Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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UNDERSHOT OVERFLOW FILTER
This invention relates to a fluid filter apparatus for use in
fluid/particulate matter
separation/concentration, and in one particular example of the separation of
fluid
from fluid entrained with waste material, particularly sewage waste.
BACKGROUND
The process of filtering fluids and their fluid borne elements can be viewed
in two
ways. In one way it is the act of removing an unwanted part of what is flowing
in
the fluid much like the action of sieving. In another way it is the act of
separating a
portion of the fluid out of the fluid and thus concentrating the remaining
fluid.
Clearly, a filter element can be used in either operation but what is clear
from the
known prior art is that regardless of the desired action there will be
blocking of the
filter to some degree. Once a portion of the filter is blocked its efficiency
reduces and
over time the filter will completely block. Maintenance of such filters is an
unwelcome and typically expensive necessity.
In this specification, a filter element used in a suitable filter arrangement
will be
described in relation to its application to storm water and sewage overflow
conditions and in particular to a concentration application in overflow
conditions. It
should however, be understood that the filter element and the filter
arrangement
concepts discussed herein, are used as an example only and that the
arrangements
described can with appropriate adjustment be used for separation and
concentration
applications.
It is a practice in some countries or with some water authorities in certain
countries
to run both storm water and sewage in the same pipe system. The pipe system
carrying sewage typically terminates at either a treatment plant or a water
outflow
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sometimes into rivers and other times into the sea. Regardless of whether
sewage
runs in pipes separated from storm water or combined overflow conditions can
arise
Engineers responsible for designing such systems can identify the maximum
volume
of combined storm water and sewage that can be accommodated into the water
treatment plant and hence the capacities of respective pipe systems flowing
into that
treatment plant.
When these maximums are exceeded, at one or more of the potential bottlenecks
in
tile pipe systems, alternative flow paths including pipes and open-air
conduits
communicate the overflow to other parts of the system or directly to an
acceptable
outflow point.
The simplest way of providing an overflow mechanism is to include a weir over
which flow volumes greater than the maximum calculated, can be directed away
from the main flow of storm water and sewage.
A simple weir has the benefit of being easy to implement and can be installed
either
at the time of the creation of the pipe system or with difficulty, retrofitted
into an
existing pipe work system.
The great disadvantage of simple weirs is that the storm water and sewage is
diverted untreated into the alternative route. In some instances, this
untreated
overflow is exposed to the atmosphere as it travels along open culverts and
the like
thus creating a potential health hazard. Furthermore, weirs create a head-loss
in the
pipe system, which is often unacceptable and requires expensive changes to the
existing pipe work.
In some systems when flow volumes are greater than the maximum that can be
accommodated, the pipe systems fill and the contents backup and at certain
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upstream inspection points, untreated overflow escapes the pipe system onto
roads,
parks and footpaths. This occurrence is liable to create health problems and
becomes
an embarrassment to the relevant water authority.
Not unsurprisingly, a common alternative to a simple weir is the use of a
filter
element which is intended to remove solid wastes from the overflow and hence
lessen the health related risks. This concentrating function is designed to
leave the
unwanted sewage in the pipe and only overflow relatively clean fluid. Such a
filter is
designed typically to come into operation only when the overflow condition
occurs.
Unfortunately, the filters and filter arrangements tried thus far, will
eventually block
and become inoperative. Surprisingly, it is not the solid sewage waste that
causes
the majority of blockages but in fact cellulose fibres are the cause. These
fibres are the
result of the breakdown of toilet paper. Cellulose fibres are long and easily
bridge
between the elements of the various filters used thus far. As time progresses,
the
fibres build up until a portion of the available flow path through the filter
is blocked.
This portion grows, eventually ceasing the flow of fluid through the filter.
Worse still, even though a first period of overflow ceases, accumulated fibres
do not
simply fall from the filter elements but harden and form a massive paper mache-
like
obstruction over portions of the filter. This built up mass is limpid-like and
does not
dislodge under normal environmental conditions and requires manual, mechanical
or high pressure back washing to remove it from the filter element.
If maintenance is not undertaken on a regular basis, expensive as it is, then
it is to be
expected that the mass will increase and the filter will eventually become
completely
blocked.
