Mixing Humid Air

Calculating enthalpy, heat, temperature and specific humidity when mixing humid air

When mixing air of condition A and of condition C, the mixing point will be on the straight line in point B.

The position of point B depends on the air volume or mass of A and C.

The mixing process can also be shown in the psychometric chart as:

The Enthalpy Balance when Mixing Air

The heat balance when mixing humid air can be expressed as:

Q_A h_A + Q_C h_C = (Q_A + Q_C) h_B (1)

where

Q = volume of the air (m³)

h = enthalpy of the humid air (kJ/m³)

By transforming (1) the enthalpy of the mixed air can be expressed as:

h_B = (Q_A h_A + Q_C h_C) / (Q_A + Q_C) (1b)

Note! It's common to use the air volume when calculating ventilation and air condition systems. Using the mass of air is of course more accurate, but within the operation limits of ventilation systems the volume approach is accurate enough. Be aware that this may not be true in industrial air handling processes such as high temperature dryers or similar.

The Moisture Balance when Mixing Air

The moisture balance for when mixing air can be expressed as:

Q_A x_A + Q_C x_C = (Q_A + Q_C) x_B (2)

where

x = specific humidity or humidity ratio (kg_water/kg_air)

By transforming (2) - the specific humidity of the mixed air can be expressed as:

x_B = (Q_A x_A + Q_C x_C) / (Q_A + Q_C) (2b)

Humid Air and Fog

When hot humid air is mixed with cold air the result may be fog.

The mixing point is below the saturation line, and the moisture in the air condensates as small droplets floating in the air. The fog process can be expressed in the Mollier diagram as:

For temperatures below 0^oC the water droplets will freeze to ice.

Resulting Temperature when mixing Humid Air

The Mixing Point is above Saturation Line

As long as the mixed air is above the saturation line (no fog in the mix), the temperature balance can be expressed by modifying the enthalpy balance as:

Q_A c_pa t_A + Q_C c_pa t_C = (Q_A + Q_C) c_pa t_B (3)

where

c_pa = 1.01 - specific heat capacity of air at constant pressure (kJ/kg.^oC, kWs/kg.K)

The temperature in the mix can be expressed by transforming (3):

t_B = (Q_A t_A + Q_C t_C) / (Q_A + Q_C) (3b)

The Mixing Point is below Saturation Line

If the mixing point is below the saturation line - water is condensated as droplets and fog is created.

The amount of condensated water can be found by following the constant enthalpy line from B to the saturation line. The condensated water is the difference between the specific humidity in point B and in the point where the enthalpy line crosses the saturation line.

The mixed temperature is where the enthalpy line crosses the saturation line.

Example - Mixing Humid Air

The Mixing Point is above the Saturation Line

1 m³ of air at 25^oC and 50% relative humidity (C) is mixed with 1 m³ air at -5^oC and 80% relative humidity (A).

From the Mollier diagram the humidity ratio of (C) is 0.0097 kg/kg. The enthalpy is 50 kJ/kg. The humidity ratio of (A) is 0.002 kg/kg. The enthalpy is 0 kJ/kg.

The humidity ratio of the mixture can be calculated as:

x_B = (Q_A x_A + Q_C x_C) / (Q_A + Q_C)

= ( (1 kg) (0.002 kg/kg) + (1 kg) (0.0097 kg/kg) ) / ( (1 kg) + (1 kg) )

= 0.0058 kg/kg

The enthalpy of the mixed air can be calculated as:

h_B = ( (1 kg) (0 kJ/kg) + (1 kg) (50 kJ/kg) ) / ( (1 kg) + (1 kg) )

= 25 kJ/kg

The temperature of the mixed air can be calculated as:

t_B = ( (1 kg) (-5^oC) + (1 kg) (25^oC) ) / ( (1 kg) + (1 kg) )

= 10^oC

Based on the information above and the Mollier diagram - the relative humidity in the mixing point is approximately 80%.

The Mixing Point is below the Saturation Line

1 m³ of air at 25^oC and 90% relative humidity (C) is mixed with 1 m³ air at -5^oC and 80% relative humidity (A).

From the Mollier diagram the humidity ratio of (C) is 0.018 kg/kg. The enthalpy is 65 kJ/kg. The humidity ratio of (A) is 0.002 kg/kg. The enthalpy is 0 kJ/kg.

The enthalpy of the mixture can be calculated as:

h_B = ( (1 kg) (0 kJ/kg) + (1 kg) (65 kJ/kg) ) / ( (1 kg) + (1 kg) )

= 32.5 kJ/kg

The humidity ratio of the mixture can be calculated as:

x_B = ( (1 kg) (0.002 kg/kg) + (1 kg) (0.018 kg/kg) ) / ( (1 kg) + (1 kg) )

= 0.01 kg/kg

Note! This is the humidity ratio including the water droplets - the fog.

The "virtual" mixing point in the Mollier diagram is where the enthalpy line - 32.5 kJ/kg and the humidity ratio line - 0.01 kg/kg - intersect.

The temperature in the mixed air can be found in the diagram where the enthalpy line - 32.5 kJ/kg - crosses the saturation line. From the Mollier diagram the mixed temperature is approximately 11.7^oC.

According the Mollier diagram the moisture ratio at this point is 0.0084 kg/kg. The moisture created as fog or droplets is

0.01 kg/kg - 0.0085 kg/kg = 0.0015 kg/kg

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