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EXTRUSION Proper cooling in extrusion is critical because it often controls output. Extruded parts can be cooled by gases, liquids, or contact with a heat-absorbing surface such as chill rolls or calibrators. Profiles, pipe, tubing, and cable jacketing are generally water cooled; other types of extrusions (such as blown film) are cooled by air or gases, either entirely or partially. Many of the same techniques that are used to optimize cooling in other processes, notably molding, apply to extruded products as well. One of the most significant is the need for turbulence of the coolant at the surface of the extrusion. When water or a gas flows at low velocity it forms layers. This is called laminar flow. Under laminar flow, the heat-transfer rate equals the surface area times the temperature differential divided by the distance from the surface. This means there is a tempera- ture gradient from the surface outward into the coolant. With laminar flow, the coolant layer nearest the extrusion, often called a boundary layer, has a low velocity because of its drag on the extrudate surface. Consequently it becomes hotter than the surrounding coolant. Since heat flow from the surface of the Extrusion Cooling: It's All About Turbulence extrudate is proportional to the difference in temperature between the extrusion and the coolant, the hotter layer at the surface of the extrudate reduces the overall heat transfer. Greater coolant velocity creates turbulence in the boundary layer and mixes the main body of the coolant with the boundary layer near the extrudate surface. Additionally, turbulence at the extrudate's surface reduces drag. Finally, increased velocity moves the hotter coolant away from the extrudate. So, an important aspect of heat transfer is the velocity of the water or gas near the surface of the extru- date or tooling component. It can be more important than the actual temperature of the coolant. Turbulent flow increases convective heat transfer, increases mass transfer and mixing, and reduces drag, all of which are all favorable to increased cooling of the extrudate. Reynolds numbers (Re) below 1000 are considered indicative of laminar flow. Numbers ranging from 1000 to 10,000 are considered a transition zone, and beyond that is turbulent flow. Turbulence is determined by calcu- lating the Reynold's number as follows: Re= ud n /V u = velocity, d n = hydraulic diameter, and V = kinematic viscosity. In the accompanying graph, the Re is compared with the Nusselt number, which is a ratio of convection to con- duction heat transfer, for a corrugated and a smooth tube. A higher Nusselt number indicates increased convective heat transfer. Comparing the Nusselt number at Re = 1000 to Re = 3000 shows an increase of more than two times the convective heat transfer. It would require a significant change in water temperature to match the effect of turbulence. Regardless of what you are extruding, in order to maximize cooling you need to generate turbulence at the heat-exchange surface. By Jim Frankland Here, comparing the Nusselt number at Re = 1000 and Re = 3000 shows an increase of more than two times the convective heat transfer. This means it would require a significant change in water temperature to match the effect of turbulence. Source: HRS Heat Exchangers The Effect of Turbulence 1 2 5 10 20 50 100 200 500 1K 2K 5K 10K 20K 50K 100K 200K 500K 1e+06 Reynolds Number Hard Corrugation Smooth Tube Nusselt Number 1000 100 10 Inlet Outlet Laminar Transition Turbulent 24 JULY 2017 Plastics Technology PTonline.com K now How