Laser Gas Kiln Help

[crb]

I bought a J9 Laser Gas Kiln last year and finally got it installed, but I can't get it to reach above about 750˚C. 
It rises over 6.5hrs to reach my maximum 35 Mbar at 570˚C and then crawls to 750˚C at 9hrs. I thought too much gas needed more air, so I've lowered the pressure, but the temperature quickly started to drop again. 
The kiln was converted from Natural Gas to LPG; its internal dimensions are 52cmsq 80cm tall. With 4 burners, it runs off 4x47kg, which has a Clesse Automatic ChangeOver Kit has a hood and flue. It came with only a couple of pages in the manual, and the lady before me hadn't fired it before. Before her, the previous owner converted it to gas (and my plumber has checked it all over and connected it up). But we can't figure out why it's not reaching higher temperatures... 
I have fired 3 different gas kilns regularly for the past 6 years, but all were installed by others, and I booked to use them, this is my first own gas kiln, and I was so excited to be self-sufficient. I have been using my previous firing schedules, but this is performing miles behind. Any help would be hugely appreciated. I have a deadline for an exhibition looming ever closer I'm sure I must be doing something wrong, I will attach pics of the set-up,
My plumber thinks that the interior needs replacing, it has cracked walls, and the front door padding has sagged, but it still does seal. Or he said he could look into making the burners orfice larger.

Edited March 12, 2022 by crb

[Bill Kielb]

This will take some doing as 24kw should be enough to power this kiln reasonably insulated. It would be helpful to know the previous orifices, the current orifices before we start calculating to figure out what the issue could be. Interesting note is propane has a little more than twice the output of natural gas so your current orifices ought to be much smaller given they are designed for propane and also designed for a higher pressure.

Some observations: if this is going to be for reduction then the hood likely needs to be a bit higher so the reduction flames don’t contact the hood. I would like to see the hood wider as well to pick up the heat and additional fumes off the kiln. I would add a weather cap to the top of your stack as well for practical reasons. Finally, the height of the stack and diameter can help this kiln with proper airflow. So in the end after all the calculation (when we know the orifices) we may recommend several minor changes so the kiln breathes correctly,  the actions of your damper are more manageable and we know we can maintain a 10:1 air / fuel ratio to get the most energy from your burners.

All this may take time to learn, so if rushed or commissioned you may want to fire in a kiln you are familiar until you dial in your own kiln.

Pictures of the inside, any other secondary air ports if on the side or bottom of the kiln and a picture of the overcut around the burners which is your secondary air port.

Edited March 12, 2022 by Bill Kielb

[Mark C.]

Yes what size are the orfices now? natural gas has larger orfices -LPG should have considerable smaller orfice  holes.

I get that its been convered but all 4 burners in one spot is a bit strange-We need to see the inside with door open as well.

You never mentioned anything about a damper-does it have one on top (I'm assuming its a updraft?)

The only downside with the hood so close is it will rust out sooner if it gets super hot and you could always have a stainless one made  like i did as they never rust-make sure to  cover your chimney cap when not in use for rain.

In terms of to small  a hood that whole shed is just for this small kiln and your shed has a wall of windows  on one side and one opens so ventaliation is looking great to me as long as you do not sit in there all closed up firing

So the issues can be

orfice size

burners to small-burner placement (all in one spot)

The stacking load of wares and bag walls

draft -layout inside to let gas flow thru load

damper issue-size of exit flue

explain in detail these issues  as best you can to help us

Photos inside are a must

[crb]

On 3/12/2022 at 6:42 PM, Bill Kielb said:

This will take some doing as 24kw should be enough to power this kiln reasonably insulated. It would be helpful to know the previous orifices, the current orifices before we start calculating to figure out what the issue could be. Interesting note is propane has a little more than twice the output of natural gas so your current orifices ought to be much smaller given they are designed for propane and also designed for a higher pressure.

