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The vertical screw conveyor-powder properties and Screw conveyor design
Alma Kurjak
Department of Chemical Engineering, Lund Institute of Technology,P.O. Box 124, SE-221 00 Lund, Sweden January 2005
1 Abstract
This Master Thesis is a study of how powder properties and screw conveyor design influence the flow
properties of a vertical screw conveyor. Studies show that different powder properties like particle
size, bulk density and particle shape have a large influence on screw capacity. Coarse powders will
flow into the screw easier than fine powders.The screw capacity will also be higher if a dense powder
is used. Particle with a round shape have lower internal friction that results in a greater screwcapacity. It was also shown that the Hausner ratio, assessed from tapped and apparent density andangle of repose are effective methods to determine the free-flowing properties of the powder. Studies
also show that the clearance and the free length of the intake have a big influence on screw capacity.
No correlation between conveying length and conveyor capacity was found.
Key words: vertical screw conveyor; powder properties; Hausner ratio; iron powders; flowability;
screw design;
2 Introduction
Different mixers can be uses forhomogenisation of powders. One of them is the
Orbiting screw mixer. Orbiting screw mixers
work excellent and the only disadvantage is
that they are expensive. One cheaper
alternative is a centre screw mixer. It consists
of a conical vessel and a screw that is
surrounding by a tube. The products to bemixed are conveyed upwards by the screw
positioned in a central mounted guiding pipe.
The combination of the rotating screw and
conical vessel results in an efficient mixing.
The aim of this thesis was to identify different
powder properties and screw design that
influences the flow properties of the vertical
screw conveyor.
3 Theory
Different powder properties can be used to
identify if powder is free flowing or not.
3.1 Particle size
Particle size has influence on flowability of apowder. In general, fine particles with very
high surface to volume ratios are more
cohesive than course particles. Particles larger
than 250 m are usually relatively free
flowing, but as size falls below 100 m
powder become cohesive and flow problems
are likely to occur.
Powders having a particle size less than 10 m
are usually extremely cohesive. [1]
3.2 Bulk density
Because powders normally flow under the
influence of gravity, dense powders are
generally less cohesive than less dense
powders. [1]
3.3 Particle shape
Particle shape has a large influence on flow
properties. A group of spheres has minimum
interparticle contact and generally optimal
flow properties, whereas a group of flakes have
a very high surface-to-volume ratio and poorer
flow properties. [1]
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3.4 Hausner ratio
The Hausner ratio is a measure of how
compressible a powder is in relation to bulk
density. It is derived from the quotient between
tapped density (TD) and apparent density (AD)
ADTDratioHausner =
Values less then 1.25 indicate good flow
whereas values greater than 1.25 indicate poor
flow. [1]
3.5 Angle of repose
The angle of repose given in table 2 may be
used as a guide to flow performance. [1]
Table 1 Angle of repose as an indication of
powder flow properties
ANGLE OF
RESPONSE
TYPE OF
FLOW
< 20 Excellent
20 30 Good
30-34 Passable
> 40 Very poor
4 Materials
4.1 Powder
Hgans AB produces two different kinds of
ferrous powders:
sponge-iron powders, and
water-atomized (unalloyed and low-alloyed) iron powders
The external shapes of both particles are
irregular and similar to one another. However,
the sponge iron particle has as its name
suggests a spongy internal structure and water-
atomized is internally compact.
In the Belgium plant, gas-atomized powders
with almost perfect round shape are produced.
