ap-3-12.dvi


Acta Polytechnica Vol. 52 No. 3/2012

Change of Pressing Chamber Conicalness at Briquetting Process in

Briquetting Machine Pressing Chamber

Peter Križan1, Miloš Matúš1, Jaan Kers2, Djordje Vukelić3

1
Faculty of Mechanical Engineering, Slovak Technical University in Bratislava, Institute of Production Systems,
Environmental Technology and Quality Management, Nám. Slobody 17, 812 31 Bratislava, Slovakia

2
Tallinn University of Technology; Department of Polymer Materials, Chair of Woodworking, Teaduspargi 5,
12618 Tallinn, Estonia

3
University of Novi Sad, Faculty of Technical Sciences, Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia

Correspondence to: peter.krizan@stuba.sk

Abstract

In this paper, we will present the impact of the conical shape of a pressing chamber, an important structural parameter.

Besides the known impact of the technological parameters of pressing chambers, it is also very important to pay attention

to their structural parameters. In the introduction, we present a theoretical analysis of pressing chamber conicalness. An

experiment aimed at detecting this impact was performed at our institute, and it showed that increasing the conicalness of

a pressing chamber improves the quality of the final briquettes. The conicalness of the pressing chamber has a significant

effect on the final briquette quality and on the construction of briquetting machines. The experimental findings presented

here show the importance of this parameter in the briquetting process.

Keywords: biomass; briquetting; pressing chamber shape, pressing chamber conicalness.

1 Introduction

The briquetting process is a very interesting biomass
treatment process. It is a very complicated process,
because many parameters influence the process and
the quality of the final briquettes. Briquette qual-
ity is defined by EU standards, and is evaluated by
mechanical and chemical-thermic indicators.

Briquette quality is evaluated mainly by density.
Briquette final density is influenced by many param-
eters. On the basis of the experience that we have
acquired and the analyses that we have made, we
can divide the parameters into the following three
groups [3]:
• material parameters
• technological parameters
• structural parameters.

2 Influencing parameters in

the briquetting process

The material parameters emerge from the properties
of the pressed material, i.e. material moisture, frac-
tion size, chemical composition of the material, etc.

The technological parameters are pressing tem-
perature, compacting pressure, compacting speed,

holding time, etc. These parameters can be changed
in the course of pressing according to the capabilities
of the briquetting machine.

The structural parameters of the pressing cham-
ber are also very important. For successful pressing
of high-quality briquettes, all the parameters have to
be in synergy. For an engineer, it is very important
to know the behaviour of all parameters. We know
that pressing temperature, material moisture, com-
pacting pressure and fraction size are very important
for briquetting. We can obtain briquettes of suit-
able quality by adjusting the optimal values of these
parameters according to the material that is to be
pressed. We know that we can achieve better results
by changing some of the structural parameters. It is
therefore is very desirable to optimize the geometry
of the pressing chamber. The main structural param-
eters influencing the final briquette density are [2,6]:
• the diameter of the pressing chamber
• the length of the pressing chamber
• the conical shape of the pressing chamber
• the friction coefficient between the chamber and
the pressing tool

• the length of the cooling channel
• the effect of counter pressure in the pressing
chamber (created in various ways)

At our institute we have designed an experimen-
tal pressing stand (see Figure 1) on which we are

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Acta Polytechnica Vol. 52 No. 3/2012

Figure 1: Experimental pressing stand with heating equipment [2]

able to perform experiments to detect the impact of
each of the parameters listed above. In this paper,
we will present our findings on the conical shape of
the pressing chamber. Why is it so important to
know the impact of changes in the conicalness of
the pressing chamber? A pressing chamber with a
conically-shaped wall is very often used in briquet-
ting and pelleting machinery. However, we know of
no studies that clearly show the interaction between
final briquette density and changes in pressing cham-
ber conicalness.

3 Theoretical analysis of
pressing chamber conicalness

The geometry of the pressing chamber is very im-
portant in briquetting. However, few studies of the
briquetting process have shown the influence of indi-
vidual parameters of this process, taking into account
the pressing conditions in the pressing chamber dur-
ing briquetting. The results presented in this paper
are based on our experience, and on a comparison be-
tween our experimental results and analyses, on the
one hand, and the existing mathematical model of
pressing, on the other.

We have attempted to find some equations or
mathematical models that can be used for calcu-
lating the pressing conditions in the pressing cham-
ber. In [1], Horrighs offers a clear description of the
pressing conditions in a cylindrically-shaped pressing
chamber. This theory is represented by the following
mathematical model (1):

pG = pk · e−
4·λ·μ·H

Dk (MPa), (1)

where pG is the counter pressure in the chamber, pk
is the axial pressure of the hydraulic press, λ is the
ratio of the main strains σr/σm , and μ is the friction
coefficient, H is the length of a pressed briquette and
Dk is the diameter of the pressing chamber.

