T I T L E PAPER WEB HEATING ON A PRESS ROLL BACKGROUND OF THE INVENTION

The present invention relates to improvements in paperma ing machines, and more particularly to an improved press for dewatering a traveling paper web.

In a papermaking machine, the web is typically made from fibers in the former and passes immediately to the press section where a series of press nips are formed which mechanically press water from the web into traveling felts and thereafter passes to the dryer section where remaining water is evaporated from the web and the web thereafter having been formed and dried is wound on large rolls. It is well known that very substantial savings in the costs of manufacture of a web can be effected if more water is extracted in the press section by mechanical pressing in that the energy required for mechanical pressing for the removal of water is much less expensive than if the water is to be removed thermally.

In the pressing operation, it is essential that the press nip maintain uniform pressure across its width so that the paper web is dewatered uniformly. One means of controlling the nip pressure is to use a controlled deflection roll which has a roll shell with a supporting shaft therein and lubricating or pressure means between the stationary shaft and the roll shell which is controllable to control the deflection and hence the nip pressure along the width of the nip. It has been discovered that by

heating the lubricating or pressure fluid, the roll shell can be similarly heated to heat the roll shell and improve the dewatering operation in the press nip.

Examples of such concept are shown and disclosed in U.S. Patent 3,726,338 and 4,679,287.

It is an object of the present invention to appreciably improve the dewatering effect of a press nip to increase the amount of water which can be removed in the mechanical pressing section of a papermaking machine.

A still further object of the invention is to utilize the effect of heat of the moisture in a paper web to improve the mechanical dewatering in a press nip.

A still further object of the invention is to provide improved means for the transfer of heat to a traveling paper web in a press section to improve the dewatering effect and permit higher pressing speeds to be achieved.

FEATURES OF THE INVENTION

In accordance with the principles of the invention, a press nip is provided either formed between two press rolls or between a press roll and opposed pressing means such as a hydrodynamic shoe. The press roll is heated to higher temperatures than heretofore thought possible by improving the heating effect of lubricating fluid directed into the interior of the- roll shell at higher temperatures than heretofore used. The effective transfer of heat to the web is enhanced by longer residence time of contact between the wet web and the press roll shell. This is accomplished by a longer contact between the roll shell and web. In one form, an extended nip press is used wherein the web is not merely passed between two rolls in a roll shell but between a roll and an extended

nip press of the type having a hydrodynamic shoe. In another form, the web is brought into contact with the press roll a substantial distance prior to the nip so that the web approaches the desired temperature even before entering the nip. This feature is utilized in one form where two nips are used and the web is maintained in contact with the press roll between the first and second nip so that the temperature of the web is increased over that of the first nip when it reaches the second nip.

In still a further embodiment, it has been discovered that improved heat transfer can be accomplished by directing steam through the web prior to its entry into the nip. The addition of steam directed against the web not only increases the temperature of the web but provides sufficient moisture at the location of contact between the surface of the web and the heated roll shell so that a more rapid temperature increase is experienced in the web from the heat of the roll shell.

Other objects, advantages and features will become more apparent in connection with the teaching of the principles of the invention in connection with the specification, claims and drawings, in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary side elevational view, taken partially in section, through a roll shell constructed and operating in accordance with the features of the present invention;

FIG. 2 is an enlarged fragmentary vertical sectional view taken substantially along line II-II of Fig. 1;

FIG. 3 is an enlarged fragmentary sectional view illustrating a seal for a controlled deflection shoe for a press roll;

FIG. 4 is a schematic end elevational view of a press roll assembly constructed and operating in accordance with the features of the present invention;

FIG. 5 is a schematic end elevational view of a modified form of the arrangement of Fig. 4;

FIG. 6 is another schematic end elevational view illustrating another form of practice of the invention; and

FIG. 7 is a schematic end elevational view illustrating still another form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figs. 1, 2 and 3 illustrate features of construction of a press roll (8) of the type having a roll shell with a shoe controlled by pressurized liquid for controlling the pressure in the nip and for controlling the deflection of the roll.