When a filter does become blocked, it merely means that filtered storm
water/ sewage will not be able to overflow as required through the filter and
that
alternative overflow mechanisms must also be put in place. These alternative
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mechanisms allow unfiltered storm water and sewage to be diverted from the
main
flow pipe system and ultimately are no better than a simple weir.
Alternatively,
incoming sewage backups the existing pipe system and overflows at some other
point further away from the treatment plant.
Thus, it is an object of the invention to reduce, minimize or eliminate some
at least of
the above-mentioned problems or at least provide an alternative, not only for
storm
water and sewage pipe works systems but also for any filter element used for
separation or concentration.
Specific embodiments of the invention will now be described in some further
detail
with reference to and as illustrated in the accompanying figures. These
embodiments
are illustrative, and not meant to be restrictive of the scope of the
invention.
BRIEF DESCRIPTION OF THE INVENTION
In a broad aspect, the invention is a filter for concentrating or separating
in a fluid
stream containing particulate matter, the filter consists of:
a plurality of solid elements in an array having gaps between adjacent solid
elements
through which spill flow of the fluid passes, and the fluid stream and filter
orientated
with respect to each other such that the spill flow of fluid through adjacent
gaps is
bounded by a dividing stream line that terminates on a solid element
intermediate
adjacent gaps, and further arranged so that the incidence of the dividing
stream line
to the plane of the array is such that particulate matter larger than said
gaps are
deflected out of the spill flow between said gaps.
In a further aspect of the invention the filter consists of at least two
chambers on the
spill flow side of the filter arranged to receive spill flow so as to maintain
substantially the same incidence along the fluid flow length of the filter
adjacent each
said chamber.
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In yet a further aspect of the invention of the filter described above, the
one or more
chambers have spill flow control means sized and positioned to allow spill
flow to
exit each chamber so as to maintain the incidence.
Suggestions and descriptions of other embodiments may be included within the
scope of the invention, but they may not be illustrated in the accompanying
figures
or alternatively features of the invention may be shown in the figures but not
described in the specification.
BRIEF DESCRIPTION OF THE FIGURES
Fig 1 depicts a top perspective view of a filter arrangement according to the
invention;
Fig 2 depicts a cutaway version of the filter arrangement of Fig 1;
Fig 3 depicts a top view of the filter arrangement of Fig 1;
Fig 4 depicts a side view of the filter arrangement of Fig 1;
Fig 5 depicts a further embodiment of the filter arrangement of Fig 1;
Fig 6 depicts a close up perspective view of the filter element;
Fig 7 depicts a side view of the principle of dividing streamline separation,
Fig 7a depicts a variant of the filter element depicted in Fig 7;
Fig 8 depicts a top view of a further embodiment of a filter assembly showing
individual partitions located above a filter element;
Fig 9 depicts a side view of the filter assembly of Fig. 8;
Fig 10 depicts a perspective cut away view of the filter element used in the
embodiments for Figs. 8 and 9; and
Fig 11 depicts a top persecutive view of the filter element used in the
embodiments
for Figs. 8 and 9.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
A number of embodiments of inline filter arrangements will be described. The
embodiments described first can be considered larger scale variants of a
specific
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portion of the later embodiments. The described embodiments described may have
advantages in applications not explicitly referred to herein but the later
embodiments
have been found to be particularly useful in the storm water/sewage overflow
situation, but of course may have use in other applications.
In the embodiment depicted in Figs.1 to 5, an inline filter arrangement 10 is
installed
along an existing storm water/sewage pipe 12 in which storm water and sewage
flow from pipe 12 to pipe 12' through the filter arrangement 10. An overflow
pipe 14
runs parallel to pipe 12' for a distance after the filter arrangement 10.
The inline filter arrangement depicted in this embodiment is preferably
arranged so
that it has the same fall as the pipe 12 between the filter arrangement inlet
16 to the
filter arrangement outlet 18.
The inline filter arrangement can be used at any location along or at the
termination
point of a storm water pipe system. The filter arrangement may also be used
other
than in-ground situations and may also be used in storm water only systems as
well
as industrial fluid filtering applications.
Fluid and entrained pollution/sewage enters the filter arrangement 10 via pipe
12
and inlet 16 so as to pass below a filter element 20 which in this embodiment
comprises a "cheese grate" screen as will be described in detail later in the
specification.
