Air Mixing (Psychrometric)

About Air Mixing Calculator (Psychrometric)

The air mixing calculator finds the condition of the single air stream produced when two moist-air streams combine adiabatically, the classic case being outdoor (fresh) air mixing with recirculated return air before it enters a cooling or heating coil. On a dry-air mass basis the mixed dry-bulb temperature, humidity ratio, and enthalpy are each the mass-weighted average of the two inlet streams.

Enter each stream's flow rate, dry-bulb temperature, and relative humidity, plus the atmospheric pressure. The tool computes each stream's humidity ratio and enthalpy from the Magnus saturation-pressure relation (the same form used in the psychrometric calculator), weights them by dry-air mass flow, and reports the mixed dry-bulb, humidity ratio, enthalpy, dew point, and a relative humidity back-calculated from the mixed condition. Flows may be entered as dry-air mass flow or as volumetric flow, which is converted to mass flow using each stream's moist-air density.

How It Works

1. Enter the flow, dry-bulb temperature, and relative humidity of both streams and the atmospheric pressure; choose a mass or volumetric flow basis.
2. For each stream the tool finds the saturation pressure Pws (Magnus), the vapour pressure Pw = RH·Pws, the humidity ratio W = 0.62198·Pw/(P−Pw), and the enthalpy h = 1.006·T + W·(2501 + 1.86·T).
3. Volumetric flows are converted to dry-air mass flow with the specific volume v = Rda·(T+273.15)·(1+1.6078·W)/P, so the weighting is always on a dry-air basis.
4. The mixed condition is the mass-weighted average: T_m, W_m, and h_m. The mixed RH and dew point are back-calculated from W_m and T_m (capped at saturation).

Worked Example

A unit mixes 1 kg/s of outdoor air at 35 °C, 40% RH with 3 kg/s of return air at 24 °C, 50% RH at 101.325 kPa. The dry-bulb mixes as T_m = (1×35 + 3×24)/4 = 26.75 °C. The stream humidity ratios are W₁ ≈ 14.11 g/kg and W₂ ≈ 9.28 g/kg, giving W_m = (1×14.11 + 3×9.28)/4 ≈ 10.48 g/kg. The enthalpies are h₁ ≈ 71.41 and h₂ ≈ 47.76 kJ/kg, so h_m = (1×71.41 + 3×47.76)/4 ≈ 53.67 kJ/kg. Back-calculating from W_m at 26.75 °C gives a mixed relative humidity of about 47.9% and a dew point near 14.8 °C.

Formulas

Mixed dry-bulb temperature

T_m = (m1*T1 + m2*T2) / (m1 + m2)

Mixed humidity ratio (mass balance)

W_m = (m1*W1 + m2*W2) / (m1 + m2)

Mixed enthalpy (energy balance)

h_m = (m1*h1 + m2*h2) / (m1 + m2)

Stream humidity & enthalpy (Magnus)

Pws = 610.94*exp(17.625*T/(T+243.04)); W = 0.62198*Pw/(P-Pw); h = 1.006*T + W*(2501 + 1.86*T)

Volume-to-mass conversion

m_dot = V_dot / v , v = Rda*(T+273.15)*(1 + 1.6078*W)/P

Standards & References

• ASHRAE Handbook — Fundamentals, "Psychrometrics" (adiabatic mixing of two airstreams)
• ASHRAE Standard 62.1 (outdoor-air/return-air mixing for ventilation)
• Magnus-Tetens saturation form (Alduchov & Eskridge)
• Conservation of dry-air mass, water mass, and energy

Frequently Asked Questions