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Mechanical Conveying of bulk solids

Bulk conveyors : belt conveyors, screw conveyor, flexible screw conveyor, vibrating conveyor, bucket conveyors / elevators, aero mechanical conveyors

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Page summary
1. Definition of mechanical conveying
2. Screw conveyors
3. Vibrating conveyors
4. Aero mechanical conveyors
5. Belt conveyors
6. Bucket conveyors


1. Definition of mechanical conveying

What is a mechanical conveyor ?

Mechanical conveying is directly opposed to pneumatic conveying in industries handling bulk solids such as powder, pellets or granules. If pneumatic conveying is not using any moving part to convey the materials, but only air, mechanical conveying need an equipment that will move the solids all along the transport path.

There are many types of mechanical conveyors, all fulfilling the same function which is to bring material from a point A to a point B, but the principles behind the design are totally different. This will allow each type of conveying to be more or less adapted to certain industries.

For example, screw conveyors are quite an instrument of choice in food industry over short distance for dosing, while bucket or belt conveyors will find much more applications in grain industries or mining industries.

An overview of the different design of mechanical conveying is given in the following table, allowing to guide the reader in selecting the right technology for his application.


Capacity Conveying distance Fragile materials impact
Clogging materials Material retention Containment
Screw conveyor High
(0-450)
Short Strong Adapted
Good
Vibrating tubes High Short Low Not adapted Almost no retention Good
Aero mechanical conveyors Low Medium Medium Will cause issues
Good
Belt conveyors Very high Very long Low Adapted
No containment
Bucket conveyors Very high Short (vertical) Strong Will cause issues
Very difficult

The webpage is giving key design aspects of each of the mechanical conveyors used in industry.

2. Screw conveyors

Screw conveyors characteristics

Screw conveyors are using a screw flight, also known as Archimedes screw, to move materials. There are 2 main types of screw conveyors :

  • Tube screw conveyors : the screw flight is inside a tube, thus it is only possible to access the screw by pulling it out of the tube.
  • Trough type screw conveyors : the screw is at the bottom of a trough, thus it is possible to open the trough lids and access the screw.

A typical screw conveyor design is shown below :

PowderProcess.net - Screw Conveyor

Figure 1 : Screw conveyor principle drawing and key components

Capacity of screw conveyors

  • Up to 400-500 m3/h, depending on screw speed, diameter, screw flight design and inclination.

Diameter

  • 100-600 mm

Conveying distance

  • In principle up to 10 m, but shorter distances are recommended, especially for industries sensitive to foreign body and / or requiring cleaning. For those industries 1-2 m is recommended. For length > 2 m, if the screw must be inspected, the trough type is preferred (one should however be careful to the safety).
  • In some cases it can be possible to consider several screw in series to reach longer distance, but one should be careful with such design as it requires a specific control and the access / maintenance is not easy.
  • Note that in some cases tubular and trough screw conveyors can be inclined (maximum is 40° but such configuration should be used with care as it has strong impact on the screw performance and accessibility).

Power

  • 1-10 kW for normal applications, up to 20 kw for specific applications. Screw can be fitted with direct drives or belt / chain drives.
  • For dosing purposes the screw drive can be equipped with a frequency converter.

Specific designs

  • Designs optimized for cleaning are available for food industries especially : easy opening covers and removal of the screw flight.
  • It is possible to have screws with 2 outlet. The drive command can be reversed to convey in one direction or another.
  • For screws used to extract materials from a silo, it is possible to consider a different pitch at the beginning of the screw to ensure a good and uniform product pick up.

Safety

  • Screw conveyors are rotating equipment, it is thus critical to prevent any access to the screw flight when it runs. This can be done through locking the inlet, outlet of the screw thanks to connections requiring tools to be opened, and to position switches on all fast opening accesses such as trough lids or openable cover so that power on the screw is cut if one of these accesses is opened.
  • Screw conveyors are sensitive to dust explosion risks, thus the design must be such that screw flight tip speed is less than 1 m/s and the bearings are protected with compressed air. Electrical equipment in the environement must also be rated for dust explosion area and the max temperature of the motor must be lower than MIT and SIT of the materials.

Flexible screw conveyors

A 3rd type of screw conveyor, besides tubular and trough screw conveyors can be designed : flexible screw conveyors. Such conveyors have a special, flexible, screw flight without a shaft and manufactured in steel that is rotating withing a plastic tubes.

This kind of design is used to adapt to specific layout constraints where a rigid screw would not fit. One must however reckon that the advantage in terms of flexibility of implementation comes with some drawbacks :

  • Not possible in practice to inspect the screw or clean it
  • Risks of foreign bodies as the screw will rub against the tube from time to time
  • Possibly strong mechanical impact on the product transferred as the screw has to rotate fast to move the materials
  • As the screw centers itself thanks to the product, the screw cannot be run empty without damaging the tube

3. Vibrating tubes

Vibrating conveyors characteristics

Vibrating conveyors are using a vibrating motor to create a movement of materials. There are 2 main types of vibrating conveyors :

  • Tubular vibrating conveyors : the product is inside a tube
  • Vibrating trays : the product is conveyed on a flat trough

PowderProcess.net - Vibrating tube drawing Figure 2 : Typical design of vibrating conveyor

Capacity of vibrating conveyors

  • Up to 20 m3/h, depending on the diameter, motor and inclination.

