Paper making and fiber liberation – Apparatus – Digester
Patent
1995-08-14
1998-04-28
Czaja, Donald E.
Paper making and fiber liberation
Apparatus
Digester
D21C 714
Patent
active
057440050
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a straining device in association with the displacement of liquid through a pulp bed, preferably in connection with the withdrawal of cooking liquid in a continuous digester for producing chemical pulp.
STATE OF THE ART AND PROBLEMS
In connection with producing chemical pulp, environmental considerations have become increasingly important. In particular with regard to the bleaching of chemically produced pulp, the trend is for rapid movement towards the use of methods which are ever more environmentally friendly. Many of these so-called environmentally friendly methods require that the strength properties of the pulp are very good on entry into the bleaching process. Thus, ever greater demands are made that the digester house process should, as far as possible, spare the fibres contained within the pulp. The possibility of providing a mild digester house process is improved by the digester being fitted with efficient straining devices, thereby permitting efficient displacement of liquid through the pulp bed inside the digester.
In particular in connection with using ever higher temperatures in the lower part of the digester (the so-called high-heat zone), it has been found to be important to have efficient strainer arrangements, something which is emphasised by the fact that, in principle, the whole digester is kept at a mainly constant temperature level, i.e. at so-called "isothermal cooking", with the temperature between the different cooking zones essentially lying preferably within an interval of four (4) degrees C. Consequently, there is a tendency for the number of strainer elements and/or the total strainer area in the lower part of the digester to increase in prospective digesters. This increase has involved the focus of attention being directed towards the arrangement of individual strainer elements, not least with regard to optimisation of fitting and of maintenance costs.
Nowadays, bar-straining devices are usually employed which comprise a number of strainer elements, often arranged in a cross-hatch pattern, with each strainer element comprising a number of vertical bars arranged in parallel, between which bars an opening is formed from which liquid can be withdrawn. A bar-strainer element which is commonly found nowadays is constructed (see FIG. 1) by the strainer bars (A), arranged in parallel, being welded into recesses in flat bars (B) which are placed transversely. In addition, at the back of these flat bars which are placed transversely there are welded on supporting pins (C) which are intended to bear against the inner wall (D) of the digester shell in order thereby to provide support for the strainer element.
Consequently, a known strainer element of this type is not self-supporting. It has been found that such strainer elements are very susceptible to incorrect installation, since the principle is based on bearing against the digester shell. If the strainer element is incorrectly installed, so that a gap arises between the wall of the digester and a/some supporting pin(s), this can lead to the whole strainer element collapsing. The reason for this is that the strainer element is very rigid and cannot therefore flex if an extreme loading should occur, with the consequent possibility of a "domino effect" of broken supporting pins, leading finally to the whole strainer collapsing. A collapsed strainer element is not only a loss per se, but also involves impaired pulp quality, while at the same time the risk is present that, in association with such a collapse, loose supporting pins can destroy equipment which is located downstream in the process.
In addition, known strainer elements are relatively difficult to clean since, normally, relatively intricate steps have first to be undertaken in order even to reach the rear surface of the strainer element for washing it. A further disadvantage of known strainer elements is that they have a relatively pronounced tendency to collect accretions, so-called scaling, both on the strainer bars and d
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Drawing 1-2212-340 RI by Kamyr Inc., Glenn Falls, NY.
Deutschman et al., Machine Design-Theory and Practice, Fig. 3-44.
Design Guidelines for the Selection and Use of Stainless Steel, Table 10.
Carlsson Jan G.
Soderquist Soren
Czaja Donald E.
Kananen Ronald P.
Kvaerner Pulping Technologies Aktiebolag
Nguyen Dean T.
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