U.S. patent application number 10/660661 was filed with the patent office on 2004-03-18 for filtration of pulp mill liquids.
This patent application is currently assigned to Andritz Inc.. Invention is credited to Phillips, Joseph R..
Application Number | 20040050510 10/660661 |
Document ID | / |
Family ID | 26906546 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040050510 |
Kind Code |
A1 |
Phillips, Joseph R. |
March 18, 2004 |
Filtration of pulp mill liquids
Abstract
A fiber mat is formed on a screen cylinder of a pressurized
fiber filtering device used to filter particles from a liquid
stream. The fiber mat is a fine particle filter to remove small
particles from the liquid stream. The fiber mat is formed by
injecting cellulosic fibers into the liquid stream flowing into the
fiber filtering device. These fibers collect on the screen cylinder
as the liquid flows through the cylinder. The collection of fibers
forms the fine filtering mat.
Inventors: |
Phillips, Joseph R.;
(Queensbury, NY) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Andritz Inc.,
Glens Falls
NY
|
Family ID: |
26906546 |
Appl. No.: |
10/660661 |
Filed: |
September 12, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10660661 |
Sep 12, 2003 |
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09843822 |
Apr 30, 2001 |
|
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6649068 |
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60211876 |
Jun 14, 2000 |
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Current U.S.
Class: |
162/41 ; 162/251;
210/739 |
Current CPC
Class: |
D21D 5/026 20130101;
B01D 33/11 20130101; B01D 33/804 20130101; B01D 37/02 20130101 |
Class at
Publication: |
162/041 ;
210/739; 162/251 |
International
Class: |
D21C 007/08; D21C
007/14 |
Claims
1. A process for removing solid particles from a liquid stream in a
pulp or paper mill having a filtering device having an inlet for
liquid having at least some undesirable materials, an outlet for
treated liquid having reduced concentration of undesirable
material, an outlet for undesirable material, and a perforated
screen element, comprising: (a) introducing a first liquid having
an undesirable material content to the inlet; (b) introducing a
second liquid having at least some comminuted fibrous material to
the inlet to form a mixture of the first and second liquids; (c)
passing the mixture of the first and second liquids through the
screen element to produce a third liquid having little or no
undesirable material; (d) discharging the third liquid from the
filtering device, and (e) practicing steps (a)-(d) so that at least
some of the comminuted cellulosic fibrous material introduced with
the second liquid is retained on the screen element to form a
permeable mat of cellulose material which acts as a filtering
medium for the undesirable material in the first liquid.
2. A process as recited in claim 1 wherein steps (a)-(d) are
practiced using a cylindrical screen element as the screen
element.
3. A process as recited in claim 1 wherein (b) is practiced so that
the second liquid is mixed with the first liquid prior to being
introduced to the inlet.
4. A process as recited in claim 1 wherein step (b) is practiced by
continuously introducing the second liquid.
5. A process as recited in claim 1 wherein step (b) is practiced by
intermittently introducing the second liquid.
6. A process as recited in claim 1 wherein in step (b) the
introduction of the second liquid is regulated as a function of the
introduction of the first liquid.
7. A method as recited in claim 1 wherein (a)-(d) are practiced so
as to sense undesirable material in the liquid and to automatically
adjust a mat formed on the screen element in response to that
sensing.
8. A system for treating a liquid stream in a pulp and paper mill
to remove undesirable material from the liquid stream, comprising:
a filtering device including a perforated screen element having a
filtering surface, an inlet, and a filtered liquid outlet; said
inlet connectable to a source of liquid containing undesirable
material; said inlet connectable to a source of liquid containing
at least some comminuted cellulosic fibrous material to the inlet;
and a bed of comminuted cellulosic fibrous material on the
filtering surface of the screen element to provide a fine screening
medium.
9. A system as recited in claim 8 wherein said screen element is a
cylindrical screen element.
10. A system as recited in claim 9 wherein said screen element
rotates.
11. A system as recited in claim 9 wherein said screen element is
stationary.
12. A system as recited in claim 8 wherein said filtering surface
is a concave or external surface.
13. A system as recited in claim 8 wherein said filtering surface
is a convex or internal surface.
