FIELD OF THE INVENTION

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

BACKGROUND OF THE INVENTION

In practice when drying web material such as for example, paper, in a paper making machine, cylinders are provided in the dryer section of a paper machine around which the web is advanced. The dryer section of a paper machine is generally provided with continuous belts of dryer fabrics which are held against approximately 50% of the cylinder surface by fabric tension. This is generally utilized in a two cylinder-high dryer section. The dryer fabric and the web material are advanced together onto a dryer cylinder, such that the dryer fabric presses against the web material and in turn presses the web material into intimate contact with the dryer cylinder surface. As mentioned previously, standard dryer sections comprise the dryer cylinders stacked single or two high. There are many other dryer section arrangements such as single-tier or multi-tier types.

It has been generally found that some difficulties are encountered in the drying process when high pressure steam cylinders are used to dry web material such as paper. It has been postulated that on some grades of paper 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 heat transfer 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 for it to condense. It is clear that, the longer the distance that the vapour has to travel, the greater the pressure generated. In the conventional dryer, a great deal of pressure is not generated because of the relatively low heat flux rates generated. When higher heat flux rates are used such as with gas heated dryers, a point is reached where the paper is heated uniformly to approximately 212°F and therefore no condensation takes place. In order for the vapour to escape it has to flow through the paper. There is a considerable resistance to this flow. Consequently when the pressure exerted by the vapour reaches the pressure that is applied by the abric on the paper, the paper lifts off the surface of the metal dryer, thereby causing a reduction in drying ability.

There are patents in the field of paper making relating to drawing a web, particularly the web edges, into close conformity with its supporting fabric and also to prevent the web from fluttering on a guide roller. Examples of the prior art are: United States patent 4,882,854, which has rotatable guide rolls having an outer cylindrical surface, on which air flow channels or grooves are formed; United States patent 5,024,729 describes a suction roll which includes a

turbulence suppression element; United States patent 3,345,757 describes a felt roller, which has a perforated outer shell; United States patent 4,677,762 is another example of a dryer arrangement. None of these have been used for the purposes to which the present invention is directed nor do any of these patents relate to heated dryer cylinders.

It is consequently desirous to provide an effective means for allowing the vapour to escape and which means is relatively easy to manufacture, simple to install and maintain and relatively inexpensive.

SUMMARY OF THE INVENTION

The present invention seeks to provide a simple and economical means for improving the drying ability of a web drying apparatus and more specifically a gas heated paper dryer.

In accordance with this invention there is provided a dryer cylinder for use in a dryer system in which a web material to be dried is held in contact with and advanced over at least part of the surface of a rotatable dryer cylinder and the surface being supported by a cylindrical shell, the cylinder comprising means formed in the dryer surface and extending into the shell such that, in use, vapour generated between the web and the dryer surface is permitted to flow into the means and away from the web so that lifting of the web due to pressure exerted by the vapour is inhibited.

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 web dryer system;

FIGURE 2 is a perspective view of a dryer cylinder according to the present invention; FIGURE 3 is a cross-sectional view of part of the dryer cylinder shown in Figure 2;

FIGURE 4 is a graph showing the average drying rate versus temperature for a dryer using a dryer cylinder according to the present invention; FIGURE 5 is a schematic end elevation of a five foot diameter dryer cylinder;

FIGURES 6 and 7 are details of the surface of a dryer cylinder according to the invention;

FIGURE 8 is a detailed, segmental sectional view of a portion of a dryer cylinder incorporating lateral flow features; and

FIGURE 9 is a schematic end view of a gas heated paper dryer illustrating its incorporation as a gas- phase reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Figure 1, a typical web dryer system is illustrated and comprises an upper tier of dryer cylinders 12 and a lower tier of dryer cylinders 16. A web W is wrapped by fabric F against a first dryer cylinder 12 of the upper tier 14 thereof. The web W leaves the heated upper dryer cylinder 12 and extends

in an open draw towards and around a lower dryer cylinder 16 of the lower tier 18 thereof.

Drying of the web is generally achieved by steam dryers and is augmented by heating the dryer cylinders 12 and 16 internally, preferably by means of a gas burner (not shown) . These burners are positioned within the cylinders to effect heating of the walls and surface thereof and thereby to heat the web in direct contact with the outer cylindrical surface. Details of such a gas heated cylinder dryer are disclosed in Applicant's U.S. Patent No. 4,693,015 issued September 15, 1987. Referring to Figure 2, a dryer cylinder 12 or 16 of Figure 1 is shown in more detail. The cylinder has an outer cylindrical surface 20. A plurality of circumferentially spaced grooves 30 are machined into the shell 22 of the cylinder 12, as may be seen in Figure 3. In use, the web W and the dryer fabric F are advanced over approximately 50% of the dryer cylinder surface 20. The web W is held in intimate contact with the cylinder surface 20 by fabric tension. At high dryer surface temperatures the web W is rapidly heated to temperatures in the order of 212°F and substantially no condensation of vapour may take place. The grooves 30 as shown in Figure 3 ensure that, in use, vapour generated between the cylinder surface 20 and the web W is permitted to flow through the grooves 30, away from the web W, thus reducing the lifting of the web off the dryer surface 20 and increasing drying. Referring to Figure 3, a preferred arrangement of the invention is illustrated in which the grooves 30 are rectangular in cross-section and penetrate the dryer shell 22 to a depth "D" of approximately 3/8 inch. Since the paper does not completely encircle

the dryer cylinder but only partially covers it, any vapour collecting between the web W and the surface 20 of the dryer is channelled or directed into the grooves 30 and is expelled from the ends of the grooves as the dryer cylinder rotates.

