GB2363805A

GB2363805A - Vacuum drying section

Info

Publication number: GB2363805A
Application number: GB0015348A
Authority: GB
Inventors: Stephen Yates Halsall
Original assignee: SANDUSKY WALMSLEY Ltd
Current assignee: SANDUSKY WALMSLEY Ltd
Priority date: 2000-06-23
Filing date: 2000-06-23
Publication date: 2002-01-09
Anticipated expiration: 2020-06-23
Also published as: GB2363805B; GB0015348D0

Abstract

In a single-felt drying section of a paper making machine, a web 10 being dried travels from one drying cylinder 14 to the next drying cylinder 18 by passing around a vacuum roll 16. The web 10 lies outwardly of a supportive drying felt 12 as it passes around the vacuum roll 16. The vacuum roll 16 has a perforate shell such that a suction pressure is exerted over substantially the entire surface of its shell. In order to maintain a high level of partial vacuum inside the shell thereby to hold the drying felt 12 and web 10 more effectively, a restrictor element 20 is positioned externally in order to restrict air flow into the roll 16 in the region of its shell not covered by the felt 12 and web 10. This restrictor element may comprise an imperforate plate 20 or a perforate plate, it may be positioned closely adjacent the roll 16 surface or else be positioned to restrict air flow into the pocket defined between respective drying cylinders (Fig. 4), and it may be associated with one or more web stabilisation boxes or seals.

Description

2363805 -P 650622 GB - EXTERNAL AIR FLOW RESTRICTOR FOR VACUUM ROLL OF

PAPER- MAKING MACHINE This invention concerns a device for use in the drying section of a paper making machine of the type comprising plural drying cylinders and at least one vacuum roll which has a perforate shell, in which respect a web travels from one drying cylinder to the next drying cylinder by passing around the vacuum roll with the web disposed outwardly of a supportive drying felt in its run from the one cylinder, while passing around the vacuum roll, and in its run to the next drying cylinder This is generally referred to as a single-felt drying section as upper and lower tiers of drying cylinders, the latter being in the form of vacuum rolls, are wrapped by the same drying felt.

Thus the paper web is sandwiched between the drying felt and each respective drying cylinder as it passes around the upper tier of drying cylinders, but is on the outside of the drying felt as it passes around the lower tier of drying cylinders When the web is on the outside it is not subject to any restraining force save its own inherent tension and this often leads to instability and even breakages of the paper web, and thus to a lessening of the paper machine's output rate It is for this reason that the lower tier of drying cylinders is conventionally in the form of vacuum rolls in a drying section of this type.

A vacuum roll is one in which a partial vacuum is applied to the inside of the roll, and thence to the drying felt and the paper web by way of a series of holes in the roll shell The drying cylinders themselves can be drilled to facilitate this operation, or specially-built rolls can be provided in both cases the paper web is supported by means of this vacuum, and the instability is reduced.

If a partial vacuum is applied to the inside of a roll with a drilled shell such that a suction pressure is exerted over substantially the entire surface of the shell, there is a tendency for the effective vacuum holding the paper web to be reduced due to air flow into the roll from that portion of the roll surface not covered by the drying felt and the paper web In order to maintain a reasonable partial vacuum in the roll, the drilled holes in the shell must be small compared to the roll surface This in turn limits the effectiveness of the vacuum in holding the paper web, and thus its effectiveness at reducing the web instability.

The object of the present invention is to provide means to restrict air flow into the vacuum roll from that portion of the roll surface not covered by the drying felt and the paper web so that a higher level of partial vacuum can be maintained inside the vacuum roll The roll would then be more effective at reducing the web instability Furthermore, a greater percentage of the vacuum roll shell could then be drilled, e g the holes could be larger or there could be more of them, and the partial vacuum still maintained, giving a greater vacuum force on the paper web and hence an improved web stability.

This object is achieved in a paper making machine drying section of the type referred to above in that a restrictor element is positioned externally of the vacuum roll in such a manner as to restrict flow of air into the vacuum roll in that region of its shell which is not covered by the drying felt and the web.

