I have used air knives to separate plugs in 'super-dense' phase and applied two in sequence to avoid plug damage. The first knife gave a short, sharp blast to cut the emerging plug and send it forward to a location where the second knife could provide the required amount of conveying air without interferring with the plug integrity. The process delivered a cohesive product with the minimum of air and virtually dust-free. The system was not pursued as being rather specialised and out of our normal market of screw type equipment, other than the the feeding device was a special screw design.

In principle, this approach could be tuned to regulate the plug mass for dispensing, but the reception point must be able to differentiate between the separate plugs. I would suspect that such a technique would be sensitive to set-up and be less flexible to variation than a controlled discharge method. I would not consider hot products less than 1000 degrees C to be any problem for a well designed screw feeder and capital cost is invariably less critical than assured performance. Unless there were other features favouring a such a custom designed solution I would tend to stick to proven equipment and, as an equipment manufacturer or consultant, certainly not undertake to beinvolved or supply this form of plant as anything other than a development project in co-operation with the user. My recommendation therefore is to stay with traditional methods unless specific experience is available or the benefits of the system are very attractive and worth the risk. ■