ROTARY KILN DESIGN: An Introduction to Considerations in Rotary Kiln Design
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ROTARY KILN DESIGN
INTRODUCTION For many years, rotary kilns have been used across various industries to drive chemical reactions by thermal processing. Rotary kilns have become very established in fields such as cement, lime, and minerals. Because these thermal processing tools have been used almost exclusively in these industries, they have been designed to meet the needs of these particular materials. However, there are countless untapped applications for the use of a rotary kiln.
Rotary kilns can be custom-engineered to meet the thermal processing needs of just about any material. At Feeco International, we are bringing custom-designed rotary kilns to new industries and applications. Unlike most rotary kiln manufacturers, our ability to run an array of batch tests allows us to be very nimble in terms of designing rotary kilns around specific problems or materials.
What follows is a general overview of things to consider before entering the process of designing a rotary kiln. This is by no means meant to be a practical guide, but rather an introduction to the many considerations involved in the design of a rotary kiln.
For more information on rotary kilns, visit our website: www.FEECO.com or contact us at
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DEFINE THE PROCESS In the process of designing each custom rotary kiln, a multitude of new questions and problems to solve arise. Because FEECO’s applications are custom, the majority of the time, we must start from square one, collecting all of the data, analyzing it, and turning this data into an effective rotary kiln design. Many people feel overwhelmed at where to start in their search for a rotary kiln. While Feeco International can do it all from idea to end product, we have put together this introductory series to not only help others understand the process that goes into designing a rotary kiln, but also to help others understand the things to consider when purchasing a rotary kiln, whether it is custom designed or not. The first step in the design of a custom rotary kiln is to define the desired reaction: what are you trying to accomplish? Just about any thermally-driven chemical reaction can be processed in a rotary kiln. Are you trying to sterilize a material, activate it, burn off its organic portion, or do you have a material conversion you need completed? Other typical rotary kiln applications include waste lime recovery, the manufacturing of proppants, mineral roasting, and phosphate production, to name a few. Defining your desired reaction will not only determine what needs to happen inside of the rotary kiln, but it will also have a lot of input as to what components will make up the rotary kiln’s design. Once you have determined what chemical reaction you are trying to achieve, you must begin looking at your material.
THERMAL ANALYSIS: MOISTURE Before a kiln can be designed, the material must go through a thermal and chemical analysis. Getting to know the material you are working with will let you know how the material is going to behave in the rotary kiln. Is your material going to melt? Vaporize? Explode? Things like the percentage of moisture a material holds, its bulk density, specific heat, thermal conductivity, chemical makeup, and so on, all have an influence on how material performs in a rotary kiln. Let’s take a look at how some of these factors take part in rotary kiln design. Unlike a rotary dryer, where the main focus is to reduce the moisture, removal of moisture in a rotary kiln is secondary to the process rather than the goal. However, it is still an issue that needs to be addressed. In many instances, it is ideal to remove the moisture before the chemical reaction takes place. Depending on the situation, some people prefer to add a rotary dryer into their process before
ROTARY KILN DESIGN
material enters the rotary kiln. Others use the rotary kiln to dry the material, greatly increasing the material’s time in the rotary kiln, and ultimately the work that the kiln is doing. Because a rotary kiln does not shower the material like a rotary dryer, the heat transfer of the gas to the material, is inefficient. In the case of an indirect fired rotary kiln, drying would take a substantial amount of time, as the material is only getting heat through the shell of the rotary kiln, and not through a stream of hot gas. There are pros and cons to each method, and ultimately, it is up to the customer to decide of material, longer retention time, and possibly what works best for their situation. additional accessories like damns?
