With primary reference now to FIG. 7, the separators 124 preferably provide two way selectable passage. Selector 124 includes an external sleeve member 129 adapted to telescopically receive conduits 120, 1268 and which is secured thereto with conventional sheet metal screws 131. Preheater conduit 128B is angularly secured to sleeve 129, terminating in an orifice 141. A valve member 143 disposed within sleeve 129 is displaceable from a bypass position to a preheat position by proper manipulation of clevis actuators 147, 148, which communicate through slots 149 to a central axle (not shown) which drives closure valve 143. In the, preheat position arcuate tab 150 (integral with valve closure member 143) prevents passage of perlite into the bypass conduit. It should be appreciated that any form of conduit selection routing system adapted to select between at least two pipes may be employed with equal utility with the present invention. Where the furnace is to be utilized only for processing perlite, the selector and preheat bypass systems may be deleted entirely by directly coupling pipes 128B and 120.