Filter underdrain

An underdrain for use with granular type liquid filtering systems including a plate of an inverted dished configuration having a plurality of laterally facing slots defined therein, the slots being defined by an upper edge and an associated lower edge, the upper edge being positioned almost directly above the lower edge such that the slots face horizontally outward, the slots being positioned upon the dished plate in a spiral array with each slot being formed by shearing of the plate material between two cutting edges positioned above and below the plate, the upper edges presenting downwardly facing arcuate edges and the lower edges presenting upwardly facing arcuate edges to define a slot having opposed facing arcuate sides, the plate may further define a flat horizontally extending imperforate annular center-section within the spiral array of slots and a flat horizontally extending imperforate annular outer-section extending about the spiral array of slots, the center-section may define a central aperture therein.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
The underdrain of the present invention is designed generally for usage 
with filtering chambers useful for the filtering and cleaning of liquids, 
particularly water, by passage of the liquid through a bed of filtering 
media usually granular as used in swimming pool filters and the like. In 
most designs the filtering chamber is formed by a filter tank which is 
partitioned into an upper section which contains the filtering media and a 
lower section from which the cleaned fluid is returned. These sections are 
partitioned or separated from one another by the underdrain. The liquid to 
be cleansed is supplied to the top of the filtering media and passes 
downwardly by the force of gravity and/or by pumping through the filtering 
media which cleans the liquid. The liquid then passes through the 
perforated underdrain into the lower section. The holes or apertures 
within the underdrain must be small enough to prevent the passage of the 
particularly chosen filtering media but large enough to admit freely the 
passage of the liquid therethrough. 
Such filtering systems must also provide a means for cleaning of the 
filtering media without withdrawing the filtering media from the filter 
tank. The usual process is a backwashing of a cleaning liquid through the 
filtering system in the reverse direction with respect to the normal 
direction of flow for filtering. 
2. Description Of The Prior Art 
One example of an underdrain configuration useful with the filtering 
devices of the present field of art is U.S. Pat. No. 3,519,135 issued July 
7, 1970. In this patent a perforated underdrain is utilized in which the 
apertures within the underdrain plate are chosen with a vertically 
extending pronged structure such that during backwashing the fluid is 
directed upwardly in a jetting action to aid in the freeing of dirt and 
other substances which have been accumulated during the immediately 
previous period of filtering operation. Other U.S. patents showing similar 
construction for particulate material filtering include U.S. Pat. Nos. 
3,142,642 and 3,625,365. These designs include various pipes, arms and 
apertured devices for providing full filtering of the liquid while still 
providing a convenient means of backwashing the filter tank. 
The present apparatus provides a unique design which overcomes many of the 
problems of the prior art since the backwashing water is directed 
outwardly toward the walls of the filter tank by the novel configuration 
of the slots within the filter underdrain. This advantage and further 
advantages of the present invention will be apparent from a review of the 
disclosure. 
SUMMARY OF THE INVENTION 
The present invention comprises an underdrain for use with filtering 
systems having granular filtering media. The underdrain may be chosen in 
the form of a plate having an inverted dished configuration with the 
concave surface thereof facing downwardly. The plate may define a 
plurality of laterally facing slots therein which are defined by a 
plurality of pairs of upper and associated lower edges such that the 
liquid to be filtered may flow through the underdrain plate whereas the 
filtering media will be held within the filtering tank thereabove. 
The underdrain of the present invention is adapted to be placed in a 
filtering tank with the filtering media held entirely thereabove whereas, 
prior art filters having various configurations such as slotted pipes and 
the like only supporting a portion of the media. The intake of liquid to 
be filtered will be at the top of the tank while the means for collecting 
filtered liquid will be located near the bottom of the tank below the 
underdrain. The slots in the underdrain plate are formed by shearing in 
the vertical plane such that the sheared edges which form the sides of the 
slots will be positioned vertically above each other. In other words, the 
aperture formed by the slot will extend solely in the vertical plane and 
will have little if any horizontal component. In this configuration 
backwashing action is greatly improved by a toroidal flow action created 
by the outwardly directed apertures. 