Some observations: if this is going to be for reduction then the hood likely needs to be a bit higher so the reduction flames don’t contact the hood. I would like to see the hood wider as well to pick up the heat and additional fumes off the kiln. I would add a weather cap to the top of your stack as well for practical reasons. Finally, the height of the stack and diameter can help this kiln with proper airflow. So in the end after all the calculation (when we know the orifices) we may recommend several minor changes so the kiln breathes correctly,  the actions of your damper are more manageable and we know we can maintain a 10:1 air / fuel ratio to get the most energy from your burners.

All this may take time to learn, so if rushed or commissioned you may want to fire in a kiln you are familiar until you dial in your own kiln.

Pictures of the inside, any other secondary air ports if on the side or bottom of the kiln and a picture of the overcut around the burners which is your secondary air port.

Hiya thank you!

The hood and bars between the kiln body came with the kiln/are part of the model. Will get back to you re orifice size - I dont know how to move the burners to measure the size will ask the plumber coming on Wednesday. And I dont have contact with the previous owner who converted it for previous orifice size I'm afraid.

I attach further pictures, the clesse changeover has a 37mbar restrictor on it - is that an issue? How do I calculate the mbar/bar I need?

I really appreciate your help thank you, 

 

 

[crb]

On 3/12/2022 at 9:21 PM, Mark C. said:

Yes what size are the orfices now? natural gas has larger orfices -LPG should have considerable smaller orfice  holes.

I get that its been convered but all 4 burners in one spot is a bit strange-We need to see the inside with door open as well.

You never mentioned anything about a damper-does it have one on top (I'm assuming its a updraft?)

The only downside with the hood so close is it will rust out sooner if it gets super hot and you could always have a stainless one made  like i did as they never rust-make sure to  cover your chimney cap when not in use for rain.

In terms of to small  a hood that whole shed is just for this small kiln and your shed has a wall of windows  on one side and one opens so ventaliation is looking great to me as long as you do not sit in there all closed up firing

So the issues can be

orfice size

burners to small-burner placement (all in one spot)

The stacking load of wares and bag walls

draft -layout inside to let gas flow thru load

damper issue-size of exit flue

explain in detail these issues  as best you can to help us

Photos inside are a must

Hi, thank you!
Sorry will get back to you re orifice size, I don't know how to get to the burners to measure it.

Has a damper ontop its a updraft, I'll attach a pictures, yes I have a storm cap which I put on when not firing. The shed is just for the kiln and the door stays open during the firing.  The clesse changeover has a 37mbar restrictor on it - is that an issue? How do I calculate the mbar/bar I need? I think my plumber should have got me the 750mbar changeover?

Many thanks,

Chloé

[Mark C.]

your 37 Mbar restictor equals about 15 inchs of water which for me is 1/2 pound of pressure which seem pretty low -that said those burners look small. I'm not a propane potter -I'm a natural gas potter-I fire with 1/4 pound pressure which is 7 inches on a water column . A pound of pressure is 28 inches

In terms of orfice size most burners unscrew -I would try that 1st-the burner closed in photo has a screw holding it it looks like

As an alternate  way You may with a chop stick and a piece of tape tape a letter drill bit taped to the stick and drop that down thru kiln floor thru orfice to find the size (letter drills are sized exactly) In the UK you may have some other system of measurement ??I know you do. I think the regulator may be  to restrictive-you should check that out. 

Hopefully someone here will chine in with propane pressures on kilns

I see your hood has fiber glued to it which will add lots of life to that metal

The other item is the flame looks like it can just go upo the side and out the flue not going thru the load-how is the kiln stacked in top 1/2?

Edited March 14, 2022 by Mark C.

[Bill Kielb]

3 hours ago, crb said:

The clesse changeover has a 37mbar restrictor on it - is that an issue? How do I calculate the mbar/bar I need? I think my plumber should have got me the 750mbar changeover?

37 mbar is plenty and a common maximum pressure for kiln service as long as your orifices are sized correctly. Your kiln is only 24kw which means 6kw (20,460 btu) per burner so very small orifices.  Your kiln is a downdraft I believe, so making sure the flue extends all the way into the kiln unobstructed will be important. It appears the exhaust gases flow up the flue originating at the bottom back of the kiln. Have a look, if this has collapsed in on itself it will likely have a significant effect.

