Kiln Element Life

[Norm Stuart]

Our kiln elements last about one year, actually 163 firings.

This consisted of:

63 ___ ^6 firings and;
100 __ firings at ^04 or less.

Most of the ^6 firings have a six hour controlled cooling between 1800 F and 1500 F.

This assumes everyone at the studio recorded their firings in our Kiln Log.

Our kiln is a Cress E23 220 Volt 36 Amp (on a 50 amp circuit and outlet).
http://products.cressmfg.com/item/ceramic-kilns/digital-electronic-...

The 8.6 kw output potential s a tremendous amount of heat for a 3.3 cu/ft kiln.

Of course the elements cycle on and off because neither the power relays nor the ceramic pieces could tolerate the kiln elements being in an on position for very long.

[Neil Estrick]

The relays can tolerate being on just fine. In fact, they are on continuously on through most of the latter part of the firing. The cycling is what kills them, as they can only switch a finite number of times before they fail. As long as they are kept cool they are essentially unaffected by being on. Some kilns do a better job of keeping them cool than others, so you'll see different relay life from one brand of kiln to another. Relays fail more often from the switch failing than from melting out.

Elements suffer the same as relays. There are two main things that affect element life- how hot they are fired, and cycling on and off. In my own semi-scientific tests, temperature has a greater affect than cycling. This is supported by the fact that glass artists firing at relatively low temperatures get many hundreds of firings out of their elements, while cone 6 potters get far fewer. Light bulbs are a good example of how cycling can affect an element. A bulb that is left on will last much longer than a bulb that is turned on and off several times a day.

[Norm Stuart]

That makes a lot of sense. We didn't change out the two power relays when we changed the worn-out heating elements, so we'll get to see how long they last.

When we first received the kiln I did one firing with a 9999 ramp to ^6 to see what it was capable of, and the relays were off only infrequently during that firing.

[Neil Estrick]

Relays are one thing you can fire until they are dead. They either work or they don't. That said, if a relay goes dead after an acceptable lifespan, then you should replace all the relays. This is because in single zone kilns all the relays fire at the same time, and in theory all have the same lifespan. So if one goes out, the others aren't far behind. I've seen two relays go out on the same day. In multi-zone kilns, where the relays are firing independently of each other, they aren't all on the same exact schedule of wear so you can often just replace the dead one. However, the difference could only be a few weeks, so I generally replace them all anyway, just to be safe. Relays are pretty cheap in the big picture, and for my customers it saves them paying for another service call a month down the road. For my personal kilns, I will usually just replace them one at a time in my small L&L (4 cu/ft), since the turnaround time is pretty quick if I have to refire due to a misfire. In my big L&L (20 cu/ft), I've never had to replace the relays, and I've been firing it for 5 years now. When one goes dead, I'll replace them all since it's been such a long lifespan. I think the longevity of those relays can be attributed to the fact that they are housed in a panel that sits about 18 inches away from the kiln, so they stay quite cool compared to those that are mounted on the kiln. I'm not looking forward to replacing them, however, since they are big 'ol honkin 50 amp relays that aren't super cheap.

Acceptable lifespans can vary, but you should be able to get at least a couple of years out of them, even with heavy use.

[Jeff Poulter]

One thing you might want to consider is going to SSR's for your kiln when your relays die(hopefully not in the on position).  They will last longer, help your elements last longer & be a little more efficient, so your electric bill might not be as high.  Their only drawback is that they do need to dissipate some heat.  You would have to put heat sinks and/or a small fan on them to get rid of the heat.  I always have a muffin fan sitting on top of the controller box whenever I am firing a kiln, just to pull the heat out & not fry any of the circuit boards.  Like Neil said, the relay is turning the elements on & off to heat the kiln at a set rate.  Basically what an SSR does is act like a dimmer switch.  Instead of cycling on & off, it just sends through what is needed to heat at that point in time.  I have already purchased SSRs for most of my kilns & will replace them as they die.  It will be a little rework of the box, since they are bigger than regular relays, with their needs of additional cooling, but I think it will be well worth the effort.  Norm, the worst thing about doing a 9999 ramp is that the elements stay on for so long & overshoots the target temp and the temp at the element goes above it's useful working limit & fries it.  The only time a program a 9999 is when I am doing a cooling segment & I want the kiln coming down as fast as it can.  jhp

