Double-felted Press

Abstract

A double-felted press for a papermaking machine or the like comprises a pair of press rolls arranged to form a nip and an endless press felt trained around a segment of each of the rolls, the press felts passing through the nip. One of the rolls is a suction roll, and the suction zone is located entirely on the downstream side of the roll, relative to the direction of rotation of the roll, from the nip, is contiguous or in proximate relation to the nip, and is within the segment of the roll over which the felt for that roll is trained. A guide roll for the felt associated with the other press roll (the nonsection-roll is normally positioned such that the felt is separated from the web being pressed before the web reaches the suction zone of the suction roll but is arranged to be moved to a position training that felt around the suction zone of the suction roll so that the two felts turn around the suction zone of the suction roll and ensure that the web runs with the suction-roll felt. Normally, the two felts are trained to run together over the suction zone of the suction roll only upon startup of the machine. BACKGROUND OF THE INVENTION This invention relates to a double-felted press of the type used in papermaking machines. A roll-type press in which each of two press rolls forming a nip between them runs within a felt loop is usually known as a double-felted press. Heretofore, double-felted presses have been used primarily in papermaking machines that operate at relatively low speeds, generally at speeds not greater than 200 meters per minute. A double-felted press usually provides greater dewatering capacity than a single-felted press inasmuch as, among other reasons, the felts on both sides of the web receive liquid from the web, and the effective distance that the liquid must flow across the thickness of the web to a felt is reduced. Because of the lesser distance (about half the thickness of the web), the resistance to liquid flow is correspondingly less, and therefore there is less chance that the hydraulic pressure developed in the press nip will exceed the shearing strength of the web and destroy the web structure by phenomenon known as "crushing." This permits a relatively high press loading in a double-felted press. In double-felted presses of the designs proposed and used heretofore, there is a tendency for liquids expressed from the web to return to it near the end of the press nip and just after the web leaves the press nip. To a relatively large extent, some of the advantage of the double-felted press is often lost through rewetting of the web in this way. Another disadvantage of presently known double-felted presses arises from the tendency for the felts associated with the respective press rolls to leave the nip with different moisture contents. Usually, the felts travel away from the press nip along divergent paths, and the web is carried on the felt that is the wetter of the two because of the higher surface tension forces of the wetter felt. Consequently, there is a substantial rewetting of the web, the rewetting tendency being higher, of course, with the wetter felt than it would be if the web followed the drier felt. With a papermaking machine that operates at a relatively low speed, say below 200 meters per minute, the web can sometimes be made to follow the drier felt by manually guiding it over to the drier felt upon startup, and thereafter the web will follow the drier felt. On high-speed papermaking machines, however, the web will not remain on the drier felt, even if it may be initially guided onto the drier felt by some means, but will return to the wetter felt in operation. It appears that the ability to operate a machine with the web following the drier felt is restricted to speeds less than 200 meters per minute, though the precise breakover point will vary somewhat depending upon the basis weight of the web and upon the dryness of the felts as they leave the press nip. One expedient for keeping the web on the drier felt at relatively higher speeds is to use a suction roll as one of the two press rolls, the suction roll having a suction zone extending through the nip and somewhat downstream from the end of the nip, the suction roll being the roll associated with the drier felt. The suction in the suction roll holds the web on the drier felt as it leaves the nip, so that the web sill thereafter run with the drier felt. Such an arrangement is, however, not entirely effective inasmuch as the benefit of having the web run with the drier felt is somewhat offset by the rewetting of the web by reason of water being sucked from the wetter felt by the action of the suction roll before the wetter felt is separated from the web. SUMMARY OF THE INVENTION There is provided, in accordance with the invention, a novel and improved double-felted press that is arranged so that the web will follow a desired one of the two felts, usually the drier felt, with a minimum of rewetting from the other felt, i.e., the wetter felt. The arrangement offers the advantage of ensuring that the web will follow the desired felt, even at relatively high machine speeds. More particularly, a double-felted press, according to the invention, comprises a pair of press rolls arranged to form a press nip and an endless press felt associated with each of the rolls, the two press felts, of course, being looped around the respective rolls and passing through the press nip. The term "press felt" is used herein in the broader sense of any suitable endless, flexible, permeable web, including fabrics made of various materials, such as textile fibers, plastic fibers or filaments, or metallic wire. One of the two press rolls is a suction roll in which the suction zone is located, in accordance with the invention, in a position entirely outside of the press nip but in proximate or contiguous relation to the downstream end of the press nip and within the portion of the suction roll over which the press felt associated with it is trained. By locating the suction zone entirely outside of the press nip, the tendency for the suction to rewet the web by sucking liquid from the felt associated with the other roll of the press roll pair is substantially eliminated. On the other hand, the suction zone is located in a position where it is effective to hold the web on the felt associated with the suction roll, which will usually be the felt which, in the particular machine, had the higher dryness of the two felts of the press. Further, in accordance with the invention, provision is made in the press for running both felts over the suction zone of the suction roll to ensure that the web, during the so-called "tail-threading" upon startup of the machine, is made to follow the felt associated with the suction roll. In other words, the web is guided in a sandwich between the two felts over the suction roll. In particular, the felt associated with the nonsuction roll of the roll pair is led from the nip to a roll that can be adjusted from a normal position in which the felt diverges from the felt associated with the suction roll upon leaving the nip and a position in which that felt is trained with the suction roll felt for some distance from the nip, such distance including the suction zone of the suction roll. Thus, the operation of tail-threading is accomplished with the two felts running together over the suction zone with the web between them to ensure that the tail of the web follows the felt associated with the suction roll at the outset. After the machine is brought up to operating conditions, the adjustable roll is shifted back to the normal position so that the felt associated with the nonsuction roll of the roll pair diverges from the nip and leaves the web supported or carried on the suction roll felt as the suction roll felt traverses the suction zone.

