Counterbalance and scraper flights

At the rear hub there is a finite clearance between the hub and the end of the conveyor. Depending upon the precise design, this clearance can be quite large. The reaction of the conveyor can shift the conveyor forward a further 

In this dead area, between the end of the conveyor and the inside face of the rear hub, cake can build up and become very compacted and eventually cause considerable wear. To combat this problem a small flight with negative pitch is often welded 180° from the main flight end. This not only scrolls cake back into the path of the main flight, but also serves to counterbalance the non-symmetry of a single lead helical flight. 

However, particularly on the larger bowls, this needs some special attention when balancing. While perfectly balanced on the balancing machine, considerable out of balance can occur when the bowl is filled with process material. This is caused by non-symmetrical buoyancy effects of single-lead conveyors. This has to be taken into account when dry balancing. Final balancing should be checked with the bowl filled with liquid. 

There can almost be an infinite number of feed zone designs with different shaped inlets and outlets, different number of outlets, erosion protection, accelerators and linings. 

Many feed zones have their discharge ports following the helix of the flight. This can allow build up of solids in dead areas. This is overcome wnth the inline feed zone that is possible when using a baffle cone (see Section 2.4.6.2). 

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