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 the Mollier diagram:

psychrometrics heating air

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 the psychrometric chart:

psychrometrics changing conditions heating

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.

psychrometrics changing condition mixing

The heat balance for the mixture can be expressed as

mA hA + mC hC = (mA + mC) h        (1)


m = mass of the air 

h = heat of the air 

The moisture balance for the mixture can be expressed as:

mA xA + mC xC = (mA + mC) xB        (1)


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 the Mollier diagram

psychrometrics changing condition fog

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 - tDP, air cools along a constant specific humidity - x - line.

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

psychrometricschangingconditionscooling.gif (3342 bytes)

It the temperature on the cooling surface is lower than the Dew Point Temperature - tDP, air cools in the direction of a point C as indicated below.

psychrometrics changing conditions cooling

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

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.

psychrometrics changing conditions steam water

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

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content

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