EP0565923B1 - Mixer for mixing two fluids, producing a constant volume flow, for feeding the headbox of a paper machine - Google Patents

Abstract

The invention concerns a mixing system for mixing two liquids at the inlet to the headbox of a paper machine with: an inlet line (A) for the first partial volume flow (a); an inlet line (B) for the second partial volume flow (b); an outlet line (C) for the mixture volume flow (c) with the flow resistance (W); a mixing angle ( alpha ) with the inlet line (A) and the inlet line (B); a main flow angle ( beta ) between the inlet line (A) and outlet line (C); a valve (S) installed in the inlet line (B) for control of the partial volume flow (b). The mixing angle ( alpha ) is selected so that the mixture volume flow (c) remains constant, independent of the partial volume flow (b).

Description

The invention relates to a mixing device for mixing two liquids at a constant mixture volume flow to supply the headbox of a paper machine.

It is known that when mixing two volume flows A and B, where A is unregulated and B is regulated, a mixture volume flow is established, the size of which is generally dependent on the mixing ratio A to B. In some technical processes, e.g. in papermaking, however, it is desirable or necessary to obtain a constant mixture volume flow irrespective of the mixing ratio of the partial volume flows A and B. This can be achieved by expensive and complex control technology.

A mixing device according to the preamble of claim 1 is known from DE-PS 40 05 281 (Fig.3).

It is proposed there to axially introduce dilution water into the expanded pressure connection of a connecting line to the headbox. In the explanations it is described that the dilution water is to be introduced into the expanded pressure connection of a connecting pipe located on the distributor. In the main claim of the patent there is even talk of that separate, central distributor in addition to the fiber suspension to supply dilution water. Both proposals assume that the direction of flow of the dilution component runs axially to the connecting line, since otherwise the dilution component does not get into the connecting line or only to a small extent. The inlet and outlet pressure of the lines are constant. The only actuator for changing the partial volume flow ratio is in the dilution water line.

This entails the following problems: Since the speeds of both partial volume flows at the mixing point have the same direction, but usually a different amount, energy is transferred from one partial volume flow to the other. The pulse set can be used to demonstrate that this leads to mutual acceleration or deceleration of the partial volume flows. Jet pumps use this effect to pump liquids or gases. If there is a flow resistance in the line following the mixing point - e.g. a throttle -, the effect of mutual acceleration or deceleration is weakened because the partial volume flows are displaced by each other before the flow resistance.

Experiments have shown that with a pressure loss at the flow resistance that is still acceptable for practical use, the acceleration of the main flow through the dilution component is so great with a proportion of the dilution component of 20% that the mixture volume flow sum of the main flow and Dilution component - increases by approximately 1%. If the proportion of the dilution component is increased to values of 50% and more, which may be necessary especially in the edge area of the headbox, the mixture volume flow change is greater than 8%. This means that a fundamental problem of such a mixing device is that the mixture volume flow changes greatly in relation to the amount metered in.

Furthermore, a mixing device for achieving the same purpose and is known from the German utility model application G 91 04 609. Here, a mixing device is presented which also serves to mix several partial volume flows in such a way that a constant mixture volume flow is created.

For this purpose, all partial volume flows are regulated depending on each other by using a complex valve control. This leads to the disadvantages that on the one hand such a valve is very complex to design and manufacture, and on the other hand that all volume flows have to be regulated. This means that a valve is also installed in the partial volume flow charged with a high fiber concentration, with all the negative effects that result, such as fiber wiping and a tendency to clog.

In addition, the parallel connection of the actuator valves requires an extremely linear behavior in order to be able to keep the mixture volume flow constant regardless of the partial volume flow ratio. The demand for good linearity requires either valves with a high pressure drop or cost-intensive control measures.

A state-of-the-art concept is to section the headbox over the working width and to supply the individual sections with suspensions of different material densities. As the material density of a section increases, the basis weight of the paper web increases at this point and vice versa.

