01
Inputs and balance basis
Enter two complete inlet states at one total pressure and positive dry-air mass flows. The model averages humidity ratio and enthalpy by dry-air flow, then solves the mixed dry bulb. This preserves water-vapor mass and moist-air energy under the stated adiabatic boundary.
Use actual dry-air mass flow when available. If only volume flow is measured, calculate each inlet density first; equal cubic flows are not necessarily equal mass flows.
02
Verified worked example
Mix 30 °C, 50% RH air with 15 °C, 80% RH air at a dry-air flow ratio of 2:1 and 101.325 kPa. The mixed state is 25.029 °C, 58.947% RH, 11.705 g/kg humidity ratio and 54.998 kJ/kg dry air.
The state lies between the inlets on the chart and the retained mass and energy residuals are effectively zero. It is not the arithmetic average temperature because moist-air enthalpy and moisture are both conserved.
03
Interpret the result in an AHU
Compare measured mixed air with the balance prediction. A material difference can indicate outdoor-air fraction error, leakage, heat gain, stratification, unsynchronized readings or sensor bias. The calculation identifies an inconsistency; it does not choose the cause.
Label Outdoor, Return and Mixed states, use a shared pressure, and record damper condition and time. A single mixed-air sensor may not represent a poorly mixed plenum.
04
Common mistakes
Do not weight by RH or temperature alone. Do not mix mass-flow and volume-flow ratios. Do not place inlets at different pressures on one simple adiabatic balance without defining the pressure-changing equipment.
Condensation during mixing requires additional treatment if the straight balance state would cross the saturation boundary. Fog, liquid carryover and wall heat transfer are not represented by the basic two-stream model.
05
Limits and next workflow
The calculation is an ideal steady, adiabatic, two-stream balance on a dry-air basis. Validation fixtures check mass balance, energy balance, endpoints and invalid flows.
Follow the airside comparison workflow to collect synchronized outdoor, return, mixed and supply readings, then use the commissioning page to interpret deviations without over-claiming.