Refining arrangement
The invention relates to a refining arrangement for refining aqueously suspended pulp between two refining surfaces, which form a refining gap and rotate relative to each other and are formed by refining strips and grooves extending there-
between, wherein the refining strips extend at least together with a directional com-
ponent radially with respect to the axis of rotation, and the pulp suspension flows through the refining gap from a radially inner, circular inlet to a radially outer,
circular outlet.
It has been known for a long time to refine pulp fibres, i.e. fresh pulp and/or recy- cled paper fibres, in order to be able to achieve the desired properties, in particular with regard to strength, formation and surface, in the fibrous web produced there- from.
In the refiners which are used in the process, the refining surfaces are formed from replaceable refining fittings, screwed to the corresponding supporting surface, be- cause of the relatively rapid wear.
To achieve the desired fibre properties, in particular the freeness, the refining fit- tings must be matched as well as possible to the pulp to be treated, also in order to prevent excessive wear of the fittings.
In addition, to increase the efficiency of the fibre treatment, the aim is optimal utilization of the available refining surface.
DE 10 2008 046592 A1 discloses a method for refining aqueously suspended pulp fibres, and also refining fittings for carrying out the method.
The method is used for refining aqueously suspended pulp fibres and uses a plurality of refining fittings
— provided with refining edges, of which one is operated on at least one stator and another on at least one rotor, and the suspension to be refined is processed between the two refining fittings.
A partial stream of the suspension conveyed towards the outlet side flows back on the rear side of the stator-side refining fitting and then reaches the refining zone again through openings which are located between the refining edges of the stator-side refining fitting.
This leads to particularly uniform and economical refining.
The object of the invention is to improve the efficiency of the fibre treatment.
According to the invention, the object has been achieved in that a plurality of return channels connecting the outlet to the inlet are arranged under at least one refining
— surface.
Returning the pulp suspension via the return channels leads to multiple treatment and therefore to homogeneous, more efficient refining.
Moreover, a lack of pulp suspension to be treated can be compensated for without difficulty by means of an appropriate return.
It is advantageous if some, preferably all, return channels have no connection to the refining gap.
Since all of the quantity of pulp suspension to be returned from the outlet to the inlet gets from the outlet to the inlet, non-uniform wear of the refining surface can be counteracted.
Furthermore, it may be advantageous to influence the quantity of pulp suspension flowing back in the return channel and therefore to influence the pressure buildup.
To this end, the return channels each have a narrowing of the flow cross section, which is dimensioned for the specific use.
Alternatively, however, the return chan-
— nels can also each have a valve influencing the throughput for active adjustment.
In order to prevent a short circuit flow via the return channels from the inlet to the outlet under unfavorable operating conditions, such as an excessive throughput or a negative pressure difference between the inlet and the outlet, the return channels
— can each have a nonreturn flap preventing a flow from the inlet to the outlet.
To assist the deflection of the pulp suspension flowing out of the return channel into the inlet area of the refining gap, the transition between the refining surface and the return channel extending under the same should extend in a curve at the outlet of the return channel, wherein the radius of curvature is preferably at least 4 mm. In addition, there should be flow barriers at the outlet, at least in front of the return channels, which extend at least with a directional component radially. Via these flow barriers, the rotation of the flow of the pulp suspension at the outlet is braked, so that the pulp suspension can flow more easily into the return channels. In the interests of simplified production of the return channels, it is advantageous if at least the refining surface having the return channels consists of multiple refin- ing sectors arranged beside one another in the circumferential direction, and some, preferably all, return channels extend between adjacent refining sectors. This means that the return channels are formed by the opposite side walls of the refining sectors. It also becomes simpler if only one refining surface rotates and the other refining surface is stationary. In this case, only the stationary refining surface should have return channels. The invention can be applied in flat refining surfaces which extend at right angles to the axis of rotation but also in conical refining surfaces in which the axis of rotation corresponds to the axis of the cone. The invention is to be explained in more detail below by using two exemplary embodiments. In the appended drawing: Figure 1: shows a schematic cross section through a disk refiner and Figure 2: through a conical refiner; Figure 3: shows a detail at the outlet 7 and Figure 4: a detail at the inlet 6 of the refiner;
Figure 5: shows a partial cross section through two adjacent refining sectors 11 and Figure 6: shows a longitudinal side of a refining sector 11. According to figure 1, a refining gap 1 is formed in the housing of the refining arrangement by a stationary refining surface that is coupled to the housing and a refining surface rotating about an axis of rotation 4, the refining surfaces extending at right angles to the axis of rotation 4. The two flat, annular refining surfaces extend parallel to each other.