Since the fiber orientation of the paper web is a function of the angle at which the jet emerges from the headbox, the fiber orientation can be influenced in a targeted manner by changing the headbox geometry, for example in the form of changes in geometry at the exit gap. Geometric changes to the headbox influence the to different degrees, depending on the point of action Amount of suspension released from the headbox in the associated section. The consequence of this is that, in the above-described conception, the basis weight at the point of engagement of the paper web also changes unintentionally when there is an intervention in the fiber orientation profile.

Practical experience and theoretical considerations with regard to the hydraulic conditions in the headbox, as well as with regard to the sheet formation mechanism in the wire section, clearly show that interventions in the fiber orientation cross profile have to be carried out far less frequently than those in the basis weight cross profile. The illustrated one-sided coupling between fiber orientation and basis weight is therefore of minor importance in the practical use of the concept explained.

The variation of the material densities in the individual sections can be achieved by assigning a mixer to each section, in which two partial volume flows of different material density are mixed with one another, and the mixture volume flow is fed exclusively to the corresponding section of the headbox. An indispensable prerequisite for not changing the fiber orientation of the section when changing the consistency is the absolute constancy of the mixture volume flow irrespective of the partial volume flow ratio set on the mixer.

If adjacent mixture volume flows are not always the same size when the material density changes, this leads to compensating flows transverse to the main flow direction in the headbox and thus to deviations of the jet exit angle from the machine direction. Since there is a direct relationship between the jet angle and the orientation of the fiber in the paper web, the amounts of the individual mixture volume flows must be absolutely the same and constant over the entire headbox width be, even if changes in the consistency of the individual sections are brought about.

Another concept for influencing the fiber orientation and the cross-sectional area profile provides for the mixture volume flow and the consistency to be changed in a locally restricted manner. The effect of the mixture volume flow change on fiber orientation is based on the relationships described above. The weight per unit area is adjusted by changing the consistency, whereby in this concept, too, the demand for absolute consistency of the mixture volume flow in the case of changes in consistency remains, so that changes in consistency do not influence the fiber orientation profile at the same time. A valve in the mixture volume flow can be installed as an actuator for fiber orientation adjustment.

The required constancy of the mixture volume flows of the individual sections when changing the partial volume flow ratios will not be satisfactorily resolvable, even with considerable control engineering effort, since the running time of the basis weight measurement signals is too long to keep the basis weight constant at the prevailing frequency of the basis weight change.

The invention has for its object to design a structurally simple, inexpensive and reliable mixing device such that the mixture volume flow c, regardless of the size of the partial volume flow b, remains constant around the basis weight and fiber orientation cross profile of a paper web largely independent of one another and locally narrowly limited influence and to avoid the above-mentioned disadvantages of the prior art.

This object is achieved by the features characterized in claim 1.

Accordingly, an essential idea of the invention is to combine two opposing effects of fluid mechanics so that the sum of two partial volume flows a and b entering a mixer remains constant regardless of the ratio of the partial volume flows to one another and with a low pressure drop across the mixer.

If the partial volume flows a and b are brought together at an angle .alpha. = 90.degree. And an angle .beta. = 180.degree. In the mixer, kinetic energy is transferred from one stream to the other in the direction of the mixed volume flow and the interrupted drawing shown in FIG. 1 is obtained Curve I.

With increasing partial volume flow, the mixture volume flow c decreases, which is due to the increase in turbulence at the mixing point. This corresponds to the negative effect.

If the partial volume flows a and b are combined under the condition α = 0 ° and β = 180 °, a Venturi effect arises which essentially leads to an increase in the mixture volume flow C with increasing partial volume flow b. This corresponds to the positive effect, which is shown in Fig. 1, curve II.

The inventors have now recognized that a combination of both effects can be achieved by skillfully selecting the angles α and β such that the decrease in the mixture volume flow due to turbulence at the mixing point is exactly compensated for by the Venturi effect. I.e. the same mixture volume flows are always obtained regardless of the partial volume flow ratio.

