Mixed Air Temperature and Enthalpy Calculator

How is air mixing calculated?

In air handling units, outdoor air is mixed with air returning from the space. The state of the mixture follows from two conservation laws: water vapour mass and energy. Both are written on dry air mass:

• W_3 = (ṁ_1 × W_1 + ṁ_2 × W_2) / (ṁ_1 + ṁ_2)
• h_3 = (ṁ_1 × h_1 + ṁ_2 × h_2) / (ṁ_1 + ṁ_2)

The mixed air temperature is solved back from these two values. On the psychrometric chart the mixing point lies on the straight line joining the two states, closer to the stream with the larger flow.

Example

Outdoor air at 1000 cubic metres per hour (35 °C, 40 percent) mixes with 3000 cubic metres per hour of return air (25 °C, 50 percent). The dry air flows are 1120 and 3496 kg/h. The mixture has a humidity ratio of 10.9 g/kg, an enthalpy of 55.5 kJ/kg and a temperature of 27.4 °C. A rough average by volume gives 27.5 °C: the gap is small here but grows with the temperature difference.

Why work with mass?

Warm air is thinner. The same volume of warm air carries less mass than cold air. The balances are therefore written on mass, not volume, and the tool works out the specific volume of each stream separately.

Fog warning

In winter, very cold outdoor air mixing with warm humid return air can put the mixing point above the saturation curve. Fog and condensation then form in the mixing box, and ice below freezing. The tool detects this and warns instead of giving a result.

Limits

The calculation assumes complete mixing in a box with no heat loss. Fan heat, duct gains and stratification are considered separately.