Abstract: An improved apparatus for extracting a side-stream flow of fine-particulate-laden effluent gases from a mechanical separator employs the use of a plenum inside the housing of the separator with a plurality of pickup openings communicating the plenum with spaces among and about the separator's collector tubes. A side-stream flow of fine-particulate-laden effluent gases is extracted from regions among and about the collector tubes by drawing these gases through the pickup openings and into the plenum, from the plenum into a ductwork communicating with the plenum, and from the ductwork through a separator where particulate matter is filtered from the side-stream flow. The method of constructing the apparatus is also disclosed.