CH680497A5 - Mechanical filter with perforated tube - has filter tube suspended in cylinder with open=end closed by butterfly valve for easy cleaning - Google Patents

Abstract

Inside the cylindrical body of a filter is supported a perforated tube covered with a layer of filter material the inner end of the tube carrying a closure plate and the outer end open to form an outlet, projecting from a supporting cap. A side outlet on the end cap communicates with the space between the body and the perforated tube. At the other end of the body is a butterfly flap forming a valve to allow solid filtered material to be discharged. USE/ADVANTAGE - Used for removing solid material from water. Sepn. is mechanical and chemicals are not required.

Description

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Description

The present invention relates to a filtering apparatus for liquids, for example water, comprising an outer casing in the form of a circular cylindrical tube, a filter body also in the form of a circular cylindrical tube and arranged concentrically with respect to the casing so as to form a filter chamber of ring section.

In the practice of industrial manufacturing processes, the problem of filtering aqueous liquids is often faced, meaning by this denomination liquids whose density and viscosity are around those of water. These liquids can be of a totally different chemical nature from water, such as e.g. of oils or derivatives: here they are equated with water to the filtering effect being, in the case of the present invention, a purely mechanical filtering of the liquid, without the intervention of any chemical or chemical-physical process (flocuation, precipitation, etc. ).

By mechanical filtering we therefore intend the fact of retaining the particles to be filtered contained in the liquid by means of a filter body provided with passage holes for the liquid of smaller size than the minimum size of the particles that you want to separate from the liquid, so that the particles of size still smaller than the ones defined above pass through the holes of the filter and are not filtered.

Different types of mechanical filtering are known and practiced today, all of which have more or less important disadvantages.

The most known and tested mechanical filtering system, applicable for the filtering of aqueous liquids with separation of particles up to 1 jxm and less, is that through the use of so-called cartridges as a filtering body. By cartridge we mean a cylindrical body of a porous filtering material, in particular paper. Characteristic of this filtering method is the fact that the particles to be filtered are more or less trapped in the empty spaces that make up the filtering material, therefore, after a certain period of time - determined by the ability to receive the material, i.e. its porosity, of the cartridge and of course the content of impurities to be filtered of the liquid and its quantity - the filter cartridge is clogged and can no longer fulfill its task, which consists in filtering a minimum quantity of liquid in a specified time. The dirty cartridge must therefore either be cleaned and recycled, i.e. made reusable - what up to a certain point can be done by washing the cartridge from the outside or by reversing the direction of the passing liquid - or be eliminated together with the filtered material that it door. In any case, a cartridge of this type has a limited life span, since it can never be completely cleaned. Its labyrinth structure, i.e. by trapping the filtered particles in the pores and channels, does not allow the filtered particles, often also having the tendency to form conglomerates inside the pores, to be then completely removed, even using the more

refined «counter-current» washing systems (ie by reversing the direction of passage of the liquid).

It follows that expensive cartridges must be eliminated together with the filtrate, which causes further costs, bearing in mind the fact that the costs of eliminating waste, especially if toxic, are normally a function of their volume. The mechanical filtering system by cartridges also has, in addition to this economic disadvantage, the disadvantage of being difficult to automate, since the cartridge replacement operation is very complex even if you have a by-pass that allows you to work with filter off.

Normally the cartridge is therefore replaced manually, with a messy operation that is not very appreciated by the operator.

For the aforementioned reason, that is, to achieve better automation of the filtering, types of mechanical filtering without cartridges have been developed and applied in practice. These are sand filtering systems, in which the liquid to be filtered passes through a more or less cleanable sand bed by reversing the direction of passage of the liquid, or systems with the use of support candles made of a relatively wide mesh network on the surface of which a filter bed is formed by means of suitable floury substances. In particular, the use of so-called fossil flour, consisting of very fine diatoms of suitable shape and size, is known to constitute this filter bed.

