Plastics Technology

JUL 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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The priority is to maximize movement of powders while minimizing dust exposure. 10 Things to Know When Specifying Vacuum Conveying Systems for Powders Vacuum conveying systems for powders and other bulk materials involve a starting point and a destination, avoiding many hazards along the way. Powders are transferred from various sources to the processing line using vacuum or negative pressure. The number one advantage of vacuum conveying is dust control, and in the plastics industry, combustible dust is a safety issue. Vacuum conveying controls dusts by eliminating hand scooping, delivering powders in an enclosed process without fugitive dust. If leaks occur, the leak is inward, unlike positive- pressure systems, which leak outward. Vacuum conveying is preferred by NFPA over positive- pressure conveying. In dilute- phase vacuum conveying, material is entrained within the air stream, with a complementary ratio of air and product. System controls allow material to convey and discharge on demand, ideal for larger appli- cations requiring movement of bulk materials from larger containers such as bulk bags, totes, railcars, and silos without a lot of manual intervention, reducing frequent container changes. Typical conveying rates in dilute phase may go up to 25,000 lb/hr, with typical conveying distances of less than 300 ft and line sizes of up to 6-in. diam. For smaller "up-and-in" conveying applications with conveying rates of less than 0.25 ft³/min, conveying line sizes start at 1.25 in. diam. To properly design a pneumatic conveying system, it is impor- tant to define the following 10 key criteria in your process: 1. KNOW YOUR BULK DENSITY As a first step, it is important to know more about the powder that is being conveyed, specifically its bulk density. This is typically described in pounds per cubic feet (PCF) or grams per cubic centimeter (g/cc). This is a key factor in calculating the size of the vacuum receiver. For example, lighter-weight powders need larger receivers in order for the material to fall out of the air stream. Bulk density of the material 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. 2. KNOW YOUR CONVEYING DISTANCE Conveying distances comprise both horizontal and vertical factors. A typical "up-and-in" system offers a vertical lift from floor-level, conveying up to a receiver over an extruder or loss-in-weight feeder. It is important to know the number of 45° or 90° sweep elbows. "Sweep," in general, refers to a large centerline radius that is typically 8-10 times the diameter of the tube itself. It is important to keep in mind that one sweep elbow is equivalent to 20 ft of linear tubing. For example, 20 ft vertically plus 20 ft horizontally and two 90° elbows equals at least 80 ft of conveying distance. Reducing the number of By David Kennedy VAC-U-MAX QUESTIONS ABOUT CONVEYING? Visit the Conveying Zone. To properly design a vacuum conveying system, you must define the upstream process that is supplying the material. In dilute-phase vacuum conveying, material is entrained within the air stream, with a complementary ratio of air and product. 48 JULY 2017 Plastics Technology PTonline.com Tips and Techniques

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