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Kiln
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Kilns are thermally insulated chambers, or ovens, in which controlled temperature regimes are produced.
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Kilns are thermally insulated chambers, or ovens, in which controlled temperature regimes are produced. They are used to harden, burn or dry materials. Specific uses include:
- To dry green lumber so that the lumber can be used immediately
- Drying wood for use as firewood
- Heating wood to the point of pyrolysis to produce charcoal
- For annealing, fusing and deforming glass, or fusing metallic oxide paints to the surface of glass
- For cremation (at high temperature)
- Drying of tobacco leaves
- Firing of material, such as clay, to form ceramics
- Drying malted barley for brewing
- Smelting ore to extract metal
- Heating limestone with clay to make cement
Ceramic kilns
Kilns are an essential part of the manufacture of all ceramics, which, by definition, require heat treatment, often at high temperature. During this process, chemical and physical reactions occur which cause the material to be permanently altered. In the case of pottery, clay materials are shaped, dried and then fired in a kiln. The final characteristics are determined by the composition and preparation of the clay body, the temperature at which it is fired, and by the glazes that may be used. While modern kilns often have sophisticated electrical systems to control the firing temperatures, pyrometric devices have been
used to provide visual indication of the firing regime since around 1000 AD.
Clay consists of fine-grained particles, that are relatively weak and porous. Clay is combined with other minerals to create a workable clay body. Part of the firing process includes sintering. This process heats the clay until the particles partially melt and flow together, creating a strong, single mass, composed of a glassy phase interspersed with pores and crystalline material. Through firing, the pores are reduced in size, causing the material to shrink slightly. This crystalline material is a matrix of predominantly silicon and aluminium oxides, and is very hard and strong, although usually somewhat brittle.
Types of Kilns
In the broadest terms there are two types of kiln, both sharing the same basic characteristics of being an insulated box with controlled inner temperature and atmosphere.
Intermittent – The ware to be fired, is loaded into the kiln. The kiln is sealed and the internal temperature increased according to a schedule. After the firing process is completed, both the kiln and ware are cooled.
Continuous, or sometimes called Tunnel. These are long structures in which only the central portion is directly heated. From the cool entrance, ware is slowly transported through the kiln, and its temperature is increased steadily as it approaches the central, hottest part of the kiln. From there, its transportation continues and the temperature is reduced until it exits the kiln at near room temperature. A continuous kiln is the most energy efficient because heat given off during cooling is recycled to pre-heat the incoming ware.
A specialty type of kiln, common in tableware and tile manufacture, is the Roller-hearth Kiln, in which ware placed on bats is carried through the kiln on rollers.
Kiln technology is very old. The development of the kiln from a simple earthen trench filled with pots and fuel, pit firing, to modern methods happened in stages. One improvement was to build a firing chamber around pots with baffles and a stoking hole, this allowed heat to be conserved and used more efficiently. The use of a chimney stack improves the air flow or draw of the kiln, thus burning the fuel more completely. Early examples of kilns found in Britain include those made for the making of roof-tiles during the Roman occupation. These kilns were built up the side of a slope, such that a fire could be lit at the bottom and the heat would rise up into the kiln.
- Anagama kiln - the Asian anagama kiln has been used since medieval times and is the oldest style of kiln in Japan. This kiln usually consists of one long firing chamber, pierced with smaller stacking ports on one side, with a firebox at one end and a flue at the other. Firing time can vary from one day to several weeks. Traditional anagama kilns are also built on a slope to allow for a better draft.
- Noborigama kiln - the Noborigama is an evolution from Anagama design as a multi-chamber kiln, usually built on a slope, where wood is stacked from the front firebox at first, then only through the side-stoking holes with the benefit of having air heated up to 600 C degrees from the front firebox, enabling more efficient firings.
- Bottle kiln - a type of intermittent kiln, usually coal-fired, formerly used in the firing of pottery; such a kiln was surrounded by a tall brick hovel or cone, of typical bottle shape.
- Catenary arch kiln, typically used for the firing of pottery using salt, these by their form (a catenary arch) tend to retain their shape over repeated heating and cooling cycles, whereas other types require extensive metalwork supports.
- Sevres kiln was invented in Sevres, France and enabled to reach efficiently high-temperature (1280 C degrees) in order to have fully water-proof ceramic bodies and easy to obtain glazes. It features a down-draft design that enabled to reach high temperature in shorter time, even with wood-firing.
- Top-hat Kiln - an intermittent kiln of a type sometimes used in the firing of pottery. The ware is set on a refractory hearth, or plinth, over which a box-shaped cover is then lowered.
- Feller Kiln brought contemporary design to wood firing by re-using the unburnt gas from the chimney in order to heat the air up before entering the firebox. This leads to an even shorter firing cycle and less wood consumption. This design requires external ventilation to prevent the in-chimney radiator from melting, being typically in metal. The result is a very efficient wood kiln firing one cubic meter of ceramics with one cubic meter of wood.