As best depicted in Figs 2, 3 and 4 the filter element 20 in this embodiment
is shown
running parallel to the lower surface 22 of the filter arrangement 10. Arrows
marked
'M' show the continuation flow of incoming storm water and sewage while arrows
marked'F' indicate the flow of filtered water (spill flow).
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In other embodiments not shown, the filter element 20 may be sloped so that it
is
closer to the lower surface 22 at the outlet end than the inlet end of the
filter
arrangement 10. In another embodiment not shown, the filter may be angled so
that
the side of the filter is higher on the side of the filter arrangement at
which wall 24 is
located and lower on the opposite side which is nearer the middle of the
filter
arrangement 10.
The height of the filter element 20 above the lower surface 22 is generally
determined
with knowledge about the range of the rate of incoming storm water and sewage
entering the filter arrangement and the particular rate is known, about 50% of
the
pipe work inner diameter, that should be reached before the incoming fluid
needs to
be concentrated so as to produce a spill flow. In this embodiment, as the
particular
incoming flow rate range is known, the height of the filter element is set
above the
lower surface 22 at approximately 50% of the diameter of the incoming and pipe
12.
This setting is such that at that a predetermined incoming flow rate, storm
water and
sewage will rise to that level and the filter/concentration action will
commence.
The final height of the filter is also dependent upon other factors, such as
filter
element 20 efficiency, the dimensions of the incoming pipe 12, the filter
assembly
dimensions in length and breadth and the outgoing pipe 12' dimensions (which
are
typically the same as the incoming pipe 12). Thus even though in Figs 1 to 5
the
length of the filter arrangement 10 seems long compared with the diameters of
the
pipe work (12,12' and 14) that length may be shorter or longer depending on
the
volumes of incoming and spill flow desired and the type of fluid or entrained
particulate matter.
Once the filter arrangement is in operation and a spill flow is produced,
filtered fluid
flowing through the filter element 20 falls off the side of the filter element
nearest the
middle of the filter assembly 10 into a overflow chamber 25 which is
preferably of
equal volumetric dimensions to that of the chamber on the filter side of the
filter
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arrangement 10. The overflow chamber 25 communicates filtered fluid to the
filtered
fluid outlet 26 of the filter arrangement, which is in fluid communication
with
overflow pipe 14. The reason the overflow chamber 25 is the same dimensions as
the
filter size dimensions is so that if necessary incoming fluid can be diverted
without
restriction.
The term "undershot overflow filter' best describes the use of the filter
element in
this embodiment as the bulk of fluid containing particulate matter
(contaminates)
flows under the filter element (continuing flow) and the filtering
(concentration)
function of the filter arrangement is used to cope with an overflow (spill
flow)
condition.
The flow of incoming fluid is arranged to submerge the filter element 20 to a
depth
where the vertical components of velocity of the fluid are small relative to
the flow
passing below the filter so that fluid passes up through the filter element
but not the
particles contained in the fluid passing below except those smaller than the
gap. This
characteristic of the filter arrangement is described in greater detail later
in the
specification and is referred to as dividing stream separation. The conditions
to
achieve dividing stream separation can be achieved in a number of ways. When
using the embodiment disclosed in Fig 1, an arrangement of weirs and/or header
tanks can be positioned downstream of the filter assembly to create some of
the
conditions required.
The filter arrangement 10 into which the filter element 20 is positioned is
used in
conjunction with weirs and/ or header tanks to control the relative velocities
of the
water flowing parallel and normal to the filter mesh so that the storm water
sewage
approaches the plane of the filter element 20, in this example a mesh, at an
angle of
less than approximately 3 % which is an expression representative of the
velocity
components normal and parallel to the plane of the filter mesh. This is
referred to as
"the incidence" in this specification.
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There is a face velocity (normal component) defined as the spill flow volume
rate
divided by the face area of the filter element from whence it came. There is a
continuation flow velocity (parallel component) defined as the mean velocity
component parallel to the filter element of that continuation flow at a point
outside
the boundary layer.
The particular incidence of 3 % in this embodiment is merely preferable and is
used
by way of example only and is not meant to be limiting in any way.
The incidence can also be varied by adjusting the depth of the spill flow
fluid above
the filter element relative to the depth of the continuation flow that
eventually flows
downstream of the filter.