The solid curve III in FIG. 1 shows the relationships measured on a real mixer. Turbulence and Venturi effect have the same effect on the mixture volume flow with a suitable choice of the angle over a large operating range, as shown in FIG. 1.

Since the flow velocities of the partial volume flows influence the turbulence at the mixing point, the angle of the equilibrium state is a function of the mixer geometry.

A prerequisite for the constancy of the mixture volume flow is the existence of the flow resistance W in the course of the discharge line C and further that the inlet pressure of the partial volume flow a in which no actuator is located and the outlet pressure of the mixer is kept constant.

In summary, the idea according to the invention is to make the energy exchange between the partial volume flows at the mixing point leading to acceleration or deceleration so large by suitable choice of the angle of the partial volume flows to one another and the pipe diameter that the mixture volume flow is always constant regardless of the partial volume flow ratio. The fact that the inlet pressure of a partial volumetric flow and the outlet pressure of the mixture volumetric flow must be constant does not constitute a restriction for the operation of the mixing device on a paper machine, since steady pressure fields are always sought in the distribution system before the headbox and in the headbox in order to ensure constant paper properties .

• 1. Die erfindungsgemäße Mischeinrichtung ist konstruktiv einfach zu verwirklichen, insbesondere dadurch, daß an das Stellglied, z.B. ein Regelventil, keine besondere Anforderungen nach Linearität gestellt sind.
• 2. Aufgrund der konstruktiv einfachen Gestaltung und des geringen regelungstechnischen Aufwandes ergibt sich eine erhebliche Kosteneinsparung in bezug auf Anschaffungskosten und Betriebskosten.
• 3. Da keine Linearitätsforderungen an das Stellglied gestellt sind, kann wenn nötig kompromißlos in Richtung Vermeidung von Faserwischbildung konstruiert werden.
• 4. Bedingt durch die Einsparung eines Stellgliedes und konstruktiv einfache Gestaltung verringert sich die Störanfälligkeit erheblich und die Betriebssicherheit steigt.
• 5. Es muß in dem Teilvolumenstrom mit der höheren Stoffdichte kein Stellglied installiert werden muß, da die Faserwischbildungsgefahr hier, gegenüber dem Teilvolumenstrom mit geringerer Stoffdichte, deutlich größer ist.
• 6. Der Druckabfall an der Mischeinrichtung ist gegenüber herkömmlichen Lösungen deutlich geringer, wodurch Pumpen mit geringerer Druckleistung eingesetzt werden können, was wiederum zur Kostenreduktion führt.

The advantages achieved by the invention are:

• 1. The mixing device according to the invention is structurally simple to implement, in particular in that the actuator, for example a control valve, has no special requirements for linearity.
• 2. Because of the structurally simple design and the low expenditure on control technology, there is considerable cost savings in terms of acquisition costs and operating costs.
• 3. Since there are no linearity requirements on the actuator, it can be designed without compromise in the direction of avoiding fiber wiping if necessary.
• 4. Due to the saving of an actuator and a structurally simple design, the susceptibility to faults is reduced considerably and operational reliability increases.
• 5. No actuator has to be installed in the partial volume flow with the higher material density, since the risk of fiber wiping here is significantly greater than the partial volume flow with a lower material density.
• 6. The pressure drop at the mixing device is significantly lower compared to conventional solutions, which means that pumps with a lower pressure output can be used, which in turn leads to a reduction in costs.

The existing task can therefore be solved by using a single valve which is specially adapted to the properties of the fiber suspension and which controls a partial volume flow of low material density.

The design of the supply and discharge lines described can take any cross-sectional shape.

Fig. 1:

Versuchsergebnisse der erfindungsgemäßen Vorrichtung.

Fig. 2:

Mischeinrichtung in Verbindung mit einem Stellglied und einem Strömungswiderstand.

Fig. 3:

Mischeinrichtung mit einem zusätzlichen Stellglied in der Ableitung.

Fig. 4:

Mischeinrichtung mit zwei Stellgliedern und zwei Strömungsviderständen.

Fig. 5:

Mischeinrichtung in räumlicher Ausführung.

