FIELD OF THE INVENTION

This invention relates to a method and apparatus for enhancing the rate of drying of a web of material passing over a dryer cylinder in a dryer section of a web making machine and in particular, to a method and apparatus for enhancing the rate of drying of a web of paper in a paper making machine.

BACKGROUND OF THE INVENTION

Web making machines, particularly paper making machines may generally be divided into three sections namely, a forming section, a press section and a dryer section. In modern paper machine former and press sections a large percentage of water is removed from the paper web. Although, water removed from the web after the press approaches 60% some water does remain in the web material and consequently the web has to be passed through a drying section which is normally composed of a plurality of steam heated dryer cylinders or drums which drives water out of the web by vaporization.

There have been a number of patents concerned with the problem of improving the efficiency of the pressing. A device for removing water or liquid from a web material is described in U.S. Patent 4,738,752 which was issued on April 19, 1988.

As mentioned earlier, even though such devices remove a large percentage of water from the web

material, it is still necessary to pass the web through a dryer section in which drying cylinders are provided around which the web material is advanced. The dryer section is generally provided with a continuous belt of dryer fabrics which are held against approximately 50% of the dryer cylinder surface by fabric tension. The web to be dried is generally fed with the dryer fabric onto the dryer cylinder surface, with the web material sandwiched between the dryer fabric and the dryer cylinder surface. The dryer fabric presses against the web material and in turn presses the web material into intimate contact with the dryer cylinder surface. There are a number of dryer section arrangements known in the art. Examples of such dryer arrangements are two-tier and single-tier types. It has been generally found that some difficulties are encountered in the drying process when high pressure steam is used to dry thick web material such as board. The reasons for these difficulties are also known.

On some grades of paper such as board there is a reduction in drying which may be attributed to vapour being generated at the interface between the paper web and the surface of the metal dryer cylinder. If this vapour is generated in sufficient volume it causes the paper to lift off the metal dryer surface thereby causing a reduction in the heat transferred between the metal and the paper.

In conventional dryers, water vapour generated between the paper and the dryer surface flows into the paper until it reaches an area which is cool enough to cause it to condense. It is clear that, the longer the distance the vapour has to travel before it condenses or leaves the web, the greater the pressure generated between the web and the dryer surface. In

conventional dryers this is not generally a problem since high metal temperatures and thus large vapour quantities are not generated.

Recently, gas heated dryers having a much higher heat flux rate than conventional dryers have been utilized. United States Patent 4,693,015 describes such a direct fired cylinder dryer. With this type of dryer, a point is reached where the web material is heated uniformly to approximately 212°F and therefore little or no condensation takes place. In order for the vapour to escape it has to flow through the web material. There is considerable resistance to this flow. Consequently, when the pressure exerted by the vapour reaches the pressure that is applied by the fabric on the paper, the paper lifts off the surface of the metal dryer thereby causing a reduction in drying ability.

Much effort has been spent in developing more efficient pressing section equipment, however, little has been done to improve the effectiveness of the dryer section of a paper making machine. There still exists a need in the dryer section for an effective means to prevent lifting of web material, especially in high temperature gas heated dryers.

SUMMARY OF THE INVENTION

Paper makers have generally considered that increasing fabric tensions above 2kN per square meter with conventional steam cylinder metal temperatures is not economically very beneficial. In contrast to this generally accepted belief, we have discovered that with metal temperatures above 200°C, higher fabric tensions are very beneficial and noticeably increase drying capacity.

It would be obvious to someone skilled in the art of paper drying that increasing the temperature of the dryer cylinder would result in an increase in drying, but the rate of improvement is expected to drop off as one goes further from the typical industry norms. Similarly, it is common knowledge that increasing the pressure applied by the dryer fabric to the web will improve the drying rate, but that the improvement drops off rapidly just above typical industry maximums. However, we have discovered that, under conditions of high surface temperature, as can be achieved with a gas heated paper dryer, increasing the applied pressure delays the drop-off in improvements substantially, so that much higher drying rates can be achieved. Conversely, under conditions of high applied pressure, for example roughly 3 to 5 times the typical industry maximum, via increased fabric tension or via pressure pad, adding high temperature would substantially reduce the levelling-off in drying rate typically seen at normal temperatures. Since the 1960's, it has been accepted in the industry that there was no improvement in drying when the fabric tension was increased above approximately 2 kN per meter. We confirmed this on an actual paper machine. It was only when we increased the metal temperature above 200°F that we found that increasing the fabric tension resulted in an increase in drying capacity. The field tests were done on fine paper and we now have reason to believe that with heavier board webs, the benefits would occur at even lower metal temperatures. It is therefore the combination of both high metal temperature and high fabric tension which result in the benefits of increased drying capacity. We subsequently discovered that there is no problems

