GB2408054A - Calender cooling apparatus - Google Patents

Abstract

A nozzle 20 for directing cooling air onto a covered roller 42 is connected to a plenum housing 10, which is connectable to air supply means (via duct 40). The nozzle 20 is of a tapered configuration and has an elongate outlet opening 26 which is in the form of a substantially rectangular slit. The nozzle interior communicates directly with a chamber within the housing 10 without any intervening inlet/outlet chamber and in such a manner that air can flow unimpeded from the plenum chamber to the exterior, via the nozzle opening, without any substantial change in direction of flow.

Description

SOFT NIP CALENDER COOLING APPARATUS

This invention concerns cooling apparatus for a calender, particularly but not exclusively a calender of the type which is commonly referred to in the papermaking industry as a soft nip calender.

Soft nip calenders are used in paper making machinery to heat a paper web surface and smoothen it without excessive compression of the web. The nip is provided between a treatable roller, which is typically a hard-chilled cast iron roller, capable of being heated to 200 C or thereabouts, and a roller, which is not heated and which has a covering layer of a resilient synthetic material. The resilient material is usually polymer, more specifically a resin composite. It is not capable of withstanding temperatures as high as those to which the cast iron roller is heated, typically between l 50 C and 200 C, but is, for the most part, insulated from these high temperatures by the intervening paper web. Thus, the typical l 5 working temperature of the surface of the polymer covered roller is in the region of 75 C to 85 C.

However, the paper web does not extend fully to the respective ends of the nip forming rollers. Usually there will be a margin of a few centimetres at each end of the rollers where there is no intervening paper web. There should be an air gap between the rollers in these margins, but this is very narrow as the paper web tends to become embedded in the polymer covering where it overlies said covering. Occasionally, there may even be contact between the rollers at these end margins.

In any event, the polymer covering is not, or is not so well, insulated from the heat of the heated roller in these end margins, and cooling apparatus is employed at these locations to reduce the risk of heat damage. If subjected to high temperatures, much above 100 C to 1 20 C or thereabouts, the end margins of the polymer covering may blister or crack, necessitating repair or replacement of the entire covering, which is costly, especially in terms of lost operating time.

The web may vary in width from job to job if the machine, of which the calender forms part, processes paper for different purposes. Even with a fairly constant width of paper the web may shift slightly from side to side as it passes though the machine.

Accordingly, it is often the case that the width of the edge margin which needs to be cooled may vary.

It is desirable that the heated roller operates at as high a temperature as possible, up to about 200 C, to achieve optimum calendering effect on the paper web. However, this is constrained by the heat reduction which can be achieved by the cooling apparatus mounted at the end margins of the rollers. For example, if the heated roller were to operate at 200 C, the temperature at the end margins of the covered roll would probably be in the region of 160 C, which is 40 C - 60 C higher than acceptable for the polymer covering. In practice, with the cooling apparatus and arrangements known hitherto, it has not been possible to achieve a 40 C - 60 C cooling effect. The maximum achievable is probably about 25 C, which means that the operational temperature of the heated roller usually cannot exceed about 1 60 C so that the temperature of the end margins can be reduced from about 1 30 C to about 105 C, for example. Obviously, the temperature at the end margins of the covered roll needs to be closely and accurately monitored and this can be done using infra red temperature sensors.

Known soft nip calender cooling apparatus produced and installed by the applicant comprises a plenum housing, connectable to an air supply means, e.g. a conduit from a fan or similar impeller, to which is mounted one or more proprietary nozzles for delivery of a jet of cooling air. These nozzles are of generally cylindrical form and have a circular outlet. Each nozzle is mounted onto a side wall of the plenum housing but with its outlet directed towards the front to deliver air in a direction outwardly of a front wall of the housing. Each nozzle incorporates its own valve, for closure and opening of its outlet, and an actuator, usually a solenoid actuator, to operate that valve. In a simple version, a pair of nozzles are usually provided, mounted on opposing sides of a plenum housing so as to direct cooling air to a common narrow band at an end of the relevant roller. In more complex versions, a series of side by side nozzles are provided on each side of the plenum housing to direct cooling air selectively, as their valves are opened, to a narrow or wider end margin of the roller. The outlets of these nozzles are optimally located at a spacing of about 50rnm from the surface to be cooled, to allow the jet of air to diverge to an adequate width to cool a width of about 50mm, particularly in the case of plural side by side nozzles to ensure all locations along the roller end margins are cooled to a substantially equal degree.

