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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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http://www.diegm.uniud.it/fmiani/PMShttp://www.diegm.uniud.it/fmiani/PMS

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