Mixed Air System

Detailed Explanation

A mixed-air system is the most common air distribution topology of modern AHUs (Air Handling Units). The mechanism works as follows:

1. Return air drawn from the space arrives at the AHU mixing box via a return fan. 2. In the mixing box, part of the return air is exhausted through the relief damper (typically 15-30%, depending on IAQ and pressure control). 3. The same volume of fresh air (makeup air) is drawn through the outdoor air damper. 4. The mixed air (return × 70-85% + outdoor × 15-30%) passes through filter, heating/cooling coil, humidifier and fan, and returns to the space as supply air.

Advantages of this topology: • Energy savings — return air is already at indoor design condition (e.g. 22°C, 50% RH); only makeup air is conditioned, reducing heating/cooling/humidification load by 70-85%. • IAQ control — the minimum makeup ratio (ASHRAE 62.1 for offices: 2.5 L/s/person + 0.3 L/s/m², i.e. about 8-10 L/s/person at typical office occupancy) guarantees CO2 and VOC dilution. • Building pressure — when return < supply, the space holds positive pressure; particulate and outdoor contamination ingress is blocked (critical in clean rooms).

Control variables: • OA (Outdoor Air) damper position • EA (Exhaust Air) damper position • RA (Return Air) damper position • Mixing air temperature — input for cooling/heating load

Mixed-air systems are standard in comfort HVAC, offices, hospitals (non-critical zones), hotels, shopping centres and similar typical buildings.

Why It Matters

The impact of a mixed-air system on humidification load is dramatic. Comparing 100% makeup vs 20% makeup for the same building:

Building: 5,000 m³ office, target 22°C, 50% RH = 8.3 g/kg Outdoor: -3°C, 75% RH = 2.4 g/kg Total supply air: 6,000 m³/h (ACH 1.2)

100% makeup scenario: • Δw = 8.3 − 2.4 = 5.9 g/kg • Air mass: 6,000 × 1.2 = 7,200 kg/h • Humidification load: 7,200 × 5.9 / 1000 = 42.5 kg/h steam

20% makeup, 80% return mixed-air scenario: • Mixed-air absolute humidity: 0.8 × 8.3 + 0.2 × 2.4 = 7.12 g/kg • Δw = 8.3 − 7.12 = 1.18 g/kg • Humidification load: 7,200 × 1.18 / 1000 = 8.5 kg/h steam

Difference: 42.5 → 8.5 kg/h = 80% load reduction. Heating, cooling and dehumidification loads also drop proportionally.

This is why mixed-air topology is standard in comfort HVAC and office systems; only critical contamination conditions (BSL-3/4 labs, isolation rooms, some pharmaceutical production) require 100% OA. At project start the "100% OA or mixed air?" question should be assessed against indoor air quality, process requirements and energy economics.

Practical Example

Consider an Ankara hospital administration block HVAC project. 8,500 m² office + outpatient clinic, 8 storeys, 350-person capacity.

Design inputs: • Target indoor: 22°C, 50% RH (winter), 24°C, 50% RH (summer) • Min IAQ supply (ASHRAE 62.1): 350 × 10 L/s = 3,500 L/s = 12,600 m³/h makeup • Total supply air (for thermal balance): ~58,000 m³/h • Makeup ratio: 12,600 / 58,000 ≈ 22% (excluding economiser mode) • Exhaust: 12,000 m³/h (from spaces + toilets) • Building positive pressure: ~+10 Pa

Winter peak scenario (-5°C, 85% RH outdoor, 2.1 g/kg): • Mixed-air moisture: 0.78 × 8.3 + 0.22 × 2.1 = 6.94 g/kg • Δw = 8.3 − 6.94 = 1.36 g/kg • Air mass: 58,000 × 1.2 = 69,600 kg/h • Humidification load: 69,600 × 1.36 / 1000 = 95 kg/h

Solution: 2 × Neptronic SKE4 60 kg/h (120 kg/h total, 25% safety margin). DI/RO water supply (mains hardness high), in-duct distribution, ±2% RH precision PID. NKT - Climate Track per-zone monitoring.

Comparison: a 100% OA design would need 432 kg/h capacity (4.5×). Capex difference ~280,000 USD, opex difference ~80,000 USD/year for electricity and steam.

Engineering Note

Critical engineering points in mixed-air system design:

• Minimum OA ratio — take the upper bound of ASHRAE 62.1 (per-person L/s) + per-area (L/s/m²) + process exhaust compensation. CO2-sensor-based demand-controlled ventilation (DCV) provides dynamic adjustment. • Mixing-box homogeneity — poor mixing of return and OA produces stratification; the cold OA band trips the freeze-stat or yields uneven moisture distribution. Mixing baffles or turning vanes are required. • Economizer cycle — when outdoor enthalpy is below indoor, the OA damper opens to 100% to provide free cooling. In this mode the humidification load grows 3-5× in proportion to the fresh-air fraction; the humidifier must be sized for the economiser-on scenario. • Freeze protection — below 0°C OA, a preheat coil or damper modulation is needed; otherwise the heating coil freezes. • Damper quality — AMCA Class 1A (3 cfm/ft² @ 1 in.wg) sealing standard; a poor damper leaks 15% OA when closed = unbalanced humidification load. • Filter load impact — dirty filters increase supply fan pressure, OA flow drops, CO2 rises; ΔP sensor with filter-change alarm is standard. • Positive pressure measurement — verified with a manometer at 5-15 Pa overpressure; if <0 Pa, outdoor contamination is drawn in. • With excessive positive pressure, humidifier efficiency loss — exfiltration causes up to 30-40% moisture loss; directly tied to building tightness.

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies TFT, Neptronic and Microwell equipment. 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.