Abstract:
A filter device includes a filter housing ( 10 ) in which at least two filter elements ( 12, 14 ) with differing filter grades of filter material ( 16 ) are arranged. The filter elements are partially separated from each other by a separating device ( 18 ), with a through channel ( 24 ) of predetermined cross-section for a hydraulic connection between neighboring filter chambers ( 20, 22 ),. The filter chambers arranged prior to the filter materials ( 16 ) in the flow direction of the fluids for filtering. The filter device permits a combination of main- and side-stream filters in the smallest possible space, whereby very good purity levels for the filtered fluid are nevertheless achieved.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a filter device with a filter housing in which at least two filter elements with filter materials of different filtration grades are mounted. The filter elements are partly separated from each other by a separating device having a passage point of assignable cross-section for a fluid-conducting connection of adjacent filter spaces. The filter spaces are situated upstream from the filter materials in the direction of flow of the fluid to be cleaned. 
   BACKGROUND OF THE INVENTION 
   U.S. Pat. No. 3,344,923 describes a filter unit with two filter elements in the form of hollow cylinders. Both elements are of the same filtration grade. The filter elements are mounted in a common housing so that their longitudinal axes are in alignment. The two filter elements are separated from each other by a separating device extending transversely to the longitudinal axes. At the same time, the filter space (unfiltered fluid space) situated upstream from the filter materials in the direction of flow of the fluid to be cleaned is divided into two subspaces, between which a fluid-conducting connection may nevertheless be established by a valve installed in the separating device. This valve is a bypass valve and initially is closed. Opening of the valve occurs only when a certain pressure difference is present between the two subspaces as a result of increased fouling and accordingly decreasing permeability of the first filter element as viewed in the direction of fluid flow. The second filter element, which serves as reserve unit, can gradually assume the function of the first filter element to ensure uninterrupted trouble-free filtering operation. No provision is made for uninterrupted filtration operation with a primary flow and secondary flow filter of different filtration grades qualities. 
   GB 860 871 A discloses a generic filter device in which the fluid flow and accordingly the pressure relationships between the two subspaces associated with the first or second filter element are affected both in the unfiltered material space and on the filtrate space side. Here, the two filtrate partial flows emerging from the two filter elements are initially conducted separately by a tapering spiral spring whose narrower end is rigidly connected to the outlet opening for the partial flow of filtrate discharged from the finer, and thus, less permeable filter element. The wider free end of the spiral spring is loosely connected to the space surrounding it into which the partial filtrate flow of the coarser, and thus, more easily permeable filter element is discharged. The latter generates at the open end of the spiral spring vortices which exert a suction effect on the interior of the spiral spring. The influx of the filtrate partial flow from the finer filter element into the filtrate partial flow of the coarser filter element is consequently promoted. The suction effect increasing with increase in the fluid flow rate ultimately causes the turns of the spiral spring, previously closed in the direction of the longitudinal axis of this spring, to separate, so that the separation of the two filtrate partial flows along the spiral spring is eliminated and the pressure decrease accompanying the vortex formation is limited. When this conventional solution is applied, only the primary flow filter is in use when the fluid passage amounts are very low and very high. Only in the intermediate moderate fluid passage areas is filtration by the secondary filter mounted parallel to the primary filter initiated, something which is detrimental to the purity of the filtrate during filtration operation as a whole. No provision is made for use of the secondary filter during the filtration operation in question. 
   SUMMARY OF THE INVENTION 
   Objects of the present invention are to provide an improved filter device for increased purity of the filtrate with every specific filtration grade of the filter elements, during the entire filtration process. 
   According to the present invention, the throttle point of the other passage point is in the form of a fluid valve provided with an assignable closing pressure which increases the prestress pressure inside the filter element with the coarser filtration grade. The result obtained in this manner is that under all operating conditions of the filter device, that is, regardless of the respective fluid throughput, a relatively high percentage of the fluid to be filtered is conducted through the filter element with the finer filtration grade, so that the purity as a whole of the filtrate flow being discharged is increased. 
   This arrangement permits combination of a primary flow filter and secondary flow filter in one filter housing. In view of the different filtration grades of the filter materials employed, the filter with the coarser filtration grade serves as primary flow filter, and thus, receives the greater part of the flow volume to undergo particle removal. The filter element with the finer filtration grade in contrast removes particles from a smaller flow volume. Such cleaning by primary and secondary current filters results in very good purity quality, even though the greater share of volume of the fluid to be cleaned is subjected to coarse filtration. This filter device is very compact in structure and in general requires a small number of structural components. This condition both increases reliability in operation and keeps manufacturing, installation, and maintenance costs low. High precision of current volume division can be achieved for primary and secondary flow filtration by the passage point of assignable cross-section inside the separation device. This passage point improves the quality of filtration of the fluid to be cleaned. 
   In a preferred embodiment of the filter device of the present invention the filter housing has an inlet point for the fluid to be cleaned and an outlet point for the cleaned fluid. The filter element with coarser filtration grade is mounted adjacent to the inlet point. The filter element with finer filtration grade is opposite the former and adjacent to the outlet point. As a result, the flow of fluid through the filter from the inlet point to the outlet point is especially favorable. 
   In another preferred embodiment of the filter device of the present invention, the separating device has a separating plate connected to the filter housing for sealing. The filter housing is divided into two adjacent housing subspaces. The ends of the adjacent filter elements are in contact with the separating plate provided with an additional passage point for connection of the interiors of these filter elements. One housing subspace containing the filter element with coarser filtration grade has the filter space which is connected to the inlet point to conduct fluid. The other housing subspace containing the filter element with finer filtration grade has the other filter space connected through this filter element to the outlet point to conduct fluid. In addition, one filter space is connected by the passage point to the other filter space to conduct fluid. An especially compact structure of the filter device as a whole is obtained. Such structure does not require a complete filter housing of its own, but is also suitable for installation on the tank side with an integrated filtering jug. 
   In another especially preferred embodiment of the filter device of the present invention, the filter elements are mounted coaxially relative to each other inside the filter housing and are introduced into the element receptacles with their free ends facing away from each other. The filter materials of the filter elements preferably rest on support elements, which are modular in structure and can be adapted to various longitudinal dimensions of the filter elements. In this way, a modular structure can be achieved on the whole for the filter device, and can be adapted cost effectively to a wide variety of applications. 
   In another especially preferred embodiment of the filter device of the present invention, one passage point includes a curved passage segment, and the other passage segment has a throttle point such that the flow resistance is increased for the filter element with the coarser filtration grade. A corresponding throttle function is also assigned to the first passage point to ensure the effectiveness of the secondary flow filter independently of the actual situation relative to fouling of the primary flow filter. In particular, the volume flows inside the filter device throttle points may be coordinated, so that reasonable division into primary flow and secondary flow filtration is achieved. Relative to the throttle point, the additional passage point is preferably in the form of a fluid valve which applies an assignable closing spring pressure to the closing element in the direction of the additional passage point. As a result, a preloading situation arises for the housing subspace with the filter element with coarser filtration grade and the fluid to be filtered is forced, as a function of the preloading selected, to pass appropriately through the first passage point acting as throttle point into the partition between the housing subspaces for secondary flow filtration. 
   In another preferred embodiment of the filter device of the present invention, at least the filter element with the coarser filtration grade is provided with a bypass valve. If complete clogging of the primary flow filter occurs, the bypass valve opens and the supply of hydraulic fluid continues to be ensured. This ability is important if the device is appropriately inserted upstream from hydraulic machine tools or devices. However, the secondary flow filtration is optionally continued only to a limited extent regardless of the bypass situation. Preferably, provision is also made such that at least one of the filter materials, preferably the filter material with the finer filtration grade, possesses moisture absorption properties. As a result, harmful moisture can be removed from fluid circulation by way of the filter device in primary and secondary flow filtration. 
   It has been found to be especially advantageous to design the passage points so that one part by volume which flows as secondary flow through the filter element with the finer filtration grade corresponds to eight to ten parts by volume in the primary flow which moves through the filter element with the coarser filtration grade. 
   Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring to the drawings which form a part of this disclosure: 
       FIG. 1  is a side elevational view in section of the filter device, as installed, according to an embodiment of the present invention; 
       FIG. 2  is a side elevational view in section of the primary and secondary flow filter units in  FIG. 1 ; and 
       FIG. 3  is an enlarged top plain view of the separating device shown in  FIGS. 1 and 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The filter device shown in  FIG. 1  has a filter housing  10 . Two filter elements  12 , 14  are mounted one above the other in the filter housing. The filter elements in question  12 , 14  have different filtration grades. These filtration grades may be determined by use of different filter materials  16  or the same filter materials  16  with different permeability values. The filter materials  16  may be in the form of hollow cylinders, but preferably have pleated filter matting, that is, matting folded like an accordion. In particular, filter materials  16  of a plastic substance may be employed in this application. 
   The two filter elements  12 , 14  are partly separated from each other spatially and with respect to external conduct of fluid by a separating device  18 . An enlarged top view of the separating device  18  is presented in FIG.  3 . For a fluid-conducting connection between the two adjacent filter spaces  20 , 22 , this separating device has a passage point  24  upstream from the filter materials  16  in the direction of flow of fluid inside the filter device for the fluid to be cleaned. The filter housing  10  has an inlet point  26  for the fluid to be cleaned or filtered and an outlet point  28  for the fluid cleaned. The filter element  12  with the coarser filtration grade is mounted on the side adjacent to the inlet point  26 , while the filter element  14  with the finer filtration grade is mounted on the side adjacent to the outlet point  28 . Both the inlet point  26  and the outlet point  28  are in the form of an inlet flange or outlet flange. These flanges are connected to the housing  10  by appropriate screw connections. The flange-like inlet point  26  may be connected to a fluid feed carrying fouled fluid inside a hydraulic circuit and tapers in extension to promote fluid inlet flow in the direction of the filter space  20 . 
