A: Doug Schieber, Carrier Vibrating Equipment, says:
In fluid-bed processing, air (or another medium) is passed upward through a bed of material, lifting and mixing the particles. As the air velocity increases, so does the pressure drop across the material bed until, at a certain flowrate called the fluidizing velocity, the material bed attains fluid-like properties and expands beyond the size of the stationary material bed. When the air velocity is further increased, the bed expands until particles are entrained by the air. This air velocity is called the entrainment velocity.
When drying is added to fluid-bed processing, the fluidizing air (or other medium) is heated. This air not only supplies heat to the wet material but continuously removes moisture from it via evaporation — that is, by converting the moisture to vapor. The evaporation, or vaporization, takes place in two major stages: constant rate and falling rate.
- In the constant-rate stage, drying is controlled by the heat-transfer rate — that is, moisture evaporates as rapidly as heat can be supplied to the wet material. Each particle’s surface moisture is evaporated to the surrounding air until dry spots begin to form on the particle surface. At this point, the surface moisture has mostly evaporated but the particle’s internal (or bound) moisture remains.
- During the falling-rate stage, the remaining surface moisture and the bound moisture are removed. In this stage, drying is controlled by the diffusion rate — that is, the rate at which the bound moisture diffuses from the particle’s interior to the surface. The diffusion rate is slower than the rate at which surface moisture can evaporate from the particle surface. Because the hot air adds sensible heat (that is, the portion of the heat load that changes in temperature during the heat-transfer process) to the process during the falling-rate stage, the material temperature tends to increase toward the hot air’s temperature, so less heat is used for evaporation during this stage. Drying during the falling-rate stage is complex, so dryer suppliers typically determine the drying rate during this stage based on their experience with drying similar materials or by running drying tests.
Because fluidization provides maximum exposure of the particles to the hot air, less fuel is required for fluid-bed drying than for many other drying methods, reducing energy costs and providing greater operating efficiency. The fluid-bed dryer also has few moving parts, reducing the dryer’s maintenance requirements compared with many other dryers whose moving parts require frequent inspection and service; examples are belt dryers, which contain belt conveyors with several moving components, and rotary dryers, which have large rotating drums. Because the fluid-bed dryer has no moving parts in contact with the material, the dryer also requires much less downtime for cleaning and for replacing worn parts than many other dryers.
Adding gentle mechanical vibration to this process provides vibratory fluid-bed drying. The vibration allows air to pass through the material bed at rates that are below the fluidizing velocity, thus reducing the dryer’s electrical power use while still maintaining the bed’s fluid-like properties. By continuously agitating the material, vibration also improves heat transfer, providing more efficient drying. The gentle vibrating action not only conveys the material forward through the dryer but helps mix and turn the material bed to achieve maximum temperature uniformity throughout the bed. This eliminates hot spots or wet spots that could produce inconsistent product quality. The continuous vibrating action also serves as a self-cleaning mechanism to prevent material buildup on dryer surfaces, reducing equipment downtime for cleaning.
The vibrating action makes the vibratory fluid-bed dryer especially suitable for materials that are difficult to fluidize because of their particle size, shape, or bulk density. An example is a material with a wide particle size distribution; in this case the vibration helps to discharge oversize particles that won’t fluidize. Using vibration in fluid-bed drying also allows the dryer to effectively handle sticky or poorly flowing materials by promoting their forward movement through the dryer. A vibratory fluid-bed dryer is ideal for fragile materials because the unit’s low-amplitude vibration and low fluidizing velocity create a gentle fluidizing action that reduces degradation. Vibratory fluid-bed drying is also ideal for temperature-sensitive materials, such as foods, or those that combust at relatively low temperatures, such as wood chips or sawdust, because the vibrating action promotes the material’s constant forward movement, preventing individual particles from being overheated or underdried.
Carrier Vibrating Equipment, Louisville, KY, designs and manufactures vibrating conveyors, feeders, screeners, fluid-bed dryers and coolers, flash dryers, spiral elevators, and other equipment.