As appeared in April 2011 PBE Copyright CSC Publishing www.powderbulk.com Answers to five common questions about vibratory fluid-bed dryers Doug Schieber Carrier Vibrating Equipment A vibratory fluid-bed dryer efficiently dries a range of bulk solid materials in a continuous, onestep process. Using a Q&A format, this article discusses the vibratory fluid-bed dryer s advantages and operation and explains how to select one for your application. The vibratory fluid-bed dryer can process a wide range of materials. Examples include fertilizers, pesticides, pharmaceuticals, salt, sugar, powdered milk, cereals, candy, wood chips, explosives, carbon black, stone, limestone, graphite, ores, sand, resins, glass cullet, fiberglass, and crumb rubber. Dryers are energy-intensive machines, and finding the right one for your application means choosing a dryer that will not only yield high-quality dried product but operate as efficiently as possible. In a vibratory fluid-bed dryer, particles receive maximum exposure to the drying air, resulting in high energy efficiency. To help you decide if this and other operating characteristics make the vibratory fluid-bed dryer right for your material, check out the following answers to some frequently asked questions about the dryer. 1What are the advantages of using a vibratory fluid-bed dryer? A vibratory fluid-bed dryer provides simple one-step processing and high operating efficiency. The dryer produces high-quality dried material and, because it has few moving parts, requires less maintenance than other dryer types. It operates continuously, minimizing the operator attention it requires. Incorporating a cooling zone after the drying zone also allows the equipment to function as a dryer-cooler. 2How does a vibratory fluid-bed dryer work? Understanding the dryer s operation will be easier if we start by looking at the dryer s components, as shown in Figure 1. In addition to the vibratory fluid-bed dryer itself, major equipment in the drying system typically includes a feed hopper and a feeder at the dryer inlet, a discharge conveyor after the dryer outlet, an air heater (such as a burner or steam coils), an inlet air manifold, an exhaust air manifold, a supply blower, a dust collector, an exhaust blower, and controls. The dryer typically has a three-piece design that includes an insulated hot air plenum, a fluidizing deck that rests on the plenum, and an insulated exhaust hood above the fluidizing deck. A drive consisting of rotating eccentric weights is attached to the hot air plenum. This entire assembly rests on soft isolation springs. The material inlet and outlet are at opposite ends of the dryer. The inlet air manifold connects the air heater to the hot air plenum, and the exhaust air manifold connects the exhaust hood to the dust collector. The supply blower before the air heater pressurizes the hot air. The exhaust blower after the dust collector creates a slight negative pressure in the exhaust hood.
Figure 1 Typical vibratory fluid-bed dryer and related system components Vibratory fluid-bed dryer Exhaust hood Exhaust air manifold Feed hopper Dust collector (baghouse) Feeder Hot air plenum Exhaust blower Inlet air manifold Discharge conveyor Air heater Supply blower Fluidizing deck (inside dryer) Adjustable weir (inside dryer) Drying operation. In operation, wet material from the feed hopper is metered into the dryer inlet by the feeder, which can be any of various types to suit the material s characteristics. The vibratory drive powers the rotating weights to vibrate the entire dryer, while hot air (or another heating medium such as nitrogen or superheated steam) is blown through the hot air plenum and forced upward through the openings in the fluidizing deck. As the vibration conveys the material toward the outlet, the hot air fluidizes and dries the particles. The dried material exits the outlet and lands on the discharge conveyor, which moves it to the downstream processing or handling operation. Meanwhile, fines entrained in the dryer s exhaust air are drawn through the exhaust air manifold into the ductwork leading to the dust collector, where the fines are collected so the clean air can be exhausted to the atmosphere. More about the fluidizing deck. The fluidizing deck is where hot air is introduced to the material and most of the material contact with the dryer occurs. This deck is typically a solid plate with drilled holes, but can also be a solid plate with nozzles, a ceramic grid, or a punched plate. The wet material from the feeder is introduced to the deck through the dryer inlet and is discharged from it through the outlet. An adjustable weir is sometimes included near the outlet, which allows small adjustments to the material residence time. More about the vibratory drive. The vibratory drive provides a vibration force with a predetermined amplitude, frequency, and angle relative to the fluidizing deck. A properly designed vibratory fluid bed will provide the minimum amount of vibration force required to consistently move and help fluidize the material. Because the magnitude of vibration force imparted to the material is mainly a function of the amplitude and frequency, it s desirable to hold the amplitude and frequency constant and vary the vibration angle to adjust the material residence time. This adjustment is particularly important when processing a temperature-sensitive material or when the dryer is used for processing more than one material. Optional cooling operation. When the vibratory fluid-bed unit is used as a dryer-cooler, it has both drying and cooling zones. In the dryer-cooler shown in Figure 2, drying occurs in the first two-thirds of the unit, which is insulated, and cooling occurs in the final third just before the outlet, which typically isn t insulated. The blower at left sends air through the air heater into the drying zones, while the blower at right blows ambient air into the cooling zone. Moist air exhausted from the drying zones flows to the dust collector and is exhausted through the stack to the atmosphere; warmer, dryer air exhausted from the cooling zone flows to the dust collector and is recycled to the air heater, reducing the energy required for heating air.
