A dryer uses much more energy than most other bulk material processing equipment. One dryer that can save energy and enhance a plant's productivity is a conduction dryer. A look at how conduction compares with other drying methods is the first step in understanding how such a dryer works and whether it's right for your drying needs. A conduction dryer can handle heat-sensitive materials, control a drying process atmosphere to ensure final product quality, or even use a waste heat source to cut energy costs.
Driving off the liquid from wet material typically requires adding heat. How this heat is transferred to the material depends on the drying method. These methods include:
- Conduction - Transferring heat and energy by contact between two surfaces
- Convention - Transferring heat by contact with a heated gas
- Radiation - Transferring heat by exposure to a hot surface
Convection is often called a direct heat transfer method because the material contacts the heating media. Conduction and radiation are typically called indirect heat transfer methods because the material doesn't contact the heating media. The indirect heat transfer in a conduction dryer suits specific materials and drying requirements.
A typical conduction dryer is a metal-walled, heat-jacketed vessel that is stationary or rotating. In some cases, the dryer is equipped with an agitator. Condensed steam, hot water, combustion gas, hot oil or electricity heats up the jacket, which transfers the heat to the dryer surface. By conduction, the heat transfers through the dryer surface to the wet material. The agitator or the vessel's rotation moves and mixes the material bed, eliminating moisture gradients within the bed and increasing the drying rate. As the material dries, the vaporized moisture exits the vessel.
The temperature of the conduction dryer's surface can range from below freezing (for freeze-drying chemicals, fine chemicals, pharmaceuticals and food products such as coffee) for drying non-heat sensitive materials such as some clays and carbonate materials that require indirect heat transfer. The dryer is typically suitable for handling materials under slight negative pressure and in an inert atmosphere. Thus the dryer can prevent volatiles from mixing with hot air during solvent recovery and can prevent easily decomposed materials from oxidizing, such as organics and food products.
Because no airflow contacts the material inside the conduction dryer, the dryer provides better dust control than many convection dryers. And unlike a convection dryer, the conduction dryer also consumes only as much fuel as needed to maintain the temperature required for drying, which means that the heat load exiting the dryer is only from the material it dries. In a convection dryer, the heat load is from both the material and hot airflow, which reduces the dryer's efficiency. And because the conduction dryer can use various heat sources, including waste heat such as steam from a boiler, the dryer can cut energy costs over the long term.
Conduction dryers are available in various types for both batch and continuous processing. A batch conduction dryer can evaporate and dry solutions, slurries, pastes and granules. It can also operate under vacuum. The dryer's fuel requirements range from 1.5-3.0 pounds of steam per pound of water evaporated, depending on the material's required final moisture content. The dryer's energy costs depend on the material and the application's requirements for agitation or evacuation. The labor for charging and discharging the material and subsequent cleanup is the dryer's major operating cost. A batch dryer is available in sizes from lab-scale to full-scale production.
Two types of batch conduction dryers are available: agitating and pan.
An agitating dryer consists of a horizontal or vertical heat-jacketed vessel with a paddle or ribbon agitator. The paddle or ribbon agitates the material to aid heat transfer from the vessel wall. The dryer can handle small production volumes of nearly any form of wet material, such as solutions, slurries, pastes and granules. The agitation makes the dryer unsuitable for some degradable materials such as tea leaves, sugar and other agglomerated, fibrous or crystalline products. The unit operates at atmospheric pressure or under slight negative pressure. The dryer is easy to clean, and the agitator's pitch can be adjusted to reduce material flow and increase the material's retention time in the dryer.
A pan dryer has a horizontal, stationary heat-jacketed vessel and a flat agitating pan. In some cases, the pan is heated. The pan vibrates or rotates to help transfer heat from the vessel wall. The dryer can quickly dry large batches of nonsticky materials such as free-flowing powders, granules, crystals and fibers, and can recover solvents. The dryer's agitation creates a grinding action that can degrade some materials, so the dryer can require a dust collector. The dryer can also operate under vacuum. Removing a washdown solution from the pan after cleaning can be time-consuming.
Heyl & Patterson designs and builds dryers to provide years of trouble-free operation, and offers facilities for laboratory testing. Because we custom engineer each MultiDisc Thermal Processor and maintain the original drawings, we can service your dryer according to its original specifications. We oversee delivery, installation and start-up of your equipment, and provide preliminary operating instructions to your personnel.
We also provide manufacturer-direct parts that meet original equipment specifications, ensuring that any upgrades or repairs will meet or exceed the original equipment's performance levels. Heyl & Patterson provides superior customer support before, during and after the purchase of every product we supply.
NEXT WEEK: Comparing and Types of Conduction Dryers
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