Note: Descriptions are shown in the official language in which they were submitted.
09200~
93-173
- MBTHOD IN CONTACT-FREF AIR-DRYING OF A M~FRT~T- WEB A8 ~ELL A8
A NOZZLE-BLO~-BOX AND A PULP DRYER THAT NA~E U8E OF THE METHOD
~- 5 B~ ROUND OF THE ~v~h.lON
:::
The invention relates to a method for air-drying material
~-~ webs, in particular material webs having a relatively high
~- grammage, such as pulp webs. In the method, air blowings are
applied in a direction substantially perpendicular to the web and 5
~10 in a direction substantially parallel to the plane of the web to
- be dried from underneath the web. The air blowings cause heat to
be transferred to the web, supported the web by air free of
contact, and stabilize the run of the web through the dryer.
~- ~ The invention also relates to a nozzle-blow-box of an air
. . . - ~~ ~ . .
- - 15 dryer through which box air blowings are applied to the material
- i ~
web to be dried. By means of the air blowings, both the transfer
.; of heat is produced from the drying air to the web and a contact-
free air support and stabilization of the run of the web are
obtained. The nozzle-blow-box comprises a box part having a
nozzle-carrier face placed against the web. A substantially V-
section groove is arranged transverse to the running direction of
the web in the middle of the nozzle-carrier face. The groove is
opened toward the web and has opposite walls in which a series of
nozzle holes are arranged. Support and stabilization air
- 25 blowings can be applied out of the series of nozzle holes such
that they are crosswise and of opposite directions in relation to
each other. Plane nozzle-carrier-face portions are placed in the
same plane with each other at both sides of the V-section groove
in the nozzle-carrier face.
The invention further relates to a pulp dryer that makes use
of the method of the invention and/or the nozzle-blow-box of the
invention.
In prior art through-dryers used in the paper and pulp
industry, blow boxes are commonly used w~ose nozzle-carrier face
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~ ~UY~IJV4
consists of a plane plate in which blow holes have been punched.
These nozzles are placed either at one side or at both sides of
tne airborne web to be dried. The no~zle-carrier-face commonly
includes a number of rows of holes, one row after the other in
the running direction of the web. The blow air flows in a space
between the web and the nozzle-carrier-face, and the blow air is
collected away through suction slots placed between the nozzle
boxes.
In the prior art direct-blow nozzle boxes of air dryers for
paper, board or pulp webs, wherein the air blowings are directed
perpendicularly to the material web to be dried, a well-known
problem is the lateral flow of the consumed air between the web
to be dried and the nozzle-carrier-face. As used in the art, the
term "lateral flow" is understood as meaning air flows parallel
to the plane of the carrier face and of the web. These air flows
are additionally parallel or opposite to the running direction of
the web. Since the air must escape from the treatment gap, a ~ -
lateral flow cannot be avoided. The lateral flow deteriorates
the transfer of heat in the prior art blow-nozzle boxes, and a
disturbing effect is increased with an increase in the velocity
of the exhaust-air flow. Further, the loss of pressure produced
by the blow box is increased when the velocity increases in the
lateral flow.
On the other hand, in view of the runnability of the web to
be dried, it is preferable to make use of the lateral flow by
shaping the blow face in the blow box and the geometry of its
nozzle openings such that a zone of negative pressure is formed
on the carrier face of the blow box. This zone of negative
pressure stabilizes the run of the web and ensures a stable and
unstrained run of the web.
With respect to the prior art most closely related to the
present invention, reference is made to the Swedish Patent No. SE ~-
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:~ ~
2 ¢~ ¢ ) 0 4
.106,152 (corresponding to U.S. ~atent No. 4,50s,053) and to
~nternational Patent Application N.. Wo 88/08950 (corresponding
to U.S. Patent No. 5,016,363).
In the blow boxes described in the Swedish patent,
triangular openings, so-called "fish eyes", have been punched
into the plane nozzle-carrier-face of the blow boxes. The front
edge of the openings the front edge, i.e. the base of the
triangle, has sharp edges. A sharp edge is not a major drawback
as long as the amount of air discharged out of the nozzle is
sufficient. However, at times, the amount of air receiv~d by the
nozzle may be reduced considerably from the dimensioned value,
for example, if the filters for drying air are blocked. In this
case, the web starts to contact the nozzle face. It has been
noticed that the sharp edges plane material out of the face of,
e.g., a pulp web, in which case both the quality of the finished
product is deteriorated and rubbish remains in the dryer.
