Filtration device

Claim 1

A filter device which comprises a tubular housing with an inlet for the fluid to be filtered in one end of the housing; an outlet for filtrate in the opposite end of the housing; a filter tube mounted inside the housing and extending essentially in the axial direction of the tubular housing, at least part of the filter tube wall forming a filter surface, and one end of the filter tube being openly connected to said inlet, whereas the other end being closed so that an annular filtrate conduit is formed between the housing and the filter tube; a first valve means arranged inside the filter tube to shut off the filter tube during cleaning periods, dividing the filter tube and the filter surface into an upstream part and a downstream part, and forcing the entire filtrate stream to pass through said upstream part of the filter surface into the filtrate conduit; the downstream part of the filter tube being provided with a discharge conduit comprising a second valve means to cause, by opening said second valve means during said cleaning periods, part of the filtrate stream from the filtrate conduit to stream back through the downstream part of the filter surface, thereby removing through said discharge conduit the rejected material settled on the filter surface, the cross-sectional area of said annular filtrate conduit being constant along substantially all of said downstream part of the filter tube to ensure, during said cleaning periods, a uniform backstreaming of the filter surface of said downstream part, and said first valve means being so located in the filter tube that sufficient filter surface is provided in said upstream part to maintain, during said cleaning periods, a filtrate flow through said outlet which is equal to the major part of the filtrate flow through said outlet during normal filtering.