14. A system for treating a liquid stream in a pulp and paper mill
to remove undesirable material from the liquid stream, comprising:
a filtering device having a perforated screen element having a
perforated filtering surface, an inlet and a filtered liquid
outlet, a liquid containing undesirable material, and a liquid
containing at least some comminuted cellulosic fibrous material,
introduced to the inlet; and a bed of comminuted cellulosic fibrous
material on the perforated filtering surface of the screen element
which provides a finer screening medium than the perforated
filtering surface of the screen element without the bed of
comminuted cellulosic fibrous material present.
15. A system as recited in claim 14 wherein said screen element is
a cylindrical screen element.
16. A system as recited in claim 14 wherein said screen element
rotates.
17. A system as recited in claim 14 wherein said screen element is
stationary.
18. A system as recited in claim 14 wherein said filtering surface
is a concave or external surface.
19. A system as recited in claim 14 wherein said filtering surface
is a convex or internal surface.
Description
RELATED APPLICATION
[0001] This application is related to and claims priority to U.S.
provisional patent Application No. 60/211,876, filed Jun. 14, 2000,
the entirety of which application is incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] In the processing of cellulose material, for example, wood
or recycled paper, liquid streams are produced that contain
cellulose fibers or other impurities that are typically undesirable
in the process in which the liquid is used. For example, cellulose
fibers must be minimized or removed from spent cooking liquors
before the liquors are forwarded to evaporators in a chemical
recovery system. Otherwise, the cellulose fibers are minimized or
removed from effluents of pulp washers or pulp presses used in
processing of the virgin pulp or recycled pulp before the effluents
can be used elsewhere.
[0003] Cellulose fibers and other undesirable material can be
removed from liquid process streams by a filtering device. One type
of filtering device that is typically used is a pressurized
screening device. These filtering pressurized screening devices are
similar to pulp-processing pressurized screening devices that
isolate or "screen" large uncooked pieces of cellulose material,
such as pins and shives, from cellulose pulp streams.
[0004] Unlike pulp-processing pressure screening devices,
pressurized screening filtering devices (typically referred to as
"fiber filters") filter undesirable material from liquid streams.
Liquid streams typically have a much lower content of cellulose
material than do the pulp streams fed to pulp screening devices.
Pressurized pulp screening devices are typically fed cellulose pulp
slurries having about 1 to 5% by weight (that is, about 1 to 5%
consistency). In contrast, fiber filters are typically fed liquid
streams having a fiber content of less than about 1% consistency,
and preferably less than about 0.1% consistency, or even less than
about 0.05%. For example, the liquid streams treated with fiber
filters typically contain fiber in the range of 50 to 250 parts per
million (ppm), which corresponds to a consistency of approximately
about 0.005 to 0.025%.
[0005] Pressurized screen fiber filters typically contain
perforated cylindrical screen baskets through which the fiber
containing liquid stream is passed to remove the fibers and other
materials. One typical fiber filter is the MODUScreen.TM. FF
Pressure Filter provided by Andritz-Ahlstrom, Inc., of Glens Falls
N.Y. This pressurized fiber filter screening device consists of a
stationary cylindrical screen cylinder mounted about a cylindrical
rotor. The stationary perforated cylinder contains circular
through-holes, having an exemplary 0.2 mm diameter. However, the
cylinder holes may be shaped as parallel bars or machined slots. In
operation of the fiber filter, the liquid to be filtered flows from
inside of a screen basket in the fiber filter, then radially
outward through the screen where the undesirable material particles
(for example, cellulose fibers), are prevented from passing through
the screen perforations. These undesirable materials captured by
the screen basket are passed out through a rejects outlet in the
fiber filter and forwarded for further processing.
[0006] The cylindrical rotor of the fiber filter is provided with a
series of uniformly spaced protrusions or "bumps". The leading edge
of the bumps acts as a hydrofoil to impart a hydraulic pulse to the
liquid flowing along the inside of the basket. The trailing edge of
the bumps creates a fluid pressure drop which momentarily draws the
liquid back through the holes to dislodge any material that may be
stuck in the holes so as to minimize screen pluggage. This type of
fiber filter has been effective in minimizing or removing cellulose
fiber and other undesirable materials from liquid streams in and
around pulp and paper mills.
