Abstract:
A filter system in which a housing and an environmentally friendly filter cartridge are coordinated to provide ease of change of the filter cartridge and reliable operation. The cartridge contains no metal parts and is readily incinerateable. The housing is provided with a number of “universality” features including a changeable bottom which allows the housing to operate as a fuel filter, a lubrication filter, or a hydraulic filter in different applications. The housing and cartridge are configured to provide support for the plastic elements of the cartridge to prevent crushing of the filter under normal and abnormal filter operating conditions. A key system associated with the cartridge and the housing provides assurance that the correct filter is installed for a particular application.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This patent application is a divisional of copending U.S. patent application Ser. No. 10/457,733, filed Jun. 9, 2003, which claims the benefit of U.S. Provisional Patent Application No. 60/387,235, filed Jun. 7, 2002, both of which are incorporated by reference in their entirety herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention pertains to filters, and more particularly to a filter system using an environmentally friendly filter cartridge.  
       BACKGROUND OF THE INVENTION  
       [0003]     Most conventional filters and filter cartridges present a disposal problem. Driven by ease of installation, many applications have gone to self-contained spin-on cartridges. These have a metal outer case, a metal base plate, and other metal components within the filter. Replaceable cartridges also have significant metal components, often in the form of centertubes or support grids for supporting the filter element, and metal endcaps. Thus, significant elements of a filter will not burn thus preventing the use of incineration for relatively complete disposal of the spent filters or filter cartridges.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     In view of the foregoing, it is a general aim of the present invention to provide a filter system capable of supporting and reliably using an environmentally friendly filter cartridge, such that when the cartridge is spent it can be incinerated. The cartridge is environmentally friendly in that it contains no metallic parts.  
         [0005]     It is a feature of the invention that plastic endcaps are utilized in the filter cartridge, and the housing has structure which cooperates with the filter cartridge to compensate for the reduction in strength of plastic endcaps over conventional metal endcaps.  
         [0006]     In a particular embodiment the invention provides a filter system based on a housing having a closed bottom and a removable cover. A filter cartridge is provided for insertion into the housing and for securing therein by locking the removable cover on the housing. The cartridge is in the form of an open center cylinder which has no metallic parts. The housing has, at its base, an upstanding annular flange which has a diameter about the same as the outer diameter of the cartridge. The cartridge carries a radial seal in the form of a depending skirt fixed to a lower endcap. The skirt has a diameter which causes the skirt to overlie the annular flange such that when the filter is in operation, internal pressure creates a radial seal between the skirt and the flange. The cartridge also has an upper endcap which carries an axial seal positioned to fit between the housing and the cover when the cover is in its locked position. The system provides cooperating supports and stops on the inside of the housing and the lower portion of the upper endcap to provide a positive seat and stop which prevents operating pressure from forcing the upper endcap into the filter beyond its seated position.  
         [0007]     In another aspect, the invention provides a filter housing of universal characteristics for cooperating with a filter cartridge to filter fluid. A housing is provided for receiving a filter cartridge which separates an unfiltered region from a filtered region in the housing. The housing has a front in which is formed an inlet port and an outlet port connected to the unfiltered region and filtered region respectively, and also connected to at least one accessory port. A removable cover is threaded onto the top of the housing and removable for allowing access to and interchange of the cartridge. The housing has bolted on removable base selected from the group consisting of a sump for fuel filter applications, and a rigidified bottom for high pressure applications.  
         [0008]     In another aspect, the invention provides a filter housing in an interconnected filter housing bank, wherein each filter housing is adapted to receive a filter cartridge which separates an unfiltered region from a filtered region in the housing. The housing has a back which includes mounting features for mounting a plurality of said housings side-by-side in a bank. The housing has a front in which is formed an inlet port and an outlet port connected to the unfiltered region and filtered region respectively. The inlet port and outlet port are vertically displaced from each on the front of said housing. Tee fittings are connected on the inlet and outlet ports. Each tee fitting has a trunk connected to the associated port and a T-arm in fluid communication with the trunk. At least some of the tee fittings having a valve for shutting off the trunk from the T-arm. The valve bank including tubing connecting the T-arms of adjacent filter housings to connect the filter housings and parallel.  
