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Vibrating conveyors design overview

Vibrating feeders for bulk solids materials (powder, pellets, granules, pills...)

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Section summary
1. Introduction
2. Function in the process
3. Important design considerations
4. Dosing with vibrating tubes / trays
5. Applications of vibrating tubes / trays
6. Common problems with screw conveyors

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1. Introduction

What is a vibrating conveyor for bulk solids ?

Vibrating conveyors are made of an empty tube, or tray, 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.

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 and can reach relatively high capacities.

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.

2. Function in the process

For what purpose are used vibrating tubes ?

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 directly product to a pneumatic (vacuum) conveying line since the absence of inserts in the tube cannot control well the flux of product and air.

3. Important design considerations

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 resonance. 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 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.

4. Dosing - Metering

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.

5. Applications of vibrating feeders

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

6. Common problems with vibrating conveyors

Vibrating conveyors are usually reliable equipment, however a certain number of issues may require some corrections :

Table 2 : common problems with vibrating tubes

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

7. Vibrating conveyor buying guide - How to select a vibrating feeder

When sourcing a new vibrating conveyor for your factory, the following questions need to be asked in order to buy the right specifications :

• What is the expected throughput? What is the product density ? This will give the size of the vibrating feeder to buy.
  
• Is the material free flowing ? If the bulk material is not free flowing, you may have to consider other kind of conveyor, on the contrary if it flows too much it can also be an issue as it will not be possible to regulate the flow.
  
• Is this a dosing application ? If so, what is the accuracy expected ? This will give you the inputs necessary for specifying the vibrating conveyor and verifying with a vendor if the dosing can be achieved.
• Is it in ATEX area ? If yes, the vibrating tube must be certified

Some possible suppliers of vibrating conveyors, check the following companies :

• Sinex conveyors
• Tetra Pak Powder Handling

(Note that powderprocess.net has no link with those companies)