with paper properties up to 300°C metal temperature. Accordingly, this combination of increased surface temperatures and increased applied pressures have shown attractively high drying rates. Thus, the present invention seeks to provide a simple and economical method and apparatus for improving the drying of a web in a dryer section of a web drying apparatus, and more specifically to dryer sections utilizing gas heated dryer cylinders or rolls.

In accordance with one broad aspect, the invention relates to a method of increasing the drying of a web material in a dryer section of a web drying apparatus, comprising; subjecting said web material to a combination of high surface temperature, such as by a gas heated paper dryer, and simultaneously increasing the pressure applied to said web, the increased pressure to be obtained by increasing the fabric tension above 8-75kN per square meter.

In accordance with another aspect of this invention there is provided an apparatus for increasing the drying of a web material in a dryer section of a web making machine, the apparatus comprising: a dryer cylinder having an outer dryer surface adapted to be heated by gas or oil; a flexible belt trained over a plurality of guide rollers such that the belt is held in contact with the moveable fabric over at least a portion of the dryer surface in a drying region; a movable fabric trained about the dryer cylinder surface for carrying the web material over the dryer

surface, the movable fabric being interspersed between the flexible belt and the web at the drying region; a pressure pad disposed on one side of the flexible belt and positioned opposite the drying region, and cooperating with the flexible belt for urging the web against the dryer surface in the drying region, such that lifting of the web by vapour generated between the web and the dryer surface is counteracted, thereby to increase the rate of drying of the web.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

FIGURE 1 is a schematic side elevation view of a typical paper making machine;

FIGURE 2 is a schematic side elevation view of a dryer section of the paper making machine of Figure 1; FIGURE 3 is a schematic elevation view of apparatus according to the present invention;

FIGURE 4 is a magnified part view of the flexible belt shown in Figure 3;

FIGURE 5 is a cross sectional view along a line 5-5 in Figure 3;

FIGURE 6(a) is a cross sectional view of a labyrinth seal;

FIGURE 6(b) is a cross sectional view of a rubbing seal; FIGURE 7 is a graph of the dryer metal surface temperature versus fabric tension;

FIGURE 8 is a schematic elevation view of a dryer cylinder illustrating the means for collection of

vapour at the sides of the belt; and

FIGURE 9 is a view taken along the lines 9-9 of Figure 8.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to Figure 1, a paper making machine is shown generally by -numeral 1. The machine has a forming section 10 which generates a continuous web of partially dewatered paper. A pressing section 12 presses the web to continue the dewatering process. A plurality of intermediate dryer sections 14 and a final dryer section 16 drive moisture from the paper web. In each of the dryer sections, the paper web is held against paper dryer rolls or cylinders 18 by fabric belts 20. At least one of the paper and dryer rolls is a direct fired gas heated dryer of t e type shown in U.S. Patent 4,693,015. A size press 24 is disposed between dryer sections to add size to the paper web. The dried paper is ironed in calender rolls 26 and wound by winder 28. Referring to Figure 2, a typical dryer section 14 is shown. This dryer section has gas heated dryer cylinders 30 and 32.

Referring to Figures 3-5 apparatus according to the present invention for increasing the mechanical pressure applied to the web 13 as it is passed over the dryer cylinder 30 of a dryer section is shown. As mentioned earlier, the dryer cylinder 30 is a direct fired dryer, which includes a generally cylindrical dryer shell. A burner (not shown) oxidizes fuel and directs hot combustion gases along the interior of the dryer shell. A flexible belt or blanket 38 is trained over a plurality of guide rollers 40 and is held in contact with the dryer surface 36 of a drying region

42. The fabric belt 20 is shown trained about the dryer cylinder surface 36 and carrying the web 13 over the dryer surface in the drying region. A pressure pad 44 is disposed on one side of the blanket 38 opposite the drying region 42.

