Connecting gas kiln to house natural gas supply

[Bill Kielb]

5 minutes ago, Mudfish1 said:

with the valve wide open I had about a 4 foot standing flame coming out of the kiln and started dropping the temp fast. I think that means I have plenty of gas flowing into the kiln, but as Russ was saying I don't have enough oxygen coming in from the venturi burners to get complete combustion

Your flames look blue, so it seems enough primary air. Yes at 5000 ft you have less oxygen per cubic foot of air, so your burners will be derated a bit from what is shown in the orifice table. Of course your natural gas supplier may also have lower energy content per cu ft of gas as well. 
Not uncommon in mid altitude supplies. It would be nice to know about how many btuh with decent coordinated gas and damper. I would suggest finding the best balance for your damper and gas setting, there  is always a balance when firing. Often finding the best damper position for a given gas pressure will be very slight movements of the damper so lots of patience is often required.

At some point you will run out of secondary air to which the over cuts on the bottom burners will be too small. I would assume as designed this thing works so damper and max gas levels are something I would suggest you tweak now.

Adding a flue pipe is a bit uncommon, typically this kiln would be installed in free air or a natural draft hood would be placed above so the existing hole in the top is probably optimal or close to optimal design for this kiln at present. Hoods use dilution air to keep them in the 500f  degree or Bvent range.  Direct connect stuff - not really great using metal at 2300 f degrees and not common for this type of kiln application. 

However if inclined, size (diameter) and height of the flue will influence the draft by stack effect - something you can look up in a table based on btuh and temperature difference. It really looks like you will get there just using a damper and proper gas pressure settings though so zI would focus there first.

[Rae Reich]

Hard to imagine a flue-plus-damper situation for that kiln. Think you’ll find that even minuscule adjustments of a damper will make a difference. Patience required in your observations and note taking. Not sure about your Venturi design, but mine do have a threaded section at the bottom to adjust primary air. Again, minor adjustments and patient observation. Try just the damper first. 

Edited February 20 by Rae Reich
Misnamed air source changed to ‘primary’

[davidh4976]

There should be adjustable shutters on the bottom of the burners to adjust the primary air. You may need to have those more open to get better combustion at higher gas flows. Once you get those openings set correctly for the top of your firing range, you can leave them alone thereafter and kick them down. Get them a bit loose while cool before the firing and then play with them at the top of your firing to see if you can improve things. They will be hot, so a long screwdriver might work on the adjustments.

Another thought is that the kiln exit hole in the kiln lid may be a little too small for higher altitude altitude. You may need to enlarge it. Not a huge amount. You can always reduce its size with a damper during firing.

For a damper, I find that on a kiln like this, a soft fire brick is easier to handle and maneuver than a piece of a shelf. Either can work.

Also note that a Type K thermocouple can be inaccurate above cone 6. I don't know if it's universal, but my use of Type K thermocouples above cone 6 is that they read low. You have to use cones and look at them through a peephole to really judge. Or use an expensive Type S thermocouple.

[Rae Reich]

A Soft brick damper is easier to slide, but can drop debris onto pots on the top shelf (speaking from experience). Keep top of kiln as clean as possible. 

[neilestrick]

It's easier to overpower these little round kilns. Sometimes you need to back off the burners to get them to go. Also, what size shelves did you have in the kiln? You usually need to use a smaller shelf than what an electric kiln of that size would use, so there's room for air to flow. A lot of people also put a shelf at the top, like 3 inches below the flue opening. Even a short pipe over the flue will increase the draw. You can even make a small chimney out of stacked soft brick.

[Babs]

Yes a hood with flue will, but hoods generally to suck up fumes on inside firing,  not needed outside firing,  can't remember the physics behind it. Why chimneys have a specific design.

Seconary and primary air intakes .

[Mark C.]

I ha ve burners like yours and you cannot adjust them much.

I have seen to much gas usually can be a problem and not using the damper to gain temp can also be a factor. More gas is usally not goin g to help but it can hinder and slow down the temp.

Finding the right mix .as Neil says maybe  a small amout of stack (chimney)  can also help.

[Mudfish1]

Folks!

Thanks for the excellent feedback and help. Based on the suggestions, it sounds like my next step will be to fire again and play with the tradeoff between damper setting and gas pressure, and see if I can coax the kiln above 2300.

David I didn't realize that a Type K thermocouple could be inaccurate above Cone 6, so I will definitely keep my witness cones in there during my next test fire. I am counting on the thermocouple to at least show if I am heating up or cooling down - as long as that works, I can play with it and use the witness cones to let me know if I am getting there.

Bill, your comment about flame color was useful. The picture I attached was from early in the firing, at fairly low gas pressure. When I am towards the end of the firing and the burners are much higher, should I still see that nice blue flame? Does that blue color mean that there is enough air coming from the venturi burners?

My understanding about venturi burners was that they were adjustable to allow more or less air to mix with the gas. I crawled under the kiln and removed the ceramic cylinder, and took pictures of just the burner and orifice assembly. The metal assembly you see in the side picture screws directly into a hole in the gas supply pipes under the kiln. You can unscrew it, but the whole assembly comes off - no moving parts. It's just a short metal can with a cross piece and disk with a hole that screws directly into the gas pipe. No moving parts.

The second picture is looking down into that metal assembly. You can see the gas pipe at the very bottom, the short metal can rising up, a metal support structure going all the way across that's welded to the insides of the metal can, and then a small circular disk with the gas orifice itself. The little disk at the bottom also won't turn or move, and just seems to have a hole in it for the gas to come out.

I don't see any way to increase or decrease the air flow - the bottom of the metal assembly is just open to the air, with that cross piece and small disk welded to the metal can, and the whole assembly just screws into the gas pipe below.

Has anyone seen this burner design? Is it basically just fixed and not adjustable?

Appreciate your ideas/comments!

[Bill Kielb]

1 hour ago, Mudfish1 said:

Bill, your comment about flame color was useful. The picture I attached was from early in the firing, at fairly low gas pressure. When I am towards the end of the firing and the burners are much higher, should I still see that nice blue flame? Does that blue color mean that there is enough air coming from the venturi burners?

Gas combustion made simple! At about 10:1 air / fuel you will get a nice blue efficient flame (natural gas, propane not so much), plenty of oxygen for efficient combustion. A nice blue base of the flame is a reasonable indication of an oxidizing fairly efficient flame so the answer to your question is basically yes it’s a decent indication enough primary air is coming in. . Venturi burners are sort of self calibrated within a pressure range of operation so the more gas pressure, the more primary air is sucked in by the Venturi effect. Your burners do not appear to have a primary air shutter and that is ok really. A nice blue color, devoid of yellow indicates you are operating within a reasonable efficient pressure  range for this burner.  

About 50%:of the air for combustion comes from the primary air and 50% secondary air. The secondary air enters around the burner or the hole in the bottom of the kiln where the burner penetrates.

when closing the damper there will be an absolute best position for a given gas pressure where you are losing the least amount of heat but still allowing enough secondary airflow through the kiln for all the gas to be burned efficiently. Close it more and flame efficiency drops, less secondary air comes in the holes around the burners.

Folks often set their gas pressure -  then find the point where performance begins to drop and open their dampers slightly more from there to get near that perfect balance. Every increase in pressure generally requires a slight opening of the damper so patience is key until you learn about where the damper needs to be for a given pressure for your kiln.

Your stuff seems to be working as designed, I would’ suggest testing with a damper first, it seems you are close ……. before making changes to the original design. Meaningful fine tuning  damper adjustments start about 1000 degrees btw.

There always seems to be a lot of confusion with combustion
Simple Bunsen burner pic below we use to teach primary and secondary air and a fuel air chart showing efficient mixtures you might find helpful.

Edited February 21 by Bill Kielb

[davidh4976]

Neil mentioned that "A lot of people also put a shelf at the top, like 3 inches below the flue opening."

Rae said "A Soft brick damper is easier to slide, but can drop debris onto pots on the top shelf (speaking from experience). Keep top of kiln as clean as possible. "

Both are good points.  I just thought I'd add a fancy drawing to illustrate what Neal described. The top shelf is reported to keep some of the heat in the kiln better but still allow the right air flow. It should also address Rae's point about debris. I don't know from personal experience since my gas kiln is a downdraft, but it makes sense to me. In your test firings, how much space is between the kiln shelves and the sides of the kiln?

[Mark C.]

As I said I have burners like yours on asmall updraft. They are a little different but non adjustable all the same. Just make sure you have a good volume of gas to supply them . Pipe is large enough all the way from meter.With monimal restrictions.

Yes put a shelve on top of load under the lid and use staggered shelve is possible on split levels. Uss a damper -you choice of material 

[Mark C.]

I forgot to add use cones all over to see whats going on. I suggest going slow and not using to much gas. You need a digital pyrometer to make sure you are climbing when it stalls if it stalls adjust the damper 1st ,If this fail and you have a piece of metal (lightweight ) pipe add it to lid to increase draft as noted in above posts or use a few soft bricks to do the same deal with damper on top. Try not to over gas it (meaning more gas is better as this is often not the case. 

