Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02356804 2001-09-04
1007-374
VENTED LEAD BLADE
Field of the Invention
This invention concerns a vented lead blade for use in a
paper making machine.
Background of the Invention
In the initial portion of the forming section of either a
single or two fabric papermaking machine, an unsupported jet of
highly aqueous stock is ejected from the head box slice orito the
surface of a moving forming fabric. The unsupported jet will
typically traverse a distance of from about 6 cm to about 40 cm
before impinging the surface of the forming fabric at the point
of impingement. The angle of impingement, a, formed between the
stock jet and the plane of the forming fabric at the point of
impingement will typically be from about 40 to about 10 . It is
well known that improved paper formation can be obtained by
minimizing both the angle a and the length of the unsupported
free jet.
As the angle a iricreases, the magnitude of the pressure
exerted by the jet on the surface of the forming fabric also
increases.
For a forming fabric moving at a speed of 20 m/sec and a jet
of pure water, the impingement pressure I in kPa can be shown to
be given by the following relationship:
I = 200 sin a.
Typical peak impingement pressures for different values of the
angle a are shown in Table 1.
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TABLE 1: Peak Impingement Pressure
Anale of Impinqement, a Pressure (I, kPa)
1 3.49
2 6.98
4 13.95
6 20.90
8 27.83
34.73
Impingement angles greater than about 5 will create peak
impingement pressures that may cause sheet marking, low retention
of paper making fines and fillers, and plugging of the forming
fabric. Therefore, the angle a should be made as small as
possible so that, ideally, the unsupported stock jet impinges on
the fabric substantially tangentially.
As the length of the stock jet increases, its outside
surface begins to break up into ridges and furrows, which will
eventually cause sheet basis weight variations. Further, finely
dispersed fibers in the stock start to reflocculate rapidly in
the unsupported jet prior to the point of impingement.
Therefore, the unsupported stock jet should be made as short as
possible to minimize these effects.
Due to the competing space requirements of both the head box
slice lip structure and the adjacent upstream rolls such as a
breast roll, it is difficult to shorten the unsupported stock jet
length without increasing the angle a. Even if the head box
slice lips can be located so that the angle a is very small, and
the free jet is nearly tangential to the forming fabric, air that
is trapped in the small wedge shaped space between the surface of
the forming fabric and the surface of the unsupported stock jet
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becomes entrained into the stock forming bubbles which are
detrimental to sheet formation.
Several proposals have been made to overcome these
difficulties. Nelson et al, US 3,440,136 discloses a method of
avoiding air entrainment by evacuating the air from the forming
zone, and flooding this area with water. However this proposal
has been found to be difficult to realize in practice. Irwin et
al., US 4,734,164, disclose a forming board for a single fabric
machine in which the first blade is slightly curved to permit the
breast roll to be lowered slightly. The difficulty with this
proposal is that air is trapped in the shallow wedge space
between the jet and the forming fabric as it passes over the
curved first blade. This air is forced into the stock as bubbles
which cause formation defects. Malashenko, US 4,802,954,
discloses a lead-in blade located ahead of the curved blade
element proposed by Irwin et al., which is said to reduce the
amount of fluid pumped by the forming fabric into the wedge
shaped space between the jet and the fabric. However, a small
wedge shaped air space remains. The pressure in this space is
controlled by a vacuum pump to reduce the jet disturbance. Ewald
in US 5,084,138 addresses the problem of excessive free jet
length by using curved turning bars to replace large diameter
breast rolls and a solid curved blade, but does not avoid air
entrapment at the wedge between the jet and the fabric.
The present invention seeks to provide a lead blade for use
in the forming shoe or forming board of a high speed paper making
machine which will eliminate or at least substantially reduce all
of the aforementioned deficiencies of the prior art by means of
a construction and arrangement which at least in part adopts
these recommendations.
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Summary of the Invention
This inventiori seeks to provide a vented lead blade for use
downstream of the breast roll, between the breast roll and the
fabric supporting elements in the forming section, and located so
that the stock jet impinges the forming fabric at, or near to,
its trailing edge. The vented lead blade does two things.
First, it serves to bend the forming fabric before it enters the
forming section. Second, it serves to vent at least a
substantial proportion of any air which becomes trapped in the
wedge shaped space between the surface of the forming fabric and
the surface of the stock jet. The vented lead blade of this
invention thus enables the forming fabric to be positioned so
that the angle a can be minimized, and, if desired, by allowing
the breast roll to be repositioned to create space into which the
head box can be moved, thus reducing the unsupported lerigth of
the stock jet. The vented lead blade of this invention can be
used in both an open surface forming section with one forming
fabric, or in the forming zone of a two fabric paper making
machine.