Methods and apparatus for adjusting both of these depths include the placement
of
valves, restrictions and weirs external and internal of the filter
arrangement. As the
incoming flow rate changes, the depths can be optimised and controlled by, for
example, using notched weirs where the depth of water upstream of the weir has
a
desired relationship to the flow over the weir or by using apertures in
multiple
isolating chambers above portions of the filter element as will be described
in more
detail later in this specification.
In some applications it may be beneficial to use a non-planar filter element
so that the
incidence of the fluid to the filter element at various regions along the
filter element
can be made uniform over the entire area of the filter element for the widest
range of
input flow rates. The incidence in this case is then referred to by way of
reference to
the velocity ratio below the local plane of the filter element ie. that
average plane in
this embodiment below a portion of the filter element.
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Thus although not shown in any of the figures the filter element may be curved
or
comprise planar sections set at different angles relative to adjacent sections
so as to
provide a curved (non-planar) shape but will have an average plane for any
particular region of the filter element for the purposes of referring to the
incidence.
A further embodiment of the filter arrangement is depicted in Fig 5 that shows
a wall
28 positioned at one side and rising above the filter element 20 along the
longitudinal
center line of the filter arrangement 10. The wall 28 causes fluid and small
amounts
of entrained pollution, that has flowed up through the filter element 20, to
flow in the
direction of the continuation flow'M' until it reaches aperture 30 in wall 28.
The
filtered fluid flow'F' (spill flow) passes through the aperture and falls into
the
overflow chamber 25 and then flows into the overflow pipe 14. Again the length
of
the filter arrangement 10 is shown to be long with respect to the diameter of
the pipe
work but this is merely an example and it could be longer or shorter dependant
on
the sewage pipe work.
In other applications the filter arrangement is likely to be different but the
spatial
arrangement of the filter element to the flows will be the same.
At the inlet region of the filter arrangement 10, the inlet 16 opens to a
continuation
flow chamber 32. This chamber has a lower surface common to the lower surface
22
of the filter arrangement such that incoming fluid flows towards the outlet 18
of the
filter arrangement along the same gradient. The incoming fluid will continue
to flow
as continuation flow'M', through the filter arrangement 10 until the incoming
flow
of water is such that the level of fluid in the filter arrangement rises above
the filter
element 20. Even though the fluid will continue to flow through the inlet 16
and
outlet 18 of the filter arrangement 10, it will also pass a portion of the
fluid (spill
flow) flowing into the arrangement through the filter element 20 thus allowing
more
fluid to pass into the filter arrangement 10.
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It has been found preferable to shape the region at the entrance of the filter
34 so as to
streamline the flow of, in this example, storm water and sewage under the
dividing
wall 36 which precedes the filter element 20. This streamlining comes into
effect
when the flow of fluid into and out of the filter arrangement is such that the
level of
the incoming storm water and sewage rises above the filter so as to smooth the
passage of the continuation flow under the filter element.
In this embodiment, as depicted in Fig 4, a rounded half circle shape 35 is
located
across the bottom of wall 36 so as to streamline the flow of water entering
the filter
arrangement 10 from the incoming pipe 12. Reduction of turbulence at the
location
above the bottom level of the wall 36 eliminates an undesirable re-circulation
region
below wall 36.
Likewise, at the end of the filter element 20 of the filter apparatus, it is
preferable to
locate a sloped ramp 37 at the bottom of the wall 36 above the filter element
that is
closest to the outlet 18. The sloped ramp is shown in Fig. 4 extending into
the outlet
pipe 12' but the ramp could be located completely within the filter
arrangement 10.
The reduction of turbulence at the outlet reduces turbulence at the end of the
filter
element that would otherwise adversely affect the effectiveness of the filter
arrangement.
Also at the inlet end of the filter arrangement 10, there exists a wall 38
(weir)
separating the continuation flow chamber 32 from the overflow chamber 25. Once
the spill flow rate is exceeded and ever increases there will be a point, at
which the
incoming flow is large enough so as to require a bypass mechanism in addition
to
that, which is provided by the filter element. In this embodiment, storm water
and
sewage in the pass forward flow chamber 32 rises above the wall 38 and drops
into
the overflow chamber 25 and out of the filter arrangement via overflow pipe
14.
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An overflow mechanism of this type could also be provided further upstream as
well
as, or as an alternative to, that provided in the embodiment described.
