GB2153508A

GB2153508A - Web drying machines

Info

Publication number: GB2153508A
Application number: GB08415687A
Authority: GB
Inventors: Peter J Fraser; R Calverly
Original assignee: Spooner Industries Ltd
Current assignee: Spooner Industries Ltd
Priority date: 1983-12-14
Filing date: 1984-06-20
Publication date: 1985-08-21
Also published as: GB8415687D0; GB2153508B; GB8333330D0

Abstract

A multi-cylinder drying machine is fitted with ventilation boxes (15) having nozzles (16) for directing drying medium through the porous fabric 13/14 and over the web (12) being dried before and after the latter leaves the surface of the drying cylinder (11). A flexible aerofoil or curved plate (17) extends from the downstream edge of the nozzle (16) to the fabric (13/14) and can be deflected against a curved wall (25) or an extension (26) of the nozzle box to allow passage of wads. <IMAGE>

Description

SPECIFICATION Improvements in multi-cylinder drying machines The present invention relates generally to the introduction of heated air to the surface of a paper or board web within a multi-cylinder drying machine and more particularly to assisting in the drying rate byfeeding air either through a permeable dryer fabric or directly on to the surface of the paper or board being dried to assist in the drying rate of the web.

The basic concept of introducing airthrough a permeable dryer fabric to assist in the drying rate of the web within the papermaking machine dryer part is well established. However, after an extensive research investigation it became evident that some form of more positive method of introducing air to the surface of the fabric was required to enable the air to reach the surface of the paper at the points where the potential for the transport of moisture from the surface of the paper web is at its greatest; i.e. where the paper web is still in contact with the heated cylinder and during the first part of the unsupported run to the next heated cylinder.

According to the present invention, a multicylinder drying machine having a permeable dryer fabric to hold the web being dried against the peripheries of the cylinders, at least at the wet end of the machine, is provided with at least one ventilation box which extends laterally between adjacent cylinders and a guide roll for the permeable fabric, which ventilation box has a nozzle which is directed towards the permeable fabric, where the latter leaves one cylinder, and a plate which extends from the ventilation box to the permeable fabric at the downstream side of the nozzle with respect to the direction of fabric movement to direct through the permeable fabric and against the web drying air which is ejected from the nozzle.

Preferably, the plate is flexible so as to be deflectable, in the event of a paper wad, to allow the latter to pass, and advantageously it may be deflected against a curved wall on an extension of the box, which wall prevents over-bending of the plate.

It is advantageous for means to be provided for adjusting the rate of flow of drying medium through the nozzle laterally of the permeable fabric to compensate for inherent uneveness in the drying rate, i.e., to compensate for "wet streaks".

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view of a papermaking machine dryer part showing a typical installation of a ventilation box applied in conjunction with both a top and bottom fabric run; Figure 2 is a detailed diagrammatic sectional view of the ventilation box; Figure 3 is a fragmentary diagrammatic perspective view of the ventilation box; Figure 4 is a view, similar to Figure 2, but showing another embodiment of the invention; and Figure 5 is a detailed diagrammatic sectional view, showing the application of the same principle to the side where the fabric is approaching the surface of the cylinder.

The sectional view of the papermaking machine dryer part illustrated in Figure 1 shows the heated drying cylinders 11 with the paper or board web 12 passing in serpentine fashion over the top cylinders and under the bottom cylinders. The paper web on the heated cylinders is overlaid by a permeable dryer fabric 13 and 14 through which the air is introduced by means of ventilation boxes 15, the fabric being held against the cylinders by hitch rolls 20. The ventilation boxes 15 extend the full length of the cylinders 11.

The detailed sectional view of the ventilation box 15 illustrated in Figure 2 shows an elongate air nozzle at 16 feeding air over the surface of an aerofoil or curved plate 17 which terminates in close proximity to the surface of the permeable fabric 13/14. The fixing brackets (not shown) which carry the ventilation boxes in the papermaking machine dryer part are adjustable.

