Extruders heat transfer

Three mechanisms influence the heat balance in an extruder heat transferred through the wall, heat transported by convection, and heat generated by viscous dissipation. The relative importance of these three mechanisms is generally expressed by the Brinkmann and the Peclet number. 

A lace of polyethylene is extruded with a diameter of 3 mm and a temperature of 190°C. If its centre-line must be cooled to 70°C before it can be granulated effectively, calculate the required length of the water bath if the water temperature is 20°C. The haul-off speed is 0.4 tn/s and it may be assumed that the heat transfer from the plastic to the water is by conduction only. 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

The temperature of molten polymer process streams is commonly measured using a thermocouple positioned through a transfer line wall and partially immersed in the polymer stream. Process stream temperature measurements that use an exposed-tip thermocouple, however, can be misleading since the temperature of the thermocouple junction is a balance between the heat transferred from the polymer stream and from the thermocouple assembly [39]. Due to the low heat transfer rate between the polymer and the exposed tip and the high thermal conductivity of the thermocouple sheath, the temperatures measured can be different by up to 35°C depending on conditions. Extrudate temperatures, however, can be accurately measured using a preheated, handheld thermocouple probe. This method minimizes thermal conduction through the probe sheath. 

Todd, D. B., Heat Transfer in Twin Screw Extruders, SPE ANTEC Tech. Papers, 34, 54 (1988)... 

As far as heat transfer is considered, Fenner [27] made a detailed comparison of the thermally fully developed flow and thermally developing flow. He indicated that the thermally developed flow will not be achieved when heat conduction effects become significant [34]. Bruker et al. [35] experimentally verified that the thermally developing flow analysis provided a more accurate description of the flow in the extruder. 

Todd [64] applied heat transfer technology from hatch mixer tanks using close-fitting agitator blades [65] to model heat transfer data from twin-screw extruders. The model that best fit his data is as follows ... 

Karwe, M. V. and Jaluria, Y., Numerical Simulation of Fluid Flow and Heat Transfer in a Single-Screw Extruder for Non-Newtonian Fluids, Numer. Heat Transfer, Part A, 17, 167 (1990)... 

Single screw extruder operating curves. The conveying characteristics of a single screw extruder can also be analyzed by use of dimensional analysis. Pawlowski [6,7] used dimensional analysis and extensive experimental work to fully characterize the conveying and heat transfer characteristics of single screw extruders, schematically depicted in Fig. 4.7. 

Heat Transfer in Underwater Pelletizing In underwater pelletizing, the melt strands are extruded directly in a water bath and chopped by a rotating, highspeed knife into short-length cylinders called pellets. Consider an LDPE extrudate at 200°C, chopped into pellets of L = D = 0.4 cm in a bath kept at 10°C. (a) Formulate the complete heat-transfer problem, (b) Estimate the time required to cool the center of the pellet to 70°C by assuming that pellet surface temperature equals the temperature of the water. 

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