Screw feeders are used for dispensing loose solids at a controlled rate from a bulk storage container, such as a silo, hopper, bin, IBC or big bag. These are normally chosen to satisfy one of two principle functions: -
1. To discharge the product from storage in a controlled manner, whilst generating an extraction pattern from the container outlet that generates the required flow regime in the stored material.
2. To meter the feed at a suitable rate, uniformity and condition to meet the demands of the receiving point.
In either case, certain basic criteria must be met.
- Deliver the design flow rate as and when required, reliably.
- Deliver the material in a suitable condition.
- Deliver to the required receiving point.
- Fit into the available space.
In Case 1, the feeder should be designed as an integral member of the storage container to optimise the geometry and performance and provide an extraction area that ensures reliable flow at an adequate feed rate and generates an extraction pattern over the feeder inlet that enables an appropriate flow regime to form in the container.
In Case 2, the emphasis is on control of the feed rate, to ensure that the output satisfies the accuracy and consistency of condition required at the point of delivery.
These differing objectives essentially separate the design process into two types of feeder. For this purpose the former is termed a ‘Screw Discharger’ and the latter a ‘Screw Feeder’. For either use the supply container and the feeder should be designed as an integral system, rather than independent items.
Screw Dischargers
The design commences with the form of the storage container to be employed, the main purpose of which is usually to hold a specific volume of bulk material in temporary storage, variously described as ‘Silos’, ‘Hoppers’ or ‘Bins’. In general ‘Silos’ are externally mounted, relatively large bulk storage containers, often circular with conical outlet sections. ‘Hoppers’ or ‘bins’ are often situated within plants and can be of rectangular or circular construction, with converging sections to the screw feeder. The storage container should be considered in two sections, that containing the main storage volume and the region approaching the feeder.
The initial task is to establish the form of global flow regime that will be developed in the container during discharge that suits the nature of the bulk material to be handled and the size of interface necessary to ensure reliable flow.
The emphasis is therefore on the extraction pattern that is generated.
For duties such as discharging the output of a plate filter or a dust collector the feed screw is merely performing a collecting duty and the extraction pattern is rarely important unless to reduce the power needed of the drive.
Mass Flow and Expanded Flow hoppers essentially require progressive extraction along the hopper outlet slot, not necessarily at a uniform rate.
However, uniform extraction and mass flow are very desirable to deal with products that tend to aerate or segregate during the filling of the hopper.
Screw Feeders
These are usually smaller installations dedicated to deliver a given rate of product. The extraction pattern from the hopper is not important and an agitator is often fitted to ensure feed reliability. Loss-in-weight feeders are generally specified where an accurate mass rate has to be dispensed or recorded.