The graph of Figure 4 shows the results of pilot paper machine tests conducted with a smooth and grooved surface dryer cylinders. The graph 42 is a plot of the results for a grooved dryer cylinder and the plot marked 44 is the results of a smooth surface dryer. With the grooved surface dryer cylinder the average drying rate is considerably higher than with non-grooved dryer cylinders at high temperature.

It is to be noted, that contact between the web and the dryer surface is reduced by the number of grooves 30 formed in the dryer cylinder. However, it is found that the overall gain in performance exceeds that lost by the reduction in contact due to the grooves at higher temperatures. In one example, approximately 5 grooves per inch were machined along the length of the dryer cylinder surface. With each of the grooves (Figures 3) having a width T of approximately 1/16 inch and a depth D of approximately 5/16 inch it has been found that there is a loss of approximately 30 percent of the surface contact area with the web. There is a corresponding increase of 30 percent in the applied pressure due to this reduction in surface contact area.

Reference should now be made to Figures 5 through 9 of the drawings. In the embodiment of Figures 5 through 8, means are provided so that the vapour can flow laterally to the front or back ends of the dryer cylinder. This enables the collection of vapour in a chamber located at the ends of the dryers. The

advantage of such an embodiment is that the vapour can either be directly recompressed or it can be passed through a re-boiler and then be compressed, subsequently to be used as a steam supply to the regular dryers of the paper machine. Such an arrangement makes the gas heated paper dryer system very efficient and the energy used in the drying process would be recovered in the form of recompressed steam. This has a further advantage in that a suitable adjustment of the fabric pressure would enable the vapour exhaust system to be operated above the ambient atmospheric pressure and this would make the recompression system less capital intensive.

Referring to Figures 5, 6 and 7, Figure 5 is a schematic end view of a five foot diameter cylinder 50 intermediate a pair of fabric rollers 52. The path of the paper 54 is shown as is the path of the fabric 56.

In a typical example, in the circumference of the five foot diameter cylinder, each circumferential groove would be subdivided into thirty-six 5 and 1/4 inch segments 60 as shown in Figure 6. Typically, the grooves are 1/16 of an inch wide by 5/16 of an inch deep. The groove is segmented every 5.25 inches on the five foot diameter cylinder to provide the thirty- six segments in the circumferential path of the groove.

As shown in Figure 6, the groove has, in each segment 60, six laterally drilled apertures 62 (two of which are illustrated) of approximately 1/8 inch diameter to allow vapour to flow laterally to the front and back ends of the dryer.

Six 1/8 inch diameter apertures in a 5.25 inch segment of the groove 60 will give a slightly lower velocity than the velocity in the groove without the

segments. Figures 6 and 7 illustrate the location of the lateral apertures 62 in the walls of the groove segment 60.

As shown in Figure 8, the dryer shell 64 is provided with the grooves 58 (not to scale) , the front and rear walls 66, 68 of the dryer shell being provided with a collector housing 70, isolated from the dryer shell by suitable seals 72. As illustrated in relation to end wall 66, ports 74 are drilled in each end wall 66, 68 of the shell so as to conduct vapour to the collectors 70 from where it is removed and forwarded to a pre-boiler, compressor or the like.

Using an arrangement as described above in combination with a suitable high density impermeable fabric, would enable the gas heated paper dryer to be used as a gas phase reactor to achieve hydrophobation with ASA.

As shown in Figure 9, a gas phase reactor is used to enhance the water resistant properties of paper. A substance such as ASA (alkenyl succinic anhydride) or AKD (alkyl ketene di er) is sprayed on to the paper web as at 76 and then enters the gas reaction chamber*. Such a process would use a special impermeable, high temperature resistant 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: Hydrophobation 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.

The number of grooves per unit width is a function of the difficulty of making narrow grooves as well as a function of the ability of the web W to lift briefly and let vapour pass to the groove, and in part a function of the lateral permeability of the web W, i.e. its resistance to the flow of vapour laterally to the groove. The depth of the groove is a function of the metal temperature and fabric tension F which causes the rate of vapour generation or evaporation of water and the vertical permeability of the web which causes the resistance to vapour flow through the web. The groove depth must be large enough so that vapour generated by evaporation can escape through the grooves so as not to exceed the pressure exerted by the fabric.

The groove widths and depths as well as the percentage of grooving will vary, depending on paper grades.

The grooved cylinders may be applied to any arrangement of dryer cylinders and dryer fabrics.

While the invention has been described in use with a continuous belt of dryer fabric, it will be appreciated that in some paper making machines the dryer fabric may be dispensed with if the web material is of sufficient strength such as in paper board production. 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. For example the same effect as the grooves may be achieved by using a flat wire wrapped circumferentially around the dryer surface and contacted to the dryer surface by metallizing or the like so as not to lose substantial heat transfer to the web.

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.