Once we know the orifice sizes we can just look up in a table to make sure you have the right amount of heat per burner. My thought looking at this is we are going to find an issue with how the damper works or how it is being adjusted. Do you have a picture of the damper?

My feeling is this kiln will be extremely sensitive to very very small damper adjustments for a given gas pressure.

First step though what are the orifice sizes. You can post them in any units though, we will adjust accordingly. So drill size, thousands of an inch, mm.  All are fine.

Nice kiln BTW!

Edited March 14, 2022 by Bill Kielb

[Mark C.]

I agree with Bill its downdraft after looking closely (I miseds that)-I do not see a damper as the top exit looks like fiber lined. Use a mirror to look down or up the flue.

[crb]

17 hours ago, Bill Kielb said:

My thought looking at this is we are going to find an issue with how the damper works or how it is being adjusted. Do you have a picture of the damper?

My feeling is this kiln will be extremely sensitive to very very small damper adjustments for a given gas pressure.

Nice kiln BTW!

Sorry not to have got the orifice sizes to you yet, don't have small enough drill bits and couldnt get them undone but I'm sure the plumber tomorrow will have something he can measure them for me!

No damper at back or in flue, its at the top previous owner slid fire bricks over the chimney slots for reduction, 

Thank you, really hope I can get it going

Chloé

[crb]

Some more images.. not sure if they're helpful!

 

 

[Bill Kielb]

17 minutes ago, crb said:

Some more images.. not sure if they're helpful!

How do you damper this kiln while firing?

[crb]

1 minute ago, Bill Kielb said:

How do you damper this kiln while firing?

The previous owner slid fire bricks/or kiln shelves over these top slots for reduction, half way either side

[Hulk]

Ah, so, the exhaust is at the bottom, in the back, eh?

[Mark C.]

The back wall is a false wall with a long slots going up. 

You will need a damper-ether brick or kiln shelve or fiber board. Bricks are the hardest to use for control

looks  like you close the hole with  those fiber scraps when done with fire? 

[neilestrick]

Were you dampering the flue at all? You'll need to. Kind of an odd setup to have the flue 90 degrees to the burners. For a single burner inlet it would be better to have the flue on the opposite side. It'll be interesting to see if you get cold spots in the front left corner.

[crb]

37 minutes ago, neilestrick said:

Were you dampering the flue at all? You'll need to. Kind of an odd setup to have the flue 90 degrees to the burners. For a single burner inlet it would be better to have the flue on the opposite side. It'll be interesting to see if you get cold spots in the front left corner.

I hadnt reached 850/900 which is when I used to start using dampers in my old kilns. Would you recommend doing before then?

[crb]

1 hour ago, Mark C. said:

The back wall is a false wall with a long slots going up. 

You will need a damper-ether brick or kiln shelve or fiber board. Bricks are the hardest to use for control

looks  like you close the hole with  those fiber scraps when done with fire? 

Yes exactly, was planning on using the strips of fiber as dampers but also have kiln bricks

[Mark C.]

Is the flue surface made from fiber or fiber board where it exits the top of kiln as bricks will abraid that surface.

I think fiber board is the best option for damper material if thats the case. Remember fiber has fibers which you shouldm not breath when moving it around

[Bill Kielb]

7 hours ago, crb said:

Yes exactly, was planning on using the strips of fiber as dampers but also have kiln bricks

I think narrow flat shelves ideally from both sides so you can close it uniformly towards the center would be ideal. There will always be a perfect amount of closure that holds the most heat in yet allows the gas flame sufficient air to burn cleanly. (Remember that 10:1 thing I mentioned some time ago) Close it too much and the combustion becomes dirty, less energy, more carbon. This is how we fire in reduction.

When you start your kiln with a very low pressure to begin warming things slowly the damper should be fairly closed, else you will waste a bunch of heat and have too much air flowing through.