         [0009]     In connection with the foregoing aspect of the invention, the filter bank also includes at least two filter housings, each requiring a filter of different characteristics. Two filter cartridges of the different characteristics, one for each of said housings, and each of the housings and filters including a keying mechanism for associating one of the filter cartridges with the associated housing and preventing installation of said one filter cartridge in the other housing.  
         [0010]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a front elevation of one form of housing constructed in accordance with the present invention;  
         [0012]      FIG. 2  is a rear elevation of the housing of  FIG. 1 ;  
         [0013]      FIG. 3  is a cross-sectional view taken generally along the line  3 - 3  of  FIG. 5  illustrating the relationship between the filter cartridge and the filter housing in a filter system similar to that of  FIG. 1 ;  
         [0014]      FIG. 4  is a front elevation showing two filter housings assembled in a filter bank;  
         [0015]      FIG. 5  shows a variation of the housing of  FIG. 1  having a different bottom constructed for higher pressure applications;  
         [0016]      FIG. 6  shows a filter housing with cover removed exposing the top of the filter cartridge;  
         [0017]      FIG. 7  is a sectional view through a filter housing showing the relationship between the cartridge and the housing and illustrating some of the system accessories;  
         [0018]      FIG. 8  is a partial view of the area of  FIG. 7  indicated by the circle  8  better illustrating the locking of the cover to the housing and compressing of the axial seal gasket;  
         [0019]      FIG. 9  is a cross-sectional view illustrating the plugged filter indicator;  
         [0020]      FIG. 10  is a view showing the valve associated with the inlets and outlets;  
         [0021]      FIGS. 11 and 12  diagrammatically illustrate the keying of filters to the filter housing; and  
         [0022]      FIG. 13  is an elevational view of a typical filter cartridge. 
     
    
       [0023]     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The disclosed housing in the present application will be described primarily in connection with fuel filters. The housing is designed for multiple parallel plumbing of fuel filters, as will be described in connection with  FIG. 4 . The housing, however, can also be used to form a full flow oil filter, a hydraulic filter or a bypass filter. Some of these applications will also be illustrated to show the universality of the filter system.  
         [0025]     Turning then to the drawings, and particularly to  FIGS. 1 and 2 , there are shown many of the basic elements of the housing portion of a filter system  20  constructed in accordance with the present invention. The system  20  is based on a housing  21  which has a plurality of ports  22  on the face thereof which allow configuration for a variety of applications. Of significance, the housing  21  is provided with a removable base  24 ; as will be described below, a number of different bases  24  can be provided to alter the application for which the filter system is used. The bottom  24  is secured to the housing  21  by a secure mounting structure, such as the threaded fasteners  25  illustrated in the drawings. Preferably cap screws  25  are used at four locations to secure the bottom in place; since the bottom will not typically be removed after the housing is dedicated to a particular installation, the attachment can be relatively permanent. The advantages of the ability to bolt a different bottom in place are achieved primarily in the manufacturing stage when a run of filters for a particular application is being made. The application shown in  FIGS. 1 and 2  is primarily for use as a fuel filter.  
         [0026]     The housing  21  has a removable cover  26 . Complementary threads (not shown in  FIGS. 1 and 2 ) on the inside of the cover  26  and on the outside of the housing  21  allow the cover to be screwed onto the top of the housing. As will also be described, an internal filter cartridge carries an axial seal gasket which is trapped between sealing faces of the cover and housing for preventing leakage between those elements.  
         [0027]      FIG. 2  illustrates four mounting bosses  30  on the back of the housing  21 . The mounting bosses are preferably threaded to allow one or more filters to be mounted on a plate, with the plate then mounted in the position where the filters will be utilized.  FIG. 4 , for example, shows a gang of two filters in which a plate  32  supports both filters. A bolt passed through the plate and into the tapped bosses  30  securely attaches the individual filters to the plate, whereupon the plurality of filters can be treated as a multiple filter bank for unitary mounting.  