The aforesaid cooling apparatus is mounted at each end of the covered roller to direct cooling air onto the respective end margin surface of that roller. It is believed to be one of the most efficient cooling arrangements hitherto available for soft nip calendars, but a cooling effect of only about 1 5 C is achieved. An additional cooling effect, taking the overall effect to about 20 C, can be achieved by mounting additional plenums with nozzles to direct cooling air onto the respective end margins of the heated roller. Yet a further cooling effect, taking the overall effect to about 25 C can be achieved by mounting additional plenums with nozzles to direct cooling air onto the end margins of the covered roller from the rear thereof, as well as from the front thereof.

A further cooling effect can be obtained by using a cooled air supply instead of ambient air.

Indeed, this is necessary, even for the normal cooling effect, in tropical climates. However, this entails using high volumes of water in the cooling process and compliance with discharge regulations for heated water.

One object of the invention is to provide cooling apparatus which will enable a greater cooling effect than hitherto to be achieved in a soft nip calender.

Another object is to provide cooling apparatus for a soft nip calender which is less expensive to produce than known apparatus.

Yet another object is to obviate any requirement for a cooled air supply.

With these objects in view, the present invention provides calender cooling apparatus comprising a nozzle connected to a plenum housing, which is connectable to air supply means, the nozzle being of a tapered configuration and having an elongate outlet opening.

The length of the outlet opening is preferably at least three times greater than its height, and possibly eight, nine or ten times greater than its height. In other words, the outlet opening is preferably in the form of a slit. Such a slit is conveniently of a substantially rectangular shape.

The elongate shape of the outlet opening in accordance with the present invention results in an enhanced air outflow velocity. It also allows for closer approach of the outlet to the roller surface to be cooled, because a space to allow transverse spread of the airflow is no longer necessary. Both of these factors result in a greater cooling effect than hitherto possible.

The nozzle itself is preferably substantially rectangular in transverse cross section. This is fairly straightforward and cost-effective to fabricate. Opposing walls of the nozzle then converge towards the outlet opening to provide the taper. The angle of convergence is preferably between 10 and 60 , mostly suitably between 20 and 40 to provide a reasonably smooth, focused outflow of air. Moreover, the opposing walls or wall regions of the nozzle preferably converge symmetrically towards the outlet opening for the same reason.

A further advantageous feature of the apparatus of the invention is that the nozzle interior should preferably communicate directly with a chamber within the plenum housing, without any intervening inlet/outlet chamber. Another preferred feature of the apparatus according to the invention is that the nozzle be connected to the plenum housing in such a manner that air can flow unimpeded from the plenum chamber to the exterior, via the nozzle opening, without any substituted change in direction of flow. This can more readily be achieved when the nozzle is mounted onto a front wall of the plenum housing and has its outlet opening directed outwardly of said front wall. All of these features reduce impedance to the flow of air to and through the nozzle, thereby further enhancing airflow velocity and cooling effect.

In preferred embodiments of apparatus according to the invention a valve member to open and close the nozzle is mounted inside the plenum housing. A solenoid actuator for the valve member is preferably mounted inside the plenum housing. Again this simplifies manufacture, reduces costs and results in a compact structure.

In many practical embodiments of the invention a plurality of nozzles having any combination of the features outlined above, may be connected to a common plenum housing. For example, from two to six nozzles may be so connected and these may conveniently be mounted side by side along one outer face of the common plenum housing.

In exceptional cases, more than six nozzles may be required, perhaps even as many as twelve, or more, and these could be mounted along one or two faces of the common plenum housing.

Respective valve members, for each of the nozzles, for independent opening and closing of each nozzle may then be mounted within the common plenum housing.

The invention also provides for a calender comprising a first roller which is treatable and a second roller which has a covering layer of a resilient material, these rollers being mounted with substantially parallel axes so as to form a nip therebetween, having cooling apparatus as specified above, with or without any of the optional features mentioned, mounted so that the elongate outlet opening of the or each nozzle is directed at an end section of the covered roller with the longitudinal axis of the or each outlet opening extending substantially parallel to the axes of the rollers.

The outlet opening of the or each nozzle is suitably spaced from the surface of the covered roller by a distance of between 5 and 40mm, more preferably between 10 and 25mm. The closer the spacing, the greater the cooling effect, so a distance of less than 30mm is desirable for that reason. However, if the distance is less than 5mm the cooling effect is not increased, possibly due to a turbulence effect.