Thus in a first broad embodiment this invention seeks to
provide a curved vented lead blade for use in a paper making
machine upstream of, and immediately adjacent to, the point of
impingement of a stock jet ejected from a head box slice onto a
moving forming fabric carrying an amount of liquid, which lead
blade has a leading edge, a shaped trailing edge portion,
including a trailing edge, and a convexly curved surface located
between the leading edge and the trailing edge portion, over
which the forming fabric moves iri sliding contact and wraps
through an angle of wrap e, the convexly curved surface and the
trailing edge portion including a plurality of grooves which
begin at the leading edge and end at the trailing edge
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constructed and arranged to vent at the lead blade trailing edge
at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet
and the forming fabric surface, and at least some of the liquid
carried by the forming fabric.
In a second broad embodiment this invention seeks to provide
a papermaking machine, having a machine direction and a cross
machine direction, which includes:
a forming fabric moving in the machine direction;
a head box including a head box slice which provides a jet
of paper making stock which impinges at an angle of impirigement
onto the forming fabric at a point of impingement;
a roll, having a cylindrical surface whose axis is in the
cross machine direction, about which the forming fabric passes,
and which is located upstream of the head box slice;
a forming sectiori, located downstream of the point of
impingement, including static support elements which define a
fabric path through which the forming fabric passes; and
a vented lead blade, located upstream of, and immediately
adjacent to, the point of impingement of a stock jet ejected from
a head box slice onto a moving forming fabric, about which a
forming fabric wraps through a wrap angle 6;
wherein the vented lead blade has a leading edge, a shaped
trailing edge portion including a trailing edge and a convexly
curved surface, located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap 6, the convexly curved
surface and the trailing edge portion including a plurality of
grooves which begin at the leading edge and end at the trailing
edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
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forming fabric from a wedge shaped space between the stock jet
and the lead blade curved surface, and at least some of the
liquid carried by the forming fabric.
In a third broad embodiment this invention seeks to provide
a papermaking machine, having a machine direction and a. cross
machine direction, which includes:
a first and a second forming fabric each moving in the
machine direction;
a head box including a head box slice which provides a jet
of paper making stock which impinges at an angle of impingement
onto at least one of the forming fabrics at a point of
impingement;
two rolls, each having a cylindrical surface whose axis is
in the cross machine direction, about each of which one of the
forming fabric passes, and which are located adjacent the head
box slice;
a forming section, located downstream of the point of
impingement, including static support elements which define a
fabric path through which the forming fabric passes; and
two vented lead blades, each located upstream of, and
immediately adjacent to, the point of impingement of the stock
jet, about the first of which the first forming fabric wraps
through a wrap angle 61, and about the second of which the second
forming fabric wraps through a wrap angle 02;
wherein each vented lead blade has a leading edge, a shaped
trailing edge portion including a trailing edge and a convexly
curved surface, located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap, the convexly curved
surface and the trailing edge portion including a plurality of
grooves which begin at the leading edge and end at the trailing
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edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet
and the lead blade curved surface, and at least some of the
liquid carried by the forming fabric.
Preferably, the convexly curved surface is essentially
circular, with a constant radius. More preferably, the shaped
trailing edge portion conforms to the convexly curved surface.
Alternatively, the convexly curved surface is not circular, and
the radius of curvature increases towards the trailing edge.
Conveniently, the convexly curved surface is not circular, the
radius of curvature increases towards the trailing edge, and the
shaped trailing edge portion is essentially flat.
Preferably, the convexly curved surface has an effective
radius of curvature that is from about 50% to about 100% of the
radius of the adjacent upstream roll. The radius of curvature
will not generally be greater than that of the adjacent upstream
roll.
Preferably, the angle 6, A1 or 82 through which the forming
fabric wraps the lead blade is from about 100 to about 40 . More
preferably, the angle of wrap is from about 15 to about 30 .
Most preferably, the angle of wrap is from about 20 to about 25 .
Preferably, in a paper making machine having two forming
fabrics, the two vented lead blades are the same, and their
convexly curved surfaces are the same shape. Alternatively, in
a paper making machine having two forming fabrics, the two vented
lead blades are not the same, and their convexly curved surfaces
are not the same shape.
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Preferably, in a paper making machine having two forming
fabrics, the two vented lead blades are the same, the angles 01
and 82 are the same, and their convexly curved surfaces are the
same shape. Alternatively, in a paper making machine having two
forming fabrics, the two vented lead blades are not the same, the
angles 81 and 82 are not the same, and their convexly curved
surfaces are not the same shape.
