GB2201173A

GB2201173A - Headbox flow control system

Info

Publication number: GB2201173A
Application number: GB08803306A
Authority: GB
Inventors: Dr-Ing Hans Dahl; Dr Elmer Weisshuhn
Original assignee: Sulzer Escher Wyss GmbH; Escher Wyss GmbH
Current assignee: Sulzer Escher Wyss GmbH
Priority date: 1987-02-23
Filing date: 1988-02-12
Publication date: 1988-08-24
Anticipated expiration: 2008-02-12
Also published as: AT391719B; FI880290L; GB8803306D0; SE8800607A0; FI880290A0; GB2201173B; SE8800607D0; ATA344987A; DE3715329A1; FI880290A7; CH671418A5

Description

1 zj 2 2M 117 3 MATERIAL FORWARD END DEVICE FOR A PAPER MACHINE The

invention relates to a material forward end device for a paper machine. It relates Particularly to such a forwardend device having a distributor through which a fibre suspension flows, and whose cross- section is reduced-ifi the direction of such flow.

-In material forward end devices of the above type, the end of the distributor with the greater cross-section is connected to an incoming fibre line, and the other end with the smaller cross-section is connected to a feedback line provided w ith a flow control device. The distributor is also provided with a guide device for distributing the suspension in the distributor which defines a discharge gap whereby the suspension is delivered over a certain web width on to the forming screen of the paper machine..

Forward end devices as described above are known from Swiss Patent No. 601-049, and its equivalent U.S. Patent No. 4 087 321. They are used for supplying a prepared fibre-suspension to a paper machine and to distribute it as evenly as possible over the whole web width.

In German Patent Specification Nos.-34 39 051.and 35

14 554 it is-proposed, inorder to produce an even-web of paper over the whole web width, to keep the hydrostatic pressure of the suspension in the distributor along its -longitudinal side constant, by measuring the hydrostatic pressure in th6 distributor at several positions ip.the direction.of the longitudinal side of the distributor and by providing several branch circuits equipped with control valves on one-longitudinal side of the distributor,,where the flow in the feedback line and the branch circuits is control - led by means of one of the control devices controlled by the measuring devices, so that there is an even hydr ostatic pressure along the distributor. An analogous effect canbe achieved, according to German Patent Specification No. 34 41 612, if a wall of the distributor is adjustable at several setting points, so that the cross-section of the distributor in the direction

2 of flow of the suspension and therefore the hydrostatic pressure in the distributor can be modified.

The above measurements are, however, not always sufficient to achieve evenness of all quality characteristics of the web of paper produced over the whole web width. In particular, in spite of keeping a constant hydrostatic pressure in the whole distribution space, asymmetry can develop in the web of paper produced, as a conseguence for example, of the fibres in the web being unevenly orientated in the suspension. other quality characteristics caused by flow conditions at the material forward end can also be inconsistent, in spite of constant pressure being. naintained over the distributor length.

The invention addresses the above difficulties, and seeks to provide a baterial forward end for a paper machine in which a more even web of paper can be produced over the whole web width. To this end, a material forward end device according to the invention comprises a distributor for receiving a film suspension from a feed source and deliver-ing such suspension to a forming screen, the cross-section of the distributor decreasing in the flow direction of the suspension from the source to a feedback connection having a flow control device; a nozzle disposed on a longitudinal side of the distributor and defining an outlet gap through which to discharge a said suspension onto a said screen; and means for taking measurements at at least two locations spaced along said longitudinal side of the distributor from which the local flow speed of the suspension through the gap can be determined, the measurement means being coupled to the flow control.device; and the control device being adjustable in response to speed differentials determined from measurements taken.

According to the invention, the local exit speed of the fibre suspension from the nozzle can be measured directly or indirectly at at least twoplaces. Thus, the variation of the discharge speed-is determined over the 1; 3 length of the outlet gap, and the control device adjusted in accordance therewith to a given value, depending on the -process requirements.

The discharge speed of the suspension can be measured directly or indirectly-by measuring the pressure and using a suitable conversion, eg that of BERNOULLI. The pressure can best be determined at at least two places over the web width and the pressure differential which can be calculated-used as the-control-parameter for the control device. The hydrostatic pressure can preferably be measured in the nozz le space or the total pressure can be measured at or just downstream of the outlet gap of the nozzle with a suitable flow measurement equipment. A suitable pressure sensor may be dispose d with a pressure sensitive surface oriented in the flow direction of the suspension. Typical sensors are piezo-electric pressure transducers, membrane pressure measuring devices, pressure head measuring devices, Pitot tubes, and non-contact-jet speed measuring-devices including those with lasers.

Surprising, we have found that by measuring the pressure in the nozzle space instead of the usual pressure measurement in the distributor,-one can achieve better evenness of the-fibre suspension emerging from the outlet gap of the nozzle space over the width of the paper machine.

The invention will now be described by way of example and with reference to the accompanying schematic drawings wherein...

-Figure 1 shows side view of a material forward end device according to the invention; Figure 2 shows one alternative arrangement of sensors; and Figure - 3 shows another alternative arrangement of sensors.

