HVAC

Adiabatic Mixing of Two Humid Airstreams

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Mix two ideal moist-air streams adiabatically at a common pressure, with no work, heat transfer, added water or condensation. Inputs are dry-air mass flows, not total moist-air mass flows or volume flows.
Each inlet uses −50 to 80 °C, 0–100% RH and 20–120 kPa absolute, with pressure above saturation pressure at dry bulb. Both inlet states must be valid even if one flow is zero. Flows must be nonnegative and their sum positive.
Using dry-air flows m1 and m2, W = (m1W1 + m2W2)/(m1 + m2) and h = (m1h1 + m2h2)/(m1 + m2). Dry-air mass, water-vapor mass and enthalpy are conserved.
For h in J/kg dry air, T = (h − 2501000W)/(1006 + 1860W) °C. Relative humidity follows from vapor pressure at this mixed temperature. Do not average inlet RH or use volume-flow fractions directly.
Convert an inlet volume flow Q to dry-air flow with m = Q/v, using that inlet's specific volume in m³/kg dry air. Mixed volume flow is the total dry-air flow times mixed specific volume; total moist-air flow includes vapor mass.
RH uses ice saturation at or below 0.01 °C and liquid water above it. A supersaturated vapor-only prediction is rejected: condensation or ice and their energy effects require a separate equilibrium model.
Enthalpy flow is referenced to the model's 0 °C datum and is not a heating or cooling load. Pressure losses, fans, stratification and incomplete mixing are excluded.