Advancing material of indeterminate length – Passive guide
Reexamination Certificate
1996-05-21
2003-01-07
Strimbu, Gregory J. (Department: 3634)
Advancing material of indeterminate length
Passive guide
C226S097100, C226S097300
Reexamination Certificate
active
06502735
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the handling of webs and especially damp or wet paper webs and more particularly to a web handling device using air currents which suitably support the web while employing significantly reduced energy required to produce the air currents.
BACKGROUND OF THE INVENTION
With the Production or processing of a web of a sensitive material, as, for example, paper or the like, it has proved advantageous for avoiding damage to the web or its surface to keep the web suspended for the duration of the drying process of the web material or its surface coating. Usually arrangements in connection with which air introduced from a blower over jets from one or both sides of the web is blown out against the web are used so that the web is borne by the air cushion so created. The air consequently circulating around the web accelerates the drying process, mostly assisted in addition by heating facilities extending over the width of the web. Since the trend in the production or processing of paper webs is toward ever higher operating speeds, while the drying rate of a unit area of web nonetheless has an upper limit, for example, owing to the maximum ability of the web material to take heat stress, this leads to the unit area having to be in a suspended state for a minimum period of time, and the webs must be guided under suspension for ever greater distances. Since, due to the relatively low strength, especially of wet paper webs, the maximally attainable distance between two successive suspension guideways is restricted, this development goes hand in hand with an ever greater number of support points necessary for the guidance of the web. On the one hand, this elevates the amount of air and therewith the energy required for guiding the web. On the other, it increases the space required by a drying device. The increased space requirement is usually taken into consideration by deflecting the paper web several times through which the floor area of the device is reduced with an equal length of freely suspended web section.
Devices with the aid of which a travelling web can be maintained or deflected in a suspended state are familiar in many forms of construction. Usually the devices include so-called nozzle boxes from which air exits in the direction of the web. Often the nozzles have the form of a slit (as for example in connection with a device in accordance with DE-PS 31 30 450). The nozzle boxes are arranged in the direction of travel of the web at a distance from one another, whereby the interstices serve as discharge pathways for the air, which then, as for example in the device familiar from GB-PS 13 07 695, basically escapes over the areas of the two web edges. The free escaping of the air over the web edges leads on the one hand to a great consumption of compressed air and therewith to a high energy demand, and on the other hand to an unstable running and unsatisfactory guidance of the web.
The device in accordance with DE-OS 29 32 794 seeks to improve on this by including air discharge nozzles running at right angles to the direction of travel of the web at the beginning and at the end of its deflection area, whereby the air current directed through them against the web induces an improved web support on the boundaries of the device and consequently prevents the web from touching the device. On the other hand, slit-like air discharge nozzles are provided in the area of the two web edges which inhibit the air discharge currents on the web edge side and consequently generate pressure heads in the web edge areas which exert a positive influence on the guidance of the web. The web deflection device described in DE-OS 29 32 794 has, however, the disadvantage that the energy demand is further increased owing to the auxiliary jets though which additional compressed air must be blown in.
The device according to DE-PS 31 04 656 represents a further development of DE-OS 29 32 794 in connection with which good characteristics with respect to the guidance and the stable running of the web are supposed to be obtained through a special selection of nozzle cross sections and their geometrical arrangement. The energy consumption to be expected is nevertheless still quite high due to the multiplicity of nozzles to be charged with compressed air.
A device for drying and/or keeping a moving web is familiar from DE 33 31 856 A1 which presents a series of air beams running parallel to one another, in which at all times a pair of elongated Coanda nozzles extending lengthwise toward the air beams are placed. Compressed air flows out of an air chamber provided in the device through the Coanda nozzles in the form of currents which provide contact-free support of the web of material guided over the device. In order to prevent an excessive amount of air from exiting on the sides of the device or on the side edges of the web of material, the first air barriers, which impede lateral air flow, are on the one hand provided at the ends of the air ducts formed between two adjacent air beams. On the other hand, the second air barriers, which border the web laterally are provided mounted on the upper side of the device and extending vertically upward this. The disadvantage in therefrom connection is that, for an effective reduction of air demand and therewith of energy consumption, the distance between the second air barriers and the web of material must be as small as possible, which is, however, possible only with a very exact guidance of the material web, as the web otherwise can be damaged by possible contacts with the air barriers.
Furthermore, devices for contact-free guidance of webs of material are familiar from GB 21 46 303 and DE 27 52 572 C2, in connection with which the special geometrical arrangement and construction of the air discharge nozzles lead to a stabilization of the travel of the web with the former. With the latter, these should make their applicability in connection with especially sensitive webs of material possible. Measures which could serve to reduce the high air requirement to be expected with both devices cannot be extracted from these writings.
A further formula for a web guidance device is familiar from DE 36 26 016 A1. With this formula, an improved guidance action on the web is achieved by providing air ducts installed at right angles to the direction of travel of the web as nozzle boxes which include lateral duct partitions forming a sharp angle to one another, whereby a series of nozzles is incorporated into each duct partition in such a manner that each nozzle lies opposite a deflector for the outflowing air formed by the other duct partition. By using the Coanda effect, a suitable air current is created for the stabilization of the web. The disadvantage with this web guidance arrangement is that the web has a tendency to arch along an isobar into the air duct, since reduced pressure prevails above each air duct on the basis of Bernoulli's law for fluid media.
This tendency is counteracted by a further familiar device available from the Krieger company, Mönchengladbach by providing auxiliary nozzles charged with compressed air on the floor of each air duct which subject the air duct to just such an additional amount of air that the isobar along which the web curves basically extends evenly over the surface of the device.
A very stable web guidance is, to be sure, obtained with this device, but the energy requirement necessitated for conveying the air, which comes to approximately 25 kW/m of web width, is quite high.
A device is known from FR-A 2 334 599 in which inlet apertures are provided between two adjacent slit-like air discharge nozzles through which additional air for supporting the web of material is drawn in by means of ejector action. The web guidance behavior should be improved by this measure.
BRIEF DESCRIPTION OF THE INVENTION
Underlying the invention is the task of carrying on development of a device suitable for its type in such a way that the energy required is considerably reduced without having to accept sacrifices wi
Kreiger GmbH & Co.
Strimbu Gregory J.
Taylor & Aust P.C.
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