Kilns 101: Understanding the Different Kiln Types
By understanding the differences and similarities between the various kiln designs, ceramic manufacturers can be better prepared to make the right selection for their plant.
It’s time to buy a new kiln. You open Ceramic Industry’s annual Data Book & Buyers’ Guide to find a kiln, but suddenly you feel overwhelmed by the different selections available. Do you need a bell, belt-type, or bending and convexing kiln? Or do you need a box, calcining, conveyor-type or decorating kiln? Maybe you need an electric, elevator, envelope or fast-firing kiln. Or maybe your best bet is a gas, infrared, high-temperature, laboratory, periodic, pusher-type, roller hearth, rotary or tunnel kiln. How do you choose?
With such a large number of options, it can seem daunting to select the right type of kiln. However, a closer look at each of the descriptions reveals that they define kilns in many different ways. Some define the kiln’s function, such as laboratory, high-temperature, fast-firing, decorating, calcining, and bending and convexing. Others describe the heating media, such as electric, gas and infrared.
The others are all sub-types of the two basic kiln types: periodic and continuous. Periodic kilns include bell, box, elevator and envelope kilns and are similar to a residential oven—the product is put inside, the door is closed, and the kiln heats and cools according to the firing cycle. Continuous kilns include belt- and conveyor-type, pusher-type, roller-hearth, rotary and tunnel kilns and are similar to a commercial baking oven—product continuously moves through the kiln, and the temperature in each area remains fixed.
Each type of kiln has its own advantages and disadvantages. By understanding the differences and similarities between each of the major styles and their sub-types, you can be better prepared to make the right selection for your plant.
Before trying to choose a kiln, you must first understand your firing objectives. In all cases, the kiln operator is looking for the same basic characteristics—uniformity and efficiency at the right temperature and atmosphere.
The kiln needs to be able to heat the product, or ware, to the temperature required for the necessary ceramic reactions to occur within the body. These reactions typically occur between 900-1,700°C (1,650-3,100°F).
The kiln must also have the right composition of gases in the atmosphere to allow the product to exit the kiln with the correct final properties. This can vary from the pure nitrogen atmosphere required for some technical ceramics to the strongly oxidizing or reducing conditions required to achieve the proper color in a whiteware glaze.
The uniform treatment of all the products in the kiln is paramount to the industrial ceramist. Only with uniform treatment will all products exiting the kiln be the same and be saleable as “first quality.” This is important whether you fire brick or electronic ceramics.
Finally, efficiency is paramount to any business; however, many companies overlook the efficiency of their new kiln when comparing the initial cost of different designs. It is important to remember that the initial cost of any kiln is low compared to the fixed costs that can be added to your budget when the kiln is not efficient. The kiln should use the least possible amount of energy and labor while meeting your production needs. You should also ensure that you will be making efficient use of the equipment—why, for instance have two small kilns that you use only 50 percent of the time when you could use one larger kiln for all of your products? Automating the kiln can also add to these efficiencies.
Once you’ve defined your firing objectives, you can begin evaluating the different kiln types.
A periodic kiln is simply a heated chamber. It contains at least one opening to allow the product to be loaded into and unloaded from the kiln. Once the unfired product has been placed in the kiln, the openings are sealed and the firing is started to heat the entire volume of the kiln. If the kiln is well designed, this is done quickly and uniformly. After the kiln has reached its peak temperature, the firing is stopped, and the kiln is allowed to cool. The kiln is then opened and the fired product is removed.
The periodic kiln is a prehistoric invention. Around 12,000 B.C. in what is now China, a prehistoric man first noticed that the earth beneath a fire pit took on different properties. It was stronger and impervious to water—thus ceramics were born.
The kiln began as a simple hole in the ground where combustible materials were heaped around the product to be fired. As techniques were developed to refine the ceramic raw materials, so, too, was the design for the kiln refined. Between 8000 and 7000 B.C., the first constructed kilns were made. These contained a firebox in the bottom, and the hot gases from the fire traveled upwards through the ware space and out a flue in the top of the kiln. Referred to as updraft kilns, these designs are still in use today.
The downdraft kiln was developed around 300 A.D. In this type of kiln, the products of combustion first traveled up inside the kiln and then were forced down and exhausted through the bottom of the kiln. This made the kiln more fuel-efficient and improved the temperature uniformity inside of the ware space.
Modern periodic kilns have a few common variants. A fixed-hearth kiln, which is still one of the most common formats for a periodic kiln, features a door on the side or top for loading the kiln. The box kiln is a classic example of a fixed-hearth kiln. Most studio pottery and laboratory kilns fall into the box category. Large industrial variants, such as the beehive and bottle-style kilns, are also still in use today.
A shuttle kiln has one or more doors and one or more kiln cars that support the ware to be fired and act as the floor of the kiln. By shuttling two sets of cars to and from one kiln, the downtime for loading the kiln can be reduced. A variant on this design, the “envelope” kiln has one or more fixed loading platforms, and the kiln shuttles between these platforms.
A top-hat kiln is a periodic kiln without swinging doors. In a small kiln, which is commonly referred to as an “elevator” kiln, the floor is often lowered out of the stationary kiln. As the load gets heavier and the kiln gets larger, it becomes less expensive to lift the kiln off the floor platform. These are referred to as “bell” kilns. These kilns are generally used for restricted-atmosphere and high-temperature applications, where a typical door creates difficulties in sealing the kiln.