Moreover, the rubbish remaining in the dryer disturbs the
threading of the pulp web. In this regard, the formation of a
"cigar" is spoken of, for the material detached out of the face
of the pulp web by planing forms a roll resembling the structure
of a cigar.
In the type of nozzle-blow-boxes employed in pulp dryers in
which the web runs above the nozzle and carrier faces of the
boxes, the function of the air blowings is both to transfer heat
from the blown air to the web and to support the web free of
contact. In view of the runnability of the web, it is preferable
to blow part of the air in a direction parallel to the plane of
the nozzle such that the web is stabilized at a distance of about
3 mm to about 6 mm from the carrier face. However, in such a
case, in the prior art nozzle-blow-boxes, the velocity of the
exhaust air in the space between the web and the nozzle becomes
high. This results in deterioration of the transfer of heat and
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20q2304
in extra and/or excessive pressure losses. The
detrimental effect of the high velocity of the exhaust
air can be reduced by making the nozzles sufficiently
narrow, but then the number of the nozzles becomes so
high that the cost of manufacture of the dryer is
increased substantially.
The present invention is directed towards the
further development of the prior art nozzle-blow-boxes
described in the afore-mentioned patents while
eliminating the drawbacks present therein.
The present invention is also directed towards the
provision of a new and improved method and nozzle-blow-
box construction by whose means it is possible to avoid
the drawbacks discussed above and to improve the transfer
of heat from the drying air to the airborne web to be
dried. This improvement of the transfer of heat can be
utilized most efficiently in the form of a smaller size
being required for the dryer. In this manner, the cost
of construction, e.g., of a pulp dryer, and the cost of
the machine hall in which the dryer will operate can be
lowered decisively, particularly the length of the
building required to house the pulp dryer.
The present invention is also directed towards
reducing the effect of the deterioration of the transfer
of heat by the lateral flow while the run of the web is
stabilized by means of the lateral flow.
In the method in accordance with one aspect of the
present invention, an air flow velocity parallel to the
plane of the web to be dried and air-supported in
connection with the nozzle-carrier face is initially kept
substantially invariable in order to improve the transfer
of heat in comparison with a plane carrier face. In this
manner, the air flow velocity is lowered in the lateral
areas of the carrier face by employing lateral areas
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2092~4
of the nozzle-carrier face that become lower in the air-
flow direction.
Accordingly, in one aspect of the present invention,
there is provided a method for air-drying material webs in a
dryer, comprising:
arranging nozzle-blow-boxes on one side of the web
such that a top side of said nozzle-blow-boxes forms said
carrier face,
arranging a groove space in a middle portion of
said carrier face and extending in a direction
transverse to a running direction of the web,
directing first air blowings to a web to be dried
through said first nozzle means in said carrier face in a
direction substantially perpendicular to the web,
directing second air blowings to the web crosswise
through said second nozzle means in a direction
substantially parallel to the web, said first and second
air blowings transferring heat to the web and supporting
the web by air such that a run of the web through a dryer
is stabilized,
maintaining an initial velocity of said second air
blowings to be substantially constant to improve the
transfer of heat in comparison with said carrier face,
and
arranging lateral areas of said plane carrier face
to become lower in the flow direction of said second air
blowings to lower the velocity of said second air
blowings in said lateral areas of said carrier face, and
increase the cross-sectional flow area between the
web and said carrier face in said lateral areas of said
carrier face such that the time of effect of said second
air blowings upon a lower face of the web is increased.
The nozzle-blow-box in accordance with one aspect of
the invention includes nozzle-carrier face portions having
extensions which comprise lateral carrier-face portions
2092G0~
placed further apart from the material web to be
supported. The velocities of the support and
stabilization air flows in the area of the carrier-face
portions are lower as compared with the velocity
prevailing in connectlon with the planar nozzle-carrier-
face portions. The nozzle-carrier face is provided with
nozzle perforations through which additional blowings can
be directed from the nozzle-blow-box in a direction
substantially perpendicular to the plane of the material
web to be supported.
