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Transfer Chute Design
Article in Bulk Solids Handling · January 2010
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Belt Conveyor Technology
Transfer Chute Design
Simulation-based Dust Prediction
A. Katterfeld, Germany, T. Donohue and C.A. Wheeler, Australia
This article describes how the discrete element method
and computational fluid dynamics can be combined
with empirical found relations for the diffuse dust
emissions to evaluate the dust generation in transfer
chutes. The results of such an analysis can not only be
used for transfer chute design – they also support the
design of accompanying dust exhaustion plants.
D
ust emission is one of the main problems associated with
the operation of transfer chutes. The design of the transfer chute influences dust generation to a large degree.
Due to the very specific design of transfer chutes, sophisticated
simulation methods such as the Discrete Element Method
(DEM) and Computational Fluid Dynamics (CFD) are necessary
to predict the material and air flow in such plants.
1
Introduction
One of the biggest environmental problems regarding transfer
chutes is the dust generation due to the fall and impact of the
material. Safety and health issues also demand low dust
emissions, especially if the transfer chute is situated in closed
buildings. Large amounts of energy and water are used for reducing the dust emission via filtering the dust polluted air or by
240
changing the bulk material properties (e.g. by changing the water content). However, both methods do not address one of the
main reasons for dust generation, the design of the transfer
chute.
A proper design of the transfer chute reduces the dust emissions without the use of expensive dust reducing equipment.
Computer simulations based on DEM allow a detailed analysis of
how the design influences the bulk material flow in a transfer
chute. However, due to the complex interactions of transfer
chute parts, bulk material and air flow, the design of a transfer
chute characterised by low dust emissions, low wear and proper
material flow is a very difficult task.
Although a more careful design might solve a lot of dust problems, dust exhaustion/reduction equipment will still be essential
for a range of application areas, where difficult bulk solid properties or certain boundary conditions limit the design options.
Although the analysis of the bulk material flow gives much of
the information regarding the dust generation, it is often necessary to also analyse the air flow in a transfer chute. For the design
of an exhaust plant it is essential to know how much dust has to
be filtered and which air flow rate is necessary to absorb all of the
dust polluted air. To provide such data it is essential to consider
the main influences for the dust generation in a transfer chute:
• Bulk material flow.
• Air flow inside the enclosure of the transfer chute.
• Dustiness of the bulk material, i.e. which mass of dust is generated by a certain mass flow rate of the material.
Computer simulations and empirical relations for the diffuse
dust emission in bulk handling can be used to estimate all three
bulk solids handling · Vol. 30 · 2010 · No. 5
Belt Conveyor Technology
Fig. 1: DEM simulation of the
transfer chute designs. Particle
colour: particle velocity:
< 0 m/s
> 7 m/s
influences and hence allow the prediction of the dust emission
considering the real transfer chute design and the real bulk material properties. A close co-operation between the Universities of
Newcastle, Australia, and Magdeburg, Germany, is established to
develop and apply the new analysis methods for dust prediction.
2
Computer Simulations
2.1
DEM Simulation
Computer simulations are increasingly used for the simulation of
material flow in transfer chutes. Due to the complex design of
transfer stations the material flow inside a station can be described analytically only for a small part of it (e.g. parabola trajectory). In recent years simulations considering single particles of
the bulk material such as DEM have become increasingly popular. It is thus not only possible to simulate and visualise the material flow but it also enables the analysis of forces and moments
acting on different parts of the transfer station. Combining the
analysis of forces and particle velocities it becomes possible to
predict the wear of such parts and optimise the design with respect to an increased lifetime [1]. Please see [2, 3] for more fundamental information on DEM and its uses in bulk handling.
For a first evaluation of the dust emission it is often sufficient to
analyse the material flow in the system. Dust is generated where
Table 1: Air mass flow rates and volumetric flow rates at different positions in the CFD model.
Condition
M1
Q1
there is an abrupt change of the particle speed or direction. These
changes cause a change in the porosity of the bulk solid and hence
the leakage/entrainment of air. A certain amount of fine particles
is exhausted together with the air and cause the dust emission of
the transfer chute. Therefore it is necessary to realise a smooth
redirection of the material flow as well as a compact material flow
– the dilution increases the porosity and causes an increase of the
air flow. The air must leave the material stream during re-compaction process on the receiving conveyor belt.
With the help of these two guidelines (smooth redirection,
compact material stream) it is already possible to evaluate the
general dust emission behaviour of many transfer chutes. Fig. 1
shows a comparison of the steady state material flow in a transfer chute with original design (left) and optimised design (right).
