Abstract: A method and apparatus for controlling a multiple-plant gas-producing facility which is subject to wide variations in the demand for its output. The invention is particularly applicable to an oxygen-producing facility for a steelmaking operation in which there are two independently operable oxygen-producing plants. It is impractical to shut down an oxygen-producing plant when its output temporarily is not needed. During periods of reduced demand conventional practice is to operate one plant at its normal rate to supply the needed oxygen and operate the other plant at a reduced rate and waste excess oxygen which cannot be liquefied and stored. The invention provides a way of combining the outputs of two plants, whereby both plants can operate at reduced rates during periods of reduced demand, and large scale waste is avoided.