Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Patent
1989-08-15
1991-04-30
Hastings, Karen M.
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
162259, 162263, 364471, 358107, D21F 706
Patent
active
050115731
DESCRIPTION:
BRIEF SUMMARY
When paper is made in a Fourdrinier paper machine having a plain wire, the slush pulp is fed on the wire on which it settles as a layer. Main part of the water content of the pulp is removed through the holes in the wire. At first the water is removed by the gravity and later on by suction produced under the wire. The water content of the pulp is typically 99% at the beginning and 80-85% at the end of the wire. The moisture is further removed in the drying section of the machine which produces the final paper. This final moisture depends on operation of the various parts of the machine and one essential quantity that affects the same is the moisture of pulp web at the end of wire.
Particularly the homogenity of the quality of the paper is affected by the change of moisture both as a function of time and accross the web. Meters based on various principles have been developed in order to determine, at the end of the paper web, the moisture and its average change as a function of time, and also the moisture profile accross the web. These devices are usually based on absorption of infrared radiation or on a corresponding phenomenon. Similar meters are also used for determination of the basis weight of the paper at the dry end. They are based e.g. on absorption of infrared or nuclear radiation.
The obtained, measured signals are further also used for feedback control of the measured quantities, the mean values of the moisture and basis weight being influenced e.g. by controlling the pressure of the headbox and the thermal effect of the drying section. Correspondingly, the transversal profile is influenced by controlling the headbox lip with the lip screws. Each one of these is controlled separately by hand; in some cases nowadays also automatically.
Correction of the moisture profile in the drying section is difficult and requires extra energy, if e.g. an excessively dried web must be remoistened at some locations. Therefore it is important to reach as homogeneous moisture as possible in the transversal direction, at the end of the wire. Further on, this value of the moisture must be correct so that the removal of water is correctly divided between the wire and the drying sections.
The moisture of the pulp web is manifested by the dry line present on the wire. As the pulp settles on the wire and water is removed therefrom, fibers accumulate at first in the lower part of the pulp layer, next to the wire. The upper part is kept dilute and resembles closely water for its properties. This dilute pulp layer disappears later, as water is removed therefrom through the pulp layer collected under it and through the wire. The borderline corresponding to the disappearance of the dilute layer can be seen at some locations because of the light reflected by the surface of the layer. In text- and handbooks this is stated as the gloss of the surface (see e.g. Suomen Paperi-insinoorien Yhdistyksen oppija kasikirja III:1 "Paperin valmistus" 1983 p. 569). The position of the dry line is to some degree also affected by the amount of wood fibers and their distribution on the wire. However, the main actor is the water and its distribution.
The dry line is usually not such a straight line and perpendicular to the longitudinal direction of the wire, as it should be in an ideal case. Its position depends on the transversal coordinate and furthermore it usually changes with the time, at least slowly. Individual spikes which express corresponding peaks of moisture are typical. Since the dryline can at some locations be observed with the naked eye, the machine tenders base their actions, especially the adjustment of the lip, on these observations. The advantage of such a control procedure is its speed. Since one does not wait for measured data from the dry end of the machine, one does not loose the dead time implied by the drying section which is at least several tens of seconds in magnitude. If one wants, on the other hand, to take a benefit of the speed reached by the stated procedure, at least one worker is continuously bound by this d
REFERENCES:
patent: 3926719 (1975-12-01), Spitz
patent: 4500968 (1985-02-01), Bialkowski
Hastings Karen M.
Niemi Antti Johannes
Niemi Ulli Riitta Annelli
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