While filtering with a sand bed is not very suitable for obtaining the safe separation of particles of a well-defined order of magnitude of a few um, the fossil flour system actually allows to guarantee the efficient filtering of particles up to about 1 um and also of perform automatic filter operation. In fact, the layer of fossil flour deposited on the candle and loaded with filtered material can be detached from the candle mechanically and automatically, after interrupting the adduction of the liquid to be filtered, by simply shaking the candle itself. The layer of filter material drags with it, falling, the filtered dirt, which is then removed in the form of a more or less dry cake, often after dehydration. This method with the formation of a filter bed has the fundamental disadvantage of requiring the use of a filter bed constituted, previously, on the candle, in the meanders of which the particles to be filtered are trapped, which can no longer be released. This means that, in addition to the costs for the filter material, there is the problem of eliminating a volume of waste material much greater than that of the actual filtrate. Since, as a rule, the costs of eliminating environmentally harmful substances - which are usually the filtrates of the filters used for the purification of industrial waters and also for certain types of mechanical processing, e.g. in electroerose-ne - they are proportional to their volume and not to their concentration, resulting in a greater cost of elimination, since you have to pay, as well as for the filtrate, also for the filtering material. These additional costs can have a significant impact

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in the economic calculation of a filtering system. In addition, the filtering substances most used today in practice, usually consisting of so-called "fossil flours" (microscopic diatoms with a very high absorption power) have aspects of toxicity so far little studied, which advise against their use.

The object of the present invention is to propose a mechanical filtering apparatus for liquids, which allows to eliminate the above mentioned disadvantages of the known systems, and in particular that it is inexpensive especially as regards the operating costs, which does not present toxicity problems and which it also offers excellent operating automation conditions.

These objects are achieved, in a filtering apparatus of the type described in the prologue of claim 1, thanks to the specifications of the characterizing part of claim 1.

Thanks to the fact that the end of the external casing corresponding to the closed side of the filtering body is closed by means of a closing valve which, in the open state, essentially leaves the whole section of the external tube free for the axial evacuation of the filtered material, is It is possible to carry out effective and rapid periodic cleaning of the surface of the filter body.

This is true in particular for the filter body having characteristics such as not to make it necessary to clean the filter by reversing the direction of passage of the liquid stream through the filter body. In this case, the filter is cleaned simply by opening the axial closing valve, which releases all the filtered material, collected in the annular chamber arranged between the outer casing and the filtering body, under the pressure of the axial thrust of the liquid to be filtered which continues to be entered into the filter. However, the axial cleaning system of the filtering device, made possible thanks to the axial arrangement of the closing valve, is also applicable when the filtering body is of the conventional type, i.e. it can be cleaned only by reversing the direction of the flow of liquid through the filtering body. same. Also in this case, the axial evacuation of the filtrate through the closing valve which essentially frees the entire section of the external tube allows rapid and radical cleaning of the filtering apparatus.

The invention is now described in more detail with the help of some preferred embodiments accompanied by the relative drawings.

The figures show:

fig. 1 is a general view of a filtering apparatus according to the invention, in section along a plane passing through its longitudinal axis and in schematic representation;

fig. 2 a variant of the filtering device according to the invention with a slightly modified system for guiding the liquid flow through the filter.

The filtering apparatus shown in fig. 1 in schematic representation shows all the construction elements which must be present at least in such an apparatus. The figure, however, must be considered purely schematic, in the sense that the actual construction can also differ considerably from what is shown here in the construction details.

The filtering apparatus according to the invention comprises a tube-shaped outer casing 1, closed on both sides so as to constitute, inside and during operation, a chamber 9 of essentially cylindrical shape. One of the sides of the cylindrical tube 1 is closed by means of a screw cover 2 equipped with a watertight seal 3, consisting of a rubber ring or in any case of an elastic material which is pressed, by the thread with which the tube 1 is equipped on its outside and the cover 2 against the front surface of the tube on one side and against an axial abutment surface 4 on the other side, with which the screw cover 2 is provided.

The screw cover 2 has, in the central area of its front side, a circular opening 5 through which an internal tube 6 passes, being guided and fixed, whose functions and specifications we will explain below.

