Good vibrations with the hopper discharge unit
In our new series, we examine typical problems of discharging and refilling and explain the corresponding technical solutions. We start with the hopper discharge unit.
Refilling sounds like an easy task, but the flow behavior of a bulk material varies with it’s particular composition. This becomes evident during filling, and is apparent during the discharging and refilling processes. Flour, starch, titanium dioxide or chalk are classic examples of extremely poor-flowing bulk materials which cannot be discharged evenly due to their compacting flow characteristics. Their high bridging tension causes them to compact in the discharge area, which slowly clogs up with the product. This can lead to failures or even damage to the unit.
Relieving the outlet, preventing product backup
The hopper discharge unit is installed underneath a hopper or silo. With its’ vibrating discharge bottom, it ensures even and continuous product discharge from the silo or hopper. The crucial element for this is the annular gap – the gap between the relief plate and the ring-shaped outer wall. Acting as a “double bottom”, the relief plate absorbs the material pressure of the product mass in the hopper to prevent the material pressure from weighing on the outlet of the discharge bottom and prevents product backup.
Norbert Marek, Head of the Spare Parts department at Brabender Technologie, explains: “If the annular gap is too large, the improvement effect for the flow characteristics provided by the relief plate is canceled out. The capacity is therefore always calculated based on the size of the annular gap. The ratio between ring gap and outlet diameter is the decisive factor here.” If required, the annular gap can be precisely adapted to the individual requirements of the application with different options.
Additional option: venting nozzles
There is an option to equip the side wall of the discharge bottom with compressed air nozzles. The blasts of compressed air ejected from the flat nozzles form a lateral air cushion between the adhering product and the hopper wall, reducing friction and activating (without fluidizing) the compacted product in the discharge area. Norbert Marek points out a possible disadvantage in this context, though: “With downstream feeding, the screw feeder can no longer ensure precise control of the product which now has a greatly changed flow behavior. Sufficient ventilation should also be ensured at all times to avoid affecting the feeding process.”
Depending on the size of the hopper discharge unit, the vibrations are generated with one or two vibration motors. “The smaller units up to model 1504 often have a higher speed and frequency, which causes the bottom to vibrate. The large units are equipped with two motors, to prevent the product being discharged unevenly. The two motors are installed in opposite positions and are self-synchronizing,” explains Norbert Marek. The vibration motor transfers the vibrations generated by unbalance weights to the discharge bottom, to which it is permanently connected. The unbalance weights consist of two half discs, whose relative position to one another determines the level of the unbalance. The unbalance is adapted to the application and discharge conditions in each case.
If the unbalance on the vibration motor is set too high, there is a risk of the product backing up or compacting in the discharge bottom, or the device can be damaged. “The vibration energy should therefore only be as strong as necessary and as low as possible,” recommends the expert. If the unbalance is set differently on each of the two motors, undefined spinning motions can destroy the device. Norbert Marek emphasizes: “The two vibration motors consequently always have to be run with the same unbalance setting.”
Less is more
“What makes our service special is that we can deliver everything from a single source,” summarizes Ralf Eikermann, Head of Customer Care. “Our unique selling proposition is that we can manufacture the hopper discharge units individually tailored to the conditions of the specific application if necessary. Customers who are unsure about the flow characteristics of their product should consult us first. We can analyze a sample beforehand to ensure that the hopper discharge unit will be precisely matched to the application.”
Less is more – this applies to the hopper discharge unit. If customers ensure economical energy input and clever use of the annular gap, “the Brabender” will reward them with a long service life.
(published in FLUX 2/2020)