Cellulose (2025) 32:1835–1850
1841
Fig. 2 Pressure profile at − 55 kPa gauge for a dwell time of 30 ms
110
100
Test boundary
90
80
70
60
50
8.47
8.52
8.57
8.62
8.67
Time (sec)
which conditions were still below atmospheric pres- sure because of the slow closing time of the sole- noid valves with respect to the spindle speed. There were more than 5 suction pulses or pressure drops , which means that pulp samples were exposed to a vacuum for far longer than desired, although the lat- ter pulses occurred at pressures closer to atmospheric conditions thereby still providing acceptable results. Shorter pulses or dwell times proved to be challeng- ing to imitate, however, a pressure drop of only 2.8% was recorded at the last pulse when compared to the first pulse, in the test boundary. Figure 3 below shows improved vacuum dewa- tering performance for a test completed with the suction box set to a total dwell time of 135 ms. The individual pulses in the figure were relatively better defined. Furthermore, there were no residual suctio n pulses unlike in Fig. 2. This indicates that the suction box performs better at longer dwell times. There was a slight departure from vacuum condition between consecutive pulses due to air leaking into the pressure vessels, which is demonstrated by a slight increase i n the absolute pressure after each suction pulse. This is especially true for the last vacuum pulse, where a pressure drop of 50% was recorded when compared
to the first suction pulse. This could have been caused by the solenoid valve bank closing before the comple- tion of the last pulse by the spindle. Pulse generation using the solenoid valve bank proved to be successful, as shown in Fig. 4. The valve bank could accurately simulate five vacuum pulses like those depicted in Fig. 3. Hand sheets were dried for 250 ms with each pulse lasting for 50 ms. The first pulse occurs during evacuation of the sample holder thereby resulting in pressures closer to atmospheric conditions. Consecutive pulses are administered at the appropriate vacuum pressure with slight pres- sure drops due to minor air leakage into the vessels. The pressure drop observed between the second and last pulse was 12%, which is much lower than that observed in Fig. 3. The dynamic configuration is capable of simulat- ing pulsation effects by suction boxes as shown by the consecutive pressure drops in the pressure profile s discussed above.
Pulp characterisation results
Table 4 provides the average values and sample vari- ance of characterisation parameters of the three pulps
Fig. 3 Pressure profile at − 55 kPa gauge for a dwell time of 135 ms
50 60 70 80 90 100
4.10
4.15
4.20
4.25
4.30
4.35
4.40
Time (sec)
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