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A
Radical Approach to the Vertical Conveyance of Bulk
Materials: the Olds Elevator 
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Paper was published in the February 2005 issue of Solids
& Bulk Handling (UK)
Lyn Bates Ajax Equipment Ltd. United Kingdom.
Until recently process engineers had only three modes
of transport available to move loose solids and bulk materials
by means of helical screws. These were by gravity mode,
as widely employed in screw type conveyors, the flood feed
form used in screw feeders, and a dynamic mode that operated
within screw elevators. Inevitably there are limitations
to each of these methods. Process engineers now have an
additional tool for elevating solids, a new elevating method
that uses a static screw, where the bulk material is moved
around the inclined face of the screw flight by the effect
of a rotating casing. It works amazingly well.
This new method of elevation has many operating advantages
that extend the scope of screw elevators beyond the normal
range of application of traditional equipment.
Screw Elevators
Traditional screw elevators are a proven and important
part of the process engineer’s armoury when faced
with the task of moving bulk materials.
However, they sometimes struggle at steep angles because
the screws have to be driven at a speed of rotation to
induce a dynamic vortex of bulk material. This is generally
effective, but the handling capacity – or product
loading – is generally low and the actual performance
depends upon the nature and state of the bulk material
that is being handled. A product that is dilated to a fluidised
condition will not elevate in a standard unit of this type
because leakage in the flight tip clearance and run-back
down the helix will negate the elevating capacity of the
machine. An inherent downside of running the screw at a
high speed is that fresh material entering the machine
has to generate sufficient flow pressure at the end of
an inclined inlet chute to overcome the centrifugal force
exerted by the existing contents. Higher speed ultimately
becomes self-defeating in a drive for increased capacity,
apart from introducing various other problems.
Addressing this difficulty by extending the steepness
of the chute or the height of the inlet port enhances the
amount of material that is elevated, but increases other
drawbacks. Deeper inlets leave more residue in the machine
at the end of a run and a greater loading height is needed
to feed material into the machine. The design of the inlet
configuration is therefore very product sensitive and the
ability to pick up product from a low level is severely
restricted, as the bottom end seal and bearing add to the
ground clearance required.
With the exception of simple, well-proven applications,
such as moving grain and plastic pellets, the design of
steep screw elevators usually involves some kind of compromise.
Mechanical feed devices, such as horizontal screw feeders,
can be used to force-feed product into a steep screw elevator,
but these introduce significant extra costs to the application.
Static Screw Elevator Design
So it is perhaps not surprising that when Peter Olds,
an Australian engineer, was faced with the task of elevating
sand to a new moulding machine in his foundry, he found
that traditional screw elevators and other conventional
devices used for this process fell short of meeting his
objectives for an economical, compact, quiet, trouble-free
machine.
After assessing his requirements from first principles,
Olds reversed the normal mechanism to drive the casing
to rotate around a stationary screw, and so developed the ‘Static
Screw Elevator’, which was installed and worked well.
Various test machines were then made and satisfactory trials
have been conducted on products as diverse as peas, beans,
bread crumbs, coal fines, dried capsicum, dog biscuits,
flour, plastic powder, wet and dry sand, rice, coffee beans,
granulated coffee, macadamia nuts, steel shot, peanut kernels
various grains and seeds, sugar, wet sulphur slurry, dry
sulphur and molasses.
Static Screw Operation
A key to the operation is that scoops on the lower end
of the machine casing feeds product into the machine. Material
entering the casing impinges on the static screw and fills
the clearance between the screw and the casing. A boundary
layer of loose product is caused to rotate by contact friction
on the casing inner wall and this seals the flight tip
clearance. Material resting on the surface of the screw
is driven up the inclined face of the flight by frictional
drag of the product on this rotating boundary layer. This
motion of product is aided at the inlet region by further
material entering the scoops. In practice, the formation
of the boundary layer to prevent the fallback of product
in the clearance between screw and casing is the first
stage of the elevating process. The absence of a dynamic
vortex of the product on the flight face and lack of ‘fallback’ in
the casing are main distinguishing features between this
apparatus and a conventional screw elevator and accounts
for their enhanced elevating capacity and gentler handling
characteristics.
