Without a full assessment, the position would be a matter of make and hope, and frankly, either of the pitches quoted may work, or neither, depending on various features. Risks of this type are often taken in solids handling, with a poor appreciation of the potential consequences.This is one of those applications that should be referred to a specialised screw manufacturer as any screw handling a fine powder at an inclination greater than 10 degrees requires special consideration due to the prospect of excess product dilatation and ‘run-back’. The screw construction is only one aspect of the installation that will influence the equipments performance. It is important to know the prior handling conditions of the material to the machine to establish the state of the bulk delivered to the conveyor and whether the screw is to control the feed rate or is required to transfer whatever is delivered to it; and if so, will this rate vary. These particulars will influence the form of the inlet and perhaps the inlet hopper holding capacity.

The question then arises whether the screw is to operate in a gravity mode or under flood feed conditions and the length of screw required. Only then can a decision made as to the flight geometry and casing construction. It should also be born in mind that the size of the receiving port does not determine the size of screw to be used, but the design of the outlet transition from the screw casing is important, whether the screw size is larger or smaller than the final opening.

It is likely that the wall contact friction will be an important design parameter and the range of ambient temperature in operating conditions. The handling system has to be considered in totality, so it is too involved to discuss in this forum. If you care to send the relevant details to me via lyn@ajax.co.uk, I will try and provide guidance, but regret that I am unable to offer a free detailed design service. ■