As illustrated in Figs. 1 and 2, a roll shell is shown with a support shoe 11 therein. The shoe 11 is provided with pressure pockets 12 to be supplied hydraulic lubricating oil for lubricating the surface between the shoe 11 and the roll shell 10 and for applying a nip control force to the roll shell. In the form illustrated, pressurized oil is delivered to a chamber 13 beneath a piston 14 supporting the shoe. Passages to the piston allow hydraulic oil to flow up to the pockets 12. The piston is supported on a central shaft 9 which extends through the roll shell and is supported on framework.

Fig. 3 illustrates a modified form of seal for a piston 15 supported on a shaft 18. A chamber 16 beneath the piston is supplied with pressurized oil, and the side of the piston is sealed by a sliding block 17 supported on a coil spring 17a which seals the block against the piston and thereby seals the pressure space 16 beneath the piston.

In accordance with the concepts of the present invention, the lubricating oil which is delivered to support the piston and to feed oil to the pockets 12 and lubricate the shoe is heated to a relatively high temperature. The press is intended to be operated at a temperature on the order of 400°F, that is, the web will be at 400°F in the press nip. For this purpose, the lubricating fluid should be heated to a temperature at least 450°F to heat the roll shell and thereby heat the traveling paper web. It is contemplated that in certain runs, the lubricating oil may be heated substantially higher on the order of 750°F to 800°F.

An important concept of the invention is the improved heat transfer to the moisture of the web to improve press drying. One concept is illustrated in Fig. 4 wherein an extended nip press is employed increasing the time of contact between the press roll and the web to increase the heat transfer between the roll shell and the web.

In Fig. 4, a web W is shown delivered to a press section which has a press roll shell 21 with an opposing press member 22. The press member 22 has an extended hydrodynamic shoe 30 shaped to build a hydrodynamic layer of liquid in the nip. Means (not shown) are employed to deliver a lubricating layer of hydrodynamic liquid to the nip. A rubber belt 31 is driven through the nip supporting a web supporting felt 23.

The roll shell is internally supported by a stationary shaft 24 supported by a framework at its ends. The shaft is provided with a hydraulic liquid supported piston 27 in a well 28 in the shaft. The piston supports a shoe 25 having lubricating pockets 26 therein facing the inner surface of the roll shell. The pockets are supplied with pressurized oil either through separate passages or through passages extending up through the piston 27 in a manner described in connection with Fig. 2.

An important feature of the invention is the delivery of a high temperature heated lubricant from a heated lubricant supply means 29. The lubricant will be delivered to the internal of the roll shell in a temperature sufficient to reach 400°F of the web in the nip N. For this purpose, the lubricant should be heated at least to 450° and higher temperatures are contemplated on the order of 700°F to 800°F.

To augment the heat transfer and help in the preparation of the web, a steam applicator 33 is positioned to receive the web as it enters the press. The web at this point is supported by the felt 23 which is supported opposite the steam chamber 33 by a support 34. Steam is directed down into the web to increase the temperature and increase the moisture at the surface. The web is then carried on the felt 23 looped over felt roll 6 and around a roll 32 which brings the web into contact with the roll shell 21 and lays the web on the surface at a location substantially in advance of the nip end. The web should be placed on the roll at least 30° in advance of the nip but a greater amount is possible depending on the type of paper being processed. The contact between the web and the heated roll shell 21 for the amount of travel before it reaches the nip N substantially increases the temperature of the moisture in the web to enhance the pressing process. When an extended nip press is used, such as shown in Fig. 4, the moisture in the web continues to be increased in

temperature throughout the pressing operation thereby further enhancing the pressing of the water from the web into the felt 23.

The object is to bring the web being pressed up to at least 400°F temperature. Problems have been experienced in heating the web to lower temperatures between 200°F and 300°F. A number of web materials tend to soften and stick solidly to the roll shell at these lower temperatures so that the temperature of the web is a very critical problem. 400°F has been found to be a good starting range for high temperature pressing in that this temperature is above the softening type temperatures.

With the prewrapping arrangement shown in Fig. 4, the prewrapping will bring the sheet temperature up past the 300°F mark, on the order of 350°F as the web enters the nip and heats further to reach the 400°F or over 400°F temperature.