- Electric kilns - kilns operated by electricity were developed in the 20th century, primarily for smaller scale use such as in schools, universities, and hobby centers. The atmosphere in most designs of electric kiln is rich in oxygen, as there is no open flame to consume oxygen molecules, however reducing conditions can be created with appropriate gas input.
- Modern kilns - with the advent of the industrial age, kilns were designed to utilize electricity and more refined fuels, including natural gas and propane. The majority of large, industrial pottery kilns now use natural gas, as it is generally clean, efficient and easy to control. Modern kilns can be fitted with computerized controls, allowing for refined adjustments during the firing cycle. A user may choose to control the rate of temperature climb or ramp, hold or soak the temperature at any given point, or control the rate of cooling. Both electric and gas kilns are common for smaller scale production in industry and craft, handmade and sculptural work.
- Microwave Assisted Firing - this technique combine microwave energy with more conventional energy sources such as radiant gas or electric heating in order to process ceramic materials to the required high temperatures. Microwave-assisted firing offers significant economic benefits
Wood-Drying Kiln
A variety of wood drying kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.
Conventional wood dry kilns (Rasmussen, 1988) are either package-type (sideloader) or track-type (tram) construction. Most hardwood lumber kilns are sideloader kilns in which fork trucks are used to load lumber packages into the kiln. Most softwood lumber kilns are track types in which lumber packages are loaded on kiln/track cars for loading the kiln.
Modern high-temperature, high-air-velocity conventional kilns can typically dry 1 inch thick green lumber in 10 hours down to a moisture content of 18%. However, 1 inch thick green Red Oak requires about 28 days to dry down to a moisture content of 8%.
Heat is typically introduced via steam running through fin/tube heat exchangers controlled by on/off pneumatic valves. Less common are proportional pneumatic valves or even various electrical actuators. Humidity is removed via a system of vents, the specific layout of which are usually particular to a given manufacturer. In general, cool dry air is introduced at one end of the kiln while warm moist air is expelled at the other. Hardwood conventional kilns also require the introduction of humidity via either steam spray or cold water misting systems to keep the relative humidity inside the kiln from dropping too low during the drying cycle. Fan directions are typically reversed periodically to ensure even drying of larger kiln charges.
Most softwood lumber kilns operate below 240 °F temperature. Hardwood lumber kiln drying schedules typically keep the dry bulb temperature below 180 °F. Difficult-to-dry species might not exceed 140 degrees F.
Dehumidification kilns are very similar to conventional kilns in basic construction. Drying times are usually comparable. Heat is primarily supplied by an integral dehumidification unit which also serves to remove humidity. Auxiliary heat is often provided early in the schedule where the heat required may exceed the heat generated by the DH unit.
Solar kilns are conventional kilns, typically built by hobbyists to keep initial investment costs low. Heat is provided via solar radiation, while internal air circulation is typically passive.
Newer wood drying technologies have included the use of reduced atmospheric pressure to attempt to speed up the drying process. A variety of vacuum technologies exist, varying primarily in the method heat is introduced into the wood charge. Hot water platten vacuum kilns use aluminum heating plates with the water circulating within as the heat source, and typically operate at significantly reduced absolute pressure. Discontinuous and SSV (super-heated steam) use atmosphere to introduce heat into the kiln charge. Discontinuous technology allows the entire kiln charge to come up to full atmospheric pressure, the air in the chamber is then heated, and finally vacuum is pulled. SSV run at partial atmospheres (typically around 1/3 of full atmospheric pressure) in a hybrid of vacuum and conventional kiln technology (SSV kilns are significantly more popular in Europe where the locally harvested wood is easier to dry versus species found in North America). RF/V (radio frequency + vacuum) kilns use microwave radiation to heat the kiln charge, and typically have the highest operating cost due to the heat of vaporization being provided by electricity rather than local fossil fuel or waste wood sources.
Valid economic studies of different wood drying technologies are based on the total energy, capital, insurance/risk, environmental impacts, labor, maintenance, and product degrade costs for the task of removing water from the wood fiber. These costs (which can be a significant part of the entire plant costs)involve the differential impact of the presence of drying equipment in a specific plant. An example of this is that every piece of equipment (in a lumber manufacturing plant) from the green trimmer to the infeed system at the planer mill is the "drying system". Since thousands of different types of wood products manufacturing plants exist around the globe, and may be integrated (lumber, plywood, paper, etc.) or stand alone (lumber only), the true costs of the drying system can only be determined when comparing the total plant costs and risks with and without drying.
The total (harmful) air emissions produced by wood kilns, including their heat source, can be significant. Typically, the higher the temperature the kiln operates at, the larger amount of emissions are produced (per pound of water removed). This is especially true in the drying of thin veneers and high-temperature drying of softwoods.
See also
External links
- from in Stoke-on-Trent, UK.
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