Filter unit having reserve filter element

Claim 1

A FILTER ASSEMBLY COMPRISING, IN COMBINATION, A HOUSING HAVING AN INLET AND AN OUTLET, A PRIMARY FILTER ELEMENT DISPOSED IN THE HOUSING; A PRIMARY PASSAGE LEADING FROM THE INLET TO THE PRIMARY FILTER ELEMENT; A SECONDARY RESERVE FILTER ELEMENT FIXED IN RELATION TO THE PRIMARY FILTER ELEMENT, AND DISPOSED IN THE HOUSING; A SECONDARY PASSAGE COMMUNICATING WITH THE INLET AND THE SECONDARY FILTER ELEMENT; A RELIEF BYPASSING PASSAGE IN THE HOUSING CONNECTING THE INLET AND THE OUTLET; A FLUID FLOW SEPARATING MEANS DISPOSED BETWEEN AND SPACED FROM THE PRIMARY AND SECONDARY FILTER ELEMENTS, PREVENTING FLOW BETWEEN THE PRIMARY AND SECONDARY FILTER ELEMENTS; A VALVE COMPRISING A SELF-CLOSING FLUID PRESSURE-RESPONSIVE SPRING DISK DISPOSED ACROSS THE SECONDARY PASSAGE, AND NORMALLY CLOSING OFF FLOW TO THE SECONDARY PASSAGE, AND NORMALLY THROUGH, BUT RESPONSIVE TO A PREDETERMINED PRESSURE DIFFERENTIAL ACROSS THE PRIMARY FILTER ELEMENT TO OPEN AND ALLOW FLOW THROUGH THE SECONDARY PASSAGE, BYPASSING THE PRIMARY FILTER ELEMENT, TO THE SECONDARY FILTER ELEMENT; AND A SECOND FLUID PRESSURE-RESPONSIVE SELF-CLOSING RELIEF VALVE DISPOSED ACROSS THE RELIEF PASSAGE, RESPONSIVE TO A PREDETERMINED PRESSURE DIFFERENTIAL BETWEN THE INLET AND THE OUTLET, TO OPEN AND ALLOW FLOW THROUGH THIS PASSAGE BYPASSING BOTH THE PRIMARY AND SECONDARY FILTER ELEMENTS WHENEVER THE PREDETERMINED PRESSURE DIFFERENTIAL THEREACROSS IS EXCEEDED.