Filter cylinder with drainage surface

A drum filter is characterized in that its filtrate compartments (20) comprise a bottom part (28,30) and a side surface (32), which is provided with a lip or curved portion (34) extending towards the inside of the compartment (20) from the edge of the side surface opposite the bottom part. The lip (34) prevents the filtrate from escaping from the compartment (20) back through the wire surface, when the drum filter (20) is rotated.

FIELD OF THE INVENTION 
The present invention relates to a rotary drum filter for treating pulp, 
which is especially suitable for treating fiber suspensions in the wood 
processing industry. The filter drum or cylinder is particularly designed 
for washing or thickening said suspensions, by filtering liquids from 
fiber suspensions. 
BACKGROUND OF THE INVENTION 
The invention relates more specifically to a so called rotary drum filter, 
the basic construction of which comprises a tank, containing the 
suspension being thickened and a wire-surfaced cylinder rotatably mounted 
in the tank. The ends of the drum or cylinder are sealed so as to prevent 
the cylinder from being filled with suspension. The cylinder has under its 
wire surface filtrate compartments, into which liquid from the suspension 
in the tank is pressed due to the hydrostatic pressure, whereby the fibers 
in the suspension are transferred with the liquid onto the wire surface 
and are thickened forming a fiber mat thereon. The cylinder in this kind 
of filter based on hydrostatic pressure can be submerged in the suspension 
in the tank only slightly below the axial plane of the cylinder. When the 
cylinder rotates slowly, approximately 1 to 3 rpm, a fiber mat is 
gradually deposited on the wire surface. The formation speed of the mat 
naturally depends not only on the thickness of the deposited mat, but also 
on how deep in the vessel the filtration takes place. The filtration 
reaches its maximum speed slightly before the bottom dead center, because, 
at this point, the hydrostatic pressure is close to its maximum and the 
build-up of a thick mat does not yet disturb the filtration. After the 
bottom dead center, the formation of the mat slows down, until the 
formation, of course, ceases completely, when the mat emerges from the 
suspension. 
As the fiber mat emerges from the tank upon rotation of the cylinder the 
filtration based on gravity begins, liquid flows slowly due to the effect 
of the gravity through the fiber mat into the filtrate compartments inside 
the wire surface. The fiber mat is removed from the cylinder at a stage, 
when it has passed the top dead center of the cylinder and again 
approaches the surface of the suspension. The mat may be removed either 
mechanically with scrapers or by injecting either a liquid or compressed 
air through the wire surface. The liquid which has flowed into the 
filtrate compartments may be discharged from the apparatus, for example, 
via the hollow shaft of the apparatus or by utilizing some other 
arrangement. The length of a filtration cycle based on hydrostatic 
pressure in an apparatus according to the above description is 
approximately 140.degree. of a revolution of a cylinder and the proportion 
of the initial filtration is approximately 30.degree. and the proportion 
of the vacuum drying (above the liquid surface of the tank) is 
approximately 100.degree.. 
The above-described basic type of a drum filter has been considerably 
improved over the decades. One significant improvement worth mentioning is 
the subjecting of the suspension in the tank to a suction applied through 
the compartments, whereby the filtration of liquid is considerably 
improved and thus the capacity of the apparatus is significantly 
increased. Generally, the suction operation is carried out by a suction 
leg in such a way that each of the filtrate compartments is connected by 
means of a separate filtrate tube to a valve which is connected to the 
suction leg and the shaft of the apparatus. The operation of the valve is 
controlled in such a way that the fiber mat is subjected to suction over a 
sector of approximately 240.degree.. Approximately 30 filtrate 
compartments are arranged within the cylinder, each of which compartments 
has been connected by a separate filtrate tube to the shaft tube of the 
cylinder surrounding the valve. Since the fiber mat generated this way is 
significantly more tightly attached to the filtrate surface than the mat 
generated in a conventional way, the thickened fiber mat must be removed 
from the filtrate surface by separate means. At this point, it should be 
noted that the suction leg may be replaced by some other apparatus 
creating subatmospheric, such as a vacuum pump. 
The filtrate compartments in accordance with the prior art have one 
disadvantage, which occurs particularly in the operation stage of the 
filter cylinder, when the pulp web is removed from the wire surface of the 
cylinder. At this point the filter cylinder described in, for example, FI 
Patent 55538 and the filter compartments therewherein have turned, in the 
mat removal stage to a position wherein the filtrate still present in the 
compartments, will flow, in the rotational direction of the cylinder, 
toward the front wall as it is unable to exit from the compartment, since 
the discharge openings of the compartment do not usually extend along the 
entire width of the bottom of the compartment. Consequently, especially 
when the suction is interrupted and the compartment turns further, the 
filtrate begins to flow against the wire surface, whereby it, of course, 
permeates the wire surface and flows back to the tank or to the pulp web 
to dilute the consistency thereof. 
