GB1559077A

GB1559077A - Method and apparatus for drying webs

Info

Publication number: GB1559077A
Application number: GB3478775A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-11-18
Filing date: 1976-11-18
Publication date: 1980-01-16

Description

(54) METHOD AND APPARATUS FOR DRYING WEBS (71) I, JOHN DEVLIN HOTCHKISS, a British subject of Old College, Swanley Village, Kent, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method and apparatus for use in drying webs of paper.

When a continuous web of paper is dried by means of steam-heated cylinders in the conventional dryer sections of a paper machine, the web does not usually dry evenly across its width. The resulting web is either too moist towards its centre or too dry at the edges, an ever-present problem often referred to as a "wet-middle"'. Additionally, it is not unusual for "wet streaks" to occur in various parts of the web, usually away from the edges of the web, but not necessarily at its centre.

There are several theoretical explanations of the wet middles, some of which have been proved by experiment. The steam to heat the cylinders is introduced through a tubular journal at one side (normally the back- or drive-side) of the machine; it is possible that this would result in preferential drying at the back side. Conversely, because dry air is drawn through the "dryer pockets" from the front side of the machine, this air becoming more humid as it passes through the pocket, there would be a tendency for preferential drying of the front side of the web. (A dryer pocket is the tunnel formed between a drying cylinder, the paper web and the dryer fabric.) Indeed, the combination of these two effects could be the primary cause of the wet middle.

It has become widely believed that the most effective measure to reduce the moisture difference is to ventilate the dryer pockets and several effective means of providing such ventilation have been devised. It has been found however, that adequate ventilation does not always solve the problem and a new tech- nique has therefore evolved, depending on the maintenance of intimate contact between paper web and drying cylinders across the width of the web.

In a conventional dryer section, the paper web is held in contact with the drying cylinders by looped fabrics, separate fabrics being used for the top and bottom drying cylinders.

These fabrics were formerly rather thick, being felts woven mainly from cotton and they had the disadvantage of being insufficiently permeable to vapours, thus limiting the evaporation of moisture from the paper web. In recent years, the dryer felts have been replaced by open weave, more permeable fabrics woven from synthetic fibres. These allow moisture vapour to pass through them while the paper is in contact with the drying cylinder and because vapour is thereby released outside the dryer pocket, ventilation becomes less of a problem.

A new problem is introduced by the synthetic fabrics in that they require greater tension to maintain intimate contact of the paper web with the drying cylinder. This increased tension is sufficient to deflect the felt rolls and possibly the drying cylinders themselves, so that the tension in the fabric is carried by the edges, while the middle becomes slack, resulting in inferior contact between paper and drying cylinder towards the centre of the web and nullifying or reducing the effects of improved ventilation so far as the wet middle is concerned. There have also been many reported cases of felt roll bearings failing as the result of the increased load. The localised slackness of the fabric also causes it to run into creases which mark the paper, damage the fabric and cause wet streaks.

One proposed means of overcoming this problem is to introduce in place of felt rolls a number of "bowed rolls" such as Mount Hope Rolls (registered Trade Mark) which have the effect of making the felt tension adjustable across its width. This method is described in the "Paper and Trade Journal", 14th January, 1975, pages 4445. However, the design and constructions of such rolls may result in a roll which is insufficiently robust to resist the stress of an adequately tensioned fabric.

The present invention provides a method of controlling the lateral uniformity of longitudinal tension in an endless loop of dryer fabric which is advaced longitudinally over a series of rolls for drying a web of paper, the method being used to compensate for roll deflection due to variations in said longitudinal tension which occur laterally between the centre and the edges of said fabric, which method comprises advancing said fabric through a gap between a pair of rolls each having a cambered or crowned surface, and maintaining the rotational axes of the rolls substantially perpendicular to the direction of advancement of said fabric whilst causing translational movement of one or both roll axes so as to vary the contact surface area between said fabric and each respective roll.

The invention also provides drying apparatus comprising an endless loop of dryer fabric for drying a web of paper, means for advancing said fabric over a series of rolls which include a pair of rolls each having a cambered or crowned surface for contacting said fabric is received, and means for supporting mounted with parallel axes of rotation and with a gap between said rolls in which said fabric is received, and means for supporting; said frame means so that the rotational axes of the rolls are substantially perpendicular to the direction of advancement of said fabric and so that translational movement of the axis of one or both of said rolls can be made so as to vary the contact surface area between said fabric and each respective roll.

Preferably, the rolls are rotatably supported on a pivoted frame. However, one of the rolls may be rotatably mounted in a fixed position and the other rotatably mounted (for example, between pivoted arms) for movement relative to said one roll.

