FEATURE NEWS:
Higher Productivity and Better Quality with Postheater in VCV-Lines
Increasing production speeds is a clear trend in today's production of XLPE insulated power cables. CV lines and peroxide cross-linking are dominantly used for medium voltage insulating and exclusively for high and extra high voltage cores. Productivity depends on extruder outputs, curing and cooling capacity. The length of a CV line is however limited, especially with vertical lines. With postheating (i.e. inductive conductor heating after crosshead), curing efficiency and line speed can significantly been increased.
Since building costs for a CV tower are remarkably high, it is vitally important to maximise line speed. Extruders, if correctly selected, can normally fulfil VCV line output needs - it is a question of curing and cooling capacity. To fully utilise layout possibilities a pressurised turn pulley is commonly used in VCV lines to extend the cooling length. Chillers for cooling water can be used but there is not much more to be done for cooling capacity.
Curing capacity is different. Inductive conductor heating can be utilised more effectively by using a so-called postheater. Today typical preheating temperature in VCV lines is 60-100 C. There are several reasons why preheating temperature is limited. These include copper oxidation, conductor tape deformation, moisture block material deterioration etc. The postheater is located after crosshead in the pressurised tube, where the conductor can be heated up without these limitations. A conductor temperature up to 180-200 C after postheater can be used. This gives a significant increase for line speed, between 20-40 per cent, depending on core and CV line layout. Fig. 2 shows one comparison for a VCV line with and without postheater. One should also notice that much shorter heating length is needed with post heater. This length can be used for cooling. Postheater can be located either right after crosshead (fig. 1) or after splice box.
Postheating also improves core quality. Results show that it improves roundness. This is due to more symmetric temperature distribution in insulation during cross-linking. Since surface of the core has much shorter residence time in high temperature (fig. 2), one may also expect better quality of outer semicon and insulation. This also means less thermal expansion and less mechanical stresses after cooling.
Company: |
Maillefer SA |
Country: |
Switzerland |
Fax: |
+41 21 691 2143 |
Email: |
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Website: |
www.mailleferextrusion.com |