kiln element proximity to insulation

Greg G · Post by **Greg G** » Mon Dec 08, 2003 10:09 pm

Can someone advise me on the distance I should keep my elements from my insulation. I'm building a top hat, fibre kiln with the elements on the top only. :?:

Brock · Post by **Brock** » Mon Dec 08, 2003 10:12 pm

They can touch it if you want, it won't make any difference. Brock

Greg G · Post by **Greg G** » Mon Dec 08, 2003 10:18 pm

Thanks Brock, but won't that make a hot spot and degrade the element at that point?
Greg

Brock · Post by **Brock** » Mon Dec 08, 2003 10:26 pm

Not in my experience. I had to add some insulation to the roof of one of my kilns, and the element has been in contact with the ceri-wool for years. Now, two of them are like that. No problem. Anyway, I've never understood what you just proposed. The element is the hottest thing in the kiln, how can it get hotter by touching insulation? Mr. Science

Tony Smith · Post by **Tony Smith** » Mon Dec 08, 2003 10:34 pm

Greg G wrote:Thanks Brock, but won't that make a hot spot and degrade the element at that point?
Greg

Whether the elements are in a groove in a brick kiln, wound around mullite or quartz tubes, or pulled tight against fiber insulation, they are always in contact with some sort of insulation (except in Bert's kiln where they hang from stainless steel hooks) and they don't seem to be affected by it. I'm with Brock on this one... it won't make a difference.

Tony

Brock · Post by **Brock** » Mon Dec 08, 2003 10:37 pm

Wow, Tony and Jim both agreed with me! Is there a full moon? Vlad

Tony Smith · Post by **Tony Smith** » Mon Dec 08, 2003 10:45 pm

Tony Smith · Post by **Tony Smith** » Mon Dec 08, 2003 10:47 pm

... and I'm not sure that Vlad Dracula ever really cared about the full moon... he was more into making sure that others got his point... regardless of what it was

Brock · Post by **Brock** » Mon Dec 08, 2003 10:48 pm

Ah. So, there's something to that then . . . Brock

Tony Smith · Post by **Tony Smith** » Mon Dec 08, 2003 11:07 pm

There's one story that is well documented of some Turkish officials who visited Vlad Dracul and refused to remove their hats in his presence, so he had their hats nailed to their heads before he sent them on their way claiming that if they wouldn't remove their hats for him, then they would never remove their hats again.... ouch!!!!

Bert Weiss · Post by **Bert Weiss** » Mon Dec 08, 2003 11:38 pm

The elements need to "breathe" When I built my first kiln I simply ran the elements through the insulation and they promptly burned out. They need air circulating around them.

Touching on one surface doesn't seem to matter.

Tony Smith · Post by **Tony Smith** » Tue Dec 09, 2003 7:20 am

Bert Weiss wrote:The elements need to "breathe" When I built my first kiln I simply ran the elements through the insulation and they promptly burned out. They need air circulating around them.

Touching on one surface doesn't seem to matter.

That makes sense. When the element heats up, it achieves an equilibrium temperature... electricity in - heat out. It needs to be able to radiate or conduct or convect heat away or it continues to heat up until it reaches the melting point. When the element is completely surrounded with insulation, the heat has no place to go, so you end up with a mini China Syndrome and the element melts down.

Tony

Phil Hoppes · Post by **Phil Hoppes** » Tue Dec 09, 2003 9:12 am

Hmm....I don't understand that one at all. My 24" Paragon has the top elements embedded in insulation. I mean these things are completely inside the fiber insualtion with only one surface, that being the top surface, exposed. I've had lots of firings with no problem. I have to believe that Paragon would have done lots of testing on this before offering it as an option.

Mr. Question

Phil Hoppes · Post by **Phil Hoppes** » Tue Dec 09, 2003 9:24 am

To your point Tony. If it is covered in insulation, the insulation simply heats up and radiates at a slower rate than that portion exposed to air. I can see where the presence or absence of air could make a difference in the rate of oxidation (if any even occurs) of the surface of the NiChrome wire as it would tend to impeed this process but I'm not understand the point of melt down. To Brock's point. You have current going through the wire. It is going to get as hot as it is going to get regardless of the amount of insulation around it. The wire has no "sense" of insulation or not. The immediate surroundings of the element cannot be hotter than the element itself. Correct me if I'm wrong here but I believe you would have to invalidate one of the basic laws of thermodynamic's to have that happen. The temperature from the element should follow a classic radiation curve, the appearance of which, will be time varying as edicted by the medium (air, insulation or insulations). Assuming you had an infinite heat source, entropy says, at equilbrium everything will be the same temperature. Sorry, but I need a better explaination than what has been said here. It's not making sence to me.

