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Two stage cooling is a process where by the air
is first cooled sensibly before it is put through the process of evaporative cooling to
below its ambient wet bulb temperature. By taking a set of conditions and applying the
process of 2 stage cooling to them we can get a clearer picture of how 2 stage cooling
works.
CONDITIONS OF AIR ENTERING COOLING SYSTEM (POINT A ON CHART)
Dry bulb temperature = 31° C
Wet bulb temperature = 20° C
Relative humidity = + 35 %
1st STAGE : SENSIBLE COOLING
Sensible cooling is usually achieved by letting the air flow over a coil
that carries water cooled by a conventional cooling tower. The air is thus cooled sensibly
which is represented as a horizontal movement to the left (point A to point B) on the
psychrometric chart. The air is cooled to below its ambient wet bulb temperature. A to point B) on the
psychrometric chart. The air is cooled to below its ambient wet bulb temperature.
2nd STAGE : EVAPORATIVE COOLING
Evaporative cooling takes place from point B up a constant line of
wet bulb temperature. If we say that the evaporative cooler is 80% efficient then the
process will take place over 80% of the distance from point up a constant line of
wet bulb temperature. If we say that the evaporative cooler is 80% efficient then the
process will take place over 80% of the distance from point B up towards the
saturation line (100% relative humidity). This will take us to a point C on the wet bulb temperature line. Point C represents the conditions of the air leaving the
evaporative cooler.
CONDITIONS OF AIR LEAVING EVAPORATIVE COOLER (POINT C ON CHART)
Dry bulb temperature = + 20 ° C
Wet bulb temperature = + 18° C
Relative humidity = + 85 %
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 C to condition D on the
psychrometric chart.
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