Kiln not reaching temperate, lid gap issue?

[MikP]

Hi everyone,

I'm refurbishing a Duncan EA820 and I'm having trouble getting it up to temperature. The kiln is a Duncan EA820, Single Phase, 240V, 24A, 5760W, 1260°C Max temp (Cone 8).

I've added a digital controller (kept the kiln sitter wired in but replaced the timing circuit and mechanical relays with an zero crossing SRR driven by a raspberry Pi), replaced the lid and replaced all 8 elements with new ones.

I've measured the supply voltage to the kiln and the current draw and confirmed its IAW with specs.

I've tried firing a cone 8 schedule but the kiln temperate maxes out at around 1080°C (approx. cone 03?) according to the thermocouple. It has been hovering around this temperature for about 2 hours now with all elements on continuously and there's been no increase in temperature.

I noticed while firing the lid has some high spots resulting in a gaps between 3mm - 5mm around the kiln which I have yet to sand down. Gap exists when the kiln is cool but I never considered sanding the high spots down as I had previously only used the kiln for bisque firing with no issues.

Is this kiln lid gap significant enough to be the most likely cause for not reaching cone 8?

Other things I've considered are:

• Inaccurate Thermocouple - I'm waiting for the kiln to cool down and i'll have a look at the witness cones. I've previously ran a 06 bisque fire with no issues.
• Kiln too loosely packed - didn't fully pack for this initial test run, would this have a significant effect on the max temp?

Any advice from the kiln gurus would be much appreciated.

Cheers,

Mik

Edited September 25, 2022 by MikP

[Bill Kielb]

just guessing here so……
Hmm, so not enough power to make temperature vs the shell losses. 1250 c recorded for a 1260c kiln. Seems reasonable actually.but maybe the green line is the set point? Assuming it’s not even close, then why is there not enough power. Brand new at 5760 Watts this barely makes cone 8. So when worn roughly 10% it will no longer make cone 8 and requires aan element change.

Assuming not enough power
Do you really have 5760 watts? ( measured accurately)

your graph does not show the firing rate of your SSR which is firing at some preset slope and never really changes if we do not include the green plot.

The lid

The gap will add to increased losses. The top and bottom of your kiln will account for the greatest percentage of losses, but this kiln will not routinely make cone 8, that is a max temp when brand new.

light load

The more wares, the more mass the more power it will take

My best guess if it’s maxing out at 1080c but as your graph shows it’s progressing at 200-260c per hour then abruptly stops it’s likely a programming issue or SSR drive related issue.

[Kelly in AK]

Bill’s the man. What he said! Seems like new elements ought to be doing better than cone 03. 

Always place the cones so you can see them through the spy hole (I know you know that now, just saying it for the next guy).

I’m out of my depth with digital controls, so this may not be relevant, but I’ve had trouble before with the thermocouple/pyrometer connection. Wiggling the plug straightened it out. 

[MikP]

35 minutes ago, Bill Kielb said:

just guessing here so……
Hmm, so not enough power to make temperature vs the shell losses. 1250 c recorded for a 1260c kiln. Seems reasonable actually.but maybe the green line is the set point? Assuming it’s not even close, then why is there not enough power. Brand new at 5760 Watts this barely makes cone 8. So when worn roughly 10% it will no longer make cone 8 and requires aan element change.

Assuming not enough power
Do you really have 5760 watts? ( measured accurately)

your graph does not show the firing rate of your SSR which is firing at some preset slope and never really changes if we do not include the green plot.

The lid

The gap will add to increased losses. The top and bottom of your kiln will account for the greatest percentage of losses, but this kiln will not routinely make cone 8, that is a max temp when brand new.

light load

The more wares, the more mass the more power it will take

My best guess if it’s maxing out at 1080c but as your graph shows it’s progressing at 200-260c per hour then abruptly stops it’s likely a programming issue or SSR drive related issue.

Hi Bill,

Green line is the target firing schedule, white line on top is is progress against firing schedule. Controller has built in schedule shifting if kiln temp lags behind setpoint to allow temperature to catch up (thus why the 2 hours @ 1080c I mentioned doesn't show up in the graph, it basically waits at the 7:20ish hour mark indefinitely). Don't believe its a controller issue/SRR issue as it works fine for bisque firings and tracks well up to 1080c. Link for the controller I'm using below if anyone is interested.

As you say I think the issue is to do with shell losses @ 1080 exceeding heating ability of coils. At first I also thought it was a worn coil issue so I replaced all 8 elements so I'm confident its not a power issue. Measured voltage at the supply is 236.3V, measured Amps were 25.6A with the new elements which is approx. 6050W (I'm thinking the higher than nameplate current draw is a result of the new elements and supply voltage being lower than 240V).

Intent is to use the kiln up to cone 6 firings but I figured I'd shoot for cone 8 as a test with the elements newly replaced.

