Sensible Cooling And Sensible Heating

Look at any psychrometric chart, and the answer will become clear.

Dear Friend,

One observation - "passing through cooling coils and hot water coils in AHU" appears to be NOT CORRECT - as the Fluid/water to be cooled or heated has to come in contact with the air, and this is not possible while the fluid/water will not come in to contact with air if coils are used.

If the air is coming in contact with fluid/water to be cooled or air is heated, then "relative humidity increases for sensible cooling and decreases for sensible heating" is as stated by you is correct, and if so:

Your question 1st Part - "How relative humidity increases for sensible cooling"

The answer is Simple. When the Relatitive Humidity of the air passing through the Hot Water is increasing, as the Hot water is absorbing Vapour from the Hot Water, it removes a Heat of 539.95 KCal./Kg of Water Vapour absorbed by the air or 2257 KJ/Kg. of Water Vapour absorbed by the air - and this Heat is known as LATENT HEAT (of Vaporisation) at Atmospheric Pressure which is 1.034 Kg/CM^2 or 14.696 PSI, or 1.013 bar.

Your 2nd part of the question is Relative Humidity "decreases for sensible heating" The answer is, It is just the REVERSE of the above i.e, when HEAT IS ADDED to the Air, the RELATIVE HUMIDITY WILL FALL.

In both the cases of COOLING or HEATING, the ABSOLUTE HUMIDITY of the AIR WILL REMAIN SAME, ONLY THE RELATIVE HUMIDITY WILL VARY, depending upon the Temp. of the Air. Once the Air is SATURATED with WATER VAPOUR, NO COOLING OF WATER WILL TAKE PLACE.

Refer Thermo Dynamics Book - Chapter PSYCHROMETRY.

DHAYANANDHAN.S