THERMAL ANALYSIS: SPECIFIC HEAT & HEAT TRANSFER
The specific heat of a material is another central factor in the design of a rotary kiln. Specific heat is how resistant a material is to heating. By definition, it is how much energy it takes to raise 1 gram of material 1 degree Celsius. The specific heat of water is very high, meaning it takes a lot of energy to cause a change in the temperature – think of how long it takes to boil your water on the stove. Metals, however, have a low specific heat, meaning it takes much less energy to raise their temperature. Similar to specific heat, the heat transfer properties of a material also play a big part in the design of a rotary kiln. How a material transfers heat will have a direct effect on how the material behaves in the rotary kiln: will it transfer its heat easily, causing even heat distribution and low retention time, or will it hold onto its heat, causing cold pockets
Another part of the thermal analysis is the thermal gravimetric analysis, or TGA. A TGA is performed on a material to determine changes in mass as a function of temperature. It describes the temperature ranges at which mass loss occurs. This is critical in determining the required temperature profile in a kiln. As an example, free water will show primary removal at around 212ºF, where tightly bound chemical water may show a mass loss upwards off 500ºF. A TGA also helps show where a reaction begins, and ends, as often, the curve on a TGA starts at a specific temperature, but does not complete until a much higher temperature. Overall, a TGA helps determine the temperature profiles required in a rotary kiln, by showing at what temperatures reactions are occurring, and for how long they need to remain at, or how much they need to increase temperature to, for reactions to finish. Additionally, while you may be trying to get your material from point A to point B, many times there are reactions that occur in between these two points. A TGA can help indicate where
ROTARY KILN DESIGN
unpredicted reactions may be occurring. FEECO also has the ability to utilize our batch rotary kiln to assist in thermal analysis. Through various testing, this small scale rotary kiln can be used to help determine temperature profiles.
CHEMICAL ANALYSIS Knowing the chemical composition of a material is a valuable asset in rotary kiln design for several reasons. One important reason is that many materials will combust inside the rotary kiln at high temperatures, creating more heat than was put into the rotary kiln. In this case, the rotary kiln needs to be designed to be able to withstand those amounts of heat. In other cases, materials may need a particular chemical atmosphere for a reaction to occur. For example, an atmosphere devoid of oxygen, or rich in carbon dioxide. Still another reason to understand the chemical
makeup of a material, and how those chemicals react together at certain temperatures, is to what is going to exit the rotary kiln in the gas stream. Gases leaving the rotary kiln typically need to be treated before their release. While a particular fuel may enter the rotary kiln, that gas may pick up whatever the material being processed is releasing, to create something new and toxic. Different gases need different treatments, so it is essential to know what gases are exiting the rotary kiln.
SIZING After the material has been thermally and chemically analyzed, we can begin sizing the rotary kiln. The size of a rotary kiln is not only a function of capacity, but also the amount of heat that will be generated or required inside of the rotary kiln from the volatizing of the material. Using the thermal analysis, we can determine an approximate retention time. Using our predetermined guides,
ROTARY KILN DESIGN
we use retention time to establish the length of the to control this. Another advantage to using an rotary kiln. indirect rotary kiln is that there are fewer exit gases that need processing, because less gas is going The diameter and length of the rotary kiln are into the rotary kiln. calculated based on the maximum feed rate, the required retention time, and what we want the bed profile(how full of material the rotary kiln REFRACTORY is) to look like. Once we have a rough design of the rotary kiln, we use several computer programs Once we have our preliminary rotary kiln size, we to help predict how the material will behave can start to think about the details of the rotary kiln in the rotary kiln we have designed. We review internals. In the case of a direct fired rotary kiln, in the combined analyses and if our design does which the hot stream of gas flows directly through not meet the appropriate criteria, we adjust our the interior of the rotary kiln, refractory is usually design accordingly. needed. Arguably one of the most critical components of a direct fired rotary kiln, the refractory is what Another aspect of rotary kiln design is deciding protects the carbon steel shell from the high whether to go with a direct fired rotary kiln, or an indirect fired rotary kiln. The difference between the two is how the heat is introduced. In a direct fired rotary kiln, the heat is introduced directly into the internals of the kiln, via a stream of hot gas. This means there is high heat transfer between the material and the heat. However, it also means that there is more gas exiting the rotary kiln that needs to be processed. In an indirect fired rotary kiln, the heat comes through the shell of the rotary kiln, and the heat transfer comes from the material being in contact with the rotary kiln shell. One of the big advantages to an indirect fired rotary kiln is that the temperature can be tightly controlled along the length of the rotary kiln. For example, if a material needs to reach a certain temperature, and be held there for a specific amount of time, an indirect rotary kiln is ideal, because it is easy
