Mixed Air Calculator

Air mixing calculations rely on mass conservation and energy balance principles within psychrometric physics. When air stream 1 (with mass flow rate m₁, enthalpy h₁, and specific humidity ω₁) merges with air stream 2 (m₂, h₂, ω₂), the combined final mass is simply m₃ = m₁ + m₂. The resulting mixed air humidity ratio (ω₃) and enthalpy (h₃) are determined by taking the mass-weighted average of the incoming streams:

ω₃ = (m₁ω₁ + m₂ω₂) / m₃

h₃ = (m₁h₁ + m₂h₂) / m₃

Vapor pressure is derived using temperature-dependent saturation curves via the Magnus formula, while specific volume equations convert volumetric flow rates into precise mass flow metrics required for accurate energy modeling.

Let us walk through a practical HVAC scenario where outdoor fresh air mixes with return air inside a commercial air handling unit. Suppose Stream 1 (Return Air) has a mass flow rate of 2.0 kg/s, a dry-bulb temperature of 24°C, and a humidity ratio of 0.009 kg/kg, resulting in an enthalpy of approximately 48.5 kJ/kg. Stream 2 (Outdoor Fresh Air) has a mass flow rate of 1.0 kg/s, a dry-bulb temperature of 35°C, and a humidity ratio of 0.016 kg/kg, yielding an enthalpy of roughly 68.2 kJ/kg.

Step 1: Calculate combined mass flow (m₃)
m₃ = 2.0 kg/s + 1.0 kg/s = 3.0 kg/s.

Step 2: Calculate mixed humidity ratio (ω₃)
ω₃ = [(2.0 × 0.009) + (1.0 × 0.016)] / 3.0 = (0.018 + 0.016) / 3.0 = 0.034 / 3.0 = 0.01133 kg/kg.

Step 3: Calculate mixed enthalpy (h₃)
h₃ = [(2.0 × 48.5) + (1.0 × 68.2)] / 3.0 = (97.0 + 68.2) / 3.0 = 165.2 / 3.0 = 55.07 kJ/kg.

Using these resulting properties, the calculator solves for the final mixed dry-bulb temperature of approximately 27.3°C.

Consistent Units: Always ensure your flow rates are either entirely volumetric (e.g., CFM or m³/s) or converted accurately to mass flow rates (kg/s) using local air density values. Mixing unit systems without conversion will result in erroneous outputs.

Account for Elevation: Barometric pressure changes with altitude. Adjust the total pressure input if your building operates significantly above sea level, as this directly affects air density and psychrometric saturation curves.