Mahdi,

The two basic requirements to feed a drier are to secure a reliable, accurate, controllable feed, without clogging or other operating or residual problem, and to deliver the product to the drier in a suitable condition for the machine.

The second feature may call for back mixing and/or a delivery system that avoids erratic fluctuations but the prime concern is to control the feed of essentially wet or damp stock. A screw feeder to deliver damp solids from a buffer hopper requires careful attention to the specific material characteristics on two counts.

1. Securing reliable flow in the hopper.

1. Avoiding build-up on the screw flights and the shaft.

Abundant references are available on hopper design but a useful hopper design guide chart is available on www.ajax.co.uk. A list of useful publications on ‘Flow in Hoppers’, ‘Interfacing Hoppers with Screw Feeders’ and ‘Powder Testing’ are also available from the company. Wall friction tests should be conducted to determine the optimum contact surface for slip and adhesion resistance and the wall inclinations necessary for mass flow, which is a ‘must’ for such duties. Avoid square corners in the hopper, select a Vee shape and terminate in vertical walls to the screw are sound principles.

The screw must have progressive extraction over the hopper inlet length and preferably well-proportioned ribbon blades with a small shaft diameter. (Ajax ‘Lynflow’ ribbons are specifically designed to deal with sticky and cohesive products). A cantilever-mounted screw offers many advantages for clean, flexible discharge arrangements. It also allows the use of a ‘plug seal’ of product, to protect prior product feed against the drier heat, resist the back-flow of vapours and give direct pressure injection, if required.

A detailed, expert design is the province of specialists and is normally specific to a given application. A wide range of techniques and special features can be incorporated for particular functions and effects. Hopper technology has advanced considerably from the days of cones, pyramids and wedges. Screws also are available in many forms and with fine variations of construction. Specialist manufacturing companies cannot be expected to provide a detailed design free. However, the above, and information in my book ‘A guide to the Design, Selection and Application of Screw Feeders’, contains much useful information and should highlight the value of an experienced supplier for such an important process duty.

Lyn Bates ■