The conversion
- 1 CFM (cubic foot per minute) ≈ 1.699 m³/h
- 1 m³/h ≈ 0.5886 CFM
One cubic foot is 0.0283168 m³; multiplied by 60 minutes that gives 1.699 m³/h. European and Asian catalogs usually quote m³/h and North American ones CFM, so convert before comparing fans. The CFM to m³/h airflow calculator does it directly.
How much airflow an enclosure needs
Drives, power supplies and PLCs heat the enclosure; a fan has to carry that heat out. From a simple heat balance on the air:
Airflow (m³/h) = 3600 × P ÷ (ρ × cp × ΔT)
- P: heat load inside the enclosure (W)
- ρ: air density, about 1.2 kg/m³ (20 °C, sea level)
- cp: specific heat of air, about 1005 J/(kg·K)
- ΔT: allowed temperature rise between inside and ambient (°C or K)
With the constants filled in:
- Airflow (m³/h) ≈ 3 × P ÷ ΔT
- Airflow (CFM) ≈ 1.76 × P ÷ ΔT (°C), or 3.17 × P ÷ ΔT (°F)
Worked example
Heat load 750 W, ambient 35 °C, maximum allowed inside 45 °C, so ΔT = 10 °C:
- Airflow ≈ 3 × 750 ÷ 10 ≈ 224 m³/h (about 132 CFM)
- Select a fan for roughly 1.2 to 1.3 times that: about 270 to 290 m³/h
Why the margin
- Catalog airflow is free-air flow with no resistance. Filters and louvers reduce it noticeably, so find the operating point on the fan's pressure-flow curve at your system resistance.
- Hot or high-altitude air is thinner, so the same volume flow removes less heat. Correct for air density with the air density calculator.
- Use the maximum heat load, from the equipment's rated losses or the power supply rating.
Room ventilation
For plant rooms, switch rooms and warehouses, size by air changes:
Airflow (m³/h) = room volume (m³) × air changes per hour (ACH)
Take the ACH value from the applicable code or design brief; the CFM calculator has a mode for this as well.
What happens when you change fan speed
For the same fan, the affinity laws apply approximately: airflow is proportional to speed, pressure to speed squared and power to speed cubed. At 80% speed, power falls to about 51%, which is why EC fans and variable-speed drives save energy. Work through different speeds with the fan affinity laws calculator.
Source
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