         [0028]     As noted above, the unit illustrated in  FIGS. 1, 2  and  4  is intended primarily in fuel filter applications. As such, the bottom assembly  24  includes a sump  34  which is typically used to collect water and other debris in a fuel filter application. Preferably the sump  34  is of see through plastic and includes a self-venting drain valve  35  for periodically removing the collected impurities.  
         [0029]      FIG. 3  is intended to illustrate the relationship between the housing  21  and the filter cartridge which it contains. For purposes of illustration,  FIG. 3  shows a cross-sectional view for a filter which is slightly different from the fuel filter, namely a lubrication filter. In effect,  FIG. 3  is a section of the lubrication filter shown in  FIG. 5 . The components are substantially the same as those in  FIGS. 1 and 2 , except many of the peripheral and accessory components associated with a fuel filter are removed. The description of the housing, unless otherwise noted, would be the same for both the fuel filter and hydraulic filter applications.  
         [0030]      FIGS. 3 and 8  illustrate certain of the internal elements of the filter system  20 . A cylindrical filter cartridge  50  fits within the housing  21 . The cartridge is inserted by removing the cover  26 , dropping a cartridge  50  in place, then reattaching the cover  26 . In the illustrated embodiment flow through the filter is from the outside in. Thus an exterior region  51  outside the periphery of the filter is dedicated to unfiltered fluid, while a region  52  within the bore of the filter is dedicated to filtered fluid. As will be understood by those skilled in this art, flow through the filter from the unfiltered region  51  to the filtered region  52  serves to remove impurities as the fluid passes through the filter medium.  
         [0031]     The cartridge  50  includes a filter element  53  having an upper endcap  54  and a lower endcap  55 . The lower endcap  55  has a radial seal gasket  56  associated therewith which interacts with an annular flange  57  on the housing base to provide a highly effective but simple radial seal  56  at the lower end of the filter separating the unfiltered region  51  from the filtered region  52 . An axial seal  102  is carried by the upper endcap  54  and is pinched between an inside shoulder  58  of the cover  26  and a mating shoulder  59  of the filter housing.  
         [0032]     An inlet port  40  penetrates the wall of the housing  21  and is in fluid communication with the unfiltered region  51 , as best shown in  FIG. 3 . An outlet port  42  is also provided on the exterior of the housing, in this case being formed in the baseplate  24  itself. The outlet port  42  communicates through an internal channel  43  (see  FIG. 3 ) to the filtered fluid region  52 . Thus fluid which passes through the inlet  40  passes through the filter element  53  to the clean region  52  in the bore of the cartridge, thereupon passes through the channel  43  in the outlet port  42  to the utilization device, usually an engine.  
         [0033]     Other peripheral elements are also illustrated in connection with  FIG. 1 , including a thermostat valve  45  which is provided primarily in fuel filter applications where it is desired to utilize a liquid heater for raising the temperature of the fuel. A warm fluid is passed through the thermostat valve  45  to coils  46  internal to the filter (see  FIG. 7  where they serve to heat the fluid). A pair of ports  49  are provided, and are associated with the thermostat  45  in  FIG. 1 . The ports  49  are connected to an internal coil  46  which is illustrated in  FIG. 7 . The unit can also be provided with an electric heater  60  (see  FIG. 7 ). Wires  62  penetrate a sealed aperture  63  in the baseplate  24  for making connections to the internal heater  60 .  
         [0034]     A further port in the housing  21  allows connection of a plugged filter indicator  48  which will be described in greater detail below. It suffices to say for the moment, the plugged filter element  48  has a port which is connected to both the filtered and unfiltered regions  51 ,  52  (in  FIG. 3 ) to measure differential pressure across the filter and, by that mechanism, determine when the filter is sufficiently plugged to raise an alarm.  