The curved plate 17 is constructed from stainless steel or a high impact, high temperature-resistant reinforced plastic and is arranged so that, in the event of a paper wad being taken round the cylinder underneath the fabric, the plate would spring away and flex round the curved wall 25 of the box extension 26 behind, thus preventing localised bending and maintaining the degree of support required for maximum impact resistance.

The curved plates 17 will be manufactured full length or in various short standard sections to enable all papermaking machine widths to be accommodated. The method of fixing the curved plates will depend on the material being used for the ultimate construction, but in most installations it is envisaged that ordinary pop rivets will be adequate; for instances where periodic replacement would be deemed necessary a bolted clamp method would be used. An alternative arrangement could also be made where the curved plate is hinged and/or independently spring-loaded.

As shown in Figure 2, air ejected from the nozzle 16 follows the curvature of the aerofoil plate 17 and passes through the porousfabric 13/14to impinge upon and flow along the surface of the web 12 both where the latter is still in engagement with the drying cylinder 11 and over the unsupported portion of the web, as indicated by arrows 37. This air flows along the web, disturbs the air boundary layer, and thereby purges the moist air from the vicinity of the web. This promotes the drying and enables higher drying rates to be achieved than have been obtained hitherto, even by employing a larger volumetric air flow. As a consequence, the energy required, both in terms of heat for heating the air and the power to drive the air blower, is substantially lower in relation to the increased drying rate which can be achieved.

The ventilation box 15 is preferably spaced by at least 75 mm from the surface of the cylinders 11.

This spacing is much larger than clearances generally provided hitherto so that damage due to paper wads, etc. can be minimized. Since it must be possible to reverse the direction of rotation of the cylinders 11 to clear a blockage, e.g. in the event of the web becoming wrapped around the cylinder 11, the free edge 17a of the curved plate 17 is curved back as shown in Figure 2 to prevent the free edge of the curved plate from snagging the fabric 13/14.

In the positions in a papermaking machine dryer partwhere a control or "profiling" ofthe airflow across the width of the machine is required, the main body 32 of the ventilation box 15 is divided into sections 33, each of which is fitted with a respective damper 30 mounted on a through shaft31, as shown in Figure 3. The upper portion 34 of the ventilation box 15 forms a supply header and extends the full length thereof and may be fed with heated air under pressure from one end or both ends. The sections 33 may be separated from one another by partitions 35 which terminate short of the nozzle 16. By fixing the dampers 30 individually at appropriate angles, the air flow from the nozzle 16 can be profiled over the width of the web.An advantage of this arrangement is that the total air flow from the nozzle 16 remains almost the same, irrespective of the damper adjustment, so that it is impossible to reduce the air input to the dryer by over-restricting the air flow from the nozzle.

The dampers 30 can be adjusted from the front side of the dryer section by coupling the dampers to suitable knobs or levers (not shown) via a mechanical linkage or Bowden cable which may be accommodated within the supply header 34 or by means of servo-motors. If the dryer is fitted with moisture sensors capable of detecting wet streaks in the finished product, the dampers may be adjusted automatically.

Figure 3 shows a main support tube 36 for the ventilation box 15. Conveniently, the tube 36 passes through the box lower extension 26. The ends of the tube 36 can be supported adjustably to enable the position of the free edge 1 7a of the curved plate 17 relative to the fabric 13/14 to be adjusted.

In Figure 4, adjustable stops 18 are shown fitted to bear on the top side of the curved plate 17 to enable the gap between the free edge 17a and the fabric 13/14to be adjusted.

Figure 4 of the drawings shows a construction in which a proportion of air is also fed to the underside of the curved plate 17 through a series of holes 19 which satifies the volume of air which is "pumped" through the fabric 13/14 at the converging nip where the fabric meets the roll 20, the holes 19 being formed in a curved wall 25 forming part of a lower extension 26 of the ventilation box 15. A seal plate 21 is fitted between the roll 20 and the lower extension 26 of the ventilation box 15 to prevent air from being "pumped" by the movement of the roll 20 back to the inlet, converging nip, side of the roll.

