More Questions and Answers
Q: When designing a pneumatic conveying system, is there a reason to prefer vacuum conveying over pressure conveying when using a bulk bag (FIBC) as the material source?
A: Gary Saunders, Smoot, a div. of Magnum Systems, says:
Yes. Typically, pneumatic conveying systems transferring from a bulk bag unloader are designed as vacuum systems, if there are no extenuating circumstances.
Pressure systems require an airlock at the pickup point. In most pneumatic conveying systems, this airlock comes in the form of a rotary valve. Rotary valves, however, leak air in pressure service. The leakage, better known as blow-by air, can interfere with a bulk bag's discharge if it has no other place to go, by turning the bag into a balloon. This can be worked around by adding a second valve and intermediate vent, but this adds height and cost.
Vacuum systems, in contrast, require their airlock at the system discharge, not the pickup point. Since the pickup point is at or near ambient pressure, typically no leakage occurs. This allows the bag to discharge more freely and at a more ergonomic height for the operator.
As you can see, while not an impossibility, design complications, costs, or height issues can quickly ensue. If you're at all unsure what design may be optimal for your application, contact an experienced design engineer.
Smoot, a div. of Magnum Systems, is a process company specializing in pneumatic conveying, packaging, and automation.