Description

DESCRIPTION OF THE DRAWING

For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the FIGURE of the accompanying drawing, which is a side view, in generally schematic form, of a portion of the press of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENT

The double-felted press shown in the drawing comprises two endless felts 1 and 3, each of which is arranged to travel in an endless path over guide rolls 5. The drawing illustrates only portions of the two felts, and those skilled in the art will readily understand that the felts travel along an appropriate path provided by additional guide rolls that are not shown in the drawing. Each of the press felts 1 and 3 will, of course, have cleaning and drying apparatus of any suitable type, also as well known to those skilled in the art, so that the felts return to the press nip in a relatively clean and dry state and capable of adsorbing liquid pressed from the web in the press nip.

The felts 1 and 3 pass together through a nip defined between a pair of press rolls 7 and 9, respectively, the roll 9 being a suction press roll. The fibrous web, designated by the reference numeral 13, to be pressed is led through the nip 11 between the two felts 1 and 3, and is subjected to the squeezing action of a nip pressure exerted by urging the rolls 7 and 9 toward each other in a suitable manner, thereby to express liquid from the web 13. Although the web 13 is shown as being delivered in an open draw, that is, unsupported by any felt or other belt, it will be understood by those skilled in the art that the web may be carried to the press on one or the other of the felts 1 and 3.

As illustrated in the drawing, the two felts 1 and 3 diverge from each other at the downstream end of the nip and are conducted along separate paths away from the nip 11, the separation point being indicated by the reference numeral 19. The web is carried from the nip on the bottom felt 3, and a short distance downstream from the separation point 19 the web 13 comes under the influence of the suction zone 15 of the suction press roll 9. As described above, the suction zone 15 is located entirely outside of the nip 11 and downstream from the separation point 19 where the press felt 1 is separated from the web. Thus, the upstream end 17 of the suction zone is illustrated in the drawing to be downstream from the separation point 19. It would be appropriate, nonetheless, for the upstream end 17 of the suction box to be placed slightly closer or perhaps in contiguous relation to the separation point 19.

With the above-described location of the suction zone, the influence of suction generated in the suction zone 15 is not such as to draw liquid from the press felt 1 with the consequence of rewetting the web as it is carried on the felt 3. However, the suction is effective to hold the web 13 on the bottom felt 3 so that the web will thereafter follow it. The felt 3, with the web 13 carried on it, may be conducted from the press to another press, to a drier, or to other components of the machine. Preferably, the suction roll is associated with the felt that is the drier of the two felts in the operation of the particular equipment, as may be designed for in the design of the machine, thereby to minimize the rewetting effect of the felt on the web as the web is carried on the felt.

In starting up the papermaking machine with which the press of the invention is associated, it is important to lead the so-called "tail" of the web along the desired path of travel in which it is to be carried from the press on the felt 3. To this end, a movable guide roll 21 is associated with the felt 1. The movable guide roll 21 is mounted, in any suitable manner, many of which will be readily apparent to those skilled in the art, so that it can be shifted between (l) a normal position, designated by the letter B in the drawing, in which the felt 1 is guided away along the normal, divergent path to the guide roll 5, and (2) a startup position, designated by the letter A in the drawing, in which the felt 1 is guided along a path in which it runs conjointly with the press felt 3 past the suction zone to a point downstream from the suction zone 15. To accommodate the alternative, startup position A of the movable roll 21, another roll (not shown) over which the press felt is guided is similarly mounted for adjustment, and the mechanism associated with the roll 21 may also operate such other adjustable rolls. Alternatively, a stretch or tension roll associated with the felt 1 may be so arranged as to accommodate the alternative position A of the adjustable roll 21.

It should be apparent that the roll 5 next to the roll 21 within the felt 1 could be eliminated, leaving only the roll 21 on the outgoing side of the nip.

With the adjustable roll 21 in position A, the web 13 is guided in a positive and certain manner in "sandwiched" relation between the press felts 1 and 3, to ensure that it will run with the felt 3 after the so-called tail-threading operation, and at any point prior to bring the machine up to full operating speed, the movable or adjustable guide roll 21 is shifted back to its normal position B. Thereafter, the web, which remains under the influence of the suction generated in the suction box 15 of the suction press roll 9, is held by the suction on the felt 3 and runs with the felt 3 along the path of travel from the press roll.

Claims

I claim:

1. A double-felted press for a papermaking machine or the like comprising:

a first rotatable roll;

a first endless press felt trained around a segment of the first press roll and movable with the press roll surface;

a second rotatable press roll positioned to form a nip with the first roll at a portion thereof along the said segment;

a second endless press felt trained around a segment of the second press roll that includes the nip between the press rolls, the second felt being movable with the roll surface;

the first roll being a suction roll and having a suction zone that (a) is located solely on the downstream part of the roll from the nip (relative to the direction of rotation of the roll), (b) is wholly outside of the nip, (c) is contiguous or in proximate relation to the nip, and (d) is within the said segment of the roll over which the first press felt is trained; and

a felt guide roll (a) associated with the second press felt, (b) located downstream from the nip (relative to the direction of felt movement) and (c) positioned and mounted for movement between a first position in which the second felt diverges from the first felt upon leaving the nip, or at least before reaching the suction zone of the first roll, and a second position in which the second felt runs with the first felt upon leaving the nip and traverses, with the first felt, the suction zone of the first roll.