A compressed air stream is cooled to a temperature suitable for its rectification within a lower pressure heat exchanger and a boosted pressure air stream is liquefied or converted to a dense phase fluid within a higher pressure heat exchanger in order to vaporize pumped liquid products. Thermal balancing within the plant is effectuated with the use of waste nitrogen streams that are introduced into the higher and lower pressure heat exchangers. The heat exchangers are configured such that the flow area for the subsidiary waste nitrogen stream within the higher pressure heat exchanger is less than that would otherwise be required so that the subsidiary waste nitrogen streams were subjected to equal pressure drops in the higher and lower pressure heat exchangers. This allows the higher pressure heat exchanger be fabricated with a reduced height and therefore a decrease in fabrication costs.