DIRECT vs. INDIRECT
ROTARY KILN DESIGN
temperatures within. A quality refractory is of the utmost importance, and many options are available, depending on the needs of the rotary kiln. Typically, there are two kinds of refractories for lining a rotary kiln: castable, and brick. Each kind of refractory has its advantages and disadvantages. The choice of refractory is dependent on the rotary kiln temperature, material chemistry, and how abrasive the material is. Castable and brick refractory are comparably priced for similar refractory compositions. However, the installation cost for brick is more since it is more labor intensive. Castable refractory comes in a powder form and is mixed with water onsite. Before the mixture can be put in place, anchors
are installed. These y-shaped anchors are similar to rebar in cement; they help give the castable lining its strength. Once these anchors are in place, the cement-like mixture is pumped into the lining of the rotary kiln, and allowed to cure for several days. Castable refractory has a similar material cost to brick. However, brick installation is much more labor intensive, as each brick is individually installed. This makes the overall cost of a brick lining more expensive than castable. The disadvantage to using castable refractory in a rotary kiln is that it is very susceptible to installation problems. When castable refractory is installed very well, it can nearly match the quality of brick. But if the castable is installed incorrectly, there can be a considerable difference in quality, and the life of the refractory can be severely compromised.
CONCLUSION ROTARY KILN DESIGN
Besides lower overall cost, the advantage to using castable refractory in a rotary kiln is that it is usually easily patched when a problem is encountered. Down time is typically minimal, because the problem area can just be cut out and new refractory can be poured back in the cavity. The second type of refractory we will look at is brick refractory. Refractory quality is measured by the percentage of alumina that it contains. Alumina gives the refractory its durability in terms of temperature resistance and strength. Brick is fired in a furnace under tightly controlled conditions that allow it to achieve better properties than a similar composition castable. Brick refractory is slightly more expensive than castable, but it does not require anchors, and it’s quality is superior. Castable is basically brick that has not been to the furnace yet. When processing a highly abrasive material, brick refractory is advisable most of the time, as castable does not have the durability to stand up against abrasive materials as well as brick. There is a downside to brick, however. Brick refractory is held in place much like the principle of a roman arch: bricks are held in place by the pressure of the other bricks pushing against each other. When a problem is encountered, typically the failed brick needs to be replaced, but when one brick is relying on the bricks around it to hold it in place, often one cannot replace just one brick, and whole sections of the refractory need to be replaced. Unlike castable refractory, the repair of a fail in brick refractory is much more involved.
This is an example of castable refractory. Anchors help to give the refractory its strength. Note the working and insulating layers, with the optional ceramic fiber backing.
to both kinds of rotary kiln refractories. However, there is more to refractory than just which material you choose. In the next part of this mini-series, we will look at the working layer, versus the insulating layer in rotary kiln refractory.
After a material is chosen for your rotary kiln refractory, you must decide the desired heat loss. When efficiency is of concern, or very high temperatures are involved, often it is desirable to use multiple layers of refractory: a “working” layer, and an insulating layer. The working layer is what is in direct contact with the material being processed. Because of this, this working layer is a dense lining that can withstand the high temperatures within As you can see, there are strengths and weaknesses the rotary kiln, and the constant abrasion from the
ROTARY KILN DESIGN
Typically the working layer and the insulating layer are made of the same material (ie. brick or castable), with varying chemistries. The working layer tends to be a higher density, stronger material that is more conductive. The insulating layer does not need these qualities, and tends to be softer, lighter, and less conductive, therefore more insulating. These two layers often vary in thicknesses, and these are determined from the needs of the rotary kiln and what material is being processed. Although, sometimes temperatures are low enough, or efficiency is of little concern, it is only necessary to use one working layer. For these reasons, refractory in a rotary kiln is often a very custom part of the design.