Since your burners like mine are fixed you only have the gas to turn up or down and the damper. My updraft is exactly like this with those only two functions. A  little damper in makes it climb slowly

Now if all this fails I would like to know how much more gas you have meaning whats left on the valve 1/4 turn or less ( FULL PORT  ball valve only)

[Mudfish1]

Folks;

Thanks for the continued interest and help during my quest for a Cone 10 firing! Mark it really helps to know that my burners are fixed (non adjustable) so that I can take that issue off my list of possible adjustments. I had read that some venturi burners are just designed to be efficient over a certain range of pressures, so it sounds like that is the case with my own little mystery kiln. 

Bill thanks for the nice illustrations of the burner flame and combustion tables. I will pay attention to the burner flame color as it enters the bottom of the kiln, that will give me a direct observation to indicate whether or not I have sufficient oxygen from the venturi effect, or if I am pushing unburned gas into the kiln. My understanding is that for any given damper setting, there should be a gas pressure that results in a good blue flame. More gas would then lead to excess fuel and a yellow flame? So is observing flame color at the burner a practical way to determine the max efficient gas flow into the kiln?

David, I appreciate your artistic skills! I won't try to compete with my own drawing, but my shelves are evenly stacked with the first layer 3" above the kiln floor to create mixing box for incoming flames. Total interior height is about 27", so shelf spacing looks like (from bottom to top)

3" (from kiln bottom)

6"

6"

4" 

which leaves about 6" space to the bottom of the kiln lid. The idea of placing a last shelf a few inches from the lid makes sense - it would certainly reduce heat loss from that vent, good suggestion.

In answer to the comments about my shelves, I am trying to keep it simple for now. I have > 1" space all around, and left a 1" gap in between shelves at each level. I didn't try to get fancy with staggered shelves, or with placing some shelves closer to the wall to block direct gas flow up the inner sides of the kiln. I've read about all kinds of creative shelf placement to block/channel/slow down the gas and force more mixing, but I wanted to master the basic kiln behavior before I got too creative with shelf placement.

I remain intrigued with the idea of adding a short flue to get some buoyancy effects and increase the draw of air int the bottom. However, I am going to bet that the kiln was designed to make Cone 10, if I am smart about the gas pressure/damper combination. I have to believe that I can squeeze another 80-100 F temp increase out of the kiln if I play with the gas/damper settings, as everyone is suggesting!

It may be friday before I get off work early enough to give this another shot. I want to start early enough that I won't be out there at midnight (again) trying to entice the kiln to get to 2300!

Thanks everyone!

 

[PeterH]

On 1/13/2024 at 11:52 PM, Mudfish1 said:

In any case, I started from everyone's useful comments and I have been doing some online homework and additional calculations. I want to fly this by everyone, let me know what you think! From reading their online material, Ward Burner Systems recommends about 10,000 - 16000 BTU/hr for each cubic foot of kiln if you are firing to Cone 10. That would mean I need 100,000 - 160,000 BTU/hr for my 10 CF kiln, or 150,000 - 240,000 BTU/hr is I want to plan for a 15 CF kiln in the future.

>>>> Please ignore, I thought I had deleted this FACTUALLY INCORRECT post <<<

Edited February 22 by PeterH

[PeterH]

Deleted.

[Bill Kielb]

7 hours ago, Mudfish1 said:

My understanding is that for any given damper setting, there should be a gas pressure that results in a good blue flame. More gas would then lead to excess fuel and a yellow flame? So is observing flame color at the burner a practical way to determine the max efficient gas flow into the kiln?

For the most part yes - My fault- I was trying to convey that observing the flame at the burner would be an important part of indicating sufficient Primary Air, however secondary air is directly influenced by the damper position. Both primary and secondary air are super important to efficient use of the fuel …… or for maximum heating. The damper will eventually influence the flow of primary air as the back pressure (when significant) will degrade the performance of the Venturi.

This is often the reason to just leave the primary air adjustments (when present) alone and just go into reduction using the damper and sufficient gas pressure to pressurize the kiln down to the lowest spyhole or even lower attempting to keep oxygen away and maintain a uniform reducing environment. Enough back pressure in the kiln will eventually influence the primary air as well.

The secondary air is often overlooked and misunderstood but is a super important part of the non powered burner process. I was trying to emphasize the difference in origination and importance of both.

Edited February 22 by Bill Kielb

[Mark C.]

Just for reference once I reach bisque temp in all my gas Kilns (I have 4)

I usually leave the gas setting alone and just use the damper to contol kiln

no need to micro adjust  gas settings thru a firing past 1800-2000

[Mudfish1]

Friends! I am happy to report a successful Cone 10+ firing in my mystery kiln! I have attached a picture of my cone packs at every level, with Cone 7, 8, 9, and 10 from left to right in each cone pack. Top of the kiln hit a hard Cone 10, and the bottom of the kiln just liquified all the cones. To fully liquify the Cone 10 like that, I am guessing I might have reached Cone 12+ on the bottom? Hard to be sure.

