The increase in consumption of high-quality steel dictates the need for more steel undergoing the vacuum process, since processing the steel melt under vacuum improves its properties by reducing gas and non-metallic inclusions in it. However, rising fuel prices and the desire to transition to carbon-free metallurgy require the industry to reduce energy intensity and, as a consequence, reduce energy consumption. This can be achieved by switching to continuous production, reducing the period of technological downtime of high-temperature equipment, the temperature of which must be maintained to increase the lining service life and improve the final product quality. But the transition to continuous steelmaking requires the development of a number of new technological units capable of functioning within the framework of the continuous steelmaking unit, including the extra-furnace processing unit for the melt. The propose of the work was development of a theoretical basis for extra-furnace processing unit of molten steel with a continuous degasser. A unit for extra-furnace processing of steel melt with a continuous U-shaped vacuum degasser is presented, which is part of a unit for continuous liquid-phase iron reduction with a capacity of 10 tons per hour for production of St3 steel. The authors studied the influence of residual pressure in a vacuum chamber on the rate of degassing and the time of a gas bubble ascent. Dimensions of the vacuum degasser were determined taking into account the productivity of the iron reduction unit. A multilayer lining was selected, and losses to the environment were assessed, taking into account convective and radiant heat transfer.