In this work, nine different iron powders wereused:
Sponge-iron powder: NC100.24, W40.24,
MH300.29, SC100.29 and M1000
Water-atomised iron powder: AT40.29,
ASC100.29 and ASC300.29
Gas-atomized iron powder: Fe6.8Si
In table 1, apparent density and flow of
different iron powder are present. [2]
Table 2 Properties of some different iron
powders
PowderAD
(g/cm3)
Flow
(s/50g)
Fe6.8Si 4.3 15
ASC100.29 3.0 25NC100.24 2.4 31
AT40.29 3.1 29
W40.24 2.5 38
SC100.26 2.7 30
MH300.29 2.9 27
ASC300 2.9 25
4.2 Apparatus
The measurements were performed with three
different screws. The first screw was used forpreliminary studies; just to see how screws
work. With the second screw almost all
experiments were done. The third screw wasused to see if the results will be the same if a
bigger screw conveyor is used, see figure 1.
Figure 1 Vertical screw conveyor
5 Methods
5.1 Methods to characterize powder
5.2 Size measurement
To determine particle size, laser diffraction,
Sympatec HELOS was used.
5.3 Bulk Density (Apparent density)
Bulk density was determined by filling the
powder through a standardized funnel into a
small cup, levelling-off the surplus powder on
top of the cup and dividing the weight of
powder contained in the cup by the cup volume
(25 cm3). [2]
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5.4 Particle Shape
To determine the shape of different iron
powders, electron microscope pictures were
used.
5.5 Hausner RatioThe Hausner ratio is derived from the quotient
between tapped density (TD) and apparent
density (AD). Tapped density was measured
using Stamp volume meter.
5.6 Angle of repose
For measuring the drained angle of repose was
used, se figure 2.
Figure 2 Measurement of drained angle of
repose
5.7 Flow
Flow rate is the time in seconds, which an
amount of 50 g dry powder needs to pass the
aperture of standardised funnel se figure 3.
Figure 3 Aperture for measurement of
Flow and AD
5.8 Methods to characterize screwconveyor design
5.9 Clearance
Tubes with different diameters were used to
see which influence the clearance has on screw
capacity.
5.10 The free length of intake
Tubes with different lengths were used to see
which influence the free length has on screw
capacity.
5.11 Conveying length
Tubes with different lengths but the same free
length of intake were used to see which
influence the conveying length has on screw
capacity
6 Results and discussion
6.1 Powder properties
Figure 4, shows how mass flow depends on
screw velocity for different powders.
0
500
1000
1500
2000
3 6 9 12
Velocity (rev/s)
Massflow(g/s)
ASC 100.29
NC100.24
W40.24
ASC300
AT40.29
M1000
MH300.29
SC 100.26
Fe.6.8Si
Figure 4 Mass flow for some different iron
powder
6.2 Size
The screw capacity is greater for coarsepowders than for fine due to the differences in
flowability.
Fine particles have large specific surface area
and are more cohesive than coarse particles. If
the powder is more cohesive, it is not free
flowing. A coarse powder will be more free
flowing than a finer powder and will thus flow
more easily into the screw.
6.3 Aerated bulk density
Bulk density is one important parameter for
screw capacity since the screw has a fixedvolume. The higher the bulk density of a
powder the more mass of the powder can be
introduced into the screw. It means that
powders with o large bulk density will have a
largest screw capacity than obtained with
powders with a low bulk density.
6.4 Particle Shape
If the shape of a particle is known then
conclusions can be made of internal friction of
the powder. In the following list powder withhigh internal friction is given first;MH300.29> ASC300 > ASC100.29 > NC100.24 >
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AT40.29 > SC100.26 > W40.24 > M1000 >
Fe6.8Si. This comparation is made on the
particle shape appearanced from SEM picture.A group of spheres has low internal friction
whereas a group of flakes has higher internal
friction. Compared to powders with lower
internal friction, powders with higher internal
friction flow into the intake area of the screw
conveyor with lower velocity. It implies that
use of powders with low internal friction
(round powder-Fe6,8Si) will result in greater
screw capacity than obtained with powders
with higher internal friction.
