|2. Position in the process|
|3. Important design considerations|
|6. Common problems with screw conveyors|
Vibrating tubes have grown popular in the field of solids handling compared to other solutions, mainly screw conveyors. Vibrating tubes allow indeed to convey or dose very gently some solids, compared to the mechanical effect of a screw conveyor on the product (breakage). They offer also very easy access and cleaning since they are only constituted of an empty tube.
Vibrating conveyors are made of an empty tube, with one inlet and one outlet (sometimes 2 outlets) and a vibrating motor that is creating specific vibration intensity and amplitudes that allow the bulk solid to flow. A typical vibrating tube feeder design is shown below :
Figure 1 : Typical design of vibrating conveyor
It should also be noted that alternative designs exist with a pan (also sometimes called tray) instead of a tube. The principle remains the same ; vibrating pan feeder design can reach higher throughput than tube design, they are also sometimes used in combination with a sieve to form a rectangular vibrating sifter.
Vibrating tubes conveyors are usually placed below hoppers where they can convey horizontally the product to another process operation. Such tubes can be very long, up to 8 m and are sometimes useful to cope with a difficult layout, although the longer the tube will be, the less easy it will be to put in place and maintain. When the conveying involves also a weighing, the vibrating feeder is actually used as a dosing equipment.
Vibrating tubes are generally not used to introduce product to a pneumatic conveying line since the absence of inserts in the tube cannot control well the flux of product and air.
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Vibrating tube feeders can be equipped with un-balanced motors or electro-mechanical motors depending on the size of the equipment and the service expected. Due to the vibrations, the support must be designed to be strong enough to avoid that it itself vibrate and transmit the vibrations to the rest of the installation (it can be very detrimental if weighing operations are closed) or even enter in reasonance. Also, to allow the right vibration which is actually conveying the product, flexible connections must be fitted at the inlet and outside of product ; the vibrating tube must be able to move freely and without transmitting vibration to the rest of the installation. If this last condition is not fullfilled, the strokes may not be correct and directed properly, which means that the powder may move slower or even compact and block if the stroke is only going in vertical direction.
Table 1 : Application of the different types of vibrating motors
Can reach very small stroke and high frequency (0.10 mm per stroke and 50-60 Hz)
Preferred for dosing
|Unbalanced motors||Larger units
Longer strokes at lower frequency (6 m/min and higher)
Preferred for conveying large volumes of powder
The throughput of a vibrating conveyor can be estimated by considering that the tube is full of powder and that the powder is moving according to the strokes applied. If the tube or the pan is known to not be full, corrections must be done.
Equation 1 : Estimation of vibrating tube or pan throughput
In order to allow the cleaning, doors can be installed at each side of the tube. They are usually equipped with a proximity switch in order make sure they are closed.
A valve must be positioned right at the outlet of the tube since the inertia of the tube is actually preventing the tube to stop sharply the delivery of product.
The throughput of vibrating tubes can range from few m3/h to 60-70 m3/h. The throughput is directly dependent to the diameter of the pipe, thus units at 60-70 m3/h reach around 40 cm pipe diameter.
Such equipment can be slightly inclined although it is definitively not the best way to operate vibrating tubes.
Vibrating conveyors are used to convey but also meter dry bulk material. However, vibrating conveyors, because of the inertia of the vibration, do not constitute the best equipment to achieve a very fine dosing in batch operation. It is possible to reduce the deviation from the target by using 2 dosing speed if the motor is on VFD and closing the discharge valve when the target is reached, but the dosing error may remain quite high and highly variable. They offer very interesting performance in continuous dosing and are often integrated to loss in weight feeder systems instead of a screw conveyor.
To be noted also that vibrating tubes are not advised for powder having bad flowing characteristics and especially that can compact easily.
Despite common belief that may consider this technology as a bit exclusive, vibrating conveyors, be it tube or pan / tray feeders, can find applications all accross industry and are actually quite versatile. Some examples are listed below :
- Food processing : flour conveying and dosing, fruit pieces or
flakes conveying and dosing
- Pharma : continuous dosing of pills, tablets
- Wood : waste woods chips
- Chemicals : carbon black or rubber conveying
- Construction : concrete, cement conveying and batching
Table 2 : common problems with
|Issue||Root cause and action|
|Inaccurate dosing - overdosing||The vibration is too important
Tune the automation system to stop the dosing before reaching the target weight
Tune the automation system to have a coarse and a fine dosing speed
|Too long dosing time / too low capacity||Check the motor vibration
Check the leveling of the vibrating tube
Check if the feed of product to the tube is correct
|Powder leakages at end covers||Gasket damaged or not properly tightened - check gasket and cover clamping|