Work on refurbishing the NAL 2 m x 2 m transonic wind tunnel was started in 1985 and completed in 1994. The pressurized dry air used at the various NAL wind tunnels is manufactured by air-compressor systems. Due to deterioration, one of them was replaced by a new oil flooded screw compressor system, as well as other equipment in 1989. This paper describes why the oil flooded screw compressor system was adopted in the design, and outlines the constructions and performance of the new system in detail.