head 92 carried on the end 93 of shaft 24 exterior of processing vessel 18 piston head 92 is disposed within a pressure cylinder 94 with head 92 separating cylinder 94 into a first pressure chamber 95 and a second pressure chamber 97 first and second fluid actuated motors 96 and 96 39 respectively are provided motors 96 96 39 are provided with fluid inlets 98 98 39 and fluid outlets 100 100 39 motors 96 96 39 form no part of this invention per se and are well known in the art motor 96 is preferably a negative pressure motor such that a suction on outlet 100 draws a fluid through inlet 98 which drives motor 96 conversely motor 96 39 is a positive pressure motor such that the admission of a fluid under pressure to inlet 98 39 passes through motor 96 39 in route to outlet 100 39 the fluid passing from inlet 98 39 to outlet 100 39 drives motor 96 39 outlet 100 is connected to first pressure chamber 95 by a first outlet motor conduit 102 a second outlet motor conduit 104 connects outlet 100 with the second pressure chamber 97 a first motor inlet conduit 106 connects an inlet 98 39 of motor 96 39 with the first high pressure chamber 95 a second inlet motor conduit 108 connects inlet 98 39 with the second pressure chamber 97 flow direction control valves 101 preferably check valves are disposed in each of lines 102 104 106 and 108 the valves 101 in.