In use, of course, respective cooling apparatus should be mounted adjacent each end of the covered roller. Additional respective cooling apparatus may also be mounted adjacent each end of the treatable roller so as to direct air flow for cooling purposes to end sections of that roller too, for enhanced cooling effect, whenever required.

Although the invention is particularly applicable to a soft nip calender which includes a polymer covered roller, the covering of which is heat sensitive, it is also applicable to other calenders, for example those which include a rubber covered roller, where heat sensitivity is an issue.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Fig. I is a side elevation of a preferred embodiment of the cooling apparatus of the invention; Fig. 2 is a front view of the same apparatus; Fig. 3 is a plan view of the same apparatus but with the lid of the plenum housing removed; and Fig. 4 is a schematic side perspective view to a reduced scale, showing how the apparatus of Figs. I to 2 may be mounted in a soft nip calender, only one roll of which is shown.

As shown in the drawings, a preferred practical embodiment of the cooling apparatus of the invention comprises a plenum housing in the form of a box 10 of substantially rectangular cross-section onto which a tapered nozzle 20 is mounted. The box 10 has a lid 12 which is fitted prior to operation, sealed by a foamed plastics strip between its interior and a lip 14 of the box 10, and secured by bolts 16.

The nozzle 20 is cast from aluminium and is of substantially rectangular transverse cross section. It has top and bottom walls 22, 24 which converge symmetrically at an angle of about 30 to an elongate opening in the form of a substantially rectangular slit 26, and side walls 28 of generally triangular shape. The slit 26 is about 35m long and 4mm wide. The nozzle 20 is bolted directly onto a front wall 18 of the plenum box 10 by bolts 21 so as to overlie an aperture 19 in that wall 18.

A valve member 30 is mounted inside the plenum box 10 upon a shah 32 to open and close the aperture 19. The aperture 19 may be circular and the valve member 30 may be a circular disc, as is apparent in Fig. 2, as that shape is easier for manufacturing purposes.

The shad 32 extends into a pneumatic cylinder 34, which is also mounted within the box upon a support bracket 35. The shah 32 and the disc 30 are actuable pneumatically to be extended to close off the aperture 19 or to be retracted from the aperture 19 by a sufficient distance to allow substantially unimpeded air flow from the plenum chamber 11 within the box 10 to the nozzle 20 and out through the nozzle outlet 26. Air supply to the pneumatic cylinder 34, via airline 37, is controlled by a solenoid valve 36, also mounted inside the box 10.

In use, the plenum box 10 is connected to an air supply duct 40, which receives air from a fan or similar air impeller (not shown). This air is ambient air at low pressure, typically about 0.7 bar, and need not be cooled. There is an adequate cooling effect when this air is supplied via the nozzle 20 without any additional pre-cooling of this air.

In practice several such plenum boxes 10 will be supplied with air from a common fan or other impeller.

The plenum box 10 is mounted, by means of a tubular beam 44, adjacent one end of a son nip calender, so that the nozzle 20 is directed towards the polymer covered roller 42 of the nip, as shown in Fig. 4. The longitudinal axis of the slit-form nozzle outlet 26 extends substantially parallel to the axis of the roller 42. The nozzle 20 is directed substantially radially of the roller 42 so that air outflow will be directly substantially perpendicularly at the roller surface and have optimal cooling effect. In practice, the air will diverge after issuing from the nozzle outlet 26. The outlet 26 is optionally placed at a distance x (Fig. 4) of between 1 Omm and 25mm from the surface of the roller 42 so that a 50mm width at the end margin of the roller surface is cooled by air from the nozzle 20. s

In practice, of course, a respective plenum box 10 with nozzle 20, is mounted at each end of the covered roller of the calender, so that both end margins are cooled.

As is conventional, infra red sensors are used to monitor temperature near the centre of the covered roller and adjacent the end margins. When the end temperature exceeds the centre temperature by a threshold value, typically of 25 C, the valve member 30 is activated by way of the solenoid valve 36, to opening the nozzle 20 to commence cooling.

Preliminary trials with the above described apparatus have shown that not only is a greater cooling effect obtained that hitherto, but that this cooling is achieved more rapidly than with the previously known cooling apparatus. Thus, with a single nozzle 20 directed at an end margin of a covered roller 42 in a soft nip calender, and starting from an initial roller surface temperature of 128 C, a temperature drop of 20 C can be achieved in as little as 15 seconds and a temperature drop of 45 C can be achieved after 5 minutes. Clearly this far exceeds the 15 C cooling effect achieved with prior art cooling apparatus, as mentioned

earlier in this specification.

