Solids Conveying Example Calculation

Total helical length of the solids conveying section (4 diameters) at the barrel, Zj 0.99 m 1.11 

7 MPa pressure, a sliding velocity of 20.2 cm/s, and a barrel surface temperature of 90 °C, the coefficient of dynamic friction is estimated at= 0.35 using Eq. 5.31. The equation is then evaluated at the conditions at the screw root of 1.7 MPa pressure, 75 °C, and a sliding velocity of 36.1 cm/s for a coefficient of friction at the screw surface of/, = 0.44. Here, / =/. 

For the modified Campbell-Dontula model, Eq. 5.13, cp is calculated as follows for 

The solids conveying rate can be now be estimated using Eq. 5.7 adapted for mass flow  

The modified Campbell-Dontula model provides an acceptable prediction of the rate at the section exit pressure. For many cases, the coefficients of friction are adjusted until an acceptable performance is obtained. This model and the other models should always be used with caution. As previously discussed, these models use a static force balance to approximate a dynamic process. 

---

The effect of channel depth on solids conveying rate is shown in Fig. 5.26 for screw and barrel temperatures of 75 and 125 °C, respectively, and at a screw speed of 50 rpm. At zero discharge pressure, the solids conveying rates were nearly proportional to the depth of the screw channel (or cross-sectional area perpendicular to the flight). For example, the conveying rates were 91 and 125 kg/h for the 8.89 and 11.1 mm deep screws, respectively. For these screws, the cross-sectional areas perpendicular to the flights were calculated at 420 and 530 mm an area increase of... 

Pi is the pressure at the base of the hopper, which was calculated in Example 8.1. Thus, P2 is 2.52 X 10 Pa (36.5 psi). The solids conveying zone could in fact extend another turn or two until the surface reaches the melting point. Pm, of... 

---