diy kiln DIY Kiln Built opinon

[lbispo]

Hi,

I would like to share my Kiln built and ask for some insights as my experience with Kiln were all in studios where others were operating it and I did not follow the whole firing cycle.

I believe it is important to start saying that I was not sure if I should post it here or at "Equipment Use and Repair", please let me know. I will try to convert the metric information to imperial, but i may fail, apologies in advance.

• I decided to make a small kiln from K28 bricks (1530C / 2800F), the bricks are 65mm(2 1/2").
• The bricks lay inside a metal drum, with a thin layer of ceramic insulation blanket, in order to reduce the gap between brick and metal, and try to add a bit more insulation;
• It is a Dihedral (new word on my vocabulary, twelve-sided polygon), and ~300mm (11 1/2"?) height, and ~300mm (11 1/2"?) "diameter", resulting in ~20L or 0.7ft3
• From commercial Kiln, I found that 20L takes 2300W, threfore using an Europe power plug (230V and 13Amps):
• I used Kanthal A1 wire (1mm,  AWG 18) aiming for 23Ohms, ie. ~12.5m (41ft).;
• I coiled the element around a 6mm  (1/4"?), resulting in a Surface load of 5.89W/cm2 (38W/in2) -> I know this is high, but I struggled to find a thicker wire, I will replace that in the future;
• The element above, also resulted in a coil pitch ("stretch factor") of 4x when using 3x layers of element inside the Kiln, again on the extreme side, it will be fixed with a thicker wire;
• I also built the controller myself, which I can share if people are curious.

With the introduction out of the way, I did a test fire on the kiln yesterday, aiming for Cone 05 1/2 -> 1015C or 1839F, as a test, since my local ceramic shop only had this cone in stock and I would like to validate against the thermocouple, type K, readings.

• The Kiln had a bottom shelf, raised 1/2" from the floor;
• the self supporting cone was placed in the middle.
• The ambient temperature was -4C (25F)

After 10hrs , it was late and the Kiln was really struggling to raise the temperature more than 870C (1600F), the temperature was going up, but Very Slowly as seen on the graph below. Therefore I concluded that it would take few more hour to reach the final temperature (if it was able to do so).

The firing schedule, not very relevant I guess, was:

• 120C -> 110C/hr | 250F -> 120F/hr
• 538C -> 250C/hr | 1000F -> 400F/hr
• 120C -> 167C/hr | 1659F -> 300F/hr
• 1015C -> 60C/hr | 1839F -> 108F/hr

Finally, my questions are:

1. Does someone have a 2300W Kiln and can share some experiences with the firing Cycle?
2. Do you see any absurd flaw that would justify the behavior?

Some pictures:

 

 

[Bill Kielb]

Just some quick early thoughts-
I guess my first impression is you are not producing enough heat to offset your losses to make top temperature, so why is that? Not enough wattage? Excessive heat loss through the bricks for the given wattage?  Not enough surface area of elements? Hmmm, does your controller cycle or at the very end keep the elements at full power?

So maybe a question or two and observation

• Can your controller drive the elements continuously on when needed?
• Have you measured the voltage and amperage in operation to confirm the wattage?
• The space between the barrel and brick seems an ideal place to add insulation

My first impression would be to insulate and increase element area in the kiln not necessarily change the wattage. As you test though you should be able to find a balance between reasonable insulation and electric needed. All assuming the controller is a non issue though.

[lbispo]

12 minutes ago, Bill Kielb said:

Can your controller drive the elements continuously on when needed?

It can drive it when needed, I constantly measured the current during the whole firing, and the graph can be found below (green line), please ignore the high spikes;

12 minutes ago, Bill Kielb said:

Have you measured the voltage and amperage in operation to confirm the wattage?

Yes, It has been measure during the whole firing, 215V@9.6A = 2064W -> I was using a long extension cord which accounts for the 15V loss, I will try again tomorrow with a better cable;

PS. I have an energy meter on my controller, but i also measure the current with the Clamp-meter, Multi-meter, in order to validate the readings;

12 minutes ago, Bill Kielb said:

The space between the barrel and brick seems an ideal place to add insulation

I have added ~7mm(1/3") ceramic insulation blanket there:

[lbispo]

Additionally, I also have the attached thermal image, where the outside of the barrel is 60C (140F) when the inside is 620C (1148F)

[Sorcery]

Needs a lid!

4 hours ago, lbispo said:

Do you see any absurd flaw that would justify the behavior?

Impatience?