The distance — between the refining surfaces is normally adjustable.
Here, the rotating refining surface is moved in the direction of rotation 12 by a shaft rotatably mounted in the housing.
This shaft is driven by a drive likewise present in the housing.
In the example shown here, the pulp suspension 5 to be refined gets into the refin- ing gap 1 between the two refining surfaces via a radially inner circular inlet 6. The pulp suspension 5 passes radially outward through the cooperating refining surfaces and leaves the following radially outer circular outlet 7. Not illustrated are the means known per se with which a force is produced in order to force the two refining surfaces against each other.
Both refining surfaces are each formed by multiple refining sectors 11 lined up beside one another in a row in the circumferential direction.
The refining surfaces are formed by a multiplicity of refining strips 2 and grooves 3 located therebetween extending at least with a directional component radially with respect to the axis of rotation 4. The cross section of the refining strips 2, also called knives, is generally rectangu- lar, but there are also other shapes.
The grooves 3 extending between the refining strips 2 likewise have a rectangular cross section and serve as flow channels for the pulp suspension 5. The groove depth is normally between 2 and 20 mm. 5 As distinct from this, the two opposite parallel refining surfaces are conical in fig- ure 2, the axis of rotation 4 coinciding with the axis of the cone.
Here, the rotating, radially inner refining surface is moved in the direction of rota- tion 12 by a shaft rotatably mounted in the housing.
This shaft is driven by a drive likewise present in the housing.
Here, too, the two refining surfaces are each formed by multiple refining sectors 11, which are supported on a supporting surface extending parallel to the refining gap 1 and are detachably connected thereto.
The cutting angle that is important to the refining results if the refining edges of the refining strips 2, which are located axially opposite the axis of rotation 4, are moved past one another during the rotation of the refining surface.
To maximize the cutting edge length, it is likewise important to keep the width of the grooves 3 at least predominantly constant.
It is important to the invention that here, as can be seen in particular in figure 6, multiple return channels 8 connecting the outlet 7 to the inlet 6 are arranged under — the stationary refining surface.
Via these return channels 8, at least some of the pulp suspension 5 can be returned from the outlet 7 to the inlet 6, so that the pulp suspension passes through the refining gap 1 once more.
In this way, the result is more efficient refining and a greater operating window as a result of the self-regu- lating pressure conditions, which lead to a high return flow being established in the event of a small throughput, and a low return flow in the event of a high throughput.
In this connection, it is also important that all the return channels 8 have no direct connection to the refining gap 1, so that the pulp suspension 5 is not led back into the refining gap 1 but entirely into the area of the inlet 6. This counteracts non- uniform wear of the refining surfaces.
In order to control the quantity of pulp suspension 5 led back in the return channels 8, a valve 9 can be arranged in each of these.
Instead of the valve 9, the return channels 8 can, however, also each have a narrowing of the flow cross section.
The cross section of the narrowing would then have to be adjusted to the specific use.
— Since, under unfavorable operating conditions, a short-circuit flow via the return channels 8 from the inlet 6 to the outlet 7 could occur, it may be advantageous, as an alternative to the valve 9 or else as an extension to the narrowing, to provide a nonreturn flap preventing a flow from the inlet 6 to the outlet 7 in each of the return channels 8.
A simple implementation of the return channels 8 results if, according to figures 4 and 5, these each extend between two refining sectors 11 that are adjacent in the circumferential direction.
This means that the longitudinal sides of the refining sectors 11 each form half of the refining surface of the return channels 8. Therefore,
the refining sectors 11 can also be cast simply, as hitherto.
Between the refining sectors 11, the refining strips 2 are interrupted, as illustrated in figure 3. This makes it easier for the pulp suspension 5 to flow into the refining gap 1.
— Because of the rotation of one refining surface, the pulp suspension 5 is also set rotating at the inlet 6 and at the outlet 7, which impairs the flow into and the flow out of the return channels 8. In order to brake this rotational flow, according to figure 3, flow barriers 10, which extend substantially radially, are therefore located at the outlet 7 in front of and beside the return channels 8.
For better deflection of the pulp suspension 5 flowing out of the return channels 8 into the inlet 6 of the refining arrangement, as indicated in figure 6, at the outlet of the return channel 8 the transition between the refining surface and the return chan- nel 8 extending under the same extends in a curve with a radius of curvature of at least 4 mm.