When would you choose an air-cooled heat exchanger over a water-cooled unit?

Air-cooled exchangers are preferred when: cooling water is scarce or expensive, water treatment costs are high, environmental regulations limit water discharge, the process temperature is above 65-70C (approach to ambient), low maintenance is desired (no fouling, scaling, or corrosion from cooling water), or for remote locations. They are typically more expensive initially but have lower operating costs. Limitations include larger footprint, higher approach temperature (10-15C to dry-bulb), and noise.