[0007] Sometimes the liquid stream introduced to these filtering
devices contains undesirable particles that cannot be easily
isolated with conventional pressurized screen fiber filters. In
particular, smaller particles are not easily isolated and tend to
pass through the screen. Particles smaller in diameter than the
diameter of the holes in the screen cylinder especially tend to
pass through the screen holes. For example, particles smaller than
the 0.2 mm diameter (about 0.008 inches) of the holes in the
Andritz-Ahlstrom MODUScreen FF Filter are not easily isolated and
tend to pass through the screen.
[0008] One method of capturing smaller particles is to reduce the
size of the screen perforations to 0.15 or 0.1 mm (that is, 0.006
or 0.004 inches), for example. However, smaller diameter screen
holes can be much more expensive and difficult to fabricate using
existing drilling or cutting technology. Another alternative is to
wrap the screen cylinder with a fine cloth, fabric, or wire screen
having a tight weave to capture the smaller particles. Such fabrics
or wire screen are prone to tearing and other damage from the
loading and high speeds under which present fiber filters are
operated. Neither of these alternatives are particularly feasible
or economical under the present state of the technology.
SUMMARY OF INVENTION
[0009] According to the present invention, a fine screening medium
is provided to the screen cylinder of a pressurized screen fiber
filtering device. This fine screening medium is formed by adding at
least some cellulose fibrous material to the filtering device so as
to form a web or mat of cellulose material on the existing
perforated surface. This web of cellulose fibers acts as a fine
screening medium to filter small cellulose fibers and other
undesirable material.
[0010] An embodiment of this invention is a process for removing
solid particles from a liquid stream in a pulp or paper mill having
a filtering device. The filtering device includes an inlet for
liquid having at least some undesirable materials, an outlet for
treated liquid having reduced concentration of undesirable
material, an outlet for undesirable material, and a perforated
screen element. A filter forming process is first used that
comprises the steps of: (a) introducing a first liquid having an
undesirable material content to the filter inlet; (b) introducing a
second liquid having at least some comminuted fibrous material to
the filter inlet to form a mixture of the first and second liquids;
(c) passing the mixture of the first and second liquids through the
screen element to produce a third liquid having little or no
undesirable material; and (d) discharging the third liquid from the
filtering device. As a result of this filter forming process, at
least some of the comminuted cellulosic fibrous material introduced
with the second liquid is retained on the screen element to form a
permeable mat of cellulose material. The mat of cellulose material
acts as a filtering medium for the undesirable material in the
first liquid. The mat is retained on the screen element and is used
to filter liquid streams flowing through the filter.
[0011] The filtering device is preferably a pressurized device
having a cylindrical screen element. For example, the filtering
device with cellulose mat may be a modified MODUScreen FF Pressure
Filter provided by Andritz-Ahlstrom. Preferably, the second liquid
containing at least some comminuted cellulosic fibrous material
(for example, hardwood or softwood fibers), is mixed with the first
liquid prior to being introduced to the filter inlet. The second
stream containing the cellulose fibers may be introduced
continuously to the first stream or may be introduced
intermittently. For example, the cellulose steam may be
automatically introduced in response to the state of the cellulose
web or mat produced on the surface of the filter element, for
example, as indicated by an electrical signal corresponding to the
pressure drop across the filter element.
[0012] The invention may also include a system for treating a
liquid stream in a pulp and paper mill to remove undesirable
material from the liquid stream. The system comprises a filtering
device having a perforated screen element having a filtering
surface, an inlet, and a filtered liquid outlet; means for
introducing a liquid containing undesirable material to the inlet;
means for introducing a liquid containing at least some comminuted
cellulosic fibrous material to the inlet; and means for forming a
bed of comminuted cellulosic fibrous material on the filtering
surface of the screen element to provide a finer screening medium
than the screen element without the comminuted cellulosic fibrous
material present. The screen element is preferably a cylindrical
screen element. The cylindrical screen element may be stationary or
it may rotate. The filtering surface of the cylindrical screen
element may be an external surface or an internal surface, that is,
the liquid may pass radially inward or radially outward through the
cylindrical screen element.
BRIEF DESCRIPTION OF FIGURES
[0013] FIG. 1 is an isometric view, partially in cross section, of
a conventional fiber filtering device with which the present
invention can be used.
[0014] FIG. 2 is a schematic diagram of partial cross section of a
fiber filtering device according to the prior art.
[0015] FIG. 3 is a schematic like FIG. 2 showing one embodiment of
the present invention.
[0016] FIG. 4 is a schematic diagram of a system in which the
present invention is practiced.
DETAILED DESCRIPTION OF FIGURES
[0017] FIG. 1 illustrates a fiber filtering device 10 in which the
present invention can be implemented. The device shown is a
conventional MODUScreen.TM. FF Pressure Filter provided by
Andritz-Ahlstrom Inc. of Glens Falls, N.Y. This device consists of
a cylindrical housing 11 containing a stationary perforated
cylindrical screen basket 12 and a cylindrical rotor 13. The rotor
13 is driven by an electric motor 14 through a conventional drive
train (not shown). The housing 11, motor 14, and drive train are
mounted in a base structure 15.
[0018] The screen cylinder 12 may be stationary or non-stationary
and the rotor 13 may be stationary or non-stationary. The exemplary
device 10 shown in FIG. 1 includes a stationary screen cylinder 12
and a rotating rotor 13. The rotor 13 includes a series of
protrusions or "bumps" 16. The leading edge of these bumps acts as
a hydrofoil to impart a hydraulic pulse to the liquid on the inside
of the basket. The trailing edge of the bumps create a pressure
drop which momentarily draws the liquid back through the holes to
minimize screen pluggage. The bumps shown in FIG. 1 are
hemispherical. However, the bumps may be implemented as a variety
of shapes and contours, including rectangular, triangular, and
elliptical shapes and contours. The screen cylinder 12 is
perforated with round holes. Although any type of perforations,
e.g., non-circular, may be used, including short or elongated slots
of rectangular or elliptical shapes. These holes in the screen
cylinder are typically less than 1 mm in diameter, preferably less
than 0.5 mm in diameter. For the MODUScreen FF of Andritz-Ahlstrom.
the holes are typically about 0.2 mm in diameter. The perforated
cylinder rotor basket 13 may be fabricated from machined plate, as
is conventional, or from a parallel-bar type construction.
[0019] In the fiber filter 10 shown in FIG. 1, liquid containing
undesirable cellulose fibers or other material is introduced
tangentially to the top the housing 11 via an inlet conduit. This
tangential inlet flow imparts a centrifugal force to the inlet
liquid flow and, in particular, to any larger, denser, "tramp"
materials (such as rocks, stones, nuts and bolts, etc.) in that
flow. The centrifugal force causes the tramp material in the flow
to collect in the periphery of the top of the housing 11 and, if
desired, is discharged from tangential outlet 17. In the embodiment
shown in FIG. 1 outlet 17 is closed by a blind flange.
[0020] From the inlet at the top of the filter, the liquid bearing
the undesirable material passes downward through an annular slot
between the stationary screen basket 12 and the rotor 13. The
annular gap between the basket and rotor is between about 2 to 10
mm. With the aid of the pressure pulsations generated by the bumps
16 on rotor 13, the liquid passes through the holes of the screen
basket 12, and collects in the annular chamber 18 between the
screen 12 and the housing 11. By passing though the screen
cylinder, the liquid is "accepted" by the filter. The liquid
"accepts" is then discharged through a tangential outlet 19, and
passes to further processing, for example, to an evaporator or
washing apparatus.
[0021] The oversized or dense material that does not pass through
the screen cylinder 12, continues to flow along the length of the
annular gap between the screen and rotor and collects in the
annular space 20 below the screen basket 12. This "reject" material
is discharged from the space 20 via reject outlet 21, and is
typically forwarded to recover useful fiber, for example, to a
refiner or the reject material may be otherwise disposed.
