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3. Development of efficient packings for columns using the theory of mass transfer;Kolev;Khim. Prom. (in Russian),1986
4. Über die optimale Form von Füllkörpern und Kolonnen;Kolev;Fat. Sci. Technol.,1990
5. R. Daraktschiev, L. Rankov, I. Andreev, Investigation of the liquid flow distribution in `honeycomb' type of packing. First National Conference on Heat Technique and Chemical Mechanical Engineering, 1969, pp. 367–373 (in Bulgarian).