Accordingly, in a second aspect of this invention, there
is provided a nozzle-blow-box of an air dryer for directing
air blowings to a material web to be dried, the air blowings
producing a transfer of heat from the drying air to the web
and contact-free air support and stabilization of a run of the
web through the air dryer, said nozzle-blow-box comprising:
a box part having a nozzle-carrier face arranged to
face the web,
a substantially V-section groove arranged in a
middle portion of said nozzle-carrier face in a direction
transverse to a running direction of the web, said groove
being open toward the web and having a series of nozzle
holes arranged in opposite walls for directing support and
stabilization air blowings to the web, said support and
stabilization air blowings being directed crosswise and in
opposite directions in relation to each other,
said nozzle-carrier face comprising planar nozzle-
carrier faces arranged on both sides of said groove and
in the same plane with each other, said planar nozzle-
carrier faces having extensions comprising lateralcarrier face portions arranged further apart from the
web, the velocity of said support and stabilization air
blowings being lower in proximity to said lateral
carrier-face portions as compared with the velocity of
said support and stabilization air blowings prevailing in
5A
~3
20q200~
proximity to said planar nozzle-carrier-face portions,
said nozzle-carrier face further comprising nozzle
perforations through which additional air blowings are
directed from said nozzle-blow-box in a direction
substantially perpendicular to the plane of the web.
The effect of the deterioration of the transfer of
heat by the lateral flow has been minimized in the present
invention by lowering the lateral portions of the nozzle
to a level lower than the planar middle portion such that
the velocity of the lateral flow is lowered. Moreover,
the lateral flows are preferably directed so that they do
not directly collide with the air jets being blown
directly on the plane face or on the lowered lateral
portions.
The lowering of the lateral areas of the nozzle-
carrier-face portions in accordance with the invention is
based on the idea that a high flow velocity of the exhaust
air between the web and the nozzle-carrier face causes a
deterioration in the coefficient of heat transfer. Thus,
the lower the space between the web and the nozzle-carrier
face, the higher the velocity of the exhaust air becomes.
The velocity of the exhaust air is increased in both
directions from the center line of the nozzles toward the
edges when more air is introduced. When the lateral areas
of the nozzle-carrier-face are lowered in accordance with
the present invention, the flow velocity in this area is
lowered.
5B
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2~9200~ ~
The nozzle-blow-box in accordance with the invention is a
combination of a nozzle with positive/negative pressure in which
the magnitude of the lateral flow that produce- the negative
pressure is selected appropriately in relation to the amount of
air in the direct blowing.
It is an important feature of the hole-no~zle field in the
carrier face in accordance with the invention that the
coefficient of heat transfer is not substantially dependent on
the distance given the relatively small nozzle-to-web distances
with which the nozzles of the present invention operates. This
feature is achieved, provided that the exhaust air does not
disturb the air jets blown out of the nozzle holes to a
significant extent. However, it is well known that, when there
are several rows of nozzle holes, one row arranged after the
other, the air discharged out of the nozzles generally passes
toward the edges in the space between the nozzle and the web.
Thus, the higher the flow velocity of the discharged air, the
greater the disturbance caused to the air jets blown out of the
holes and the greater the deterioration of the coefficient of
heat transfer.
In a preferred embodiment of the nozzle-blow-box in
accordance with the invention, air jets are directed from the
walls of a V-section groove placed in the middle of the nozzle
face in a direction crosswise in relation to one another at
continuous rounding points between the plane carrier face placed
at each side of the walls of the V-section groove. The air jets
are tangential to the rounding points so that they turn and
become parallel to the plane portions of the carrier face as a
result of the Coanda effect. In accordance with the Bernoulli
principle, a zone of negative pressure is formed between the web
and the carrier faces which stabilizes the web at a certain
distance from the carrier face. The distance is generally from
~5
2~92~0~
~ut 3 mm to about 6 mm.
