THE PSYCHROMETRIC CHART
- EVAPORATIVE COOLING AS READ ON THE PSYCHROMETRIC CHART -

Evaporative cooling takes place along lines of constant wet bulb temperature or enthalpy. This is because there is no change in the amount of energy in the air. The energy is merely converted from sensible energy to latent energy. The moisture content of the air increases as the water is evaporated which results in an increase in relative humidity along a line of constant wet bulb temperature. By taking a set of conditions and applying the process of evaporative cooling to them we can get a clearer picture of how this process happens.

 

 

CONDITIONS OF AIR ENTERING EVAPORATIVE COOLER (POINT A ON CHART)
Dry bulb temperature = 31 C
Wet bulb temperature = 20 C
Relative humidity = +  35 %
High sensible heat (energy)
Low latent heat (energy)

EVAPORATIVE COOLING
Evaporative cooling takes place from point
A up a constant line of wet bulb temperature. If we say that the evaporative cooler is 90% efficient then the process will take place over 90% of the distance from point up a constant line of wet bulb temperature. If we say that the evaporative cooler is 90% efficient then the process will take place over 90% of the distance from point A up towards the saturation line (100% relative humidity). This will take us to a point B on the wet bulb temperature line. Point B represents the conditions of the air leaving the evaporative cooler.

CONDITIONS OF AIR LEAVING EVAPORATIVE COOLER (POINT B ON CHART)
Dry bulb temperature = 21 C
Wet bulb temperature = 20 C
Relative humidity = + 90 %
Low sensible heat (energy)
High latent heat (energy)

FINAL CONDITIONS OF AIR IN AREA BEING COOLED
The final conditions of the air in the room or area being cooled by the evaporative cooler will depend on the heat loading in the area. If the area is not effected by any other heat conditions then the condition of the air in the room will be very close to the conditions of the air leaving the evaporative cooler. If there are any additional heat loadings in the room (e.g. large numbers of people or machinery) then the temperature of the air leaving the evaporative cooler will tend to rise by a few degrees. This would be represented by a horizontal movement to the right from the condition
B to condition C on the psychrometric chart. B to condition C on the psychrometric chart.