turning now to the drawing , a cooking extruder 10 in accordance with the invention includes an elongated , tubular , multiple - section barrel 12 presenting juxtaposed , intercommunicated chambers or bores 14 , 16 , and a pair of elongated , helically flighted , axially rotatable , juxtaposed , intercalated screws 18 and 20 within the bores 14 , 16 . the barrel 12 includes an inlet 22 and a spaced outlet 24 which communicate with the bores 14 , 16 . although not shown , it will be appreciated that a restricted orifice die is normally positioned across outlet 24 for extrusion purposes . additionally , the drive ends 26 of the screws 18 , 20 are operably coupled with a drive assembly ( not shown ) for axially rotation of the screws 18 , 20 , which typically includes a drive motor and gear reduction assembly . in more detail , the barrel 12 includes , from right to left in fig1 and 3 , a series of tubular sections connected end - to - end by conventional bolts or other fasteners . specifically , the barrel 12 has an inlet head 28 , a first short steam restriction head 30 , a first steam injection head 32 , a second short steam restriction head 34 , a mid - barrel adjustable valve assembly head 36 , an adjustable steam outlet head 38 , a second steam injection head 40 , and third short steam restriction head 42 . as illustrated , each of the heads 28 - 34 and 38 - 42 is equipped with endmost , radially enlarged connection flanges 28 a - 34 a and 38 a - 42 a , and all of the heads 28 - 42 have aligned through - bores which cooperatively form the barrel bores 14 and 16 . the head 36 likewise has through bores mating with those of flanges 32 a and 38 a . the heads 32 and 40 of barrel 12 are each equipped with two series of steam injection ports 44 or 46 , wherein each of the ports houses an elongated steam injector 48 or 50 . the two series of ports 44 in head 32 are located so as to respectively communicate with the bores 14 and 16 of the head ( see fig4 ). similarly , the two series of ports 46 in head 40 also respectively communicate with the bores 14 and 16 of this head . importantly , the ports 44 and 46 are oriented at oblique angles relative to the longitudinal axes of the corresponding bores 14 and 16 . in practice , the ports are oriented at an angle from about 30 - 85 degrees , more preferably from about 30 - 60 degrees and most preferably about 45 degrees , relative to these axes . moreover , the ports 44 , 46 are preferably oriented in a direction toward the outlet 24 . more specifically , and referring to fig5 , it will be seen that each representative port 44 presents a longitudinal axis 52 . if this axis 52 is orthogonally resolved into components 54 and 56 , the component 54 extends in a direction toward outlet 24 . the mid - barrel adjustable valve assembly head 36 is of the type described in u . s . patent application ser . no . 11 / 279 , 379 , filed apr . 11 , 2006 and incorporated by reference herein . briefly , the head 36 includes opposed , slidable , flow restriction components 58 and 60 , which can be selectively adjusted toward and away from the central shafts of the extruder screws 18 and 20 , so as to vary the restriction upon material flow and thus increase pressure and shear within the extruder 10 . on the other hand , the steam outlet head 38 has a steam outlet 62 with an adjustable cover 64 permitting selective escape of steam during the course of extrusion . in some instances , a vacuum device ( not shown ) can be used in lieu of cover 64 for more effective withdrawal of steam and / or reduction in processing pressures . the screws 18 and 20 are identical to each other , and thus only one of the screws need be described in detail . referring to fig3 , it will be seen that the overall screw 20 broadly includes a central shaft 66 with helical flighting 68 projecting outwardly from the shaft 66 . however , the screw 20 is specially designed and has a number of novel features . these features are best described by a consideration of certain geometrical features of the screw 20 and its relationship to the associated bore 16 . in particular , the shaft 66 has a root diameter r d defined by the arrow 70 of fig3 , as well as an outermost screw diameter s d defined by the screw flighting 68 and illustrated by arrow 72 . in preferred practice , the ratio s d / r d of the of the outermost screw diameter to the root diameter is from about 1 . 9 - 2 . 5 , and most preferably about 2 . 