FIELD OF THE INVENTION
This invention relates to a filter element for a liquid
filter of the type employing a filter medium, such as a fabric bag or sleeve through
which liquid is to pass in order to effect filtration, and wherein a filtration
aid such as diatomaceous earth is generally used to form a layer on the outside
of the filter medium during operation of the filter. During operation, dirt becomes
deposited on the layer of filtration aid, rather than directly on the filter medium.
More particularly, the invention relates to a filter element
of the type comprising a support frame adapted to be covered by a fabric bag or
sleeve as well as to a liquid filter including at least one, and generally a series
of such filter elements.
BACKGROUND TO THE INVENTION
Filters that employ a filter medium, generally in combination
with a filtration aid in order to avoid premature blinding of the filter medium
and thereby to extend the cycle time between back washing procedures, generally
operate on the principle that when the filter is turned off, the filtration aid
and dirt collected thereby fall to the bottom of the container. When the filter
is started up again, inlet fluid serves to stir up the filtration aid usually together
with at least some dirt from the bottom of the filter container so that the filter
aid becomes coated on to the filter medium once more.
One type of filter that has proved to be highly successful
is that described in my issued South African Patents Nos 93/0761 and 2001/2144.
This type of filter utilises a series of generally parallel elongate filter elements
that have a plastics injection moulded frame having, in cross-section, a number
of longitudinally extending ribs radiating from a central axis and the outer edges
of which support the fabric sleeve. The construction of the frame is dictated, to
some extent, by the tool design and, as a consequence, the operatively upper end
of each frame has heretofore been fitted with a moulded collar encircling the upper
end of the frame and having a flange that serves to support the downwardly extending
filter element in its operative position relative to a divider plate that supports
the filter elements in a generally vertical orientation.
This construction, whilst being substantially successful,
has proved to result, in some cases, in the possibility of stresses caused by forces
exerted on the filter elements, in particular during start-up, leading to failure
of the frame in the region of its upper end, and in particular at the collar. This
results from the fact that the upper end is the sole support region of the filter
element in use.
In addition, if the fabric bag or sleeve is not held adequately
tightly over the upper end of the frame, the fabric of the sleeve can be pushed
in, by liquid pressure, to the flow path between the radiating ribs and collar with
a consequent decrease in flow from the interior of the relevant filter element and
an accompanying decrease in filtration efficiency.
OBJECT OF THE INVENTION
It is accordingly an object of this invention to provide
a filter element frame that is adapted to strengthen the support of the frame at
its upper end and also, optionally, to diminish the likelihood of the fabric of
the bag or sleeve being pushed into the flow path past the collar edge.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a liquid
filter element frame comprising an elongate injection moulded body having a plurality
of support ribs radiating from a central axis thereof and a support collar encircling
the outer edges of the ribs at the operatively upper end thereof, the frame being
characterised in that the collar has a plurality of integral radially inwardly directed
support rails having inner ends cooperating with the moulded body between adjacent
support ribs thereof.
Further features of the invention provide for the moulded
body to comprise a plurality, typically six, radiating support ribs in which case
the collar has an equal number of inwardly extending support rails the inner edges
of which are received neatly in a corner formed between each pair of adjacent support
ribs; for the support rails to be flat in general construction with a tapered inner
edge for neat accommodation in the corner between an associated pair of support
ribs; for the support rails to extend downwards beyond the operatively lower end
of the collar; for the operatively outer edges of the lower ends of the support
rails to curve inwards towards the centre thereof; and for the operatively upper
end of the collar to have an integral outwardly directed support flange.
Preferably, the elongate injection moulded body consists
of a series of collinear injection moulded units each having an integral support
ring interconnecting the outer edges of the support ribs at the operatively lower
end thereof and an operatively upper end that, in the case of all but the uppermost
unit, receives the lower edge of the support ring of the super-jacent unit. In such
a case the operatively uppermost unit receives the collar of this invention with
its integral support rails and support flange at its upper end. The collar is bonded
to the uppermost unit by any suitable means such as ultrasonic welding or, more
conveniently, utilizing solvent welding or a suitable adhesive combined with a tight
fit of the collar on the element.