With additional cooling apparatus having single nozzles directed also at the heated roller, also at an optimum distance from the roller of from 10 to 25mm, comparable cooling effects have been achieved, with the maximum exceeding 50 C.

The foregoing is illustrative, not limitative of the scope of the invention and many variations in detail are possible. In particular additional nozzles may be mounted to the plenum box, preferably side by side along the same front wall of the box in order to cool a greater width of the respective roller and its end margin. Thus each additional nozzle, where these are identical to the illustrated embodiment, can add an extra 50mm to the width which is cooled. Such nozzles advantageously have respective valve members under independent control of respective solenoid valves, all mounted within the common plenum box. The nozzles can then be selectively opened to cool an additional end margin width when paper webs of narrower extent are being fed through the calender. s

Claims (23)

1. Calender cooling apparatus comprising a nozzle connected to a plenum housing, which is connectable to air supply means, the nozzle being of a tapered configuration and having an elongate outlet opening.

2. Apparatus according to claim 1 wherein the length of the outlet opening is at least three times greater than its height.

3. Apparatus according to claim 1 or 2 wherein the outlet opening is in the form of a substantially rectangular slit.

4. Apparatus according to any preceding claim wherein the nozzle is substantially rectangular in cross section.

5. Apparatus according to any preceding claim wherein the nozzle has opposing wall regions which converge towards the outlet opening at an angle of between 10 and 60 .

6. Apparatus according to claim 5 wherein the opposing wall regions of the nozzle converge symmetrically towards the outlet opening.

7. Apparatus according to any preceding claim wherein the nozzle interior communicates directly with a chamber within the plenum housing, without any intervening inlet/outlet chamber.

8. Apparatus according to any preceding claim wherein the nozzle is connected to the plenum housing in such a manner that air can flow unimpeded from the plenum chamber to the exterior, via the nozzle opening, without any substantial change in direction of flow.

9. Apparatus according to any preceding claim wherein the nozzle is mounted onto a front wall of the plenum housing and has its outlet opening directed outwardly of said front wall.

10. Apparatus according to any preceding claim wherein a valve member to open and close the nozzle is mounted inside the plenum housing.

Apparatus according to claim 10 wherein a solenoid actuator for the valve member is also mounted inside the plenum housing.

12. Apparatus according to any preceding claim and including a plurality of nozzles, as defined therein, connected to a common plenum housing.

13. Apparatus according to claim 12 wherein from two to six nozzles are connected to the common plenum housing.

14. Apparatus according to claim 12 or 13 wherein the plural nozzles are mounted side by side along one outer face of the common plenum housing.

15. Apparatus according to any of claims 12 to 14 wherein respective valve members, for each of the nozzles, for independent opening and closing of each nozzle, are mounted within the common plenum housing.

16. A calender comprising a first roller which is treatable and a second roller which has a covering layer of a resilient material, these rollers being mounted with substantially parallel axes so as to form a nip therebetween, and cooling apparatus as defined in any preceding claim mounted so that the elongate outlet opening of the or each nozzle is directed at an end section of the covered roller with the longitudinal axis of the or each outlet opening extending substantially parallel to the axes of the rollers.

17. A calender according to claim 16 wherein the outlet opening of the or each nozzle is spaced from the surface of the covered roller by a distance of between 5 and 40mm.

18. A calender according to claim 17 wherein the outlet opening of the or each nozzle is spaced from the surface of the covered roller by a distance of less than 30mm.

19. A calender according to any of claims 16 to 18 wherein the cooling apparatus is mounted so that air will flow from the outlet of the or each nozzle towards the surface of the covered roller in a direction substantially radially of the covered roller.

20. A calender according to any of claims 16 to 19 wherein the cooling apparatus is mounted so as to have a front wall of the plenum housing facing towards the covered roller and the or each nozzle is connected to and projects directly from that front wall towards the surface of the covered roller.

21. A calender according to any of claims 16 to 20 wherein respective cooling apparatus is mounted adjacent each end ofthe covered roller.

22. A calender according to any of claims 16 to 21 wherein respective cooling apparatus is also mounted adjacent each end of the treatable roller so as to direct air flow for cooling purposes to end sections of that first roller, whenever required.

23. Soft nip calender cooling apparatus substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.