Abstract:
A screw press with twin screws is provided with discontinuities or interruptions  7  in the flighting  8  of the screws  2.  Stationary resistor teeth  9  protrude at these interruptions into the flow of material being pressed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improvement in a screw press, specifically to the configuration of the screws used in a twin screw press. 
     Screw presses have been used for centuries. Common applications are extracting juices and dewatering fibrous materials. Basically a screw press is a machine in which a helicoid screw rotates inside a cylindrical perforated screen. Entering material is subjected to gradually increasing pressure as it moves toward the exit end of the press, forcing the liquid phase to expel through the screen. 
     Compression is generally achieved in a screw press by a combination of three features: (a) the diameter of the shaft of the screw is increased, forcing material against the screen; (b) the pitch of the flights of the screw is reduced, compressing the material; and (c) back-pressure is created within the screen chamber by use of a cone (door, choke, plate, stopper or plug) located at the cake discharge end of the press. 
     A major innovation in screw press design was patented in 1900 by Valerius D. Anderson. His U.S. Pat. No. 647,354 describes how interruptions can be made in the flighting of the screw. This results in improved performance on slippery or slimy materials such as spent brewers&#39; grain, orange peel, fish, and rendering offal. The use of screw presses with interrupted flights rapidly came to dominate in these applications. 
     Flighting is the helicoid (corkscrew) surface that wraps around the shaft of a screw, somewhat like threads on a rod. 
     An improvement to the Anderson design was the addition of stationary resistor teeth. These are mounted, fixed to the frame of the machine, so that they protrude through the screen at the locations where the screw flighting is interrupted. The teeth are in the flow of material passing through the press. They can range in length from very short to reaching almost to the screw shaft. They cause stirring which both reduces co-rotation (slippage) tendency and puts wet material against the screen. 
     The l/d ratio is the ratio of screw diameter to screen length. Short l/d ratios, compared to those of continuous screw presses, are typical of interrupted flight machines. 
     A weakness of the single screw press is that, lacking positive displacement, excessive slippage can occur in the press. Thus difficult (slimy) materials can tend to co-rotate with the screw, resulting in a loss of both throughput and dewatering capacity. The amount of compression that can be applied to a material is limited by its tendency to slip, even in interrupted screw machines. 
     In recent decades another type of screw press has been found to offer stronger dewatering performance. This is a double (twin) screw press that features a pair of overlapping screws mounted side-by-side. Since the screws have continuous flighting, relatively positive displacement is created. This 
     semi-positive displacement, combined with optional features such as increasing shaft diameter, tightening pitch, and discharge back-pressure, result in a tighter squeezing press. 
     The principal disadvantage of the conventional twin screw press is that it requires (a) constant flow of (b) consistent material. If the solids consistency of the feed material decreases, this sloppy material will purge from the discharge of the press. A similarly bad situation occurs if the consistency increases: excessive dewatering occurs and the press jams or locks on a solid mass of cake. Screw and gearbox failures can result when this occurs. 
     Because of their heavy pressing action these continuous flight presses characteristically require high torque and rigidity, which results in large gearboxes, shafts, bearings, and flighting. These characteristics put conventional twin screw presses at a cost disadvantage. 
     In a continuous flight screw press there is almost no stirring action within the press. This is true even with twin screw designs. “In at three o&#39;clock, out at three o&#39;clock,” is how they are described. A disadvantage of this is that high l/d ratios are required in order to allow enough time for liquid in the material against the screw shaft to migrate to the screen surface. A higher l/d ratio results in a longer, more costly machine than one with a low l/d ratio. 
     SUMMARY OF THE INVENTION 
     The object of the subject invention is to gain, in one machine, the respective advantages of interrupted flight screw press and twin screw presses. The invention combines the high capacity and low horsepower, relative to size, of the interrupted screw press, along with the strong, positive throughput and dewatering features of the twin screw press. 
     The subject invention uses twin overlapping interrupted flight screws with stationary resistor teeth mounted at the interruptions. The interruptions and teeth stir the material being pressed, which has the effect of both reducing co-rotation and placing fresh, wet material against the screen surface. Thorough dewatering is achieved with the low l/d ratios typical of interrupted screw presses. 
