Every mixed-air AHU does the same psychrometric thing: combine return air with outdoor air upstream of the cooling coil. The mixing point determines the entering coil condition, which determines the coil duty, which determines whether the building actually maintains its design temperature and humidity. This walks through the math, the chart, and the typical Indian design failures.
The mixing equation
For two air streams at steady state, mass balance gives:
w_M = (m_RA × w_RA + m_OA × w_OA) / (m_RA + m_OA)
DBT_M = (m_RA × DBT_RA + m_OA × DBT_OA) / (m_RA + m_OA)
h_M ≈ 1.006 × DBT_M + w_M × (2501 + 1.86 × DBT_M)
The mixing point lies on the straight line connecting RA and OA on a psychrometric chart, at a fractional position equal to the OA mass fraction.
Worked example: Delhi composite climate, office
Inputs:
- Return air (RA): 24°C / 50 % RH → w_RA ≈ 9.4 g/kg, h_RA ≈ 47.9 kJ/kg
- Outside air (OA, May design): 41°C / 25 % RH → w_OA ≈ 11.6 g/kg, h_OA ≈ 71.0 kJ/kg
- Mass flow split: 80 % RA / 20 % OA (typical 8 L/s/p OA on 30 % occupancy)
Mixing:
- w_M = 0.80 × 9.4 + 0.20 × 11.6 = 9.84 g/kg
- DBT_M = 0.80 × 24 + 0.20 × 41 = 27.4°C
- h_M ≈ 51.7 kJ/kg
The coil now sees 27.4°C / 9.84 g/kg ≈ 45 % RH at entering. To deliver 13°C / 90 % RH supply air (~9.0 g/kg, 33.6 kJ/kg), the coil must remove 51.7 – 33.6 = 18.1 kJ/kg of enthalpy. Multiply by mass flow to get coil duty in kW.
Worked example: Mumbai warm-humid, hotel guestroom
Same architecture, different climate:
- RA: 22°C / 55 % RH → w_RA ≈ 9.1 g/kg, h_RA ≈ 45.3 kJ/kg
- OA: 33°C / 70 % RH (Mumbai design) → w_OA ≈ 22.0 g/kg, h_OA ≈ 89.4 kJ/kg
- Mass split: 70 % RA / 30 % OA (hospitality higher OA fraction for IAQ)
Mixing:
- w_M = 0.70 × 9.1 + 0.30 × 22.0 = 13.0 g/kg (!!)
- DBT_M = 0.70 × 22 + 0.30 × 33 = 25.3°C
- h_M ≈ 58.5 kJ/kg
Coil now has to drop humidity from 13.0 to ~8.5 g/kg, a 4.5 g/kg latent removal. That’s a 6-row coil at 5.5°C water, not a 4-row at 7°C.
The Mumbai mixed air enters the coil at 13 g/kg — higher than Delhi’s 9.84 g/kg even though Mumbai’s room is at lower w (9.1 vs 9.4). Why? Because Mumbai’s OA at 22 g/kg dominates the mix.
Why the chart matters
Plotting RA, OA, and M on a psychrometric chart shows you immediately whether the mixing point is on the wet side of the room. In Mumbai, M sits at 25.3°C / 13 g/kg — well above and to the right of R. The coil has to take a long psychrometric “leg” down + left. In Delhi, M sits at 27.4°C / 9.84 g/kg — above R but at almost the same humidity. The coil mostly does sensible work + a small dehumid.
This is the core insight: the coil’s psychrometric leg is determined by mixing, not by climate alone. A poorly chosen OA fraction in warm-humid can blow up the coil duty even if the climate looks “moderate” on the OA condition alone.
How MEPVAULT Psychrometric Analyzer simulates this
Open Psychrometric Analyzer → → mode = “mixing”
Inputs: RA flow + DBT + RH, OA flow + DBT + RH, altitude. Output: mixed DBT, RH, w, h, OA fraction. Then switch to “cool_dehumid” mode with mixed condition as the inlet to compute coil duty.
From the Field — Engineer’s Notebook
On a 2023 BPO retrofit in Pune, the original AHU was sized at 35 % OA (per IGBC enhanced ventilation credit). Post-COVID, the client increased OA to 50 % for IAQ. The supply air condition didn’t change in the BMS, but humidity creep was reported within 2 weeks. We re-mixed: 50 % OA at Pune monsoon design (32°C / 80 % → 22 g/kg) pushed mixed w from 13.5 to 16.0 g/kg — coil ADP couldn’t reach 8.5 g/kg leaving even at 7°C water + 4 rows. Solution: pre-cool OA via desiccant wheel (DOAS retrofit) to drop OA w from 22 to 12 g/kg before mixing. Mixed w returned to 11.4 g/kg, coil hit target. The OA change wasn’t in the AHU’s design envelope and only the psychrometric walk-through caught it.
5 common mistakes
1. Mixing at the wrong fraction. OA fraction in your psychrometric is mass-fraction (kg/s), not volumetric (L/s). They differ when DBT differs across streams.
2. Plotting the mixing line above the saturation curve. This signals you have fog inside the AHU — the OA was so cold/wet that mixing produced supersaturated air. Real-world: drain pan overflows + coil corrosion.
3. Treating reheat as part of the cooling coil duty. Reheat happens downstream of the coil and is a separate energy + duty. Account for it explicitly in the coil duty + reheat budget.
4. Ignoring leakage in the AHU casing. A 5 % return-side leakage on a hotel AHU at 30 % nominal OA actually delivers 33-35 % OA. Casing leak class affects the mix.
5. Forgetting the bypass damper. Some AHU designs include a bypass that re-mixes downstream of the coil — this changes the supply state. Document the airflow path on the design drawing, don’t assume it.
Designer’s checklist
- [ ] OA + RA mass flows confirmed (kg/s, not L/s as nominal)
- [ ] Mixed air condition computed on chart, not just numerically
- [ ] Coil entering condition matches mixing output ± 0.5°C
- [ ] AHU casing leakage class specified (≤ Class L2 per EN 1886)
- [ ] OA-fraction flexibility planned (manual + BMS controlled bypass damper)
- [ ] DOAS pre-conditioning evaluated for warm-humid zones
- [ ] Mixing point not above saturation curve at any operating condition
- [ ] Bypass damper documented on supply/return airflow drawing
Pairs with: Psychrometric Analyzer, Psychrometrics Tropical India