16 HEIMBACH
Pulp Paper & Logistics
Fig. 10 Fabric damage through excessive jet impact
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water pressure – damage the fabrics. The properties of the water jet depend on the quality and precision of the nozzles and on constant water pressure. The latter should be at least 20 bar in the case of standard nozzle diameters (0.8 to 1.0 mm), but not exceed 35 bar. Good water quality will help with fault free functioning of the nozzles by preventing clogging or deposit build up and thus maintaining good jet quality. Therefore nozzles with standard jet diameters of
Fig. 11 Usual layout: Water jet between the return rolls
between 0.8 to 1.0 mm need to be equipped with an efficient water filtration screen (usually before the high pressure pump) of 50 µm (300 mesh). Another point to take into consideration is the impact of the water temperature. This should always be in the range of the production temperature, to avoid an uneven temperature profile with its resultant negative effect on cross direction profiles in the sheet. Poor quality water jets also
carry the risk of damaging the fabric: turbulent jets can cause the machine and cross machine
direction strands to vibrate quite intensely (Figure 6) causing the strands to wear at the cross-over points and leading to fibrillation. This can reduce the permeability and machine direction stability in localised areas of the fabric. Modern Ruby nozzles, with their high jet quality and long life performance, will usually eliminate most of the problems described above. A regular check-up of the water jets is recommended in order to identify and rectify any problems at an early stage. Defective, contaminated or partly- blocked nozzles will lead to a change in the water jet properties (Figure 7), causing uneven coverage across the fabric width
Fig. 12 Water mist in the space inside the fabric return loop
(Figure 8) and poor cross direction profiles in the sheet. The use of a hand stroboscope is
recommended to assess the water jet quality of the high-pressure showers. The stroboscope is adjusted to about 50 to 60 Hz and is held flat above the shower pipe and directed towards the jets exiting from the nozzles. This will allow observations that would be impossible under normal lighting conditions. At a fabric changeover a short trial run of the showers should be conducted. This should be done before the new fabric is installed, otherwise it will be impossible to see all the water jets. To conduct the trial run during a machine
Fig. 13 Variation: Water jet onto return roll
Fig. 15 Variation: Inner and outer water jet
Fig. 14 HD water jet onto fabric return roll May 2013
Fig. 16 Ideal angle of the water jets
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