Definition

Air drawn into the AHU or directly into the space from outside the building, never previously circulated indoors; it directly drives the mixed-air ratio and the humidification load.

Detailed Explanation

Outdoor air (OA), or fresh air, refers to air drawn into the HVAC system from outside the building that has not yet circulated indoors. In a typical AHU, outdoor air is mixed via a damper with return air; the resulting mixed air passes through filters, heater, cooler, humidifier, and fan, then is delivered to the space as supply air. The outdoor-air fraction ranges from 100% (full fresh air) to 10–20% (high recirculation); the choice depends on hygiene, odour, CO2, contamination, and energy priorities.

Ventilation standards (ASHRAE 62.1, EN 16798, REHVA) define minimum outdoor airflows by sector: 8–10 L/s/person in offices, 20+ ACH at 100% outdoor for hospital ORs, 20–60 ACH for ISO 7 pharma cleanrooms, 100% outdoor for restaurant kitchens. Outdoor airflow is the largest driver of the humidification load, because outdoor air enters with a large absolute-humidity difference.

Why It Matters

In humidification-capacity calculations, outdoor-air fraction is the second-most critical input after airflow. With 100% outdoor air in winter, outdoor air typically holds 2–3 g/kg of absolute humidity (e.g. 0°C / 80% RH ≈ 3 g/kg); for an indoor target of 8.3 g/kg (22°C / 50% RH) the humidifier must add about 5.3 g/kg. The same plant running on 50% outdoor + 50% return ends up with a mixed-air state of ≈ 5.7 g/kg; the humidifier adds only ≈ 2.7 g/kg — capacity demand drops by 50%.

That is why the question "which airflow will be humidified?" is matched in importance by "how much is outdoor and how much is return?" An incorrect assumption (e.g. treating the whole flow as outdoor) oversizes the unit by 2–3×; correct calculation cuts CAPEX by 30–50% and reduces electricity proportionally.

Practical Example

A printing house in Ankara targets 22°C / 55% RH (≈ 9 g/kg) for offset; the January design condition is −9°C / 80% RH (≈ 1.5 g/kg). Total supply airflow is 50,000 m³/h. Scenario A: 100% outdoor — humidification load ≈ (9−1.5) × 50,000 × 1.2 / 1000 ≈ 450 kg/h of steam. Scenario B: 30% outdoor + 70% return — mixed-air absolute humidity ≈ 0.3×1.5 + 0.7×9 = 6.75 g/kg; load ≈ (9−6.75) × 50,000 × 1.2 / 1000 ≈ 135 kg/h — 30% of Scenario A. NKT always recommends maximum recirculation that hygiene constraints allow.

Engineering Note

Outdoor airflow is sized by ASHRAE 62.1 (per-person minimum + per-area minimum), EN 16798-1 (categories I–IV), or REHVA Guidebook 14. In Türkiye, TS 4196 and TS EN 13779 are the references. Hygiene-critical facilities (hospitals, pharma, food) often require 100% outdoor air; in that case, humidification, heating, and cooling loads are planned at their peak. Heat recovery (HRV/ERV) can save 70–80% energy, but the humidification-load calculation must use the post-recovery mixed-air state.

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies the Neptronic SKE4 resistive steam humidifier, the Neptronic SKS4 steam-to-steam humidifier and the Neptronic SKH high-pressure atomisation system. It supports unit selection with capacity calculation and technology matching based on space conditions, target temperature and RH (or dew point) and moisture load, and provides technical service and spare parts. Design, installation and commissioning remain within the scope of the facility's designer and contractor.