Embodiments of such a device can be in several forms The simplest form is that of a single imperforate plate positioned close to that portion of the vacuum roll surface not covered by the drying felt and the paper web In a more complex embodiment, the restrictor element, also including a plate, may be constructed or positioned so as restrict flow of air into a pocket defined between the respective adjacent drying cylinders, the web runs from the one cylinder to the vacuum roll and from the vacuum roll to the next cylinder and the region of the perforate shell of the roll which is not covered by the drying felt.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic side elevation of a small portion of a single felt drying section of a paper making machine fitted with a first embodiment of a restrictor element in accordance with the invention; Figure 2 is a similar sketch showing a second embodiment of restrictor element fitted in the machine; Figure 3 is a similar sketch showing a third embodiment of restrictor element fitted in the machine; Figure 4 is a similar sketch showing a fourth embodiment of restrictor element fitted in the machine; and Figure 5 is a diagrammatic perspective view of the restrictor element shown in figure 4 mounted above a vacuum roll.

Referring firstly to Figurel, a paper web 10 is shown carried on a drying felt 12 from a first drying cylinder 14, around a vacuum roll 16, to a second drying cylinder 18 The drying cylinders 14,18 are mounted on the same plane in an upper tier of a drying section of a paper making machine, while the vacuum roll 16 is mounted therebelow in a lower tier In this respect, there will in practise be a plurality of cylinders in the upper tier and a plurality of vacuum rolls in the lower tier, and the web 10 and the drying felt 12 will be guided around them alternately in a similar fashion to that shown in the illustrated portion Thus the paper web 10 is sandwiched between the felt 12 and each of the cylinders 14,18, but is on the outside of the felt 12 as they pass to, around and from the vacuum roll 16.

The vacuum roll 16 has a perforated shell and the perforations are distributed throughout Therefore when the interior of the roll 16 is connected to a vacuum source in operation of the machine suction pressure is exerted over the whole of its surface Air is drawn into the interior of the roll 16 primarily where it is not covered by the felt 12 However, in accordance with a first embodiment of the invention, a restrictor element in the form of an imperforate plate 20 is mounted above this region of the roll 16 The plate 20 has an arcuate lower surface lying closely adjacent that portion of the roll 16 which is not covered by the felt 12.

Thus the plate 20 forms a partial seal around the open portion of the vacuum roll 16, the only areas for air ingress being small gaps 22 around the periphery of the plate 20 Thus the amount of air that can flow into the vacuum roll 16 through the portion of the shell not covered by the drying felt 12 and the paper web 10 is much reduced, and hence the ability of the roll 16 to maintain a partial vacuum is much improved.

In a modified embodiment of the invention the imperforate plate 20 may have some form of sealing device (not shown) attached to each end, creating an even more effective seal between the imperforate plate 20 and the vacuum roll 16 In this case, the airflow into the roll 16 through the open area not covered by the drying felt 12 and the paper web 10 would be reduced to virtually zero The sealing device could be either a contacting or non-contacting type.

Referring now to Figure 2, in a further embodiment of the invention a restrictor element, also in the form of a plate 30 similar to that in the first embodiment above, is constructed as an attachment to or part of a web-stabilising box 32 of known type, which is mounted on the open draw of drying felt 12 and paper web between the first drying cylinder 14 and the vacuum roll 16 In this respect the same reference numerals are used for parts which are identical to those shown and described in Figure 1.

A web-stabilising box is a known device that provides a partial vacuum on the inside of the drying felt loop over said open draw so as to hold the paper web onto the drying felt, thus improving web stability The web-stabilising box 32 can create the partial vacuum either by directly applying a vacuum by means of an evacuation fan, or by inducing a vacuum by blowing air against the direction of motion of the drying felt 12 In such a case, the imperforate plate 30 that creates the airflow restriction could be constructed as an integral part of the web- stabilising box, or as a separate entity.

As illustrated in Figure 3, in a modified embodiment of the invention an air restrictor plate 34 is being built in conjunction with two webstabilising boxes 36,38 one on the drying felt down-run and one on the drying felt up-run Again, the imperforate plate 34 that creates the airflow restriction can be an integral part of the construction of either or both of the web-stabilising boxes 36,38, or as a separate entity.

