There are two main criteria for selecting Mass Flow as a flow regime in a bulk storage container. The first is to guarantee that there are no 'dead' pockets of storage where the material may deteriorate in flow property or quality in some way. It is Not usually chosen for this reason if the process entails a total batch discharge of all the contents, for example. This mode of discharge is frequently quoted as a way to redress segregation but it is not a complete answer, and some experience and special techniques are apprproriate in sensitive cases.

The other reason is to secure the flow benefits of mass flow with regard to the size of orifice needed to guarantee reliable flow. In such cases, an 'expanded flow' form of geometry may be the most efficient shape. This is one where the section adjacent to the outlet has mass flow characterisitics, but at a cross section that is greater than the 'critical rathole' diameter, the construction may convert to a non-mass flow form.

Also to be taken into account for optimum performance of bins and hoppers are the alternate forms of construction. For example, pyramid shapes have terrible characteristics for total wall slip, mass flow, cleaning and contamination and the like, so these are rarely used in quality pharmaceutical applications. Cones are widely used, in spite of their relatively poor flow characteristics. Vee shaped are better, but conventionally require a feeder to serice a slot outlet. More sophisiticated shapes are available that overcome these drawbacks, but cost more to fabricate. Overall, expert advice is useful to optimise selection, as no one shape fits all duties. Advice is free from tech@ajax.co.uk and some technical publications will help you to steer clear of many common problems and hazards.

Lyn Bates ■

Bearing in mind the major importance of blend homogeneity and the ramifications for product quality if segregation or flow problems occur, the answer to your question should be - mass flow. However, in practice this is not necesarily the case (whether by lack of accepted design procedures for flow, or by ignorance of basic flow behaviour when specifying equipment). It is not unusual to find far more attention being paid to the quality of external finishes on vessels and handling equipment than to that at the bounding flow surface (which is where it actually counts!).

The widespread misunderstanding of flow properties and discharge behaviour is manifested as poor plant performance and unnecesaarily high reject levels in a large number of production facilities.

Richard Farnish

Consulting Engineer

The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, London

http://www.bulksolids.com ■

The pharmaceutical hoppers are mostly small. For such hoppers the usual hopper design don’t work. Therefore the IPT have develop so called ideal flow silos – the silos without the cone where the diameter of the discharge equipment is equal to the silo diameter. Such – ideal flow silos – are suitable for storage of free flowing and cohesive powders. This means that the same design can be used for all available powders and it is not necessary to design the powder for each material separately. Contact us for more information.

PLEASE VISIT OUR WEBSITE AND CONTACT US FOR FURTHER DEVELOPMENT OF A COOPERATION. WWW.IPT-ONLINE.COM.

Best regards

IPT

Dr. Ivan Peschl ■

Dear PEP,

True flow similation can be performed using Discrete Element Modeling (DEM). Visit our website for an introduction. Virtually any shape can be modelled with surface degradation, mixing behavior, core flow, mass flow or other derivatives that may occur.

Lawrence Nordell

COnveyor Dynamics, Inc.

www.conveyor-dynamics.com ■