PETpla.net Insider 05 / 2015

PREFORM PRODUCTION 37 PET planet insider Vol. 16 No. 05/15 www.petpla.net mould metal is greatly jeopardised by the slow speed in the contact area. Systems with laminar flow show a large increase in coolant temperature through the system. Turbulent flow, on the other hand, allows frequent contact between coolant and mould metal. Because of the constant mixing of the coolant, its temperature does not spike up, keeping the important temperature difference between the coolant and the hot metal high. Turbu- lent flow is not so much dependent on a high inlet pressure but rather on the pressure difference between the inlet and the outlet. A pressure drop of 5 bar (70 psi) is sufficient to achieve this effect. Sizing return water line diameters to 25mm (1 in) bigger than supply lines often reduces back pressure that might be present in the system. A good sign of turbulent flow is a temperature differ- ence of 1.5°C (3°F) between the inlet and outlet points. (Measurements must be taken by thermocouples inside the coolant flow and not on the pipes.) Increasing pump size in the central location or at each machine (booster pumps) is the most likely remedy for any deficiencies. If exact calculations are required, the so-called “Reynolds number” must be determined. This dimension- less number takes into account the viscosity of the fluid, the mass veloc- ity, and the length of the pipes. A good resource for this can be found at: www.processassociates.com/process/ dimen/ dn_rey.htm. Reynolds numbers above 2000 indicate turbulent flow. 3.8.1 Antifreeze The use of antifreeze in cool- ing water deserves a special note. Anti-freeze has two functions in a water system. As the name implies, it reduces the freezing point of water. Water tem-perature at the contact point in the evaporator of the chiller will be several degrees lower than that at the exit point of the system. Hence, it may reach its freezing point even though the chiller is adjusted to a tem- perature well above it. The required antifreeze concentration will depend on the chosen water temperature. Recommended cooling water tem- perature for PET moulding is 8–10°C (46–50 °F). Antifreeze also has an anticorro- sive effect. However, using antifreeze for corrosion protection requires con- centrations of about 25%, which is the reason why this concentration level is recommended in some manuals. Unfortunately, the ability of antifreeze to absorb heat from mould parts is inferior to that of water, and the effi- ciency of the chiller system can be compromised. Furthermore, antifreeze is more viscous than water and so increases pump loads and electricity costs to run them. It is therefore preferable to adjust the antifreeze concentration only with respect to lowering the freezing point as necessary and to add corrosion protection by other means with regular water treatment. The best approach is to have the water tested by a certified laboratory and have qualified techni- cians proscribe a proper mix of chemi- cals that address corrosion as well as algae and mineral suppression. Antifreeze cannot be used in a cooling tower because the perfor- mance of a cooling tower is based on evaporation. Adding antifreeze continuously into cooling tower water would be expensive and is illegal in many jurisdictions as the evaporating antifreeze is an environmental hazard. Freeze prevention is accomplished instead by using a thermostat to cycle the tower fan and the tower recircu- lation pump on and off based on a temperature set point between 21 and 29 °C (70 and 85 °F). Antifreeze comes in two formula- tions: ethylene glycol and polypropyl- ene glycol. The former is a controlled substance that may not be released into the municipal water system. The latter is environment friendly and should be used wherever available. 3.8.2 Water flow balance Equally important is the fact that cooling water reaches each core and cavity at the same time and flow rate. Examining Fig. 3.25, it is easy to see that the top cavities will receive less water flow and pressure than the bottom ones because the pres- sure drops when the water flows up to them. As a result, cycle time will have to be adjusted so that the warmer top cavities can be demoulded while the Want the secret to bottle light weighting? Process Pilot ® automated blowmolder control system takes the guesswork out of material distribution management and can help ensure the success of even the most aggressive light weighting program. Featuring 24/7 blowmolder control, the Process Pilot system continuously maintains proper material distribution, on every bottle, to an unmatched level of accuracy. 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