[Neil Estrick]

I disagree. While it may overshoot it slightly, it's not really any worse than a normal firing. When firing to cone 6, the elements are usually on for a considerable amount of time as you approach the final set point, every bit as hot as the 9999 rate. And even if it overshoots a little bit, it's not like it'll burn out an element. You wouldn't overshoot by more than 1/2 cone, which will not do any significant amount of damage to the elements. The greater danger of firing at 9999 is that the ramp is so fast that the kiln tends to fire quite unevenly.

Jeff Poulter said:

Norm, the worst thing about doing a 9999 ramp is that the elements stay on for so long & overshoots the target temp and the temp at the element goes above it's useful working limit & fries it.  The only time a program a 9999 is when I am doing a cooling segment & I want the kiln coming down as fast as it can.  jhp

[Jeff Poulter]

Here are a couple of quotes from Terry Fallon from over on the Crystalline Glaze Foum who designs kilns & accessories:

Mr. Russel

Skutt is a good kiln and they have a good watt loading, but 64 firings on a set of APM’s is dismal. 

I have worked with some production Potters on getting better life out of their elements, both with K-A1’s and K-APM wire.
Here is what I have found.

K- APM will handle the frequent /\10 firing better than K-A1, no doubt about it.
But I have not found anyone who gets double the number of firing for using them compared to what they got with K-A1’s. When factoring the triple cost of the K-APM’s 
The K-A1’s is my prefer Element Wire. 

What can you do to get the biggest BANG for the buck?

STOP USING 9999 ON YOUR HEATING RAMPS
There is a big DISADVANTAGE to 9999 “ it cooks the elements.”

Here‘s how. The elements can run 400°f or more higher than the temperature of the kiln.
If the kiln only need to advance slowly the Elements temperature lead will be small 100°f to 200°f ahead. This is OK the elements upper temperature limit is in order of 2450°f.
But when you program in 9999 this happens. At 2000°f the elements extend the temperature lead to the maximum they can obtain, you guessed it to their maximum temperature 2450°f and above. 
If the elements would last 100 hours at 2000° they will only last 20 hours at 2450°
If it takes One hour for the kiln to reach your top temperature and you programmed in 9999 you used up Five hours of element life in just one hour.

Now if you program in a rate of rise that in the kiln’s limits say 150°f to top temperature 
It may take longer to reach top temperature, maybe one and a half hours, but the elements never made it to their upper limits this saving is big.


Never use 9999 on a heating ramp! Always stay under what the kiln can actual do. 
If you don’t hear the relays clicking you are burning up element life times 5!

For more bang use a larger wire gauge on the next set of elements.
To go all out switch to Solid State Relays.

There are some members here that use SSR’s system with larger element wire, maybe we will hear from them, on there experiences.

§terry 

Mr. Russell

Your Achilles heal with your relays, is not an isolated problem.
There is a world of people with the same problem. Most to date have gone to Mercury Displacement Contactors, because of their extended life spans.
However very soon, Mercury Displacement Contactors will be illegal to purchases, much like Freon is today.

The reliability and life expectancy of SSR’s is in years and some in decades.
No moving parts and expected too exceed 100,000,000,000 cycles compare that too 500,000 cycles for a good contactor and 250,000 for a good relay.
If you fire just 14 hours a week, you should see a relay fail within two years or less for the china c r a p 
With SSR’s you should expect the same failure every 40 years.
They have there own Achilles heal, they need a good line side power, Lighting strikes and Voltage spicks will kill them. A good set of MOV’s used on the line and on across each SSR’s use, will stop all but the direct lighting strike. 


