Plastics Technology

JUL 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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elbows in your system should be an important goal. Powders do not flow like liquids, and vacuum conveying systems should mini- mize the number of back-to-back elbows. If vertical vacuum con- veying runs exceed 12-15 ft in batch mode, the material in that line will stop conveying and fall back down. For this scenario, it is important to design a "line-clearing valve" to make sure that the vertical leg is clear before the batch process stops. 3. KNOW YOUR CONVEYING RATE When calculating conveying rates, it is important to know how many pounds or kilograms per hour will be conveyed. Additionally, define whether the process is batch or continuous. For example, if a process requires conveying 2000 lb/hr of product, but the batch process requires conveying 2000 lb every 5 min for 1 hr, this really amounts to 24,000 lb/hr. This is the differ- ence between 2000 lb in 5 min vs. 2000 lb over a 60-min period. It is important to know the demands of the process in order to prop- erly size the system to determine the conveying rate. 4. KNOW YOUR BULK MATERIALS CHARACTERISTICS In the plastics industry, there are many different bulk material characteristics, particle shapes, and sizes. Powders can be fine, cohesive, or floodable. Other characteristics include pastilles, pel- lets, flakes, granules, and so on. Understanding particle size and size distribution is very important when sizing the receiver and filter assembly, for either mass-flow or funnel-flow distribution. Other considerations include defining whether the material is free-flowing, abrasive, or combustible; whether it absorbs moisture; and whether there potentially could be chemical compatibility issues with con- veying hoses, gaskets, filters, or process equipment. Additional char- acteristics include materials that are "smoky" like talcum powder, which has a high fines content, requiring more filter area. For non- free-flowing materials with a high angle of repose, special consider- ations need to be given to the receiver design and discharge valve. 5. KNOW HOW YOU RECEIVE THE RAW MATERIALS When designing a vacuum-conveying system, it is important to clearly define how the materials are received and introduced to the process. There are many ways of introducing materials into a vacuum- conveying system; some are more manual, while others are more suitable to automation—and all require attention to control dust. Material can be received in paper bags, fiber drums, barrels, bulk bags, super sacks, rigid intermediate containers, silos, day bins, railcars, bulk trucks, or upstream process equipment. To maximize dust control, bulk-bag unloaders incorporate enclosed vacuum- conveying lines, and bag-dump stations integrate dust collectors. Material is conveyed from these sources by filter receivers and then conveyed to the process. Bulk material characteristics play a vital role in properly sizing vacuum receivers and filter assemblies Lighter-weight powders need larger receivers in order for the material to fall out of the air stream. Material bulk density is also a factor in calculating the size of the conveying line, which in turn determines the vacuum producer and conveying velocity. Higher-bulk-density materials require faster transport velocity. @plastechmag 49 Plastics Technology C O N V E Y I N G

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