The invention is explained in more detail with reference to the drawings. It also shows the following:

Fig. 1:

Test results of the device according to the invention.

Fig. 2:

Mixing device in connection with an actuator and a flow resistance.

Fig. 3:

Mixing device with an additional actuator in the discharge line.

Fig. 4:

Mixing device with two actuators and two flow resistors.

Fig. 5:

Mixing device in spatial execution.

1 shows the measurement results of a mixing device corresponding to that shown in FIG. 2. The partial volume flow ratio a / b is plotted on the abscissa, and the mixture volume flow c is shown on the ordinate. Curves I, II and III represent the test results. I shows the results at a mixing angle of 90 °. II shows the test results at a mixing angle of α = 0 °. III shows the results at an optimal mixing angle of 80 °.

Fig. 2 shows a mixing device according to claim 2. A rectilinear feed line A is shown, which merges into the discharge line C at an angle of β = 180 °. At the meeting point between supply line A and supply line C, supply line B, also a straight supply line, is added at a mixing angle. An actuator S is installed in the feed line B and regulates the size of the partial volume flow b. The partial volume flow b is led through the actuator via the supply line B to the mixing chamber M, in that the partial volume flow a coming through the supply line A meets the partial volume flow b and is discharged as a mixture volume flow c through the discharge line C. A flow resistance W is further stylized in the derivation C, which contains a necessary prerequisite for the functioning of the mixing device.

Fig. 3 shows a mixing device as described under Fig. 1, however, in addition to the built-in the partial volume flow control device S₁ there is a further control device S₂, which is installed in the course of the derivative C following the flow resistance W.

Fig. 4 shows a mixing device, as described in Fig. 3, but in addition, in the derivative C built-in resistor W₁ a second flow resistance W₂ is installed in the course of the feed line A.

Fig. 5 shows a mixing device similar to Fig. 2, but this time the lines are not in one plane, but are spatially arranged. Part a of FIG. 5 shows a top view, in which the angle γ, the angle between A and C is shown, and part b shows the mixing device in a side view.

Claims (11)

1. A mixer for mixing two fluids at the feed to the headbox of a paper machine, comprising:

   a delivery pipe (A) for the first part-volume flow (a);

   a delivery pipe (B) for the second part-volume flow (b);

   a discharge pipe (C) for the mixed volume flow (c) with flow resistance (W);

   a mixing angle (α) with delivery pipe (A) and delivery pipe (B);

   a main flow angle (β) between the delivery pipe (A) and the discharge pipe (C);

   a control element (S) which is built into the delivery pipe (B) for controlling the part-volume flow (b),

   characterized in that

   the mixing angle (α) is chosen so that the mixed volume flow (c) remains constant independently from the part-volume flow (b).
2. A mixer according to claim 1, characterized in that the mixing angle (α) lies in the region of 0°≤α≤90°.
3. A mixer according to claim 1, characterized in that a separate flow resistance (W) for the mixed volume flow (c) is built into the path of the discharge (C).
4. A mixer according to claim 1 to 3, characterized in that the main flow angle (β) is 180°.
5. A mixer according to claim 1 to 4, characterized in that the pipes (A), (B) and (C) are not in one plane, thus establishing an additional space angle (γ).
6. A mixer according to claim 1 to 5, characterized in that the angles (α) and/or (β) andlor (γ) are changeable during operation.
7. A mixer according to claim 1 to 6, characterized in that a control element (S₂) for regulating the mixed volume flow is provided in the discharge pipe (C).
8. A mixer according to claim 1 to 7, characterized in that the flow resistance (W) is established by a headbox.
9. A mixer according to claims 1 to 8, characterized in that the delivery pipe (A) is provided with a second flow resistance (W2).
10. A mixer according to claim 1 to 9, characterized in that the flow resistances in the delivery and discharge pipes are variable.
11. A mixer according to claim 1 to 10, characterized in that at least one of the pipes A, B and/or C have a variable narrowing in the region of the mixing area.

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