gas/flame to create (in my view) a balloon of reduction(fuel rich) atmosphere. If the damper were closed, it would choke out the flow.

Starving the fuel of oxygen, it seeks oxygen from any source. As the temperature rises,the atmosphere expands(blowing up a balloon) so opening the damper is like leaning out the choke on a car(old style) and keeping this reduction envelope maintained within the ware chamber. Too small an envelope and oxygen can slip in and oxidate the ware.

When all this happens seems to be most critical before the melting point of the lowest melting element in the glaze.

In general it seems that around 1500f is good reduction for my copper reds, others may have another sweet spot for their glazes but 1800f is about the max. Too much reduction causes black coring or carbon trapped in the clay body, making it more brittle.

A reduction is kept till about cone 9-10, based on flame length & color at the bottom peep in a downdraft kiln.

A yellow licking flame tells me the atmosphere is seeking oxygen, there for in reduction.

Others may have other ways of expressing this but this is how I describe my reduction.

Wyndham

[Mark C.]

With my Oxyprobe the reading with damper open when kiln is firing up is zero's when the damper goes in a little the numbers slowly climb as the damper goes in more.

I love this simple system-when we have a power outage it works just as well as power is not part of the system.

I have 3 kilns that work on this design on natural gas-one is fired about 30 some fires a year-so simple also turns into one less thing to work on.

Mark

[jrgpots]

If you created a hole in the side of the chimney above the damper during the reduction phase, would this also reduce the draw of the chimney and add to the chamber's reducion?

 

Is this ever done? I have been thinking of adding an adjustable opening in the chimney above the damper that I could close during oxidation and open during reduction. Bernoulli's principle would predict that ambiant air would be drawn into the chimney through this hole and thereby reduce the draw from the chamber.

 

Jed

[Mark C.]

Yes its been done

I built a copper heat exchanger above the damper that heated out hot tub in the 70's.

You would slip the copper coils in  when the kiln was going with a pump going pushing water thru the coils so it whould not boil.

Worked great till the power went out one night.

I also used this hole to help with a broken damper shelve once getting it out. I have put an inspection port in almost in all the stacks I have made since.I never had any luck doing what you discribe as I want my chimmneys to suck well and that just adds air and slows down the draw. 

In my venturi natural draft kiln you want the stack to draw so this defeats the purpose.

Now if you have a power burner thats another story as the stack suck no longer is a big factor

Mark

waiting for two glaze fires to cool today

[neilestrick]

If you created a hole in the side of the chimney above the damper during the reduction phase, would this also reduce the draw of the chimney and add to the chamber's reducion?

 

Is this ever done? I have been thinking of adding an adjustable opening in the chimney above the damper that I could close during oxidation and open during reduction. Bernoulli's principle would predict that ambiant air would be drawn into the chimney through this hole and thereby reduce the draw from the chamber.

 

Jed

 

Yes, it is called a passive damper. It spoils the draft. Common in wood burning kilns.

[jrgpots]

Once again, my great idea already has a name. It's kind of reinventing the wheel.

 

On-the-other-hand, it means I'm beginning to understand...

 

Thanks