During a backwashing operation fluid is passed through the filtering media 
in a direction reverse with respect to the direction of filtering. With 
designs heretofore the apertures in the underdrain plate allowed channels 
to develop through the filtering media such that the backwashing fluid was 
of minimal use in cleansing the granular filtering media substance. Also, 
a "dead sand area" was developed around the walls of the tank which was 
not cleaned due to flow of the backwashing liquid vertically solely 
through the channels which allowed the accumulation of dirt and debris in 
the media even after the backwashing operation. 
One of the objects of the present invention is the full cleansing of all 
sand or other filtering media within the filter tank during a single 
backwashing operation. This complete backwashing operation is achieved by 
the orientation of the slots within the underdrain plate facing outwardly 
toward the walls of the filter tank away from the axis or center of the 
underdrain plate itself. In this manner backwashing fluid will be urged to 
flow toward the walls and upward along the walls in a toroidal motion such 
that a thorough mixing of the filtering media will occur. Another 
advantage of this design will be the thorough mixing of the sand or other 
filtering media above the underdrain to thereby fully homogenize the 
larger and smaller media particles to facilitate subsequent filtering 
operations and to aid in filtering variously shaped and configured debris 
when operating in the filtering mode. 
In order to aid in manufacture, the slots may be formed in the underdrain 
plate in a spiral array. However it should be appreciated that any design 
of laterally extending slots would be able to achieve the advantages of 
the present invention and therefore are anticipated herein. 
The underdrain plate may include a flat horizontal imperforate annular 
center-section which defines a central aperture therethrough to facilitate 
mounting of the underdrain within the filtering tank and to allow use of 
the underdrain plate with systems having a central flow conduit. Also the 
present invention may include a flat horizontal imperforate annular outer 
section to aid in mounting or handling of the underdrain plates. 
It is an object of the present invention to provide a filter underdrain 
which suspends filtering media thereabove while allowing full liquid flow 
therethrough. 
It is an object of the present invention to provide a filter underdrain 
which when operated in a backwashing mode cleans all of the filtering 
media located thereabove since a large percentage of the filtering media 
is directly supported by the slotted surface of the underdrain. 
It is an object of the present invention to provide a filter underdrain 
which is sturdy in construction and can withstand high operating 
temperatures. 
It is an object of the present invention to provide a filter underdrain 
which is strong enough to resist the development of high internal 
pressures within filtering systems. 
It is an object of the present invention to provide a filter underdrain 
system which is less prone to stoppage and breakage caused by corrosion of 
other underdrains having different configurations and formed from 
different materials. 
It is an object of the present invention to provide a filter underdrain 
system which homogenizes the filtering bed thereabove during backwashing 
operations. 
It is an object of the present invention to provide a filter underdrain 
which prevents channeling of the filtering media during backwashing 
operations. 
It is an object of the present invention to provide a filter underdrain 
which allows a low pressure drop thereacross because of uniformity in 
slotted openings. 
It is an object of the present invention to provide a filter underdrain 
which prevents the by-pass of sand because of the uniformity of slot 
openings. 
It is an object of the present invention to provide a filter underdrain 
which is easy to manufacture and inexpensive to manufacture. 
It is an object of the present invention to provide a filter underdrain 
requiring little maintenance thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention may include a downwardly 
facing concave circular underdrain 10 which is positioned within a filter 
tank 16. The underdrain 10 provides a lower support means for holding the 
filtering medium 20 such as sand or the like thereabove within the 
filtering chamber 18 defined by tank 16. 
In one embodiment a liquid supply 12 may extend into the upper portion of 
chamber 18 and a liquid return 14 may extend within the lower area of tank 
16. With this configuration liquid to be filtered passes through supply 12 
into chamber 18. This liquid then flows through the filtering media 20 
which collects debris and contaminants therefrom. This filtered liquid now 
passes through the slots 22 defined within underdrain 10 and is collected 
by liquid return 14 by passing through the orifices 36. 
Each of slots 22 is defined by an upper edge 24 and a lower edge 26. 
