971,868. Centrifugal separators. DOUGLAS AIRCRAFT CO. Inc. Sept 23, 1963, No. 37284/63. Heading B2P. Solid particles are classified by the opposing actions of centrifugal force and an inward current of working fluid, the classifying zone comprising an outer annulus between inwardly converging stationary walls 130, 182, and an inner annulus between inwardly diverging walls of a rotor with vanes 158, the working fluid being impelled to flow as a free vortex into the outer annulus and taking up a constant radial velocity in the inner annulus, particles to be classified being fed with zero radial velocity into the junction between the inner and outer zones. Particles are fed into a hopper 28 at the top of the machine and fall into the feed pipes 146, Figs. 3 and 4, mounted on the rotor. From the pipes 146 particles enter a rotating annular chamber, containing spreading bars 154 and exit vanes 156 between which the particles emerge to strike a replaceable stationary ring 202 and to fall as a curtain into the central region of the separating zone. Once taken up in rotary motion the particles separate; heavy particles progressing against the flow of fluid into a series of peripheral outlets 106 leading to an annular outlet chamber 30; light particles proceeding inwardly with the fluid flow to turn round the U-section portion 150 of the rotor and be impelled by rotor blades 160, 164, into a lower annular trough bounded by the outer wall 86 and an inner conical wall 78. At the bottom of the trough is a series of radial outlets 92 leading to an annular outlet chamber 34 for the light particles. The working fluid is recirculated upwardly in towards the centre of the rotor past a sleeve throttle valve 230 and the feed pipes 146, and between impeller vanes 144 which drives the fluid back into the classifying zone round the stationary member 130. A grid of separators 174 is arranged side by side in circular array to clean returning fluid of residual light particles. As best seen in the alternative sections of Figs. 7 and 8, the grid comprises hollow bars with vertical slots to catch the radially incident fluid. Inside the bars the fluid throws out the residual particles which fall out the open bottoms of the bars and into the light particle collecting trough. To vary the separating point the sleeve valve 230 may be raised or lowered to raise or reduce the speed of the circulating fluid, A finger wheel 256 projects at the base of the machine and through gear wheels 248, 246, rotates the threaded sleeve 236 to control the valve. A geared down indicator wheel 258 is driven from the sleeve by gears 262, 260. Recirculating fluid, in the illustrated embodiment the fluid is air, is drawn into the machine beneath the top cover 120 at the peripheral gap 122. The shallow blades 142 at the top of the rotor impel the fluid outwards and upwards to pass through a hole 176 into the space between outer wall 108 and inner casing 90. From the bottom of this space passages, which are not shown, pass downwardly into the cavity 178 to escape through the gaps 184 and 186 between the stationary and rotary parts of the machine. By this means air losses are made up and the gaps are kept free of fine particles. The coarse particles ejected from the annular chamber 30, and the fine particles ejected from the annular chamber 34, are separated from the working fluid by means of cyclones attached by clip fastenings to the outlets of the chambers. The use of water as a working fluid requires the provision of a reservoir tank on the top of the machine (see Fig. 9, not shown). The machine is mounted on a coaxial motor housing.