Vibrating tubes

Section summary
1. Introduction
2. Position in the process
3. Important design considerations
4. Dosing

1. Introduction

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.

A typical vibrating tube design is shown below : - Vibrating tube drawing

Figure 1 : Typical design of vibrating tube

It should also be noted that alternative designs exist with a pan instead of a tube. The principle remains the same ; pan design can reach higher throughput than tube design, they are also sometimes used in combination with a sieve to form a rectangular vibrating sifter.

2. Position in the process

Vibrating tubes 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 tube 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.

3. Important design considerations

Vibrating tubes 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

Motor type Applications
Electromagnetic Small units

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 tube 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. - Vibrating tube throughput

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 troughput 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.

4. Dosing

Vibrating tube, because of the inertia of the vibration, do not constitute the best equipment to achieve a fine dosing. 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.

To be noted also that vibrating tubes are not advised for powder having bad flowing characteristics and especially that can compact easily.

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