Phil

Bert Weiss · Post by **Bert Weiss** » Tue Dec 09, 2003 9:53 am

Phil Hoppes wrote:To your point Tony. If it is covered in insulation, the insulation simply heats up and radiates at a slower rate than that portion exposed to air. I can see where the presence or absence of air could make a difference in the rate of oxidation (if any even occurs) of the surface of the NiChrome wire as it would tend to impeed this process but I'm not understand the point of melt down. To Brock's point. You have current going through the wire. It is going to get as hot as it is going to get regardless of the amount of insulation around it. The wire has no "sense" of insulation or not. The immediate surroundings of the element cannot be hotter than the element itself. Correct me if I'm wrong here but I believe you would have to invalidate one of the basic laws of thermodynamic's to have that happen. The temperature from the element should follow a classic radiation curve, the appearance of which, will be time varying as edicted by the medium (air, insulation or insulations). Assuming you had an infinite heat source, entropy says, at equilbrium everything will be the same temperature. Sorry, but I need a better explaination than what has been said here. It's not making sence to me.

Phil

Phil

I am aware of the insulation/element system that you describe where the element is embedded. I think this requires a particular material embedding the element and maybe even a particular element metal. Kanthal has different oxidizing qualities than nichrome.

While I can't provide the scientific explanation, I do know that my element fried quickly when embedded in HD Board and mineral wool. It was obviously from getting too hot. I now thread my elements through some kind of kiln furniture or the tubes from knob and tube wiring.

Phil Hoppes · Post by **Phil Hoppes** » Tue Dec 09, 2003 10:17 am

Bert,

I understand. What happened to your elements obviously...happened. What I'd like to understand is the real reason why. Material types both in elements and insulation probably have everything to do with the answer. You've had a case where you can't embbed the element in insualtion as it fries it. I've got a case where it appears to work fine. I'm just trying to be careful as to the advice on the board that others read as in this case we clearly have two very different things that are going on here and for lack of a scientific explaination to explain the root reasons, we are both right wrt an answer to the original question. You mention Kanthal, I'm not familiar with that material, is that what you are using for your heating elements? The kiln I'm building (see Photo's area under Jackie's post) I'm using nichrome elements both in quartz tubes for the top and mullite rods with doughnut supports for the sides. On the sides, my elements are in contact with the insulation on the side walls but I know it's not a problem. Joppa's been selling these elements longer than I think I've been alive without people having problems so I don't think, for nichrome and fiber insulation, touching at least on one side is a problem.

Phil

Ron Coleman · Post by **Ron Coleman** » Tue Dec 09, 2003 10:42 am

With any heating element design, the critical factor in element longevity is watt density. The higher the watt density, the higher the element temperature. When your design is near the top end of the watt density scale, the ability for the element to give up heat becomes critical in how long it will live.

Ron

Phil Hoppes · Post by **Phil Hoppes** » Tue Dec 09, 2003 11:31 am

Ron,

That makes sense. Also I was thinking about it and the insulation/no insulation around the element probably has more to do with longevity when the element is actually off rather than on. In the off state, the immediate envrionment around the element will be cooler or hotter depending on the material around the element. In the case of air, it will be cooler and will allow the element itself to cool. In the case of an insulator, it will be hotter. I suspect in Bert's case, the kanthal remained hotter and it reduced it's life.

From a mechanical engineering standpoint this is an interesting problem. At first glance, I might be inclined to think that repetitive cooling and heating around the element might be harder on it mechanically due to the thermal cycling as opposed to simply staying hot. That being said, all failures are mechanically related and have an activation energy that is directly porportional to heat so I could see where staying hot longer could also make it fail. I'm sure it is related to the mechanical properties of the element material, which, in turn, relates to what you were saying Ron about the energy density within the element itself.

Phil

Brock · Post by **Brock** » Tue Dec 09, 2003 12:02 pm

The aforementioned elements in my kilns are Kanthal, and have been for over 20 years. One broke from moving the kilns, and a few have burned out at the 2-bolt connector between the element and the hi-temp wire to the stove switch. Otherwise, no problems touching insulation. Brock

Tony Smith · Post by **Tony Smith** » Tue Dec 09, 2003 12:05 pm

Phil,

I think we are talking about design issues here. Elements are typically rated for an operating temperature in free air. This operating temperature depends on free air convection to take some of the heat away from the element. When you remove the free air convection, the temperature of the element goes up. If you replace free air convection with conduction, as in the case of the refractory cement that Paragon uses, you bring the temperature back down again. In order to fully understand the situation, you need to know the thermal conductivity of each of the materials (air vs refractory cement vs ceramic fiber). If you remove the free air convection and insulate the element, the temperature of the element will rise until it reaches an equilibrium with the conduction to the air and insulation in contact with the element as well as some radiative coupling to the insulation. If that equilibrium temperature is higher than the melting temperature of your element, then you "burn out" your element.

Tony