22 HEIMBACHANDRITZ
Pulp Paper & Logistics
Figure 3: Overview of nip lengths and nip area
represented as line diagrams in Figure 3 with the nip lengths in blue, the area in orange. To clarify, the length is exactly the distance in MD where the rolls meet; the width is limited by the measurement area. This results in what is described as the nip area (in square cm), which shows where the rolls press together. Since the lengths (and therefore also the area) were all in order the attention had to be focused on force and pressure – which are also very significant parameters. Heimbach frequently points out to customers that conclusions based merely on nip length (and area) are not sufficient. Facts cannot be considered to be reliable unless force is taken into consideration.
Unequal force distribution Simply put, in the area of higher pressure there is no change in nip length any more, which means that misinterpretation is possible. For this reason, and in order to avoid this, TASK always measures area – and force. The important thing is to establish the actual force that has been exerted to calculate the pressure (pressure = force/area). Another line diagram (Figure 4) shows the pressure in blue (kN/sq cm), with the force (kN) shown in orange. The facts then became clear: With equal nip length the pressure (towards the centre) has increased. Thus an increased effective force was detected. This can also be seen in the original image (Figure 5), which shows in graphic form that the
Figure 4: Pressure and force in detail
force in the central area was significantly higher than at the edges (FS/DS).
Correct crowning These insights correspond to the CD moisture profiles of the felts that were shown at the beginning. Clarity was achieved, thanks to measurements. Heimbach TASK was able to prove that the press roll showed over-crowning (Figure 6) – a fact that previously had not been considered. The customer is now going
to pass on the results of Heimbach’s measurements to its roll manufacturer who will then calculate how the over-crowning can be rectified. The aim is of course a 100 per cent parallel roll gap that guarantees the best uniformity.
What this ultimately means is that when rolls are perfectly crowned the surfaces of press felts enable their full potential can be used, in other words: maximum dewatering is guaranteed. In this way the optimum from the nip is extracted – and that is ultimately the aim to leave the press as dry as possible.
Figure 6: Classic over-crowning
Figure 5: Original image of the unequal force exerted November/December 2017
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