[0022] As described above, the prior art, as exemplified by the
filtering device 10 shown in FIG. 1, is limited to removing
undesirable material having a size greater than the perforations in
the screen cylinder 12. Smaller particles of undesirable material
could be removed by reducing the size of the perforations in the
cylinder, which places an excessive burden upon the fabrication
process, or placing a cloth-type barrier over the cylinder, which
is prone to damage. Both of these alternatives are undesirable. The
preferred alternative according to the present invention is
illustrated in FIGS. 2 and 3.
[0023] FIG. 2 illustrates a partial, cross-sectional view of the
cylindrical screen 12 and the cylindrical rotor 13 having spherical
bumps 16 shown in FIG. 1. In the conventional design of such fiber
filter devices, the liquid bearing the undesirable fiber and other
material passes into the annular gap 22 between the screen and
rotor as shown by arrow 23. As the liquid passes through the
annular gap 22, the liquid passes (flow arrows 29) radially outward
through the holes (or perforations) 24 in cylinder 12. This liquid
then passes to the accepts outlet 19 (shown in FIG. 1). Desirably,
the fiber 25 and other undesirable materials 26 are prevented from
passing through the screen holes and continue down the length of
the annular gap 22 as shown by arrow 27 to be discharged from
reject outlet 21 (shown in FIG. 1).
[0024] As shown in FIG. 2, particles 28 smaller than the holes 24
in the screen cylinder 12, can pass through the holes 24 and be
included with the accept flow discharged from outlet 19 (see FIG.
1). These small particles 28 may be small cellulose fibers
(referred to as "fines"); pigments, fillers, and other additives
from recycled paper or paper machine white water; scale; products
of corrosion; metal-containing compounds; or any other smaller
particulate materials found in pulp and paper mill liquid streams.
These small particles 28 can interfere with the operation of the
devices to which the accept flow is passed, for example, these
particles or their accumulation, can interfere with proper
operation of evaporators, heat exchangers, pulp cooking equipment,
and pulp washing equipment.
[0025] According to the present invention, the concentration of
these small particles is minimized. FIG. 3 illustrates a partial
cross-sectional view of a cylindrical screen 30, similar to the
screen 12 shown in FIG. 2. The construction, operation, and hole
size of the screen 30 is essentially identical to the screen 12 as
shown in FIG. 2. However, a layer or mat of cellulose material 31
is introduced to an internal surface of the screen cylinder 30.
This layer of material 31 acts as a further barrier to the passage
of small particles 32 through the screen perforations 33 and into
the accept flow shown by arrows 34. The small particles 32 are of
the same diameter as the particles 28 that pass through the holes
in the screen cylinder, but for the fiber mat 31. This layer of
material 31 is comprised of overlapping cellulose fibers introduced
to the inlet of the screen housing 11 of FIG. 1. Because of the mat
of cellulose fibers on the screen cylinder, the content of fine
particles 32 in the accept flow discharged from outlet 19 (in FIG.
1) is minimized. The absence of these small particles will prevent
the small particles from interfering with the operation of the
equipment to which the accept flow is sent, and will reduce the
detrimental effect due to small particles on the operation of that
equipment.
[0026] FIG. 4 illustrates one system 40 in which the present
invention can be practiced. FIG. 4 includes a fiber filter 41, for
example, an Andritz-Ahlstrom MODUScreen FF, which receives a liquid
flow containing comminuted cellulosic fibrous material, for
example, softwood fibers or other solid contaminants, from source
42 and forwards a filtered liquid having little or no comminuted
cellulosic fibrous material to a process 43. The source of fiber
containing liquid 42 may be a cellulose pulp digester, either
continuous or batch, a pulp washer, pulp wash press, paper machine,
or any other source of liquid that contains cellulose fibers or
other solid contaminants that are preferably removed. The process
43 may be a recovery process, for example, an evaporator; a washing
process, for example, a diffusion washer or drum washer: or any
other pulp or paper mill process that can benefit from the removal
of cellulose fibers and other undesirable material from the liquid
stream which is fed to it.
