Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
2000-08-17
2002-04-02
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S206000, C162S207000, C162S253000, C162S263000, C162S359100, C162S290000, C034S452000, C034S456000, C034S463000, C034S117000, C034S118000
Reexamination Certificate
active
06365004
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The object of the present invention is a method and apparatus for drying a paper web or the like, as defined in the preambles of the independent claims presented below.
This means that the object of the present invention typically concerns a method and apparatus for drying a paper web or other similar web in the dryer section of a paper machine or the like, in which dryer section the web is dried against the heated cylinder surfaces of the drying cylinders and by means of air impingement drying with at least one air impingement unit.
Another object of the invention concerns the optimisation of paper web drying in the dryer section of a paper machine or the like, the dryer section comprising a drying section consisting of one or more drying cylinder groups and at least one air impingement unit. The invention is, however, intended to be applicable also to other types of dryer sections, if necessary.
It is previously known to use twin-wire web transfer and/or single-wire web transfer in the multicylinder dryers of a paper machine. During the past 15 years, single-wire web transfer has been used to an increasing extent, in which transfer there is only one dryer wire in each drying cylinder group, supported on which wire the web passes through the entire group so that the dryer wire presses the web with the help of the drying cylinders against the heated cylinder surfaces, the web remaining on the side of the outer curve on the turning cylinders or rolls. Thus, in single-wire web transfer, the drying cylinders are outside the wire loop and the turning cylinders or rolls are inside it. Previously known is the type of dryer sections which consist only of so-called normal single-wire web transfer groups in which the drying cylinders are in the top row and the turning cylinders or rolls in the bottom row.
In order to heat up the drying cylinders, steam is introduced inside them and the temperature of the drying cylinders is controlled by regulating the steam pressure and/or the rate of flow of the steam. It is also possible, although rarely implemented, to control the final moisture content reached in the dryer section by regulating the speed of the machine. In such a case the pressure of the steam is kept constant—in machines with limited drying capacity usually at maximum pressure—which means that the final moisture content can be increased or reduced by slowing down or speeding up the passage of the web through the dryer section.
The most commonly used steam pressure regulating systems are so-called cascade regulation and thermo-compressor regulation, which are described, for example, in the following publication: TAPPI NOTES, Practical Aspects of Pressing and Drying, Short Course, 1990.
One problem with these conventional-type dryer sections, where drying is carried out entirely by means of drying cylinders, by using either single-wire web transfer or twin-wire web transfer, has related to the regulation of drying efficiency. In order to achieve the desired final moisture content of the web, the drying efficiency of the drying cylinders is generally regulated by regulating the pressure of the steam supplied to the cylinders. This type of regulation is relatively slow and does not, therefore, react at optimum speed, for example, to sudden changes of moisture content in the web originating in the press section or wire section. Particularly in connection with a change of paper grade, start-up and web breaks, final regulation of drying to the optimum level by regulating the pressure of the steam is slow, due to the considerable mass of the drying cylinders. Methods used in regulation have included dry matter measurement based on IR measurement after the last drying cylinder, and feedback for controlling steam pressures, usually to the main steam cylinder group of the dryer section. This type of regulation has, as such, functioned without problems, where it has been a question of standard production at constant speed and no web breaks have occurred. Regulation problems occur, however, in connection with a change of paper grade, web breaks, or the paper machine start-up phase.
In connection with a change of paper grade, problems are caused in conventional dryer sections by the fact that each drying cylinder has a high thermal capacity, due to its great mass, which means that the temperature of the drying cylinders changes slowly. Because of this, the temperature changes in the drying cylinders have not been rapid enough where a change of grade is concerned. In some cases, the changes required relating to the regulation of drying efficiency have been made by changing the loads in the press section, but this will also change the qualities of the paper, which is obviously usually not desirable. Furthermore, when the loads in the press section are changed, the cross-direction profile of the paper web will also change, which means, for example, that there will often be defects in the moisture profile. Due to the foregoing reasons, a web of incorrect final moisture content or quality standard may be reeled on the reel-up in connection with a change of paper grade. According to solutions known from the prior art, it takes some 15-20 minutes after the change of grade before a balanced state is again reached. With paper machine speeds of, for example, 1500-1800 metres per minute, a large amount of paper, that is, paper of incorrect quality, is produced during this time. On a wide machine the amount may be 10-20 tonnes.
During a web break, on the other hand, problems arise, for example, due to the fact that the drying cylinders overheat when no paper that would transfer thermal energy away from the cylinders is supplied to the dryer section. Excessively hot cylinders cause problems in tail threading after a web break, as the tail threading cord adheres to the hot cylinders. In addition, excessively hot cylinders overdry the tail threading cord, which makes the cord brittle and causes it to lose its strength properties, which may cause problems in tail threading. Furthermore, at the stage when the tail threading cord broadens after a web break, it takes a long time before the drying cylinders return to an equilibrium temperature due to the drying cylinders' slow ability to change their temperature, that is, their high thermal capacity. In case of a web break, there was previously no other way of bringing the situation under control than by reducing steam pressures for the duration of the web break. The result of this has in turn been that the final moisture content after a web break has not conformed to the desired values. Furthermore, it has taken a long time before the situation could be returned to correspond to normal running conditions.
At the paper machine start-up stage, the steam pressures suitable for a particular paper grade are usually first taken from a memory in which adjustment values that have proved good in earlier, corresponding running situations have been collected, for example, in tables, and the steam pressures in the drying cylinders are controlled by means of the above data. The steam pressures selected and their time staggering, or change, may also be based on computational models and values obtained thereby. Conventionally, when the web is first taken to the machine, the steam pressures used are slightly below the optimal pressures, and after this the steam pressures are increased to the desired level. The high thermal capacity due to the great mass of the drying cylinders makes start-up slow, so that it takes a long time before the desired situation is reached. This is problematic because during the start-up stage a large amount of paper of an incorrect type is produced.
The problems described above are thus mainly due to the fact that the thermal capacity of the drying cylinders is high and there is a long time-lag before they achieve the temperature changes required.
The dryer section is the part of the paper machine that uses the most energy. It can be said that as much as over two thirds of the energy consumption of a paper
Hamstr{haeck over (o)}m Kristian
Happonen Harri
Kuhasalo Antti
Lipponen Juha
Nurmi Järkko
Halpern Mark
Metso Paper Inc.
Nixon & Vanderhye PC
Silverman Stanley S.
LandOfFree
Method and apparatus for drying a paper web does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for drying a paper web, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for drying a paper web will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2915876