Kiln Design - An Introduction to Electric Kiln Heating by
Jeremy Willis, P. Eng.
If you are planning to build your own electric kiln, element design is one of the most important considerations. Even if you plan to buy your heating elements ready made, it is very beneficial to have an understanding of the basics of element design. This article will help to provide an introduction to electric kiln design. Areas discussed are different types of heating elements available, heat distribution within a kiln and an introduction to the design of coiled wire elements.
The most common element types are: metal sheathed elements, such as used in a residential oven; open coil wire elements, usually used for kilns for pottery and glass; heavy gauge wire and strip, used mostly for industrial heat treating applications; and silicon carbide heating rods and molybdenum disilicide elements, which are used for glass melting and high temperature ceramics.
Metal sheathed elements see limited use in low temperature applications for glass, having a temperature range of 750oF/400oC to 1600oF/870oC (temperatures given are for the heating element itself, the maximum furnace temperature will be less, by an amount depending upon the specific design). The temperature limit depends primarily on the sheath material, which can range from steel to more expensive high temperature alloys. These elements are essentially self-supporting, and have the benefit of providing protection against electrical shock if touched inadvertently. Design considerations with respect to power requirements are similar to open coil elements, but the fabrication process is much more sophisticated. The coil is of very fine wire, which is embedded in mineral oxide powder inside a metal sheath. The complete assembly is then rolled to a smaller diameter to compress the oxide and keep the coil from contacting the outer sheath. The finished assembly is then formed to the required shape.
The most common element type for pottery, as well as for glass annealing, fusing, slumping and decorating, is open coiled resistance wire. There are two primary types of resistance material that are used: nickel-chrome (NiCr) and iron-chrome-aluminum (FeCrAl). The grade of NiCr used in these applications is 80-20 (80% nickel, 20% chrome), as it has the highest temperature capability, 2190oF/1200oC. NiCr has the advantage of not deforming when it is heated, so that it requires only intermittent support. FeCrAl also comes in a variety of grades, with a highest temperature of 2550oF/1400oC (Kanthal's A-1 alloy, for example). This is the standard wire for pottery kilns. As well as a higher temperature capability, FeCrAl requires less wire to achieve the same heat as NiCr, which translates into lower element costs (approximately 30% less for the material component). FeCrAl elements must be completely supported, however, as they will deform when hot. Usually grooves are built into the walls of a furnace to fully support the element coils. Alternatively, elements can be supported inside glass tubes or on ceramic tubes through the center of the coil, which gives improved heat radiation, but can add significantly to the furnace cost. FeCrAl also becomes brittle once fired and must subsequently be handled with care. FeCrAl is magnetic; NiCr is not, which is the easiest way to tell them apart. Coiled wire elements are relatively easy to fabricate, and some people wind their own, though commercially fabricated elements are readily available.
Resistance wire is only marginally suitable for glass melting. The maximum wire temperature of FeCrAl is 2550oF/1400oC, but the maximum furnace temperature this provides is approximately 2450oF/1350oC, and a maximum melt temperature of approximately 2280oF/1250oC. This means that for typical melt temperatures the elements will be working at their top limit, which will wear them out relatively quickly. Vapours from the melt can also be corrosive to the element material. To protect elements used under these conditions there is an element conditioning process that is desirable for FeCrAl elements, which requires heating the elements, in air, above 1830oF/1000oC, and holding for several hours. This develops an aluminum oxide coating on the outer element surface, which can dramatically improve element life.
Silicon carbide (SiC) heating elements are used for higher temperature ceramics and can be used for glass melting, with a maximum element temperature of 2900oF/1600oC. These are formed out of rigid silicon carbide compositions, and are most commonly fabricated as round bars with connections at both ends, or as "bayonets" with connections at one end only. SiC elements are more expensive than resistance wire elements, but offer higher temperatures, and higher power concentration. SiC elements typically operate at low voltages, and are usually supplied by transformers that reduce the supply voltage (it is possible to design to match the element configuration to the supply voltage, but often this is not possible due to other furnace design considerations). SiC elements vary in resistance with temperature and they also lose power as they age: either the furnace will lose power with time, or there must be compensation for the aging effect. Typically, elements are selected which will give more power than necessary, and then over time the supply voltage is adjusted to compensate for aging. In the past this was done with multi-tap transformers, now it is done with SCR's. The combination of series connections and aging raises another problem. When an old element fails a new element cannot be inserted in the series connection, so the whole group of elements must be replaced (or furnace usage time must be logged, so that elements of similar age are used together). It is recommended that an experienced vendor be involved in the design process for these elements.
Another electric element type used for high temperature ceramics and glass melting is molybdenum disilicide (MoSi2). These are also rigid elements, typically fabricated in a "U" shape, which is suspended vertically. These elements have a maximum temperature of 3360oF/1850oC, and do not suffer from the aging associated with SiC elements. Their resistance does vary with temperature, however, and SCR power control systems are required. MoSi2 elements are popular in commercial applications, due to high power output and reduced maintenance costs over SiC, but they do represent a higher initial cost. Again, it is recommended that an experienced vendor be involved in the design process.
