Thursday, June 18, 2009

Element Design --- The End

I just thought I would try to end this thread by not going mathematical, but by speaking about the parameters that you need to be able to keep in mind when you are designing elements for a kiln. Anyone who is interested in how we did Papa Bear just let me know and I will do a separate post on that.

At any rate this will just be a review and crystallization of what we have done so far.

The most important variable is what is called watt loading, or the amount of watts per square inch that the elements have to generate to fire the kiln. Larger diameter wire allows for lower watt loading figures, but you have to be able to fit the wire into the kiln, and larger diameter wire also has less resistance per linear foot -- it's easier for the electrons to pass through a larger wire. So as you make the diameter of the wire larger, you need more of it to achieve the resistance that you need, and it may be so much wire that it won't be able to fit. We found that 12 gauge wire worked well for us, and that Kanthal A1 was the alloy that is the most economical and useful at the same time. Kanthal also makes an APM element but the raw wire cost is almost 5 times as much. It is special wire and I feel like it is not worth the extra cost as long as you have low watt loading. I think the watt loading for Papa Bear is around 12 watts per square inch. If we had decided to use 15 or 16 gauge wire, the APMs might have been a good choice, as they can go to a higher temperature.

We also have to take into consideration the diameter of the element. The diameter should be, according to Kanthal, about 5 to 6 times the diameter of the wire. Once we wind the element so that all the wire is touching we need to be able to pull the element to just over twice its wound length. The distance from the top of one wire to the top of the next wire is called the pitch and we want it to be just over 2 times the diameter of the wire. So we should be able to put a piece of element wire of the same size in between the stretched coils and move it back and forth a little.

We need to know how large our kiln is and how many kilowatts it will take to fire it. The Robert Fournier book has this information.

And we must keep in mind what happens when elements and banks of elements are wired in series and parallel.

Here is Galen Olmstead changing the element holders so that Papa Bear can accept the larger elements. We had to grind the element holders on the lap wheel so that we will be able to easily install the elements. Galen is beginning graduate school at UF this fall and he makes some wonderful sculptural objects.


Thursday, June 11, 2009

Element Design --- Answers to Questions -- Part 5

I was thinking that this thread has gotten out of hand by being so technical --- and no pictures --- which is why I shortened it but here are the answers to Dan and Alex's questions.

When I got Papa Bear I had discussed my needs with L&L and they sent this large 29 inch diameter kiln with 18 gauge elements, and the elements had almost no stretch to them. The kiln fired fine for a few firings and then it would not get to temperature quickly enough. I will mention that zinc crystalline glazes are quite caustic to elements and kilns so this was a new problem for them and that they have been really open to learning and overcoming the problems that we have. But anyway the kiln was underpowered and I just started using it for earthenware and bisque. It now will not even do that so something had to be done. The kiln was rated for less than 40 amps and that was not enough to make it useful.

I did not want to learn about element design because it hurts to try to keep all that stuff in memory at one time. But I have this kiln that is totally useless, and I wanted to bring it up to useful.

My smallest kiln, Baby Bear, has 137 firings on one set of elements so I know that there is a way to design elements so that they last a long time, but I don't know how to do it. I'll get a picture today of the elements of Baby Bear. There are twice as many element holders in Baby Bear as a normal kiln.

These new elements for Papa Bear, 12 gauge Kanthal A1 wrapped around a 3/8 inch mandrel and 8 ohms cost 72.00 each and the kiln needs 8 of them so a good part of this is wanting elements that last a long time. But it is also important for the kiln to be nimble -- able to reach temperatures quickly (both up and down) -- say at least 200F/hr at the end. Eventually this figure will not be able to be reached and we will have to replace the elements, but we want to be able to use them for as long as possible as 576.00 is 576.00.

By the way, if you have a newer Bartlett controller and you press 5 while the kiln is firing, it will tell you how fast you are going. Mini tidbit but impressive for onlookers.

It is true that my work requires more precision than most, but the principles apply to anyone who fires to cone 9 or 10 --- your elements will wear out at some point. Ray Gonzalez, who is the tech person at UF, says that kilns fired to cone 6 and below rarely need element changes.

One thing that is a good question is whether the kiln manufacturers should design their kilns for us. I say no, it is not their responsibility. They have to compete with one another and they have to make the best product that they can within those parameters, but they make "hobby kilns" and we have to adapt them to our purposes.

It's true that the L&Ls are made like a tank. If you stacked boxes of clay on top of an L&L stand, you would be able to go until the stack fell over. That stand could probably hold 500 pounds or more. Every other kiln that I have had has had a less robust stand, and I have had a few. Not that the stand does not still work, but I did get a feeling of confidence the first time I held an L&L stand.

Tonight I will go over the math stuff again but I am the cook today for the community and it basically takes all day to shop and cook. I cook every other Thursday. Yesterday was Anne's birthday and she requested black bean soup with amarillos and tostones and salad so that is what it is. Maybe a photo of the whole meal tonight if I can remember.


Wednesday, June 10, 2009

Element Design --- Our Final Idea --- Part 4

Well those elements are ordered and we now just have to wait for them to come. I sent the controller box down to Terry so he could install the Solid State Relays. These relays cycle on and off every 200 milliseconds so the elements do not get that much hotter than the kiln.

Here is what we decided to do. There are 8 elements in all in the kiln, three in each side section and two in the floor. We divided up the kiln into 4 sections of 2 elements each. This is quite different from how the kiln was originally wired, which was a 3, 3, 2 configuration.

