Filter unit having filter elements in series and in reserve

Claim 1

A FILTER ASSEMBLY COMPRISING, IN COMBINATION, A HOUSING HAVING AN INLET AND AN OUTLET; AND, DISPOSED IN THE HOUSING, A PRIMARY FILTER ELEMENT TO RECEIVE FLOW FROM THE INLET; A RESERVE FILTER ELEMENT ADJOINING THE PRIMARY FILTER ELEMENT IN A CONSECUTIVELY ENDWISE POSITION WITH RESPECT TO THE PRIMARY FILTER ELEMENT; SEPARATING MEANS EXTENDING FROM THE JUNCTURE OF THE PRIMARY AND SECONDARY FILTER ELEMENTS TO THE HOUSING, AND DIVIDING THE HOUSING INTO AT LEAST TWO INLET FLOW CHAMBERS INCLUDING A PRIMARY INLET FLOW CHAMBER AND A RESERVE INLET FLOW CHAMBER, THE PRIMARY FLOW CHAMBER OPENING ONTO THE UPSTREAM SIDE OF THE PRIMARY FILTER ELEMENT, AND THE RESERVE FLOW CHAMBER OPENING ONTO THE UPSTREAM SIDE OF THE RESERVE FILTER ELEMENT, SAID SEPARATING MEANS NORMALLY CLOSING OFF THE RESERVE FLOW CHAMBER FROM THE INLET AND THUS ISOLATING THE UPSTREAM SIDE OF THE RESERVE FILTER ELEMENT FROM INLET FLOW; AND A RELIEF VALVE ASSOCIATED WITH THE SEPARATING MEANS AND RESPONSIVE TO A PREDETERMINED PRESSURE DIFFERENTIAL ACROSS THE PRIMARY FILTER ELEMENT TO OPEN AND PERMIT FLOW TO PROCEED INTO THE RESERVE FLOW CHAMBER, THUS TO EXPOSE THE UPSTREAM SIDE OF THE RESERVE FILTER ELEMENT TO FLOW FROM THE INLET, AND TO PERMIT FLOW TO CONTINUE TO THE OUTLET VIA THE RESERVE FILTER ELEMENT.