Referring now to Figures 4 and 5, in a further embodiment of the invention air flow restriction elements are provided which are not located directly above the open area of the vacuum roll 16 not covered by the drying felt 12 and the paper web 10 In contrast to the foregoing embodiments, the whole of a pocket 40 (see Figure 1) created by the down-run and up-run of the drying felt 12 and the vacuum roll 16 is closed off by means of imperforate plates 42 located at respective ends of the pocket 40 and a further imperforate plate 44 mounted between the two adjacent drying cylinders 14,18 across the full width of the pocket 40 so as to close off the pocket 40 from the top These three imperforate plates 42, 44 restrict air flow into the pocket 40, and thus the flow of air into the open area of the vacuum roll 16 not covered by the drying felt 12 and the paper web 10 is correspondingly restricted.

In the prior art, a device similar to that formed by the plates 42,44, called a phantom box, has been used in conjunction with a vacuum roll of a different type that does not apply a partial vacuum to that portion of the shell not covered by the drying felt and the paper web, but its purpose in that case was different.

In further embodiments (not shown) of the invention having full-pocket imperforate restricting elements as described above some form of sealing device may also be provided around the periphery of the imperforate plates The airflow into the pocket could then be restricted to a minimum, and the airflow into the vacuum roll through the open area of its shell not covered by the drying felt and the paper web would then also be restricted to a minimum.

In yet further embodiments of the invention which are not illustrated the plates or the sealing elements, when provided, may themselves be perforate with the intention of allowing a reduction in the partial vacuum created in the vacuum roll or in the pocket Furthermore, additional means, such as slides, may be provided to enable the size or number of the perforations in the plates or the sealing elements to be varied as required This would allow the airflow to be deliberately increased so as to provide the optimum level of paper web support without creating unnecessarily high air speeds in the small gaps between the plates and the vacuum roll or the plates and the periphery of the pocket, or without creating unnecessarily high forces on the sealing elements between the plates and the vacuum roll or the plates and the periphery of the pocket.

The invention is not limited to the details of the aforementioned embodiments and many variations are possible, particularly as regards the precise configuration and mounting of the restrictor elements.

Claims

1 A paper making machine having a single-felt drying section comprising plural drying cylinders, at least one vacuum roll which has a perforate shell, in which respect a web travels from one drying cylinder to the next drying cylinder by passing around the vacuum roll with the web disposed outwardly of a supportive drying felt in its run from the one drying cylinder, while passing around the vacuum roll and in its run to the next drying cylinder, and means for applying a partial vacuum to the inside of the vacuum roll so that a suction pressure is exerted over substantially the entire surface of the shell of the roll, characterised in that a restrictor element is positioned externally of the vacuum roll in such a manner as to restrict flow of air into the vacuum roll in that region of its shell not covered by the drying felt and the paper web.

2 A machine according to claim 1 wherein the restrictor element comprises an imperforate plate.

3 A machine according to claim 1 wherein the restrictor element comprises a perforate plate.

4 A machine according to claim 3 wherein the restrictor element further comprises means for varying the size or number of perforations in the plate.

A machine according to any preceding claim wherein sealing means are provided between the restrictor element and the vacuum roll to further reduce flow of air in that region of the shell of the vacuum roll which is not covered by the drying felt and the paper web.

6 A machine according to claim 5 wherein the sealing means are perforated or are capable of adjustment to vary the flow of air allowed therethrough.

7 A machine according to any preceding claim further including one or more web-stabilisation boxes with which the restrictor element is associated or combined.

8 A machine according to any preceding claim wherein the restrictor element is constructed and/or positioned so as to restrict flow of air into a pocket defined between the respective adjacent drying cylinders, the web runs from the one cylinder to the vacuum roll and from the vacuum roll to the next cylinder and the region of the perforate shell of the roll which is not covered by the drying felt and the web.

Amendments to the claims have been filed as follows 1 t A paper making machine having a single-felt drying section comprising plural drying cylinders, at least one vacuum roll which has a perforate shell, in which respect a web travels from one drying cylinder to the next drying cylinder by passing around the vacuum roll with the web disposed outwardly of a supportive drying felt in its run from the one drying cylinder, while passing around the vacuum roll and in its run to the next drying cylinder, and means for applying a partial vacuum to the inside of the vacuum roll so that a suction pressure is exerted over substantially the entire surface of the shell of the roll, characterised in that a restrictor element is positioned externally of the vacuum roll and extends substantially the entire width of the vacuum roll in such a manner as to restrict flow of air into the vacuum roll in that region of its shell not covered by the drying felt and the paper web.