Air Mixing Calculator

When two airstreams combine adiabatically, both dry air mass and energy are conserved. The mixed humidity ratio and enthalpy are the mass-weighted averages of the two streams: W_mix = (m₁W₁ + m₂W₂) / (m₁ + m₂) and h_mix = (m₁h₁ + m₂h₂) / (m₁ + m₂), where each mass flow m is the airflow in CFM divided by that stream’s specific volume. Weighting by mass rather than by volume matters because a hot stream is less dense than a cold one at the same CFM, so it carries less air than its volume suggests. On a psychrometric chart the mixed state always falls on the straight line joining the two inlet states, dividing that segment in inverse proportion to the mass flows — the mixed point sits closer to whichever stream carries more air. The most common application is the mixed air condition at a coil inlet, where outdoor ventilation air mixes with return air. When two nearly saturated airstreams mix, the mixed state can fall above the saturation curve. That condition cannot exist: the excess moisture condenses out and the air leaves saturated at 100% relative humidity, which is why mixing warm humid air with cold air produces visible fog.