What happened? You shut it down?

Up slowly is up. Slowly is good!

Sorce

 

[lbispo]

7 minutes ago, Sorcery said:

Needs a lid!

it has a lid, made with the same brick, 65mm (2 1/2") thick.

 

It was 3am and I didn't want to run the first test unattended, so I turned it off, after 8 hours.

[Bill Kielb]

5 hours ago, lbispo said:

Additionally, I also have the attached thermal image, where the outside of the barrel is 60C (140F) when the inside is 620C (1148F)

So great answers, it’s down to power to loss ratio. I have attached an L&L document relating heat loss, power, temperature here: https://hotkilns.com/sites/default/files/pdf/BTUS-Easy-Fire.pdf

This may prove helpful (3.41 btu. Per watt which you likely know already)  so the key observations I take from these is generally kilns designed with three inch brick require about  2-3 times more power than their top end cone 6 losses to successfully fire a load for their given volume to cone six. Generally these kilns are initially designed new at about 110% capacity meaning if they lose 10% of their heating ability they require an element change else they cannot make cone six in an acceptable timeframe. This  generally means their heating rate at the top end of heating slows down to  say approximately  20f - 30f degrees per hour for the last 200-250f (100-120c) of the firing. At this speed generally things begin to overfire and or the controller simply shuts down with a firing rate error. It’s important to point out because it still is a positive rate, just not enough to successfully fire to cone.

Based on these numbers you may find your kiln is significantly under powered for its volume.

L&L provides the kiln wall sf figures as well so that may prove more useful to you. I have found this to be pretty handy for approximating removal of  heat as well. Anyway for similar kiln construction, the apparent easiest way I see to improve performance would be to reduce losses which to me says inches of fiber insulation would be fairly easy in your design. 1/3” is likely little benefit.

Finally, your infrared is odd as most kilns I scan lose far more heat out the top than the sides so I am unsure what that picture reveals but 40c to me might be significant and unexpected.

Finally looking at your controller output, the slope of the line before the top end anomaly (about the 1d mark) is about the same as after so having this bit of controller issue (localized  undershoot) at this point definitely affected the firing. I assume you will tune this out.

@lbispo
Last  add 15v drop  is very high especially if you are marginally powered to begin with and as it heats it will only increase.

Hope that gives you some ideas.

[lbispo]

6 minutes ago, Bill Kielb said:

So great answers, it’s down to power to loss ratio. I have attached an L&L document relating heat loss, power, temperature here: https://hotkilns.com/sites/default/files/pdf/BTUS-Easy-Fire.pdf

Thanks for sharing, the losses values are reassuring and it aligns with my calculations from EngineeringToolbox which gave be 645Btu/h@600C(1112F).

• My internal area is 0.414m2 (640.15 inch2) @ 2300W = 3.6W/inch2 -> similar ish to e18S

20 minutes ago, Bill Kielb said:

Based on these numbers you may find your kiln is significantly under powered for its volume.

I'm not sure if I fully understand the rationale, as the W/inch2 seems to be aligned with the table you sent and originally, I based my choices on KMT-614 and Ecotop 20 both with ~20L (0.7ft3)@2300W

 

22 minutes ago, Bill Kielb said:

I see to improve performance would be to reduce losses which to me says inches of fiber insulation would be fairly easy in your design.

Agreed, I also consider a 400V@8A circuit which is fairly available on my current installation, but that would require a new element. I also already have the ceramic blanket, so the first option may prove to be easiest.

 

24 minutes ago, Bill Kielb said:

Finally, your infrared is odd as most kilns I scan lose far more heat out the top than the sides so I am unsure what that picture reveals but 40c to me might be significant and unexpected.

On top of the cover i have 1/2" of refractory mortar, maybe this is keeping the heat inside.

 

Thanks for the inputs, and if you have more comments, keep it coming.

 

 

[Bill Kielb]

9 minutes ago, lbispo said:

keep it coming

Sorry, I did add a couple points on the controller, slope of the line  and the large voltage drop above. In theory it’s about 6% of a design that only has 10% excess capacity.

[neilestrick]

3 hours ago, lbispo said:

My internal area is 0.414m2 (640.15 inch2) @ 2300W = 3.6W/inch2 -> similar ish to e18S

An e18S pulls 24 amps in order for it to get to cone 10. Kilns that size that only go to cone 6 pull about 20-21 amps. If yours is pulling 9.6 amps, it's very underpowered.