         [0035]     While all of the elements associated with the system  20  will not be described in detail, they do emphasize an inventive aspect of the invention, namely the extreme flexibility which is provided by the multiple ported housing with adaptable baseplate and changeable (and keyed) filter cartridge arrangement.  
         [0036]      FIG. 4  illustrates the features of the invention which simplify arranging individual filter housings  21  in a bank. Using the mounting bosses  30  on the back of the filter, a plurality of filter housings  21  are mounted in a single bank  70 . The bank is based on the plate  32  to which the filter housings  21  are bolted, and itself has mounting holes  72  for securing the filter bank  70  to structure. It is seen that the inlet ports  40  and outlet ports  42  are in different horizontal planes when the filters are mounted in a gang, and this simplifies interconnection between filters and also connections to external elements. The inlet and outlet ports  40 ,  42  are provided with special tee type fittings in accordance with the invention. Referring concurrently to  FIG. 4  which shows the filter bank and  FIG. 10  which shows one of the tee fittings, it will be seen that each of the tee fittings includes a trunk  74  which is connected to the associated port  40  or  42 . Fluidically connected to each trunk  74  is a T-arm  76  having oppositely directed ports  77 ,  78  referring to  FIG. 4 . With the tee fittings arranged in this manner, it is relatively simple to plumb the filter units in parallel. Connector tubes  80  are connected between associated ports  77 ,  78  as illustrated. The ports  77 ,  78  are preferably  0 -ring and threaded ports for tube connections, so that tube sections  80  are relatively easily and reliably connected between the two. Ports not used can readily be plugged. With the arrangement illustrated in  FIG. 4 , a plug  82  is located in the left hand most port  77  on the outlet side, and in the right most port  78  on the inlet side. The left hand most port  77  on the inlet side is connected to a supply as suggested at  83 , and a right hand most port  78  on the outlet side is connected to a conduit which runs to the engine as suggested at  84 . With the illustrated configuration, both housings  21  are connected in parallel to accept unfiltered fuel from the supply  83  and supply filtered fuel via conduit  84  to the engine.  
         [0037]     It is feature of the invention that individual filters in the housings  21  can be changed without shutting the engine down. To that end, each of the tee connectors  73  can be provided with a valving arrangement indicated generally at  86 . The valve is arranged to allow shut-off of the connection between the trunk  74  and the T-arm  76 , but without interfering with flow through the T-arm. Thus, if one of the filters is to be shut-down for changing a filter, the associated valves  86  can be moved to the closed position, at which point there is no flow into or out of the associated filter, whereas the flow continues from the supply to the engine through the other filter or filters in the bank. With the valve  86  shut-down, the cover can be removed, the filter changed, the cover replaced, then the valve reopened so that normal operation can continue. A filter can be changed without shutting down the engine.  
         [0038]     In contrast to relatively expensive ball valves used in the past, the present invention utilizes a simple plug valve as best illustrated in  FIG. 10 . A threaded portion  88  of the valve body  89  is fit with a threaded plug  90 . A threaded plug has a closure section  92 . In the position illustrated in  FIG. 10 , the valve is closed, since it will be seen that the flow from the trunk  74  to the T-arm  76  is not possible, having been blocked by the plug portion  92 . When it is desired to open the filter to flow, the plug is rotated counterclockwise, to draw the plug  90  further out of the valve, moving the plug member  92  to a position where it does not block the passage connecting the trunk to the T-arm.  
         [0039]     The ability to change filters without shutting down the engine is a desirable situation in many applications, such as on an electric generating system or a marine application, or other types of applications where the engine runs continuously. By their nature, fuel filters are changed more frequently than other filters, and being able to change them without interrupting the flow of fuel is desirable. It is also noted that plugs are inserted in most of the feature ports in the system of  FIG. 4 . This is not only to simplify the drawings and to illustrate the parallel plumbing between inlets and outlets, but also to emphasize the point that in its simplest application, the filter can be supplied without most of the special features, for relatively simple, for example, warm weather, applications.  