Preferably upper edge 24 is positioned approximately directly above lower 
edge 26 such that the aperture opening of slot 22 extends laterally and is 
defined almost solely within a vertical plane. The upper edge 24 is 
preferably of a arcuate vertical cross-section facing downwardly or 
outwardly and the lower edge 26 is preferably of an upwardly facing 
slightly arcuate cross-section to define the slot 22 therebetween as a 
long narrow slot extending laterally. 
The underdrain 10 of the present invention may include a flat horizontal 
imperforate annular cross-section 28 which may define a central aperture 
30 therethrough to facilitate mounting of the underdrain within the filter 
tank 16 or to facilitate usage of the underdrain 10 with commonly used 
filtering systems having a central conduit extending vertically 
therethrough. The underdrain 10 may also include a flat horizontal 
imperforate annular outer-section 32 which extends about the periphery of 
the underdrain to provide support to the edges thereto and facilitate 
handling thereof. Extending between the center-section 28 and the 
outer-section 32 the slots 22 may be configured in a spiral array or a 
plurality of circular arrays to facilitate backwashing and filtering 
operations. 
The embodiment shown in FIG. 1 illustrates the particular advantages of the 
present design when operating in the backwashing mode. With such filtering 
systems it is desirable to periodically backwash the system to cleanse the 
filtering media 20. During this operation backwashing fluid is supplied 
through liquid return 14 to pass through orifices 36. This backwashing 
fluid then passes upwardly through slots 22 to pass vertically upward 
through the filtering media 20. With prior art designs this backwashing 
has often resulted in the forming of channels which provide the least 
resistent vertical flow path for the backwashing fluid which greatly 
minimizes the cleansing effect of the backwashing operation. The present 
design utilizes particular configurations for slots 22 in which the upper 
edges 24 are positioned approximately directly above the lower edges 26 
and face outwardly. This outward facing is achieved by the upper edge 24 
being positioned directly above the lower edge 26 in the downwardly facing 
concave configuration. In this configuration the backwashing fluid flows 
as shown by arrows 38 in FIGS. 4 and 5. As the backwashing fluid flows 
upward and through slots 22 it is guided to flow in an outward direction 
toward the walls of the filtering tank 16 and then upward along the walls. 
In this manner the "dead sand area" which has heretofore existed adjacent 
the interior walls of the filter tanks is eliminated. The filtering media 
20 is thoroughly mixed by this lateral backwashing flow which fully 
homogenizes the variety of granular sizes existent within the filtering 
media 20 while also cleaning all of the filtering media. A homogeneous 
media 20 is desirable in order to aid in fully filtering all types and 
sizes of contaminants existing within the fluid to be filtered. 
The slots 22 of the underdrain 10 may be formed by shearing of the virgin 
plate in a vertical plane. In order to aid in manufacture it may be 
desirable to form the slots one at a time and in this manner a spiral 
array of such slots may occur. This spiral array of slots is desirable 
since a lower fluid pressure drop will therfore exist across the 
underdrain plate 10 due to the uniformity of slot openings. This 
uniformity of slot openings also prevents by-pass of filtering media 
therethrough. Prior art designs have overcome this filtering media by-pass 
problem by holding the filtering media within a radial array of arms which 
contain the filtering media therein. Water to be filtered passes into the 
center of the array through the media and passes to the collector through 
apertures in the radial arms. Such a system does not have the advantages 
of the present design since all the media is not supported by the 
underdrain. The present design has advantages over that system since the 
uniformity of slot configuration allows the entire filtering media 20 to 
be placed directly upon the underdrain 10 and therefore there is less of a 
pressure drop across the underdrain while at the same time filtering media 
by-pass is prevented. This combination of purposes, as well as the 
advantages of backwashing through a sand bed or filtering bed which is 
fully supported by the underdrain, has not been shown heretofore in the 
prior art. 
While particular embodiments of this invention have been shown in the 
drawings and described above, it will be apparent that many changes may be 
made in the form, arrangement and positioning of the various elements of 
the combination. In consideration thereof, it should be understood that 
preferred embodiments of this invention disclosed herein are intended to 
be illustrative only and not intended to limit the scope of the invention.