Another thing to consider is that K28 bricks do not insulate as well as the K23 bricks that most commercial electric kilns are built from. K28 are heavier and more dense, so they're absorbing more of the heat you put into the kiln than they would in a typical kiln, and you're going to need even more power to compensate for that. HERE is a chart that shows the difference between the bricks of different ratings. This is one specific brand, but will be similar to all brands.

[lbispo]

2 minutes ago, neilestrick said:

cone 6 pull about 20-21 amps. If yours is pulling 9.6 amps, it's very underpowered.

That is an inaccurate statement, please note that I'm in Europe and as mentioned, 9.6Amps at 230V which is ~2300W... The same as 20Amps at 115V.

[neilestrick]

2 minutes ago, lbispo said:

That is an inaccurate statement, please note that I'm in Europe and as mentioned, 9.6Amps at 230V which is ~2300W... The same as 20Amps at 115V.

Based on your comparison to an e18S it is not inaccurate, as the e18S size kilns pull about 24 amps, or 5,000-6,000 watts depending on wiring configuration. However, in re-reading your measurements, it's not as big as an e18S, which is about 2.3 cubic feet. I see the measurement you posted in the your initial description is 0.7 cu/ft, in which 2,300 watts should be about enough, assuming it's insulated as well as the commercial kilns that run on the same wattage, which I think it's not. I think that with the K28 bricks you've got too much mass to heat up. Can you add an element to the top ring, or are you limited to the current amperage draw? How about removing the top ring and seeing if it can get to temp then? Also, any little gaps in the bricks will affect the insulating ability, so try to get them tightened up with tight seams along every joint.

[Bill Kielb]

2 hours ago, lbispo said:

That is an inaccurate statement, please note that I'm in Europe and as mentioned, 9.6Amps at 230V which is ~2300W... The same as 20Amps at 115V.

I think that the electrical statement from Neil was just an over site or confusion in models etc... 2300 watts is 2300 watts. His point about the bricks is super relevant. The difference in thermal conductivity between K28 and K23 is significant at all temps but on the order of 50% more conductive at 800c.

So L&L’s measurements of known performing kilns  using k23 is likely spot on and your design should have significantly more losses because of the k28 brick.

So voltage drop, additional shell losses,  and the controller undershoot likely make this a poor performer at top temp. Fortunately fiber around  the outside is easy to  try and now you can calculate the minimum amount of fiber necessary with some certainty.

The joints are also fairly relevant so a perfect spot for radiant reflection then insulation.

[lbispo]

8 minutes ago, neilestrick said:

Can you add an element to the top ring, or are you limited to the current amperage draw?

Thanks for the inputs, I really appreciate it. I'm not limited to the current amperage, but since it was my first run, I was wondering what were the experiences from other using 2300W elements. I have more Kanthal wire and can make a bottom element, or top. I also have 3" top layer that can be removed, making the overall volume smaller to 16.3L (0.57 ft3)

13 minutes ago, Bill Kielb said:

The difference in thermal conductivity between K28 and K23 is significant at all temps but on the order of 50% more conductive at 800c.

That is indeed a valid point, that I didn't pay much attention, however it was hard to find those bricks in my area and from other countries the freight becomes crazy. My supplier only mention the thermal conductivity (not Kx, i assumed it). It is 0.25 W/mk @ 350C

18 minutes ago, Bill Kielb said:

controller undershoot

Good catch, but that hiccup on the temperature, it was because I changed the micro-controller code so it would alarm if I fall asleep, it was late.

--

For the record, I'm running the kiln now again, because I'm stubborn, red is temperature, it took 7hrs to reach 900C (1652F). Tomorrow I will wrap the whole thing with 1/2" ceramic blanket, accordingly to the calculation it should drop the losses to e18S levels.

[Bill Kielb]

3 minutes ago, lbispo said:

Tomorrow I will wrap the whole thing with 1/2" ceramic blanket, accordingly to the calculation it should drop the losses to e18S levels.

 

If you have the insulation I would not be afraid of having the most efficient kiln in the neighborhood. 1” really is not all that much in my mind. 

[lbispo]

Ops... the blanked is 1" (25mm)... Got confused with the units thing. I probably have enough to do 2x... I can remove the top 3" of the kiln and only fire cup coasters

If relevant, my calculations are: (probably not right)

• Area = 0.413m2
• internal temp = 900C
• external temp = 160C
• Brick thermal conductivity = 0.31
• Loss = 1467btu/h (or W)

With 25mm blanket

• Blanket thermal conductivity is 0.16 @ 800C
• Loss = 838btu/h (or W)

It could be better.

