Reliable feeding requires proper refilling
In Brabender Technologie’s new Technical Center, filling and refilling processes can be accurately tested using the second refill level. “Many users underestimate how critical these processes are for weighing, especially during continuous production”, Ralf Eikermann, Head of Customer Care, relates. That is why installation and maintenance of all the equipment should be made by a single organization.
“We are familiar with this issue from our service visits”, Ralf Eikermann tells us of his team’s experiences. “Customers often purchase feeders that can be incorporated with their existing refilling systems. For large high-rate feeding equipment, a robust steel structure is integrated with the feeder system to support the weight of the equipment and bulk material. For example, when a large bag is emptied into the refill hopper, it generates a substantial dynamic force that can temporarily affect weighing.” This impacts feeding accuracy, “although steel is strong, it is also flexible. We can’t see it with the naked eye, but if the refill device comes into contact with the weighing unit, the resulting force confuses the scales. They feed in higher quantities, as they are measuring a gain in weight.” The screw’s rotational speed then increases. “Consequently, customers call to tell us that their feed rate is inaccurate. However, the fault lies in the structural design because the feeder scales weigh more than just the bulk material.”
The venting problem
At a major customer event on September 20, 2018 in Duisburg engineers at Brabender Technologie set up three demonstrations of typical refilling problems. Insufficient equipment support was one of them. Another demonstrated a typical problem that occurs when powder is fed. The problem was obvious – a feeder normally feeds powder very uniformly, however, during refill the bulk material discharged uncontrolled at a high pressure out of the screw tube. What happened? The dust sock had blinded and therefore the screw tube was the pressure release point. When the feeder’s hopper is filled, the displaced air needs to escape. That would normally happen via a dust sock. If this is clogged, then the screw tube is the only remaining option for the air to escape, causing material to discharge in an uncontrolled manner.
“It’s something we come across again and again during our service visits: customers refill feeders very fast and in very large quantities to keep the volumetric feeding phase as short as possible during the gravimetric cycle”, Ralf Eikermann relates from experience. The Head of Customer Care responds, “But they achieve exactly the opposite – there is a lot of air in a large empty feeder hopper and this air must escape during the refilling process. If the dust sock is undersized, the air pushes towards the only open point, the screw tube causing a temporary uncontrolled high feed rate. The solution to this problem – either the sock needs to be cleaned regularly or utilize a self-cleaning jet filter.
A jet filter removes dust from the air that is displaced out of the hopper during filling. In contrast to normal dust socks it is also self-cleaning – it automatically cleans the filter with a pulse of compressed air that blows trapped ingredient back into the process. In a continuous process, jet filter cleaning occurs at the end of the refill during the material stabilization phase without interfering with the gravimetric cycle. When very expensive or toxic materials are involved, a jet filter is a must, but it can also be a good option for other powders and fine pellets. “The best thing for users to do is seek our advice”, Jochen Keesen, Head of the Technical Center, recommends. “We can help select the right filter and cleaning intervals.”
Maintenance is key
In the third demonstration line, a feeder was attached to a vacuum conveyor. Both units are separated by a knife gate valve. “But this valve has to be monitored regularly and maintained”, relates Jochen Keesen, who set up the demonstration. “Otherwise product residues or a defective valve could result in the valve not fully closing and thus leaving a small gap.” The vacuum conveyor then causes negative pressure inside the feeder, which has an effect on the scales. “As a result, the weight reading is reduced because there is permanent upward suction.” This is displayed on the measurement screen as follows: the screw speed initially decreases briefly and then increases sharply, because this suction makes the hopper seem empty. Once the pressure evens out again, too much material is fed initially, because the screw speed is still too high.
Many questions were asked during the demonstration in the Technical Center showing that Brabender Technologie had increased the visitors’ awareness of “refilling” problems. “Continuous processes are becoming increasingly popular”, says Ralf Eikermann. “For us that means increasing customer awareness of refilling issues.”
(published in FLUX 2/2018)