The critical overflow rate will likely be defined by the water authority
engineers
based on calculated needs and the capacity of the existing pipe work system to
provide an alternative route for the overflow fluid that unfortunately will be
unfiltered.
In an example of an overflow mechanism being provided external of the filter
arrangement, the wall 38 will be extended to the roof of the filter
arrangement or be
otherwise configured so as to ensure that no overflow of unfiltered fluid
enters the
overflow chamber 25 or overflow pipe 14. Unfiltered overflow will then need to
be
directed by an upstream weir to another filter-like arrangement or to an
acceptable
outfall or even a different treatment plant to that which it would otherwise
have been
expected to be directed to.
The filter element 20 comprises in this embodiment, a "cheese grate" mesh, a
detailed
illustration of which is provided in Fig 6.
Referring to Fig 4, arrows indicate how fluid containing sewage flows under
the filter
element (mesh) 20. While doing so, a certain portion of the fluid flows
backward
through the mesh (relative to the continuation flow) and then flows above the
mesh
20. It has been found that the flow pattern above the mesh 20 is a good
indicator of
how evenly the mesh is operating. One filtered fluid flow arrangement is to
have the
filtered flow (spill flow) move sideways off the mesh 20 and into the overflow
chamber 25. When the mesh 20 is working evenly, the water has little or no
flow in
the same direction as the main flow, i.e. no stream-wise flow and it only
moves
sideways. A stream-wise component above the mesh 20 seems to indicate that
there
is some re-circulation. The sideways movement appears to result from the
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momentum of spill flow falling into the overflow chamber 25 and water surface
tension forces.
In this embodiment the flow above the mesh is shown moving in the same
direction
as the continuation flow, both flows being in the direction of respective
outlets 26
and 18. The water flowing above the mesh of course, is filtered and first
falls into the
overflow chamber 25 and then eventually exits the filter arrangement 10
through
outlet 26 into overflow pipe 14.
Fig. 6 depicts a bottom view of a portion of one mesh type (referred to herein
as a
cheese grate mesh) with the arrow'M' showing the direction of the continuation
flow
of the storm/ sewer fluid.
Fig 7 pictorially depicts the mechanism that produces the results described
above
and that is evident in other embodiments that are yet to be described in this
specification. The mechanism is referred to herein as "dividing streamline
separation" .
A portion of a cross section of the "cheese grate" mesh and or a slat-like
grate array
as will be used in another embodiment as depicted in Fig 7.
Solid lines show the mesh/ grate elements and the dotted lines in the water
are
virtual in the sense that they do not exist but will in this particular
example, assist the
depiction of the method of operation of the filter. Line 70 represents the top
of the
continuation flow chamber 32 in particular, the lowest portion of partition
wall 36.
Unfiltered fluid is shown flowing into the region below the filter element 20
and
dotted line 72 defines a height X1 of fluid which will flow upwards into the
gap
between grate element (a) and grate element (b).
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Upwards flow of water (spill flow) and not entrained particulate matter,
occurs as a
result of a number of influences. Control of the previously mentioned
incidence
angle of the continuation flow onto the mesh along its length is an important
influence on the successful use of the filter arrangement. Dividing streamline
separation contributes to non-blocking of the mesh because the fluid borne
particles
passing through the mesh are much smaller than the separation between the mesh
apertures, viz. the dimensions of X1 and larger particles are drawn further
along the
filter arrangement. This arrangement appears counter-intuitive and indirect
contrast
to the way in which existing screens and filters work that eventually trap
particles
between their elemenfs because the particles are being drawn towards and into
the
gaps between solid elements of the mesh/ grate and are larger than the
provided gap.
Particles are larger than the gap bridge across the gap between mesh elements
whatever their spacing or orientation.
Therefore in this filter arrangement, any entrained particle or cellulose
fibre flowing
in the stream having less than height X1 will pass through the filter element
while
particles and other fibres, the bulk of which primarily fit below dotted line
72, will
flow past that particular portion of the filter element.
However, fluid and particles which flow fully within dotted Iines 72 and 74
that
defines height X2, will flow through the gap between grate elements (b) and
(c).
Likewise, the fluid and particles flowing fully within the dotted lines 74 and
76 that
defines height X3 will flow through grate elements (c) and (d) respectively.
Generally, particles having a centre of gravity below a respective line will
be drawn
past the streams heading towards gaps in the filter element.