[neilestrick]

15 minutes ago, lbispo said:

I can remove the top 3" of the kiln and only fire cup coasters

Maybe there's a big market for coasters?  I was just thinking as a test to see if it can get to temp if it's smaller. It may help you figure out if just another element is needed, or if you need to change out the existing elements.

The fiber will help with heat loss, but those bricks are also more dense than K23 so you've got more mass to heat up in the first place.

Welcome to the forum, BTW. 

[Bill Kielb]

1 hour ago, lbispo said:

For the record, I'm running the kiln now again, because I'm stubborn, red is temperature, it took 7hrs to reach 900C (1652F).

For the record me too!   I think you will have success, fixing the extension cord alone has to improve things by at least 3% if not much more. Let us know your results it’s rare to confirm theory and practical results so very interesting and very educational.

[lbispo]

Thanks for the warm welcome.

I'm pleased to share that the theory proved to be right. I lousily wrapped the thing with 25mm (1") blanket and fired up to 981degC (1800F), I cranked the temp rate at the beginning but at the end it was 45.7 C/hr.

• Cones: 012, 06 and 05 1/2

Today I will put some load and bisque with a 12hrs schedule.

Have a good weekend

 

[Bill Kielb]

Nice work! A win for science. So next random thought you are likely performing near that 110% kiln design which  I spoke of, but that was a K23 density design that achieves 110% reasonably loaded with wares and you have some extra mass with the K28’s that I think Neil mentioned.. My next thought is you may still need to improve the performance a bit due to the extra mass of the K28’s. Your  kiln may perform empty like others perform somewhat filled.  May being the operative word ............. but you are however free to make this as efficient as you like with solid tested theory to help guide you.

For now, I hope it works well and the firing is even, if not then better / more radiators not necessarily more wattage which I think helps with your element loading..  Just another thought to consider which may help -  most of the energy transference after red heat (late in the firing) is radiant, followed by conduction, then very litlle by convection.

Really good work IMO staying focused! 5-6 hrs! I guess that insulation stuff really works, thanks for sharing.

[lbispo]

Thanks for the comments... I believe now it starts the dilemma, as it is feasible to improve it, I may as well make it larger ... never ending project

[neilestrick]

Definitely an improvement! If you plan to glaze fire to cone 5/6, at bisque temps the kiln should be able to maintain a rate of 350F/175C/hr. Have you tried putting it full speed to see what it can do, or was this firing already there?

Is the metal drum inside the blanket? You're going to degrade the metal quickly if so. It would be better to have all the insulation inside the drum. Or do away with the drum and put metal banding around the bricks and then wrap the blanket and cover it with wire mesh or sheet metal to protect it. Also, if that's a wood floor under the kiln, put a couple layers of cement board under the stand for fire safety.

Keep us posted as you continue to make improvements.

[lbispo]

1 hour ago, neilestrick said:

If you plan to glaze fire to cone 5/6, at bisque temps the kiln should be able to maintain a rate of 350F/175C/hr.

This is good information... I have not put it in full power to see what it can do. I was running bisque at 60C/hr.... It was cycling the relay very seldom, therefore I do not believe it would coupe with 175C/hr

1 hour ago, neilestrick said:

Is the metal drum inside the blanket? You're going to degrade the metal quickly if so. It would be better to have all the insulation inside the drum. Or do away with the drum and put metal banding around the bricks and then wrap the blanket and cover it with wire mesh or sheet metal to protect it. Also, if that's a wood floor under the kiln, put a couple layers of cement board under the stand for fire safety.

Yes, the metal drum is inside the blanket, I do not like it, I'm thinking about ways around it... My cat also is planning to have a go with the blanket, that will be a mess and upsetting, so I will need to do something about it. The concrete brick is there.

 

I just bisque fired with ~4/5kg of stuff, here is how it looks:

4 segments, this is the rate measured: 71C/hr, 210C/hr, 127C/hr, 60C/hr

[lbispo]

24 minutes ago, lbispo said:

I just bisque fired with ~4/5kg of stuff, here is how it looks:

4 segments, this is the rate measured: 71C/hr, 210C/hr, 127C/hr, 60C/hr

12.2kWh of energy consumption

[Bill Kielb]

24 minutes ago, lbispo said:

I just bisque fired with ~4/5kg of stuff, here is how it looks:

Really nice! Seems to have influenced the firing some