First, using no damper, I fired the kiln up to about 2100 F by having my gas pressure as high as it would go with no standing flame from the vent. I wanted to see how hot I could get with an oxidizing atmosphere. If I avoided any standing flame from the vent, the kiln seemed to stall at around 2100 F. 

Then I slowly adjusted my gas pressure upwards and used the pyrometer to simply make sure that kiln temperature was increasing. I started to get a small standing flame from the vent, but I ignored it. I just watched the pyrometer. Every time the kiln stalled, I increased the gas just a bit. It was a painstaking process, but by making tiny adjustments to the gas pressure, I was able to hit a max temp of 2303 F based on my pyrometer. Less gas would make the temp start to drop. More gas would make the temp start to drop. So with no damper, I had found the optimal gas flow rate to maximize kiln temp.

Note that at this point of maximum temp, I had a significant standing flame coming out of the vent (see picture). That means my kiln reached peak temp with enough gas flow to have significant reduction going on. This surprised me - I had assumed having extra unburned gas flowing through the kiln would cool things down, but the data was clear - I reached peak temp with a nice standing flame from the vent. 

I then did some experiments to test out the effect of a damper. Keeping the kiln stalled at that peak of 2303 F, I slid the damper on just a fraction of an inch. That made the temp drop. If I slid the damper on more, that just made the temp drop more.

I took off the damper and got back to 2303 F, and then tried sliding on the damper while simultaneously decreasing gas pressure. I assumed I would be able to play with more damper/less gas and maintain the peak temp, but it didn't work that way. The damper made the temp drop. Deceasing the gas made the temp drop. I could not find a combination of more damper/less gas that reached my peak of 2303 F - every combination of conditions was cooler. 

So that part of controlling the kiln is still a mystery. I blew past Cone 10 with no damper and a significant reducing flame coming from the vent. Any use of a damper made the kiln cooler no matter what else I did.

Given my results and what I described, I would appreciate thoughts on how to play with that damper/gas flow combination of factors.

In the meantime, I can certainly get this little kiln to Cone 10 temp just by adjusting the gas flow!

Thank you to everyone that's been reading my posts and giving me excellent ideas and suggestions on what to do.

 

 

 

[neilestrick]

Congrats!

Flame out the top does not necessarily mean reduction. Does the clay from your cone packs look reduced?

[Rae Reich]

Probably be helpful to use red stoneware for the cone packs to better gauge reduction. 

[Mudfish1]

Hi Neil;

I always thought if you get a flame out the top, it meant that you had a reducing atmosphere in the kiln?

Good idea on the clay color, when I do a real firing I will know how much reduction I got. I use a lovely iron-rich stoneware clay that gets nicely buff colored when it's reduced.

Richard

[Bill Kielb]

7 hours ago, Mudfish1 said:

I do a real firing I will know how much reduction I got

Another technique some use  for later is to mix up a copper red glaze for reduction and place samples throughout the kiln to see just how well you reduce and maintain it. This oxblood color goes ugly green / grey pretty quickly with any sneaky oxygen infiltration. It teaches the importance of keeping the kiln in full reduction top to bottom while providing a way to map the reduction performance within the kiln. Only a handful of metals are significantly affected by reduction btw.

 

Edited February 28 by Bill Kielb

[Rae Reich]

2 hours ago, Bill Kielb said:

This oxblood color goes ugly green / grey pretty quickly

We call that color ‘snot red 

[Mudfish1]

Copper red glaze test tiles is a good idea - I will mix up a batch and make sure I have some test tiles in my next firing. I fired a large West Coast gas kiln for many years, but when the studio closed I moved to my garage, switched to a Cone 8 electric kiln, and reformulated all my glazes to Cone 8. Now that I am moving back to Cone 10 gas firings, I will dust off my old glaze recipes and get used to my small gas mystery kiln. I had a copper red formula that was great because it was beautiful in reduction, but also a lovely green in oxidation. Hopefully I can blow the dust off my notebooks and find it!

For anyone that has a small gas kiln, is it hard to get good reduction throughout such a small volume? 

Thx

[Rae Reich]

10 hours ago, Mudfish1 said:

is it hard to get good reduction throughout such a small volume? 

Small changes, many notes.

If there are sweet spots, you can plan around them. Nice to have a red that is acceptable in oxidation. 

Edited February 29 by Rae Reich