In addition, on the horizontal part of the carrier face in
the present invention, attempts are made to avoid direct
collisions between the jets of direct blowing and the air jets
that flow in the lateral direction.
According to the present invention, the lateral areas of the
nozzle-carrier face of the nozzle-blow-box have been lowered so
that the velocity of the lateral flow is lowered as the cross-
sectional flow area becomes larger. In this manner, the heat-
transfer effect of the blow jets coming from the holes of direct
blowing placed in the lowered inclined and~o- ~traight nozzle-
carrier-face portion is improved.
The nozzle-blow-box in accordance with the invention is
suitable for use for drying a web both in on~--ized/two-sided
drying, in the case of low-grammage webs ( < 200 g/sq. m), and
both underneath and above the web. In the ca~e of heavy webs,
such as pulp webs, the nozzle-blow-boxes in accordance with the
invention are preferably suitable tor lower nozzles together with
direct-blow boxes that operate ~b upper nozzles, or alone as
lower nozzle boxes in one-sided drying.
A further advantage is achieved by means of the geometry of
the blow-carrier faces of the nozzle-blow-boxes in accordance
with the invention. This advantage is that a smooth blow face
with no sharp edges is obtained as the air of the lateral flow is
introduced out of the central V-section groove while guided by
rounded faces. If the web contacts the nozzle face, there are no
sharp edges which will tear the web and cause a deterioration in
the quality of the finished product and rubbish to remain in the
dryer which will disturb the threading of the pulp web.
The transfer of heat to the web can be improved by about 5
to about 10% by means of the present invention, This improvement
can be ; -~iately taken and put to use in the form of a reduced
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209200~
size of the dryer whlch substantially lowers the cost of
the investment of the dryer and the machine hall. Also,
this improvement indirectly reduces the number of
production interruptions and improves the operating time
ratio of the dryer. The advantages mentioned above are
particularly important in the case of large and
complicated pulp dryers. The nature of the improvements
has been quantified by measurements that have been
carried out and that will be described in more detail
later.
In a nozzle-blow-box in accordance with the
invention, a V-section groove is used in the middle of
its carrier face. Blowings parallel to the carrier face
are applied crosswise through the groove. By means of
this arrangement, a rigid mechanical construction is
obtained in addition to a favorable blow/heat-transfer
technique. The V-section groove rigidifies the nozzle
carrier-face efficiently without any other rigidifying
structures which would otherwise be necessary.
It is a minor drawback of the blow face of the blow
box in accordance with the invention that it is somewhat
more difficult to manufacture than a uniform plane face.
However, this drawback can be solved by means of
development of the manufacturing technology.
In an additional aspect, the present invention
provides a pulp dryer for drying material webs,
comprising:
nozzle-blow-boxes arranged at horizontal distances
from one another to form spaces therebetween, said
nozzle-blow boxes being placed at a distance from a
material web to form treatment gaps and directing air at
least in a perpendicular direction to the web to support,
dry and stabilize the webj the air being removed from
said treatment gaps via said spaces, said nozzle-blow-boxes
being arranged in the running direction of the web one after
B
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2092004
another in a horizontal plane such that rows of said nozzle-
blow-boxes are formed in the dryer,
a hood through which the web to be dried runs through the
dryer supported by air, the web being arranged to run over
said rows of said nozzle-blow-boxes in horizontal backward and
forward runs arranged in a vertical orientation in the dryer,
and
reversing rolls for reversing the running direction of
the web between said horizontal runs.
The following drawings are illustrative of
embodiments of the invention and are not meant to limit
the scope of the invention as encompassed by the claims.
Figure 1 is a schematic vertical sectional view in
the machine direction of a pulp dryer in which the method
and of a set of nozzle-blow-boxes in accordance with the
invention are utilized.
Figure 2 is an axonometric view of the modular
construction of a pulp dryer in which the method and of a
set of nozzle-blow-
8A
20~21D0~ ~
es in accordance with the invention are utilized.
Figure 3 is a schematic vertical sectional view in the
machine direction of a set of nozzle-blow-boxes in accordance
with the invention and of a set of boxes of direct blowing placed
above the set of boxes.
Figure 4 is an axonometric illustration of a nozzle-blow-box
in accordance with the invention and of the principle of its
blowings.
