Definition

The air-flow velocity inside an HVAC duct (m/s); directly affects steam absorption distance, sensor performance and noise levels.

Detailed Explanation

Duct velocity is the speed of air through an HVAC duct section, calculated as V = Q / (3,600 × A) (Q in m³/h, A in m², V in m/s). In duct design the velocity is a balance between duct size, pressure loss, fan energy and noise: low velocity means a large duct section, high velocity means more pressure loss and noise.

Why It Matters

In steam humidification, duct velocity works in two directions. (1) For the same humidification duty, higher velocity carries the steam further while it mixes and the absorption distance grows; high velocity does not shorten it. (2) At lower velocity the same steam goes into less air, the RH after humidification rises and the air approaches saturation, which also lengthens the distance and increases the risk of wetting surfaces. Velocity is therefore assessed together with the RH after humidification. The common design range at the distributor position is 2.5–4 m/s; the actual absorption distance is set from manufacturer data.

Practical Example

If 8,000 m³/h passes through a 1,500 × 600 mm duct, the velocity is 8,000 / (3,600 × 0.9) ≈ 2.5 m/s, within the common design range for a distributor. The same flow through a 600 × 400 mm duct would run at about 9.3 m/s; pressure loss and noise rise, and steam travels further while it mixes. The velocity at the distributor is verified on site with a pitot tube or thermal anemometer, averaging several points across the section.

Engineering Note

Velocity is measured as the average of several points across the section rather than a single point (for example the log-Tchebycheff method). The distributor is placed in a straight section where flow is evenly spread, not where the velocity profile is uneven, such as a fan outlet or just after an elbow. On units such as the Neptronic SKE4, an airflow switch prevents steam injection when the fan stops.

NKT Application Link

In Neptronic humidifier and distributor selection, NKT asks for the air velocity in the section where the distributor will sit as a key input; velocity and RH after humidification are assessed together and the distributor type is selected from manufacturer data.