Free Mixed Air Calculator

Mixed Air and Its Role in HVAC Systems

In heating, ventilation, and air conditioning (HVAC), accurately determining the properties of mixed air—the combination of outdoor fresh air and return air—is essential for efficient system design and energy performance. The HVAC mixed air calculator presented here eliminates the need for manual psychrometric chart readings by computing key parameters such as dry‑bulb temperature, enthalpy, humidity ratio, dew point, and specific volume in a few clicks. Whether you are an engineer or a technician, this mixed air temperature calculator streamlines the design and troubleshooting of air‑handling units.

What Is Mixed Air?

Mixed air is the result of blending two distinct air streams: the outdoor (or outside) air intake and the building's return air. Each stream has its own temperature, humidity, and flow rate. The resulting mixture exhibits properties that lie between those of the two sources, determined by their proportions. This mixing process is a cornerstone of HVAC operations because it allows the system to pre‑condition air by reusing already‑conditioned return air, reducing the energy required to heat or cool the fresh outdoor air.

Fundamental Assumptions and Formulas

Air mixing is treated as an adiabatic process: the heat exchange with the surroundings is negligible, no work is performed, and changes in kinetic and potential energy are ignored. Under these conditions, the conservation of energy and mass leads to a straightforward set of equations.

Let subscripts 1, 2, and 3 refer to outside air, return air, and mixed air, respectively. The mass flow rates ($\dot{m}$), enthalpies ($h$), and humidity ratios ($\omega$) satisfy:

• Dry‑air mass balance: $\dot{m}_1 + \dot{m}_2 = \dot{m}_3$
• Vapor mass balance: $\dot{m}_1\,\omega_1 + \dot{m}_2\,\omega_2 = \dot{m}_3\,\omega_3$
• Energy balance: $\dot{m}_1\,h_1 + \dot{m}_2\,h_2 = \dot{m}_3\,h_3$

From these equations, the mixed air’s enthalpy and humidity ratio can be computed directly if the properties of the two inlet streams are known. The full set of mixed air properties—dry‑bulb temperature, relative humidity, specific volume, dew point, and vapor pressure—can then be obtained from psychrometric relations.

Using the Mixed Air Properties Calculator

The mixed air properties calculator simplifies the entire process. You only need to provide, for each of the two inlet streams, either two independent psychrometric properties (e.g., dry‑bulb temperature and relative humidity) and the volumetric flow rate. The calculator then:

1. Computes the mass flow rate, enthalpy, humidity ratio, specific volume, and other missing properties of each stream.
2. Applies the mixing formulas to derive the properties of the resulting mixed air.
3. Displays all results in a clear table.

Step‑by‑Step Example

Consider an HVAC system with the following conditions:

• Outside air (stream 1): dry‑bulb temperature $32\,^\circ\text{C}$, relative humidity $60\%$, volumetric flow rate $20\ \text{m}^3/\text{min}$.
• Return air (stream 2): saturated at $14\,^\circ\text{C}$ (i.e., $100\%\ \text{RH}$), volumetric flow rate $50\ \text{m}^3/\text{min}$.
• Mixing pressure: standard atmospheric pressure ($101.325\ \text{kPa}$).

After entering these inputs into the air mixing calculator, the tool immediately returns the complete set of properties for each stream and for the mixture:

Outside Air Properties

Property	Symbol	Value
Humidity ratio	$\omega_1$	$18.025\ \text{g/kg}$
Dew point temperature	$T_{\text{dp1}}$	$23.27\,^\circ\text{C}$
Specific volume	$v_1$	$0.8895\ \text{m}^3/\text{kg}$
Mass flow rate	$\dot{m}_1$	$0.37474\ \text{kg/s}$
Enthalpy	$h_1$	$78.33\ \text{kJ/kg}$
Vapor pressure	$P_{\text{v1}}$	$2.854\ \text{kPa}$

Return Air Properties

Property	Symbol	Value
Humidity ratio	$\omega_2$	$9.965\ \text{g/kg}$
Dew point temperature	$T_{\text{dp2}}$	$14\,^\circ\text{C}$
Specific volume	$v_2$	$0.8265\ \text{m}^3/\text{kg}$
Mass flow rate	$\dot{m}_2$	$1.0083\ \text{kg/s}$
Enthalpy	$h_2$	$39.255\ \text{kJ/kg}$
Vapor pressure	$P_{\text{v2}}$	$1.598\ \text{kPa}$

Mixed Air Results

Property	Symbol	Value
Volumetric flow rate	$V_3$	$70\ \text{m}^3/\text{min}$
Dry‑bulb temperature	$T_{\text{db3}}$	$18.93\,^\circ\text{C}$
Relative humidity	$\%\,\text{RH}_3$	$88.79\%$
Humidity ratio	$\omega_3$	$12.15\ \text{g/kg}$
Dew point temperature	$T_{\text{dp3}}$	$17.04\,^\circ\text{C}$
Specific volume	$v_3$	$0.8436\ \text{m}^3/\text{kg}$
Mass flow rate	$\dot{m}_3$	$1.383\ \text{kg/s}$
Enthalpy	$h_3$	$49.84\ \text{kJ/kg}$
Vapor pressure	$P_{\text{v3}}$	$1.9414\ \text{kPa}$

The mixed air enthalpy calculator functionality automatically uses the psychrometric relations to produce these numbers, saving you the time of plotting points on a chart. Moreover, the tool is flexible: you can also enter the mixed air properties together with one inlet stream to solve for the unknown inlet stream.

Conclusion

By replacing tedious chart readings and manual formula applications, this HVAC mixed air calculator makes the analysis of air mixing fast and reliable. It handles everything from mixed air temperature and enthalpy to dew point and specific volume, making it an indispensable companion for anyone working with air‑side HVAC design.