Figure 5 is an axonometric view of the construction of an
upper direct-blow box in accordance with the invention and used
in a method of the present invention.
Figure 6 shows an embodiment of a carrier face of a nozzle-
blow-box in accordance with the invention and of the blow nozzles
of the carrier face in more detail, together with important
dimensioning parameters.
Figures 7A, 7B, 7C, 7D, and 7E illustrate different
variations of different embodiments and dimensions of bevel and
step formations of the nozzle-carrier laces ot ~ nozzle-blow-box
in accordance with the invention.
Figure 8A shows a nozzle-blow-box as shown in Figs. 4 or 5,
viewed from a side of the nozzle-carrier face.
Figure 8B is an enlarged schematic vertb:~l sectional view
in the machine direction of a preferred geometry and dimensioning
of the V-section groove of the nozzle.
Figure 9 illustrates different relative coefficients of heat
transfer of the embodiments illustrated in Figs. 7A to 7E as a
function of the distance of the ~eL at a first air-blow velocity.
Figure 10 illustrates the corresponding measurement results
in a manner corresponding to Fig. 9, at a second, higher air-blow
velocity.
_,.. .~ , ,, ,,. ,, ~
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209200~
.
~ ntion are utilized. The basic principle of the embodiment of
Fig. 2 is, e.g., similar to that illustrated in Fig. 1. The
dryer-blower module comprises blower towers 21 and blowers, which
are provided with blade wheels 22. The module construction
includes heating radiators 24 through which the blow air is
passed into the gap between the upper nozzles and the lower
nozzles, i.e. into web gap 25. Further, the module construction
includes air filters 26. At the operating side of the blower
module, a ten~;ng bridge 28 is arranged, in connection with which
there are servicing gates 27 for the blower motors and servicing
doors 29 for the blower modules. Fig. 2 shows the circulation of
the drying air as illustrated by the arrows, and also the nozzle-
blow-boxes 30,40 in accordance with the invention and the web
gaps 25 between them.
With respect to the above description of Figs. 1 and 2, it
is to be emphasized that only one field of application of the
method and of the set of nozzle-blow-boxes 30,40 in accordance
with the invention are described therein. The method and the set
of nozzle-blow-boxes 30,40 in accordance with the invention can
also be applied in numerous other environments and also in
devices other than pulp dryers, for example in board and paper-
web dryers. However, pulp dryers are the most advantageous and
primary field of application of the invention wherein several
different and distinct advantages of the invention are most
appropriately and completely utilized.
Fig. 3 is a schematic illustration ol a s~t of nozzle-blow-
boxes 30 in accordance with the invention and ~f an opposite set
of boxes 40 of direct blowing. In the followin~J description, the
shorter name "lower box" will be used for the no~zle-blow-boxes
30, because they are preferably placed undern~th the
horizontally running web W. Free spaces 30a are arranged between
the lower boxes 30 and, in a corresponding way, tree spaces 40a
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2~92~0~
are arranged between the direct-blow boxes 40. The blow air is
, ssed through spaces 30a and 40a further through the heating
radiators 24 shown in Fig. 2 and is carried or propelled by the
blower 22 back to the blow boxes.
As shown in Fig. 3, the web W to be dried, typically a pulp
web, runs as a horizontal run through the web gap 25. The web
gap 25 is defined as the distance between the lower boxes 30 as a
lower point and the direct-blow boxes 40 as an upper point. The
lower boxes 30 are preferably uniformly spaced in one horizontal
plane. The direct blow-boxes 40 are also uniformly spaced in a
horizontal plane. The web W, which is usually heavy (the weight
of a wet pulp web may be up to about 2000 g/sq.m), is supported
on the blow boxes 30 by means of the blowings B2 and B3.
Blowings B1 are applied to the web W through nozzle holes 42
placed in the horizontal lower walls of the direct-blow boxes 40
in a direction perpendicular to the plane of the web W. The web
W is dried from above by means of these blowings B1.
