Mixed Air Temperature Calculator

The mixed air temperature calculator finds the equilibrium condition produced by combining two air streams with known dry-bulb temperatures, humidity ratios, and dry-air mass flow rates. It is designed for a common HVAC and thermodynamics approximation: two streams enter a well-mixed control volume at steady state, no heat is exchanged with the surroundings, and no water condenses or evaporates. Under these conditions, the contribution of each stream is proportional to its dry-air mass flow. Temperature is calculated as a flow-weighted mean. The temperature of stream one is multiplied by its flow, the temperature of stream two is multiplied by its flow, and the sum is divided by total flow. Humidity ratio is balanced in exactly the same way because it expresses water-vapor mass per unit mass of dry air. The resulting ratio is therefore the total incoming water vapor divided by total incoming dry air. The displayed flow fractions make the weighting explicit and help users verify which stream dominates the mixture. This method is intentionally simpler than the full moist-air enthalpy balance used by a detailed psychrometric calculator. At ordinary comfort-conditioning temperatures and modest humidity ratios, the specific heat of moist air changes only slightly, so the flow-weighted temperature is a useful engineering estimate. For high temperatures, very humid processes, or precision equipment selection, balance moist-air enthalpy instead and solve the final dry-bulb temperature from the mixed enthalpy and humidity ratio. Humidity ratio is not the same as relative humidity. A ratio of 0.010 kg/kg means that each kilogram of dry air carries 0.010 kilogram, or ten grams, of water vapor. Relative humidity additionally depends on temperature and pressure. This calculator reports the conserved humidity ratio rather than attempting to infer relative humidity without a pressure input. Once the mixed temperature and ratio are known, a psychrometric chart or separate humidity calculator can provide relative humidity, dew point, and wet-bulb temperature. Typical uses include blending outdoor ventilation air with return air, estimating supply conditions upstream of a coil, combining process exhaust and make-up air, analyzing parallel duct branches, and teaching conservation of mass. Always use flow rates on the same basis and in the same units. If only volumetric flow is available, convert each value to mass flow using the density of that stream. The calculator provides a quick, transparent balance for preliminary design and field checks.