The situation regarding the pressing conditions in
the conical chamber is somewhat complicated. The
pressed material in the chamber is subjected to multi-
axial pressing. This increases the pressing quality:
it increases the briquette density and also the me-
chanical properties of the briquettes. However, the
tool wear is also increased. We have often replaced
a cylindrical pressing chamber by a conical pressing
chamber and obtained briquettes of higher density.
However, there is no mathematical model for a con-
ical pressing chamber. We are therefore attempting
to design a mathematical model for a conical pressing
chamber.

For an analysis of the influence of changes in con-
ical pressing chambers, we used the theory of for-
ward extrusion [4] as a basic metal volume moulding
technology. In this definition, the force and pressure
distribution is very close to the theory of biomass bri-
quetting. A simple scheme of the main parts of a con-
ical pressing chamber is shown in Figure 2 (left and
middle). This type of pressing chamber is often used
in the structure of briquetting machines. The press-
ing chamber consist of 3 basic parts — a cylindri-
cal part, a conical (reductive) part and a calibration
part. The material is filled into the cylindrical part
and then starts to be compacted by the pressing pis-
ton. The main pressing of the material takes place in
the conical part. The pressure and the conical cham-
ber cause a multi-axial pressing effect. Some holding
time while the pressed briquette is under pressure is
necessary in order to eliminate pressing expansion.
The calibration part gives the final shape to the bri-
quette and provides the holding time under pressure
and temperature.

Figure 2 (right) provides a better description of
the pressing conditions in a conical chamber, and
shows all the acting forces and pressures. The max-
imum attained axial pressure PK depends on the
length of the pressing chamber L, on the shape of
the pressing chamber, on the size of the vertex an-
gle of the conical chamber, and on the friction condi-
tions between the pressed material and the wall of the

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Acta Polytechnica Vol. 52 No. 3/2012

Figure 2: Main parts of a conical pressing chamber (left and middle), and the pressing conditions in a conical
pressing chamber (right) [2,4–6] (PK – axial pressure of press (MPa), PG – counter pressure in chamber (MPa),
PR – radial pressure (MPa), PM – axial pressure on the briquette (MPa), d0 – input diameter of the pressing
chamber (mm), d1 – output diameter of the pressing chamber (mm), d – diameter of the pressing chamber in
cross-section (mm), μ – friction coefficient (–), L – length of the conical part of the pressing chamber (mm))

Figure 3: Pressure distributions in space (left) and plane pressure distributions (right) on a conically-shaped
element

chamber. The surface friction drag is given by the ra-
dial pressure PR, by the cross factor of pressure PM ,
which affects the wall of the chamber, by the fric-
tion coefficient μ, and by the length of the pressing
chamber L. The diameter of the chamber decreases
linearly according to the length of the chamber L.

For a description of the pressing process, we have
to start with a description of the pressures acting on
an element of dx thickness cut in a conical shape (see
Figure 3 left). In the vertical direction, the axial com-
pacting pressure PM , acting in the opposite direction,
elicited PM + dPM . The friction also increases the
pressure perpendicular to the wall of chamber PR.

To make a balanced equation, we will need to
know the sizes of the surfaces on which the pressures
act. On the basis of the plane pressure distributions
(see Figure 3, right) we were able to write the follow-
ing equation (2):

PM · SV 2 + PM · SV − (PM + dPM ) · SV 2 − (2)
μ · (PR + PM · sin α) · dSP L · cos α = 0

In order to implement equation (2) it will be neces-
sary to define the border conditions and to define the

final shape of a mathematical model suitable for opti-
mization methods. This mathematical manipulation
has not yet been finalized, so we are not yet able to
present a mathematical model for a conical pressing
chamber.

4 A experiment to measure
the impact of conicalness

The next step was to measure the impact of conical-
ness. We attempted to measure the interaction be-
tween changes in pressing chamber conicalness and
final briquette density. For this experiment we used
an experimental pressing stand. It was necessary to
prepare some new components — new chambers with
different wall conics (see Figure 4).

For the experiment we prepared three new cham-
bers with 1◦, 2◦ and 3◦ degree conic walls. The
results were evaluated in terms of final briquette
density. We compared briquettes pressed in a con-
ical chamber with briquettes pressed in a cylindrical
chamber under the same conditions.

