For years Vincent has tried various means to improve screw press performance by force-feeding material into the press. The goal has been to increase throughput capacity while simultaneously lowering press cake moisture.

In 1993 progress was made at Cargill using a screw conveyor to drive shredded orange peel into a press. In the next few years, this force-feeding "Supercharger" was sold on nine different jobs. Most of these were citrus applications, although the Supercharger was also used on spent brewers grain, hydrolyzed feathers, and soybean protein fiber.

The results were decidedly mixed. Up to a point the Super- charger would usually force material into the press. The problem was that there was always a point at which we could no longer drive additional material into the press. A Super- charger speed would be reached where the inbound material would co-rotate with the screw of the Supercharger. There were times, when this speed was reached, that capacity would fall off and a decrease in amps of the press main drive motor would be registered. We definitely were not getting more material through the press or doing more work on the material.

Late last year remarkable improvement was made at a Minute Maid citrus juice plant. Twin Screw Superchargers were supplied on 50 hp VP-22 presses doing second pressing. The twin screws (one left hand, the other, right) overlap within an oval trough so as to form a positive displacement machine. This has eliminated the problem of co-rotation. There has been no problem keeping the presses loaded at the maximum 50 hp rating over a wide range of operating conditions.

It is clear that 60 hp or 75 hp drives would be useful. However, it is not likely that this change will be made. This is because the new presses have created a condition whereby the customer now has difficulty disposing of all of the press liquor being generated!

The Supercharger is recommended for firm materials that want to retain their moisture, such as citrus peel, diffused sugar beets, and spent brewers grain. The Supercharger is not suitable for low consistency materials that can be pumped to the press. Nor is the Supercharger right for materials characterized as containing loose water, such as paper fiber and plastic waste.