The screw lid 2, which has the general shape of a cap with a front part in which the hole 5 is made and a cylindrical part ending with the aforementioned closing screw, also has, in the cylindrical part preferably, but not necessarily, a the inlet opening 7 of the liquid to be filtered, equipped with a cylindrical fitting to which an adduction tube of the liquid to be filtered can be connected in a suitable way.

Inside the external casing 1 there is contained, essentially coaxially, a filtering body 8, also in the shape of a tube. A filter chamber 9 with an annular section is thus formed between the inner wall of the outer casing 1 and the filtering body 8, which is traversed by the liquid to be filtered in the manner which will be explained below.

The filtering body 8 is in turn constituted by a tubular and radially rigid internal support body 10, equipped with openings 11 of large dimensions with respect to the maximum dimensions of the material to be filtered. In fig. 2, the support body 10 is schematically represented, in section, as a cylindrical tube equipped with circular holes 11. This is however only one of many other possible solutions for the internal support body 10, which must only be radially rigid as well to be a good support for the body of actual filtering material that surrounds it from the outside. The support body 10 can thus be made of a rigid tube having openings of any shape, however of such dimensions as to prevent as little as possible the free passage of the liquid to be filtered through the wall of the body 10 itself. A solution (not shown), also often used for this type of support body 10, is that of a spiral-wound iron wire spring: in such a solution the large passage openings are then constituted by the free space between the coils of the spring, a space that also has the shape of a spiral.

The fact that the internal tubular support body 10, in addition to having open 5, is certainly essential

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of large dimensions as mentioned above, is equipped with a mechanical resistance capable of supporting the total pressure acting on the liquid to be filtered without deforming, i.e. without crushing or collapsing.

Outside the internal support body 10 there is now a body of filtering material 12, which completely surrounds the whole area of the internal support body 10 which has holes through which the liquid passes. In practice, the body of filter material 12 covers the cylindrical surface of the support body 10 like a sock. As regards the front end part of the internal support body 10, different solutions may be provided, all equally acceptable within the scope of this invention. In the solution of fig. 1, the tube of the internal support body 10 is closed frontally, in its side which ends inside the casing 1, by means of a lid 13 of material impermeable to liquid, such as for example. a plate disc welded directly on the end of the internal support body 10. This solution with the closed end of the body 10 is however shown only by way of example, since it can certainly also be foreseen, within the scope of the invention, that the internal support body 10 is closed at its end by a rigid support which also has large openings which allow the liquid to pass, similarly to what the internal support body 10 does. In the case shown in fig. 1, with the end of the support body 10 closed liquid-tight, the body of filter material 12 will have the shape of a cylinder inserted over the internal support body 10 so as to co-prime all the holes 11 present on the circular surface of the body of internal support 10. If the internal support body 10 has a front end ending in the casing 1 permeable to the passage of the liquid, due to the presence of holes or other, the body of filter material 12 must also completely cover this front end terminal.

The body of filter material 12, shown in fig. 1 in dashes, covers the support body 10 everywhere where it has passage holes or openings 11. In practice, in the example of execution of fig. 1, up to a collar 14 with which the internal support body 10 connects with the internal tube 6. The connection solution between the internal support body 10 and the internal tube 6 via the collar 14 is purely schematic and can therefore , in practice, to be resolved differently from that shown here. The fixing of the tube of the filter material body can thus take place e.g. for mechanical locking by means of a clamping nut equipped with a conical clamping surface (not shown) or e.g. by welding along its entire circumference (also not shown) or in any other suitable way. However, this fixing does not constitute a feature of the invention and can therefore be carried out as best believed.

Also by way of example, it is shown how the external casing 1 is connected with the cover 2 and this with the tube 6: all this is secondary in the context of this invention and can therefore be solved in any suitable way, capable essentially to ensure the watertight integrity of the body of the filtering device.

On the other end of the outer casing 1, the one shown on the right in fig. 1, the filtering device is closed by means of a drain valve 15. In the example of fig. 1, the valve 15 has the shape of a butterfly valve rotating around an axis 16 transversal with respect to the longitudinal axis of the filtering apparatus. The valve 15 works against two abutments 17 and 18 of the external casing 1, arranged, with respect to the valve plate 15, on two opposite sides, so as to allow the valve to overturn around its axis for opening the appliance. of filtering and the discharge of the filtered material accumulated on the surface of the body of filtering material 12 and in the chamber 19 formed between the terminal part of the filtering body, e.g. between the cover 13, and the valve 15.

Within the scope of the present invention, the type of valve 15 used to close the open part of the outer casing 1 does not however play a role, therefore the butterfly valve shown here can be replaced by any other suitable type of openable closure.

Essential for the present invention is the arrangement of the individual elements described above inside the outer casing 1 of the filtering apparatus, which provides that:

a) the filtering body 8 has the shape of a tube closed at one end 20 and open at the other end 21,

b) the filtering body 8 is guided and fixed from its open side 21 to a front wall 22 of the external casing 1 - formed in the example of fig. 1 from a front part of the screw cover 2 - where the outlet of the filtered liquid is located, which coincides with the open end 21 of the filter body 8,

c) the end 23 of the external casing 1 corresponding to the closed end 20 of the filtering body 8 is closed by means of a closing and discharge valve 15 which, in the open state, essentially leaves the whole section of the external tube 1 free for the axial evacuation of the filtered material collected in the chamber 19,

d) the outer casing 1 has an intake opening 7 for the liquid to be filtered, arranged at the opposite end to that 23 where the closure and discharge valve 15 is located, i.e. arranged on the same side of the outer tube 1 where it is located the outlet of the filtered liquid 21.

As mentioned above, the type of closure and discharge valve 15 is irrelevant to the effects of this invention: only the fact that the opening of the valve 15 must give vent to the whole section of the external tube 1 for the evacuation of the material is essential filtered collected.

The advantage of the arrangement of elements constituting the filtering apparatus object of the present invention is evident from the operating mode which we will now briefly describe.

The liquid to be filtered is introduced into the filtering device through the inlet opening 7 e

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it then passes, first passing through the body of filtering material 12 and the internal support body 10 which together constitute the filtering body 8, inside the filtering body 8. In this passage the liquid is filtered, since the particles to be filtered larger than the maximum dimensions allowed for the passage from the body of filtering material 12, they are retained on the surface of the latter, on which they tend to collect. The filtered liquid - still by definition still containing all the solid particles smaller than the maximum dimensions allowed for the passage from the body of filter material 12 - is then evacuated through the open end of the filter body 21 which, in the embodiment of fig. 1, coincides with the open end of the inner tube 6.

Normally the liquid to be filtered is pushed under pressure through the inlet 7 and then pushed through the filter body 8, so a certain overpressure also reigns inside the filter body itself and is sufficient to ensure the evacuation of the filtered liquid through the opening 21: however, it is also conceivable to work in depression, by applying a suitable suction to the open end of the filtering body 12, with which something the liquid to be filtered is sucked through the filtering apparatus, which is found itself in depression during normal operation. Also conceivable, and in some cases preferable, a combined operation, with liquid to be filtered sent under pressure through the inlet opening 7 and aspirated in depression through the open end of the filtering body 8 after the filtering has taken place. These different operating systems of the filtering apparatus according to the invention can have certain specific advantages, in particular the last mentioned if applied to a particular embodiment of the invention such as that of fig. 2 which will be described below. In any case, the various variants mentioned above for feeding the liquid to be filtered and for evacuating the filtered liquid do not play a particular role in the effects of the present invention. Essential is the fact that the filtering apparatus comprises an annular shaped chamber 9 crossed by the liquid to be filtered, at least in part, axially and ending in a chamber 19 arranged at the closed end of the filtering body. The body of filter material 12 used in the inventive filtering apparatus is then preferably such as not to be subject to clogging by the filtrate.

According to a preferred form of body of filter material 12, the filtrate deposited on the surface of the body of filter material 12 cannot anchor on the surface of the body 12 itself, since the holes with which it is provided prevent such an "anchoring", due to the penetration respectively to the interlocking of the filtrate particles in the cavities of the body of filter material 12.

