How does air temperature affect fan performance?
Hotter air is thinner. At 60°C, density is about 0.90 of the 20°C value (at the same pressure altitude); the fan develops about 10% less pressure and moves about 10% less mass at the same rpm. Since heat transfer rides on mass flow, a fan that just meets the target at room temperature misses it in a hot soak — correct the requirement, or the selection, for density.
The numbers
Density at atmospheric pressure: ≈1.204 kg/m³ at 20°C, ≈1.127 at 40°C, ≈1.060 at 60°C, ≈1.000 at 80°C. Volume flow (CFM) barely changes — the fan moves volume — but mass flow = ρ × Q scales with density. Pressure capability scales with density too, so the whole P-Q curve compresses vertically at high temperature. Combined effects: high temperature plus altitude multiply (e.g. 0.90 temperature × 0.82 altitude ≈ 0.74 at 60°C and 2,000 m).
| Air temperature | Density (kg/m³) | Mass flow ＆ pressure | vs 20°C |
|---|---|---|---|
| 20°C | 1.204 | 100% | — |
| 40°C | 1.127 | ≈94% | −6% |
| 60°C | 1.060 | ≈88% | −12% |
| 80°C | 1.000 | ≈83% | −17% |
What to correct
Convert the thermal requirement to mass flow (kg/s) once, then compare candidates on mass flow at the worst-case temperature, not on volume CFM at ambient. Check the fan's rated ambient and motor current at that temperature — a fan working against hot, thin air draws different current than its 20°C catalogue value. And validate at representative temperature: a hot-soak test with the real duty cycle, measuring component temperatures and the fan's own inlet temperature.
- State design temperature in the RFQ: 'delivers X kg/s at 60°C, 1,000 m'.
- Remember the fan cools itself with the same thin air — its motor runs hotter.
- Where the fan sits in the hot exhaust stream, its own inlet density is the one that matters.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
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