Sanding down the lid to minimise the gap will be my next step but I wanted to sanity the idea that the gap could cause that much drop in max temp while I wait for the kiln to cool 

[Bill Kielb]

I get 6100 w and some change. Is the graph true and it’s heating at a decent rate and just stops at the magic 1080c? Normally the SSR would be pulsed on / off by the PID logic to maintain a heating rate. At 1080c can you confirm it is on 100% of the time and the kiln does not climb? Also would be curious of the drop across the SSR just to understand how much power was being consumed by it. Easy enough to just jump this out and see what the firing temperature rate of rise ends up to be for real.

Its hard to believe it’s moving upward in the hundreds of degrees c per hour and just quits at some point.

[PeterH]

9 hours ago, Bill Kielb said:

My best guess if it’s maxing out at 1080c but as your graph shows it’s progressing at 200-260c per hour then abruptly stops it’s likely a programming issue or SSR drive related issue.

I assume that this graph shows the progress to 1080C (and the kiln just hangs from there, but this isn't shown) ... is it coincidental that this glitch happens at the end of one ramp and the start of another?

If the kiln was simply running out of steam  wouldn't  the angle of the ramp gradually reduce -- unless there was a sudden change in the thermal situation (like the lid springing a little further open).

[neilestrick]

Not a lid gap issue. Like Bill said, something else is going on. Is this the first time it has done this, or has this phenomenon been repeated? It's possible an element connection fried or something like that. I would turn it off and open up the control box and make sure there aren't any wiring issues, and if not then start looking at the digital system. Maybe a high limit or a hold in the program accidentally?

[MikP]

On 9/25/2022 at 3:26 PM, Bill Kielb said:

I get 6100 w and some change. Is the graph true and it’s heating at a decent rate and just stops at the magic 1080c? Normally the SSR would be pulsed on / off by the PID logic to maintain a heating rate. At 1080c can you confirm it is on 100% of the time and the kiln does not climb? Also would be curious of the drop across the SSR just to understand how much power was being consumed by it. Easy enough to just jump this out and see what the firing temperature rate of rise ends up to be for real.

Its hard to believe it’s moving upward in the hundreds of degrees c per hour and just quits at some point.

I've caused a bit of confusion as the controller UI doesn't display the schedule shifting well, it 'squishes' the measured temperature onto the target profile so it appears as though it climbs and stops abruptly. I've pulled the raw data from the controller and plotted it below to clarify:

 

Target profile in red, measured temp from thermocouple in blue. Schedule shifting takes the target profile and shifts it right until the measured kiln temperature catches up.

Kiln can't meet the ramp rate (approx 200c / hour) at around the 800c mark and maxes out at 1080c, SRR is on continuously at this stage. Kiln sitter timer switched the kiln off at 17h (a bit excessive but I wanted to confirm the temperature plateau).

Voltage drop across SRR is 1V, voltage supply to SRR drops to 228V (down from 236V) when elements are firing. Element and section resistances match up to values given in the service manual (820-1/820-2 for 240V supplies):

Based on this, I'm inclined to think its a shell loss issue. Am I barking up the wrong tree?

Edited September 26, 2022 by MikP

[Bill Kielb]

Assuming these numbers are correct, when your element resistance rises by 10%, generally they are worn and need replacement and your kiln will not make top temperature. The design of many of these kilns (new) is 110% excess wattage to offset the shell losses.

Wattage and losses drive the entire thing. Power or wattage = voltage(squared)/resistance. Power decreases linearly as resistance goes up but power decreases exponentially as voltage goes down. So a 5% decrease in voltage gets you approximately a 10% decrease in wattage. Add some shell leaks and you can’t make temperature.

The numbers say your decrease in power is definitely a huge issue because of the depressed voltage. Is the wiring to the kiln adequately sized or does your mains voltage drop to 228v? Additionally, measuring the actual voltage and amperage at top temp accurately so we know the real hot resistance of the elements would probably reveal even less power is available as the kiln heats up.

Your controller has interesting steps and droops in it as well which could indicate significant overshoot and under shoot while firing. The percentage on / off is all over the place so it’s PID parameters likely don’t match this load well.

Seal up your kiln, but at that low of voltage I don’t believe you have enough energy. Your kiln is running on 10% less wattage to begin with.

Edited September 26, 2022 by Bill Kielb

[Johnmicheal]

I can't answer too the digital side of your problem, but I can say that it's not a leakage issue. Every kiln I fire has a lid gap or worse. I have fitted lids in the past, but on the whole of it, I don't think it's a priority unless it's causing an issue with venting. In which case I just adjust for that also.

[jbruce]

Hi @MikP. I'm the author of this software. If you're still having trouble, lmk and I'll do what I can to help. It's best to open an issue on github at https://github.com/jbruce12000/kiln-controller/issues

From what I've read, it looks like the elements need replacing. We can help with PID tuning after you get the heating problem sorted.

For everyone else, this software acts like an on-off switch until it gets N degrees away from the set point, so in this case, since it is way under the set point, the controller is telling the elements to heat at 100% continuously. Once it is within the window, it uses a PID to control temps.

[jbruce]

@MikP there is also some software to graph a run https://github.com/jbruce12000/kiln-stats

so you don't have to do it manually unless that's your jam. I use the graphs to help diagnose problems and help folks with tuning. You can see the sample is not tuned well.

