Part of the PT Freeport Indonesia (PTFI) mine production is in the process of transitioning from the deep ore zone (DOZ) mine to the deep mill level zone (DMLZ) mine. The DMLZ mine is the next lift in the caving series and lies 500m below the current DOZ footprint.In order to reduce the need for a very expensive additional ventilation system for the DMLZ mine, it was decided to reconfigure some infrastructure and upgrade the performance of the three existing DOZ mixed flow ventilation fans in situ, reducing the new fans required to two. This was far less costly than providing complete new installations.These fans are installed in individual underground chambers in DOZ mine exhaust drifts and connected to the DMLZ mine exhaust drifts system in 7m7m airways size, through sets of 6m diameter by 500m high raises.The upgrade process involved replacement of motors, fan shafts and bearings. The existing 1600kW DOZ mixed flow fans are 3500mm diameter and operate at a speed of 595rev/min and were originally designed to deliver 390m3/s at 3.3kPa. Efficiency optimisation may be required at a later date.The upgraded DMLZ fans are required to deliver 425m3/s at 3.7kPa and it was necessary to increase in motor size and speed to 2200kW at 710rev/min. This paper will discuss the fan upgrade project and in particular the integration of an unusual ventilation system, which is based on the semi parallel operation of multiple large fans.CITATION:Sani, R, Sianturi, A and Lownie, K, 2017. Operation of deep mill level zone mixed flow fans after the first fan upgrade - aparallel fans case study, in Proceedings Australian Mine Vent Conference 2017, pp 285-292 (The Australasian Institute of Mining and Metallurgy: Melbourne).