Several material characteristics must be included in the design of a rotary dryer that operates efficiently and reliably. In addition to a material’s specific heat, its heat transfer properties must also be taken into account during the dryer sizing and design process.
Material Specifications: Heat Transfer Properties
Conductors and Insulators
A material’s heat transfer properties refer to how heat can move from one source to another. A material can be categorized as either a conductor, meaning it transfers heat very well, or an insulator, meaning it holds on to heat well (and subsequently is resistant to transferring it to other materials).
Metals (especially non-ferrous metals) are excellent thermal conductors, while wood is a poor conductor, making it an effective insulator.
Moisture Retention
In addition to whether a material transfers heat well or not, materials also differ on where they carry their moisture. Some materials – glass for example – hold their moisture on the surface, while others, such as clay, carry their moisture on the interior.
In theory, these two materials could have the same moisture content and require the same percentage in moisture reduction, but require very different systems to reach their objective.
Driving Force
Materials that carry their moisture on the surface will generally dry faster than those that carry it internally. Some materials, however, may dry fairly quickly, until their moisture drops below a certain point. Once this critical point is reached, the moisture becomes more difficult to draw out.
This is known as the material’s “driving force,” which states that the closer the remaining moisture gets to zero, the harder it is to draw out. Driving force diminishes as the percentage of moisture in a material reaches zero, therefore requiring additional time to fully dry. Sand is a common example of this; it dries readily at first, but getting the last remaining moisture out becomes more difficult.
Design Parameters Influenced by Heat Transfer
The unique heat transfer properties of a material can have a variety of effects on the overall design of a rotary dryer. Common design parameters that are influenced by heat transfer properties include:
Air Flow Configuration
Air flow configuration can be co-current or counter current. In the co-current configuration, material and combustion gases flow in the same direction, both entering at the inlet end of the dryer. This puts the hottest gases in contact with the wettest material.
Conversely, in the counter-current configuration, material and process gases flow in opposite directions to each other; one entering at the inlet end, the other entering at the discharge end. This puts the driest material in contact with the hottest combustion gases.
In the case of sand, for example, a counter current configuration would best serve the process, because of the sand’s driving force; the hottest gases would be in contact with the material at it driest point, so the increased temperatures at the discharge end can draw out that difficult-to-remove moisture.
Retention Time & Temperature
In general, materials that are conductors will require much less energy to dry. Similarly, materials that are considered insulators will require significantly more energy to dry. This will be reflected in the time and temperature profiles, which ultimately play a part in burner selection and dryer sizing.
Conclusion
FEECO is the leading provider of custom rotary dryers. All FEECO dryers are designed with the characteristics of the material to be processed in mind in order to provide an optimal drying solution built with longevity in mind. For more information on our custom rotary dryers, contact us today!