The particles to be filtered therefore remain on the surface of the body of filter material 12 and are transported towards the closed end 20 of the filter body 8 by means of the more or less pronounced axial current which is formed in the annular chamber 9. This axial current, whose force can be influenced in various ways, therefore it drags the filtered particles more and more towards the chamber 19, where they collect, first filling the chamber 19 and then, progressively, the part of the annular chamber 9 closest to the chamber 19. This axial movement of the stream of liquids can be influenced and sustained in various ways, the simplest being that of placing the filtering apparatus in a vertical position with the end 23 of the external casing 1 housing the closing valve 15 facing downwards, thereby of gravity will tend to deposit the filtered particles in the chamber 19. This tendency to axial movement d However, the flow of liquid in the annular chamber 9 can also be favored with other expedients, one of which, particularly favorable, will be described below in relation to fig. 2.

The use of such a body of filter material 12 is not however mandatory within the scope of the present invention, since the same operating mechanism of the filtering apparatus, with collection of the filtrate in the chamber 19 and progressive filling of the chamber 9 starting from the end facing the open part 23 of the casing 1, can also be made with other types of filter material bodies having properties similar to those of the aforementioned filter material body, i.e. made in such a way as to prevent the passage of the particles to be filtered without however, imprison them in the maze of complex layers, forming curved channels, etc. This mechanism is also a function of the material to be filtered and is not an object of the present invention.

Important for the present invention is only the fact that, thanks to the inventive arrangement chosen of the elements that constitute the filtering apparatus, the filtrate collects in the chamber 19 and in the part adjacent to it of the chamber 9. When the filtering material collected, naturally containing still a certain percentage of liquid, it has reached a sufficient quantity (for example it has filled the annular chamber 9 up to a certain desired "height") then the filtering apparatus is cleaned. This takes place in a very simple way and constitutes the main advantage of the present invention. In fact, it is sufficient to open the shut-off and drain valve 15, which has the characteristic of essentially leaving the entire section of the outer tube 1 free. The pressurized liquid, which continues to enter from the inlet opening 7, aided in this case - case of an arrangement of the filtering device vertically with the valve 15 facing downwards - by the force of gravity, it will push all the filtered material collected out of the opening left open by the valve 15.

Precisely the fact that this opening is completely opened in practice - if you neglect, e.g. in the case of the butterfly valve 15 of fig. 1, the thin central area that will be occupied by the valve 15 itself in its open position - allows the evacuation of the material axially with respect to the filtering apparatus, which guarantees rapid removal of the filtrate without the need to interrupt the supply of liquid to the filtering device. Indeed, this adduction is necessary, since it is thanks to it that it forms on the head of the

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filtrate column, the thrust pressure used to evacuate the filtrate. It is already noted here that the evacuation of the filtrate from the chamber 19 and from the circular chamber 9 does not necessarily have to be total, i.e. such as to completely empty these chambers. In fact, it can also be only partial, i.e. such that a part of the chamber 9 and / or the chamber 19 still remains full of filtrate. In this case the pressurized liquid acting on the filtrate "column" collected in the chambers 9 and 19 pushes this column towards the opening 23 of a certain section, after which the valve 15 is closed again. In this case, a certain portion (or even all) of the surface of the body of filtering material 12 will have been freed, while the filtrate will remain in the chamber 19 to form a sort of buffer. This "partial" cleaning method, which moreover is not the subject of this invention, has the advantage that the filter is never completely emptied, so that there is never a spill of liquid from the open side 23 of the external casing 1. unfiltered which should then be recovered.

This working method is therefore an empty calibration of the filtering apparatus, which can, in some cases of application, be particularly advantageous.

Fig. 2 shows a preferred embodiment variant of the filtering apparatus according to the invention, in which the liquid stream supplied to the apparatus itself is forced to travel through the annular chamber located between the external casing and the material body filtering in the axial direction, so as to drag the filtrate particles towards the "open" end of the outer casing.