Diameter

  • 80-400 mm

Conveying distance

  • In principle up to 5-6 m.
  • In some cases it can be possible to consider several conveyors in series to reach longer distance, but one should be careful with such design as it requires a specific control and the access / maintenance is not easy.
  • Vibrating conveyors can accept a slight inclination of max 5° upwards and 10° downwards.

Drive

  • Vibrating conveyors can be fitted either with electro mechanical drives (magnetic vibrators), especially useful for fine dosing
  • Or they can be fitted with unbalanced electrical motors, more adapted to high conveying capacities.

Specific designs

  • Very small vibrating conveyors (typically vibrating trays) can be used for precise dosing, for example on Loss In Weight Feeders

Safety

  • Vibrating conveyors are quite safe by design from an occupational safety point of view as they don't have rotating parts (except the drive, but encapsulated).
  • Electrical equipment in the vicinity of vibration conveyors must also be rated for dust explosion area and the max temperature of the motor must be lower than MIT and SIT of the materials.

4. Aero-mechanical conveyors

Aero-mechanical conveyors characteristics

Aero mechanical conveyors, also called cable conveyors, when looked at quickly from the outside seem similar to a pneumatic conveying system : one sees a conveying tube with a pretty flexible layout, going form a point A to a point B. However, the principle of conveying is totally different : aero mechanical conveyors are using discs, linked with chains or cables, to create pockets within the tube and drag the materials all along the conveying tube. It actually means that 2 tubes are necessary, one to bring the product, and the other to return the discs, empty, to the pick-up point.

Principle of working of aero mechanical conveyor

Figure 3 - working principle of aero mechanical conveyors

Capacity of aero mechanical conveyors

  • Up to 40 m3/h, depending on the diameter and motor

Diameter

  • 80-250 mm

Conveying distance

  • In principle up to 50 m.
  • Up to 10 m upwards

Drive

  • Chain or belt drives

Specificity of design

  • Aero mechanical conveyors are well adapted to fragile granulated that should not be damaged during conveying.
  • One cannot exclude that the disc wear over time, thus leading to a risk of foreign bodies more or less sensitive according to the industry served.

Safety

  • Aero mechanical conveyors have moving parts, thus the inlet and outlet must be guarded so that operators cannot get their hand trapped by the chain, cables and discs.
  • The conveying discs are made of plastic, and usually have a moving speed < 1 m/s, thus avoid the risks of sparks. Electrical equipment in the vicinity of aero mechanical conveyors must be rated for dust explosion area and the max temperature of the motor must be lower than MIT and SIT of the materials.

5. Belt conveyors

Belt conveyors characteristics

Belt conveyors are using a rubber belt to transport materials. Belt conveyors can be flat or inclined.

Capacity of belt conveyors

  • The capacity depends on the speed of the belt, its width and the height of the layer of material that can be transported
  • Max 2-3 m/s

Dimension

  • Up to 1-1.5 m wide

Conveying distance

  • Variable, can be very long in industries such as mining. Several conveyors can be considered in series.

Drive

  • 6 kW for a section of 6 m

Specificity of design

  • Belt conveyors, especially those working at high speed, put a lot of stress on the belt, thus specific instrumentation should be foreseen, such as speed controller, alignment sensors.
  • The belt tensioning must always be correct, thus a tensioning system must be part of the design.

Safety

  • The equipment must be the object of a dust explosion risk analysis, especially for the belts running at high speed.
  • Electrical equipment in the vicinity of belt conveyors must be rated for dust explosion area and the max temperature of the motor must be lower than MIT and SIT of the materials.

6. Bucket elevators


Bucket conveyors characteristics

Bucket elevators are particularly used in heavy industries : mining, cements, power plants or in grain industries. They allow to bring up at high capacity materials that would not be possible to convey otherwise.

Capacity of bucket conveyors

  • Up to 200 m3/h, depending on the size and speed of the buckets

Size

  • Up to 0.5 m wide buckets

Conveying distance

  • Conveying is always upwards, up to 50 m

Drive

  • Chain or direct coupling drives

Specificity of design

  • Buckets are mounted on a rubber belt.

Safety

  • Bucket elevators were in the past the source of several explosions, thus a strong dust hazard analysis must be performed for every unit. If not possible to prevent explosions, most manufacturers are now proposing to include explosion vents.
  • Electrical equipment in the vicinity of bucket conveyors must be rated for dust explosion area and the max temperature of the motor must be lower than MIT and SIT of the materials.