3How do fluid-bed processing and vibration enhance drying? Fluid-bed processing. 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 (discussed later in this article). 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 fluidbed 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. Figure 2 Vibratory fluid-bed dryer-cooler Duct for recycling warm, dry air from cooling zone to air heater Cooling zone Drying zones Blower supplying ambient air for cooling
Vibratory processing. 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. By continuously agitating the material, vibration also improves heat transfer, providing more efficient drying. 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. 4How can a vibratory fluid-bed dryer be customized for my application? A vibratory fluid-bed dryer can be customized in several ways. Handling dusty materials. If your material contains a significant amount of dust, you can select one or more types of dust separation equipment to remove it. For instance, if the dust will contain larger particles, you can select a cyclone that will remove and return the larger particles to the material stream exiting the dryer while passing fines onto a baghouse or cartridge dust collector. 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 Appropriate fire and explosion protection and suppression systems can be added to your dryer and related dust collection equipment to meet National Fire Protection Association (NFPA) and OSHA explosion-safety requirements for handling combustible dust. Cooling. If you want to cool the dried material before it s discharged from the dryer or recycle the dryer s warm, dry exhaust air, you can use the last portion of the dryer as a cooling zone, making the unit a dryer-cooler, as discussed previously. The cooling zone can be cooled by either ambient air or chilled air. Ambient air will typically cool the dried material to 20 F above ambient, so on an 80 F day, ambient air can cool the material to 100 F. Or the cooling air can be chilled by a refrigeration system after the blower if you need a lower material discharge temperature relative to ambient. Whether the cooling air is ambient or chilled, the warm, dry air exhausted from the cooling zone can be recycled to the system s air heater to conserve the energy required for heating air (Figure 2). You can also use the vibratory fluid-bed unit strictly as a cooler to reduce the temperature of a very hot material, such as material discharged from a rotary or fluid-bed calciner, which can be 1,000 F or more, before it passes to downstream equipment or packaging. The vibratory fluidbed cooler s construction and operation are similar to that of the dryer. This dryer-cooler produces dried sand for use as a proppant in oil and gas drilling; the unit receives wet sand from a belt feeder and discharges the dried sand to a bucket elevator that carries it to a screening operation. Using multiple heating zones. The vibratory fluid-bed dryer can be designed with multiple temperature-controlled zones for processing temperature-sensitive materials. In such a unit, it s not uncommon to have three or four zones, each with an independently controlled air temperature.
Using other heating media. If you need to use a heating medium in the dryer other than air, you can operate the dryer in a closed-loop system. This allows you to recycle the heating medium, improving the dryer s energy efficiency. Typical media other than air are nitrogen, which can be used to reduce material oxidization or combustion and explosion risks or to recover solvents (such as isopropyl alcohol or ethanol) from the feed material, and superheated steam, which can be used for the same applications or simply to conserve energy. Meeting sanitary and cleaning requirements. To meet your material s sanitary or other industry requirements, the dryer can be constructed of stainless steel or other alloys. The dryer can also be equipped with clean-in-place components to simplify and speed routine cleaning. An important part of this collaboration is providing detailed information about your material and drying requirements to the supplier. The supplier can save you time and money during the selection process by testing samples of your material in a vibratory fluid-bed dryer in its test facility. You can witness the tests in person or, in many cases, view a video of them; some suppliers also provide field-rental dryers so you can obtain test data at your plant. The data recorded during the tests includes moisture content, fluidizing velocity, material temperature, exhaust air temperature, and material residence time. The supplier s analysis of this data and the material characteristics and behavior charted during the tests will help you choose your dryer s components, size the dryer, and develop operating parameters for your application. PBE For further reading Find more information on fluid-bed drying in articles listed under Drying in Powder and Bulk Engineering s comprehensive article index (in the December 2010 issue and at PBE s Web site, www.powderbulk.com) and in books available on the Web site at the PBE Bookstore. You can also purchase copies of past PBE articles at the Web site. 5What should I expect from the supplier during dryer selection? Selecting the right vibratory fluid-bed dryer for your application requires considering several details about your material and drying requirements. For one thing, the behavior of particles in a fluid bed depends on their size, shape, and other characteristics. Drying time for a given material also depends on multiple factors, including the feed material s moisture content and temperature, the final product s desired moisture content, and others. Collaborating with the supplier s experienced design engineers can help you make sense of these complex details and ensure that the dryer you choose not only yields a consistent, high-quality product but also operates efficiently. Doug Schieber is sales manager at Carrier Vibrating Equipment, Box 37070, Louisville, KY 40233-7070; 502-969-3171, ext. 271, fax 502-969-3172 (dschieber@carri ervibrating.com, www.carriervibrating.com). He holds BS and MS degrees in mechanical engineering from the University of Louisville in Kentucky. An important part of this collaboration is providing detailed information about your material and drying requirements to the supplier. Many suppliers provide an application data sheet for this. The material data you ll need to provide includes particle size, particle shape, bulk density, specific gravity, chemical composition, ph level, chemical reactivity, volatile characteristics, temperature limits, and fire and explosion hazards. Information about your drying requirements includes the moisture content and temperature of the material at the dryer inlet, desired moisture content and temperature of the material at the dryer outlet, fuel required for the air heater, material feed source, material destination, ambient air condition, required drying capacity, and dryer location.