If you want a screw feeder to meter material reliably, DON'T GET THE MATERIAL INTO A FLUIDISED STATE!! ■

The clearance between the screw and the casing is virtually irrelevant with material in a totally fluid condition as the material can swirl round the spiral form of the flight driven by the pressure from the head of fluid product . Extending the length of the choke section and/or fitting double start flights with a close clearance to the casing can make a difference in marginal cases, but the best approack is to address the conditions that are causing flooding.

Apart from initial filling conditions from a pneumatic conveyor or the free fall of the material delivered into the hopper that causes a loose condition of the bulk, the most usual reason for flushing is the collapse of arches because the flow channel is inadequate for reliable flow, so the interface situation to ensure live drawdown and mass flow are the features that are needed to be addressed. There is also the prospect of installing devices to accelerate de-aeration in the flow channel. The application would seem to call for the examination of the total hopper and screw arrangement by a specialist to determine the best way forward. If you want an initial assessment, send full details of the installation to lyn@ajax.co.uk ■

If the material can fluidize, then the most reliable way of feeding with a screw is to install a rotary feeder at the screw discharge. (The rotary feeder is just an airlock with bigger tip clearance.)

When the material is not fluidized, the screw acts as the feeder and the rotary feeder just dumps out whatever you put in. When the material is fluidized, the screw just acts like a pipe and the rotary feeder acts as the feeder. This has been done many times on coal ash bins fed by pneumatic conveyors.

Doesn't matter what you do, some materials will just fluidize sometimes, usually after the bin is emptied and is starting to refill. ■