Thus it can be seen from the pictorial representation in Fig 7 that there
exists a
dividing streamline 100 that terminates on each solid element of the filter
element
and that flows that gaps between the solid. elements is bounded by those
stream
lines. Particulate matter that fits between the solid elements and the bounds
of the
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dividing streamline ie. one dimension being X1 will flow between the gap
between
adjacent solid elements with the spill flow. Particulate matter that does not
fit will
not be drawn into the gaps and thus not bridge the gap and cause a partial
blockage
of the filter element.
A filter configuration of this type has been found to be non-blocking in most
conditions including low and high flow rates and regardless of the nature of
the
entrained contaminates, for example and in particular, long cellulose fibres
such as
those described previously. The dotted lines depicted are merely schematic and
will
vary in shape and spacing dependent on flow rates (continuation and spill) so
as to
reflect the principle as understood and described herein.
The "cheese grate" mesh has been found useful in certain circumstances in that
there
is little or no collection of debris, however other mesh or grating types
could be used
and have a similar or better results. For example, the mesh or grate may be
formed of
a parallel array of rectangular, square or round bars which are located
laterally with
respect to the flow path.
A yet further embodiment of the filter arrangement is depicted in Figs 8 and 9
that
show ten (10) independent compartments above an elongate filter element. The
filter
element comprises an array of parallel louvres located laterally with respect
to the
continuation flow and with each generally planar filter element angled with
respect
to the flow so that the gaps between louvres are directed in an opposite
direction to
the continuation flow M as is depicted in Fig 10. The arrangement of the
louvres in
cross-section is not unlike that depicted in Fig 7 so that a condition for
dividing
streamline separation can occur.
However, there are a number of characteristics that need to be controlled if
the
benefit of a non-blocking undershot overflow filtering arrangement is to be
achieved
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and optimised. These characteristics have been identified at this time by way
of
modelling and experimentation.
Referring to the filter element depicted in Fig ~ and in particular an element
in the
form of an array of louvres, the length L of the louvre, the thickness T, the
gap G and
the pitch P of the individual louvres of the grill are all variables relevant
to some of
the characteristics of such an a filter arrangement. It is preferable to
flatten the lower
edge of the louvres, which lie at the interface of the filter element to pass
forward
flow as is depicted in Fig 7a. This modification is intended to lessen the
possibility of
turbulence at this region.
Pass forward flow is tangential and spill flow is normal to the plane of the
louvre
panel. The face velocity is the mean spill flow velocity component that is
normal to
the plane of the filter element 20. It is numerically equal to the spill flow
volume rate
divided by the face area of the panel of the filter element as described
previously.
It is a preferable feature to maintain the face velocity substantially
constant over the
length (along the direction of flow) of the panel of the filter element, in
this example
the array of louvres.
Spill flow is that flow of filtered water filtered fluid leaving the filter
apparatus and
the spill ratio is a ratio of the volume of filtered outflow (spill flow) over
the total
fluid flowing into the filter assembly. Thus a filter assembly accepting 21
litres per
second having a spill flow of 10 litres per second has a spill ratio of
approximately 48
per cent. The continuation flow would be 11 litres per second.
The higher the spill ratio, the better the performance of the filter as long
as the filter
elements does not collect debris and become blocked.
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It may be that it is possible to optimise the filter arrangement for non-
blocking at a
relatively high spill ratio but that at a lower spill ratio blocking or
fouling could
occur.
It has been recognised that there are a number of variables that need to be
optimised
to achieve the best possible combination of spill ratio and filter non-fouling
across the
broadest possible range of input flow rates wherein the spill ratio is greater
than 1.
One of those characteristics is the incidence of the continuation flow to the
plane of
the array of louvres. This incidence leas been found to be preferably in the
range 1 to
3 % as described previously and if maintained along the length of the filter
element
then fouling is very unlikely or non-existent. Maintenance of a constant
incident
would appear to enhance the principle of dividing stream separation discussed
previously and lead to a maximum of spill flow from a filter of any given
size.
However, to achieve uniformity of this characteristic it is firstly necessary
to keep the
filter length relatively short as wall friction and stream wise gradients of
velocity
cause variations in the pressure differential across the louvre array, leading
to non-
uniform incidence. The pressure differential across the louvre array is the
driving
force producing the face velocity. As the length of the filter element
increases, the
ratio of energy loss to the total energy of the fluid increases and a non-
linear gradient
of total energy occurs. The angle of incidence becomes non-uniform along the
length
of the filter and if too high, causes fouling, and if too low, reduces the
average face
velocity.