Material forms a boundary layer on the wall at a lower
rotational speed than that needed to create a product seal
by centrifugal force alone. Two extra factors cause the
material to hold against the casing wall and prevent it
from falling down through the flight tip clearance. These
are the pressure arising from the material being moved
up the sloping face of the screw flight and also the fact
that the bottom plate supports the base of the annulus
layer lining the casing. The elevating capacity of the
machine is determined by the inlet geometry, but is ultimately
limited by the ability of the subsequent conveying section
to transfer the product along the screw face. The crucial
feature of 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.
The lack of fallback means that bulk material is moved
more coherently, and therefore more gently, and at a higher
rate that in a normal screw elevator running at the same
differential speed between the screw and the casing. Most
machines made to date utilise twin scoops on the casing.
This is superficially attractive as a symmetrical feature
to avoid out-of-balance forces on the casing. A single
scoop version offers advantages for more delicate handling,
more 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, but optimisation
needs to be tempered with a margin of safety to deal with
variable product conditions.
Benefits of the Static Screw Elevating Approach
The benefits of static screw elevating include:
- Performance is more predictable, more reliable, more
product friendly, more product tolerant and much less
maintenance prone than conventional elevators.
- The pick up height is at the lowest point of the casing,
and can be at virtually floor level.
- Pick-up scoops give a positive, controlled input of
material into the casing of the elevator. The transfer
rate is volumetrically proportional to casing speed,
so the machine can be used as a variable rate feeder.
- The collection area extends to a swept diameter as
the tip of the scoop(s), providing a relatively large
effective flow channel to a mechanically extracted region.
The feed hopper can diverge from the scoop tips as a mass
flow, expanded flow or a non-mass flow channel, to suit
the nature of the bulk material being handled, thereby
maximising the hopper holding capacity.
- Feed material can be entered from any sector of the
machine’s periphery.
- As the discharge end has no bearings, seals or a drive,
discharge can be close to the ultimate headroom.
- The discharge can be divided into two or more sectors,
with the discharge rate to each being directly proportional
to the percentage of the casing periphery taken by the
respective chutes.
- There is virtually no limit to the length of the machine
although a practical limit of 10m is recommended at present.
- Bearings are totally external to the product flow and
accessible for maintenance, if required.
- The drive can be located at any position on the casing,
to suit easy mounting, wiring and access.
- The static screw allows the stiff casing tube to rotate
without generating internal rotational forces on the
screw or the product other than transfer by contact friction,
so shear and dilated vortex motion in the conveyed product
is reduced, therefore delicate products can be handled
with little damage.
- Fallback is prevented by product lining the casing,
which also tends to centralise the screw and hold it
from casing contact. This results in a higher conveying
rate and less product damage than with standard screw
elevators. (Smaller casing clearances may also be employed
than with standard screws).
These benefits address many of the limitations that currently
inhibit the selection of a screw elevator in some process
applications. The static screw elevator is therefore expected
to extend the scope of application of screw equipment to
include duties for which a conventional screw elevator
is not suitable of or cannot fit within the particular
site constraints.
Static Screw Planning Considerations
When process engineers consider the benefits of the static
screw elevator they have to note some drawbacks, which
must be clearly identified to draw a balanced picture of
its capabilities.
- The construction is larger in cross section because
of the framework needed to support the casing bearings.
- The machine tends to retain more residual product at
the end of a run than a conventional screw elevator,
but it is easy to empty by removing the bottom plate.
- The rotating casing has to be guarded where assessable
by personnel.
- The equipment presents a more cumbersome appearance
than the simple tube of a conventional screw elevator.
- Larger bearings are required to fit around the casing.
Split bearings are convenient for maintenance, but relatively
expensive.
- The constructional features make the machine more expensive
to manufacture than a standard screw elevator, however
this is usually well offset by its advantageous features,
improved reliability and performance. The self feeding
aspect of the static screw saves any need for an additional
screw feeder. Also when efficiency comparisons are made
the static screw requires a smaller diameter screw and
energy requirements are also significantly less.
For further information…
The static screw elevator is an important innovation that
not only creates new opportunities for solids and bulk
handling but also offers process engineers high levels
of performance and reliability. Ajax Equipment has licensed
the static screw elevator technology in the UK and has
constructed two test and demonstration machines. For further
details or a trial with the Static Screw Elevator contact
Ajax Equipment on 01204 386723 or sales@ajax.co.uk.
In the US. contact Olds Elevator LLC at 978 887
2871 or sales@oldsusa.com |
