Paper making and fiber liberation – Apparatus – Combined
Reexamination Certificate
2000-10-17
2002-04-16
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Apparatus
Combined
C162S201000
Reexamination Certificate
active
06372094
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The object of the present invention is a pulp drying line and a method for drying pulp, as defined in the preambles of the independent claims presented below.
It is previously known to dry chemical pulp by means of drying cylinders, in which case a conventional web with a dry matter content of 45-50% is formed from the pulp coming from the headbox on the wire and press section, the said web being finally dried to the desired dryness on the cylinder dryer section. It is, however, difficult to break up this pulp again in connection with paper manufacture.
To remedy the problem, it is proposed in Finnish patent number 58020 that the pulp web coming from the wire and press section be dried to a dry matter content of only approximately 60-70% in the conventional manner with cylinders, after which the actual final drying is proposed to be carried out by means of hot gas blown through the pulp. This means that the conventional pre-dried pulp web coming from the cylinders will have to be shredded and defibrated to form a loose pulp which allows through-blowing. Of the loose pulp thus obtained a porous air-permeable pulp web is formed between two horizontal wires, and the said web is finally dried. Shredding and defibration of the viscous pre-dried pulp web into a fine loose pulp is not, however, always easy.
In the case of FI 58020, final drying takes place gradually, so that in the first part of the dryer section hot gas is blown through the pulp web from the blow box above the web to the suction box below the web, from where the exhaust air is conducted, via a heater, to the blow box adjacent to the suction box below the web and second in order, from which air is blown further up through the web to the suction box second in order above the web. From the second suction box the air is blown further in a corresponding manner through a third pair of blow/suction boxes, etc. At the end of the dryer section, the exhaust air is taken from the last suction box back to the beginning of the dryer section through an air duct having a length corresponding to the entire dryer section. In this case, therefore, the air circulation is very long, since the entire drying air flow passes first in a wave-like fashion through the long dryer section and then finally back to the beginning of the dryer section. Any changes in the air flow at the beginning of the air circulation will affect air flow throughout the entire dryer section. The system does not allow for local adjustment of air blowing.
The pulp drying solution described above, in which pre-drying is carried out by means of drying cylinders and final drying in a horizontal dryer section, requires a conventional web to be formed of the pulp at two separate times—first, before cylinder drying, and second, before fan drying. The drying line with its cylinders and horizontal final dryer section is long and thus takes up a lot of space.
The aim of the present invention is to achieve an improved drying line and method for drying pulp.
A particular aim in this case is to achieve a drying line construction which takes up less space and is more economical as regards its construction costs.
A further aim is to achieve a method by means of which still relatively moist pulp coming from the pulp press can be dried efficiently without conventional pre-drying by means of drying cylinders.
To achieve the above aims, the method, apparatus and chemical pulp drying line relating to the invention are characterised by what is defined in the characterising part of the independent claims presented below.
A typical pulp drying line relating to the invention for drying chemical pulp with a dry matter content of less than 3% comprises a pulp press, a defibrator, a web forming section and the actual dryer section. The pulp press used may, for example, be a roll press in which the pulp is pressed in a nip between two rolls to a dry matter content of about 30-50%, typically 40-50%. From the pulp press the pulp is taken, for example, by means of a screw conveyor, to the defibrator. According to a preferred solution of the invention, the pulp is taken directly from the press to the defibrator without being formed into a web in the meantime, which thus saves the cost of web forming at this stage. The pulp can be conveyed by means of the screw conveyor, which means that the pulp can also be dewatered at this stage.
The pulp pre-dried in the defibrator is defibrated to form a loose pulp suitable for web formation. The loose pulp is taken to the web forming section, where the pulp is formed into an air-permeable pulp web on a forward-running air-permeable forming base, typically a wire. The loose pulp flakes and/or fibres are spread by spreading means on the forming base, to form an air-permeable layer.
In the actual dryer section, which comprises a forward-running air-permeable support fabric, such as a wire, which supports the pulp web as it passes through the dryer section, and blowing means or the like, by means of which drying air or gas is blown through the pulp web in order to dry it.
A typical chemical pulp drying line relating to the invention comprises, in the actual dryer section,
blow boxes, through-blowing cylinders such as drying drums, or other similar means for blowing drying air through the pulp web to be dried, and
suction boxes, suction cylinders or air-removal means for removing the air that has passed through the pulp web and absorbed moisture into it from the vicinity of the pulp web.
In a first solution relating to the invention, blow boxes and suction boxes are fitted in the dryer section to form narrow vertical drying gaps extending through the dryer section in cross-web direction. The gaps are only about 30-100 mm, preferably 50-70 mm, wide in the machine direction. The drying gaps are delimited on one side by blow boxes fitted across the machine, and on the other by one or more suction boxes extending across the machine.
The dryer section preferably incorporates several drying gaps in succession, in which case the pulp web to be dried is arranged to pass through the successive drying gaps alternately upwards and downwards. At the same time, hot drying air is blown from a blow box on the first side of the web to the opposite suction box on its other side. The drying air is supplied to the blow boxes from fan towers adjacent to the actual dryer section. A pressure difference of typically about 200-800 Pa is arranged across the pulp web, due to which the drying air flows through the web.
While passing through the web, the drying air cools down and becomes wet. From the suction box the cooled air containing moisture is taken to the heater in the fan tower, from where the air, once heated, is taken back to the blow box on the first side of the web. Some of the wet cooled air coming from the suction box is replaced by fresh dry air.
In this first solution relating to the invention, the drying air circulation has been arranged so as to be local, and is thus very short. Drying air circulation takes place by means of fan towers integrated into the dryer section. The fan towers supply drying air to the blow boxes from single vertical compressed air chambers connected to the ends of the blow boxes and situated adjacent to the machine. A compressed air chamber is preferably arranged to supply drying air to two separate blow box units fitted in succession in the machine direction and comprised of blow boxes placed on top of one another.
The fan towers collect the return air from the suction boxes into a return air chamber connected to the ends of the suction boxes and fitted adjacent to the dryer section, thus maintaining underpressure in the suction boxes. One return air chamber is preferably arranged to collect return air from two separate suction boxes fitted in succession in the machine direction.
In this first dryer section relating to the invention, several pressure chambers and vacuum return air chambers are typically arranged alternately and in succession on both sides of the machine. The pressure chambers and return a
Heikkilä Pertti
Sairanen Paavo
Chin Peter
Hug Eric
Metso Paper Inc.
Nixon & Vanderhye P.C.
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