Retruder
BACKGROUND OF THE INVENTION 
     The invention relates to a retruder for continuously conveying, mixing and/or kneading media at elevated pressure. The term extruder has become established for apparatus of a similar type. This term is fundamentally incorrect for the present invention, because of the reversal of the direction of conveying, instead of a circumlocution the abbreviation retruder (reverse extruder) is proposed. 
     Continuous conveying, mixing and/or kneading of media is part of the basic operations of process engineering. The material for mixing and/or conveying can be very different substances. The chemical and physical properties of these substances, their state of aggregation and their behaviour under pressure and heat can be correspondingly different. The solutions provided by the engineering of the prior art, which extend from kneading chocolate composition, producing semifinished plastics by extrusion and processing flowable plastics granules with a glass fibre content for injection moulding to the disposal of biologically active waste, correspond to this variety. 
     For all these tasks there are known machines which have, depending on the requirements, cooled or heated housings and at least two rotating, mutually meshing conveying screws which produce a pressure build-up by forcibly conveying the medium in the individual, closed windings of the screw channel, without having a reactive effect on the mass transport. The seals and the bearings are the weak points of these constructions, since the conveying pressure produces high axial pressures. In particular, this disadvantage becomes important when processing abrasive media or when the high pressures in the medium for the purpose of increasing the frictional forces in the medium are themselves advantageous. This is the case with kneading tasks if the efficiency of kneading is increased or for example in the case of a frictional reactor which is to be operated as a thermo-mechanical process unit for preparing biomass, at high operating pressures. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to enable and to increase the pressure build-up and the frictional forces within the medium of a machine having at least two rotating, mutually meshing conveying screws forcibly conveying the medium in the screw channel without the disadvantageous consequences for the bearing seals and for the bearings. 
     The advantages achieved by the invention reside substantially in the increase in the kneading output, the service life and the availability of forcible conveyors for existing tasks and moreover the possibility of providing new processes. This is due to the increase in the permissible conveying pressure in accordance with the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by way of example with reference to the attached diagrammatic drawings, in which: 
     FIG. 1 shows the perspective illustration of the screws of a retruder. 
     FIG. 2 is an end view of the retruder; 
     FIG. 3 is a cross-sectional view. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The retruder 1 in FIG. 1 is illustrated without a housing and only shows the two mutually meshing reversible screws 2, 3. The term &#34;reversible screw&#34; indicates the particular property of these screws, since both reversible screws change their direction 14 of conveying to the reverse direction 15 by the inversion of the pitch angles, which is a function of the pitch 5 and the circumference of one of the two reversible screws 2, 3, in the region of the reversing points 20, 21 and/or 30, 31. The absolute values of the pitch in the region of the direction 14 of conveying and the pitch of the reverse direction 15 can be different depending on requirements. A further important property of the two reversible screws 2, 3 is the fact that they rotate in opposite directions 12, 13 and in meshing manner. In the region of the reversing points 20, 30 and 21, 31 of the reversible screws 2, 3 the meshing is performed such that the reversing windings of the reversible screws 2, 3 roll on one another in the manner of double helical gearwheels. 
     The depth of the screw channel (the difference between the external radius of a screw and the core radius of the respective one of the reversible screws 2, 3) and the flank angle 4 form the cross-section of the screw channel as a function of the pitch angle. The cross-section of the screw channel can decrease with the flank angle 4 in the direction 14 of conveying and cause an increasing compression of the medium. 
     FIGS. 2 and 3 show the screws 3, 4 within a housing 6 having a charging (inlet) port 10 and a discharge port 11, spaced along the length of the housing. 
     From charging port 10, the medium passes into the screw channel of the reversible screws 2, 3 and is conveyed as a result of the rotation in opposite directions forcibly in the individual turns, which are closed by the windings of the screw channel, the housing 6 inner wall and the meshing screw, as far as the reversing points 20, 30 and 21, 31 in accordance with the direction 14 of conveying. The mass transport conveyed in the reverse direction 15 presses against this mass flow from the plane of rotation of the common reversing points 20, 30 and 21, 31. Depending on the counter-pressure of the reverse direction, the frictional force in the medium is increased and the axial load on the screw bearings is reduced. The progressively decreasing flank angles of the reverse direction 15 allow the mass transport of the medium to advance to the removal port 11. It should be noted that discharge port 11 is generally transverse to the longitudinal axes of the screws. In an extruder, the discharge port is generally in line with the screw axes. As shown in the drawings, the inlet and discharge ports 10, 11 are diametrically spaced. Because of the counter-pressure in the reverse direction 15, the pressure on the seals of the adjacent bearings is considerably reduced so that the risk of damage to the bearings is correspondingly lessened. 
     The mass flow of the retruder 1 permits charging 10 and removal 11 perpendicular to the reversible screws 2, 3. Depending on the wish of the user, charging 10 and removal 11 can be in opposite from what is illustrated, or indeed lie along the same line (not illustrated). Depending on requirements, the housing of the retruder 1 can be provided with a casing which is common and/or divided as desired and which permits heating and/or cooling of the retruder. Finally, the possibility of forming the reversible screws as a monobloc should also be pointed out. Thanks to this robust construction of the retruder, common sense will set the limits on output.