Equipment Description:Press Rolls
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The Press Rolls are the pairs of rolls which provide the nip in the Felt run. Their primary purpose is to continue the process of sheet de-watering. The first Phase was in the sheet forming stage, using gravity then vacuum. This second Phase uses a combination of absorbent felts and suction rolls containing vacuum boxes inside. There are many different designs of press rolls, the main objective is to remove as much water as possible using the minimum amount of energy, control the sheet compaction and maintain the sheet quality. Since the 1980’s many faster modern machines have extended nip presses. These units have a larger footprint in the nip, this means that less pressure is needed per square unit because the pressure is applied for a longer time. |
Press Fabrics The felts used to carry the sheet through the presses. Nip Pressure: This is the loading between two rolls. Edge guides: These units control the run of the felt. Shoe Press: This is an extended nip press. Draw: This is the tension between each section of the paper machine. Roll hardness: Press rolls can have different hardness covers. |
 Photo courtesy of IMDConsulting |
Press rolls need to have a strong construction. They are usually hollow, the shell structure phosphor bronze, the suction rolls contain a suction box which has to be set in the optimum position to make sure the vacuum is applied to the correct place. If it is too early, then it is before the nip, too late and it will pull the felt down after exiting the nip. The vacuum box is in the suction roll to take the water away from the sheet via the felt as the sheet and felt(s) are compacted in the press nip. Loading for press rolls is usually applied hydraulically and can be controlled independently at the front side compared to the back side. Because of the high roll loads, the rolls need to be cambered. This means that in profile the centre of the roll will be a higher diameter than the edges. When pressure is applied, the deformation results in an even nip pressure across the face of the rolls. The extended nip rolls have a different suction arrangement. Instead of a hard shell, there is a polymer belt which runs around a shell frame allowing the ‘shoe’ to provide the extended nip. |
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