Absorption Distance

Detailed Explanation

Absorption distance is the distance over which steam leaving the distributor fully mixes into the airstream and the visible mist disappears. Steam leaves the distributor as an invisible gas; on meeting cooler air part of it condenses into micron-sized droplets and forms the white plume. Further downstream the droplets re-evaporate; the point where the plume ends marks the end of the absorption distance. The non-wetting distance quoted by manufacturers is the usually shorter distance beyond which surfaces no longer get wet.

The main conditions that set the distance: (1) RH after humidification: the closer the air gets to saturation, the longer the distance; this is the most decisive condition. (2) Air velocity: for the same humidification duty, higher velocity carries the steam further while it mixes and the distance grows; at lower velocity the same steam goes into less air, pushing it towards saturation, which also lengthens the distance. (3) Air temperature: cold air approaches saturation sooner. (4) Distributor geometry: splitting the steam across many tubes or nozzles shortens the distance. Neptronic data give an absorption distance under 1.5 m for S.A.M. and S.A.M.E2 and under 1 m for Multi-Steam™; the actual project value is set with the manufacturer's selection software or charts.

Why It Matters

Every surface placed inside the absorption distance gets wet, with three consequences: (1) water on duct walls and floor: corrosion, mould and hygiene risk; (2) wet sensors, filters or coils: the sensor reads unmixed steam and control becomes unstable, filter media gets wet; (3) dripping onto equipment below the duct.

The straight duct length after the distributor is therefore checked when the distributor is selected. If space is short, the solution is a shorter-distance distributor (S.A.M.E2 or multi-tube Multi-Steam™) and keeping the RH after humidification low in the duct. Increasing air velocity does not shorten the distance; for the same humidification, higher velocity carries the steam further.

Practical Example

Suppose 20 kg/h of steam is injected into a 1.0 × 0.6 m duct with air arriving at 22 °C and 40% RH (about 6.55 g/kg). At 4 m/s the airflow is 8,640 m³/h and the RH after humidification is about 52%. At 2 m/s the same steam goes into 4,320 m³/h and the RH rises to about 63%, and at 1 m/s to 86%. In air this close to saturation the plume lengthens and the risk of wetting duct surfaces grows, which is why duct humidity is capped with a high-limit sensor.

Engineering Note

There is no general formula for absorption distance; manufacturers give it through selection software and charts based on their own test data. The inputs are inlet RH, RH after humidification, air velocity, air temperature and the number/spacing of dispersion tubes. The high-limit humidity sensor and the final filter are placed outside this distance, and the control sensor in the room or return duct. The visible mist can be checked by eye: where it ends, the distance ends.

NKT Application Link

In Neptronic distributor selection, NKT checks the absorption distance against manufacturer data using the duct dimensions, airflow, air conditions and the straight duct length after the distributor, and shares the placement recommendation with the design and installation team.