Heyl & Patterson: Fluid Bed Dryer Fundamentals

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Fluid Bed Dryer Fundamentals



by David Phillips

Heyl & Patterson, USA


The ancient Greek mathematician Archimedes is best known for inventing a screw pump mechanism to transfer low-lying water into irrigation ditches. He also developed the scientific rule in which a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces. Archimedes' Principle basically means that objects will either float or sink in a fluid depending upon their density, and is considered one of the fundamental laws of physics. This is one of the ideas behind the functionality of fluid bed dryers. In fluidized bed systems, air or another gas streams upward through a bed of bulk material, lifting the material and causing the particles to float and behave like a fluid. Archimedes may have been one of the most brilliant scientists in classical antiquity, but he likely never imagined how his principle would be applied toward industrial drying equipment.


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A fluid bed dryer features high operating efficiency, ease of use and low maintenance costs, and is an attractive option for bulk solids drying applications. When determining if a fluidized bed dryer is the right equipment for a given application, the most important material properties to be aware of are the particle size distribution, specific gravity, particle shape and material handling characteristics. Drying requirements also include the required feed rate, feed moisture content, discharge moisture content, material temperature limit and available fuel source. All of this information is used to engineer a dryer that will successfully handle any application.

Based on these factors, a heat and mass balance can be calculated for the application's air flow requirements. This will also determine whether a circular or rectangular dryer is best suited to the available space and the required discharge moisture content, along with the dryer size.

The dryer must have a certain area to provide an adequate fluidizing air velocity. Known as the minimum fluidization velocity, this is the air velocity that will fluidize the largest particles in the material. The air velocity should not reach the terminal velocity of the smallest particles, which is the velocity at which they will become entrained in the exhaust air flow and be removed from the dryer. Ideally, the fluizidation velocity should be slightly above the largest particles' minimum fluidization velocity but less than the smallest particles' terminal velocity.


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Determining the ideal fluidization velocity is why it is important to know the material's particle size distribution. If this is not known, then the material can be tested, either by a dryer supplier or an independent material characterization lab. Having a supplier test the material in a pilot plant fluid bed dryer can provide more accurate results.

Next, the dimensions of dryer's main body and freeboard section must be selected. The right height will achieve the material's ideal fluidized bed depth, and will typically be from several inches to a few feet, depending on the application. The bed depth determines the material's residence time in the dryer, which is critical to achieving the application's required drying time.

The width of a rectangular dryer or diameter of a circular dryer is determined by considering any particle entrainment that may occur in this section. If the dryer's fluidization velocity exceeds the smallest particles' terminal velocity, then choosing a freeboard section wider than the drying chamber will slow the air velocity, reducing particle entrainment. The freeboard section does not usually need to be much wider than the drying chamber. For example, air moving from a circular dryer's three-foot-diameter drying chamber into a four-foot-diameter freeboard section will slow by 44 percent.

A supplier typically will custom design the remaining components in the drying system based on the application requirements. Once the design itself is completed, the supplier will provide guidance in the installation and start-up of the entire drying system.

Heyl & Patterson manufactures fluid bed dryers in conventional designs for powders and granular materials, as well as unique designs for materials which exhibit characteristics not normally conducive to fluid bed processing, such as sludges, filter cakes and agglomerates. Heyl & Patterson dryers have several distinct features and benefits which make them particularly desirable for applicable processes, including high thermal efficiency, completely pneumatic fluidization, relatively small equipment footprint, uniform product quality and low initial capital cost. Both circular and rectangular styles are available, and applications can be tested at Heyl & Patterson's pilot plant lab facility.

For more information about fluid bed dryers, contact Heyl & Patterson.

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