Hello Lianjie,

This caking problem starts with moisture arriving in humid air with the bulk material at an elevated temperature. As soon as contact is made with a surface below dew point, the water vapour condenses and reacts with the particles to form crystal bridges sticking to the surface. The bed thickness increases with time and can eventually form a stable insulating layer, but the damage is done by this time and problems can also move down-stream.

It is rarely practical to dry the ambient air or pre-cool the product so usual approaches are to prevent condensation by insulating or heating the outer surface of the hopper, or fitting a UHMDP liner to the inner surface. Such a line provides a degree of insulation but also offers a hydrophobic surface to which moisture will not adhere.

Generally, a mass flow form of hopper is preferred for hygroscopic, damp, cohesive or products that tend to cake in order to avoid excessive residence periods of storage. The effect of potential blockages should be addressed by using flow channels of well adequate dimensions or incorporating ‘poking holes’ or lump traps for intransigent situations.

If screw feeders are to be used they should employ progressive extraction ribbon flights that are designed to resist the build up of material on the shaft. If you care to send dimensions of the existing equipment to lyn@ajax.co.uk I will try to advise more directly.

It is often practical to include lump-breaking facilities in discharge or conveving screws to reduce the size of indeterminate agglomerates that can form in materials that cake. If the product has to be reduced to a grain-sized powder state it may be necessary to pass the material through a grid fitted with agitated blades. Wedgewire screens are good for avoiding clogging in these circumstances.

with regards

Lyn Bates ■