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Engineering ToolBox > Changing Air Conditions by Heating, Cooling, Mixing, Humidifying or Dehumidifying the Air

Overview of the basic changes of air conditions by heating, cooling, mixing, humidifying and dehumidifying air by adding steam or water. The different processes are shown in psychometric diagram and the Mollier chart.

The most common changes of air conditions are

• Heating of Air
• Mixing of Air
• Cooling and Dehumidifying of Air
• Humidifying by Adding Steam or Water

Heating of Air

The process of heating of air can be expressed in the Mollier diagram as:

Heating of air moves the air condition from A to B along a constant specific humidity - x - line. The supplied heat - dH - can be read in the diagram as shown.

The heating process of air expressed can also be expressed in the psychrometric chart as:  

• Calculating enthalpy change and temperature rise when heating air.

Mixing of Air of different Conditions

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

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

The heat balance for the mix can be expressed as:

m_A h_A + m_C h_C = (m_A + m_C) h_B         (1)

where 

m = mass of the air 

h = heat of the air 

The moisture balance for the mix can be expressed as:

m_A x_A + m_C x_C = (m_A + m_C) x_B        (1)

where 

x = water content in the air

When hot air is mixed with cold air the result may be fog. When the mixing point is below the saturation line, parts of the moisture in the air condensates as small droplets floating in the air. The fog process can be expressed in the Mollier diagram as: 

• More about Mixing Air

Cooling and Dehumidifying Air

With a cold surface in the air, the air may be cooled and even dehumidified.

If the temperature on the cooling surface is higher than the dew point temperature - t_DP, the air cools along the constant specific humidity - x - line.

The cooling process of air can be expressed in the Mollier diagram as: 

It the temperature on the cold surface is lower than the dew point temperature - t_DP, the air cools in the direction of a point C as shown below.

Vapor in the air condensates on the surface, and the the amount of water condensated will be x_A - x_B.

• More about Cooling and Dehumidifying - Latent and Sensible Heat 

Humidifying , Adding Steam or Water

If water is added to air without any heat supply, the air condition will change adiabatic along a constant enthalpy line - h. The dry temperature of the air will decrease as shown in the Mollier diagram below.

If steam is added to the air, the air condition will change along a constant dh/dx line as shown above. 

• More about Humidifying Air