Screw conveyors are a very popular equipment in solids processing industry to move and / or dose products. Their principle allows a simple and quite robust operation, although the design must be well detailed in order to avoid issues that could lead to damages to the product and to the equipment itself (generation of foreign bodies). Screw conveyors are used with product causing no or medium abrasion but should be avoided with products causing high level of abrasion ; the screwm rotating in the product, is indeed very sensitive to abrasion phenomena.
A screw fitted directly to a hopper upstream is often referred in the litterature as screw feeder while the equipment alone is often referred as screw conveyor.
A typical screw conveyor design is shown below :
Figure 1 : Screw conveyor principle drawing and key components
Screw conveyors are usually placed below hoppers where they can convey horizontally the product to another process operation. When the conveying involves also a weighing, the screw conveyor is actually used as a dosing equipment.
Screw conveyors can also introduce solids in a pneumatic conveying system. In most of the case, they are used in vacuum transport since there is no pressure problem, but some designs can also used to feed the product towards a pressure conveying line. For this particular purpose, the screw flight design must allow a compression of the solid so that the plug formed avoids the pressurized air of the conveying line to come back through the screw. This cannot be used for all products, thus, for pressure conveying, airlock rotary valves are usually preferred to screw conveyors for introducing the product.
Screw conveyors can be very long reaching >5 m, however this kind of design can only be used with non sensitive product and when no cleaning is required. It is indeed not possible to remove the flight of such screws, and if some designs exist to still access the inside (trough design - show example), the efficiency of cleaning is reduced and the time to clean will be long. Besides, the length of the screw makes it prone to bending and to possible metal / metal contacts with the housing, risking to pollute the product with metal chips. As a reference, a screw flight of 4 m can bend up to 7-8 mm from the shaft straight axe, if no proper provision have been taken to allow for such bending, metal / metal contact can happen.
It appears better, for hygienic applications, to request the following characteristics :
- Short screws
- Extractible flights
- 2 bearings / although cantilevered designs are available for very short screws of 1-1.5 m
- Flat operation
- Tip speed 1 m/s and pressurized bearing seals - for ATEX prevention
Inclined screws are tempting because they allow to solve many process problems especially when retrofitting installations, one should however be very careful that these can of screws are usually difficult to access and extract for cleaning, may be also more sensitive to bending and metal metal contacts, and are not very efficient (efficiency is decreasing with the angle).
Table 1 : Screw conveyor capacity as a function of its inclination
|Angle of screw (degrees)||Percent of max capacity (%)|
Screw conveyors are rotating equipment, which them a risk for operators who would try to reach the inlet of outlet of the screw. For this reason, a special attential must be paid to the inlet and down pipe. If they are equipped with flexible especially, those flexible should be dismantable only with a tool and the operator trained to stop the machine if such dismantling is necessary. In case it is not possible, a cross in the pipe preventing to reach the screw flight can be installed, provided that it does not prevent the flow of product. In any case, a risk analysis must be carried out by the plant operator to make sure the access is properly protected.
The screw flight can take different shape and pitch according to the application in which it is used.
- Standard flight : constant pitch = 1 diameter ; works for most of the applications
- Short pitch : used for inclined screws, can also be used for materials that are easy to fluidize
- Enlarging pitch : smaller pitch at beginning of the screw, can be used for feeders below a hopper to provide a constant feed.
- Shortening pitch : large pitch at the beginning and shorter pitch at the end, this will create compression, it is not a common design and can lead to a lot of thrust on the screw, power requirement, and possibly mechanical damage. If the screw is properly designed, this screw flights can be used as feeding devices towards a pressure system, since the powder compressed at the end of the screw will act as a plug preventing the gas to leak towards the screw.
- Ribbon : used mainly for sticky materials
Other designs are available with paddles or cut flights when there is a need for mixing the material.
In applications requiring high hygiene degree of finition, the shaft must be plained, not hollow, and the flight must be fully welded and polished. This in order to avoid that material get trapped in non accessible areas and pollute later good products.
The design of the shaft and the motor must be well aligned so that the motor is able to run the screw and start it from 0 speed or run it overloaded, when it requires the most torque. Conversely, the shaft must be designed to sustain such a torque and not break when it is applied.
When used in dosing applications, the screw must be equipped with a motor on VFD in order to run in coarse speed and fine speed (see dosing page).
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