Reverse pitch

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

The Ekato intermig impeller has reverse pitch on the inner and outer blades and they are almost always used with multiple impellers. They are used at high D/T and promote a more uniform axial flow pattern than other turbine impellers. They are advertised to be very effective for solids suspension, blending, and heat transfer in the medium viscosity range. Lower Nrc limit not given by Ekato (9), perhaps 5. 

Miyaoka T, Seno H, Itoga M, Horiguchi J. Reversible pitch perception deficit caused by carbamazepine. Clin Neuropharmacol 2000 23(4) 219-21. 

The Sorticanter has a scroll with reversing pitch on one side, which skims a floating solids layer off the top of the carrier hquid, usually an aqueous brine of intermediate density. Sinking solids are scrolled out similarly to normal decanter centrifuges, and liquid is discharged under pressure via a stationary but variable impeller. The Sorticanter is utihzed in the plastics recycling industry. 

Reverse pitch elements are used to generate backpressure and therefore create sections of 100%i fill that, for example, can be used to separate unit operations, or totally fill a mixing section. 

The polymer melt is forced from the metering section into a bypass flow channel by incorporating a multi-flighted screw section with reversed pitch and shallow channels between the metering section and the extraction section. The bypass channel has one or more adjustable restrictions to control the rate of flow into the extraction section. The polymer flows from the bypass channel into the beginning of the extraction section. Maddock and Matzuk concluded that bypass venting allows a wider range of operation, is less susceptible to instabilities, and is less operator-sensitive. 

Wheat bran and sodium hydroxide were blended at room temperature in a separate reactor one hour before each experiment. Tlie initial L/S ratio was seven and the mixture was stirred for five minutes. The L/S ratio was increased to ten just before the introduction of the mixture into the twin screw extruder with a Nemo excentric-screw pump. Straw was introduced in the extruder s first section with a screw feeder. Straw was mixed with the alkaline dough in the first zone of the barrel through the neutral pitch element and the reverse-pitch screw element successively. The washing water was injected downstream from this zone, and the mixture was conveyed through the second reverse pitch located just downstream from the filtration module. The filtrate was collected and kept in a cold room before further processing, while the refined cellulosic fibres were gathered at the barrel outlet. 

FIGURE 831 Flow in a Kenics static mixer. The mixer consists of a pipe with heUcal elements of alternating reverse pitches. 

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