Air Mixing Calculator
Combine two or more airstreams — each with its own flow rate and moist-air state — into the resulting mixed condition. Standard for AHU outdoor-air/return-air mixing box design and multi-stream duct junctions.
Inputs
Mixed-Air Result
Stream contributions
| Stream | T_db | RH | Dry-air flow | Mass fraction |
|---|
Formula & Calculation Basis
The mixed state is computed server-side by the PsychroLib v3 API (ProExergy) as a mass-weighted (not volume-weighted) average of humidity ratio and enthalpy across all dry-air mass flows, then re-solved into a full psychrometric state.
| Symbol | Meaning | Unit (SI) |
|---|---|---|
| ṁ_da,i | Dry-air mass flow rate of stream i | kg/s |
| W_i | Humidity ratio of stream i | kg/kg dry air |
| h_i | Specific enthalpy of stream i | J/kg dry air |
| ṁ_da,mix | Total mixed dry-air mass flow | kg/s |
| W_mix, h_mix | Mass-weighted mixed humidity ratio and enthalpy | kg/kg, J/kg |
| x_i | Mass fraction of stream i = ṁ_da,i / ṁ_da,mix | – |
Calculation sequence: resolve total pressure → convert each stream's volumetric flow (using its own density) or accept its dry-air mass flow directly → call /api/v1/calc/mix with all streams' states and flows → API returns the mass-weighted mixed PsychroState, total dry-air mass flow, and per-stream mass fractions.
Assumptions & Limits
Included effects
- Adiabatic, steady-state mixing at a common pressure — no heat gain/loss across the mixing plane.
- Mass conservation of dry air and water vapor; energy conservation via enthalpy weighting.
- Each stream may be specified by any single humidity input (RH, wet-bulb, dew point, humidity ratio, vapor pressure, or enthalpy).
Excluded effects
- Duct/damper pressure losses, imperfect mixing, and stratification at the mixing plane.
- Condensation during mixing when the resulting state would fall below saturation (fog-point mixing) — verify RH ≤ 100% in the result.
- Different pressures per stream — all streams are mixed at one common atmospheric pressure.
Valid input range: same dry-bulb, humidity, and pressure limits as the underlying psychrometric state calculation; at least 2 streams with positive flow required.
Engineering Notes
- For a typical AHU mixing box, enter outdoor (fresh) air as one stream and return air as the second — the mixed state is your entering-coil condition for downstream cooling/heating coil sizing.
- Mixing is linear in humidity ratio and enthalpy, but not linear in dry-bulb temperature or RH — always mix via W and h, never by simple T or RH averaging (this calculator handles that automatically).
- If the mixed RH result exceeds 100%, the two streams are on either side of the saturation curve and physically condense on mixing (common with very cold, near-saturated outdoor air meeting warm humid return air) — treat the mixed point as the fog-point line, not a stable state.
- Check the resulting mixed dry-bulb against your minimum outdoor-air damper/freeze-stat setpoint to avoid coil freeze risk in cold climates.
- Method reference: PsychroLib (ASHRAE Fundamentals 2017, Ch.1) mass/energy-weighted mixing, as implemented by the ProExergy PsychroLib v3 API.
- API reference: ProExergy.com.
Revision: v1.0 — initial release, supports 2–6 airstreams with volumetric or dry-air mass flow input and any humidity descriptor per stream.