The optimum location of heating elements in a furnace will be influenced by the furnace dimensions, and by the intended usage. It is quite common to see commercially designed kilns for glass with elements in the roof only, while pottery kilns normally heat from the sides. Slumping of glass, particularly of large flat pieces, is typically done in large shallow kilns with heat from the roof. As the kiln shape becomes taller and narrower it makes more sense to heat from the side (pottery kilns can usually be used for glass, if the dimensions of the kiln suit the application). Heat is transferred in three possible ways: conduction (through physical contact of the element and the material being heated); convection (through the air in the kiln); and by radiation (directly from the elements to the material being heated). At lower temperatures convective heating is predominant, as the temperature increases radiation becomes more prevalent. For this reason element placement is less critical for low temperature applications, as there will be some mixing effect. Direct venting (whereby a small volume of air is continuously drawn from the kiln during firing) is particularly beneficial in low temperature kilns as it helps to move air around the kiln (it is also very desirable for decorating, especially decal firing, as it refreshes the oxygen in the kiln). At higher temperatures it becomes more important to distribute the heat within the chamber through element design, to provide for even radiant heating.
It is recommended that element calculations be worked out before furnace construction is started, as the size, number and length of the element support grooves (or tubes) required will be affected by the element design. Also, if a three-phase supply is to be used, it is helpful to have the number of elements divisible by three. The first design parameter is the size of the furnace. As a general rule, do not make the furnace larger than necessary, as this will unnecessarily increase the power consumption. To determine the power required, in watts (W), calculate the interior surface area of the furnace, and then multiply this by the appropriate factor for the furnace usage. For glass annealing a multiple of 2-2.5 W/in2 (.31-.39 W/cm2) is sufficient, a typical multiplier for pottery kilns is 3.9 W/in2 (.6 W/cm2). A certain amount of power is required to reach the desired temperature; any additional power will increase the speed of heat up of the furnace. For this reason it may be desirable to design extra power into an annealer, for instance, in order to provide for quick re-heat once the door has been opened to add additional pieces.
The amount of power possible may be constrained by the available supply. Some houses, for instance, have as little as 60 amp services, and even commercial studios can quickly reach their limit if a lot of electrical equipment is being used. Amperage is equivalent to watts divided by volts, typical household voltage is 240 V, typical commercial is 208 V or 480 V (600 V in Canada). For three phase systems divide watts by volts and then divide by 1.73 (for balanced loads). The selection of voltage and phase will not affect furnace performance; it is the watts that are important. Once the desired furnace wattage is calculated, determine the required amperage for the available voltage, and compare this to what is available. Leave some margin for fuses/circuit breakers: electrical codes generally allow heating loads to be only 80% of the fuse rating, to avoid spurious fuse failures. If a three phase supply is available (typical in commercial spaces and in institutions) it is desirable to design the elements to suit, as this helps even out the power demand (which helps to reduce utility costs) and reduces the load amps, which can save money on supply components (cables, disconnects, etc.). Single-phase kilns can usually be converted to three-phase (with the same voltage) without changing the elements, and three phase kilns can always be changed to single phase. Utilizing a 480 or 600 V supply (208 V supplies are typically transformed down from 480/600 V) can also save costs, partly through lower amp draw, and partly through removing the necessity of a transformer � 480 or 600 V is only practical for larger furnaces, however, in excess of approximately 10 ft3 (280 L).
The most important factor in element design is the wire surface loading - how much power (heat) is being radiated from the surface area of the wire itself. The optimum surface loading is mainly a function of furnace temperature - ranging from 30 W/in2 for 1500oF to 15 W/in2 or less for kilns firing to 2370oF (4.6 W/cm2 for 800oC to 2.3 W/cm2 for 1300oC). Surface loading is related to the wire gauge selected, typical gauges used range from 10 to 18 (the higher the gauge number the finer the wire), with heavier gauges generally being preferred for longer life at higher temperatures. Designing with lower surface loading will also increase element life. Different combinations of parallel or series element connections will provide a selection of wire gauge choices. (In a parallel connection both ends of each element are fed the supply voltage, such that each element is operating from the full voltage, in a series connection the supply voltage is supplied to each end of a chain of elements). Other considerations in element design are the diameter of the coil - outside coil diameter should ideally be 10-14 times the wire diameter, and the stretched length of the coil - there should be at least one wire diameter between the coils (2 to 1 stretch) and ideally 2-5 wire diameters (3-6 to 1 stretch).
The process of calculating wire gauges for different element connections is not difficult, but some experience is helpful - this job can be done by your element vendor or, if more information is desired on the details of element calculation, contact your wire supplier for the necessary formulas and tables.
Jeremy Willis is a professional engineer and has been designing electric kilns, for artisans and for industry, since 1984. He is a managing director of The Pottery Supply House Limited and the manager of Euclid's Elements.
Copyright The Pottery Supply House Ltd.