     The use of twin overlapping screws achieves a degree of positive displacement, resulting in tight squeezing and a great reduction in slippage. This improves dewatering of slimy materials. On the other end of the scale, there is enough give to the interrupted configuration that jamming is minimized. In addition it has been found that slicing action resulting from the interruptions in the screw flighting reduces the torque (horsepower) requirement for proper dewatering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic top view of a twin screw press. The overlapping screws  2 , with continuous feeder flighting  1  in the inlet hopper  21 , and a discharge cone  5  are illustrated. 
     FIG. 2 shows a side view of the same screw press. The motor  17  is seen driving the gearbox  18 . The gearbox is mounted on the frame  19  of the screw press. Through one or two shaft couplings  20  the gearbox drives the screws  2 . Material is feed into the press through the inlet hopper  21 . Press liquor drains from drain  22  while press cake falls from the cake discharge  23 . 
     FIG. 3 shows a side view of a pair of typical interrupted flight screws  2 . The shafts  15  have continuous flighting in the inlet area of the press; this serves to feed material into the screened compression portion of the screws. The flighting becomes interrupted to allow spaces  7  for the insertion of resistor teeth  9 . The flighting  8  of the compression stages can be double flighted  24  or single flighted  25 . 
     FIG. 4 shows a sectional side view through the main screen area  4  of the press. The fixed resister teeth  9 , positioned in the gaps  7  between the compression flights  8 , are illustrated. 
     FIG. 5 shows a sectional end view through the screen area of the press. The screws  2  with their shafts  15  are shown overlapping within the screens  14 . Resistor teeth  9  are shown, top and bottom, protruding into the flow stream of the press. The resistor teeth are shown mounted in a typical arrangement using support bars  16 . 
    
    
     Additional screen areas may be provided both in the inlet hopper of the screw press and on the face of the discharge cone. Screens are typically made of punched sheetmetal, drilled plate, and profile bar (wedgewire). The type of screening and screen opening is not material to this patent. 
     DESCRIPTION OF THE INVENTION 
     With reference to the accompanying drawings, there is shown a screw press with twin overlapping screws  2  with interrupted compression flights  8  and fixed resistor teeth  9 . 
     As shown in the figures, material is fed into the feeder portions  1  of the screws  2  through an inlet hopper  21 . This material is fed from the inlet hopper into the main screen chamber  4  where the dewatering occurs. Expelled liquid (press liquor) drains through screen  14  to a collection pan  12 , while dewatered material (press cake) discharges past a restraining cone  5  at the solids discharge end. 
     Screws  2  consist of helicoid flighting  1  and  8  mounted on shafts  15 . As shown in FIGS. 3 and 5, the screws  2  overlap each other. These are driven in opposite directions of rotation in order to progressively move material through the press. The flights on one screw are right hand, while the other screw is left hand. The amount of overlap, from near zero to where the flights of one screw almost hit the shaft of the other, is varied by application requirements. 
     Furthermore, the screws must be driven in a synchronized manner in order to prevent mechanical interference. Normally this is achieved with the use of either a twin output gear box or, as illustrated, separate spur gears  13  mounted on the screw shafts. 
     The figures show a screw press in a horizontal configuration. Vertical configurations, with material flowing either upward or downward, can also be employed. The pressing action remains essentially the same. 
     The figures show a screw press with five stages of compression with resistor teeth on two sides of each screw. From three to seven stages of compression are typical in production machines. Also, resistor teeth can be placed on only one side of a screw or on only one of the two screws. 
     The position of restraining cone  5  is shown automatically adjusted by means of a pneumatic cylinder  11 . Springs, bladders, counterweights, and hydraulic cylinders can also be used to provide automatic adjustment of back pressure during press operation. The discharge cone need not be conical: flat and pyramid configurations are also used. 
     While the present invention has been described with respect to certain preferred embodiments, it is to be understood that the invention is not limited thereto, but rather is susceptible of numerous changes and modifications which will occur to those skilled in the art. Consequently, the present invention is not to be limited to the details shown and described herein, but is intended to cover all such changes and modifications as are encompassed by the scope of the appended claims.