Preferably, said relative movement is effected in accordance with the wetness of the web of paper measured with respect to a direction transverse to its path of advancement. For example, the apparatus may include means responsive to signals representing a moisture content profile measured across the paper web. The relative movement may be controlled in accordance with the peak moisture content or the difference between the maximum and minimum moisture contents. Said relative movement may also be affected to compensate for weft creepage and associated narrowing of a dryer fabric in applications where wetness (for example, moisture content) is less or not important.In this case, such relative movement may be caused in accordance with signals, for example, derived from photoelectric cells, which represent a detected degree of creepage of said fabric in a direction transverse to its advancement Weft creepage is automatically prevented by the adjustment of fabric tension due to said relative movement.

The amount of crown or camber to be applied to the rolls depends on a number of factors, but the principal factor is the difference between the lengths of the paths followed by two points on the fabric, one in the middle and one on the edge of the fabric. This in turn depends on the combined deflection of all the rolls and cylinders around which the fabric passes. The diameters, lengths and fiexural moduli of all the components are involved, also the materials of construction of the fabrics and the various conditions of temperature and humidity affecting the dimensional instability of the fabric.

The effect of a pair of crowned rolls approaching their maximum adjustment is that the fabric following a complete S path will contact each roll through half its circumference, when the difference in path length between the edge and middle ofthe fabric will be: 2 (R-r) where R is the radius of each roll at its midpoint and r is the radius of each roll at a point corresponding to the edge of the fabric. If the required difference in path length is 1, then the amount of crown as defined by the difference in radius is given approximately by:- Amount of crown- 2a it being assumed that the two rolls in a pair are of the same dimensions.Thus, a pair of rolls having crowns of 1 centimetre will accommodate a differense in expansion of the centre-line of a fabric greater than that of its edges, up to a maximum of about 2 ir or 6.28 centimetres.

If the largest practicable amount of crown is insufficient to compensate for the differential stretch of the fabric, it is necessary to include more pairs of crowned or cambered rolls in the run of the fabric.

Because the peak of the moisture profile does not necessarily coincide with the centre of the paper web, it is preferable to use at least three pairs of crowned or cambered rolls, one in alignment with the path of web movement through the drying section, one offset towards one side (e.g. the back) of the drying section and one offset towards the opposite side (e.g. the front) of the drying section. In addition, or in some cases as an alternative, a pair of narrower rolls may be capable of lateral movement with respect to the path of advancement of the dryer fabric so that the crown or camber may be moved over a certain distance towards the back or towards the front of the dryer section. A combination of these devices is capable of dealing with a two humped moisture profile, which sometimes occurs.

The overall tension of the dryer fabric is controlled by orthodox tensioning methods and is not affected by the devices herein des cribed, except that the uniformity of tension provided by this invention should permit an overall tension lower than that otherwise required.

It is common practice on paper machines to provide a means of preventing the fabrics from creeping towards the centre of the machine; the means adopted is normally a spiral spreader, a device which can be incorporated in the crowned or cambered rolls of this invention if it is necessary. However, the use of crowned or cambered rolls in accordance with the invention will of itself eliminate weft creep age and the associated narrowing of fabrics.

Preferred embodiments of this invention are capable of providing an even tension in a paper machine dryer fabric, thus promoting uniform pressure of paper web against drying cylinders which in turn provides even drying and a substantially flat moisture profile across the paper web. Important consequences of the flat moisture profile are: 1) The average moisture content of the paper can be increased, thus reducing the weight of raw material per unit area of paper.

2) The printability of paper is improved by providing the ideal moisture content and a more uniform moisture content.

3) The frequency of paper breaks on web fed printing presses and other paper convert ing machinery is reduced.

4) The efficiency of paper production is improved.

An embodiment of the invention will now be described with reference to the accom panying drawings, in which: Figs. 1 and 2 are elevational and front views of part of a drying apparatus, Figs. 3 and 4 are similar views of a similar part of said apparatus having rollers of re duced length, Figs. 5 and 6 show how the dryer fabric is fed through the rollers of Figs. 14, and Fig. 7 is a schematic view of a dryer sec tion of a paper making machine showing possible positions for the parts shown in Figs.

1.

Referring to the drawings, apparatus for adjusting the tension across the width of a dryer fabric 2 comprises one or more pairs of crowned or cambered rolls 1 or 5 carried on arms 3 which are rotatably mounted in a frame 8 comprising a pair of arms 3. The frame 8 is pivotally mounted on shafts 4 ex tending from the middle of each arm 3. The inclination of the frame 8 is such that the dryer fabric follows a "S" shaped path over and between each pair of rolls 1 or 5. Frame 8 is coupled to an adjusting mechanism (not shown) whereby it may be caused to pivot about shafts 4 to accentuate or reduce the shape of the "S" and thereby control the lateral uniformity of longitudinal dryer fabric tension, i.e. between the middle and edges of the dryer fabric.Such adjustment is made from time to time while the paper machine is running in accordance with the observed moisture profile of the paper web as determined by testing, or preferably by the use of instruments which continually record the moisture content at different points on the paper web extending transversely of the direction in which the paper web is advanced. The adjustment of the crowned or cambered rolls 1 or 5 could be controlled automatically by instruments sensitive to the moisture profile.

