Cooling Fan Airflow Requirement

Formula

Description

The required airflow to cool an enclosure or heatsink depends on the total heat dissipation and the allowable air temperature rise. The constant 1.08 converts between watts and CFM·°F and accounts for the volumetric heat capacity of air at standard conditions (1.08 = ρ × cp in BTU/(min·ft³·°F) converted to W/(CFM·°C)). This is a first-order estimate for sizing cooling fans in electronics enclosures, server racks, and telecom cabinets. The actual required airflow may be higher due to altitude derating, recirculation, and non-uniform heat distribution.

Variables

• CFM — Required airflow in cubic feet per minute
• P — Total heat dissipation (W)
• ΔT — Allowable air temperature rise from inlet to exhaust (°C)

Practical Notes

A typical ΔT of 10-15°C is used for electronics cooling. At altitude, air density decreases (about 12% per 1000 m), requiring proportionally more CFM. For metric: 1 CFM ≈ 1.7 m³/h. Common fan sizes: 40 mm (5-10 CFM), 80 mm (20-40 CFM), 120 mm (40-100 CFM). Fan curves (pressure vs. flow) must be matched to system impedance (backpressure) for accurate airflow prediction.