Dryer section

Details Dryer section Dryer section

2001-02-20

2003-02-25

Lazarus, Ira S. (Department: 3749)

Drying and gas or vapor contact with solids

Process

Gas or vapor contact with treated material

C034S456000, C034S458000, C034S092000, C034S094000

Reexamination Certificate

active

06523278

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a dryer section, comprising at least one dryer group that makes use of single-wire draw and/or twin-wire draw and at least one suction roll with which the web to be dried is in direct contact.
BACKGROUND OF THE INVENTION
As is known from the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires, which press the web, one from above and the other one from below, against heated cylinder faces. Between the rows of drying cylinders, which are usually horizonal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, of low strength. This is why, in recent years, ever increasing use has been made of said single-wire draw, in which, in each group of drying cylinders, there is one drying wire only, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, and on the reversing cylinders or rolls between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside the wire loop.
When paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried by means of normal groups with single-wire draw from the side of its lower face, and if such asymmetric drying is extended over the entire length of a forward dryer section, the drying takes place so that first the side of the bottom face of the paper web is dried, and, when the drying makes progress, the drying effect also spreads to the side of the top face of the paper web. Thus, the dried paper is, as a rule, curled so that it becomes concave when viewed from above. In view of controlling the tendency of curling of paper, dryer sections have been used in which there are inverted dryer groups.
With respect to the prior art related to the control of curl, reference is made to FI Patent Application 971301, in which a method is described in the dryer section of a paper or board machine for the control of the curl of paper. In the method, steam treatment and/or moistening of the paper web is employed, and the operations carried out in order to control the curl of the paper web are carried out in a number of stages and/or when the temperature of the web is lower than 85° C., preferably lower than 75° C., and/or when the dry solids content of the web is in a range k
1
. . . k
2
, wherein k
1
=the ultimate dry solids content−7%, and k
2
=the ultimate dry solids content+3%. In said patent application, a paper or board machine is described which comprises at least a headbox, a former, a press, and a dryer section, in which steam boxes or moistening devices are employed for the control of the curl. There are at least two curl regulation devices in view of providing a curl regulation taking place in a number of stages.
The paper web can also be dried in dryer sections partly or fully based on impingement drying, and with respect to the prior art related to such dryers, reference is made to FI Patent Application 971713, in which a dryer unit and a dryer section that makes use of the unit are described. In the dryer unit, there is a drying-wire loop, and the dryer unit comprises a large-diameter impingement-drying and/or through-drying cylinder, which has been fitted in the interior of said drying-wire loop, and on the top of which cylinder/in the vicinity of which cylinder, at both sides of the cylinder, smooth-faced heated contact drying cylinders have been fitted, whose diameter is D
2
<D
1
. The impingement-drying and/or through-drying cylinder has been placed in spaces primarily below the floor level of the paper machine hall and is provided with a blow hood that can be opened and closed, so that removal of broke out of connection with said hood takes place primarily by the force of gravity. The central axes of the heated contact drying cylinders placed in the vicinity of, and at both sides of, the impingement-drying and/or through-drying cylinder have been placed in the vicinity of, or above, the floor level of the paper machine hall, and the wrapping sector b of the paper web to be dried on the outside face of said drying wire over said impingement-drying and/or through-drying cylinder is b>180°.
With respect to the prior art related to impingement drying, reference is also made to FI Patent Applications 963733 and 951746 (U.S. patent application Ser. No. 08/945224).
In the FI Patent Application 963733, a dryer section of a paper machine is described, which comprises drying modules, in which there is a drying-wire loop guided by guide rolls. The drying modules comprise a large-diameter impingement-drying and/or through-drying cylinder, which has been fitted inside the drying-wire loop. The outer mantle of the impingement-drying and/or through-drying cylinder is provided with grooves and/or it is permeable to the drying gas, and on the sector of contact with the drying wire a drying hood has been fitted, in whose interior, in the vicinity of the outer face of the web to be dried, there is a nozzle field or equivalent, through which a set of drying gas jets can be applied against the free outer face of the web to be dried. In the dryer section, a number of successive drying modules are employed, which have been fitted above the run of the paper web so as to apply drying energy to the paper web primarily from the side of its top face, and of the total drying energy of the dryer section, the major part, about 60 . . . 100%, is applied to the paper web by means of said drying modules from the side of the top face of the paper web to be dried.
In the FI Patent Application 951746, a dryer section concept for a paper/board machine is described, in which the dryer section comprises a number of groups of drying cylinders, which groups are exclusively provided with single-wire draw, on whose support the web is guided, meandering in loop shape, from a suction cylinder onto a drying cylinder and from the drying cylinder onto a second suction cylinder and from it further onto a second drying cylinder and further in the group. At least some of the drying cylinders comprise impingement drying units or equivalent in their connection, through which units a heated medium, preferably air or steam, is passed through the wire into contact with the web to produce a two-sided drying effect and to increase the drying capacity. The impingement drying units have been fitted in the end of the dryer section in such an area of dry solids content of the web that curl of the web can be affected and prevented by means of impingement drying.
With respect to impingement drying, reference is made further to FI Patent 100,013.
With respect to the prior art, reference is also made to FI Patent Application 960925, in which paper machine dryer sections provided with intermediate calendering are described.
Further, reference is made to a prior-art dryer section, in which the suction roll is provided with a drying-wire circulation of its own and which is described in the U.S. Pat. No. 3,816,941.
With respect to the prior art, reference can also be made to the U.S. Pat. No. 3,345,757, in which a dryer section is described in which the web runs around a drying cylinder and there is a blow hood on the cylinder, in connection with which hood an air-conditioning felt guide roll has been fitted.
From the JP publication 5-222692, a solution is known in which heatable suction rolls are used, against which the web is in direct contact. In this cited paper, suction rolls are also described which have a common contact and vacuum area in which a vacuum is applied to the web in the sector that is covered by the paper web. In addition to this, in the paper, impingement drying applied against said suction rolls is described.

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