The invention will now be described with reference to the
attached schematic drawings in which:
Figure 1 shows part of ari open surface paper making machine
including a vented lead blade;
Figures 2 and 3 show parts of two different twin fabric
paper making machines;
Figures 4, 5 and 6 show the effects of wrap angle and vented
lead blade effective radius;
Figures 7, 8 and 9 show three different groove arrangements
for the vented lead blade top surface; and
Figure 10 shows four different groove shapes.
In Figures 1 - 6 only the parts of the paper making machine
required for an understanding of this invention are showri.
Figure 1 shows a paper making machine which includes a
vented lead blade according to the teachings of this invention.
The slice lips 1A and 1B of the head box deliver a jet of stock
2 onto the forming fabric 3 at the impingement point I. The
forming fabric 3 passes around the roll 6, wraps the vented lead
blade 5, and then passes over the first static support element 4
in the forming section. As the moving forming fabric 3 and the
stock jet 2 converge, air 8 in the wedge shaped space between the
surface of the stock jet 9, the surface 10 of the head box lip
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1B, and the surface of the forming fabric 11 becomes trapped and
enters the forming fabric 3. This air, together with some of the
liquid in the forming fabric at this point, passes through the
forming fabric 3 into the grooves 12 in the top surface of the
vented lead blade 5.
In this arrangement, since the axis 15 of the roll 6 is now
located out of the fabric path 7, and since the vented lead blade
occupies far less space than the roll, it is possible to
maintain the angle of impingement close to zero, and to shorten
the unsupported length D of the stock jet 2 significantly.
Further, since the trailing edge 14 of the vented lead blade 5 is
closely adjacent the impingement point I, the trapped air is
prevented from interfering in the formation process within the
stock in the forming section. The grooves 12 are located and
dimensioned so that at least a substantial proportion, and
preferably all, of the air passing through the forming fabric,
together with any liquid carried with the air from the liquid on
the forming fabric at this point, is vented as at 13 from the
vented lead blade at its trailing edge 14.
Figures 2 and 3 show a paper making machine which has two
forming fabrics, and which incorporates two vented lead blades
according to the teachings of this invention. In describing
these two Figures the terms "upper" and "lower" refer only to the
orientation shown: in practice in a twin fabric machine the
forming section is often oriented vertically, or at some angle
thereto.
Referring first t.o Figure 2, the slice lips 1A and 1B
deliver a jet of stock 2 onto the lower forming fabric 31 at the
impingement point I. The forming fabric 31 wraps the lower
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vented lead blade 51. As the moving forming fabric 31 and the
stock jet 2 converge, air 81 in the wedge shaped space between
the surface of the stock jet 91, the surface 101 of the head box
lip 1B, and the surface of the forming fabric 111 becomes trapped
and enters the forming fabric 31. This air 81, together with
some of the liquid in the forming fabric 31 at this point, passes
through the forming fabric 31. into the grooves 121 in the lower
vented lead blade 51. The accumulated air and liquid 131 is
vented from the lower vented lead blade 51 at its trailing edge
141. The construction as regards the lower forming fabric 51 is
thus essentially the same as Figure 1. Downstream of the
impingement point I a second forming fabric 32 converges with the
upper side 92 of the stock 2 on the lower forming fabric 31.
Further air is trapped in the space 82 between the lower face 112
of the upper forming fabric 32 and the surface 92 of the stock 2.
This additional air, and some of the liquid in the forming fabric
52, passes through the forming fabric 52 into the grooves 122 and
is vented at the trailing edge 142 of the upper vented lead blade
52.
Figure 3 shows an alternative arrangement to that of Figure
2. The arrangement of the lower forming fabric 31 is the same.
The upper forming fabric 32 converges with the stock jet 2 at the
same point as the lower forming fabric 31, more or less at the
impingement point I. For the upper forming fabric air is now
trapped in the space bounded by the upper surface 92 of the stock
jet, the lower surface 112 of the upper forming fabric :32, and
the upper surface 102 of the head box lip 1A. All of the trapped
air, together with some of the liquid in the two forming fabrics,
is vented through the grooves 121, 122 at the trailing edges 141,
142 of the two vented lead blades 51, 52.
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In both of these twin fabric machines, the two vented lead
blades will often be the same, and have the same convexly curved
shape. However in some circumstances it may be desirable to use
two different vented lead blades, which may have different
convexly curved shapes. It is also then possible that the two
wrap angles 61 and Z will likely be similar, they may not be the
same.
Figures 4, 5 and 6 show the relationship between the angle
6 through which the forming fabric 3 wraps about a vented lead
blade, the effective radius of curvature of the vented lead
blade, and the radius of the adjacent upstream roll. Each of
these Figures uses the same numbers as Figure 1, and thus
considers a single fabric machine. Similar considerations apply
to a twin fabric machine.
