When two streams of air with different properties and flow rates are mixed, the properties of the resulting mixture (enthalpy, dry bulb temperature and saturation ratio) can be determined by simple proportional mass and energy balances.

In most cases, heat loss from the system is negligible and the system can be modeled at adiabatic. On a psychrometric chart, the condition of the exiting air stream must be on the line connecting the incoming air streams. The distance along that line is determined by the ratio of the mass flow rates of the incoming streams.

Examples of steady flow mixing process include the outdoor air – return air mixing chamber of an air handling unit; warm stream – cold stream mixing in dual stream systems (double duct or multi-zone); and supply air – recirculated air mixing in fan powered terminals.

Governing Equations:

The governing equations for this process are: