Screw assembly for cooking extruders operating with significantly reduced specific mechanical energy inputs
Screw assemblies are provided for cooking extruders characterized by operation using significantly reduced specific mechanical energy inputs, wherein the screw assemblies include first and second elongated, axially rotatable screws which are substantially coextensive in length, with the screw flighting presenting a pair of axially spaced apart sections of short pitch length and an intermediate section of greater pitch length. The intermediate section has an axial length greater than about four times the length of the axially spaced apart sections. The screw assemblies permit incorporation of significantly greater amounts of steam into comestible products during cooking thereof, as compared with prior art designs.
1. A screw assembly for use in a twin-screw cooking extruder and comprising:
first and second elongated, axially rotatable screws each having an elongated shaft with outwardly extending helical flighting along the length of each shaft, the flighting of each shaft being intermeshed with the flighting of the other shaft,
said first and second screws being substantially coextensive in length,
the flighting of each of the first and second shafts presenting a pair of axially spaced apart sections of short pitch length, and an intermediate section having a pitch length greater than the pitch lengths of either of said short pitch length sections,
said intermediate section having an axial length greater than four times the length of each of said axially spaced apart sections,
said screws having an S D /R D ratio of from about 1.9-2.5, wherein S D is the outermost screw diameter of the flighting of said first and second shafts, and R D is the root diameter of said first and second shafts.
2. The screw assembly of claim 1 , the flighting of said intermeshed intermediate sections having an axial gap distance there between of from about 0.1-0.4 inches.
3. The screw assembly of claim 2 , said gap distance being from about 0.15-0.35 inches.
4. The screw assembly of claim 1 , wherein said S D /R D ratio is about 2.35.
5. The screw assembly of claim 1 , wherein the ratio of the pitch length of said intermediate section to the pitch lengths of either of said first and second sections is from about 3-6.
6. The screw assembly of claim 1 , said screw flighting oriented for co-rotation of the first and second screws.
7. The screw assembly of claim 1 , the flighting of each of said first and second shafts having equal screw diameters.
8. The screw assembly of claim 1 , the root diameters of said first and second shafts being the same.
9. The screw assembly of claim 1 , the flighting of said first and second shafts being single flight.
10. The screw assembly of claim 1 , wherein the opposed ends of said intermediate section abut said pair of short pitch length sections.