         [0040]      FIG. 5  shows additional versatility in the system, primarily in the ability to interchange bottom plates to change the application to which the filter is directed. Whereas the filter units of  FIGS. 1, 2  and  4  all had a sump bowl and were arranged for fuel filter application, the filter unit  21   a  of  FIG. 5  is arranged for hydraulic and oil filter applications. The primary difference between the filters, in addition to dispensing with most of the peripheral attachments, is the provision of a different baseplate  24   a . In contrast to the baseplate  24  which included a provision for a fuel bowl, the baseplate  24   a  has a rigidified bottom  91  adapted to resist substantially higher pressures. Thus, the housing, being configured for higher pressures, is adaptable to ordinary lubricating applications as well as higher pressure hydraulic applications. In contrast to the relatively low pressures encountered in fuel filter applications, pressures as high as 150 psi can be encountered in lubrication applications, and as high as 800 psi in hydraulic applications. The base  24   a  also has, similar to the channel  43  of  FIG. 3 , an internal connection between the outlet port  42  and the filtered region. Similarly the baseplate  24   a  will have a raised flange  57  for cooperating with a radial seal gasket on the hydraulic filter to be inserted in the housing of  FIG. 5 .  
         [0041]      FIG. 6  illustrates the system with the cover  26  removed. It will be seen that with the cover removed, the upper portion of the cartridge  50  is exposed, particularly the upper endcap  54  thereof. Also shown is the upper axial seal gasket  102  which is compressed between the flange  59  formed on the housing and a corresponding flange  58  (see  FIG. 8 ) on the cover. The cartridge  50  conveniently has its own handle  106  to allow ready removal of the cartridge from the housing once the cover is removed.  
         [0042]      FIG. 7  is primarily illustrates the variety of accessories which can be provided with a cartridge according to the present invention. The fluid heater  46  connected to ports  49  has already been mentioned. An auxiliary electrical heater  60  connected to external wires is also provided. The self-venting drain valve  35  at the lower end of the fuel bowl  34  is illustrated in cross-section. An electrical water-indicator  104  is also included which can sense water in the material in the fuel bowl and light a signal light.  
         [0043]     The cartridge  50  is an integral part of the system. It is environmentally friendly in that it contains no metallic parts. As such, a centertube, when one is needed, is provided by the housing. For example,  FIG. 3  illustrates a centertube  120 , carried by the base  24  and supporting the otherwise unsupported inner bore of the cartridge  50 .  
         [0044]     The cartridge includes endcaps  54 ,  55 , which are normally made of metal. In the present case, however, they are made of plastic. In practicing this aspect of the invention, provision is made for accommodating for the reduced strength of these materials. For example, the upper endcap  54  would tend to bow with pressures inside the filter. To alleviate that a pressure equalization aperture  122  is provided in the rim of the upper endcap  54  to allow the high pressure from the unfiltered zone  51  to appear at the top of the upper endcap  54 . Having thus relieved that problem, however, in a plugged filter condition, the high pressure on the top of the filter would tend to force the upper endcap  54  downwardly, attempting to crush the filter element of the otherwise unsupported filter. To prevent that, the upper endcap  54  is securely supported in the housing structure itself. Thus it will be seen that the upper endcap is provided with angled projections such as ribs  124  which serve to center the filter by riding over projection  125  in the housing. Projections  124  have lower shoulders  126  which seat on a ridge  127  on the interior of the housing  21 . Thus, when the filter is properly positioned, the shoulders  126  will seat upon and travel no farther than the ridge  127 . Even in a plugged filter condition, the pressures will not be capable of driving the endcap  54  down further than this position, preventing crushing of the filter in this situation.  
         [0045]     With respect to gasketing, it will be seen that the upper axial seal gasket  102  is a simple disc-like structure which is carried in a groove  102   a  in the upper endcap. Preferably the rubber gasket is sized so that it is snapped in place in the groove  102   a  for reliable retention.  