SUMMARY OF THE INVENTION 
It has been possible to solve the problems of the prior art apparatuses 
with the present invention in such a way that by providing the filter 
cylinder in accordance with the present invention it is possible to remove 
all liquid from the filter compartment without the risk of the filtrate 
flowing back to the pulp web. 
The filter cylinder in accordance with the present invention is 
characterized in that the filtrate compartments comprise a bottom part and 
at least one side wall, the leading side wall of each compartment with 
respect to the direction of rotation of cylindrical part is provided at 
its upper edge remote from the bottom part with a curved portion extending 
towards the inside of the compartment or with a projecting lip or other 
overhanging portion, which curved portion or lip prevents the filtrate 
from escaping from the compartment back through the wire surface, when the 
cylinder is rotated.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
According to FIG. 1 a filter apparatus mainly comprises a tank 10, which 
may be either completely open from the top or covered with a hood, and a 
cylindrical filter cylinder 12, which is arranged on shaft 14, which is 
mounted at its ends with bearings and seals 16 relative to the tank 10. 
The body of the filter cylinder 12 comprises end plates 18 attached to an 
extension of the shaft 14, which plates at the same time prevent entry of 
the suspension to the inside of the cylinder, and filter compartments 20, 
by means of which the ends of the cylinder are connected to each other and 
which form the primary structure of the cylinder. The cover of a filter 
compartment may be either a perforated plate, on which the wire surface 
serving as a filter surface is located, or the wire surface 22 itself. If 
the cover of the filtrate compartment is a perforated plate, it will also 
serve as a member bearing the stresses of the cylinder. A great number of 
filtrate compartments 20, approximately 30-40, has been arranged at the 
periphery of the cylinder. The bottom surfaces (30, 28) of the filtrate 
compartments 20 are inclined either only towards one end of the cylinder 
or from the center in both directions towards the opposite ends thereof, 
mainly along the entire length of the cylinder, which, in most of the 
cases, is more than 5 meters. Filtrate tubes 24 leading towards the shaft 
14 of the cylinder are connected to the bottom surface 28 of the filtrate 
compartments 20 at the end of the cylinder (or at both ends thereof, when 
the bottom surfaces of the compartments are inclined towards both ends). 
Adjacent to the filtrate tubes 24 is a distribution chamber 26, to which 
the filtrate tubes 24 are connected in either one or two rows mainly 
according to the diameter of distribution chamber 26. Distribution chamber 
26 is mainly required for guiding the filtrate flowing from the filtrate 
tubes to the suction leg without the need to constrict the sectional area 
of the flow. In the distribution chamber the sectional area of the flow is 
changed from the shape of a round filtrate tube to a rectangular one 
defined by the radial intermediate and end walls of the chamber. A valve 
apparatus 40 for guiding the filtrates of tubes 24 in a desired manner is 
located inside the distribution chamber 26. By means of the same valve it 
is possible, during the operation of a conventional filter, to also 
conduct liquid or gas for the removal of the fiber mat through filtrate 
tubes 24 to filtrate compartments 20. 
Filtrate compartment 20 comprises, according to FIG. 2, a bottom part 28 
substantially parallel to the periphery of the filter surface, to which 
part 28 a filtrate tube 24 is connected. An inclined bottom part 30 is 
joining the edge of the bottom part 28, and extends in the rotational 
direction (arrow B) of the filter cylinder and advantageously forms an 
angle of approximately 165.degree. with the bottom part 28. A 
substantially radial wall 32 is joining the front edge of the bottom part 
28 in the rotational direction of the drum filter. The outer edge of wall 
32 is provided with an inwardly extending lip or curved portion 34 
pointing towards the filtrate compartment, the purpose of which curved 
portion 34 is to prevent the filtrate in the compartment from escaping 
through a filter surface 22 back to the fiber mat or, after the mat is 
removed, to the tank when the cylinder turns to a position, in which the 
pulp web is removed from the filter surface (rotated approximately 
60.degree. downwards and to the right from the position shown in FIG. 2). 
The bottom part 30 extends upward advantageously to about half the height 
of wall 32 of the next filtrate compartment. Bottom part 30 is attached to 
wall 32, for example, by welding. The construction thus produced is very 
resistant against different stresses it may be subjected to. Additionally, 
curved portion 34 will strengthen the compartments against bending 
stresses. Curved portion 34 also forms a suitable location for attachment 
of filter surface 22. 
Curved portion 34 formed in the filtrate compartment 20 will retain inside 
all of the filtrate which might otherwise drain back to the pulp web 
during the removal stage of the web. The filtrate will remain in the 
compartment 20 until the compartment submerges under the surface of the 
suspension and the filtration based on the hydrostatic pressure begins, at 
which point the filtrate is either discharged with the air from the 
filtrate tubes or during the stage when suction is applied. 
As it will be appreciated from the above description, it has been possible 
to eliminate or minimize the defects of the prior art apparatuses by 
providing an apparatus, which is easy to produce and by means of which the 
filter cylinder becomes stronger than those of the prior art.