The pressure pad 44 is comprised of an arcuate housing 46 which defines a pressure chamber 48. Pressure within the pressure pad chamber 48 exerts a downward or inward force on the blanket 38 which in turn presses the fabric 20 and the paper web 13 into intimate contact with the dryer surface 36 as the blanket, fabric and web are advanced over the dryer surface. The pressure medium in the chamber 48 may be by a gas such as air. As shown in Figure 3, a gas phase reactor 50 may be used to enhance the water resistant properties of the paper web 13. A substance such as ASA (alkenyl succinic anhydride) or A D (alkyl ketene dimer) is sprayed on to the web 13 as at 50 and then enters the gas reaction chamber*. Such a process would use a special impermeable, high temperature fabric which would have to be used with the proper tension.

The use of hot extended nips as self-sealing reaction vessels, for example for gas phase modification of dry paper webs, was set out in a paper by Ernst L. Back and Signar Danielsson entitled "Hot Extended Press Nips As Gas Phase Reactors. Example: Hydro hoba ion With ASA." (Proceedings, the Helsinki Symposium on Alternate Methods of Pulp and Paper Drying, June 4-7, 1991, Helsinki, Finland).

Gas heater paper dryers can be used to achieve the above process.

Details of the drying region may best be seen in Figures 4 and 5, and particularly in Figure 5 wherein

the edges of the pressure pad housing 46 are not directly in contact with the blanket 38, but are however sealed to the blanket by labyrinth or rubbing seals 49, 51, shown in Figures 6a and 6b respectively. The structure of these seals will not be discussed in detail as they are well known in the art.

As mentioned earlier, with high heat flux rate dryers, vapour generated in the web 13 has to flow through the web material in order to escape. Due to the considerable resistance to this flow by the web, vapour pressure between the web 13 and the dryer surface 36 increases to a point where the web 13 begins to lift off the dryer surface thereby causing a reduction in drying. With the apparatus of the present invention, the pressure pad 44 in conjunction with the blanket 38 exerts a mechanical pressure on the web 13 specifically in the drying region 42. Most of the vapour is releasable from the web in this drying region 42 between the blanket 38, fabric 20 and the web 13 and, in order to allow more effective removal of vapour from the web, the blanket 38 may be provided with lateral or cross grooves 52 formed on a side facing the web 13.

Referring to Figure 4, the cross grooves 52 are shown on one side of the blanket 38 and running in a direction normal to the direction of advancement of the blanket over the dryer surface 36. Although in this preferred embodiment these grooves are extending shown in a lateral direction, grooves may also be provided in the longitudinal direction (not shown) of advance of the blanket 38. By providing grooves 52 in the blanket 38, pressure in the vapour release area 50 may be maintained at around atmospheric pressure or at some other optimal pressure, while the pressure on the

opposite side of the blanket 38 is increased beyond atmospheric pressure in order to maintain the web in intimate contact with the dryer surface 36.

Reference should now be made to Figures 8 and 9 as well as Figure 5. With the grooves 52 extending laterally in the underside of the belt 38, vapour released from the web 13 may be collected at the terminal end edges of the grooves 52 by arcuately shaped collection chambers 45 and 47 located adjacent the ends of the dryer cylinder 30 and in communication with the edges of the grooved belt 38. In the illustrated example shown in Figure 8, one chamber such as 45 would be fixed in its location whereas chamber 47 at the other end of the cylinder would be retractable in the direction of the arrow shown in Figure 9 to allow threading operations to take place. In such an example, the front side of the chambers 47 would be cambered to allow retraction by screw jacks or the like to place the chambers clear of the cylinder 30 for purposes of threading.

Vapour collected in the manner shown in Figure 8 may then be recompressed from a reboiler and used in other conventional steam dryers in the paper machine system or the vapour may be used to preheat the web 13 before the press section

The pressure pad 44 may also be segmented (not shown) across its width in order to provide profiling in the evenness of the finished paper moisture.

Typical pressures used in the pressure pad are in the range of 2 to 5 psi.

The release of vapour from the web 13 may also be increased by increasing the tension in the fabric 20 when using high metal temperatures such as with a gas heated dryer. By increasing the tension in the fabric

to 8.5 kN/m, in combination with high metal temperatures it has been found that the vapour released from the web is considerably greater than that released at lower fabric tensions. Referring to Figure 7, a plot of improved contact heat transfer coefficient for increased felt tension is shown. Therefore, by increasing fabric tension, a significant increase in drying can be obtained when the increased fabric tension is used in combination with high metal temperatures.

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

The terms and expressions which have been employed in the specification are used as terms of description and not of limitations, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claims to the invention.