Airflow and K-Factor Calculation in AHUs (Calculator)

The most practical way to measure airflow in an air handling unit is to read the differential pressure at the fan inlet cone and multiply it by the k factor supplied by the manufacturer:

Q = k · √Δp

Where does the k factor come from?

The k value is specific to the fan/AHU model and is determined by the manufacturer in the laboratory. It is found on the nameplate or datasheet. A k value from another unit cannot be used — it differs even between models from the same manufacturer.

Why is density correction essential?

Manufacturers quote k against a reference air density of 1.2 kg/m³. When air temperature or altitude changes, density changes and the measurement deviates:

• In 40 °C air the density falls to 1.123 kg/m³ → the flow reads 3.4% different
• At 1000 m altitude the effect grows further

Commissioning measurements taken without correction are therefore systematically wrong.

Reverse calculation: finding the k factor

If you have a calibrated flow measurement, you can derive the k value from the same relation. This is useful on older units whose nameplate is no longer legible.

SFP: the measure of efficiency

Specific Fan Power (SFP) is the electrical power the fan consumes per m³/s of air. Duct design, filter selection and fan efficiency all feed into it directly. A low SFP means a permanent saving in running costs.

Watch out during measurement

Filter fouling, damper position and duct tightness directly affect the result. At very low differential pressures (below 20 Pa) the measurement error rises sharply.

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