Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
l~Z2S'~O
ELD OF THE INVENTION
This invention relates to a filter with a rotary filter drum for
dewaterin~ material in the form of liquid-containing stock mixtures, for
example, fiber suspensions of cellulose pulp. Such filters can be used
advantageously for washing the material. The invention, more precisely,
relates to a doctor device for taking off a material web from the filter drum.
BACKGRO'J~D OF THE INVENTION
For the dewatering of fiber suspensions of cellulose pulp, filters
are used, which comprise a filter drum intended to rotate partially immersed
in the suspension. Such a filter drum consists of an outer shell, which is
provided with strainer apertures and on which a filter cloth is attached.
Typically, an inner shell is also provided ~Yhich has no such apertures. Be-
tween these two shells, partition walls are arranged which e~tend substan-
tially rectilinear in the longitudinal direction of the drum and divide the
intermediate space between the shells into so-called suction cells. At so-
called valveless vacuum filters, the suction cells are connected to suction
or drain ducts, which are bent rearward, seen in the direction of drum ro-
tation, and in which vacuum is created at the rotation of the drum. At so-
called drop leg filters, the suction cells are connected to a stationary
2Q suction pipe via one or more val~es. Due to the vacuum in the cells, the
liquid in the fiber suspension flows into the cells and is removed from the
material on the outside of the filter drum, so that a pulp web is formed.
O~ing to said vacuum, the pulp web is further dewatered when it has been
moved up and out of the suspension by the rotation of the drum. The pulp
web is then removed from the filter cloth at a po~nt which usually is located
--1--
~'
~:. - , : . ..: , :.. .
-
l~ZZ54~
some distance above the suspension le~el on the downward directed side of
the drum. This web removal is made possible in that the vacuum in the cells
at this point is replaced by atmospheric pressure.
The invention can also be applied to so-called open deckers, at
which the pulp web is formed due to the fact that the suspension level out-
side the drum is substantially higher than the filtrate level within the drum.
To remove the pulp web, different devices can be used, for example,
grooved or smooth take-off rolls. The simplest take-off device is a doctor
located with small clearance, about 2-7 mm, to the filter cloth. The removal
process is usually facilitated in that fluid, such as air, steam, or liquid
is sprayed in~o the gap between the doctor and the filter cloth. In addition,
in order to prevent appreciable deflection, the doctor is made to rest on a
holder, which is stiff against deformation and is supported at its ends.
In recent years, filters for increasing production have been con-
structed, requiring long drums and doctors often extending up to 6 meters in
the axial direction of the drum. ~ith very long filter drums of lQ to 12
meters in length, the holder is constructed in two pieces~ which are also
supported at their ends. Owing to the small clearance between the doctor
edge and the filter cloth, it is of greatest importance that -the carrying
holder is not deformed, because otherwise the doctor blade would damage the
filter cloth.
Even when the holder is constructed with great rigidity, deforma-
tions can still occur as a result of heat stresses. Before the start of
the filters, for example, in so-called brown stock washing plants, the filter
drums often are rotated in hot liquor of about 90 C, and at the same time,
--2--
.
. .: ' . '- ~ '~ :
; . . . : .
l~ZZS4~
the filter cloth is spray-cleaned with a fluid, for e~ample, liquor of sub-
stantially -the same temperature or steam. At filters in bleaching plants,
the liquor is replaced by hot water, preferably white water. The filter drum
is thereby substantially heated, and the heat spreads primarily to the lower
surface of the holder, i.e., the side which faces the filter drum. The lower
surface thereby expands more than the upper surface. Consequently, the
doctor assumes an arc-shape, i.e.~ the distance to the drum gets smaller at
the center of the doctor than at its ends. In order to prevent jamming be-
tween the doctor and the filter drum, the distance must be increased to an
undesirable degree, which results in the pulp web being taken off at the ends
of the doctor in an improper fashion. Heat stresses in doctors and their
holders have often resulted in the filter cloths, which are very expensive,
being destroyed.
SU~IARY OF THE INVENTION
According to one aspect of the present invention there is provided
apparatus for removing a web from a rotary filter drum comprising a doc~or
holder and a doctor blade mounted on said doctor holder, said doctor holder
including a hollo~Y chamber for receiving a regulating fluid to regulate the
tenlperatUre of at least a portion of said doctor holder, the improvement
comprising nozzles mounted on said doctor holder and directed toward said
drum, said nozzles communicating with said hollow chamber such that said
regulating fluid is sprayed therefrom.
