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 of material, longer retention time, and possibly additional accessories like damns?
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 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. For more information on FEECO’s batch rotary kiln and how we use it in the design of a rotary kiln, see Feeco’s Batch Rotary Kiln is helpful in Rotary Kiln Design Applications.
FEECO Rotary Kiln Series: