Mechanical action considerations on tannery drums

12 September 2004

Abstract This paper is part of the final reports for the Chempen project and analyses the factors influencing mechanical action, comparing different types of vessels in a context of possible chemical penetration variables. Partners of this European founded project are International Tanning (Ireland), BLC (UK), AIICA (Spain), University of Pisa (Italy), Tarnsjo Garveri AB (Sweden), Catalano Europea (Spain), Pere de Carme (Spain), Grad Chemicals (UK), and Genesis ecotec (Italy). Vessel's influence on mechanical action How can a vessel influence penetration? The first analysis shows that the main variables which may have an influence on mechanical action are: 1. Type (a vessel may be a traditional drum, a Y drum, a square drum, a paddle, a mixer, Y disposition washing machines etc) 2. Size (width and diameter) 3. Speed 4. Load 5. Internal disposition (pegs, shelves, combined, and their size) 6. Float recycling system (flux, turbulence, recycled volume) 7. Material (wood, stainless steel, polypropylene) 1. Type of vessel We will concentrate only on drums in this paper. We commonly find two types of drums: traditional drums (Figure 1) and Y type (Figure 2). Y type may be found as three-storey drums with a stationary float such as the washing machine type. Other vessel types include the square drum, mixer type such as Challenges, paddles and others. Each type of vessel will have a different mechanical action on leather, needing different float relations to run or move the skins. Because our partner tanneries just have traditional drums, we will concentrate on them, while making some references to Y type drums. 2. Vessel size The fundamental parameters we must consider in drums are their internal diameter and length. Both parameters have obvious influence in the capacity of the vessels, while diameter will define a peripheral speed at a certain angular speed. 3. Vessel speed The speed of a drum will define different types of mechanical action movement related to the maximum mechanical action speed (mmas). mmas is the speed at which the centrifugal effect of a given leather mass will be compensated for by its weight, falling from the highest part of the vessel. In Figure 4, we can observe the mechanical effect at different proportions of the mmas. The figure on the left shows a drum running at 25% of the mmas where the centrifugal effect is very low and mostly the friction is between the pieces of leather with gentle action of pegs and shelves. In the middle, we can observe a drum running at 50% of its mmas where we notice a certain fall effect but not from the highest portion of the vessel. Peg action at this speed will cause a mild bend and compression of the leather, while the low fall of the mass will generate a low hit compression. At 100% of the mmas (Figure 4 right), the mass falls from the highest portion of the vessel, generating the maximal compression. At this speed, the action of the pegs will also cause a strong compression by shock and bend. In case the speed is over 100% of the mmas, the centrifugal force will overpass the mass and so the mechanical action will be reduced with consequent energy waste. It is important to note that apart from the deformation strength with speed, the drum will achieve a certain hit frequency. In all these cases compression/release movement will generate a certain peristaltic pump effect that is one of the clue factors of penetration by mechanical action. 4. Vessel load The leather mass will be responsible for the falling hit which will generate the said compression/ release mechanism responsible for the peristaltic effect. Logically, the higher the leather mass, the higher the compression and peristaltic effect. This means that the compression pressure will be a combined speed/mass effect. 5. Vessel's internal disposition A typical internal drum disposition may involve pegs, shelves (or a mix of pegs and shelves) and also big shelves as we can observe in Figure 6. Pegs and shelves help the leather mass to overcame inertia. Pegs are more indicated to avoid knots but their action is more aggressive. There are different types of pegs which may vary in size and shape, being from rounded cylindrical to flat shelf-like. Shelves have reduced tearing and deformation effects. Big shelves generate a high peristaltic and compression effect at low speeds, in which case mmas cannot be applied. As we saw in point 3, a combination of speed and internal disposition will generate a characteristic mechanical effect due to the contact hit, generating a certain compression/release effect. 6. Float recycling system, recycled volume and flux The main effect involving a recycling system in penetration is the recycling volume which will define a float-in-vessel volume which will, of course, be different to the float-in-process volume. A low float-in-vessel volume will generate a higher chemical penetration due to a higher hit pressure and the consequent peristaltic and compression effect on the leather. A high float-in-vessel volume will produce an attenuation of the hit pressure, due to the Archimedes principle with a reduction of the peristaltic and compression effect, which will reduce penetration. A low float-in-process volume means a high concentration of the chemicals in respect to the water, propitiating penetration by an osmotic effect. 7. Vessel construction material The most currently used materials for drums are wood, polypropylene and stainless steel. In our experience, we did not find any formal differences in penetration rates between these three types of material, just technical differences such as heat conductivity, abrasion and chemical resistance, and chemical absorption by the material itself. Regarding abrasion and chemicals absorption, stainless steel and polypropylene show more advantages than wood. Both are long lasting materials with a smooth surface and are easy to clean. With reference to thermal isolation, polypropylene is slightly more efficient than wood and much more efficient than stainless steel. 8. Mechanical action calculation on traditional drums The movement of the drum with the help of pegs and shelves to overcame inertia will generate an increasingly high centrifugal effect on the leather mass causing it to fall from its higher part (mmas). Proportions of the mmas can be suggested for each step of the process, depending on the desired effect we want to obtain. For example, during chrome leather dyeing, we need a high hit of the leather mass so as to achieve the maximum compression and peristaltic effect of the leather to complete penetration in the shortest time. The opposite effect will occur during the beamhouse process and particularly with lime addition, where it is important to achieve a certain friction between the skins while, at the same time, limiting the abrasion by walls of the vessel. In Figure 9 we can observe a description of movements and speeds. One formula may be applied to calculate the percentage of the maximum mechanical action speed to define a mechanical parameter in a certain process or experience. This formula (Figure 10) may be applied only in pegs and short shelf configurations, because the big shelf drums work under a different logic: shelves in this case are not only a media to overcome inertia as was said before but a media for picking up the leather mass. In the formula, sD is the maximum mechanical action speed, wL is the weight of the load, L is the drum's internal width, and Δ the drum's internal diameter. Applying this formula, we created the following graphic (Figure 11) which is a 3D representation showing the evolution of the percentages of the mmas, with respect to the volume occupied by the load and that of the mechanical efficiency. In this way we may calculate the optimal speeds for each vessel and the proportions of the mmas advisable for each process. In Figure 12, we may see the typical speed ranges for each of the traditional processes. 9. Mechanical properties of Y drums Y drums will have different movement logic as we can see in Figure 13. In this case, the mechanical action involves the following steps: a: rotation of the drum with lift action; b: first fall with very low/no float; c: rotation movement down; d: second fall with float involving rotation of the drum with lift action; e: rotation movement with friction. In the case of Y drums, the important measurement parameters to consider are, as we can see in Figure 14, the distance d, and as in a traditional drum the width and the loaded mass. Acknowledgements: Italprogetti Engineering, Prof Jose Maria Adzet

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