The invention also provides filter elements including a
filter element frame as defined above as well as a liquid filter including such
In order that the invention may be more fully understood
one embodiment thereof will now be described with reference to the accompanying
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:-
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
- Figure 1
- is a schematic sectional elevation of a liquid filter assembly of the type in
which the expedient of the invention is embodied;
- Figure 2
- is a general schematic sectional elevation of one filter element having a frame
composed of a series of collinear plastics injection moulded units the uppermost
one of which is fitted with a collar according to the invention;
- Figure 3
- is a similar but enlarged exploded view of the operatively upper end of the
filter element frame;
- Figure 4
- is a similar elevation showing the collar in the installed position on the uppermost
unit and showing also the next unit down;
- Figure 5
- is an end view of the collar alone;
- Figure 6
- is an end view of an injection moulded frame unit from the operatively upper
- Figure 7
- is an end view of the collar in the installed position on the upper end of the
associated frame unit; and,
- Figure 8
- is a partly sectioned elevation of the operatively lower end of a filter element.
The type of liquid filter assembly with which this invention
is concerned typically comprises a container (1) of a circular cylindrical shape
and having a concave lower end (2) and a correspondingly convex lid (3) removably
secured thereto. Towards the upper end of the container is a transverse, generally
horizontal, divider plate (4) which supports the upper ends of an array of generally
vertical elongate filter elements (5) each of which has a filter medium in the form
of a fabric sleeve (6) supported on a supporting frame (7) therefor.
Located in the side wall of the container towards the bottom
thereof is an inlet (8) fitted with a one way non-return valve (9) and connected
through a control valve (10) to an inlet connection (11) for the liquid to be filtered.
The upper end of the container has an outlet (12) for filtered
liquid through the side wall thereof located above the level of the dividing wall
In use, liquid enters through the inlet (8); passes through
the fabric filter medium of the sleeves; and thence up the filter elements (5);
into the upper region of the container (1) on the opposite and thus upper side of
the dividing wall (4); and out through the outlet (12). In the type of filter under
consideration the fabric filter medium supports, in the operative condition, a finely
subdivided filtration aid that is typically diatomaceous earth.
At the end of a filtration cycle the diatomaceous earth
and the dirt that has been trapped thereby fall to the bottom of the container and
must be stirred up in a substantially even manner when the filter is next started
It will be understood that the elongate filter elements,
being supported only at their upper ends, are subjected to certain stresses and
strains particularly during start-up and that the operatively upper end of the elements
must accept the force that is applied to the elements.
The frame (7) is elongate and is made up of a series of
collinear injection moulded units (13) shown most clearly in Figure 3 and each of
which has, in cross-section, six radiating support ribs (14) the outer edges of
which support the sleeve (6). The support ribs (14) are interconnected at their
outer edges by a ring (15) at one end, the operatively lower end thereof, with the
ring encircling the ribs and being moulded integral therewith. The operatively lower
end of the composite frame terminates in such a ring (15a) (see Figure 8).
As provided by this invention the operatively upper end
(16) of the uppermost frame unit (13) is fitted with a collar (17) that has integral,
equally angularly spaced, inwardly directed support rails (18). The support rails
have convergent innermost edges (19) that are made to fit tightly between the innermost
ends of the support ribs (14) so that each support rail is offset angularly by 30
degrees to each of the two adjacent support ribs of the injection moulded frame
unit. As shown in Figure 4, the collar has an internal flange (20) against which
the upper end of the injection moulded unit abuts in the installed position.
The support rails extend appreciably below the lower end
(21) of the collar, as indicated by numeral (22), so that a strong mounting for
the operatively upper end of each filter element frame is provided.
The collar also has an integral outwardly directed support
flange (17a) at its upper end.
The collar units are also injection moulded from plastics
material and are permanently secured to the upper end of the associated injection
moulded frame unit by any suitable means such as solvent welding or other bonding
agent or, alternatively, by ultrasonic welding. In any event the collar unit is
made a tight fit onto the end of the injection moulded frame unit so that no play
is possible between the two parts in the operative condition.
The filter element frame is finished at the operatively
lower end utilizing a disc (23) made of injection moulded plastics material and
having a rounded edge (24) as shown in Figure 8 and as more fully described in my
granted South African patent No 2000/2145 the disclosure of which is incorporated
herein by reference.
It will be understood that the radially extending support
rails that engage the operatively upper end of the support frame of the filter element
form an extremely firm and robust attachment for the frame to the divider plate
and, according to tests conducted the date, the collar of this invention exhibits
appreciably superior strength over previous attachment collars used heretofore.
Also, the arrangement of the support rails extending below the lower edge of the
collar itself is effective to prevent any filter fabric that may be loose from being
pushed by liquid under pressure into the flow passages through the collar and thereby
at least partially blocking them.
It will be understood that numerous variations may be made
to the embodiment of the invention described above without departing from the scope
hereof. In particular, it may be found adequate to provide, for example, only three
inwardly directed support rails in the case of six support ribs on the filter element
frame unit itself although the remaining three apertures may be subject to drawing
filter fabric into them in the event that it is too loose.