This illustration shows an example of brick refractory with a working layer, and an insulating layer.
material. However, when it comes to refractory, the denser it is, the less insulating capabilities it has. This means that even though there may be a tough, durable, thick working layer in place, the heat can easily pass through it to the shell of the rotary kiln. For this reason, an insulating layer is needed beneath the working layer (See Figures 1 and 2). The insulating layer does just that; it insulates the shell of the rotary kiln so the high temperatures cannot reach the shell and damage it, or the nearby components.
Sometimes when insulation is extremely critical, an optional third layer of ceramic fiber backing is used. Though there are various kinds of this backing, this thin, but very efficient layer is similar to fiberglass insulation found in a house, but it is much more compressed. However, the decision to employ this layer comes with some responsibility. Should a crack in the refractory occur and go unnoticed, it is possible for the high heat inside the rotary kiln to reach this backing and actually burn it up. This would create a gap between the refractory itself, and the shell of the rotary kiln, which could cause disastrous problems. Due to this potential of increased risk, this third layer is not always appropriate.
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ROTARY KILN DESIGN
Protecting the life of your refractory is crucial not only to running at high efficiency, but also to maintaining your rotary kiln. Once your rotary kiln refractory is installed and in use, it is important to take the extra steps to protect it. A well-installed, high quality refractory can have a lifespan of many years. But there are several culprits that can cut refractory life short. Unfortunately, aside from a refractory, refractory failure is hard to spot. The good news is, many of the things that can cause refractory failure are preventable. The biggest source of refractory failure is what is called “cycling.” Cycling is simply the heating up and cooling down of a rotary kiln. Each time the rotary kiln is heated, the refractory expands with the drum. As the rotary kiln is cooled, the refractory Refractory failure can have disastrous results. Even a small crack also retracts. If a kiln is constantly being turned can allow heat to reach the rotary kiln shell. It is important to routinely temp gun the exterior of the rotary kiln shell, ensuring on and shut down, the refractory can easily be that the temperature is consistent for the entire circumference of the drum. stressed, causing cracks. Cracks can also occur from heating or cooling the kiln too quickly. It handle such aggressive corrosion. Similarly, this is important to try to reduce cycling as much as failure can also happen when a rotary kiln is used possible, keeping shut downs to a minimum. for something the refractory was not designed for. Sometimes there are unknown components in a Another source of refractory failure is chemical material, and when a feedstock is changed, these incompatibility. refractory is not designed to be able unknown components can attack the refractory, to withstand certain chemicals. A big offender of again, causing excessive wear. this is chlorides. Chlorides can aggressively attack refractory, causing excessive wear because of Aside from regular inspections, one easy way to their corrosive nature. When these chemicals are help extend the life of your rotary kiln is to check identified up front, refractory can be designed to for hotspots on a regular basis. This can be done by
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ROTARY KILN DESIGN
picking a spot on the rotary kiln shell, and holding a temp gun in place. As the rotary kiln rotates, that spot should be the same temperature for the entire circumference of the shell. There would be trouble if you’re reading 400º, 400º, 700º, 400º. A hotspot on the shell of the rotary kiln indicates a failure in refractory. Left unnoticed, this could lead to severe damage to the rotary kiln shell. In addition to circumference temperature being the same in a given location, there should be a gradual shift in temperatures from one end of the kiln to the other, not a drastic change.