The fluid may be either a cleaning fluid sprayed onto the drum to
clean the drum, or a fluid sprayed into the gap between the doctor and the
drum. Both may be employed where the doctor holder is provided with separate
-3-
:
: . '
~lZ~54~
internal chambers for supplying the respective fll~icls to respective sets ofnozzles.
With this arrangement, the different parts of the holder are main-
tained at substantially the same temperature. The fluids can be air, steam,
or liquid.
According to another aspect of the present invention there is
provided a method for removing a web from a rotary filter drum which employs
a doctor blade mounted on a hollow doctor holder, comprising the steps of
supplying fluid to a chamber ~thin said hollow doctor holder for controlling
the temperature of at least a portion of said doctor holder, supplying said
fluid to nozzles mounted on said doctor holder for directing said fluid to
a portion of the drum where said web has just been removed so that said Muid
is also used to clean said drum.
BRIEF DESCRIPTION OF THE DRAWINGS
The following is a detailed description o-f a presently preferred
embodiment, to be taken in conjunction with the accompan~in~ drawin~s, wherein:
Figure 1 partially illustrates a rotary drum "~hich is provided
~ith a doctor device in accordance with the present invention; and
Figure 2 is a sectional view, alon~ II of Figure 1, illustrating
in detail the doctor holder of the present invention.
DETAILED DESCRIPTION OF PREFERRED E~lBODI~ENT OF THE INVENTION
The filter comprises a rotatable filter drum 1, which is partially
immersed in a vat 2 containing a fiber suspension 3. A doctor blade 4 is
located adjacent the drum for removing the pulp web from the drum, which by
vacuum adheres to the filter drum land follows along ~th it. The doctor
blade 4 is attached to a doctor holder 5 which is hollow and defined by
-
.- -
.
ll;~ZS~O
upper and lower walls 6, 7, respectively, and by end walls.
~ t the forward portion of the holder 5, i.e., at the portion located
closest to the drum where the pulp web is taken off, spray nozzles 8 for air
are provided. Nozzles 8 are attached to the holder 5 and communicate with a
forward chamber 9 located in the forward portion of the holdcr. In the end
walls of the holder, the forward chamber 9 communicates with supply conduits
10 for supplying air under pressure, in the range of 0.1 to 3 kp/cm , and
preferably about 0.3 kp/cm .
On the rear portion of the holder 5, spray nozzles 11 are provided
for spraying liquid, preferably liquor. These nozzles 11 communicate with
a rear chamber 12, which occupies the greater part of the holder cavity.
Chamber 12 communicates with supply conduits 13 for supplying liquid.
In the embodiment shown, the doctor holder 5 can be moved up and
do~n about a pivot axle coinciding with the axis of the conduits 10, 13.
It is thereby possible to easily clean the nozzles 8 when necessary. The
conduits 10, 13 alternatively may consist of flexible hoses, which are con-
nected beside the pivot axle. The gap between the doctor blade 4 and the drum
1 is adjustable by means of bolts 14 or the like, which co-operate ~ith and
adjustable relati~e to fixed stop member 15.
In operation, before the filter is started, the filter drum is
rotated in hot liquor of about 90 C, and at the same time, liquor of the same
temperature is supplied through the conduit 13 to the rear chamber 12 in the
doctor holder 5. Such liquor is sprayed against the filter drum 1 through
the nozzles 11 for cleaning the drum. The upper and lower walls 6, 7 of the
doctor holder 5 are thereby heated uniformly, so that the holder 5, as well
--5--
~Z~540
as the doctor blade 4, do not deform, and their straight shape is maintained.
When the filter is operated, the spray cleaning of the filter cloth
can continue without interruption, as there usually is demand for such
cleaning. Other~ise, the liquor supply may be stopped, because the pulp web
during operation is passed over the upper wall 6 of the doctor holder and
thereby maintains it at about the same temperature as that of the lower wall.
The removal of the pulp web is facilitated in that air is sprayed
through the nozzles 8 in the gap between the doctor blade 4 and the drum 1.
A latitude of modification, change, and substitution is intended
in the foregoing disclosure, and in some instances, some features of the
invention will be employed without a corresponding use of o-ther features.
Accordingly, it is appropriate that the appended claims be construed br~adly
and in a manner consistent with the spirit and scope of the invention herein.
, :
- ' ' ~" `
