by EPA Dosiertechnik GmbH, Germany

 

Since antiquity, the Archimedean screw has been used for conveyor tasks. In the modern bulk material industry, conveying and metering screws have become indispensable, although they have some disadvantages.

By design, the material to be transported is compressed or squeezed, there are rotating parts in the product stream, which make cleaning difficult, and the sealing of the drive shaft is always a weak point with regard to product contamination. In many cases, high friction between the screw and the trough wall impairs the product, in addition, screw machines operate in a narrow performance range of only approximately 1:10.

Nevertheless, screw conveyors cannot be replaced in many applications, but with just as many, it is also possible with vibrating conveyors (feeders).

 

Technical principles of vibratory feeders

Oscillating conveyor drives are magnetically or pneumatically driven two-mass vibration systems with a drive base (A - the larger mass), a conveyor table with mounted conveyor trough and gutter assignment (B–use-mass) and leaf spring elements (C - they connect the utility Mass with the drive base) (See Figure One).

The product on the channel is accelerated by the oscillation with defined frequency and amplitude (Fa) and travels a small distance on the channel, which correlates directly proportional to the stroke of the channel.

Due to the force of gravity (Fg), the product remains after acceleration over the distance (stroke) on the channel (Fh), by renewed acceleration with the drive frequency, the process is repeated (See Figure 2).

The product is thus transported with the drive frequency and the stroke of the channel, without being mechanically stressed.In the following, we focus on electro-magnetically driven vibratory conveyors, only here the drive can take place with resonance frequency, which is essential for a linear performance.

A linear performance is, in turn, the prerequisite for achieving good metering results with vibrating troughs.

The useful mass and the leaf spring elements form a vibration system with a narrow-band, mechanical resonance frequency. Any change in the use-mass or leaf spring elements shifts this resonant frequency.

The stable operation of a conventional conveyor drive has close working limits, because the drive frequency and the resonance frequency must be close together for a useful vibration deflection.

Changing the useful mass (eg the trough load) therefore forces a complex adjustment (eg by addition attach weights or replace leaf spring elements). Inadequate balance results in reduced delivery rate (vibration amplitude is too small) or unstable operation (swinging amplitude rocks). Another drawback of the conventional technique is that the delivery rate is not linear to the control voltage.

To compensate for these disadvantages, our vibratory conveyor drive is permanently driven at the resonant frequency of the entire system. The optimum drive frequency is determined and readjusted by means of an integrated linear speedometer generator.

This speedometer consists of a generator coil (attached to the vibration drive) and a permanent magnet (attached to the conveyor trough) and detects the oscillation frequency and amplitude between the vibration drive and the conveyor trough, thus providing the reference signal for the two control loops:

The frequency control circuit synchronises the drive frequency to the resonance frequency of the vibration system (it is not adjusted as usual, the resonance frequency).

The amplitude control circuit controls and limits the energy and guarantees an exactly proportional behavior of the oscillation amplitude to the setpoint. Disturbing influences such as load changes are corrected, so that the conveyor trough is always controlled with the optimum frequency even in continuous operation.

The amplitude (stroke of the channel) can be set between 0.4 and 4mm. Outputs of 50 g/h to approximately 10 t/h can be achieved via this stroke (with a bulk density of 1kg/l). The setting range of the resonance-driven channels is 1: 1000 and is linear to the specified setpoint.

With regard to the product to be conveyed, this technology offers various advantages:

1. Length of the conveyor trough

The channel shape can be individually adapted to the product and the geometric requirements of the process. Thus, conveyor channels can vary in length between 50cm and four metres. The large spectrum is made possible by the use of several synchronously operating vibrators.

2. Width of the conveyor trough

For different tasks conveyor channels can be calculated so that constant discharge widths of up to two metres are possible. For applications that require a uniform application of product, there is virtually no comparable technical solution.

3. Discharge, shape of the gutter

As a rule, classic rectangular trough moulds are used. Depending on the application, delivery pipes with quick-release fasteners are also used for easy cleaning, and delivery pipes made of glass are also possible. For particularly large performance ranges of up to 1: 1000, V-shaped troughs are used.

4. Conveyor troughs as a separator

By using one or more screening stages within a gutter, conveyors can also be used as separators and integrated directly into the product stream. This makes classification into upper and lower grain possible. Multi-stagesystems can alsobe realised.

5. Cleaning, FDA, ATEX

When using a vibrator conveyer there are usually no other parts (rotating screw, etc.) in the product stream. This makes conveyor systems with dosing channels less prone to external contamination.

What's more, the gutter system is very easy to clean and FDA compliant. Versions with complete encapsulation of the product flow via PTFE inlays and plastic pipes are also possible. Thus, products that cannot tolerate contact with metal, can be conveyed.

Systems with gutters can be easily sealed/encapsulated or operated under light pressure (about 20mbar). Thus, the certified operation within the ATEX zones 2/22 is easily possible.

Use of conveyor troughs in conjunction with "loss in weight" controls for demanding dosing tasks

In combination with a suitable gravimetric control system, all conveyors can be used for highly precise metering of bulk solids (accuracy better than 0.5%).Using the resonant-controlled vibratory trough in metering devices controlled by the LWF Integral (LWF) method results in extremely good short-term accuracies.

This results from the fact that in one second, depending on the determined resonance frequency, 35-65 drops of the product can be realised, which is hardly conceivable when using dosing screws.

By using the LWF Integral control, the container weight is continuously measured and the dosing capacity with the vibrating chute is controlled in such a way that the quantity of conveyed material corresponding to the setpoint value is always deducted from the container.

The decreasing setpoint curve is processed as an integral function according to the required throughput rate. A deviation of the decreasing container weight from the calculated setpoint curve directly intervenes as a controlled variable for the drive. The dosing is thus following a high-precision beam, constantly readjusted.

Conclusion

The trough is not the first choice for all applications, but in many scenarios it is clearly superior to available alternative technologies. Due to the low-maintenance design, not only the purchase is significantly cheaper compared to screw systems, also the maintenance-free operation keeps the total TCO low.

The achievement range of> 1: 100 is unsatisfied. This cannot be realised with any comparable system.In particular, this can become very relevant for later performance or recipe adjustments of a plant and thus also reduce costs in the long term.

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