A continuous kiln is constructed as a long, usually narrow tunnel. The temperature and sometimes the atmosphere changes constantly along its length, with the highest temperature zone occurring somewhere near the middle of the kiln. A steady stream of ware is conveyed through the kiln, and this ware is heated and cooled in the same way as in a periodic kiln.
Unlike the periodic kiln, the continuous kiln is a modern invention. The first continuous kilns appeared in the late 19th century as an offshoot of the industrial revolution. Ceramics were increasingly being produced in high-output manufacturing facilities rather than by individual artisans, and the continuous kiln was invented to make the ceramic manufacturing process more efficient.
The constant stream of products exiting the kiln made the production flow better than in a batch kiln, where a large cascade of products occurred every time a firing cycle was completed. The products were now automatically conveyed to an operator for unloading, instead of requiring a separate operator to first haul them out of the freshly cooled kiln. As a result, the labor required to load and unload the kiln was significantly reduced. The continuous kiln was also more fuel-efficient—since the lining of the kiln always stayed at the same temperature, only enough heat for the products and their supports, as well as to overcome losses through air infiltration, was required.
The major variants of continuous kilns are the tunnel, pusher slab, conveyor-type and rotary kilns. The tunnel kiln features a kiln floor composed of a series of cars. Ware is placed on the cars, which are constantly moving through the kiln. On the outside of the kiln, transfer cars are used to move the kiln cars to one or more return tracks, where the cars are loaded and unloaded. Tunnel kilns are the most common continuous industrial kilns and are found in factories in nearly every industry.
The pusher slab kiln features a design similar to a tunnel kiln but has a fixed floor or hearth. Heavy refractory plates containing the ware are pushed along the surface of this hearth. These kilns were popular before the invention of the roller-hearth. Today, they are primarily used for special applications, such as for certain electronic ceramics that cannot be fired in other kiln types due to their small size, slow firing cycles and aggressive ingredients.
Conveyor-type kilns, such as the roller-hearth kiln, are inventions of the last 40 years. Like a pusher slab kiln, these kilns also have a fixed floor, but the hearth is composed of a conveyor. In the roller-hearth, the “conveyor” is a series of ceramic or alloy rollers that extend through the walls of the kiln to an external drive system. The roller-hearth kiln is typically used for very fast firing cycles. With roller-hearth advancements, airlocks can be placed within the kiln to allow different atmospheres to be used at different times as the product progresses through the kiln. The belt kiln, another type of conveyor kiln, uses an alloy or ceramic belt for the conveyor. However, these have limited applications because of their temperature and load restrictions. Belt kilns are most often used for decorating dinnerware.
Rotary kilns are another variant used mainly for pre-processing raw materials. In the rotary kiln, the product must be in the form of small pellets so that it can be fed into one end of a rotating tube. These pellets are then heated and fall out of the other end, ready for further processing.
Choosing the Right Type of Kiln
The best type of kiln for your plant depends on your production. Before you choose a kiln, consider how you must produce. Are you producing consistently or in bursts? Are all of your products roughly the same size, or do they vary greatly in size and mass? Does each product require its own unique firing cycle, or can all products be fired the same way? If the firing cycles differ, how different are they? Once these questions are answered, the type of kiln you need will become obvious.
Continuous kilns are best for consistent, high-volume production. They are more fuel-efficient than periodic kilns, since only the product and kiln furniture must be heated. It is easier to achieve uniform temperatures in a continuous kiln than a periodic kiln, since there are fewer heat sinks and the products are all heated in the same way. Because of this, a well-designed continuous kiln is able to run a faster cycle than a well-designed periodic kiln. By their nature, continuous kilns are more easily loaded and unloaded than a periodic kiln.
However, continuous kilns are limited in their flexibility. As their name implies, these kilns operate continuously and must therefore be loaded and unloaded around the clock. This requires either 24-hour staffing in the plant or an automatic buffering system to keep the kiln fed during the off hours. Continuous kilns are also limited in the amount of change that can be made to their temperature curves. They are built to maintain a profile along their length, and physical constraints are designed into the kiln. Drastic changes to the profile are not possible.
Periodic kilns are best for inconsistent or low-volume production. They are very flexible, as a different curve can be run in a periodic kiln every time it is started. With a periodic kiln, you can produce two or more completely different products in one kiln without having non-productive kiln time or performing the time-consuming adjustments required with a continuous kiln. Since periodic kilns operate intermittently, they are better for production that has large peaks and valleys. They can be left idle when there is no ware to be produced and then restarted as soon as demand improves.
However, periodic kilns are less efficient than a continuous kiln with the same capacity. Since the whole of the kiln must be heated and cooled at the same time, more energy and time are required to process each piece of product. This is especially true at high temperatures, since the whole of the kiln lining must be constructed to resist the peak kiln temperature. In a continuous kiln, only a small fraction of the lining must resist the peak temperature.
Understanding the different types of kilns can help you decide whether you need a periodic or continuous kiln. However, even with a basic understanding of the different kiln types, it can be difficult to know exactly which sub-type to choose. It is often best to begin working with a reputable consultant or kiln supplier as early as possible in the selection process so that you can use their experience to help you determine which kiln is the most appropriate for your production process.
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