14 Extruder.pdf

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Temperature sensing: T of product sensed just behind die (melt temperature). Temperatures along barrel also often monitored with thermocouple probes. Pressure sensing: pressures just behind die are useful. Breaker plate: perforated metal discs between end of screw and die. Stops hard uncooked pieces from entering die and plugging. Also enhances back-mixing. Estimated moisture loss mC p(T 1 − T 2) = m(M 1 − M 2 )ΔH v M 2 = M 1 ΔH v − C p (T 1 − T 2 ) ΔH v m =mass flow rate of food, Cp=ht capacity, M=moisture content, ∆Hv=latent heat of vaporization (at ambient pressure), 1 = before die 2 = after die. Food Properties During Extrusion 200 Temp (°C) 0 Flour H 2O Feed Zone Venting Plasticizing Zone Pressure Build-up Zone Die: shaped holes through which exudate emerges. May form rods, spheres, donuts, tubes, strips, etc. Expansion occurs as product leaves because of rapid release of pressure from the end of the extruder to ambient conditions. Flashing of moisture occurs (H 2 O was above normal TB). High P, High T Product 50 Moisture (%) 0 Flour H 2O Feed Zone Atmospheric Pressure T > T b Venting Steam Steam leaves product and expands matrix. As moisture content decreases, structure solidifies. Plasticizing Zone Pressure Build-up Zone
1 Density (g/cm 3 ) 0 Flour H 2 O Feed Zone Venting Plasticizing Zone Pressure Build-up Zone 50 Pressure (bar) 0 Flour H 2 O Feed Zone Venting Plasticizing Zone Pressure Build-up Zone
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Page 7: Screw Sections Feed section: has d

14 Extruder.pdf

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