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1. (WO1986003687) FILTRES
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C LAIMS

1. A filter unit comprising a housing element (111) having a first (116) and a second inlet (114) for fluid and a first (118a) and a second outlet (113) for fluid, means (112,119) in said housing defining a first path of flow for fluid between the first inlet and the first outlet of the housing, means
(126,122,125) in said housing defining a second path of flow for fluid between the second inlet and the second outlet of the housing, and a filter member in said housing capable of filtration to remove particles of dimensions greater than 5 micrometres, said second path of flow including a path through said filter and a by-pass (130) about said filter having a resistance to flow of fluid no greater than that of said filter whereby in normal use a proportion only of fluid passing through the second flow path passes through said filter.
2. A unit as claimed in claim 1 wherein the by-pass includes a variable flow restrictor means (130,131), resiliently loaded in a manner to tend to increase restriction of flow .
3. A unit as claimed in claim 1 wherein the by-pass is permanently open.
4. A unit as claimed in claim 1 wherein the second outlet (113) is disposed adjacent to the first inlet (116), and the first outlet (118a) is disposed adjacent to the second inlet (114).
5. A unit as claimed in claim 1 for connection between a source of fluid to be filtered and a coarse filter, wherein said means bounding the second path of flow has an internally threaded bore (113) at said second outlet and an externally threaded stem (114) at said second inlet, said first inlet and second outlet being at a first end face (115) of the housing, and said first outlet and second inlet being at a second and opposed end face (118) of the housing.
6. A unit as claimed in claim 1 wherein said housing element is a part of a filter housing (74) also containing a coarse filter (83).
7. A unit as claimed in claim 6 wherein the housing comprises a mounting plate member (71) having an axially located threaded bore (62) for mounting on a complimentary threaded stem and constituting said second outlet and having one or more fluid inlet passageways (75) constituting said first inlet, a body portion (74) closed at one end and mounted on said mounting plate member at the other (75) and an axially extending tube (70) extending from adjacent the said closed end of the body portion to said threaded bore. 8. A unit as claimed in claim 7 wherein the body portion is snap fitted to the mounting plate by engagement of snap coupling elements (75) provided on the body portion and mounting plate.
9. A unit as claimed in claim 1 wherein the by-pass is provided by one or more axially running external grooves (79) in the tube.
10. A unit as claimed in claim 7 wherein the coarse filter (83) coaxially surrounds the tube (70) adjacent the mounting plate member (71) and the said filter capable of filtering 5 micrometre particles coaxially surrounds the tube (70) between the coarse filter and said closed end (74).
11. A method of filtering a fluid comprising passing the fluid through a filter unit as claimed in any preceding claim such that fluid passes simultaneously, through the said filter and the by-pass path.
12. Filtration apparatus comprising means defining a flow path between an inlet and an outlet for fluid to be filtered, said flow path containing in series a coarse filter for treating at least substantially all of the fluid passing through the flow path and a fine filter capable of removing particles of 5 micrometres and greater, wherein the flow path includes a by-pass around said fine filter for a proportion of fluid passing through the flow path which by-pass offers no greater resistance to flow than said fine filter.