The narrower rolls of Figs. 3 and 4 are capable of lateral movement with respect to the path of advancement of the dryer fabric 2 so that the crown or camber may be moved over a certain distance towards one of the edges of the dryer fabric to deal with a two humped moisture profile (measured across the paper web).

Fig. 7 schematically illustrates two possible positions 18 for pairs of rollers in accordance with the invention. The two positions 18 illus trated are the minimum requirements for such a dryer section, i.e. one in the top run of fabric and the other in the bottom run of fabric. In this figure, the pairs of rollers are shown as being mounted in a pivoted frame as described above. The dryer section of Fig.

7 includes top and bottom drying cylinders 11, 12 respectively and top and bottom por tions of dryer fabric 13, 14 respectively. A paper web 15 is looped alternately around cylinders 11, 12. The dryer fabric 13 is also supported on felt rolls 16 and felt tension rolls 17.

WHAT I CLAIM IS: 1. A method for controlling the lateral uni formity of longitudinal tension in an endless loop of dryer fabric which is advanced longi tudinally over a series of rolls for drying a web of paper, the method being used to com pensate for roll deflection due to variations in said longitudinal tension which occur later ally between the centre and the edges of said fabric, which method comprises advancing said fabric through a gap between a pair of rolls each having a cambered or crowned sur face, and maintaining the rotational axes of the rolls substantially perpendicular to the direction of advancement of said fabric whilst causing translational movement of one or both roll axes so as to vary the contact surface area between said fabric and each respective roll.

2. A method according to claim 1 wherein said relative movement is effected in accordance with a moisture content profile measured across the paper web in a direction transverse to its path of advancement 3. A method according to claim 1 wherein said relative movement is effected in accordance with a detected degree of creepage of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. cribed, except that the uniformity of tension provided by this invention should permit an overall tension lower than that otherwise required. It is common practice on paper machines to provide a means of preventing the fabrics from creeping towards the centre of the machine; the means adopted is normally a spiral spreader, a device which can be incorporated in the crowned or cambered rolls of this invention if it is necessary. However, the use of crowned or cambered rolls in accordance with the invention will of itself eliminate weft creep age and the associated narrowing of fabrics. Preferred embodiments of this invention are capable of providing an even tension in a paper machine dryer fabric, thus promoting uniform pressure of paper web against drying cylinders which in turn provides even drying and a substantially flat moisture profile across the paper web. Important consequences of the flat moisture profile are:

1) The average moisture content of the paper can be increased, thus reducing the weight of raw material per unit area of paper.

4) The efficiency of paper production is improved.

1.

said fabric in a direction transverse to its advancement.

4. A method according to any one of the preceding claims in which said rolls are rotatably supported on a pivoted frame, the inclination of said frame being adjusted to cause relative movement between both of said rolls and said fabric.

5. A dryer section for a paper making machine, the dryer section comprising an endless loop of dryer fabric for drying a web of paper, means for advancing said fabric over a series of rolls which include a pair of rolls each having a combined or crowned surface for contacting said fabric, frame means on which said rolls are mounted with parallel axes of rotation and with a gap between said rolls in which said fabric is received, and means for supporting said frame means so that the rotational axes of the rolls are substantially perpendicular to the direction of advancement of said fabric and so that translational movement of the axis of one or both of said rolls can be made so as to vary the contact surface area between said fabric and each respective roll.

6. A drier section according to claim 5 wherein said rolls are rotatably supported on a pivoted frame.

7. A drier section according to claim 5 wherein one of said rolls is rotatably supported in a fixed position and the other roll is rotatably supported between pivoted arms for movement with respect to said one roll.

8. A drier section according to any one of claims 5-7 including means responsive to signals representing a moisture content profile measured, in use, across the paper web, transversely to the direction of its advancement, for causing said relative movement.

9. A drier section according to any one of claims 5-7 including means responsive to signals representing a detected degree of creepage of said fabric in a direction transverse to its advancement for causing said relative move ment.

10. A drier section according to claim 6 wherein said rolls are mounted in the frame means substantially as herein described with reference to Figs. 3 and 4 of the accompanying ing drawings.

11. A drier section according to claim 6 wherein said rolls are mounted in the frame means substantially as herein described with reference to Figs. 3 and 4 of the accompanying drawings.

12. A paper making machine including the drying section according to any one of claims 5-11.

13. A paper making machine according to claim 12 including first, second and third pairs of said rolls each mounted in its respective frame means, the frame means of the first pair of rolls being supported so that its pair of rolls are centrally located with respect to the width of said fabric, and the frame means of the second and third pairs of rolls being mounted so that the respective pairs of rolls are offset towards respective edges of said fabric.

14. A paper making machine according to claim 12 or 13 including a pair of rolls which are narrower than said fabric, said rolls being capable of lateral movement with respect to the path of advancement of said fabric.

15. A paper making machine substantially as herein described with reference to Fig. 7 of the accompanying drawings.