A good screw design depends on a variety of factors
When designing a screw conveyor (horizontal, diagonal or vertical), designers at Klinkenberg have to take into account the various flow characteristics of the powder, granulate, chips or fibres. Experience has shown that the various characteristics such as particle size, bulk density and particle form can have a major effect on the capacity of a screw conveyor. A product with coarse particles will flower more easily than a product with fine particles. A spherical particle gives less friction, resulting in better flow behaviour and a bigger screw capacity. By closely examining the characteristics of powders, granulates, chips or fibres, you can predict whether they will flow well or not.
The size of the particles has an effect on a product’s flow capacity. In general, small particles are more cohesive. Particles larger than 250 µm usually flow freely, but when the particles are smaller than 100 µm, they cluster together so closely that the flow can be impaired.
Most bulk goods flow due to the effect of gravity. In general, dense products are less cohesive than less dense products, which means that flow characteristics can be different.
The shape of a particle has a major effect on a product’s flow characteristics. A rounded particle has fewer points of contact with other particles, which produces a better flow behaviour. A particle shaped like a flake has a high surface to volume ratio and a poorer flow behaviour.
The Hausner ratio indicates how compressible a product is in relation to the bulk density. It is derived from the quotient between tapped density (TD) and apparent density (AD). Hausner Ratio =TD/AD
Values of less than 1.25 indicate good flow characteristics. A higher figure will indicate a worse flow characteristic. The Hausner ratio gives a good indication of the products flow behaviour.
The angle of repose of bulk goods already gives a good indication of the flow behaviour. The table below shows some indications of the angle of repose in relation to the flow behaviour.