CONTROLLING THE TREATMENT OF FIBROUS MATERIAL
Description
The invention relates to a method for the open-loop or closed-loop control of an apparatus for treating fibrous material at least partially on the basis of its drive power, wherein the apparatus has a housing, in which at least one first treatment tool and one second treatment tool are arranged, the treatment tools are each fastened on a base plate, have a rotationally symmetrical shape, are arranged coaxially with respect to one another, rotate relative to one another about a common axis and delimit a treatment nip which is flowed through by the fibrous material, the nip width of which treatment nip can be changed via an axial displacement of at least one base plate of a treatment tool.
The invention likewise relates to an apparatus for treating fibrous material with a housing, in which at least one first treatment tool and one second treat- ment tool are arranged, the treatment tools are each fastened on a base plate, have a rotationally symmetrical shape, are arranged coaxially with respect to one another, rotate relative to one another about a common axis and delimit a treatment nip which is flowed through by the fibrous material and the nip width of which can be changed by means of an actuator via an axial displace- ment of at least one base plate of a treatment tool in order to carry out the method.
— Apparatuses of this kind are described in EP 0 406 225 A2 and WO 00/43590 Al, for example.
By virtue of the relatively high consistency of the fibrous material during treatment, intensive mechanical treatment is possible with apparatuses of this kind, although the treatment tools, which can be moved relative to one an- other, do not come into contact but instead move past one another with a small clearance.
Considerable forces arise during this process.
Apparatuses of the abovementioned kind are used, for example, to improve the quality of chemical pulp, TMP or fibrous material.
There is a well-established practice of grinding pulp fibres, i.e. fresh pulp and/or recycled paper fibres, to enable the desired properties, particularly in respect of strength, porosity, formation and surface quality, to be achieved in the web of fibrous material produced from them.
In the case of the refiners, dispersers and deflakers that are used in processing fibrous material, the treatment surfaces are formed by replaceable fittings screwed to the corresponding base plate on account of the relatively high wear.
To achieve the desired fibre properties, in particular freeness, and also to pre- vent excessive wear on the fittings, the grinding fittings must be optimally matched to the fibrous material to be treated.
Moreover, to increase efficiency in the treatment of fibres, the aim is to make optimum use of the available treatment surface.
US 2004 / 0112 997 Al as well as DE 2 939 587 Al and DE 3 602 833 Al disclose the practice of one-time measurement or calculation of the idling power before start-up and of using it as a basis for machine control.
In all cases, the efficiency of treatment decreases if the nip is too large.
If the nip is too small, on the other hand, there is the risk of excessive electric power consumption and contact between the treatment tools.
It is the object of the invention to allow safe and efficient operation of these apparatuses by the simplest possible means.
According to the invention, the object was achieved by the fact that the axial force which acts on the displaceable base plate is measured and the width of the treatment nip is open-loop or closed-loop controlled at least in a manner which is dependent on the drive power and axial force. Knowledge of the axial force makes it possible to make judgements on the flows between the treatment tools and on the treatment of the fibres in the treatment nip. This, in turn, makes it possible to influence the treatment and thus the parameters of the fibrous material after treatment in a more selective way. The flow of the fibrous material suspension in the treatment nip takes place substantially in a radial direction, although, because of the rotation, there is also a directional component in a rotational direction and, in the case of a conical arrangement of the treatment tools, a directional component in an axial direction. It is advantageous here if the width of the treatment nip is set in such a way that an optimum axial force corresponding to the fibrous material to be treated — results. For a certain fibrous material, the optimum axial force can be deter- mined during operation by changing the width of the nip, measuring the axial force and detecting the effects on the parameters of the fibrous material after treatment. Asan alternative, however, it is likewise possible, in setting the optimum axial force, to have recourse to empirical values with a similar fibrous material. It should also be taken into account here that the optimum axial force depends on the desired parameters after the treatment of the fibrous material. If the fibrous material is ground in the apparatus, a reduced nip width leads to intensified elongation of the fibrous material, and an increased nip width leads to intensified fibrillation of the fibrous material.
It is furthermore possible, during operation of the apparatus, to extrapolate a parameter change of the fibrous material in the case of a change in the ratio between the drive power and axial force.
The machine controller can then make the required adaptations on this basis.
Thus, in the case of a parameter change of the fibrous material, the nip width of the apparatus, in particular, should be reset.
In respect of the apparatus, it is essential that it has a measuring unit for de- tecting the axial force which acts on the displaceable base plate.
The invention is particularly suitable for apparatuses which are designed as refiners for grinding or as dispersers for dispersing the fibrous material.
