How should I select a fan for a high-ambient product?

Use the application ambient, not room temperature. Above roughly 60°C, every element of the fan is stressed: bearing lubricant films thin, drive electronics run closer to their limits, cable insulation and plastics age faster, and motor current can rise. Selection at high ambient means checking the fan's rated ambient, the life claim at bearing temperature, and a validation that runs the real duty cycle.

What to check on the datasheet first

Operating temperature range (many standard DC fans are rated to 70°C or 85°C, but only with stated life at that temperature), the L10 life and the temperature it is referenced to, ball-bearing construction for elevated temperatures, and any derating of speed or current near the top of the range. If the datasheet publishes life only at 40°C, ask for the extended-temperature curve — do not extrapolate silently.

Design-side levers

Keep the fan out of the exhaust of hot components; draw air from the coolest available inlet; give the motor its own conduction or airflow path; specify PWM so the fan idles in cool conditions and only ramps during hot soaks; and select a slightly larger, slower fan instead of a small fast one — the slower fan runs cooler, quieter and lives longer at the same delivered mass flow.

• Prefer lower rpm at larger diameter for high-ambient enclosures.
• Shield the fan cable from hot surfaces; check insulation rating.
• Validate with thermocouples on the bearing housing or motor winding, not only air temperature.

How to validate

Run the sample in the real assembly at worst-case ambient and duty cycle. Record supply voltage, current, PWM duty, tach speed, and temperatures at the motor and the critical components, until temperatures stabilise. Then check the measured bearing temperature against the fan's life-versus-temperature reference. A sample that passes at 25°C lab air proves nothing about a 70°C rooftop cabinet.