6.5 Hausner Ratio
Iron powders have different Hausner Ratios,
see table 3. Free flowing powders are less
cohesive and have Hausner Ratio close to 1while less free flowing powder have Hausner
Ratio > 1.25. If a powder is free flowing, it
will easier flow into the screw. Use of free-
flowing powders will result in a greater screw
capacity than obtained with not free-flowing
powders
Table 3 Hausner Ratio and angle of repose for
different iron powders
PULVER HAUSNER
RATIO
ANGLE OF
REPOSE
Fe6.8Si 1.10 25.8M1000 1.10 33.4
ASC100.29 1.27 34.6
W40.24 1.12 34.9
NC100.24 1.26 35.3
AT40.29 1.17 36.0
SC100.26 1.27 -
MH300.29 1.28 47.2
ASC300 1.34 48.2
6.6 Angle of repose
Iron powders have different angels of repose,see table 3. A comparison between angle of
repose and screw capacity, figure 4, for
different iron powders shows that powders
with low angle of repose get a higher screw
capacity than powders with a high angle of
repose. It was expected because powders with
a low angle of repose flow more easily into the
screw.
6.7 Flow
No correlation between flow and screw
capacity was found. It seems as if this method
is not sufficient to describe screw capacity.
6.8 Screw conveyor design
6.9 Clearance
At a large clearance a back flow of bulk
material opposite to the conveying direction
occurs followed by reduction in conveyorcapacity, see figure 5.However, if clearance is
small milling and jamming can take place
between screw and casing. Clearance is also
necessary for smooth running of the conveyor.
Therefore, it is important to find the smallest
clearance at which no milling and jamming
process takes place.
0
1000
2000
3000
4000
2 4 6 8 10 12
Clearance (mm)
Massflow(g
/s)
60 Hz
70 Hz
80 Hz
Figure 5 Mass flow for ASC100.29 at differenceclearance
6.10 The free length of intake
At a very low speed, it is enough to have a
short free length of intake to obtain maximum
output, se figure 6. As speed increases, the
vortex formed in the screw limits the amount
of powder that can enter the screw. To
compensate for this, a larger free length of
intake is necessary at higher speed to obtain
maximum output.
0
100
200
300
400
500
600
700
800
900
1000
3 5 7 9
Velocity (rev/s)
MassflowASC100.2
9(g/s)
11
I = 6 cm
I = 12.5 cm
I = 17.5 cm
Figure 6 Mass flow at different length of intake
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6.11 Conveying length
It is difficult to say which influence conveying
length has on the conveyor capacity. It seems
as if there is no correlation between conveying
length and conveyor capacity, see figure 7.
Probably conveying length has no influence onthe capacity. Perhaps, the arrangement used in
this work was not big enough to see any
differences.
100
200
300
400
500
600
700
3 5 7 9
Velocity (rev/s)
Ma
ssflowASC100.2
9g/s
11
Conveyor lenght 28.5 cm
Conveyor lenght 33.5 cm
Conveyor length 40 cm
Figure 7 Mass flow at different conveying
lengths
7 Conclusions
Powders with coarse particles will flow into a
screw easer than powder with fine particles.
This results in a greater mass flow.The screw
capacity will also be higher if dense powder isused. Round powder, have lower internalfriction that results in a greater screw capacity.
Hausner Ratio and angle of repose are most
likely efficient methods to measure if powder
is free flowing or not.
The clearance and the free length of intake
have a large influence on screw capacity. No
correlation was found between conveying
length and conveyor capacity.
8 Acknowledgments
Special thanks to my supervisors Ingrid Eriksson
at Hgans AB and Anders Axelsson at the
Department of Chemical Engineering, Lund
Institute of Technology for guiding me throughthe whole work during my Master thesis.
I would also like to thank all at Hgans AB
for useful help and advice on various
problems.
9 Reference
[1] Aulton Michael E.:
Pharmaceutics The Science of Dosage
Form Design p.197-210 and 133-135
[2] http://www.diegm.uniud.it/fmiani/PMS
CHOOL/CHAPT03.PDF
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