ap-3-12.dvi Acta Polytechnica Vol. 52 No. 3/2012 Idealized Compression Ratio for a Screw Briquetting Press Peter Biath1, Juraj Ondruška1 1 Institute of Manufacturing Systems, Environmental Technology and Quality Management, STU Bratislava, Námestie slobody 7, 812 31 Bratislava Correspondence to: peter.biath@stuba.sk Abstract This paper deals with issues in determining the ideal compression ratio for a screw briquetting press. First, the principles of operation and a basic description of the main parts of a screw briquetting press are introduced. The next section describes the pressing space by means of 3D software. The pressing space was created using a Boolean subtract function. The final section of the paper measures the partial volumes of the pressing chamber in CATIA V5 by function of measuring. The measured values are substituted into the formula for the compression ratio, and the resulting evaluations are presented in the diagram in the conclusion of this paper. Keywords: screw briquetting press; compression ratio; calculation of compression ratio. 1 Introduction Briquetting is the most widely-used waste com- paction technology. Briquettes made by screw bri- quetting presses are of the highest quality. In this paper, we will deal with the screw briquetting presses produced within the project “Developing progressive technology of biomass compaction, production of pro- totypes and highly productive tools” co-financed by the European Structural Funds. The research is com- plex, ranging from a study of elementary biomass particles to the production of a new design and struc- ture for briquetting machines and briquetting ma- chine tools. The purpose of the study is to design and verify a prototype for a new progressive briquet- ting machine design, and to develop highly produc- tive tooling. The paper attempts to clarify the procedure for calculating the idealized compacting ratio during compaction in our screw briquetting press. The re- sults will clarify how the material changes in volume during compaction (although only idealized), and will be of great benefit to further research and develop- ment of screw presses. 2 Description of a screw briquetting press In developing the design of a new press, we apply re- search results for the parameters that influence the biomass compaction process. When managing the technology and constructing the pressing parameters, it is important to achieve high-quality production in a variety of factors. The following experiments and the optimization to be implemented on the ma- chine will reflect real compaction technology in prac- tice. The research area focuses on the principle of screw presses due to the achievement of the high- est with a view to achieving the highest quality pro- duction using this principle. However, one big dis- advantage of this type of machine is its short bear- ing and worm tool life, due to the high axial pres- sures. Figure 1: Screw briquetting press 13 Acta Polytechnica Vol. 52 No. 3/2012 Figure 2: Basic parts of the new screw press (1 – drive, 2 – storage node for screw and spindle, 3 – pressing chamber, 4 – feeding system) Figure 3: Pressing chamber (1, 2 – nozzles; 3 – pressing screw, 4 – feeding screw, 5 – collet, A, B, C, D, E – location of compaction) The basic structure of a briquetting press is shown in Figure 2. The design of the machine is double- chambered to eliminate sharp rises in axial force. The primary parts of the machine are its main drive, the storage node for screw and spindle, the pressing chambers, the cooling channels and the two feeding systems. The core of the machine is held on frames which are similar in dimensions to euro pallets for better manipulation during transportation. It is not necessary to anchor the machine during operation, because the frame of the machine does not transmit any workloads. The machine is developed to be modular, making it easy to change it from a double-chamber version to a single-chamber version simply by removing one part of the machine — from the drive side, without any other modifications. The single-chamber struc- ture is very helpful, especially in experiments and measurements, because we can measure a wide range of operating parameters, including the entire work- load, during operation. 3 Pressing space The pressing space in compaction machines is cre- ated from moving and stationary parts. In our case it comprises the pressing screws (positions 3, 4; Fi- gure 3) and the nozzles (positions 1, 2; Figure 3). The pressing space in our equipment is quite com- plicated, because there are two screws (for pressing and for feeding) in each side. The first screw is for feeding (4). It transports material into the throat of the pressing chamber and overwhelms the second “pressing” screw (3). There is compaction of the ma- terial even in the feeding screw, so we will include it in the pressing space in order to determine the total pressing space of the machine. 14 Acta Polytechnica Vol. 52 No. 3/2012 The compaction process starts at position A, where the feeding screw moves the material from the hopper to its grooves. At this moment, the bulk ma- terial has the same density as the material in the hopper. However, when the material begins to enter the conical part of the feeding screw (B) its volume decreases and its density increases. The compacted material at the end of the feeding screw moves to the overloaded space of the pressing screw (C) with- out changing density, but only in ideal conditions. In this space we will consider the same density, because the first part of the pressing screw is feeding (the screw and the nozzle are cylindrical in shape with- out any change in shape or into the conical parts). In the subsequent space, the material is again com- pacted (D) by the taper of the nozzles and the change in the pressing screw geometry. The compacted ma- terial then moves to the