www.powderbulk.com As appeared in April 2013 PBE What to consider when selecting a convection dryer Dan Poirier Buhler Aeroglide A convection dryer is by far the most commonly used drying equipment in bulk solids plants. Because the dryer is available in several types, choosing the right one for your process can be challenging. You can simplify the task by following this article’s advice about what factors to consider during dryer selection and the importance of working closely with the dryer supplier. A sidebar provides additional guidance on selecting a convection dryer to meet food safety requirements. B ulk solid materials are dried to meet a range of product goals. Many materials are dried to bring about physical or chemical changes or reduce shipping costs. Food products are usually dried to enhance shelf life. No matter why you need to dry your material, there are many factors to consider when selecting a dryer and its features. This article focuses on what to consider when choosing a convection dryer, in which heat is transferred by the direct contact of a hot gas (typically air) with the material. In a convection dryer, the hot air typically flows across the material or through a stacked material bed. Several types of convection dryers are available, and no one type can handle all applications. For instance, some dryers are wellsuited to drying small particles, some can’t handle sticky materials, and others can handle only liquids. Your mate- rial’s characteristics will dictate the ideal convection dryer for your application. The following sections discuss how to match the dryer type to your material and other application factors when specifying the dryer. During the selection process, it’s best to work closely with an experienced convection dryer supplier. The supplier’s understanding of the science of thermal processing will help you choose a dryer with the features you need to achieve your desired drying results with the best possible efficiency. Consider how your material will be handled in the dryer Table I briefly summarizes the operation of common convection dryers and their applications. As you can see, the most obvious difference among the dryers is in how they handle material. Some convection dryers, such as conveyor, tray, and tunnel dryers, handle the material very gently. In these dryers, the particles are stacked on a moving belt or on trays, and drying air circulates around the particles. Other dryers, such as fluid-bed and rotary dryers, are much rougher on the material: The drying air fluidizes the particles — in some dryers, with high-energy air jets — or the dryer tumbles the particles. While some materials will be damaged if they’re fluidized or tumbled, other materials may require a rotary dryer’s tumbling action to break up clumps so that all surfaces will contact the drying air. These differences often make a convection dryer’s material handling method the primary consideration in selecting a dryer type. Table l Operation and applications for common convection dryers Dryer type Operationa Applications Conveyor Particles are stacked on a perforated conveyor belt that moves through the dryer as air flows through the particle bed; dryer may consist of a single conveyor or multiple conveyors that are stacked or staged. Materials consisting of stackable particles from 2 to 25 millimeters, such as extruded foods and feeds, cereals, fruits, vegetables, nuts, seeds, industrial fibers, and synthetic rubber and polymers Flash Small particles are entrained in a fast-moving hot airstream and dried as they’re pneumatically conveyed through the dryer. Materials consisting of particles below 0.5 millimeters that are easily entrained in air, such as wood fibers and tobacco Fluid-bed Particles are stacked in a bed that’s fluidized by upward airflow through the bed. Materials consisting of discrete particles usually below 5 millimeters with relatively uniform size distribution, such as polymers and pharmaceuticals Impingement Material is conveyed through the dryer on a flat conveyor or rotating drums while high-velocity air jets impinge on the material. Flat materials, such as sheets, webs, and flat snack products Rotary Particles are tumbled inside a rotating drum while air passes through the drum. Friable (crumbly) materials, such as biomass and many waste by-products Spray Liquid material is sprayed into a hot airstream and forms a powder as the atomized droplets dry into solid particles. Liquids such as dairy, dyestuff, and detergents Tower Material is introduced through the top of a tower and flows through a heated section, then a cooled section, while air passes through the tower. Grains such as rice, corn, and soybeans Tray or tunnel Material is laid on trays (often manually) that are stacked and introduced into the dryer and air flows across or through the trays; tray dryer operates only in batches and tunnel dryer can operate in batches or continuously (with tray stacks moving through the dryer). Hard-to-handle materials or materials that must be processed in batches, such as pharmaceuticals, fruit, and natural pet treats Note: All dryers operate continuously unless otherwise noted. a Determine the best heat source for the drying air The convection dryer’s drying air can be heated by any of several heat sources, including burners, steam coils, thermal oil coils, or waste heat from another process. Fuel availability at your location will often dictate which heat source you use. Natural gas is a clean-burning fuel that can often be directfired in the airstream, even in food dryers. As a result, it’s the easiest fuel to use in convection dryers. Other fuels, such as oil or biomass, are often burned in a boiler instead of directly in the dryer. While