As a random numerical example (these numbers are made up and not specific to your kiln) , I start my kiln at 1-2 mbar, the damper(s) would be nearly closed. The kiln is using little gas and only needs a small amount of air to operate efficiently and excess air will cool the kiln. Maybe only a few inches of flue needs to be uncovered so we don’t let out more heat than necessary. As the kiln becomes warmer, more air will naturally rise through this opening in the damper and you will need to turn up the gas pressure a bit as the rate of heating slows ……. and open the damper slightly more to find the new efficient spot at the new gas pressure.

As I fire, I continue to need more energy so I raise the pressure to let’s say 10 mbar and open the dampers to find my best firing speed which will be when the burners have just the right amount of air and not so much excess air as to let the heat out or cool the kiln. This refinement or dance with gas and damper is how one achieves the most heat with the least fuel. At peak output, (31 mbar for you) there will still be a point where the damper needs to be shut to achieve the most efficient heating and just the right amount of airflow to fire efficiently and not let too much air pass through the kiln and out the flue.

Folks become proficient with their kilns and maybe simplify it to (again made up numbers) 10 cm damper to start until 5 mbar, then 15 cm to 10 mbar, then 30 cm to 25 mbar …. Etc…..

They generally are trying to fire at a speed, maybe 75 - 250c per hour or whatever they and their wares are comfortable with. Reduction occurs as one closes the damper just enough to decrease the airflow causing the gas flame to begin burning dirty, lots of carbon monoxide,  pressurizing the kiln and as a result the kiln slows down its heating.

I will stop there because it sounds complicated but it’s really simple in practice and is part of learning your kiln and firing a gas kiln efficiently. The hard part in the beginning is understanding the relationships and patience with settings. Often the difference between heavy reduction and nearly no reduction with high efficiency will be mm of damper movement.

I wouldn’t worry about the burners to one side, I believe Laser still makes these style kilns with a burner off to one side and the draft across the back. It’s likely performed well for years so I would expect a learning curve with respect to even firings but seems to have a successful history of working just fine.

 

Edited March 16, 2022 by Bill Kielb

[neilestrick]

If you don't have one, get a pyrometer so you can check the rate of climb after you make adjustments. Also remember that with many kilns, a very small adjustment to the damper can have a very large effect, so make small adjustments, like 1cm at a time, and give it a minute or two to adjust to the new settings before assessing the results making further changes.

[crb]

On 3/14/2022 at 4:16 PM, Mark C. said:

As an alternate  way You may with a chop stick and a piece of tape tape a letter drill bit taped to the stick and drop that down thru kiln floor thru orfice to find the size (letter drills are sized exactly) In the UK you may have some other system of measurement ??I know you do. I think the regulator may be  to restrictive-you should check that out. 

I did this! A 0.85mm drill bit fit into the orifice of the gas burner

[Bill Kielb]

On 3/17/2022 at 10:24 AM, crb said:

I did this! A 0.85mm drill bit fit into the orifice of the gas burner

Assuming you are close that equates to 0.033” to let’s say .035” inches which translates to about 7600 to 8850 btu (3.3kw - 3.7  kw)  at 11” of water column (prox 20 mbar) which seems a little light. So at approx 30 mbar each burner is likely short of the design 6kw each. Your orifices were likely sized as 6 kw @ 37 mbar. So you will need to turn this up to max pressure to get the 24kw of energy expected.  If you ever need more it’s simple to change the orifices to the next size or two larger.  

Knowing what we know about your damper and the kilns capacity I think it would be good to learn how to operate the damper and gas pressure as described earlier to maximize your heating and minimize fuel use. The orifices and operating pressure seem very reasonable at this point which is good. Now on to learning the nuances of firing your kiln.

Edited March 18, 2022 by Bill Kielb

[crb]

Thank you so much for all your help, I managed to find someone who has the same kiln as me and the manual!! So the correct regulator is in and she is firing beautifully.  @Bill Kielb thank you for your detailed firing tips I followed it today and looking forward to opening the results on Saturday! Thank you also @Mark C. for all your help!

[Mark C.]

So what fixed the issue-just the regulator or something else ??

[crb]

The regulator -- the manual specifies up to 300mb so 30mb was going to be tricky to get to top temp I think...