How does altitude change fan selection?

Air density falls with altitude and the fan moves less mass. At 2,000 m, density is roughly 0.82 of sea level (ISA), so the same fan develops about 82% of the pressure and delivers about 82% of the mass flow at the same rpm. If your thermal budget is written in mass flow — and heat transfer is — you must size up for elevation.

The numbers to apply

Standard atmosphere density ratios: ≈0.90 at 1,000 m, ≈0.82 at 2,000 m, ≈0.74 at 3,000 m. Fan pressure scales with density, so the whole P-Q curve scales down vertically. To recover the same mass flow on a resistance-dominated system, you typically need a fan one size up, a higher-speed variant, or an extra fan — a rough multiplier is the inverse of the density ratio (e.g. ÷0.82 ≈ ×1.22 at 2,000 m, before margin).

What changes besides raw flow

Lower density also lowers the fan's ability to generate pressure across filters and heat sinks, so the operating point slides back toward free air. Electronics suffer too: reduced air density reduces convective cooling on PCBs, and corona/arc-down ratings of high-voltage supplies degrade. For DC motor drivers, check the controller's altitude rating. Cooling margin must be validated at representative density — an altitude chamber or a derated bench test.

• Re-check fan rated voltage/current at altitude — cooling of the motor itself also drops.
• Filter loading behaves differently: pressure developed across the filter is lower at altitude.
• If the product ships globally, validate at the worst-case customer site, not the lab.