All the parts of fig. 2 which correspond to parts of fig. 1 having the same function have been indicated with the same reference numbers and are no longer described in detail here.

The embodiment of fig. 2 is characterized in that it has a tube-shaped filter body 8 which has, inside it, a support tube 24 closed along its entire length except in its terminal area 25 adjacent to the closed side 13 of the filter body 8, where there are holes for passage 26 of the filtered liquid. The support tube 24 is further made to come out of the external casing 1, so as to form the open end 27 through which the filtered liquid can flow, corresponding, in function, to the open end 21 of the filtering body 8 of the fig. 1.

The advantage of the embodiment of fig. 2 is to force the liquid to be filtered to travel the filtering chamber 9 in an axial direction from the inlet opening 7 to the terminal area 25 of the tube 24. Naturally the axial displacement of the liquid can take place both externally to the filter body 8, i.e. in the annular chamber 9, which - at least partially - inside the filter body 8, i.e. in the annular cavity 28 located between the internal wall of the filter body 8, respectively of the internal support body 10, and the external wall of the support tube 24. Which of the two ways the liquid chooses will then depend on the pressure losses existing on the surface of the body of filter material 12. In the first line the liquid to be filtered chooses, for its axial displacement, the external chamber 9, especially if the surface of the body of filter material is already partially "closed" by a layer of filtrate deposited. Thanks to its axial movement along the chamber 9, the liquid will therefore tend to drag towards the closed end of the filter body 8, i.e. towards the collection chamber 19, ie the particles deposited temporarily - but without getting stuck - on the surfaces of the body of filter material in its region closest to the inlet opening 7. This is an extremely desired self-cleaning action of the filter within the operating mode explained above of the filtering apparatus according to the present invention.

By way of completeness, fig. 2 shows another type of closure valve 29 also particularly suitable for the filtering apparatus according to this invention. The valve 29 replaces the valve 15 of the solution of fig. 1 and presents, with respect to the latter, some not negligible advantages which will be briefly explained here. However, the type of valve used to close the open part 23 of the external casing 1 does not constitute a particular condition for the realization of the present invention. The valve 29 is a slider valve which, in fig. 2, is also represented, like all the other components, in a purely schematic way. The valve 29 has a slider 30 which, in the closed position of the valve shown in continuous lines in fig. 2, completely closes the opening 23 of the outer tube 1. The slider 30 is moved by a cylinder unit 31 with piston 32, operated by means of a fluid under pressure or in any other suitable way. When the piston 32 moves into the cylinder 31 in its maximum rear position, shown in dashes in fig. 2, it carries with it the slider 30, completely freeing the opening 23 for the passage of the filtered material collected in the chamber 19 and / or in the annular chamber 9. The evacuation of the filtrate then takes place in the same way described in relation to the solution of fig. 1.

The type of slider valve 29 has the advantage of not needing any seat in the interior of the outer tube 1, which can therefore remain completely smooth and in particular not have any rise 17, 18 as is instead necessary for a butterfly valve . Furthermore, the slider valve 29 allows to shorten the total length of the tube 1 of the filter apparatus. This advantage with respect to the butterfly valve 15 is however compensated by the greater size that a slider valve 29 requires outside the apparatus, for the necessary length of its actuation member. This can be disadvantageous especially when batteries complete with filtering devices are used. The choice of the type of shut-off valve therefore depends on some factors related to the specific use of the filtering device and is mainly dictated by considerations related to the size and danger of fouling of the valve by the escaping filtrate.

The speed ratios inside the filtering device according to the invention naturally play a very important role in the proper functioning of the filter.

So it proved to be optimal to choose the diameters

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Fundamentals of the appliance according to the following formula:

di2 = (d22 - d32) ± 20%

where the symbols mean:

di = internal diameter of the adduction pipe, resp. of the entrance opening 7

d2 = internal diameter of the external casing 1, and d3 = external diameter of the filtering body 8

Thanks to this relationship, which constitutes a relationship between the passage surface of the liquid in the adduction tube 7 and that of the filtering chamber 9, it is possible to size the spaces covered by the stream of liquid in the filtering apparatus so that the passage allow correct operation as explained above.