Figs. 4, 6, and 8A and 8B illustrate the construction of the
lower boxes 30 in greater detail. A transverse groove 32 is
arranged in the middle portion of a carrier face 31 of the lower
boxes. Groove 32 passes across the width of the web W and is
opened toward the web W in preferably a V-section. The opening
angle of the V-section groove 32 is denoted with a. Angle a is
generally from about 50~ to about 90~, preferably a is from about
60- to about 80'. The inclined walls of the V-section groove 32
are preferably plane and arranged to turn and join a horizontal
plane portion 34 of the carrier face at an angle k by means of
rounded portions 31b having a curve radius R. As shown in Fig.
6, between the angles a and _, there is a relationship a + 2b =
180-.
Both of the inclined plane faces of the V-section groove 32 .
have rows of blow holes 33. These blow holes 33 are arranged and
2()~ OO'1
d ~cted so that the air jets ~ are blown through the blow holes
33 in a direction tangential to the :ounded portions 31b between
the plane faces. The rounded portions turn the air jets B3 by
the Coanda effect onto the plane portions 34 of the carrier face
31 and make the jets parallel to the plane portions. The blow
holes 33 are placed in the opposite sides of the v-section groove
32 and arranged in a staggered arrangement in relation to one
another (Fig. 8) such that the blowings B3 are interlocked with
one another crosswise in opposite directions. Thus, one set of
the blowings B3 is directed parallel to the running direction of
the web W and to its plane, whereas the other set of the blowings
is directed parallel to the plane of the web W but in a direction
opposite to the running direction of the web W.
In accordance with the Bernoulli principle, the blowings B3
induce a zone of negative pressure between the web W and the
carrier face 31. This zone stabilizes the web W at a certain --~
distance H from the carrier face 31. The distance H is
preferably from about 3 mm to about 6 mm, in which case the air
drying of the web W is most efficient.
Lowered lateral portions 35 are placed in both of the
lateral areas of the carrier face 31 over its length L in the
direction of the web. The height of these lowered lateral
portions 35 is lower in relation to the web W than the height of
the middle-plane portions 34 of the carrier face 31. According
to Fig. 6, lateral portions 35 are inclined plane bevel parts
whose distance in relation to the plane parts 34 at the edges of
the nozzle box 30 is denoted with h2.
In the nozzle-blow-box in accordance with the invention, the
air velocity is first substantially invariable in connection with
the plane carrier-face portions 31 in the web W treatment gap 25
underneath the web W. The air velocity is lowered in connection
with the carrier-face portions 35,35b,35d,35e which are stepwise
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2092ao4
continuously lowered in height when moving towards the edges
of the box 30 and towards spaces 30a in the treatment gap 25. In
this manner, the transfer of heat can be intenblfied
considerably, as will be described later in connection with the
test results illustrated in Figs. 9 and lO. The intensification
of the transfer of heat comes largely from th-- ~act that the air-
flow velocity parallel to the plane of the web ~ is lowered
considerably on the lowered carrier-face portions 35,35b,35d,35e. 5
This intensifies above all the heat transfer ot the direct
blowings B2.
In a pulp dryer in accordar7~ ~ith the invention, the lower
box 30 and the direct-blow box 40 as shown in Figs. 4 and 5 are
placed one above the other and one facing the other so that faces
41 and 31 of the boxes 40,30, respectively, are substantially
parallel to one another and generally horizontal. Rounded
portions 43a may be arranged at the edges of the faces 41 of the -~
direct-blow boxes 40. Corresponding rounded portions 31a maybe -
arranged at the edges of the carrier faces 31 of the lower boxes
30.
The opposite faces 31 and 41 on the lower box 30 and on the
direct-blow box 40 are provided with nozzle perforations 42,36,
respectively. A preferred distribution of the perforations 36 on
the blow box 30 is illustrated in Fig. 8. Perpendicular blowings
Bl,Bz are directed against the web W through the perforations
36,42 and the drying of the web W is promoted by means of these
perpendicular blowings. The direct blowings B2 will have a
longer time of effect on the lower face of the web W as a result
of the decrease in the air-flow velocity on the carrier-face
portions 35 because of an increased cross-sectional flow area.