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Acta Polytechnica Vol. 52 No. 3/2012

Figure 4: Example of a new chamber with a conical wall (left), and the cross-section of pressing stand (right)
(1 – pressing chamber; 2 – flange; 3 – start-up chamber; 4 – counter plug; 5 – piston; 6 – pressed material;
7 – chamber with a conical wall; 8 – sleeve connector; 9 – mounting screw)

Figure 5: Negatives of internal chamber space representing different geometries

Table 1: Experiment results — density of pressed
briquettes, in kg/dm3

T1 = 20
◦C T2 = 85

◦C T3 = 120
◦C

α = 0◦ 0,852 1,152 1,207

α = 1◦ – 1,221 1,216

α = 2◦ – 1,236 1,224

α = 3◦ – – –

∗α represents the conicalness of the chamber

We chose the following conditions for this experi-
ment: pressed pine sawdust material, material mois-
ture 8 %, fraction size 1 mm, compacting pressure
159 MPa. The pressing was done without the use of

additional heating equipment, i.e. without a pressing
temperature effect. The measurements were carried
out in laboratory conditions, at temperature 20 ◦C.
For each setting we pressed 7 briquettes. These were
measured, and then we were able to calculate their
density. Table 1 presents the results, with only the
average briquette density value. The average values
were compared.

For pressing without additional pressing temper-
ature (under laboratory conditions) we did not ob-
tain any briquettes. The first column of Table 1
(20 ◦C) therefore only shows the briquette density ob-
tained by pressing with a cylindrically-shaped cham-
ber. The problem was the very high friction force
between the pressed material and the chamber wall.
The pressing forces in the pressing chamber are dis-
tributed as a laminar flow in a cylindrical pipe (see

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Acta Polytechnica Vol. 52 No. 3/2012

Figure 6: Pressing force distribution across the pressing chamber

Figure 6, left). The maximum axial pressing force is
applied in the axial axis of the pressing chamber, see
Figure 6, middle and right. The experimental press-
ing stand is inserted into the hydraulic press, which
can exert a maximum pressing force for 10 tons. In
our case, this corresponds to compacting pressure
318 MPa. We also used also the maximum pressure,
but without success.

From our experience, we know that the friction
force for briquetting can be reduced by lignin plas-
tification. The friction force can be reduced by in-
creasing the pressing temperature. For this purpose,
we used heating equipment affixed to the pressing
chamber. The heating equipment was controlled by
a regulator that works on the basis of a signal coming
in from the temperature sensor. Lignin is a material
component of all types of biomass. In the briquet-
ting process, it has the function of a gluing medium,
strongly joining the particles of the material into a
compact briquette.

We then decided to repeat the experiment at a
different pressing temperature. The results are pre-
sented in Table 1. With a higher pressing tempera-

ture we were able to obtain briquettes. We can add
that the briquette density increases as the wall angle
in the chamber increases, at each temperature level.
We also proved that increasing the pressing temper-
ature has a positive impact on the friction forces be-
tween the pressed material and the chamber wall.
However, we found that a chamber with a 3◦ degree
wall cannot be used in our conditions. The experi-
ment with this chamber was not successful, because
the mounting screws were destroyed during pressing.
The maximum strength of the hydraulic press was
also insufficient to extrude the pressed briquette from
the conical chamber. During the second squeezing,
the screws were destroyed.

Another interesting finding during the experiment
was that we were able, during pressing, to recognize
three compacting pressure values. The first value rep-
resents pressing, the second represents overcoming
the friction force, and the third represents extrud-
ing the pressed briquette from the chamber. The
following figures present a graphic record of these
three pressures values, as recorded by the hydraulic
press.

Figure 7: Graphic record of briquette pressing in conical chambers at 120 ◦C pressing temperature

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Acta Polytechnica Vol. 52 No. 3/2012

Figure 8: Comparison of recognized pressures for pressing in conical chambers (blue columns represent the
pressure needed to overcome the friction force, red columns represent pressing, and yellow columns represent
the pressure needed to extrude the pressed briquette from the chamber

These figures prove that during pressing with con-
ical chamber acting higher pressures as with cylindri-
cal chamber. This proves that higher briquette den-
sity can be obtained by pressing in a conical chamber.
We can state that it is possible to increase the pres-
sures acting in the chamber by changing the degree
of conicalness of the chamber. However, it can be
seen that higher friction forces act in a conical cham-
ber with a higher degree of conicalness. The friction
forces can be reduced by a higher pressing temper-
ature. As the pressing temperature increases, the
compacting pressure action decreases.

A future study should investigate the unit pro-
duction costs (energy costs and production costs).

5 Conclusion

The main aim of this paper has been to present the
results of our experiment to detect the impact of the
conical shape of pressing chambers. We also wanted
to show the importance of this type of parameter for
the briquetting process. In the near future, we aim to
design a mathematical model for a conically-shaped
pressing chamber. Of course, more experiments will
need to be made. With this model, we will be able
to calculate the optimal length of a conical pressing
chamber in accordance with the standards for the fi-
nal density of briquettes.

Acknowledgement

This paper is an outcome of the project “Develop-
ment of progressive biomass compacting technology
and the production of prototype and high-productive
tools” (ITMS Project code: 26240220017), on the
basis of the Operational Programme Research and

Development support funding by the European Re-
gional Development Fund.

References

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[2] Križan, P.: Process of wood waste pressing and
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[3] Šooš, L’., Križan, P.: Analysis of the interaction
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