   In addition, the inlet point  26  is mounted closer to the separating device  18  in the filter housing  10  than to the upper cover plate  32  of the filter housing  10 . The cover plate  32  is in turn connected by a screw connection (not shown) to the rest of the hollow cylindrical filter housing  10  and is sealed to this hollow cylinder wall by a conventional O-ring. The filter device may be ventilated by a sealing connection  34  on the upper end of the cover plate  32 , or, for example, may be connected to a fluid fouling indicator (not shown). In any event, after removal of the cover plate  32 , it is possible to replace the filter unit itself containing filter elements  12  and  14  with a new unit when fouling reaches a particular level. In addition, the cover plate  32  has on its inner side mounting connectors  36  pointing inward which may be joined together in one piece in the form of a connector ring. 
   The separating device  18  essentially comprises a separating plate  38  connected to the interior of the filter housing  10  to effect sealing. For this purpose, the separating plate  38  has a grooved annular channel  40  extending over its outer lateral circumference and a conventional O-ring  42  received in channel  40  to effect sealing. The separating plate  38  is preferably in the form of an injected plastic element, and thus, can be cost effectively manufactured. By the separating plate  38 , the filter housing  10  is divided into two adjacent housing subspaces  44 , 46 . The adjacent filter elements  14 , 16 , with their ends facing each other as shown in  FIG. 1 , are frontally in contact with the separating plate  38  provided for connection of the interiors  48  of these filter elements  12 ,  14  to another passage point  50 . One housing subspace  44  with the filter element  12  with the coarser filtering grade has the filter space  20  connected directly to the inlet point  26  to conduct fluid. The other housing subspace  46  with the filter element  14  with the finer filtering grade, on the other hand, has the other filter space  22  connected by way of this filter element  14  to the outlet point  28  to conduct fluid. The filter elements  12 , 14  are mounted coaxially with each other inside the filter housing  10 , that is, along the common longitudinal axis  52 . 
   The longitudinal dimension selected for the upper filter element  12 , as seen in the line of sight to  FIGS. 1 and 2 , is somewhat larger than that of the subjacent or lower filter element  14 . In addition, the free ends of the filter elements  12 , 14  facing away from each other are introduced into element receptacles  54 , which in turn are connected to support pipes  56  on the inside of the filter elements  12 , 14 . The support pipes  56 , to form support elements, have individual support leaves  58  delimiting free fluid passage points therebetween. The support pipes  56  may be built in modular form, that is, they may have individual segmental elements that may be joined to each other and mounted in sequence as a function of the longitudinal dimension required. In addition, the support pipes  56  are of plastic materials and internally have support studs or flanges  60  extending diametrically toward each other. The structure of support pipes  56  is conventional, so that no detailed discussion thereof is needed at this point. The support studs  60  may impart a specific direction to the flow of fluid inside the respective filter element  12 , 14 . This flow favors energy conservation in filtration operation. The support studs  60  do not hamper passage of fluid from the filter element  12  into the following passage point  50  or transition of the interior  48  of the filter element  14  in the direction of the curved flange-like outlet point  28 . The filter elements  12 , 14  are cylindrical in form and have hollow cylindrical interiors for conducting fluid. 
   On the ends adjacent to each other, the filter elements  12 , 14  are received correspondingly into an annular recess  62  in the separating plate  38 . For this purpose, the separating plate  38  has a corresponding receptacle extension on the side of the external circumference, that is, one which extends upward and downward. In the direction of the interior, the filter elements  12 , 14  are supported again by the end areas of the support pipes  56 , which in this area are in contact with the separating plate  38 . The ring channel  40  is in turn fastened to the annular recess  62  and is reinforced by diametrically opposite studs  6  and  64 . As shown in  FIG. 3  in particular, the passage point  24  is in the form of a curved passage segment which extends between two adjacent connection studs  64 . The other passage point  50  forms a throttle point such that the flow resistance is increased for the filter element  12  with the coarser filtration grade. Throttle point  50  is in the form of a fluid valve  66  having the closing element  68  pressed in the direction of the additional passage point  50  with an assignable spring closing pressure. The closing element  68  has guide flanges  70  distributed on the circumference side which permit precise control of the closing element  68  inside the lower support pipe  56 . In addition, a support plate  72  of the fluid valve  66  is rigidly connected to the lower support pipe  56 , and a closing spring  74  in the form of a pressure spring rests with its one of its free ends against this support plate  72  and with its other free end on a radial recess in the closing element  68  of the fluid valve  66 . 
