Design of a screw feeder
At the design of a screw feeder the goal is to let the capacity increase evenly over the length of the screw. The design procedure is as follows:
·the wall friction of the product on trough and screw blade and the internal friction of the product are measured.
·the capacity profile around the maximum pitch is calculated. On the basis of this value the maximum applicable pitch is fixed.
·the desired capacity and this pitch now determine: number of revolutions, screw diameter and core diameter at the discharge.
·The increase of the capacity per length of screw is fixed now. The filling in of core diameter and pitch can start.
·The core diameter at the start of the screw is chosen. For short screws this may be the same as the diameter at the discharge, but mostly a bigger core has to be chosen. The minimum pitch is calculated.
·The starting value of factor Z is chosen, in combination with the ’stickiness’ of the product. The resulting step in capacity can be smoothened with an extra blade with the starting pitch.
·The pitch increases until optimum transport is reached.
·If the core diameter at the start is bigger than at the end, the reduction of the core is determined.
·In the last part the pitch increases again, until the earlier determined value of the maximum applicable pitch is reached.
·In the transport section of the screw (after the bunker) the pitch increases slightly, to reduce the filling degree, and avoid jamming.
The result of this procedure is an equal withdrawal of the product over the length of the screw. See fig.4, in this figure also the capacity profile for a screw with a stepped pitch is given. With a well designed screw there will be no stagnant areas in the silo and the risk of bridging and flooding is reduced to a minimum.
On the basis of the dimensions of the screw, the pitch profile and the product characteristics the torque and power needed can be determined. Here the difference between a screw feeder and a transport screw becomes evident. Through the higher filling degree of the screw (beneath the silo 100%, after that at least 80%) and because of the pressure of the product in the silo on the product in the screw, the torque and power needed is much higher than for a transport screw. In selection of a drive this is very important.
The start-up torque can be estimated on the basis of the silo pressure. It needs to be investigated if the motor, reductor and frequency regulator can yield this torque. See figure 3 below for an example.