Fig. 5 illustrates a press arrangement where a press roll shell 35 is employed and the opposing press member in this case is another press roll 36. The web is presteamed by a steaming chamber 38 and the web is carried on a felt beneath the steaming shoe. A roll 38a applies the web to the roll shell 35 prewrapping it so that heat transfer begins immediately and the web reaches the desired temperature by the time it reaches the nip N. The roll shell is heated by an internal lubricating fluid supplied by a heater 39. The internal support for the roll shell 35 is similar to that as shown in Fig. 4 and need not be described in detail.

Figs. 6 and 7 illustrate a modified form wherein increased heat transfer to the web is achieved by keeping the web wrapped on a press roll in a travel from a first nip N-l to a second nip N-2. The first press N-l is a light pressure press which removes water but also insures

that the web is in solid contact with the press roll shell

41 to travel around the roll shell prior to reaching the second nip N-2. Nip pressures for the nip N-l on the order of 100 pli are acceptable, although a range of 100 to 200 pli may be employed.

In the second press nip, higher pressures are used on the order of 400 to 800 pli.

Referring in greater detail to the arrangement of Fig. 6, a web W is formed on a forming wire 40 and transferred at the transfer roll 45 onto a felt 46. the felt carries the web into the first nip N-l where it is pasted onto the outer surface of the heated roll shell.

The roll shell is internally supported on a stationary shaft 42. Controlled deflection of the roll shell for the first nip N-l is achieved by a shoe 44 within the roll shell and the shoe is supplied with heated pressurized oil from a heater 43.

The roll shell is supported opposite the second nip N-2 by a hydraulic shoe 44a which is supplied with heated pressurized oil from a heater 43a. The heated oil will heat the roll shell on the order of 450°F.

Following the second nip N-2, the web is separated from the roll shell and passes down onto the first drum 49 of a dryer. The water pressed from the web in the second nip is transferred to a felt 47 supported on a press roll 48.

In the arrangement of Fig. 7, the web is similarly carried on a roll shell 50 between two nips N-l and N-2. A single felt lies outside of the web in its travel on the roll shell.

A web W is formed on a forming wire 57 and picked off the wire by the felt 55 with the felt being carried on a press roll 54. The web travels from the first nip to the second nip N-2 where it is pressed by a press roll 55a transferring water to the felt 55. The web is separated to pass down into the first roll 56 of a dryer section.

With the web traveling in contact with the roll shell for the distance between the two press nips, the web will approach the temperature of the roll shell by the time it hits the second nip. With the longer contact with the roll shell, the roll can be carried at 400°F to bring the web to that temperature.

The arrangements illustrated in Figs. 7 do not have the presteaming arrangement but that could be employed in this arrangement as well. By the presteaming, the speed of getting the sheet up to temperature is increased and steam is penetrated into the paper sheet as soon as possible. While moisture is added, it provides a deep penetration of the sheet. In some arrangements, the presteaming principally may be used without the prewrapping and in other arrangements, the prewrapping used without the presteaming. Depending upon the type of paper being used and the speed of operation, it is preferred that both be used to insure the desired temperature of the web in the press nip.

With this type of arrangement, the typical dryness of the web coming into the press is on the order of 20% to 40%. With effective pressing such as accomplished with an extended nip press, the dryness can approach in the range of 60% and sometimes as high as 75%. With heavier weight paper, the dryness decreases and with heavier weight paper, it becomes desirable to utilize a larger degree of prewrap to get the temperature of the sheet to the desired degree by the time the sheet enters the press nip.

All of this is accomplished by heating the lubricant within the roll shell to maintain the roll shell at a good heat transfer temperature. There, of course, will be some heat loss going through the shell since there is a substantial heat transfer to the moisture of the web.

It is desirable to maintain a very high temperature for the lubricants inside the shell, but care must be taken so that the lubricating properties do not break down such as can occur at temperatures over 800°F. Also, arrangements have to be provided to protect bearings which are located between the stationary shaft and the roll shell in installations.

With the arrangement illustrated, because of the ability to increase the transfer of heat to the web prior to entry into the nip, practical commercial speeds can be attained for pressing. Presses capable of operating on the order of 5000 feet per minute can be utilized and higher speeds can be achieved.

Thus, it will be seen that I have provided an improved pressing arrangement which meets the objectives and advantages above set forth and provides for an improved method and apparatus for hot pressing of paper web for improved mechanical dewatering.