Mixed Air Calculator

Mixed Air in HVAC Systems

Mixed air is the stream formed when outside air and return air combine in an HVAC system. Engineers need the resulting humidity ratio, temperature, and mass flow to size coils, fans, and dampers. This calculator applies adiabatic mixing balances for two air streams.

Mixing Formulas

Mass flow balance:

$$\dot{m}_3 = \dot{m}_1 + \dot{m}_2$$

Humidity ratio mixing:

$$\omega_3 = \frac{\dot{m}_1 \omega_1 + \dot{m}_2 \omega_2}{\dot{m}_1 + \dot{m}_2}$$

Temperature mixing (dry bulb approximation):

$$T_3 = \frac{\dot{m}_1 T_1 + \dot{m}_2 T_2}{\dot{m}_1 + \dot{m}_2}$$

For precise psychrometric work, enthalpy mixing replaces the simple temperature average. Related tools: Mixed Air Temperature Calculator and Ideal Gas Pressure Calculator.

Frequently Asked Questions

What is humidity ratio?

Humidity ratio (ω) is the mass of water vapor per mass of dry air, usually in g/kg. It describes how moist an air stream is.

Is air mixing adiabatic?

Yes, in HVAC mixing chambers heat loss to surroundings is small. Energy is conserved through enthalpy balance rather than simple temperature averaging.

What units should mass flow use?

Use consistent mass flow units such as kg/s. Volumetric flow can be converted using specific volume from a psychrometric chart.

How do I get humidity ratio from relative humidity?

Use a psychrometric chart or dedicated psychrometric calculator with dry bulb temperature and relative humidity at your barometric pressure.

What is stream 1 vs stream 2?

Stream 1 is typically outside air and stream 2 is return air from the conditioned space, though you can enter any two streams being mixed.

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Mixed Air in HVAC Systems

Mixing Formulas

Frequently Asked Questions

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