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processing feature | Top tips for twin-screws


FIGURE 8: TURBULENT FLOW DELIVERS FAR GREATER HEAT TRANSFER FROM THE PIPE WALL THAN LAMINAR FLOW


this means is that the water flows all the way through the barrel without removing much heat. With turbulent flow caused by high pressure delivery,


there is a high degree of transverse momentum exchange, which breaks up the boundary layer. As a result, the violent fluid motion causes much greater heat transfer from the pipe wall to the fluid[4]


. Given the fixed geometry of the extruder’s cooling


system, the easiest way to induce turbulent flow in the barrel cooling bores is to increase the delivery pres- sure. There are a wide variety of cooling recirculation systems used for extruders, with supply pressures typically ranging from 20 psi to 60 psi. To achieve turbulent flow, a pressure of approximately 120 psi is desired. This can be attained relatively easily by changing the pump in the recirculation system to a high pressure type. Almost all extruder cooling system components (hoses and valves) are rated for at least 150 psi, so 120 psi still provides some factor of safety. The advantage will be immediately apparent with a


process which is highly exothermic. Zone overrides can often be greatly reduced or even eliminated. A side benefit of turbulent flow is that it inhibits fouling of the cooling bores from scale build-up.


470 460 450 440 430 420 410 400


9.45 10.00 10.15 10.30 10.45


FIGURE 9: RAISING THE TEMPERATURE OF COOLING WATER IN THE BARRELS WILL GIVE SMOOTHER TEMPERATURE CONTROL


34 COMPOUNDING WORLD | May 2012 11.00 0 8


Set barrel cooling water temperature at 50°C


Many people tend to think because the function of the water is to cool the barrels, the water should be as cold as possible. But to an extruder barrel at 220°C, even water at 80°C is cool, and water at 18°C is absolutely frigid!


The problem with running very cold process water is


that it can cause overcooling, which frequently shows up as zones cycling up and down, in a saw-tooth pattern. The reason: the temperature in the barrel rises normally until the controller calls for cooling, opening the solenoid valve. When the valve opens, even for only a short duration, the cold water removes too much heat. This causes the temperature to plummet to far below set-point, which turns on the heater again. For most plastics processes a water temperature of


approximately 50°C seems to work well. With a higher water temperature, the cooling rate will be slower and less likely to overcool. The result will be smoother temperature control (Figure 9).


About the author


Bert Elliott is engineering manager at Leistritz Corporation in New Jersey, USA. ❙ www.leistritz.com


COMING NEXT MONTH: Part Two of Bert Elliott’s Top Tips article will contain 10 more helpful hints for getting the most from your twin-screw extruder


.


Topics up for discussion in the next instalment include: flushing of barrel cores; better barrel alignment; gear oil selection; purging techniques; useful advice for faster product and colour changeovers; optimising the degree of fill; key process indicators; and more.


References


D.B. Todd, Plastics Compounding Equipment and Processing, Hanser, New York, (1998).


W.F. Stoecker, Design of Thermal Systems. McGraw-Hill, New York, NY (1971).


C. Martin, Extrusion Technology and Troubleshooting, 2, Brookfield, CT (2001).


V.L. Streeter and E.B. Wylie, Fluid Mechanics, McGraw-Hill, New York, (1975).


www.compoundingworld.com


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