Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2001-05-11
2002-12-17
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S460000, C034S508000, C034S621000, C034S117000, C034S123000
Reexamination Certificate
active
06493962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a drying section of a machine for producing a material web, in particular a paper or cardboard web. The invention further relates to an airborne web dryer for such a drying section.
2. Discussion of Background Information
In the multi-cylinder drying sections that have been common up to now, the paper web is guided over steam-heated cylinders or over steam-heated cylinders and wire suction rolls for the purpose of drying. Especially at the beginning of drying, when the moist paper web does not yet have sufficient strength, problems often occur with respect to web guidance, which can particularly be attributed to the fact that the web, which is still moist and poorly ventilated, sticks to the smooth contact surfaces, which are absolutely necessary for a sufficient heat transfer. In order to be able to remove the web from the cylinders, a sufficient web tension is necessary, which can be produced before or directly after the initial smooth contact surfaces by pulling, i.e., corresponding differential speeds. However, this often leads to web tears and an overstretching of the web edges. Therefore, as a rule, care should be taken that the drying occurs more slowly, i.e., e.g., a lower steam pressure is present at the beginning, which also means that the necessary drying section becomes longer overall. The problems mentioned above occur more intensely at higher web speeds.
SUMMARY OF THE INVENTION
The present invention provides a drying section of the type mentioned at the outset which, at high speeds, simultaneously guarantees as optimal a drying rate as possible at the beginning of the drying and a secure web guidance. Moreover, an airborne web dryer is provided that is appropriate for such a drying section.
In particular, the instant invention includes at least one web guidance device through which the material web is guided along with a wire, and in which the material web can be impinged with pressurized gas in such a way that, in the web travel direction, it is alternately lifted up from the wire and rests against it.
Because of this construction, the material web, which is still relatively moist, must no longer be prematurely removed from a smooth surface. As a result of avoiding smooth contact surfaces at the beginning of drying, the danger of web tears and of an overstretching of the web edges is virtually eliminated. In particular, high drying rates are now possible as well, whereby the total length of the drying section is shortened correspondingly. Since the web must not be subjected to high removal stresses caused by pulling until it has higher dry contents, the stressing of the web after the pressing section is minimized. Finally, correspondingly higher machine speeds are possible.
In a preferred practical embodiment of the drying section according to the invention, at least one pressurized gas-driven web guidance device embodied or formed as an airborne web dryer is provided. Such an airborne web dryer, e.g., can be provided directly after the pressing section and before the first drying cylinder. As a result of the combination of the airborne web dryer with a drying wire, even a web with a lower strength (for example, a web with higher moisture, e.g., before cylinder drying) can be dried.
While the material web can be deflected by a corresponding gas pressure distribution in the respective web guidance device, e.g., in a waveform, and can, e.g., be stabilized in a sine wave, the wire in the respective pressurized gasdriven web guidance devices tends to have a progression at least essentially in a straight line.
The web guidance device appropriately includes several nozzles arranged one behind the other for the purpose of impinging the material web with pressurized gas. Preferably, such nozzles are provided on both sides of the material web. Here, in particular, several banks of nozzles can be provided which extend in the direction of the web width.
In a preferred practical embodiment of the drying section according to the invention, at least one pressurized gas-driven web guidance device is arranged before the first drying cylinder.
In particular, air can be used as the pressurized gas.
It is also advantageous for at least one impingement dryer provided with a drying hood to be provided, by which the material web can be impinged with a hot air and/or a hot steam. Here, the material web is preferably guided over at least one support roll, in particular a larger support roll, with an impingement dryer assigned to it.
Again, such an impingement dryer is preferably arranged before the first drying cylinder.
Before the material web is pulled off a smooth heated surface for the first time, it can be predried by at least one airborne web dryer and, optionally, at least one impingement dryer. The gas and/or air drying appropriately occurs until a sufficient dry content is reached at which the necessary web tension can be applied in a problem-free manner.
In a preferred practical embodiment, the material web already has a dry content of at least approximately 58% to approximately 65% upon leaving the first drying cylinder or being pulled from a smooth heated surface for the first time. Here, as already mentioned, a part of the gas and/or air drying can also occur by an impingement dryer or an impingement hood.
Advantageously, at least one pressurized gas-driven web guidance device is provided subsequent to the last pressing nip of the pressing section.
In certain cases, it is appropriate for the material web, subsequent to the last pressing nip of the pressing section, first to be guided by at least one pressurized gas-driven web guidance device and subsequently by a support roll, in particular a larger support roll, with an impingement dryer assigned to it before it is guided to the first drying cylinder.
The drying speed in the region of a respective web guidance device embodied as an airborne web dryer and/or a respective impingement dryer can advantageously be regulated using the pressurized gas temperature, speed, and/or moisture.
The air permeability of a wire assigned to a respective web guidance device driven by pressurized gas is advantageously selected to be as high as possible. Thus, in a preferred embodiment, the air permeability of the respective wire is greater than approximately 300 cfm and preferably greater than approximately 500 cfm.
It is also advantageous for the material web to be guided in a closed draw from the last pressing nip of the press section to the first drying cylinder.
In certain cases, it is appropriate for the material web to be transferred directly from the pressing section to the wire assigned to a pressurized gas-driven web guidance device. However, the transfer of the material web from the pressing section to the wire assigned to a pressurized gas-driven web guidance device can also occur by means of at least one transfer belt or the like.
The material web can be removed from a felt (upper or lower felt) guided through the last pressing nip of the pressing section. However, removal directly from a pressing roll assigned to the last pressing nip of the pressing section is also conceivable.
In order to achieve the desired dry content, several airborne web dryers and/or several impingement dryers can also be provided.
A given pressurized gas-driven web guidance device can be arranged horizontally, vertically, or diagonally. In the latter case, the material web can be guided in the pressurized gas-driven web guidance device running diagonally, either generally diagonally upwards or generally diagonally downwards.
If several pressurized gas-driven web guidance devices are provided, at least a part of these web guidance devices can each have its own wire assigned to it.
In certain cases, it is also advantageous for at least one wire to be provided that is guided through several pressurized gas-driven web guidance devices.
In order to hold the material web on the wire, suction rolls, transfer foils and/or suction boxes can be provided before and/or after the pressur
Bowden Eddie
Kahl Peter
Mayer Roland
Oechsle Markus
Procter Robert
Greenblum & Bernstein P.L.C.
O'Malley Kathryn S.
Voith Paper Patent GmbH
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