Question

Air at 25 C and 750mmHg has a dew point of 16C. If you want to remove 50% of the initial moisture in the air (at a constant pressure of 750 mmHg), to what temperature should you cool the air?

Answer 1

To remove 50% of the initial moisture content, we need to make the specific humidity half of the previous value.

Refer to the following Psychometric chart.

To obtain the initial point,

Dry a vertical line from X-axis at Dry bulb temp 25 C.

Draw a horizontal line corresponding to the dew point temperature 16 C towards the curve on the left.

Both lines are indicated by red lines. Point A is the required point.

Initial specific humidity - extend the line to the Y-axis on right to obtain specific humidity ratio.

H₁ = 12 g H2O/kg dry air.

We look for half of it. H₂ = 6 g H2O/kg dry air.

The temperature will be be correspoonding to the 100 % Relative humidity, since water is removed only once the RH=100 %. Thus draw a horizontal line all to the 100% RH curve to get the temperature here.

Here, Temperature = same as Wet bulb temperature = 6 Celsius

Dehumidification occurs along the blue path. i.e. first reaching 100% Relative humidity at same dew point temp, and then cooled .

ANSWER = 6 degree celsius