Modeling incomplete adiabatic mixing of return and outdoor air streams in an air handling unit

The mixing of different air streams is a process commonly found in air handling systems. One of the most common examples of this process is found in the air handling unit mixing box where outdoor air is mixed with return air. In the adiabatic mixing process, the energy leaving the mixing box equals the energy entering the mixing box. In order to calculate the energy in the mixed air stream, it is only necessary to know the energy in the outdoor and the return air streams. As a result, it is not necessary to consider whether the mixing process is ideal or not. Because of this fact, the efficiency of the mixing process has largely been ignored. Since no real process is 100% efficient, there is a need for equations to describe incomplete or real mixing processes. A simple model that represents non-ideal adiabatic mixing of two air streams is presented. This simple model is used to develop equations that define the maximum and minimum temperatures downstream of an air handling unit mixing box. These equations are then used to derive an equation that defines the minimum level of Mixing Effectiveness that is required to prevent freezestat trips.