The above named criteria can be easily evaluated via the simulated particle flow and the particle velocity (particle colour).
2.2
CFD Simulation
The interaction of bulk material and the surrounding air is important in many fields of bulk material handling and process engineering (e.g. pneumatic conveying, fluidised bed technology,
etc). Hence, a lot of work has already been done in the field of
coupling DEM and CFD [4-8]. A direct (simultaneous) coupling
of DEM and CFD is especially necessary if the particles – fluid/gas
interaction influences the particle velocity which is usually the
case for high fluid/gas velocities or small particles. Simultaneous
coupled DEM-CFD simulations show an even higher numerical
effort than DEM or CFD simulation itself.
Due to the high mass flow rate in transfer chutes, it is usually
necessary in DEM simulations to neglect the mass fraction of the
fine bulk material that is influenced by the air flow in the transfer
chute, with the flow of the large particles not usually being affected by the air flow in the transfer chute. Hence, a simultaneous coupling of DEM and CFD seems unnecessary. A more applicable and efficient approach involves a serial combination of
DEM and CFD simulations together with entries out of the well
known continuum approach [9]. First results of such a combined
analysis are already discussed in [10].
M2
Q2
M3
Q3
M4
Q4
M5
Q5
M6
Q6
Vent 1
+0.8
(no pressure) +0.96
-0.87
-1.05
+0.95
+1.14
-0.98
-1.18
+0.1
+0.12
N/A
Vent 1
(-50 Pa)
+2.03
+2.44
-0.87
-1.05
+0.95
+1.14
-0.98
-1.18
-1.12
N/A
Vent 2
+0.97
(no pressure) +1.17
-0.87
-1.05
+0.95
+1.14
-0.85
-1.02
N/A
-0.2
-0.24
Vent 2
(-50 Pa)
+1.13
+1.36
-0.87
-1.05
+0.95
+1.14
-0.11
-0.13
N/A
-1.09
-1.31
3
Vent 2
(-100 Pa)
+1.23
+1.48
-0.87
-1.05
+0.95
+1.14
+0.26
+0.31
N/A
-1.56
-1.88
For the combined analysis of DEM and CFD it is necessary to run
two CFD simulations: firstly the air flow in the bulk material
Combined DEM-CFD Simulation
bulk solids handling · Vol. 30 · 2010 · No. 5
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Belt Conveyor Technology
feeding and receiving belt was considered in the DEM simulation
and is visualized by the stream line profile in Fig. 3, left.
The boundary conditions for the simulation were chosen to
reflect the movement of air around the moving material stream.
These air movements include air being induced into the material
stream as it falls and also air being exhausted as the stream impacts with the receiving belt. For this simulation, the porosity in
the impact zone on the receiving belt was variable but still had a
relatively ordered structure. This ordered structure caused the air
flow within the material stream to be much more ordered which
meant it was necessary to apply an external pressure boundary
condition to the top surface of the receiving conveyor belt. This
ensured the airflow in the region of the impact zone more closely resembled what we would expect in real circumstances (large
amounts of air exhausting from the material stream).
Fig. 2: 3D CAD
model of the combined DEM-CFD
simulation model
stream and secondly the air flow around the bulk material stream.
For the CFD calculation of the air flow in the material stream the
following information from the DEM simulation is essential:
• The idealized geometry profile of the bulk material flow.
• The porosity distribution inside the material flow.
In addition to this, it is necessary to assume several boundary
conditions in the CFD simulation along the bulk material profile
to define the interaction of the air stream inside and outside of
the bulk material stream. Previous work in [10] considered a simple fall of bulk material from a feeding belt conveyor to a horizontal plate, where no enclosure around the material stream was
considered. For a more complex transfer chute example, this
study simulates an enclosure around the bulk material stream
and on the top of the receiving conveyor (Fig. 2). This simplified
example is chosen to show which further analysis possibilities are
offered by an extended CFD analysis. During these preliminary
studies it is not intended to model a specific transfer station
from industry rather just to show the general capabilities of this
approach. Some of basic model parameters are:
• Falling height (height difference between feeding and receiving conveyor): 2 m
• Belt width: 1100 mm
• Bulk mass flow rate: 215 kg/s = 774 t/h
Furthermore, two circular vents with 400 mm diameter, having different positions and a variable pressure drop are considered in the model (see Fig. 2). The highest amount of dust will be
generated during the impact on the receiving conveyor. Hence, it
can be easily estimated, that Vent 1 – in opposition to Vent 2 –
will not exhaust a large amount of dust laden air. The general
material flow is not affected by the enclosure. Hence, only the
4
Air Flow Boundary Conditions
Some of the important boundary conditions for the calculation
of the air flow in the enclosure (outside the bulk material stream)
are shown in Fig. 3, right. The air flows designated in this figure
can be explained as follows:
• M1 – This is an zero relative pressure opening which allows air
to flow into or out of the enclosure (depending on the solution obtained from the other specified boundary conditions).
