Extruder adiabatic

Extruder, adiabatic Also called autothermal. Describe a process or transformation in which no heat is added to or allowed to escape from the system under consideration. It is used, somewhat incorrectly, to describe a mode of a process such as an extruder in which no external heat is added to the extruder. Although heat may be removed by cooling to keep the output temperature of the melt passing through the extruder at a constant and control rate. The screw develops the heat input in such a process as its mechanical energy is converted to thermal energy. 

Experimental and simulation results presented below will demonstrate that barrel rotation, the physics used in most texts and the classical extrusion literature, is not equivalent to screw rotation, the physics involved in actual extruders and used as the basis for modeling and simulation in this book. By changing the physics of the problem the dissipation and thus adiabatic temperature increase can be 50% in error for Newtonian fluids. For example, the temperature increase for screw and barrel rotation experiments for a polypropylene glycol fluid is shown in Fig. 7.30. As shown in this figure, the barrel rotation experiments caused the temperature to increase to a higher level as compared to the screw rotation experiments. The analysis presented here focuses on screw rotation analysis, in contrast to the historical analysis using barrel rotation [15-17]. It was pointed out recently by Campbell et al. [59] that the theory for barrel and screw rotation predicts different adiabatic melt temperature increases. 

The specific energy from Eq. 10.13 is reported in J/g for convenience in analyzing the process. If a change in a process causes the specific energy to increase by 50 J/g, the troubleshooter can translate this to an adiabatic temperature increase of 20 °C because many unfilled resins have heat capacities near 2.5 J/(g °C). This temperature change calculation, however, is an approximation because extruders typically do not operate adiabatically. 

Transport of energy in the screws was modeled previously for single-screw extruders [30-32] and for twin-screw extruders [33]. In order to predict the axial screw temperature in a single-screw extruder, heat conduction along the screw has to be modeled. The model developed by Derezinski [32] included heat conduction from the barrel through the screw flights to the screw surface, heat conduction from the polymer to the screw root, and heat conduction in the axial direction. The model showed that the screw does not behave adiabatically and that the steady-state heat conduction in the screw depends greatly on the size of the extruder. 

Recall that in the process of low-pressure moulding, thermoplastic polymers (polyolefins, polyamides, or their compositions) are loaded into a cylinder of the adiabatic extruder, plasticized and injected into a mould at low pressure. The formed article is cooled in the mould and removed due to shrinkage phenomena. 

Several side feeds of deep cooled monomer will lower the base temperature to which the adiabatic temperature rise has to be added. A fraction of the monomer is fed at the beginning of the extruder and has to be heated to the starting temperature of the reaction hence successive side feeds can be metered at much lower temperatures. 

The specific heating and cooling power is an often-underestimated source of error. It is physically possible for a large production extruder to require less heating and/or cooling power than a small laboratory extruder in terms of drive power (Fig. 11.15). Therefore, laboratory extruders are often operated adiabatically or only moderately heated in order to ensure comparability with a large extruder. 

Fig. 3. Variation of extrusion pressure with extrudate velocity. Extrusion temperature M °C, nominal draw ratio IS, die diameter 15.5 mm, sample R (for sample description see Table 2). R mes at lower and higher velocities are isothermal and adiabatic, respectively ...

A polymer melt of density p and specific heat capacity is extruded through a die, across which the viscous resistance of the melt causes a pressure drop AP. Show that under adiabatic conditions (no heat loss to the wall of the die) the temperature rises by AT, where... 

Autothermal extrusion (adiabatic or autogenous extrusion) n. In screw extruders, a steady state of operation in which the increase in enthalpy of the plastic from feed throat to die entry is equal to the net... 

If the Brinkmann number is much smaller than the Damkohler IV number (or Daiv/Br 1), the heat released by viscous dissipation can be neglected with respect to the heat of reaction. If the Graez number is small, the heat removed by the cooled wall will be negligible, and the process is nearly adiabatic. This will generally be the case in situations where large extruders are used. 

In large extruders the convective heat flow generally dominates the conductive heat transfer to the wall (Gz 1). The process approaches an adiabatic situation, and all the heat produced by the reaction or by viscous dissipation results in a temperature rise. In this case the equations for temperature rise can be simplified even further. 

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