The handling characteristics of residential waste is likely to depend much upon the location from which it is collected. Warren Spring Laboratories in UK did considerable experimental work in the late 70's on handling domestic waste and developed a 'plug flow' system based upon direct injection into the conveying pipe, (no rotary valves or other pressure barrier), from a supply hopper into a pipe approx. 200mm diameter by means of a screw feeder. This technique employed an air knife that separated a solid plug of the material and projected it as a coherent mass along the pipe with minimum dilatation. (Ajax equipment supplied the screw to deal with the difficult problem of promoting flow in the supply hopper and producing a compacted plug virtually impervious to the leakage of air)

This method is highly efficient on air consumption because it 'fires' the slug like a bullet. There are various design 'secrets' to the technology, such as (1), Using twin air knives to effect plug separation and the supply of the subsequent conveying air in two stages to prevent plug damage. (2) Relaxation of the tube diameter, (a) To avoid regenerative wall friction during the plug compaction process and (b) To relax wall contact to allow transfer on a Bingham layer in the conveying tube and negotiate bends without excess wall pressures. (3) design of the integrated hopper/screw feeder to overcome the strong arching tendencies of variable, elastic natured product and provide a powerful, balanced, end thrust to the delivery end of the screw.

Wall friction is easily measured. See www.ajax.co.uk.

One of the operating virtues of this method is the ability to handle virtually all the miscellaneous junk that is collected in domestic waste.

As with all conveying systems, the 'systems' aspect is crucial to effiecient performance. Probably the most difficult part in this system design is then geometry of the hopper/feeder element.

Lyn Bates ■

We are manufacturers of Waste Recycling Equipment. Please visit our site at www.trommel.co.uk which may be helpful to you.

We certainly do not use pneumatic conveyance of waste material due to the unknown and varying characteristics of this material. Conventional belt conveying is used. Raw waste will go up a plain 15 degree conveyor - shred it and it won`t, a chevron belt has to be used.

Our Blowers are used to remove ligtweight contaminates from screened material, this is the nearest we get to using air.

If you have any questions we would be pleased to pass on our experiences.

Regards

Ron Gill ■