Indirect fired rotary kilns are an effective method of processing materials, but sometimes it is necessary to take additional measures to ensure that the rotary kiln is processing efficiently. Indirect fired rotary kilns create heat transfer by conduction through the shell of the rotary kiln, rather than by means of a hot gas stream. Because all of this heat transfer is occurring through the shell, it is essential to have good, even contact between the material and the rotary kiln shell itself. This will assure that the transfer of heat is as efficient as possible. Helping to create this efficient heat transfer can be done with what is called a “bed disturber.”
A bed disturber, often custom designed to create maximum, material-specific efficiency, is anything affixed to the inside of the rotary kiln that helps to mix the bed of material. Ideally, the bed should tumble, turning over and minimizing dead spots, or temperature variations within the bed. Unfortunately, not all materials tumble well which results in a slipping bed with poor mixing and large temperature variation. Bed disturbers, whose design can vary considerably, can be attached to the interior of the rotary kiln, to disturb the bed and turn it over. However, what seems like a simple task, can get complicated quickly, as thermal stressors come into play.
A common bed disturber is merely a bar, that runs the length of the interior of the rotary kiln. Material pushes up against the bar, building up and rolling over it, so material that was on the top of the bed now gets redistributed to the bottom of the bed. The disadvantage to using a bar bed disturber is 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 rotary 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.
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.
There are various types of bed disturbers, but they are all designed to do the same thing: turn the bed over to maximize the transfer of heat from the rotary kiln shell to the material.
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FIGURE 1. In a bed that just 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. |
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FIGURE 2. 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. |
FEECO Rotary Kiln Series:
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- Feeco’s Batch Rotary Kiln is helpful in Rotary Kiln Design Applications - Direct Fired Rotary Kiln vs. Indirect Fired Rotary Kiln: What’s the Difference? - Rotary Kilns vs. Rotary Dryers: What’s the Difference? Rotary Kiln Design: Rotary Kiln Refractory: Indirect Fired Rotary Kiln Applications: |
is the author of this post. She has been part of the FEECO Marketing Team for 2 years, and has gained her knowledge from interviewing FEECO engineers, as well as spending time in the FEECO testing & tolling facility. At FEECO -- We Build BIG Stuff! Check out our website and social networks below to see some of the equipment we manufacture...