Each of these pairs of elements will be wired in series within the pair, and all 4 pairs will be wired in parallel to each other. The elements will be 8 ohms each so each section will be have 16 ohms of resistance and 15 amps of current, and all 4 sections will generate 60 amps.

The wire is going to be 12 gauge Kanthal A1 wound around a 3/8 inch mandrel. This gives us a very good length to the element -- when it is stretched the pitch is just about perfect. And the watt loading figure is very low so these guys should last a long time.

Let me know if I need to be more specific about any of this -- how we did it, why we did particular things.......


Little Lemma Dilemma

Thank you Lee for the nice pun.

In my "Little Lemma" post near the bottom I mention about how my thinking had changed and now I have to admit, that was from right to wrong. I got out the Fournier book and saw that to get the distance it takes for a wire to wrap around a mandrel, you add 2 radiuses to the diameter of the mandrel, not two diameters, as I had. Sorry for this.

After thinking about it so many ways, and consulting my friend Terry Fallon, we have finally decided on the elements for the kiln.


Friday, June 5, 2009

Little Lemma

In one of my past lives, I worked towards a PhD in Mathematics, and could not resist throwing in a math term. Never did finish it but I have a Master's Thesis around here that is unintelligible to me now. Did I actually understand that?

A lemma is a small theorem which is used to prove a larger theorem. 

Suppose that we are going to wrap an 11 gauge wire around a 3/8 inch mandrel. How much wire do we need to go around once?

OK. What do we know? We know that 3/8 inch is .375 inches and that the wire diameter is .091 inches. We also know that the circumference of a circle C = πd where d is the diameter of the circle and π is 3.14. 

So what is this diameter? It has to take into account the diameter of the wire. It certainly looks to me like that diameter is .375 + .091 + .091= .557 and that the circumference is .557π = 1.75 inches. That is, it takes 1.75 inches of 11 gauge wire to wrap around a 3/8 inch rod one time. 

Here is my thinking. It has changed since I started writing this so I might be wrong. 1.75 inches is the circumference of the rod plus two wire diameters so it is the outside circumference of the wire wrapped around the rod. 

If we measure around the outside of the wire we get our answer. And anything less is not going to meet up.


Thursday, June 4, 2009

Element Design --- Wire Size and Pitch --- Part 3

This has been a very interesting time as I am trying to understand what makes a great element and, of course, get great elements in my kilns. The whole thing has been quite time consuming, but I feel like if I understand how to do this, I can maximize the life of my elements and the usability of my kilns. 

Here are my thoughts how to design elements for Papa Bear, which is a 29 inch diameter L and L kiln with 2 sections. There are 3 elements in each of the sections and 2 elements in the floor, so there are 8 elements in all. Since we want to have 60 amps, which will give us 14,400 watts, we divide the kiln into 3 different sections of 20 amps each. Each of these sections will be wired in parallel to each other. 

But within each section the elements will be wired in series. So the top section will have 3 elements wired in series, the bottom section will have 3 elements wired in series, and the bottom of the kiln will have 2 elements wired in series. That makes 8 elements in all. 

Let's look at the top section. We have 240 volts, we want 20 amps, so we need 12 ohms. (240/20 = 12) Since we have 3 elements in series each element needs to be 4 ohms. 

Now there are a lot of ways to get an element of 4 ohms depending on the diameter and type of wire, but I have decided that I am going to use either 11 gauge or 12 gauge Kanthal A1 wire. 11 gauge is thicker (.091 inches) than 12 gauge (.081 inches) and I would rather use 11 gauge than 12 gauge because it will last longer. 11 gauge wire also has less resistance than 12 gauge wire --- .106 ohms/foot as opposed to .134 ohms/foot. It's easier for the electrons to travel through a thick wire. 

However I have been told that 11 gauge wire is very difficult to work with because of the strength needed to wind the element, and also to install the wound element in the kiln. I don't have experience with this, but I do like the idea of using 11 gauge. 

Let's calculate the length of the wire needed for 4 ohms. 11 gauge is 4/.106 = 37.74 feet of wire and 12 gauge is 4/.134 = 29.85 feet of wire. 

Now we have to wind these wires on a mandrel and there are a couple of things we want to keep in mind, these courtesy of Kanthal.$file/1-A-5B-3%20UK%20resistance%20alloys.pdf?OpenElement

First of all we want the diameter of the element itself to be between 5 and 6 times the diameter of the element wire. This is so the heat can get out of the back of the element, and also so the element does not fall over when it is hot -- if the diameter is too big, the element won't be able to stand up when it is soft. 

Secondly, we would like to be able to stretch that element so that we have close to 1.25 element diameters between each pair of adjacent coils. We want to be able to take a piece of wire that the element is made of, insert it in the coils, and be able to wiggle it around a little. This is so the heat from the back of the element can make it out. Kanthal defines the "pitch" of the element to be the distance between the top of one coil and the top of the next coil and they specify that the pitch should be between 2 and 3. Our pitch above was 2.25. We don't want the elements to be stretched too much so we want to keep the pitch below 3 for an ideal element. 

When Papa Bear arrived the elements were of probably 17 or 18 gauge (They measure .040 inches) and the pitch was approaching zero. Just an unstretched element laying in the holders. I was only able to get a few cone 10 firings from them and now they will not even go to bisque. 

Next -- the winding of the elements.