6I. C: MφLL er. Electirc Preheating:A A New Method for Laying Pre - insulated District Heating Pipes. H & VENGINEER 1990(APRIL) : 13 - 15.
7Anon. Electric preheating. A new method for laying pre -insulated district heating pipes. ABB Review, n 1, 1990,p33 - 38.
8Jensen, Leif Erweiterung der Fernwaerme - versorgung in Daenemark durch Kraft - Waerme - Kopplung ( Expansion of district heating supply by combined heat and power in Denmark). Fernwaerme International, v 19, n 6, Nov - Dec, 1990,472,475 - 476.