35 . the individual sections of the screw flighting 68 also have different pitch lengths along screw 20 , which are important for reasons described below . additionally , along certain sections of screw 20 , there are different free volumes within the bore 16 , i . e ., the total bore volume in a section less the volume occupied by the screw within that section differs along the length of the screw 20 . in greater detail , the screw 20 includes an inlet feed section 74 , a first short pitch length restriction section 76 within head 30 , a first longer pitch length section 78 within head 32 , a second short pitch length restriction section 80 within head 34 , a second longer pitch length section 82 within heads 38 and 40 , and a third short pitch length restriction section 84 within head 42 . it will thus be seen that the pitch lengths of screw flighting 68 of screw sections 76 , 80 , and 84 are substantially smaller than the corresponding pitch lengths of the flighting 68 of the screw sections 78 and 82 . in preferred practice , the pitch lengths of screw sections 76 , 80 , and 84 range from about 0 . 25 - 1 . 0 screw diameters , and are most preferably about 0 . 33 screw diameters . the pitch length of 78 and 82 range from about 1 - 2 screw diameters , and are more preferably about 1 . 5 screw diameters . these geometrical features are important in achieving the ends of the invention , and specifically permit incorporation of significantly greater amounts of steam into the material passing through extruder 10 , as compared with conventional designs . in essence , the restriction heads 30 and 34 , and 34 and 42 , together with the short pitch length screw section 76 , 80 and 84 therein , cooperatively create steam flow restriction zones which inhibit the passage of injected steam past these zones . as such , the zones are a form of steam locks . additionally , provision of the heads 32 , 38 , and 40 with the longer pitch length screw sections 78 and 82 therein , between the restriction zones , creates steam injection zones allowing injection of greater quantities of steam than heretofore possible . the longer pitch screw sections 78 and 82 result in decreased barrel fill ( not necessarily greater free volume ), and thus create steam injection zones . finally , the orientation of the injection ports 44 and 46 , and the corresponding injectors 48 and 50 therein , further enhances the incorporation of steam into the material passing through extruder 10 . this combination of factors within extruder 10 allows significantly greater steam to be injected , as compared with conventional extruder design . in the later case , only about 3 - 5 % steam may be injected , with any excess simply passing through the extruder and exiting the barrel inlet . however , in the present invention , about 6 - 8 % or more by weight steam may be successfully injected without undue injected steam loss , based upon total weight of dry material ( i . e ., the total weight of the dry ingredients containing only native water , less any water normally added to the dry ingredients prior to passage thereof into the extruder ) within the barrel 12 at any instance taken as 100 % by weight . this is particularly important in the case of certain animal feeds which are improved by additional cooking within the extruder without imposition of excessive shear and mechanical energy cooking . although the extruder 10 illustrated in the figures includes the use of an adjustable valve assembly head 36 and steam outlet head 38 , the use of such heads is not required . the head 36 can advantageously be used as a further restriction against steam loss , and the head 38 can be used in instances where mid - barrel steam venting is desired , e . g ., where denser products are desired . furthermore , while the illustrated embodiment is in the form of a twin screw extruder , it will be understood that the principles and constructional features of the invention may be used in the context of single screw extruders as well . finally , the extruders of the invention are cooking extruders , which are designed to at least partially or fully cook material passing there through , while also forming the material into discrete shapes . as such , the extruders are equipped with screws which impart shear and mechanical energy as a part of the cooking process , which is augmented by steam injection . further , although not shown , such cooking extruder barrels may be equipped with external jackets for introduction of heat exchange media to indirectly heat or cool the material passing through the extruders .
an improved extruder is provided which permits successful introduction of very high quantities of injected steam into material being processed , on the order of 6 - 8 % or more by weight steam . the extruder includes an elongated extruder barrel having at least one elongated , axially rotatable , helically flighted extrusion screw therein . the barrel is equipped with obliquely oriented steam injection ports along the length thereof , housing steam injectors . the barrel includes relatively high free volume steam injection heads having therein screw sections of relatively long pitch length , together with steam restriction heads on opposite sides of the injection heads having therein relatively short pitch length screw sections .