[0027] The fiber filter 41 includes an unfiltered liquid inlet 44,
a tramp material outlet 45, a rejects outlet 46, and an accepts
outlet 47. The inlet 44 receives a stream of liquid from conduit
48. The tramp material outlet 45 discharges to conduit 49 and
typically to disposal. The rejects outlet 46 discharges to conduit
50 and to further processing or disposal. The accepts outlet 47
discharges to conduit 51 which feeds the filtered liquid to the
downstream process 43. Dilution liquid may also be introduced to
device 41 as is conventional.
[0028] The system of FIG. 4 includes a source of comminuted
cellulosic fibrous material 52, for example, hardwood or softwood
fibers, which is fed to the inlet 44 of filter 41 via conduits 53
and 48. As described with respect to FIGS. 2 and 3, the fibers from
source 52 are introduced to filter 41 to provide an additional
filtering medium 31 (see FIG. 3) that minimizes or prevents the
passage of fine particles through the filter 41, specifically
screen 30 of FIG. 3. Source 52 may be any source of cellulose
fibers, for example, a high-density pulp storage tank or a pulp
washer or pulp press. The fibers from source 52 may be any form of
fibers that will form a filtering medium, e.g., a mat, on the
screen surface such as screen 30 of FIG. 3 within filter 41,
including hardwood and softwood fibers; recycled fibers, including
fibers from waste paper (MOW, ONP, etc.) and old corrugated
container (OCC); fibers from agricultural waste, such as bagasse;
and fibers from grasses, such as straw or hemp; or any other form
of fibers. The fibers in conduit 53 may be supplied as a slurry at
any available fiber to slurry weight concentration (e.g.,
consistency), for example, from 0.1% to 20% bone-dry (BD)
consistency. The fibers are preferably supplied at a consistency
less than about 12% BD, and even more preferably, less than about
5% BD.
[0029] The flow of fiber-containing slurry or liquid in conduit 53
may be regulated by a flow control valve 54. The flow control valve
54 may control the flow in conduit 53 in response to a control
signal 58 received from a flow controller 55. This flow controller
55 may control the flow in conduit 53 based on a manual operator
input or based on other control parameters in the pulp and paper
mill. For example, the controller 55 may control flow in conduit 53
as a function of the pressure drop across the screen 30 (see FIG.
3). This pressure drop may be obtained from the pressure indicator
signal 56 received from the pressure indicator in conduit 48 and
the pressure indicator signal 57 received from the pressure
indicator in conduit 51. The difference in these pressures
indicates the pressure drop across the filter screen. In one mode
of control, an increase in this pressure difference indicates that
a sufficient layer of mat of fiber has been established on the
screen, and that less or no further fiber need be introduced to the
filter. A drop in the pressure difference across the screen
indicates that more fiber should be added to re-establish the
filtering fiber mat.
[0030] The flow through valve 54 may also be controlled as a
function of the flow in conduit 48 as indicated by flow sensor 59
or the rate of flow through conduit 53 as indicated by flow sensor
60. These flow sensors 59, 60 are conventional flow detecting
devices, such as magnetic-type flow meters, or "mag" meters. The
control signal 61 from sensor 59 and the control signal 62 form
sensor 60 provide a flow indication to controller 55. For example,
the controller 55 may control the amount of flow passing through
valve 54 as a function of the flow through flow meter 59. As the
flow through meter 59 increases, the flow of fiber-containing
liquid through valve 54 can be increased. Also, the sensor in
conduit 51 having a control signal 57 may also be turbidity meter,
that is, an optical sensor which indicates the clarity of the
liquid passing through conduit 51. As the clarity of the liquid
decreases, suggesting an increase in undesirable material in the
liquid passing through conduit 51, the flow of fiber-containing
liquid through valve 54 can be increased to increase the filtering
effect and increase the clarity of the liquid in conduit 51. Other
control mechanisms will be apparent to those familiar with the
art.
[0031] In the above disclosure all specific ranges within a broad
range are also included herein. For example, 1-20% includes ranges
such as 1-3%, 2.4-8%, 1.36%, and all other narrower ranges within
the broad range.
[0032] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
and methods included within the spirit and scope of the appended
claims, and as broadly as allowed by the prior art.
* * * * *