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Conveying
Productivity benefits
Getting conveyor operations right can have significant productivity
benefits, notes Dr Eddie McGee, technical director, Ajax Equipment
First the basics: never confuse the function of a screw conveyor the conveyor. In this
with that of a screw feeder or screw elevator. A screw conveyor case the screw
operates in gravity mode where the material slides down the conveyor can be
face of the screw flight as an inclined plane. Screw feeders, on designed with a
the other hand, work by flood mode, where the material totally counter balanced
fills the casing cross section. A screw elevator works in dynamic covers such that the
mode where the material behaves as a fluid vortex, spirally cover is easily lifted up
advancing because rotation of the screw is impeded by contact by the operator. In
friction with the inner wall of the casing. those cases where you
Common conveyor problems are related to the length of the want to minimise the
conveyor, angle of incline, types of material being conveyed, cleaning volume within
interface with other equipment, outlet design and ease of access the screw, particularly
for cleaning. Long conveyors are sometimes specified which will with sticky pigment
usually need intermediate hanger bearings to support the screw. type products which
Hanger bearings are susceptible to failure due to overloading might 'stick' in
caused by a supply feed surge and they also present a flow significant quantities to
The screw conveyor can be designed with a
obstacle to the powder. One way of avoiding this is to the underside of the
counter balanced covered top
incorporate a reduced capacity feed control section such as lid, it is possible to fill the space above the screw with a sleeve
reduced pitch flights and stepped screw diameter to prevent the welded to the underside of
in-feed overfilling the initial screw section. However, if reliable
operability of plant is important, without the regular maintenance
hanger bearings demand, then robust support shaft design
becomes increasingly important and it is often easy to reach 6m
conveying length or more.
Longer lengths can then be served by in-series arrangements
of screws. If the conveyor features an open intermediate outlet
then careful outlet design and flight geometry is essential to
prevent carry over from the intended outlet. Screw conveyors
are often used horizontally but can also be used for gentle
inclines, up to 20 degrees without too much alteration to the
design. Above this and performance begins to tail off; above 30
degrees output rapidly falls away, literally. 'Fall back' of material
into the prior pitch space significantly increases the cross
BLOWER EXPERTISE
sectional loading of the conveyor. Shorter pitch construction and
EASE OF INSTALLATION //
casing form can provide more effective transfer.
DEPENDABILITY // LOW NOISE
Conveyors are widely used for handling loose dry bulk solids Fully packaged air blowers, spare parts and maintenance services,
but they can also be used for damp filtercakes and pastes.
units for hire. Aeration and air scour to pneumatic conveying
and vacuum pumps. UK distributor for Robuschi RPD blowers.
The flow characteristics of the material should be reflected in
Donkin SG high efficiency turbo compressors for higher duties.
the choice of screw. There are many types of screw flight
forms for mixing, pre-breaking, compacting and other
functions. For example, it is best to use ribbon flights for
damp, sticky and cohesive bulk products but not for stringy
materials. Ribbon flights resist clogging with damp products -
the trick is getting finish quality and construction proportions
Howden Process Compressors
particularly flight diameter, width and pitch right. to ensure
Park Road
Holmewood Industrial Park
smooth conveying operations. Chesterfield S42 5UY
In many processes it is required that the conveyor has high
United Kingdom
containment capabilities and doesn't let any dust out, yet there
Tel: +44 1246 859 053
Fax: +44 1246 859 054
has to be ready access to the screw for cleaning operations by
Email: hdb.sales@howden.com www.howden.com
the operator without the need for a mechanic to disassemble
www.solidsandbulk.co.uk August 2007 • Solids & Bulk Handling 35
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