[Babs]

The colour perking through that gap looks higher than 1050. Whst are your cones saying?

Are all your elements heating?

[Bill Kielb]

1 hour ago, jbruce said:

From what I've read, it looks like the elements need replacing. We can help with PID tuning after you get the heating problem sorted.

Just an FYI, he also has low voltage. 228v fully loaded. Probably needs that stabilized before you tune the loop

[jbruce]

@MikP if you ever suspect the pid controller is contributing to the issue [and I don't think so in this case], you can remove it and try to fire using manual controls to see if you can reach beyond 1080c.

[Babs]

What did your cones tell you?

A bank of cones in line eith that spyhole would be handy.

Cone 8 max so firing there continually will wear everything fast.

What does the thermocouple look like.

Just that, when freaking about my controller malfunctioning it was a simple slight bend in rod which hits the on lever when lid is closed, ok in lower range of temp but when everything heated it did not function properly.

[MikP]

Thanks for all the suggestions. I sealed up the lid gap and did another run, max temp did increase to 1141C (up from 1080C with the gap) but still well shy of the 1250 target, results graphed below.

Had a range of cones from 03 to 8 in there to check accuracy and they more or less lined up with thermocouple readings.

@Bill KielbYou're right about this being a low voltage issue. The Nominal voltage down under is 230V with a -6%, +4% tolerance which already puts me under the 240V rating of the kiln. Wiring to kiln is adequately sized IAW Aussie standards (they allow for a 5% drop).

Not much I can do about the voltage supply, could upsize the feeder cable from the mains but I doubt cost of doing so would justify the increase in wattage I'd get from reduced voltage drop. 

I'm thinking my best option to increase wattage would be to play with the resistance values for the heating elements. The replacement elements I got were 14 AWG Kanthal A1, figure I could go to 13AWG and size for a current draw of 30A @ 240V. I'm working on a small DIY kiln for glaze testing to see if my element design math checks out, if that goes well I'll have a go at some custom elements for this project.

@jbruce thanks for the offer for tech support, haven't focused on tuning yet as I wanted to verify max temps first, will have a go at tuning before the next bisque fire.

Edited October 14, 2022 by MikP

[Bill Kielb]

23 hours ago, MikP said:

@Bill KielbYou're right about this being a low voltage issue. The Nominal voltage down under is 230V with a -6%, +4% tolerance which already puts me under the 240V rating of the kiln. Wiring to kiln is adequately sized IAW Aussie standards (they allow for a 5% drop

Sorry to hear that actually. 3% rule of thumb here is usually allowed but for kilns for obvious capacity reasons less is suggested. No point in starting with a brand new kiln in effect worn 3%. There is a special rule for continuous resistive loads to protect the breakers from repeated heating in North America. The rule here would be no less than 125% of the rated load and not to exceed 150%. Often this takes care of the voltage drop issue, a nice side benefit. Most countries address these loads similarly to protect the protection devices so you may want to research. Electricians not used to working on kilns are often not familiar and tend to under size things.

Most often the rules for maximum loading on a breaker fit well with this standard. So for example, here for normal breakers you are only allowed to size them so normal loading does not exceed 80% of the rating. Which, interestingly is the reciprocal of 1.25 or 125%. Actually intuitively who would put a 30 amp load on a 30 amp breaker? It does happen though.

For your use you may want to have elements re-wound to make the original wattage at your rated voltage. (Total kiln R= V(squared) / Watts) Of course all protection and wiring would need to be upgraded as necessary and I would suggest clear re-labeling is a must. BTW we just went through this in another thread and as an FYI, kilns with 4 watts per interior square inch are usually most robust. Hope that helps!

Previous thread : https://community.ceramicartsdaily.org/topic/30261-paragon-dtc-800-kiln/?do=findComment&comment=228236

 

Edited October 15, 2022 by Bill Kielb

[Bill Kielb]

On 10/14/2022 at 11:09 AM, MikP said:

Not much I can do about the voltage supply, could upsize the feeder cable from the mains but I doubt cost of doing so would justify the increase in wattage I'd get from reduced voltage drop

Interesting late notice when looking at the Duncan service manual. 208 v elements are 8.66:ohms which places you at 6000 w when operated at 228v which is 26 amps. Here that current draw would require at least a 30 amp breaker and number 10 wire rated for 30 amps. So gauging the elements down but having them wound as if 208 v could be a reasonable solution to the low voltage.

Edited October 16, 2022 by Bill Kielb

[Min]

On 10/14/2022 at 9:09 AM, MikP said:

I'm thinking my best option to increase wattage would be to play with the resistance values for the heating elements. The replacement elements I got were 14 AWG Kanthal A1, figure I could go to 13AWG and size for a current draw of 30A @ 240V. I'm working on a small DIY kiln for glaze testing to see if my element design math checks out, if that goes well I'll have a go at some custom elements for this project.

Have you contacted Euclids with an email with kiln and electrical specs that you do have and ask them about element design? They will do the math and let you know gauge etc. or if they think the kiln will be underpowered with your current low voltage.