last part of the pressing chamber (E). Here the pressing pressure is generated only by back-pressure, which is caused by the change in the collet (5) section. However, in this paper we will pay attention to creating the compression ratio by screw compaction only. Thus the pressing space that we are studying ends up at the last thread of the pressing screw. However, we should not forget that the output of this experiment will be idealized, meaning that, if we neglect the resistance and fric- tion within the pressed material and the friction of the compacted material in the pressing chamber, the following will apply: if the screw turns one revolu- tion then the material will move by the value of one thread pitch. We used CATIA V5 3D software to create the pressing space. We identified the components which create the pressing space (the hopper, the feed screw (4), the feed screw nozzle (1), the pressing screw (3), and the pressing screw nozzles (1, 2, 3,)). In assembling the model, we created components (tubes) that create the theoretical fill of the pressing space in both screws. The tube dimensions must be designed in such a way that the smallest diameter of the tube must be smaller than the smallest diameter of the screws (screw grooves), and the largest diame- ter of the tube must be larger than the largest diam- eter of the individual nozzles (longitudinal grooves in the nozzles). The dimensions of the tubes were designed with respect to the Boolean subtract op- eration, where we need to intersect the bodies and then subtract the intersections from one another. Fi- gure 4 shows the pressing space that is created, with and without screws. After this, the pressing space will be divided into smaller parts, and then we will measure its volumes. Figure 4: 3D model of the pressing space 4 Measuring volume of the parts in the pressing space To find the compression ratio, it is necessary to di- vide the model of the pressing space transversely into smaller parts. The resulting smaller volume parts will be measured. Transversely dividing the model results in one quarter of the screw pitch. The feed screw is double-grooved with a pitch of 120 mm, and the pressing screw is single-grooved with a pitch of 31.8 mm. The step in dividing the feed screw is 30 mm, and for the pressing screw the step is 7.95 mm. We do not list the individual measured values in the paper but only mention them in the diagram in the next section. Figure 5: 3D model of the pressing space — volume parts 15 Acta Polytechnica Vol. 52 No. 3/2012 Figure 6: Diagram of the idealized compression ratio of a screw press 5 Measurement evaluation The compression ratio is the ratio between the bulk material volume before compaction and the volume of the compacted material after or during compact- ing. The value of the compression ratio indicates how much the input volume decreases during the com- pacting process. The feeder compression ratio is the ratio of the given hopper material volume to the par- ticular measured volumes in other feeding screw sec- tions. zpP i = VDP max VDP i , (1) where (zpP i) is the compression ratio in any given screw section of the feeder, (VDP max) is the maxi- mum input volume of pressed material in the feeder, (VDP i) is the volume of the measured part in any given screw section of the feeder. It is necessary to determine the compression ra- tio separately in mechanically separate phases of the process. The resultant compression ratio in the press- ing space of the pressing chamber is the product of the compression ratio in the feeder and the partial compression ratios of the pressing screw. zpLi = VDL max VDLi · zpP max, (2) where (zpLi) is the compression ratio in a given screw section of the pressing space, (VDL max) is the maxi- mum input volume of pressed material in the press- ing space, (VDLi) is the volume of the measured part in a given screw section of the pressing space, and (zpP max) is the maximum compression ratio in the feeder. By substituting the measured values into the for- mula we find the character of the compression ratio, which is shown in the diagram below. Figure 6 is divided into two parts: the first presents the com- pression ratio in the feeding space, and the second describes the compression ratio in the pressing space. The undulation of the character of the compression ratio is influenced by the change in the geometry of the screws and nozzles. 6 Conclusion The values and the character of compression ratio in this paper are idealized. In the real process we can- not consider it as completely relevant, for example, to determine real compression by a direct calcula- tion, because multi-axial compaction processes are very complex and difficult to describe theoretically. The behavior of the material during compaction is influenced by technological and design parameters. The results of this experiment will be compared with real experiments and measurements. The results that are obtained will be used to create a basic idea about the processes inside the chambers during compaction on this screw press. Further research should deter- mine the real compression ratio in this machine. Acknowledgement This paper reports on work done in the project “Developing of progressive technology of biomass compaction and production of prototypes and highly productive tools” (ITMS code of the project: 26240220017), supported by the Research and De- velopment Operating Programme funded by the Eu- ropean Regional Development Fund. References [1] Biath, P., Ondruška, J., Šooš, L’.: Theoretical cal- culation of PLG 2010 press compression ratio. Briquetting and pelleting 2012. Bratislava : STU, 2012, s. 117–123. ISBN 978-80-227-3641-1. [2] Matúš, M., Križan, P.: Development of a pro- gressive machine design for biomass briquetting. Briquetting and pelleting 2012. Bratislava : STU, 2012, s. 197–202. ISBN 978-80-227-3641-1. 16