SummaryThe concept is discussed of cooling a power cable by the natural convective flow of a coolant in the conductor duct. The following quantities are determined at a given distance from the conductor duct entrance; gas/liquid content (void fraction), heat-transfer co-efficient, temperature, flow quality and pressure. For a given reservoir head and cable loading (i.e. heat input) the permissible length of cooled cable is determined such that certain constraints are obeyed.Two coolants are considered, water and Freon-12. Water is held at a pressure of about 10’2 Bar in the reservoir which provides a low inlet temperature and a suitable phase-change temperature in the duct, i.e. one with a suitable margin below the cable operating temperature. Over part of the duct length, heat is absorbed by the fluid as sensible heat and over the remainder as latent heat in the two-phase flow with no temperature rise. Freon-12 is pressurised to about 15 atmospheres and similar methods are used for the design calculation as for water. The possibility of a“dry-out”occurring 1s examined and is seen to be a design constraint for the Freon-12 system.