steam coils can also be used, they can only heat the air to less than 347ºF (175ºC) unless they have very high steam pressure. This can make some convection dryers, such as rotary dryers, impractical when only steam is available because they require higher-temperature air to operate efficiently and reduce the dryer size. Sometimes, free heat from another process is available to heat the dryer air. For instance, conveyor dryers for drying charcoal briquets use hot exhaust air from upstream furnaces as the sole heat source. Consider energy efficiency Unfortunately, evaporating water from wet material takes a significant amount of energy, so whatever dryer you choose, it will likely be the highest energy consumer in your plant. There are many ways to minimize your convection dryer’s energy use. Recirculating the drying air to minimize the amount of warm air exhausted from the dryer is the first step. Beyond that, you can also preheat makeup air to the dryer or recover heat at the dryer exhaust to significantly reduce energy consumption. Another often-overlooked but key way to reduce energy consumption is to make sure that the dryer is operating properly. Meeting food safety requirements with a convection dryer Food safety doesn’t end with the design of the dryer itself: The dryer is just one step in your process. Choosing the wrong dryer can result in unforeseen problems else- where in the production line. If you’re selecting a convection dryer for food products, partner with a supplier that understands food safety requirements. —D. Poirier F ood safety is highly regulated and critical for food producers, so drying food products comes with more requirements than drying other products. Food safety considerations for convection dryers include minimizing flat surfaces where fines can accumulate, ensuring good access for cleaning, and designing the equipment for washdown. This conveyor dryer for drying breakfast cereal uses multiple conveyor beds and is equipped with large access doors to simplify cleaning. [Editor’s note: Find more information about reducing a convection dryer’s energy use in the author’s previous Powder and Bulk Engineering article “Big dryer, small footprint: Minimizing your convection dryer’s environmental impact,” April 2008, pages 27-31; also see “For further reading” later in this article.] Consider drying uniformity Drying your material to a uniform moisture content is critical. Nonuniform drying can lead to product spoilage or inferior quality if the dried product has more or less than the required moisture content. It can also lead to lost revenue if you’re forced to overdry products to compensate for nonuniform drying. The revenue gained by reducing lost product weight due to overdrying can often pay for a new dryer in less than your required payback period. A simple way to determine if a dryer will dry uniformly is to ask the supplier whether every particle will experience the same processing parameters as it passes through the dryer. If some particles spend less time in the dryer than others, they’ll be underdried. Likewise, if some are exposed to lower-temperature air or less airflow than other particles, they’ll be underdried. Consider long-term supplier support Proper dryer operation is critical not only to achieve good energy efficiency, but to ensure good product quality and drying uniformity. You’ll need support from your dryer supplier to ensure that your dryer is operating properly. The supplier should offer training and come to your plant to help workers learn how to operate the new dryer. Because downtime costs can rapidly add up when a dryer breaks down, the supplier should also be able to supply spare parts quickly. When you select a dryer, you’re not only buying the equipment, you’re entering into a long-term relationship with the dryer supplier. You’ll be living with the equipment and the supplier for decades to come. Consider life-cycle cost Finally, of course, cost will come into your dryer selection equation. Be sure to consider not just the dryer’s capital cost, but, more importantly, its life-cycle cost. The cost of running a dryer is far greater than the capital cost. For instance, you could easily spend over $6,000,000 in today’s dollars on fuel alone to run a $500,000 dryer over its 20- A biomass burner (at right) is the drying air’s heat source for this rotary convection dryer. year lifespan. The costs of maintenance downtime, revenue losses resulting from nonuniform drying, and inefficient energy use could surpass the dryer’s capital cost in less than 1 year if you choose the wrong dryer. PBE For further reading Find more information on convection dryers in articles listed under “Drying” in Powder and Bulk Engineering’s article index (in the December 2012 issue and at PBE’s website, www.powderbulk.com) and in books available on the website at the PBE Bookstore. You can also purchase copies of past PBE articles at www.powderbulk.com. Dan Poirier is director of process engineering at Buhler Aeroglide, 100 Aeroglide Drive, Cary, NC 27511; 919851-2000, fax 919-851-6029 (dan.poirier@buhler group.com, www.buhlergroup.com/drying). He holds a masters degree in applied sciences from the University of Ottawa, Canada. Drying workshop to be held at PBE’s 2013 Midwest Conference & Powder ShowTM A 3-hour technical workshop on drying, “Understanding factors that affect dryer performance,” will be held at PBE’s upcoming Midwest Conference & Powder ShowTM in Columbus, Ohio, May 21-23. Workshop instructor Karl Jacob from Dow Chemical will take a practical approach, focusing on key aspects of reliable and optimal dryer operation. For more information and to register, visit www.pbepowdershow.com.
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