• M2 – This is the air flow of air induced by the falling stream of
bulk solid (this boundary condition is specified from the results of the CFD simulation for the bulk material stream).
• M3 – This is the air flow of air being exhausted due to the rapid
compaction of the falling stream upon impact (this boundary
condition is specified from the results of the CFD simulation
for the bulk material stream).
• M4 – This is an zero relative pressure opening which allows air
to flow into or out of the enclosure (depending on the solution obtained from the other specified boundary conditions).
• M5 – This is a relative pressure specified opening, where the
pressure was either 0 or -50 Pa.
• M6 – This is a relative pressure specified opening, where the
pressure was either 0, -50 Pa or -100 Pa.
The results of CFD simulations are shown in Fig. 3 and Table 1.
Fig. 4 shows the air flow inside the enclosure while using Vent 1
(Vent 2 closed), left, and while using Vent 2 (Vent 1 closed), right.
The results shown in Fig. 4 were for the case of -50 Pa relative pressure drop. In Fig. 4 left it can be clearly seen that the air, exhausted
by Vent 1, does not come from the impact zone of the material
stream on the receiving belt conveyor, rather it comes from the
Fig. 3: Velocity stream lines of
air which is induced in falling
material stream (left); mass
flow rates measured at different
positions in the CFD model
(right).
242
bulk solids handling · Vol. 30 · 2010 · No. 5
Belt Conveyor Technology
opening above the incoming conveyor belt. Hence, the dust content of the exhausted air will be very little. The right image in Fig. 4
shows that almost the whole amount of the exhausted air of
Vent 2 comes from the impact zone. Hence, it can be estimated
that a large amount of the dust polluted air is flowing into Vent 2.
Table 1 shows the general mass flow and volumetric flow of
the air inside the enclosure at different positions and for different test cases. Positive values mean air flowing into the enclosure. The overall behaviour of the air flow, which can be explained
by the help of the streamline figures in Fig. 4 can be confirmed by
the shown data.
For this model it can be assumed that the air with the highest
dust content can be found on top of the receiving conveyor (region M3 in Fig. 4, right, an average volumetric flow of 1.14 m³/s exhausts from the material stream on the receiving belt). Vent 2 exhausts the air from this region, and, depending on the pressure
drop of the vent, from the opening above the receiving belt (M4).
Putting the results from Table 1 into context, a pressure drop of 50
Pa for Vent 2 shows a volumetric flow of 1.31 m³/s, and while this
is more than the 1.14 m³/s coming from region M3, there is still an
outflow at the opening above the receiving belt (M4) of 0.13 m³/s.
This means that there is a possibility of dust escaping the enclosure. If we then consider the case of a 100 Pa pressure drop at
Vent 2, the volumetric flow through Vent 2 increases to 1.88 m³/s
out of the enclosure, which results in the air flow at the opening to
be reversed so that air is drawn in through this opening. This indicates that no dust polluted air escapes from the enclosure.
Table 2: Weighting factor for dustiness
Material Property
a
High dust generation
√105
Medium dust generation
√104
Low dust generation
√103
Imperceptible dust generation
√102
Extra moist / low-dust material
√100
5
___
___
___
___
___
Empirical Prediction of Diffuse Dust
As it was shown, the DEM simulation allows the prediction of
the areas with highest dust generation and the combined DEM
and CFD simulation allows the calculation of the air flow out of
these regions. Hence, it is possible to estimate an optimal position and operating condition for an exhaustions plant.
However, no information is given about the mass of dust which
has to be filtered by the exhaustion plant because this is a bulk
material property. Although the industrial and political demand
is given due to the many problems caused by dust emission, only
vague methods exist to predict the dust amount. A standardised
laboratory test for the determination of the dustiness properties
does not exist, although several works are published which describe devices which could be used for such an analysis [11, 12].
bulk solids handling · Vol. 30 · 2010 · No. 5
243
Belt Conveyor Technology
Fig. 4: Velocity stream lines of air
inside the enclosure, indicating
the origin of the air.
