Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2000-07-12
2002-07-16
Doerrler, William C. (Department: 3744)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S444000, C034S448000, C034S452000, C034S114000, C034S117000, C034S119000, C034S120000, C034S124000
Reexamination Certificate
active
06418639
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for a dryer section of a paper or board machine having a single-wire draw.
The present invention relates to a device in a dryer section of a paper or board machine having a single wire draw.
BACKGROUND OF THE INVENTION
As is known from the prior art, in multi-cylinder dryers in paper machines, twinwire 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 horizontal 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 each group of drying cylinders has 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 placed 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.
It is known from experience that, if 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 the 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 is also spread 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. From the point of view of runnability of the paper machine, however, a dryer section with full support over its entire length and based on normal groups with single-wire draw, without inverted groups, would be a particularly justified solution.
With respect to the prior art related to the present invention, reference is also made to the U.S. Pat. No. 5,600,898, in which an arrangement related to the control of curling in the dryer section of a paper machine is described. It is a drawback in said arrangement that it does not permit the use of a runnability component operating with the principle of blowing, which is a definite requirement when running takes place with open wires at high speeds.
OBJECTS AND SUMMARY OF THE INVENTION
One of the objects of the present invention is to provide a solution which permits a dryer section based on normal groups with single-wire draw.
In the solution of method and solution of equipment in accordance with the present invention, in its commonest embodiment, the web support function and the web impingement drying function have been accomplished by means of the same solution of equipment, which comprises one continuous hood, i.e. box construction, for the device. In accordance with the invention, said equipment extends into the pocket space between the drying cylinders and the suction roll that operates as a reversing roll so that air is removed out of said pocket space and/or an ejection blowing is produced in said pocket space along the wire, in which case the web is affixed to the wire face by means of a vacuum. Since, favourably, wires of high permeability are employed, application of the vacuum to the web in this way is possible. In accordance with the invention, by means of the same solution of equipment extending into the pocket space, impingement drying is also carried out. Preferably air, favourably heated air, or steam is employed. Within the scope of the present invention, an embodiment is possible in which, through the impingement drying unit, a part of the impingement drying air is passed through the interior of the box construction to the end of the box construction and/or further as an ejection jet and/or as a closing jet and/or as an exhaust blow jet at the inlet and outlet side of the suction roll in the vicinity of the wire/web. Thus, in a preferred embodiment of the invention, the impingement drying hood extends onto the drying cylinder and further into the pocket space.
In the embodiments defined in the sub-claims of the invention, it is suggested that the device be employed in certain areas of the dryer section, for example in the areas of the dryer section in which the dry solids content of the web is higher than 60%. In such a case, impingement drying is employed in particular for prevention and control of the curl of the web.
The invention is characterized in what is stated in the patent claims.
The present invention is applied in a dryer section in which a normal single-wire draw is applied at least partially. The dryer section may also be such that it is provided with impingement drying units.
Out of the impingement drying units, hot air/steam is blown through the wire onto the paper. By means of such an impingement drying unit, a considerable increase in the evaporating capacity is achieved. In such a case, the evaporation takes place to an increasing extent from the wire side on the cylinder. Owing to the increased evaporating capacity, the dryer section can be made of shorter length, and in this way economies can be obtained in the cost of construction of the hall. Owing to the increased evaporating capacity, the concept of the present invention can also be applied to modernizations, in which the available space is often quite limited.
From the patent application JP 222 691/1993, a dryer section is known in which there is an impingement drying hood above all of the upper cylinders. Thus, it has been known from the prior art to enhance the evaporation taking place on a cylinder by ventilating the rear side of the wire or by blowing hot air partly through the drying wire.
By means of studies carried out on test devices, it has been established that the evaporating capacity that can be achieved depends highly extensively on the permeability of the wire. In order that an increase in the evaporating capacity could have economic significance, the permeability of the wire must be preferably in the range of 2000 . . . 20,000 m
3
/h/m
2
(cubic metres per hour per square metre), preferably 4000 . . . 10,000 m
3
/h/m
2
. The permeability, i.e. permeability to air, of a wire H is the flow rate as cubic metres of air per hour that passes through an area of a size of one square metre of a wire when the pressure difference across the wire is 100 Pa.
Formation of pressure in a closing nip is prevented so efficiently that no bag formation in the web takes place. One possible alternative solution is a runnability component, which prevents passing of air into a closing nip by means of suction and permits the use of an open wire, in which case evaporation from the top side of the web can be enhanced, for example, by means of solutions of the type of impingement drying hoods. It is possible to employ a suction box that fills the whole pocket at the suction roll, which suction box is provided with preventive blowings at the edges in order to prevent leakage of air into the pocket.
In the preferred case, the necessary portion of the dryer section or the whole dryer section consists of assemblies with the construction described above. It is a great advantage of this solution that there is no need for an inverted group, which is difficult in view of cleaning, and it is, nevertheless, possible to dry the web efficiently and even to regulate the drying capacity at the top side and bottom side of the web by means of the speed and temperature of the impingement drying air.
In the present invention, impingement drying hoods are preferably placed in connection with those cylinders only at which they provide a considerable effect either in the control of curl o
Juppi Kari
Kaihovirta Juha
Milosavljevic Nenad
Saarikivi Pekka
Virta Raimo
Doerrler William C.
Metso Paper Inc.
Shulman Mark
Steinberg & Raskin, P.C.
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