drum. This chain and sprocket drive is used for small rotary drums, running up to 75 horsepower. It is not suitable for larger rotary kilns running above 75 horsepower, but is ideal for smaller jobs, as it is cost-effective, and easy to run. The gear drive is best for heavy duty situations, running above 75 horsepower. Similar to the chain and sprocket setup, instead of a sprocket wrapped around the girth of the drum, this setup has an actual gear around the drum. This gear meshes with a small gear drive, which rotates it. This type of drive is more expensive, but is a necessity when dealing with a heavy duty application. Unlike most other rotary kiln components, there is not a need for BEARING & DRIVE COMPONENTS any customization in terms of the mechanicals of Bearing and drive components for a rotary kiln are the drums. The need for one or the other is solely similar to those on a rotary dryer. There are two dependent on how much horsepower is required. types of arrangements for rotating a rotary dryer or rotary kiln: a chain and sprocket, or a gear drive. BED DISTURBERS For either arrangement, there are two tires on the rotary drum that sit on top of trunnion wheels. Indirect fired rotary kilns are an effective method of This point of contact is what is supporting the processing materials, but sometimes it is necessary rotary drum. Thrust rollers run on either side of the to take additional measures to ensure that the wheels, help to keep the rotary drum from moving rotary kiln is processing efficiently. Indirect fired too far up or down hill. A chain and sprocket rotary kilns create heat transfer by conduction arrangement works much like a bicycle. There is a through the shell of the rotary kiln, rather than by large sprocket wrapping around the rotary drum means of a hot gas stream. Because all of this heat with a chain on it that goes to the reducer and transfer is occurring through the shell, it is essential motor. The spinning motor turns a gear box, which to have good, even contact between the material spins a small sprocket that is attached by the chain and the rotary kiln shell itself. This will assure that the to the large sprocket wrapping around the rotary transfer of heat is as efficient as possible. Helping
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In a bed that only slips along the interior of the rotary kiln, the inner part of the bed never really gets exposed to the rotary kiln shell, creating major temperature discrepancies.
Fixed to the interior of the rotary kiln, a bed disturber turns over the bed of material by creating a place for the material to build up and tumble over, redistributing what was on top to the bottom, and vice versa.
to create this efficient heat transfer can be done whose design can vary considerably, are can be with what is called a “bed disturber.” attached to the interior of the rotary kiln, to disturb the bed and turn it over. However, what seems like A bed disturber, often custom designed to a simple task, can get complicated quickly, as create maximum, material-specific efficiency, thermal stressors come into play. is anything affixed to the inside of the rotary kiln that helps to mix the bed of material. Ideally, the A common bed disturber is merely a bar, that runs bed should tumble, turning over and minimizing the length of the interior of the rotary kiln. Material dead spots, or temperature variations within the pushes up against the bar, building up and rolling bed. Unfortunately, not all materials tumble well over it, so material that was on the top of the bed which results in a slipping bed with poor mixing now gets redistributed to the bottom of the bed. and large temperature variation. Bed disturbers, The disadvantage to using a bar bed disturber is
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ROTARY KILN DESIGN
that they can sometimes bend and break with the thermal stresses of the rotary kiln. A rotary kiln naturally has gradients of temperature, usually cooler on the ends of the kiln, and hotter in the middle. This gradation in temperature causes thermal expansion in some places on the rotary kiln shell, but not in others. Because of this, the bar, welded to the shell, is being pulled in different directions, which can cause it to bend or break. When this kind of thermal expansion is at work, it is usually best to look at alternative bed disturbers.
DAMS For various reasons, it is often desirable to increase retention time in the rotary kiln. In order to do this, the loading, or how much material is in the rotary kiln at one time, needs to be increased. This is done by adding what is called a dam to the interior of the rotary kiln. A dam in a rotary kiln works much like a dam in a river. When a dam is put into place, material builds up behind the dam, forcing retention time to increase. Material then spills over the dam, and discharges from the rotary kiln. Internal dams can also be used if a discharge end dam is not sufficient.
Another type of bed disturber is similar to flights in a rotary dryer. These flight-like bed disturbers are welded with one weld point each, to the inside of the rotary kiln. This kind of bed disturber was designed to accept the different thermal expansion stressors, making it ideal for drums with various temperature gradations. Dams are useful when bed depth needs to be increased, or when retention time needs to be There are various types of bed disturbers, but they increased, using a kiln that was designed for a are all designed to do the same thing: turn the particular retention time. bed over to maximize the transfer of heat from the rotary kiln shell to the material.
Dams are put in place when retention time needs to be increased using the same size rotary kiln. Dams allow the loading to be increased, which increases retention time by forcing the material to build up in the rotary kiln.