   The fluid valve  66  is shown in its closed position in  FIGS. 1 and 2 . The lower end of the upper support pipe structure  56  positioned above valve  66  is shown in outline. The support studs or flanges  60  of the upper support pipe  56  do not disrupt the operation of the fluid valve  66 . The fluid valve  66  is provided with an assignable closing pressure which increases the preloading pressure in the interior  48  of the upper filter element  12 . The fouled fluid passing through the corresponding filter material  16  is then slowed by the increased preloading pressure. Also, the tendency of the fouled fluid to flow through the first passage point  24  in the separating plate  38  to the additional filter element  14  with finer filtration grade becomes greater. This arrangement accordingly prevents occurrence of filtration in the primary flow only by the upper filter element  12 , with hardly any filtration in the secondary flow by the lower filter element  14 . 
   The filter element  12  with the coarser filtration grade is provided on its upper end with a bypass valve  76  to prevent operational failures as soon as the primary flow filter with the upper filter element  12  becomes completely clogged. In such a case, the bypass valve  76  opens and permits flow of the uncleaned fluid in the primary flow through the interiors  48  of the upper filter element  12  and lower element  14  to the outlet point  28 . The bypass valve  76  is comparable to the fluid valve  66  in structure. The description of valve  66  also applies to the bypass valve  76 . 
   To eliminate moisture from the fluid, the filter material  16  of the lower filter element  14  has moisture-absorbent layers. The filter material  16  of the lower filter element  14  may also be made entirely of moisture-absorbent layers. 
   The filter elements  12  and  14  are geometrically more or less identical except for their length and filtration grades. Particularly, internal and external diameters correspond to each other. In addition, the filter elements  12 ,  14 , together with the associated valves  66  and  76  and with their support pipes  56  and the support plate  38  form one structural unit. This structural unit may be replaced in its entirety by a new such unit. Replacement is effected by way of the upper cover plate  32 . This structural unit may be disposed of in environment-friendly fashions, incinerated, or recycled. So that the structural unit will also be retained securely in the housing  10 , not only is it supported by the separating plate  38  on the housing side, but, in the area of the lower outlet point  28  a ring-shaped filter element receptacle  78  is provided onto which the structural unit may be slipped by its lower filter clement  14 . The interior  48  of the lower filter element  14  is sealed fluid-tight from the lower filter space  22  by an O-ring seal. The filtration grade selected for the filter materials  16  is such that a coarse filtration grade value of 20 μm, for example, is selected for the upper filter element  12 , while the lower filter element has a relatively fine filtration grade of, for example, 2 μm. 
   With the filter device of the present invention, a primary flow filter  12  is accommodated with a secondary flow filter  14  in a filter housing  10  having only one inlet  26  and one outlet  28 . The filter housing  10  may also have exclusively one compartment (this design is not shown), for example, if use in a tank with integrated filter cup is involved. The division into primary and secondary flow filters may be even further graduated, with more than two filter elements  12 , 14  (not shown) positioned in parallel being mounted in sequence. More finely graduated filtering capabilities may accordingly be provided for multiply separable volume flows. Again, the filter elements may in turn be designed to be in multiple parts, to the extent that this should be necessary out of considerations of practicability. In any event, the filter device illustrated makes it possible to cause a hydraulic flow of large volume of a relatively coarse filtration grade, for example, 20 μm absolute, and a hydraulic flow of low volume, for example 2 μm absolute, to flow through the device in parallel. The volume flows are coordinated by internal flow control by the orifices or throttles in relation to the passage points  24  and  50 . Introduction of additional throttles or orifices as a function of the flow control desired would also be possible. 
   In order to ensure efficiency for the secondary flow filter  14  independently of fouling of the primary flow filter  12 , the throttle device is integrated into the outlet of the primary flow filter  12  and in the housing  10  between the upper and lower parts of the filter element. The filter device provides a compact operating, installation, and service unit to be provided for applications both in the high-pressure and the low-pressure area (in return-flow and in secondary-flow lines). 
   In the embodiment illustrated, the passage points  24 , 50  are designed so that for one part by volume flowing in the secondary flow through the filter element  14  with finer filtration grade, there are to eight to ten parts by volume flowing in the primary flow through the filter element  12  with the coarser filtration grade. For the secondary flow operation in question, one fouled quantity of flow from the upper filter space  20  is diverted from the primary flow direction by the passage point  24  in the separating plate  38 , and in this way is transferred to the lower filter space  22  for finest filtering by the secondary flow filter  14 . All fluid components filtered by the filter materials  16  in any event reach the outlet side  28  of the device through the interiors  48  of the filter elements  12 , 14 . 
   While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.