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 (typically 8-12 L/s/person per ASHRAE 62.1) 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. The NKT engineering team evaluates the "100% OA or mixed air?" question at project start through the triad of IAQ + process requirements + 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. • Economiser cycle — when outdoor enthalpy is below indoor, the OA damper opens to 100% to provide free cooling. In this mode the humidification load grows 5-12×; 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 (1 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. • Without positive pressure, humidifier efficiency loss — exfiltration causes up to 30-40% moisture loss; directly tied to building tightness.

NKT Application Link

NKT delivers humidification and dehumidification solutions integrated into mixed-air AHU systems:

1. In-duct steam distribution — Neptronic SKE4 + SAM-e steam absorption manifold; even distribution into the mixed-air stream, ±2% RH precision, hygienic. 2. Low-pressure-drop adiabatic — Neptronic SKG4 ultrasonic or SKH3 high-pressure mist, integrated inside the AHU, cooling + humidification bonus. 3. NKT - Climate Track AHU integration — T/RH/dewpoint monitoring at supply, return, OA, mixed-air points; automatic capacity adjustment at economiser transitions. 4. PID control strategy — outdoor enthalpy + indoor design + makeup-ratio dynamic equation; humidifier output 5-100% modulation. 5. BMS/SCADA Modbus integration — API compatible with BMS/PLC automation system, building automation system, Johnson Controls, Trane AI.

Sample configuration: a 50,000 m³/h supply AHU requires 90 kg/h at 20% OA, 380 kg/h in economiser mode. NKT selection: 2 × SKE4 60 + 2 × SKG4 120 hybrid, adiabatic as the main source in economiser mode (low energy), steam support for winter peak (±2% precision). Annual energy ~380 MWh, 42% energy savings.