The German VDI standard 3790 “Emission of gases, odours and
dusts form diffuse sources: Storage, transhipment and transportation of bulk materials” [13] describes a general method for the
prediction of the dust generation caused by the continuous or
discontinuous fall of the bulk material. Although the purpose of
the standard is to calculate the dust pollution close to ports or
mines, the bulk material classification system can also be used for
a rough estimation of the dust generation inside transfer chutes.
The VDI 3790 uses the following empirical found equations for
the determination of the generated dust qdust per metric tonne
of the discharged material:
( )
v2
qdust = qnorm · 0.5 · ___
4·g
1.25
in [g/t]
(2)
with the standardised emission factor qnorm given by
qnorm = 83.3 · Q-0.5 · a in [g·m3/t2]
(1)
where:
About the Author
mass flow rate
weighting factor for the material dustiness
material velocity at the main impact point
gravitational acceleration
[t/h]
[-]
[m/s]
[m/s2]
The weighting factor a is given in Table 2. The characterisation
of the bulk material regarding dust generation seems to be quite
random, but a bulk material catalogue based on practical experiences is given in the Appendix of the Standard containing more
than 100 bulk materials. However, due to the influence of water
content and other parameters, it must be critically proven how
the material has to be categorised. Assuming dry hard coal as the
bulk material for the example of the transfer chute described in
Section 2.2 the following parameters can be calculated:
• Impact speed of the material on the receiving belt: v = 6.3 m/s
• Standardised emission factor: qnorm = 97.7 g·m3/t2
• Dust per tonne of bulk material transferred: qdust = 48 g/t
• Dust mass per hour operation: Qdust = 37 kg/h
About the Author
About the Author
Dr. T.
Donohue
A/Prof. Dr.
A. Katterfeld
Dr. C.A.
Wheeler
Dr. Timothy Donohue completed an
undergraduate degree in Mechanical
Engineering with
1st Class Honours in 2003 at the University
of Newcastle, Australia. He received his
Ph.D. from the same institution, working
closely with the Centre for Bulk Solids and
Particulate Technologies. His particular area
of focus is on permeability, with a specific
interest in fibrous materials. More recently
his work has been in the area of computer
simulations of particles.
Prof. André Katterfeld received his
Ph.D. with his work
on the functional
analysis of tube
chain conveyors from the Otto-von-Guericke-Universität Magdeburg, Germany in
2005. In 2009 he was appointed Junior-Professor for Continuous Conveying Technology at the Institute for Logistics and Material Flow Technology of the same University.
Prof. Katterfeld is a guest lecturer at the University of Newcastle, Australia, and he is also the co-founder of the CeParTec GmbH.
Dr. Craig A. Wheeler
(BE, PhD, MIEAust) is
currently a Lecturer
in the School of Engineering at the University of Newcastle, Australia. He worked
as a Mechanical Engineer for BHP Billiton
for 11 years and then as Research Fellow
at the Centre for Bulk Solids and Particulate Technologies for 4 years. He was
appointed as a Lecturer in Mechanical
Engineering in 2002 and undertakes
industrial consulting activities through
TUNRA Bulk Solids Research Associates.
Contact:
Otto-von-Guericke Universität Magdeburg
Jun.-Prof. Dr.-Ing. André Katterfeld
Institut für Logistik und Materialflusstechnik
Universitätsplatz 2, 39106 Magdeburg, Germany
244
Q
a
v
g
Tel.:
Fax:
++49 (0) 391 67 12245
++49 (0) 391 67 12518
E-Mail: andre.katterfeld@mb.uni-magdeburg.de
bulk solids handling · Vol. 30 · 2010 · No. 5
Conclusion
This study has shown how DEM and CFD simulation can be combined to analyse the bulk material flow and the air flow inside
transfer chute enclosures. Together with an empirical model for
diffuse dust emissions it is possible to calculate the basic parameters for dust exhaustion plants. This is fundamental in the optimisation of such plants. Furthermore this analysis allows a reduction of the dust generation because it allows the consideration of the transfer chute design. The ongoing research at Newcastle and Magdeburg covers the simulation and dust prediction
of more complex transfer chutes. Future work will improve and
extend the assumptions for the CFD simulation. Furthermore,
the development of improved laboratory tests for the determination of the dustiness property of bulk materials is covered by
this research.
n
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