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Changing the State of Air 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

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The 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 visualized in the Mollier diagram as:

psychrometrics - heating air

The heating process moves the air condition from state A to state B along a constant specific humidity - x - line. The specific heat supplied - dH - to the air is as indicated in the diagram.

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

psychrometrics - change state of air by heating

Mixing Air of different States

When mixing air of state A and state C - the mixing point will be on the straight line between the two states - in point B.

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

psychrometrics - change state of air by mixing

A heat balance for the mixture can be expressed as

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

where 

m = mass of the air (kg)

h = heat of the air (J/kg)

The moisture balance for the mixture can be expressed as:

mA xA + mC xC = (mA + mC) xB                             (2)

where 

x = specific humidity in the air (kgh2o/kgdry_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 condense to small droplets floating in the air. The "fog process" can be expressed in the Mollier diagram as

psychrometrics - changing state of air and creating fog

Cooling and Dehumidifying Air

With a cold surface 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 then condensate on the surface and the air as a whole 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 as

psychrometrics - change state of air by cooling

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 state of air by 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 state of air by adding steam or water

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

When adding saturated steam at atmospheric pressure the temperature rise is very small - in general less than 1oC. For practical purposes the process of adding saturated steam at atmospheric pressure approximates the horizontal temperature line in the Mollier diagram.

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  • Engineering ToolBox, (2003). Changing the State of Air by Heating, Cooling, Mixing, Humidifying or Dehumidifying Processes. [online] Available at: https://www.engineeringtoolbox.com/air-heating-cooling-mixing-d_26.html [Accessed Day Mo. Year].

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