Fig. 8B is a schematic illustration of a preferred
embodiment of the geometry and a dimensioning example of the V-
section groove 32 described above. The geometry shown in Fig. 8B
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!
i ~ymmetric in relation to a transverse vertical center plane K-
K. It is important in the design of the V-section groove 32 that
the air jets F1 and F2 which are blown from the opposite sides if
the groove 32 can be directed tangential to the rounded portions
31b connected with the edges of the groove 32. These air jets
will, by the Coanda effect, turn and become parallel to the
carrier face 34. The area between the groove 33 and the carrier
face 34 must be specifically rounded in such a way that the air
starts following the carrier face 34.
Figs. 7A-7E show alternative embodiments of the carrier face
of the blow box 30. The nozzle box 30A as shown in Fig. 7A
comprises a carrier face 31 in which plane portions 34 are
arranged at both sides of the V-section groove 32. Plane
inclined bevel portions 35 are arranged after the plane portions
34 in the air-flow direction.
Fig. 7B shows a particularly advantageous blow box 30B, in
which plane portions 34b of the carrier face are arranged at both
sides of the V-section groove 32. ~;-ep portions 37 are arranged
after the plane portions 34b in t~e ~ir-flow direction. Step
portions 37 are perpendicular both to the first plane portions
34b of the carrier face and to plane portions 35b of the carrier
face that follow after the step portion 37 in the air-flow
direction. The initial parts 34b of the carrier face 31 are
preferably parallel to one another and in the same horizontal
plane. In a corresponding way, the lateral portions of the
carrier face 31 are preferably parallel to one another and in the
same horizontal plane.
Fig. 7B also shows a preferred dimensioning example of the
nozzle box 30B. According to Fig. 7B, the height h2 of the step
portion 37 is about lO mm. Generally, the height of the step
portion h2 may vary from about 7 mm to about 15 mm.
In Fig. 7C, a nozzle box 30C is illustrated as a reference,
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w~ ~h box has a fully plane carrier face 31c. Properly speaking,
this nozzle box 30C is not a preferred embodiment in accordance
with the present invention, and it is illustrated in this
connection for the sake of reference only. The results of a
S comparison including this reference nozzle box are illustrated in
Figs. 9 and 10, which will be described in more detail later.
Fig. 7D illustrates a blow box 30D in accordance with the
invention, which box has relatively long plane carrier-face
portions 34d and relatively short and steep, inclined lateral
portions 35d. In Fig. 7D, a preferred dimensioning example is
also provided.
In Fig. 7E, an alternative modification of the blow box as
shown in Fig. 7B is illustrated. Blow box 30E has relatively
long plane carrier-face portions 34e and step portions 37 which
are followed by relatively short carrier-face portions 35e in the
air-flow direction. Fig. 7E also shows an example of the ;
construction of blow box 3OE.
Fig. 8A shows the relative locations and staggering of the
nozzle holes 33 in the V-section groove 32 so that the blowings
B3 being blown in opposite directions are blowr crosswise. The
perforations 36 in the nozzle-carrier face 31 are placed in four
lines or rows, one line after the other, as stagyered in such a
way that the blowings B2 and B3 neither meet ea~ other nor
disturb each other. The mutual spacing of the rlozzle holes 33 is
generally from about 20 mm to about ~,o mm. In a corresponding
way, the mutual spacing of the nozz~- holes 36 in the transverse
direction and in the machine direction is usually from about 40
mm to about 100 mm.
Further, as to the dimensioning of the blow boxes shown in
Figs. 6 and 7, it can be stated as follows, with reference to the
denotations in Fig. 6. The angle a of the V-section groove 32 in
the middle of the carrier face is generally from about 50~ to
c It 90-, in which case the angle k of the Coanda faces 31b is
from about 45~ to about 65~. The height h1 of the V-section
groove 32 is optimally obtained from the equation h1 = (2 - 5) x
~ wherein ~ is the diameter of the nozzle holes 33 in the walls
of the V-section groove 32. The diameter ~ of the nozzle holes
33 is selected relative to the diameter of the direct-blow
nozzles 36 in the carrier face so that the air quantity in the
carrier-blowings B3 blown through the nozzle holes 33 is about
30% to about 60%, preferably 35~ to about 45%, of the overall air
quantity of the blowings B2 and ~.