The material forward-end device shown in Figure 1 has a distributor i, which consists of an incoming line 2 from 4 a source, a conical - part 3 with a cross-section which is reduced in the direction of flow S and a feedback line 4. A guide device 5 for the liquid material is connected to a longitudinal side of the conical part 3 of the distributor, which guide device has a large number of channels 6. As described in US 3 725.197, for example, the channels 6 may widen in steps or in another suitable way. The channels 6 of the guide device 5 open into a nozzle space 7, which has two lips 8. The lips 8 form an outlet gap for fibre suspension between two side plates 9. Usually at least one of the lips 8 is adjustable to vary the width of the gap, and the side plates 9 can also be movable if desired.

A pressure sensor 10 is provided inside the nozzle space at at least two positions as shown over the web width, forsexample at or over the side plates 9. The sensors can be made as piezo-electric pressure transducers or as membrane pressure sensors, for example. The pressure sensitive surface of the pressure sensors 10 is best orientated in the direction of flow of the suspension, so that these pressure sensors record the hydrostatic part of the suspension pressure.

The two pressure sensors 10 are each connected to one input of a control mechanism 11, the output of which controls a flow control device 12 of the feedback line, depending on the measured differential pressure of the two pressure sensors iO. The control device 11 can have a different circuit at its input, ie a differential amplifier-which is made as a charge amplifier if the pressure sensor 10 is made as a piezo-electric transducer.

The control of flow in the feedback line 4 by means of flow control device 12 occurs so that the differential pressure on the two side parts of nozzle space 7 measured by the two pressure sensors 10 is about zero, or has a given value. In this way, it is possible, even in cases where there are pressure differences in the distributor for various reasons, to achieve optimum outflow conditions of the fibre suspension in the nozzle space and th!refore 2j, 1 1 9 a more even quality of the web of paper over the width.

Instead of two or more pressure sensors in a differential circuit, distributed_over the web width, a suitable differential pressure sensor can be provided at suitable place in the nozzle space, which determines the measured differential pressure or the pressure gradient along the web width and passes it on to the control device.

Instead of pressure sensors to measure the ydrostatic pressure of the flowing suspension, pressure head measurement devices can Also be used, which determine the total pressure, including the hydrodynamic part. Figure 2 shows a simple version_ in which a Pitot tube 13 is provided on both side& in the nozzle space 7, which is connected to a common differential pressure measurement device 14, which controls the flow control device.

Figure 3 shows an arrangement in which one Pitot tube 15 provided directly in front or downstream of the outlet gap 8 at either side. Each Pitot tube is connected to a transducer which initiates the common control device. Instead of Pitot tubes, jet speed measuring devices working without contact can al so be used, which determine the jet sp eed over the web width, which is connected with the hydrodynamic pressure according to the formula:

p = dV2 2 (where p = pressure,- d density, v - speed), ie which also supplies a value equivalent to the pressure when required. These jet speed measuring devices can work_ without contact and without any effect on flow, using laser technology, for example.

-1

Claims

CLAIMS:

1. A material forward end device for paper machine comprising a distributor for receiving a film suspension from a feed source and delivering such suspension to a forming screen, the cross-section of the distributor decreasing in the flow direction of the suspension from the source to a feedback connection having a flow control device; a nozzle disposed on a longitudinal side of the distributor and defining an outlet gap through which to discharge a said suspension onto a said screen; and means for taking measurements at at least two locations spaced along said longitudinal side of the distributor from which the local flow speed of the suspension through the gap can be determined, the measurement means being coupled to the flow control device; and the control device being adjustable in response to speed differentials determined from measurements taken.

2. A device according to Claim 1 wherein said longitudinal side of the distributor extends substantially parallel to the flow direction from the source, and wherein the nozzle is oriented to discharge a said suspension therefrom in a direction substantially perpendicular to said longitudinal side.

3. A device according to Claim 1 or Claim 2 wherein the nozzle is adj ustable to discharge a said suspension over a range of widths onto a said screen.

4. A device according to any preceding Claim wherein the nozzle is adjustable to vary the width of the outlet gap.

5. A device according to any preceding Claim including a control mechanism for receiving said measurements and instructing the flow control device.

6. A device according to Claim 5 wherein the control device comprises a difference circuit for determining said speed differential from said measurements.

7. A device according to any preceding Claim including a differential amplifier coupled to the flow control device.

8. A device according to any preceding Claim wherein the measurement means comprises pressure sensors.

i X 11

9. A device according to Claim 8 wherein each pressure sensor has a pressure sensitive surface oriented in the flow direction of a said suspension in the nozzle.

10. A devic- e according to Claim 8 or Claim 9 wherein each pressure sensor comprises a piezo-electric transducer.

11. A device according to Claim 8 or Claim 9 wherein each pressure sensor is a membrane pressure measuring device.

12. A device according to Claim 8 or Claim 9 wherein each pressure sensor is a pressure head measuring device.

13. A device according to Claim 8 wherein each pressure sensor is a non-contact jet speed measuring device. __

14. A.deviCe according to Claim 13 wherein the measuring devices have a laser.

15. A device according to any preceding Claim wherein the measurement means are located in the nozzle.

16. A device according to Claims 15 wherein the measurement means are located in the outlet gap.

17. A device according to any of Claims 1 to 14 wherein the measurement means are located proximate the nozzle in the path of a said suspension discharged from the outlet gap.

18. A material forward end device for a paper machine substantially as described herein with reference to the accompanying drawings.

19. A paper machine including a material forward end device accOrding to any 1 -preceding Claim.

1 published 1988 at The Patent Office, State House, 66,71 High Holborn, London WClR 4TP. Purther copies may be obtained from The Patent Offace, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.