Unlike screw systems, vacuum conveyors can curve around reactors and deliver particles to an upper mezzanine with a 90° bend. While not as low-cost as screw conveyors, they are relatively inexpensive and require little maintenance. Vacuum lines even purge themselves (with a vacuum, of course) when switching between products. They also simplify dust control, because any leak will suck in air rather than blow out powders.
Vacuum conveyors also have disadvantages, notes Joe Morris, vice president of sales and marketing for Cyclonaire, York, Neb. They are limited in length, generally to no more than 100 ft, and usually to moving material to a single point because switching destinations requires a vacuum receiver at each location as well as a complex system of valves.
Also, vacuum conveyors that rely on high airflow can pulverize fragile products. In such lean-phase systems air speed can exceed 1 mile per minute. This is fast enough to turn tablets into fine powders the first time they reach a 90° bend.
Use of lower air speed can solve this problem. Intermediate-phase systems, for example, provide just enough vacuum to entrain some powders while letting others drop to the bottom of the line. As these powders build up, they impede airflow. This increases suction and air speed at that point in the line, pulling the powders forward in gentle waves. The result is smooth and relatively gentle transport.
However, this approach doesn't work well with mixtures of fine and coarse particles. Dense-phase conveying solves this problem by ratcheting down air speeds further. This causes powders to build up until they plug the line. The vacuum eventually pulls the plug of material slowly through the line. "It moves so slowly you can convey very delicate products without damaging them," says Hayes.
Even the slowest vacuum conveyors can achieve large increases in productivity by enabling companies to switch to more flexible intermediate bulk containers (FIBCs), also known as bulk bags, from smaller packages such as 50-lb bags. The 1-ton bulk bags are simpler to handle and store. In addition, companies often can get substantial discounts for purchasing material in FIBCs.
More importantly, though, use of bulk bags simplifies process automation. Instead of manually handling 50-lb bags, say, to fill a hopper above a mixer, a vacuum line from the bulk bag can charge the material. Adding the vacuum system simplifies process flow and eliminates labor and the risk of injury from lifting.
"You can add sensors and it will automatically dose it, monitor the hopper, and send a message to the control room when it is time to get a new bag. It's cleaner and more convenient," says Morris. "We had people say they wanted to load up a weekend's worth of product. If something goes wrong, the system calls you on the phone. Otherwise, you come back in on Monday," he adds.
Even in low-volume applications, where bulk bags make little sense, vacuum systems can simplify process flow (Figure 1) and reduce the potential for injuries caused by lifting and unloading (Figure 2).
Many companies opt for vacuum systems to save labor, Heller notes. "If they can save a job here or there in terms of cleaning or maintenance, they want to do that, too," he adds.