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A Radical Approach to the Vertical Conveyance of Bulk Materials: the OLDS ELEVATOR™

Figure 5 – 150 mm (6”) by 7.5m and 7 m lift OLDS ELEVATORS™ at Bean Growers Australia.

3. New vertical elevator design: the OLDS ELEVATOR™

Operating Principle

Figure 6 – In-feed pick up scoops are attached at the bottom of the elevator.

Figure 7 – The bulk material can be easily picked up near ground level (left), being fed by the two rotating in-feed pickup scoops at the base of the casing (right).

Figure 8 – Enlarged view of the boundary layer that dynamically seals the annular clearance gap between the screw and rotating casing.

Elevators

Figure 9 - Comparison of elevation mechanisms of (left) the OLDS ELEVATORT where fall back is prevented by the sealing boundary layer of rotating product and (right) the conventional rotating screw conveyor showing product fall back in the screw clearance gap.

The new vertical elevator design has only one moving part in contact with the bulk material, a tubular casing with its attached in-feed scoops and delivery slinger, that rotates around a static screw. Generous clearance is provided between the static screw's diameter and the tube (figure 6). This clearance is an important design feature that prevents damage to the bulk material, casing wear, metal-on-metal contact and improves energy efficiency. In-feed pick up scoops at the lower end of the rotating casing gather the bulk material from the feed supply into the rotating casing. The elevator is therefore self-feeding at a controlled rate as it rotates. The top of the casing has an overlapping 'slinger' seal that prevents material leaking from the surrounding discharge chute. Bulk material in the feed hopper typically covers the pickup scoops (figure 7). As the casing rotates, material is directed into the rotating casing by the pick up scoops that rotate with the casing. Friction against the inner wall of the casing rotates the material and causes product resting on the screw flight to be driven gently up the inclined face of the screw. Back leakage of materials down the screw clearance is prevented by the rotating boundary layer of material that forms on the inside wall of the casings and seals the annular clearance between the casing and the static screw diameter. The bulk material itself forms an effective dynamic seal and stabilizes the central position of the screw to inhibit casing contact (figure 8). This contrasts dramatically with the dynamic leakage that takes place in a conventional screw elevator that offers little resistance to the whirling potential of the rotating screw (figure 9).

The bulk material flows smoothly and continuously up the screw, to discharge evenly over the 'slinger' into the surrounding discharge chute (figure 10). The delivery rate is directly related to the rate of rotation of the casing. Therefore consistent control of the product delivery rate is possible – in fact from any rotational speed above 0 RPM.

Further explanation and comments

The in-feed scoops control the rate of handling and deliver material into the casing with a pressure that initiates motion up the lower portion of the screw flight. The internal mechanism then takes over to move the material along the inclined face of the screw flight by frictional drag of the product on the rotating boundary layer. This sealing and elevating motion of the product is reinforced at the inlet region by further material entering the in-feed scoops. The collection and transfer of material takes place in settled conditions, rather than the dilated turbulence of a dynamic vortex. The density difference and quiescent movement allows a greater mass of material to be elevated.

The effectiveness of the mechanical extraction of material from the feed hopper by the rotating scoops contrasts sharply with the resisting pressure offered by prior contents in the rotating screw of a conventional screw elevator. Instead of needing a steep inlet chute at one side of the casing to generate the horizontal pressure to overcome the centrifugal force of rotating product, the new elevator offers a flow-gathering dimension larger than the casing diameter virtually at floor level. This enables difficult flow materials to be offered to the machine at low level via a mass flow inlet section on the feed hopper, whereas the bottom bearing and seal of a standard screw elevator, followed by a steep inlet chute on one side of the casing that extends some distance up the length of the casing inevitably means a high rim height and/or low holding capacity of a conventional feed hopper, even for free flowing products. Difficult flow materials usually require a separate feed screw to deliver the bulk material into a standard form of screw elevator.

Figure 10 - Top of the 7.9m (26 foot) elevator being tested at Olds Engineering, showing discharge hopper, drive pulley, bearing, and Stainless steel 100mm (4") O.D. rotating tube.

It was initially thought the casing speed would need to generate sufficient centrifugal force to maintain the annular boundary layer to create an effective seal against back-leakage. In reality, the elevator delivers product at any rotational speed above zero RPM in a flooded condition, similar to a horizontal screw feeder operating in flooded mode. The machine capacity is determined by the mechanics of in-feed scoops, but is ultimately limited by the ability of the subsequent conveying screw section to transfer the product along the screw face. A key design feature is therefore to optimize the relationship of these features. A crucial feature of screw helix angle is that the frictional drag imposed by shearing contact between the product resting on the screw flight surface and the rotating boundary layer must exceed the frictional resistance and lifting effort of the product sliding up the flight surface. In practice, slippery and spherical products such as beans, steel shot and round macadamia nuts elevate easily by this method. As internal friction is normally considerably higher than sliding friction on a smooth metal surface, a ratio of screw pitch equal to the screw diameter can normally be employed.

Most machines made to date utilize twin in-feed scoops on the rotating casing. This is superficially attractive as a symmetrical feature to avoid out-of balance forces on the casing. A single in-feed scoop version (with an opposite balance weight) is beneficial for delicate handling, sensitive feed control and to enlarge the swept pick-up area for difficult flow or larger particle size products. It is essential to ensure that the elevating capacity is not exceeded by the collecting mechanism otherwise some material in the flow path of the scoop will move around or over the scoop instead of into the casing. Optimization needs to be tempered with a margin of safety to deal with variable product conditions. A feature of the elevator is that, as with a normal screw elevator, it remains full of product unless the machine is run on for a while after the feed hopper empties, and even then will not completely self-clear. This means that a free flowing product will tend to run back down the casing when the elevator stops and the machine has to re-start with a section of the casing full. Long elevators may have a slightly shorter pitch in the lower section of the screw, to give a better wedging action and allows material to accelerate away when it reaches the longer pitch section.

CONTINUE >> Contrasting the OLDS ELEVATOR™ with conventional screw elevators: A Radical Approach to the Vertical Conveyance of Bulk Materials: the OLDS ELEVATOR™

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