The relationship between the speed of entry of the liquid into the filtering aggregate and the speed of exit from the same can also be formulated in an optimal relationship, given by the respective diameters. That relationship is

d4 ä where di = internal diameter of the adduction pipe, resp. of the inlet opening 7, and d4 = internal diameter of the liquid outlet opening (21,27).

The filtering apparatus according to the invention has proven to be particularly suitable for filtering liquids of the aqueous type, that is, having a density close to that of water, containing materials to be filtered in suspension of a solid nature and not subject to conglomeration and whose dimensions are between about 50 firn and 1 um.

In particular, the filtering apparatus described here has proven to be suitable for filtering dielectric liquid in EDM machines, which work with a relatively high production of filtrate and which require perfect separation of the particles from the circulating dielectric.

The filtering device according to the invention has, in addition to the aforementioned advantage of not requiring the use of any support substance to be thrown together with the filtrate, also that of allowing continuous cleaning of the filter without the need to interrupt the liquid flow , since the cleaning operation is precisely carried out under the thrust of the incoming liquid. The small amount of unfiltered liquid which may possibly escape when the cleaning operation is carried out does not play any role on the total operating budget, especially since this liquid can be easily recovered by known means. The filtering apparatus described here therefore allows the machine to be connected to it without interruption and without having to resort to double filters connected by a by pass.

Claims (7)

claims 1. A filtering apparatus for liquids, for example water, comprising an external casing in the shape of a circular cylindrical tube, a filter body also in the form of a circular cylindrical tube and arranged concentrically with respect to the external casing so as to form a section filter chamber annular, characterized in that a) the filtering body (8) has the shape of a tube closed at one end (20) and open at the other end (21), b) the filter body (8) is guided and fixed from its open side (21) to a front wall (22) of the external casing (1), where the outlet of the filtered liquid from the filtering apparatus is located, which coincides with the open end (21) of the filter body (8), c) the end (23) of the external casing corresponding to the closed end (29) of the filtering body (8) is closed by means of a closing valve (15) which, in the open state, essentially leaves the whole section of the tube free external (1) for the axial evacuation of the collected filtered material, d) the outer casing (1) has an adduction opening (7) of the liquid to be filtered arranged at the opposite end to that (23) where the closing valve (15) is located, i.e. arranged on the same side of the outer tube (11) where the outlet of the filtered liquid is located (21). Filtering device according to claim 1, characterized in that the closing valve is a shutter valve movable in a plane perpendicular to the longitudinal axis of the external tube. Filter apparatus according to claim 1, characterized in that the closing valve (15) is a butterfly valve oscillating around a transverse axis (16) with respect to the external tube (1) and intersecting the longitudinal axis of the tube (1) same. Filter apparatus according to claim 1, characterized in that the tube-shaped filter body (8) has, inside, a support tube (24) closed along its entire length except in its terminal area ( 25) adjacent to the closed side (13) of the filtering body (8), where there are holes for the passage (26) of the filtered liquid, and the fact that the support tube (24) comes out of the external casing (1) forming the outlet (27) of the filtered liquid from the filtering device. Filter apparatus according to claim 1, characterized in that the following relation holds: di2 = (d22-d32) ± 20% di = internal diameter of the supply pipe (7), d2 = internal diameter of the external casing (1), d3 = external diameter of the cylindrical filtering body (8). Filter apparatus according to claim 1 or 4, characterized in that the following relation d4Sdi applies where the symbols mean: di = internal diameter of the adduction pipe (7) 5 10 15 20 25 30 35 40 45 50 55 60 65 Use of the filtering apparatus according to one of claims 1 to 5, characterized in that the apparatus is used for filtering the dielectric liquid in EDM machines. 5 10 15 20 25 30 35 40 45 50 55 60 65 8 7 13 CH 680 497 A5 d4 = internal diameter of the liquid outlet opening (21; 27).