Every owner/operator of a kiln knows one thing about elements: They are the things that are very difficult to change and fail at the most inopportune time. Their design and theory of operation are however not generally understood. This article is intended to present the basics of element design and answer some of the basic questions of elements such as, “Why do I have to buy a different element for 208 volt kilns than 240 volt kilns?” “Why is the size of that element different from the one in my other kiln?” and “What makes elements last longer?”
We will begin with an overview of heat generation and kiln design. Some basic math equations are required to sufficiently explain the basic concepts of physics that apply to kiln design. Then we will turn to the specifics of element design including mandrel size and wire gauge. Lastly, the question of why elements wear out and melt will be discussed.
An element’s place in kiln design
A kiln is box that gets hot in order to make other items hot. Heat is required to make the kiln hot. Heat is generated by turning electric energy into thermal energy. Elements are the means to that end. The size of the kiln, the type of insulation, and the desired operating temperature determine the amount of thermal energy required to obtain the temperature. Therefore a larger kiln (10 cubic feet) requires more heat to obtain the same temperature than a smaller kiln (6 cubic feet), and more heat is required to go to cone 6 than cone 04. Heat generated by the elements is measured in watts. Kiln wattage is calculated by multiplying the voltage by the amperage:
Wattage = Voltage x Amperage
(All kiln manufacturers have this data located somewhere on the switch box.) For example, a kiln rated at 240 volts and 45 amps would have 10,800 watts (45 x 240=10,800). If the elements actually cycled on for five hours during a firing, the kiln would use 54,000 watt hours (10,800 watts x 5 hours) or 54 kilowatt hours. Power companies sell in kilowatt-hours. The amperage is calculated by dividing the voltage by the ohms in the element.
Amps =Voltage / Ohms.
For example, an element of 10 ohms with 240 volts running through it would have 24 amps (240 / 10 = 24). If you combine the two equations you get:
Wattage = Voltage / Ohms x Voltage
The meaning of this equation in relation to elements is that as ohms of the elements go down, the amperage increases. Subsequently, as the amperage increases, the wattage increases. Therefore, the lower the ohms, the “hotter” the element at a set voltage. The reverse of this is also true; as ohms increase, the amperage decreases and along with lower amperage comes lower wattage. Ohms are determined by the elements.
Once the size of the kiln is determined, the required watts are calculated, and the voltage is specified, the elements can be designed since the required ohms can now be calculated using the above formulas. The next step is to design an element with the determined ohms that will have a long operating life. The first decision is the type of element material used.
Elements are made of one of three types of material: Iron Chrome (FeCrAl), Nickel Chrome (NiCr), and powdered metal (APM). Iron Chrome is the most popular because of its high temperature rating, about 2500 degrees F, its Alumina Oxide coating adheres well to the element, and its higher surface load (discussed below) enhances element life. Nickel Chrome elements are used mostly for temperatures below 2000 degrees F. APM elements use powdered metal technology to make a very durable element. The drawback to APM wire is its cost, which is about five to seven times that of Iron Chrome. For the remainder of the article I will discuss the design for coiled iron chrome elements in recessed grooves of insulated firebrick, as it is the most common design in ceramic and pottery applications.
Everyone wants elements that last a long time. The element design by the kiln manufacturer is an important first step toward long element life. There are two basic concepts of element design. First, the element must be able to dissipate heat into the firing chamber fast enough to prevent the element itself from becoming too hot and melting. Second, as the element becomes hot, it becomes very soft and loses its shape and must not be permitted to fall or lean on another element coil because of the first limitation.
Therefore in designing an element, two important parameters are monitored: the element pitch and the watt density. The element pitch is the number of wire diameters between coils. The pitch should be above 2 wire diameters i.e. there should be at least two wire diameters of distance between the coils. This permits enough distance between the coils to prevent them from sagging and touching one another and enough space for air to move around them to transfer the heat energy from the element to the air in the firing chamber.
The other design parameter is surface load or watt density measured in watts per square inch of element surface area. This should be below 20 watts per square inch for kilns that fire to 2000-2350°F. The closer the individual operates the kiln to 2350 F, the lower the watt density should be. This is because the closer the firing chamber temperature is to the temperature at the element, the less thermal energy is moved from the element. If the element becomes too hot, it melts and breaks.
Wire gauge and mandrel size
Watt density and pitch are controlled in element design by the wire gauge and the mandrel size used to wind the element. Each wire gauge has a set number of ohms per linear foot of element wire. Thicker wire has less resistance than thinner wire and hence, thicker wire has less ohms per linear foot. Therefore, 12-gauge wire has less resistance than 15-gauge wire. The logic behind this is the narrower the “pipe,” the more resistance there is to move something (electrons) through it. Therefore a 10-ohm element of 12-gauge wire is longer than a 10-ohm element of 15-gauge wire. This logic would suggest smaller gauge wire because less would be used and therefore the cost would be less. However, thicker element wire has a property that makes it a better design choice, longer element life.
Thicker wire has longer element life because it has more resistive alloy (Iron Chrome) in relation to its volume to form a new oxide coating thereby delaying element breakdown. Thicker elements are better. As in life, there is always a but. Thicker wire causes problems with pitch. Thicker wire requires more space between elements because the pitch is measured in diameters of wire.
Mandrel size is the way to control pitch. Here is how this works. Remember we already have defined the ohms the element must be. Therefore we need to use a certain length of element wire to obtain the desired number of ohms. For example, a 15-foot piece of element wire with 1 ohm per foot will have 15 ohms in total. Now we must wind this wire around a metal rod. The number of times a 15-foot wire goes around a ¼ diameter rod is more times than it will go around a ½ inch rod. More coils will be on the ¼ inch rod than on the ½ inch rod, and when the element is stretched to the same length, the coils will be closer together and have a lower pitch. Therefore, one way to mitigate the drawback to thick element wire is to increase the mandrel. But the mandrel cannot be increased beyond the size of the groove in the insulated firebrick.
Why elements wear out
If one took a cross section of an element when it was hot, one would see that the outside is an oxide coating, and the middle section is molten and soft. This is why elements sag when hot. The oxide coating is the protective layer for the element. A good oxide coating is required for long element life. Hence, firing with peepholes open is very good for the elements as oxygen is introduced into the firing chamber and replenishes the oxide coating. On the other hand, reduction firing is not good for element life as the atmosphere is deprived of oxygen. As the element fires, the oxide coating breaks or flakes off and more alloy is used to create the oxide coating. As the alloy is transformed into an oxide, the amount of material is decreased making the element diameter smaller. This is the same affect as lowering wire gauge. As the ohms/resistance increase, the wattage output of the kiln decreases. In order to produce the same heat out of an element with higher ohms, the element must have power running through it for more time. As the element runs more, it causes further breakdown of the element, and the cycle continues. Eventually the element cannot produce enough thermal energy, and the kiln will take too long to reach temperature or it will not reach temperature at all. The elements will require replacement.
Element life is related to wire gauge, watt density, pitch, and as seen above, the number of hours the elements are actually on. The number of hours elements are actually on can be controlled to an extent by overpowering a kiln. For example, a 3 cubic foot kiln which is fired to cone 06 regularly is well powered with 7,200 watts. In general, the elements will have a long life assuming the other design parameters were met. But if the kiln was made with 8,000 watts, the elements would be running for less time to reach the same cone 06 firing and therefore the element life would increase slightly. However, as the elements become overpowered, they tend to creep out of the grooves more. Also, as elements become overpowered their watt density increases as more wattage is produced by less wire, thereby increasing the watts per square inch of surface area of the wire. So movement in this direction is also limited by the characteristics of the element wire.