The site is a copper operation in the Peruvian Andes, sitting at 3,900 m. Two screw compressors were supplied: one dedicated to a drill rig, one covering the workshop and the maintenance bay.
Why altitude changed the specification
Air at that elevation is roughly 40% thinner than at sea level, so a machine sized on standard figures would have arrived underpowered. Capacity was corrected for the site elevation before the model was chosen, the motor was re-rated for the thinner cooling air, and the controller was configured for the local atmospheric pressure.
Result
Both units hold their rated pressure at working temperature, and the drill rig runs a full shift without the pressure decay that had been forcing the previous setup to pause.