Modifying the tilt of the filter element can initially and at certain
continuation flow
rates be beneficial, but beyond a certain angle (which is different for
different flow
rates) the positive effect can transform into a negative effect such that face
velocity
not only reduces but reverses, ie, the filtered fluid above the filter element
is sucked
back into the continuation flow.
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18
Unfortunately, to have a high spill ratio and an appropriate face velocity,
the angle of
inclination of the filter element becomes too great and too much friction loss
occurs
with the consequences described above.
Compartmentalising the filter, that is, shortening the filter length and
keeping the
parameter of spill ratio, uniform face velocity and incidence constant within
the
region below the filter and respective partition helps to achieve a desirable
outcome.
The face velocity can be difficult to control.
It is possible, to at least in an active system, control the spill flow from
each
compartment. However, an active system is not an ideal arrangement as it adds
complexity and cost to a filter arrangement that in the stormwater pipe system
is
mostly remote of maintenance personnel. Such a system using moveable weirs or
valve actuations is possible in appropriate application such as industrial
filtering
where specific spill ratios may be required and maintenance of the
mechanical/hydraulic elements is less of an issue.
Of particular benefit in some application and particularly in the sewage
application
used as an example herein is the provision of a passive means that at least
within
acceptable boundaries, controls the spill flow from each chamber thereby
maintaining the desired incidence without the need for mechanical/hydraulic
elements.
The challenge therefore is to passively account for different spill flows
where there
exists a wide range of input flow rates.
Some of the devices that may be used as an outlet control for the spill flow
control
feature include a broad crested weir, an orifice, a combination of the two, a
v-notch
control and a slit. A simple circularly shaped orifice is preferred. The
outlet control
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19
device is preferably located above the filter element on a sidewall of a
respective
chamber. The circular orifice should be positioned so as to provide a head of
water
(acting like a weir) above the level of the filter element at a height such
that not only
is the spill flow from each chamber known (because of the known aperture
area), but
which also maintains a head in the chamber that is beneficial to the
maintenance of
the incidence and the inter-related face velocity. Additional orifices above
an~
existing one can be used to regulate spill flow when higher flow rates into
the filter
arrangement are encountered. Those orifices may be the same or different to
those
previously mentioned.
Fig 11 depicts a filter arrangement of the above configuration in operation
and the
spill flow can be seen emitting from three circularly shaped orifices located
one
above the other in each of the chambers used in that embodiment.
In the arrangement depicted in Figs 8, 9 and 10 the spill flow aperture is
shown as
circular holes arranged substantially one above the other however a slot of
appropriate dimension could also be used for the purposes described.
Consideration can also be given to the aspect ratio of the filter elements
such that the
chamber used to encompass a filter element is wider than it is long. It would
also be
possible to use a cylindrical filter element such that the fluid to be
concentrated is
swirled into the inside of the cylinder and chambers are arranged about the
outside
of the chamber to create ideal conditions for dividing streamline separation
to occur.
A conically shaped filter with a truncated apex may also work.
Such arrangements of course may well have more difficulty in being retrofitted
to an
existing pipe system, but may provide flexibility in certain circumstances.
Indeed, any configuration of filter element 20 which exhibits the non-blocking
characteristics desired, using a dividing streamline separation principle is
likely to be
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useful in certain circumstances including as mentioned previously in
industrial
applications requiring non-blocking filters to separate fluid from fluids
entrained
with particulate matter (particulate that may include long thin strands of
material).
It is a matter of experimentation with filter shapes and slopes, spill flow
rate, spill
flow aperture shape and location, face velocity, incidence as well as filter
element
sizes and aperture sizes to identify the most ideal arrangement for various
installation conditions as well as configuring the most suitable continuation
and spill
flow rates for a range of input flow conditions.
It will be appreciated by those skilled in the art that the invention is not
restricted in
its use to a particular application described. Neither is the present
invention
restricted in its preferred embodiment with regard to the particular elements
and/or
features described or depicted herein. It will be appreciated that various
modifications can be made without departing from the principle of the
invention.
Therefore, the invention should be understood to include all such
modifications
within its scope.
