Changing Air Condition by Heating, Cooling, Mixing, Humidifying or Dehumidifying Processes

Basic air condition processes - heating, cooling, mixing, humidifying and dehumidifying by adding steam or water - psychometric diagrams and the Mollier charts

Most common air condition processes are

• heating air
• mixing air
• cooling and dehumidifying air
• humidifying air by adding steam and/or water

Heating Air

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

Heating 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 can also be expressed in a psychrometric chart as:  

• Calculating enthalpy change and temperature rise when heating air.

Mixing Air of different Conditions

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

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

The heat balance for the mixture 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 mixture can be expressed as:

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

where 

x = specific humidity in the air

When hot air is mixed with cold air it results in fog if the mixing point is below the saturation line of the air. When there is fog parts of the moisture in the air are condensated to small droplets floating in the air. The fog process can be expressed in a Mollier diagram as

• More about Mixing Air

Cooling and Dehumidifying Air

With a cold surface is exposed to moist air the air close to the surface may be cooled down below the saturation line. Humidity in the air close to the surface will condensate on the surface and the air will be dehumidified.

If the temperature on the cooling surface is higher than the Dew Point Temperature - t_DP, air cools along a constant specific humidity - x - line.

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

It the temperature on the cooling surface is lower than the Dew Point Temperature - t_DP, 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 changes adiabatic along a constant enthalpy line - h. The dry temperature of the air decreases as shown in the Mollier diagram below.

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

• More about Humidifying Air

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