         [0046]     The lower gasket  56  is similarly fit within a groove  56   a  in the lower endcap, and snapped in place for reliable retention as shown in  FIG. 3 . The lower gasket has a skirt  130  which overlies an upstanding annular flange  132  in the base of the filter housing. With pressure in the unfiltered region  51  being higher than pressure in the clean region  52 , the operating pressure in the filter tends to force the skirt  130  against the flange  132 , maintaining the effectiveness of the seal. This same pressure which forces the skirt  130  against the flange  132  also forces the skirt  130  against the endcap  55 , rendering the seal even more reliable. Finally, positioning the lower radial seal gasket  56  at the outer periphery of the housing lends additional benefit in keeping the lower endcap  55  with a slight pressure differential in the downward direction. The bottom of the endcap  55  is in the clean region  52  of the filter and thus at a lower pressure. The upper endcap  54  is at a somewhat higher pressure due to its connection through the dirty region and the pleats. Thus the differential force will tend to force the lower endcap  55  downward slightly, keeping the filter element in tension, which is an acceptable condition to a pleated paper filter.  
         [0047]      FIG. 8  shows additional detail of the main seal between the cover  26  and the housing  21  which prevents fluid in the housing from exiting the housing. As seen, it is an axially compressed seal  102  and is compressed between the shoulder  58  on the cover and the similar shoulder  59  on the housing. The lid has female threads  140  which mate with male threads  142  on the housing. As best shown in  FIG. 8  the lid  26  has a long mating section  143  below the female threads  140  which have a close fit between the outside diameter of the housing male threads  142  and the inside diameter of the mating section on the lid. By arranging the structure in this way, the lid automatically becomes aligned with the housing  21  and it forces the lid  26  to be threaded correctly and avoids cross threading. Large diameter threads are prone to cross threading, and the elongated section where the lid first has a smooth section which fits over the threads in the housing, before the respective housing threads are mated, helps to prevent this from occurring.  
         [0048]     A significant feature of the invention is the fact that the upper and lower seals  102 ,  56  on the cartridge are attached to the cartridge itself. Thus they are placed in their proper location by the manufacturer. Because of the positive snap fit of the gaskets, they are very difficult to remove, and thus the gaskets are in their proper place when the cartridge is dropped into the housing. This feature makes the changing of cartridges for the filtering system almost full proof. The mechanic does not require any knowledge as to where to place loose gaskets in the system, because there are no loose gaskets. This makes changing of the cartridges fast, easy, avoids errors, and is almost as simple as changing a spin-on canister.  
         [0049]     Since the main seal between the housing and the lid is an axial seal  102 , the lid  26  is easy to spin on and off. In addition, it is only necessary to tighten the lid hand tight. If the main seal were a radial seal, the lid tightening would require a wrench and additional effort. With respect to user convenience, this is a significant positive feature.  
         [0050]     When considering the universality of the housing and filter arrangement, a keying system which associates the particular type of filter with a particular housing can also be an important feature. For example, in the ganged situation of  FIG. 4 , the two filters in the two parallel housings might have slightly different characteristics. In other ganged situations, where the filters are running individually but not in parallel, it will be typical to have different filters in adjacent identical housings. A keying system which prevents the installation of the wrong filter would be of material benefit.  
         [0051]      FIGS. 11 and 12  illustrates a keying feature which can be used in the practice of the present invention. The inside of the upper endcap  54  (see  FIG. 12 ) is provided with a plurality of key positions, best illustrated in  FIG. 11 . It will be seen that at a given radius from the center of the endcap  54 , a plurality of key positions  160  are provided. The illustrated embodiment includes eight key positions in a single ring. More or fewer key positions per ring, as well as additional rings can also be provided, but it is believed that the eight key positions, which can provide the sixteen possibilities illustrated in  FIG. 11 , is adequate for most applications.  
