Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
2001-05-30
2002-07-23
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S253000, C162S263000, C162SDIG004, C034S446000, C034S526000
Reexamination Certificate
active
06423184
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER
FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The invention concerns a method for regulation of the initial part of the dryer section in a paper machine, in which method moisture is removed from the paper web in the dryer section and in which method the paper web is dried in the dryer section, in which method the paper web to be dried is passed from the press section into the dryer section into the first group of drying cylinders, in which the paper web is dried against heated faces of drying cylinders.
Further, the invention concerns an equipment for regulation of the initial part of the dryer section in a paper machine, which equipment has been fitted in connection with the press section and with the dryer section in the paper machine.
As is known from the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw the groups of drying cylinders comprise two wires, which press the web, one from above and the other one from below, against heated cylinder faces. In twin-wire draw, between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular in the stages of drying in which the web is still relatively moist and, therefore, of low strength. This is why, in the course of the last 15 years, ever increasing use has been made of the single-wire draw, in which each group of drying cylinders comprises just one drying wire, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against heated cylinder faces, and on the reversing cylinders or rolls placed between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls inside the loop. In order to heat the faces of the drying cylinders, steam is passed into their interior, and the temperature of the surface of a heated drying cylinder is regulated so that the pressure of the steam present in the interior of the cylinder is regulated.
In view of the cylinder drying and in view of the runnability of the paper web and of the quality properties of the paper, the first drying cylinders placed in the beginning of the dryer section are highly important. If the temperatures of the first drying cylinders are too high, the paper web has a tendency to follow the face of the cylinder, which causes problems of runnability and thereby restricts the running speed. Likewise, the brightness of the paper web becomes lower, the porosity increases, the roughness increases, formation of dust increases, and the bulk may become lower. On the other hand, if the temperatures of the first drying cylinders are too low, the drying cylinders in the initial part of the dryer section are inefficient.
During threading of the web, even though the steam pressure has normally been switched off for this time or the steam pressure has been lowered to a level lower than a normal running situation in respect of the cylinders in the initial part of the dryer section, most commonly by lowering the pressure in the principal steam group, the lead-in strip has adhered to the drying cylinders, because the faces of the first drying cylinders are hot since the energy-consuming paper web is not present on them, in which case, thus, the web does not consume the thermal energy of the drying cylinders. Adhering of the lead-in strip has caused threading problems and thereby lowered the efficiency of the paper machine.
In the prior-art multi-cylinder dryers, in the initial part of the dryer section, it has been a further problem that it has not been possible to use a temperature sufficiently high in view of optimal drying, because, when the paper web is in direct contact with the hot faces of the drying cylinders, at temperatures higher than a certain level the web tends to adhere to the hot faces of the cylinders, which results in web breaks and standstills. It has also been noticed that excessively hot contact drying cylinders have detrimental effects on the quality properties of the paper. On the other hand, excessively low temperatures result in lowered drying efficiency.
At the first drying cylinders, problems have also arisen from the fact that, when the paper web starts becoming warm, it stretches, and when it stretches, it loses some of its tension, in which connection the web tends to follow the drying cylinder, which may cause problems of runnability. Traditionally, these problems have been solved so that a difference in speed has been arranged between the press section and the dryer section in order to maintain the web tension. Further, differences in speed have been employed between the first dryer groups in order to produce an adequate tension. However, excessive differences of draw have negative effects on the properties of the final product, because, when the paper is pulled, in particular its strength properties both in the machine direction and in the cross direction change.
One prior-art solution of the type described above is known from the publication EP 0,769,587. In said prior-art arrangement the draw applied to the web is increased constantly so that a number of short dryer groups arc employed in the initial part of the dryer section, which is unfavourable in view of the costs, because, among other things, extra drives, wires and wire guides are needed.
Important factors which affect the temperatures of the first cylinders in a dryer section include, among other things, the paper grade to be produced, the web moisture after the press section, and the temperature of the paper web. Usually, information on these factors in combination with the steam pressures in the first drying cylinders employed in cylinder drying has been obtained empirically only.
The current development in paper manufacture is about to lead in a situation in which there is no open draw between the press section and the dryer section, but a closed draw is concerned, in which case no high differences in speed can be employed, for which reason the problems described above in relation to the stretch of the web are emphasized.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a solution for the problems described above.
It is a further object of the invention to provide a drying method and a dryer section that makes use of said method in which adhering of the web to the cylinders in the initial end of the dryer section is prevented and in which, at the same time, improved quality of the paper and improved runnability of the paper machine are achieved.
In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in that, in the method, the surface temperatures of the first drying cylinders in the dryer section are measured, that, in the method, the dry solids content/moisture content of the paper web and the temperature of the paper web before said first drying cylinders are measured, and that, in the method, in view of producing the desired running situation, based on the measurement results obtained, the surface temperatures of said first drying cylinders and/or the dry solids content/moisture content and/or the temperature of the paper web before said first drying cylinders is/are regulated.
On the other hand, the dryer section in accordance with the invention is mainly characterized in that the equipment comprises measurement devices fitted in connection with the first drying cylinders in the dryer section for measurement of the surface temperatures of said first drying cylinders, a measurement device for measurement of the dry solids content/moisture content of the paper web and of the temperature of the paper web before said first drying cylinders, and a control device for regulation of the surface temper
Huhtelin Taisto
Kaihovirta Juha
Lipponen Juha
Vähätalo Harri
Chin Peter
Hug Eric
Lathrop & Clark LLP
Metso Paper Inc.
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