Definition
The diameter of water droplets leaving a nozzle in an adiabatic humidifier (microns, μm); a design parameter that directly drives evaporation time, wetting risk and hygiene safety.
Detailed Explanation
Droplet size is the single numerical indicator of atomisation quality. Unit: micron (μm); 1 μm = 0.001 mm. Adiabatic humidification bands: 5–15 μm "fine mist" (high-pressure misting, ultrasonic), 15–30 μm "medium" (mid-pressure), 30–80 μm "coarse" (low-pressure spray, wetting risk). Measurement: laser diffraction (Malvern Spraytec, Sympatec) for field validation. Manufacturers report D50 (median droplet) and DV90 (largest 10%).
Why It Matters
Droplet size drives three critical parameters: (1) EVAPORATION TIME — per Stokes's law; 10 μm ≈ 0.2 s, 30 μm ≈ 2 s, 100 μm ≈ 25 s. Faster evaporation → shorter absorption distance → more compact duct design. (2) WETTING RISK — above 50 μm droplets fall under gravity (terminal velocity 0.15 m/s @ 100 μm); landing on duct walls or floors causes hygiene problems, corrosion, slip hazard. (3) HYGIENE — fine mist (< 15 μm) is respirable with the air; if contaminated, Legionella risk. That is why fine-mist systems mandate RO + UV pre-treatment.
Practical Example
A print hall in Istanbul chose an evaporative humidifier (Neptronic SKV, cellulose pad — no droplet size, evaporation happens on the media surface). Standard hygiene class, 22°C / 55% RH. The alternative was a high-pressure misting system (SKH) with D50 = 12 μm, DV90 = 22 μm. Absorption distance 0.8 m, in-duct humidification stable ±3% RH. RO + UV pre-treatment added €4,500. Outcome: SKV gives simpler hygiene (water never contacts air) + lower capex; SKH gives more flexible placement + adiabatic cooling. The customer picked SKV because hygiene risk is lower.
Engineering Note
Droplet size is read from the manufacturer spec but must be field-validated by laser sizing (especially for hygiene-critical sites). Manufacturer tolerance is typically ±20%; field validation should sit within ±5%. If DV90 > 50 μm is measured, pump pressure has dropped or nozzles have worn — maintenance is triggered. At high RH (>70%) and low temperature (<15°C), even fine droplets do not evaporate; in those conditions evaporative is not recommended and the design should shift to steam.
NKT Application Link
In NKT commissioning protocol, laser droplet sizing is a mandatory step for high-pressure misting systems; D50 and DV90 are recorded in the commissioning report. Annual periodic maintenance re-measures; a deviation > 15% triggers nozzle replacement or pump service.