         [0052]     Referring primarily to  FIG. 12 , it will be seen that a single key  162  is provided on the underside of the illustrated endcap in a given position. While only a single key is shown in  FIG. 12 , as contrasted with the three keys of  FIG. 11  it is believed that the single key  162  will adequately illustrate the invention without overcomplicating the drawings. The key  162  is in a fixed angular position with respect to the key circle  163  (the circle in which the keys are located). The key  162  projects into the internal bore of the filter element  53 .  FIG. 12  shows a portion of the housing centertube  164  having a top surface  165  which is substantially solid except for a key opening  166 . The upper surface  165  of the centertube  164  has a plurality of key positions in a key circle  167  in the same pattern as illustrated in  FIG. 11 . However, instead of projections  162 , the keys in the upper surface  165  are apertures to receive the projections.  FIG. 12  shows a single aperture  166  positioned in the key circle  167  to engage the single projection  162  positioned in the key circle  163 . Thus, when the filter is installed in the housing, the keys  162  will align and allow the filter to reach the seated position, allowing the cover  26  to be placed on the housing and operation to continue. If the wrong filter inserted, the filter will not seat, and the user will be incapable of completing assembly.  
         [0053]     This feature is particularly significant when using a universal housing as illustrated in this application. For example, two housings might be used side-by-side to provide a primary and a secondary fuel filter system. Both housings would be of the same diameter and height, but would require different filter cartridges. The keys will prevent the cartridges for one of the housings from being installed incorrectly in the other housing. It would be a simple matter to have several different key configurations to suit various applications and indeed various customers.  
         [0054]     A further significant feature of the invention is the plugged filter indicator  48  (see  FIG. 1 ). A detailed cross-sectional view of the plugged filter indicator is shown in  FIG. 9 . The indicator has a button  180  at the top which is spring loaded by way of a spring  182 . A magnet  184  is fit within the top end of a piston  185  which rides in a cylinder  186 . The cylinder  186  is ported to the high pressure region  51  of the filter via a conduit  187 , and to the low pressure region  52  of the filter housing via a conduit  188 . As such, the differential pressure across the filter is also applied across the piston  185 , with the high pressure side on top. A calibrated spring  190  is arranged between the body  191  of the housing and a cavity  192  within the piston  185 . Thus the piston  185  is normally biased to the upward position, with the magnet  184  very near a thin wall  195  which separate the magnet  184  from an armature  196  in the button  180 . When they are closely positioned, the magnet attracts the armature  196  and keeps the button  180  withdrawn into its associated housing. However, when the differential pressure across the filters build up to a level determined by the calibration of the spring  190 , the piston  185  is driven downwardly, separating the magnet  184  from the armature  196 . The button  180  is then released and remains in the upward position, even if the piston  185  again returns to its top position. To reset the plugged filter indicator, it is necessary to manually depress the button  180  to re-latch it to the magnet  184 .  
         [0055]     The plugged filter indicator is thus easily ported into the universal housing, is highly reliable and does not introduce complications to the overall structure. In addition, it is relatively economical and has very little possibility of creating leaks in the system or otherwise deleteriously affecting the system operation. The calibration spring  190  can also be changed for different applications. For example, a relatively light spring would be used in fuel applications where the differential does not get much higher than say five to seven inches of mercury. When the housing is set up for a lubrication application or hydraulic application, heavier springs are used which would change the set point to somewhere in the range between 15 to 50 psi. This ready ability to alter the uncomplicated plugged filter indicator is believed to be a significant advance over the prior art.  
         [0056]     It is emphasized that the filter cartridge to be used with this system is an environmentally friendly filter made up of elements which can all be incinerated.  FIG. 13 , for review, shows, in elevation, one form of that filter.  FIGS. 3 and 13  illustrate the filter cartridge to include upper endcap  54 , lower endcap  55  having a filter element  53  supported therebetween. The filter element is potted into channels in the respective endcaps using conventional potting techniques. The lower radial seal gasket  56  is set in a groove  56   a  in the lower endcap. Similarly, the axial seal gasket  102  is set in a groove  102   a  in the upper endcap  54 . The pressure equalization port  122  is also illustrated. It will thus be apparent that once it is spent, filters such as that shown in  FIG. 13  can be removed from the housing, collected and incinerated, and will leave very little residue.  
         [0057]     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.  
         [0058]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0059]     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.