I believe the term implied is ‘Choking’, not ‘clocking’, although this term is normally synonymous with ‘Logging’ or ‘clogging’, which is the build-up of sticky or cohesive material sticking to the shaft and flight volume to rotate with the screw and negate axial transfer. However, the problem is clearly one of stalling the screw due to excessive torsional resistance, which is not surprising given that the three screws are of uniform pitch over an exposed length of six feet, followed by a further twelve feet of confined transfer without expansion relief when handing a strongly interlocking bulk material.

This installation demonstrates that brute force is not a good approach. There is a huge torque available from a 10 HP drive at 4 rpm, so a stalled condition is imposing a massive bending load on the overhung drive shaft and seen to be bending the screw the screw tube that, although of substantial strength, is quite a long span. The design of such systems requires a sensitive balancing act for a bulk material that just loves to hang together - between securing a reliable flow from the hopper, without imposing large overpressures on a big, confined shear area. An outlet size 6 foot x 3 foot superficially appear large and will pass a huge flow rate, but the wood chips are quite capable of arching and rat holing over this cross section, depending on the geometry and wall friction value of the flow channel and this might expose another problem if the extraction is cured and deep beds of material are formed.

These wood fragments do not lend themselves to bulk strength measurement by standard shear testing equipment so the design of the hopper and discharge equipment calls for some experience. With this rectangular outlet it is most likely that the hopper immediately above is a pyramid shape, and unlikely to be of mass flow design because of the difficulty of stimulating wall slip in corner gullies. Apart from the reduced flow efficiency, the overpressure transferred through a non-mass flow orifice that is active, is greater than that from a correctly designed mass flow channel.

Retrofitting this equipment is quite a challenge, but requires an assessment of the overall hopper and screw design. Basic steps are to replace the screws with ones having progressive pitch and possibly stepped shaft construction to avoid dead regions of extraction and offer expansion relief in the continuation conveying section. If gears are fitted between the screws to transmit power from the centre to the outside screws, it would help to replace equal gears with a small central pinion and larger outside gears to slow the two outer screws in compensation for the increased output of the centre one with a larger exit pitch. Review the hopper approach to see if it can be reconfigured to expand around an overpressure relief insert, so that the screws tend to clear an unconfined repose slope, rather than shear the mass. Perhaps weld horizontal rolled steel angles,’ toe- in’, to the higher walls, if the span is too large to support an arch, to give a short 45 degree surface to reduce flow pressures.

This problem is far from unique and has been brought out in a number of biomass applications, causing serious financial setbacks to projects that were considered attractive. Specialists are available and their value should not be underestimated. ■