Figure 4 also shows another way of achieving a control of the air flow across the width of the machine. A slide valve 22 is fitted on a horizontal division plate 23. This plate is provided with an array of holes at constant pitch along the length of the ventilation box, i.e. transversely of the permeable fabric 13,14, whereas the slide valve 22 is punched with a pattern of holes where every 3rd or 4th pitch is reduced, thus enabling by a small movement of the slide valve, the open area for the supply of air to the lower section of the ventilation box to be controlled over its full length. The movement of the slide valve 22 may be attained in various ways, depending on the degree of accessibility of the particular papermaking machine, e.g., by Bowden cable, manual screw adjustment or pneumatic positioner.For access to the slide valve etc. within the ventilation box, the top is removable.

The detailed sectional view of the ventilation box 45 illustrated in Figure 5 shows the principle of the curved aerofoil plate 17,17a applied to both the ingoing and outgoing side of the fabric run. The aero-foil plate 17a is constructed like the aero-foil plate 17 to converge up to the permeable fabric 13,14. In the event of a paper wad, the plate 17a can be deflected against a curved wall 25a of the extension 26a to the ventilation box 45. The curved wall 25a may or may not be provided with discharge holes like the holes 19 of the curved wall 25 in Figure 4. Other parts like those of the embodiment of Figures 2 and 4 are provided with like reference numerals.

An advantage gained in feeding air to the outgoing converging nip at 24 between the permeable fabric 13,14 and the heated cylinder is that the pressure in the converging nip 24 can be increased which, in turn, will increase the contact pressure between the paper web and the heater cylinder surface and assist in the heat transfer from that surface.

Claims

1. A multi-cylinder drying machine having a permeable dryer fabric to hold the web being dried against the peripheries of the cylinders, at least at the wet end of the machine, and provided with at least one ventilation box which extends laterally between adjacent cylinders and a guide roll for the permeable fabric, which ventilation box has a nozzle which is directed towards the permeable fabric, where the latter leaves one cylinder, and a plate which extends from the ventilation box to the permeable fabric at the downstream side of the nozzle with respect to the direction of fabric move menu to direct through the permeable fabric and against the web drying air which is ejected from the nozzle.

2. A drying machine as claimed in claim 1, in which the plate is flexible so as to be deflectable, in the event of a paper wad, to allow the latter to pass.

3. A drying machine as claimed in claim 2, in which the ventilation box has an extension provided with a curved wall against which the plate may be deflected to prevent over-bending of the plate.

4. A drying machine as claimed in claim 1,2 or 3, in which the plate is curved to an aerofoil shape so that the drying air follows the curvature of the plate.

5. A drying machine as claimed in any of claims 1 to 4, in which the ventilation box is spaced by at least 75 mm from the adjacent cylinder.

6. A drying machine as claimed in any of claims 1 to 5, in which the free edge of the plate is curved back away from the fabric.

7. A drying machine as claimed in any of claims 1 to 6, in which means are provided for adjusting the rate of flow of drying medium through the nozzle laterally of the permeable fabric to compensate for inherent uneveness in the drying rate, i.e., to compensate for "wet streaks".

8. A drying machine as claimed in claim 7, in which the ventilating box is divided into sections, each provided with its own damper to enable the flow rate through the nozzle to be varied transversely of the machine.

9. A drying machine as claimed in claim 7, in which the ventilation box has a division plate provided with an array of holes along the length of the ventilation box and a slide valve provided with a pattern of holes co-operates with the division plate, the pattern of holes in the slide valve being different from the array of holes in the division plate, whereby adjustment of the slide valve varies the rate of flow of drying medium to be varied over the length of the ventilation box.

10. A multi-cylinder drying machine constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.