Headbox and process for the metered addition of a fluid...

Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...

Reexamination Certificate

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C162S216000, C162S258000

Reexamination Certificate

active

06464837

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the metered addition of a fluid medium into a material suspension stream of a headbox in which the material suspension stream in the headbox is distributed across the machine width, fed into a plurality of turbulence-producing conduits, and supplied to the headbox nozzle. Furthermore, a total pressure loss &Dgr;P
total
is caused in the region of the turbulence-producing conduits, including at least one intake pressure loss &Dgr;P
intake
created at the entrance to the conduits followed by a graduated pressure loss &Dgr;P
grad
created at a graduated pressure loss area defined by at least one change in cross-section inside the conduits. Furthermore, the metered addition of a fluid medium occurs in the region of the conduits, between the conduit entrance and the conduit exit.
The invention also relates to a headbox for a paper or cardboard machine having at least one material suspension supply running the width of the machine, at least one turbulence producer having a plurality of conduits through which the material suspension flows, a plurality of metered addition points for a fluid medium in the region of at least one of the conduits and an attached headbox nozzle.
2. Discussion of Background Information
DE 35 14 554 A1, describes a conventional headbox design, wherein
FIG. 6
shows a headbox having a suspension supply across the width of the machine and a turbulence producer having several conduits through which a material suspension flows. The conduits have a plurality of metered addition points for introducing a fluid medium into one part of the conduits. Moreover, a headbox nozzle is attached in the turbulence producer.
the German patent also discloses a corresponding process for the metered addition of a fluid medium into a material suspension stream of a headbox in which the material suspension stream distributed across the machine width utilizes a plurality of turbulence-producing conduits which are fed by these to the headbox nozzle. In the region of the turbulence-producing conduits, a total pressure loss is produced that arises from at least the intake pressure loss in the conduits and the subsequent graduated pressure losses from cross-sectional change inside the conduits. The metered addition of the fluid medium occurs in a part of the conduits between the conduit entrance and the conduit exit.
In this conventional design, the main pressure loss of the turbulence-producing conduits occurs in the entrance region of the turbulence inserts, i.e., before the metered addition points of the fluid medium. As a result of this arrangement, changes in the flow rate through the conduits during the metered addition affect the basis weight cross profile and fiber orientation cross profile in a negative manner. One attempt to solve this problem is shown in
FIG. 6
of the application cited above, in which a fluid is sprayed into the lateral distributor of the headbox near the entrance opening of a turbulence tube, i.e., before the location of the main pressure loss. This design provides for a very small influence of the metered addition on the total flow rate of the turbulence conduits, but it also has the disadvantage that no clear assignment of the added fluid to a certain turbulence conduit is given, with the working width of the metered addition also depending on the amount of the fluid added.
SUMMARY OF THE INVENTION
One object of the invention is to disclose a process for the metered addition of a fluid medium into a material suspension stream of a headbox in the region of the turbulence-producing conduits, in which the metered addition of the fluid medium has the least possible influence on the total flow rate of the respective conduit. It is also an object of the invention to disclose a headbox that is appropriate for performing the process according to the invention.
It is recognized that a fundamentally necessary pressure loss occurs in the conventional headbox designs at a turbulence insert having turbulence conduits and/or turbulence tubes chiefly by intake pressure loss at the transition between the lateral distributor of the headbox and the turbulence insert. Because only a low pressure gradient occurs in the part of the turbulence insert that is connected at the outlet side, metered fluid additions in this region have an intense and undesirable effect on the total flow rate of the respective conduit. The solution to this problem is to substantially reduce the intake pressure loss in relation to the pressure loss arising in the further course of the conduit, by an appropriate design of the turbulence conduit in the entrance region. This allows for the possibility of arranging the metered addition in the conduit at the location where the main pressure loss is placed behind the combination point of the added fluid stream with the main suspension stream inside the conduit. The influence, as a result of the amount of fluid in the metered addition which is added to the main stream, has only a very small effect on the total flow rate.
The invention provides for a process for adding a metered addition of a fluid medium into a material suspension stream of a headbox, in which the material suspension stream in the headbox is distributed across the machine width in a plurality of turbulence-producing conduits that supply the material suspension to the headbox nozzle, such that in the region of the turbulence-producing conduits a total pressure loss &Dgr;P
total
is caused which arises from at least one intake pressure loss &Dgr;P
intake
at the entrance to the conduits and which provides for a graduated pressure loss &Dgr;P
grad
by means of at least one cross-section change inside the conduits, in which the metered addition of the fluid medium occurs in the region of at least one of the conduits, between the conduit entrance and the conduit exit. The invention can ideally provide that at least approximately 50% of the total pressure loss &Dgr;P
total
is produced downstream of the metered addition of the fluid medium. Preferably, the portion of the pressure loss that occurs downstream of the metered addition of the fluid medium should amount to no more than approximately 75% of the total pressure loss &Dgr;P
total
.
One embodiment provides for approximately 50%-60% of the total pressure loss &Dgr;P
total
to be produced downstream of the metered addition of the fluid medium.
This design of the headbox allows the metered addition of fluids, e.g., dilution water, into the turbulence insert of a head box wile causing only the most minimal changes in the local total flow rate at the turbulence insert and, it also allows for a clearly defined width of effect of the metered addition, in the range of normal output quantity, which is essentially independent of the amount of the metered fluid.
According to the invention, it is advantageous for the total pressure loss &Dgr;P
total
to be in the range of approximately 0.3×10
5
Pa to 8×10
5
Pa, with a total pressure loss in the range of approximately 4×10
5
Pa to 8×10
5
Pa being preferred (Pa representing pascal pressure units). This relatively high total pressure loss allows for a further reduction in the influence of the local total volume stream in the case of different metered additions, for example.
It may also be advantageous for the intake pressure loss &Dgr;P
intake
to be kept low by a venturi cross-section design in the conduit since no separation zones to the inflow ensue.
A further possibility for influencing the pressure loss distribution is by using a wave contour located downstream of the metered addition of the fluid medium in the respective conduit in order to produce the pressure loss. By utilizing the plurality of waves in the wave contour, a high pressure loss can be produced in the narrowest space with a desired turbulence structure, i.e., turbulence with short wavelengths.
Another advantageous embodiment of the process provides for the metered addition of the fluid medium in at least one cond

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