Fan and duct: airflow, operating point and forced convection

A fan only cools if it can push enough air through the enclosure: grilles, filters, ducts and heatsinks all cost pressure. The Fan & duct mode of the Thermal tab works out how much air your heat load needs and where the fan's datasheet curve meets the resistance of your air path. It uses the same engine as the ohmforge-thermal-calculator tool (mode “Fan & duct”), so the tab and the tool give identical numbers. Estimates, not certified design — noise is not modelled.

1. The air your heat load needs

Open Instruments → Circuit calculators…, choose the Thermal tab and set Calculation to “Fan & duct: airflow and operating point”. Heat load to carry away (W) is 60, Inlet air temperature (°C) 30 and Allowed air temperature rise (K) 10. That needs 18.44 m³/h of air — Q = P/(ρ·cp·ΔT) with the air density at the inlet temperature. Altitude (m) lowers the density, so higher up you need more volume for the same heat.

2. The fan curve

Fan curve from the datasheet takes one point per line: flow and static pressure, from shut-off (flow 0) to free delivery (pressure 0). The example is an 80 mm fan with 48 Pa at zero flow and 85 m³/h free flow. Pick the units of your datasheet under Flow unit (m³/h, CFM or L/s) and Pressure unit (Pa, inH₂O or mmH₂O). Number of identical fans with Several fans puts fans side by side (flows add) or one after the other (pressures add); Speed ratio applies the fan laws, for example 0.5 for a fan slowed to half speed.

3. The air path

Air path (one component per line) describes everything the air passes, lengths in mm:

grille dp=6 at=60
duct L=250 w=120 h=60
fitting elbow90
filter dp=18 at=60
fitting exit

Filters and grilles take the pressure drop from their datasheet at a rated flow; ducts use the Darcy–Weisbach friction of their size; fittings use typical loss coefficients at the duct velocity. A heatsink w=60 h=25 L=80 fins=15 t=1 line adds a fully ducted plate-fin heatsink.

4. Read the operating point

The chart draws the fan curve, the system curve, their crossing and a line at the flow you need. Here the fan settles at 54.93 m³/h and 22.91 Pa, almost three times the air needed, and the Air outlet temperature is 33.4 °C. The component table shows that the filter takes 64 % of the pressure — the place to improve first. For a turbulent duct the table also compares the Colebrook and Swamee–Jain friction factors (Turbulent friction factor chooses which one the result uses).

Try Speed ratio 0.5: the flow drops to 27.45 m³/h, still enough. If the fan curve crosses the system curve more than once, a warning points to the stall region of the fan.

5. Hand the airflow to the board map

Board length along the air flow (mm, optional) with the free channel around the board gives an approximate forced-convection coefficient: about h = 14.3 W/(m²·K) for a 160 mm board in the 120 × 60 mm channel. Press Use h = 14.3 W/(m²·K) in the board map: the tab switches to “Board thermal map” with that h on both faces. On the sensor-node example board the hottest point falls from 41.7 °C in still air to about 38 °C.

6. Tips

• Enter static pressure curves, not total pressure; most datasheets give static pressure.
• Two fans in parallel only double the flow into an open path; against a filter the gain is smaller.
• The flat-plate h is approximate: tall components trip the air flow and usually raise it locally.