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SEALS Holding the appropriate temperature within a rotary kiln is what allows the desired chemical reaction to occur. Sustaining that temperature, however, can be difficult if the right seal is not chosen. Almost all rotary kilns run at a negative pressure, meaning gas doesn’t leak out, but rather air leaks in. Because kilns are always running at a higher temperature than the ambient air, any ambient air leaking into the rotary kiln will cause the temperature inside of the rotary kiln to drop. Not only will this result in an unnecessary amount of energy being used and wasted, but if the leak is severe enough, it could potentially disrupt the chemical reaction. This is why it is crucial to have a quality seal. Sealing the ends of a rotary kiln can be a difficult task, because there is always going to be something rotating attached to something stationary. This breeching, or the stationary part, is typically where leakage will occur. So how do you seal a moving part against a stationary part? One answer is a leaf seal. Leaf seals are the standard seal used on rotary kilns and rotary dryers. But how does a leaf seal work? Leaf seals are similar to a fanned out deck of cards. The “cards,” or leaves, are made out of a spring steel. These fanned out leaves are bolted to
the breeching, and the springy leaves are forced to push against the seal/wear plate of the rotating kiln, naturally keeping pressure on the rotary kiln to create a good seal. Several variations of this seal are also available, such as the purged double leaf seal. The purged double leaf seal is typically used in situations where maintaining the atmosphere inside the rotary kiln is extremely critical. For example, in cases where the atmosphere inside the rotary kiln cannot tolerate oxygen from ambient air leakage. In this case, the purged double leaf seal would be a viable alternative to a standard leaf seal. The purged double leaf seal is made up of two components. The first is two sets of seals which consist of two layers of leaves on top of each other. The second component is an inert purge gas, such as nitrogen, which is introduced between the two sets of seals. This purge gas pushes outward to ambient, so that there is a flow of gas going out, and therefore, no oxygen is allowed to flow in.
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ROTARY KILN DESIGN
As a leader in the thermal processing industry, The same is true for a change in the ambient Feeco International knows how critical it is to run conditions. A drastic change in temperature can efficiently. We can supply leaf seals and purged throw a rotary drum out of alignment. double leaf seals to suit your needs. Another potential problem area, typically specific to rotary kilns is refractory failure. Recognizing MAINTAINING YOUR ROTARY DRUM potential refractory problems is much more Rotary drums are a valuable investment with the difficult than recognizing problems with the rotary potential to have a long productive life with proper drum itself. Unfortunately, many people are not maintenance. While protecting this investment aware they have a problem until their refractory is not hard, it does take know-how, and routine fails. If not attended to, refractory failure can have maintenance. Luckily, this routine maintenance disastrous results. and knowledge go a long way in recognizing potential problems, and spotting when a rotary Learning to spot refractory issues in many cases drum is in need of some first aid. is as simple as using a temperature gun. Simply picking a spot on the rotary drum as it rotates, One of the most prevalent problems that can and making sure it is the same temperature for cause rotary drum issues is mis- alignment. When the entire circumference of the rotary drum tells a rotary drum is installed, whether it be a rotary you there are no hot spots in that area. If your cooler, rotary dryer, or rotary kiln, great care is temperature gun were to read 600, 600, 600, 900, taken to ensure that the rotary drum is in perfect 600, this would be an indicator that heat is making alignment. While many rotary kilns and rotary dryers it through the refractory to the shell of the rotary are running on a slope, aligning them makes sure kiln. This could mean a loss of refractory. Routinely that there is even pressure on all wheels. Signs that temp gunning spots on the exterior of the rotary a rotary drum is out of alignment include abnormal kiln can help keep up on some potential refractory wear patterns or grooves on wheels and tires, issues, and help in preventing major failures. and/or constant running on one thrust roller and not the others. Watching for signs like these can Learning to look for some of the above issues can help prolong the life of your rotary drum, and tell help extend the life of your rotary drum, and also you when it is time for realignment. help avert catastrophic failures and downtime. Aside from caring for the rotary drum, routine Knowing when to realign a rotary drum can also maintenance should be done on the mechanical help in its longevity. If there has been a change components of the rotary kiln. Lubricating the tires, in product, the rotary drum should be realigned. performing an oil change on the gear box, and 3913 Algoma Rd. Green Bay, WI 54311 • Phone: (920)468.1000 • Fax: (920)469.5110 • Email:
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checking potential problem spots for misalignment should all be a part of routine maintenance checks on rotary drums. While it’s not fool-proof, doing all of these things can only help in prolonging the life of your rotary drum.
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