What to Know When Selecting Drying Equipment
Asking questions about the material to be dried, requirements for drying, production and final product quality, and your physical plant will help you define what you do — and don’t — need in a process dryer.
Because an industrial dryer is a major capital investment for any organization, it pays to select and test the unit carefully. Dryer life can be measured in decades, so the selection of inefficient equipment can have a long-term impact on the economic health of the facility that houses it. The selection of drying equipment is predominantly an exercise in which knowledge, experience and science all play important roles. Because of the complexity of the drying process, many factors need to be considered and weighed.
Before choosing an industrial dryer, conduct a preliminary analysis of your drying needs. Ask questions about your material, the production and final product quality requirements, and your plant.
Material. What are the material’s physical characteristics when wet or dry? What is the particle size distribution? Is the material toxic, flammable, corrosive or abrasive?
Drying Requirements. Is the material’s moisture bound (chemically trapped inside the particles), unbound (not attached to the particles, which is also called free moisture) or both? What are the material’s initial and final moisture contents? What are the maximum permissible drying temperature and probable drying time for the material? What is the drying curve? Will the drying temperature need to fluctuate midway through the drying process?
Production Requirements.Does the material need to be batch or continuously processed? What quantity of material must the dryer handle per hour? What retention time will achieve a high-quality final product? How will the processes before and after drying affect the dryer choice?
Final Product Quality Requirements.Can the material shrink, degrade, over-dry or become contaminated during drying? How uniform must its final moisture content be? What should the final product temperature and bulk density be? Will the dried material give off dust or require solvent recovery?
Plant. How much space is available in the plant? How warm, humid and clean is the plant air? What fuel and power sources, wet feed sources and exhaust gas outlets are available in the plant? What levels of noise, vibration, dust and heat loss are permissible in the plant according to local zoning and environmental regulations?
By answering these questions, you will rule out many dryers that are not suitable for your application. For instance, a material’s physical or handling characteristics will eliminate some dryers. A steam-tube rotary dryer is not a good choice for a wet, sticky material such as mica. The dryer moves material by rotating and rolling it, and such passive conveying cannot move a sticky material out of the inlet area before it cakes on the vessel wall and steam tubes. A screw conveyor or indirect multiple disc dryer is a better choice. Either unit provides positive displacement that can quickly move the mica away from the inlet and toward the outlet.
Next, consider the footprint or required floor space of the remaining dryers that meet your requirements. Rule out any dryer that does not fit the existing space or would require a costly plant renovation or expansion. Compare the capital and operating costs and performance of the remaining candidates.
If you are selecting a high-performance dryer to upgrade your existing drying process, consider whether existing ancillary items such as storage and receiving equipment, conveyors and pollution control equipment can handle the new dryer’s increased capacity.
Once the field of dryer choices has been narrowed, test the dryers with your material under your operating conditions.
Dryer tests can be conducted in a pilot plant lab facility and should establish:
• The optimal operating conditions for your material.
• How well each dryer handles the material’s physical characteristics.
• The material’s quality and characteristics after drying.
• The appropriate dryer size.
Based on these results, a dryer manufacturer can develop detailed recommendations for equipment that will meet your drying needs. With recommendations in hand, you can select a dryer and drying options. Factors to discuss include the dryer’s installation and operating costs, the dryer’s operation and maintenance requirements.
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