In each of these Figures, the convexly curved surface 16 of
the vented lead blade 5 has a constant radius, the forming fabric
3 is received tangentially from the adjacent upstream roll 6, and
the impingement point I is adjacent the trailing edge 14 of the
vented lead blade S. These three Figures are also shown to
essentially the same scale.
In Figure 4, the radius R4 of the vented lead blade surface
16 is the same as the radius R1 of the roll 6, which is some
distance from the head box slice lips 1A, 1B. The vented lead
blade 5 then has to be relatively wide in order to bend the
forming fabric 3 into the forming path 7. If R1 and R4 are both
approximately 46 cm, the angle of wrap 04 is about 18 . This
large radius also implies that the length D4 of the unsupported
stock jet 2 cannot be shortened very much.
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In Figure 5, the radius RS of the vented lead blade surface
16 is smaller than the radius R, of the roll 6, which is still
some distance from the head box slice lips 1A, 1B. The vented
lead blade 5 then still has to be relatively wide in order to
bend the forming fabric 3 into the forming path 7. If R1 is
approximately 46 cm, and R5 is approximately 23 cm, the angle of
wrap 6, is about 36". This smaller radius R. allows the length DS
of the unsupported stock jet 2 to be shortened.
In Figure 6, the radius R. of the vented lead blade surface
16 is smaller than the radius R, of the roll 6, which is now
closer to the head box slice lips lA, 1B. The vented lead blade
is much narrower in order to bend the forming fabric 3 irito the
forming path 7. If R, is approximately 46 cm, and R6 is
approximately 23 cm, the angle of wrap 66 is about 18 . This
smaller radius R. allows the length D. of the unsupported stock
jet 2 to be shortened still further.
It can thus be seen that the vented lead blade of this
invention provides considerable flexibility in the geometry of
the machine wet end.
The curved surface of the lead in blade is vented by means
of grooves which begin at the extreme upstream edge of the blade
and are continuous to its downstream edge. Three groove
arrangements are shown in Figures 7, 8 and 9. In each Figure,
the forming fabric moves in the direction of the arrow A. In
Figure 7, the grooves 17 are regularly spaced the same distance
apart, and are essentially parallel to the arrow A. The
disadvantage with this arrangement is that there is some risk of
the forming fabric tracking across the blade. This is avoided in
Figure 8, where although the grooves 18 are still regular and
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parallel, they are at an angle R to the arrow A. Alternat:ively,
two sets of angled grooves 18A, 18B can be used as shown in
Figure 9. Within each set, the grooves are parallel to each
other and regularly spaced, and each set is at an angle (3 to the
arrow A. Typically, the angle R in Figures 8 and 9 will be from
about 2 to about 30 .
The purpose of the grooves in the vented lead blade surface
is to provide spaces through which at least a substantial
proportion of the air is vented which enters the forming fabric
from the wedge shaped space essentially between the forming
fabric and the jet surface. At the same time, at least some of
the liquid in the forming fabric as it enters the forming section
will also be vented.
Figure 10 shows four possible groove structures for this
purpose. For simplicity, the blade surface 17 has been shown
flat; in practice it will be curved. Groove 19 is of constant
width for its entire length from the leading edge 17A to the
trailing edge 17B. As the fabric passes over the vented lead
blade surface 17, air is continuously entering the forming
fabric. It is thus often advantageous to use a groove with an
increasing cross section, such as the groove 20 which widens, or
the groove 21 which deepens, or the groove 22 which both widens
and deepens, in each case from the leading edge 17A to the
trailing edge 17B. Typically, the grooves 19, 20, 21 and 22 are
from 2 - 8 mm in width, and 1 - 8 mm in depth.
In operation, the forming fabric moves in sliding contact as
it wraps the vented lead blade. At the point of contact of the
forming fabric with the upstream leading edge of the lead blade,
for example at 17A in Figure 10, the fabric may form a small
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contact angle that is ideally 0.5 or less. It is preferred that
the forming fabric is tangential to the vented lead blade surface
at this point. The leading edge of the blade can be curved to
minimize any frictional effects at this point; a suitable edge
radius is from about 0.0025 mm to about 0.0065 mm. The forming
fabric should exit the vented lead blade tangentially to the
trailing edge portion of the blade surface, substantially in the
path defined by the downstream fabric support elements. The
downstream path of the forming fabric should be parallel to and
in the same plane as the underside of the stock jet, so that the
angle of impingement of the stock jet onto the forming fabric is
substantially zero.
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