The length L1 ~f the bevelled or step-formed lateral
portions 35,35b,35d,35e in the carrier face 31 is selected
according to the equation L1= (0.1 - 0.3) x L, preferably L1 =
(0.2 - 0.25) x L, wherein L is the total length of the blow box
30 in the machine direction. Generally, length L is from about
300 mm to about 500 mm. The difference in height hz of the
bevelled portions 35,35d or of the step-formed portions 35b and
35e is selected from about 7 mm to about 15 mm, preferably h2 is
about 10 mm.
Figs. 9 and 10 graphically illustrate test results obtained
with nozzles as shown in Figs. 7A-7E. In Figs. 9 and 10, the
vertical axis represents the relative heat transfer coefficient
~R and the horizontal axis represents the distance of the web W
from the carrier face 31, expressly from its plane portion 34.
In Figs. 7A-7E, the letter symbol after the numeral "7"
corresponds to the curves A-E in Figs. 9 and 10. ~-
Of the nozzle arrangement described above, versions as shown
in Figs. 7A-7E were made and the transfer of heat was examined in
a static test device by blowing hot air against a plane metal
face. The efficiency of the transfer of heat w~s obtained by
measuring the heating rate of the plate by mearls of temperature-
measurement detectors inlaid in the plate.
.~. .
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. ~
209~Jo4
In Figs. 9 and 10, the measured relative heat-transfer
coefficients ~R are illustrated as a function of the distance H
between the web and the carrier face 31 of the nozzle box at two
different blow velocities. According to the results, a lowering
h1 of the lateral portions 35,35b,35d,35e provides an increase in
the range of about 5% to about 10% in the coefficient of heat
transfer as compared with a plane carrier face (Fig. 7C, carrier
face 31c), when the distance H equals the normal airborne
distance of a pulp web W (between about 3 mm and 6 mm). In
contrast, at larger distances H, a lowering or decrease of the
lateral portions 35 does not give any corresponding advantage.
The increase was highest in the case of the nozzles at which the
lateral portions 35,35b of the carrier face 31 had been lowered
most, on the average (Figs. 7A and 7B). The measurement results
provided in Fig. 9 were obtained with a blow velocity of w~h is -
about 26 m/s of the blowings B2 and B3. The results given in
Fig. 10 were obtained with a corresponding blow velocity of w~h
is about 34 m/s, while the temperature T~h of the blow air was
about 150~C. As shown in Figs. 9 and 10, there are substantially
large differences in the relative heat-transfer coefficient ~R
exactly at the optimal airborne web W distances, i.e. when H is
between 3 mm and 6 mm.
The simulation and measurement method employed in the
measurements of Figs. 9 and 10 has been described in more detail
in a paper by P. Heikkila and I. Jokioinen, "Airfoil Dryer Heat
Transfer", published in The Helsinki Symposium on Alternate
Methods of Pulp and Paper Drying in Helsinki, June 4-7 (1991).
Based on the measurements described above, the most
advantageous embodiment of the invention is, according to the
present-day opinion and on the basis of the available measurement
results, the blow-nozzle box 30A as shown in Fig. 7A. According
to the measurement results of Figs. 9 and 10, a carrier face
18
2~200 1
,35b with a steep step formation ~7, as sho~n in Fig. 7B, is
optimal in view of the transfer of heat. However, a nozzle-blow-
box 30A as shown in Fig. 7A, whic~ 5 provided with continuously
lowering ramp-formed lateral portions 35 of the carrier face, is
preferable in an overall consideration because the risk of
formation of a "cigar" is lower for this nozzle-blow-box 30A as
the geometry of the blow face does not include sharp angles.
Thus, according to a present-day estimate, a nozzle-blow-box 30A 5
as shown in Fig. 7A (also in respect of its dimensions) is the
most preferable embodiment of the invention in a situation in
which the distance, e.g., of a pulp web W from the horizo~tal
portion 34 of the carrier face 31 is about 5 mm.
The examples provided above are not meant to be exclusive.
Many other variations of the present invention would be obvious
to those skilled in the art, and are contemplated to be within
the scope of the appended claims.
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