§terry

[Neil Estrick]

Interesting. Thanks for the data. I'm going to see if I can confirm this.

[Norm Stuart]

The 9999 ramp on the Cress E-23 is considerably faster than the Cress FAST speed, it puts out a tremendous amount of heat for a 3.3 cu/ft kiln. Essentially as the wires age, the kiln still has plenty of reserve capacity.

The Cress E-23 puts out 8,600 Watts at 36 amps, similar to Jeff Poulter's Skutt which puts out 8,400 Watts at 35 amps. Our very old Cress with a kiln-sitter coughed along at 5,200 Watts. Someone asked us to take a look at their really old Cress kiln - while much larger than the size of a refrigerator due to all the insulation, actually operated on 110 Volt.

Even the FAST profile is a ramp of 570 F per hour until 200 F below set-point, then 200 F per hour until set-point.
As the Cress Manual suggests, we've never used the FAST speed for anything other than gilding.

Even choosing MEDIUM-FAST for a cone 6 firing is taking a chance, with a ramp profile of 500 F per hour until 1,100 followed by 400 F per hour until 250 below set-point, then 175 F per hour.

The new bisque profiles at 100 F per hour through the critical 1,000 F to 1,100 F are way slower than our prior Cress with kiln-sitter. That kiln-sitter was a professional bisque cracker.

Neil Estrick said:

I disagree. While it may overshoot it slightly, it's not really any worse than a normal firing. When firing to cone 6, the elements are usually on for a considerable amount of time as you approach the final set point, every bit as hot as the 9999 rate. And even if it overshoots a little bit, it's not like it'll burn out an element. You wouldn't overshoot by more than 1/2 cone, which will not do any significant amount of damage to the elements. The greater danger of firing at 9999 is that the ramp is so fast that the kiln tends to fire quite unevenly.

Jeff Poulter said:

Norm, the worst thing about doing a 9999 ramp is that the elements stay on for so long & overshoots the target temp and the temp at the element goes above it's useful working limit & fries it.  The only time a program a 9999 is when I am doing a cooling segment & I want the kiln coming down as fast as it can.  jhp

[Norm Stuart]

Now you tell me, after I had to see how fast our new kiln was at 9999 ramp.

Jeff Poulter said:

What can you do to get the biggest BANG for the buck?

STOP USING 9999 ON YOUR HEATING RAMPS
There is a big DISADVANTAGE to 9999 “ it cooks the elements.”

If the elements would last 100 hours at 2000° they will only last 20 hours at 2450°
If it takes One hour for the kiln to reach your top temperature and you programmed in 9999 you used up Five hours of element life in just one hour.

[Jeff Poulter]

OK, I lied! I have user 1 on my e18t programmed to do a 9999 ramp to 100c for when I am reheating a large pot to re-spray it. I don't hardly use it so I forgot. jhp

[Neil Estrick]

So then why don't manual kilns burn out their elements really fast? You turn it in 'High' and the elements run at full blast for several hours until the sitter shuts it off. Yet they don't burn out any faster than elements in digital kilns...

[Neil Estrick]

The max continuous operating temp for Kanthal A-1 is 2550F. Melting point is 2730F. So if I run it at 9999 to 2350, and the elements are 400 degrees hotter or more, that puts them beyond the melting point, yet they survive for another firing. Even at a slower ramp the elements will be full on for extended periods in order to get to 2350. So, respectfully, I don't buy it that the elements run that hot. Yes, they do run hotter than the interior air temp of the kiln, but according to a kiln manufacturer I spoke with, it's more like 100 degrees hotter.

Do you really think they would make it possible to burn out your elements by firing at full speed?

Yes, I believe there is truth to the idea that slowing down the firing a bit will extend the life of your elements by allowing them to cycle down. But when they cycle at high temperatures, they are off for a very short time, and on for a very long time, so I don't think it would make nearly as much of a difference as claimed. And like I said, manual kilns are on 'High' for hours without shortening the element life.