Conveying rate

Conveying characteristics also will provide useful information when an existing plant needs to be upgraded to achieve say, a higher conveying rate of solids. For example, it will be possible to determine whether the system and the material will be able to cope with the increased pressure and/or air flow requirements (i.e., whether the combination of pipe size and blower/compressor rating will be sufficient). 

As previously discussed, the solids conveying rate decreases as the discharge pressure increases for all materials studied. Eor HDPE resin, the solids conveying rate decreased almost logarithmically with discharge pressure, as shown in Eig 5.12. This is a characteristic of most crystalline polymers, and has been well documented in prior literature [21] as well as predicted by all of the major models [1,14,19,20, 22, 23]. The HDPE resin demonstrated the greatest solids conveying pellet flow... 

The dependence of rate on the discharge pressure was considerably less for HIPS resin as compared to HDPE resin. As shown in Fig 5.13, the solids conveying rate for HIPS resin decreased to a lesser extent with increasing discharge pressure, and the rate did not seem to be dependent on temperature in this temperature range. This result is consistent with the dynamic coefficient of friction for HIPS resin, as shown by Fig. 12.17. As shown in Fig. 12.17, the coefficient did not depend to a high level on temperature or velocity in this low temperature range. 

Several of the most commonly used resins were studied at a screw and barrel temperature of 35 °C. As previously discussed, this temperature condition is comparable to the conditions in the feed casing or Section 1. This is just the start of solids conveying as conveying continues into Section 2 where the inside barrel wall temperatures are considerably higher. In order to visualize the contrast between the six different polymers tested, the solids conveying rates as a function of discharge pressure for these select resins are presented in Fig. 5.14. 

Figure 5.22 Solids conveying rate as a function of barrel and screw temperature for the shallow screw (8.89 mm) at a screw speed of 50 rpm and at zero discharge pressure...

Figure 5.25 Solids conveying rate as a function of discharge pressure and screw speed for the shallow (8.89 mm) screw. The barrel and screw were maintained at temperatures of 125 and 100 °C, respectively...

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... 

Figure 5.26 Solids conveying rate as a function of discharge pressure for the shallow/...

Figure 5.27 The effect of flight radii size on solids conveying rates for a barrel temperature of 75 °C, a screw temperature of 50 °C, and a screw speed of 50 rpm...

The soiids conveying rate data as a function of discharge pressure at a screw speed of 50 rpm and barrei and screw temperatures of 75 °C are provided in Tabie 5.1. 

Table 5.1 Experimental Solids Conveying Rates (Fig 5.24) as a Function of Discharge Pressure at a Screw Speed of 50 rpm and Barrel and Screw Temperatures of 75 °C for the LDPE Resin. Data were Collected Using the Shallow Screw for the Dow Solids Conveying Device...

Figure 5.32 Solids conveying rate data calculate using the modified Campbell-Dontula model for the Dow solids conveying process using the shallow screw, 75 °C barrel and screw temperatures, and a screw speed of 50 rpm. The solids conveying rates measured from the experimental device are provided...

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

As discussed in Chapter 5, the solids-conveying rate for a specific screw is directly proportional to screw speed. That is, if the screw speed is increased by a factor of two the solids-conveying rate will nearly double. The melting flux of the screw as measured in kg/(h-m2) at the barrel wall, however, will not increase at the same rate. Typically, the melting flux will increase by 40% for a doubling of the screw speed [1]. In order to complete the melting process, additional area at the... 

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