In the case of special embodiments, the treatment tool and the base plate may also be formed integrally.
If the axial force is transmitted from the actuator via an axially displaceable shaft to the base plate, as is customary especially with dispersers, the measur- ing unit can be arranged on this shaft and/or between the actuator and the shaft and/or in the actuator.
If the axial force is transmitted substantially directly from the actuator to the base plate, as is known from refiners, the measuring unit can be arranged in the actuator and/or between the actuator and the base plate.
The actuators can be embodied on a hydraulic, pneumatic, electric or manual
— basis.
In the case of the measuring units too, it is possible to have recourse to known force measuring devices.
Here, the total drive power consumed by the apparatus is made up of the idling power and of the specific apparatus drive power applicable to the intended treatment intensity.
5 As the operating time of the respective treatment tools progresses and hence also the wear of said tools, in particular of the profile thereof, increases, the current idling power of the apparatus decreases.
Consequently, the total power consumption should be reduced accordingly.
In this context, changes in the axial force during operation provide useful in- formation on the state of wear of the treatment tools.
The invention will be explained in greater detail below with reference to an exemplary embodiment.
In the attached drawing, the figure shows a schematic cross section through a refiner.
The paper stock 1 is forced directly into the central, i.e. radially inner, region of the refiner fitting, which is formed by the two treatment tools 3, 4.
While one treatment tool 3 does not rotate and is thus designed as a stator, the
— other treatment tool 4 is rotatably mounted in the housing 2 of the refiner.
The treatment tools 3, 4 each have a rotationally symmetrical shape, wherein the two annular grinding surfaces are arranged parallel to one another, and the nip clearance between these surfaces can be set by means of an axial displace-
ment of the non-rotating treatment tool 3.
Here, the rotating grinding surface is moved in the direction of rotation by a shaft 10 rotatably mounted in the housing 2. This shaft 10 is driven by a drive, likewise present in the housing 2.
In the example shown, the fibre suspension 1 to be ground passes via an inlet, through the centre, into the grinding nip 6 between the grinding surfaces of the two treatment tools 3, 4. The fibre suspension 1 passes radially outwards through the interacting grind- ing surfaces and leaves the adjoining annular space through an outlet.
Both grinding surfaces are formed by a plurality of grinding plates, which each extend over a circumferential segment of the corresponding grinding sur- face.
When lined up side-by-side in the circumferential direction, the grinding plates give a continuous grinding surface.
The grinding plates and thus also the grinding surfaces are generally formed by a multiplicity of substantially radially extending grinding bars 9 and grooves situated in between.
Moreover, the treatment tools 3, 4 are fastened on corresponding base plates 7, 8. In contrast to the example shown here, the treatment nip 6 may extend not only perpendicularly but also, in the case of conical refiners, obliquely to the axis of rotation 5. The axial position of the non-rotating treatment tool 3 can be changed by means of an actuator 11, e.g. in the form of a screw jack element, with a cor- responding effect on the width of the treatment nip 6 and the pressure condi- — tions therein.
It is essential to the invention that the axial force F exerted directly on the displaceable base plate 7 by the actuator 11 is measured by a measuring unit 12, e.g. in the form of a load cell arranged between the actuator 11 and the — base plate 7, and the nip width between the treatment tools 3, 4 is closed-loop controlled at least in a manner which is dependent on the drive power and axial force F.
The intensity of the treatment of the fibrous material 1, i.e. the grinding power in this case, is determined substantially by the specific drive power of the apparatus.
However, the technological processes in the treatment nip 6 are influenced by way of the axial force F.
The width of the treatment nip 6 is therefore set in such a way that the result is an optimum axial force F corresponding to the fibrous material 1 to be treated and to the desired parameters of the fibrous material 1 after treatment.
For this purpose, the machine controller can measure the parameters of the fibrous material or of the web of fibrous material produced from the latter, preferably online, and change the axial force F until an optimum is established in the parameters.
If, after this optimization, there is subsequently a change in the ratio between the drive power of the apparatus and the axial force F, it is possible to extrap- — olate a change in the parameters of the fibrous material 1 fed in.
In this case, the width of the treatment nip 6 should be reset.
As an alternative, as likewise illustrated in the figure, it is also possible to make only the rotating base plate 4 axially displaceable.
In this case, the ac- — tuator 11 would act on the shaft 10 connected to the rotating base plate in order to change the width of the treatment nip 6. Here too, the measuring unit 12 could be arranged between the shaft 10 and the actuator 11.