ADIABATIC SATURATION

Specific humidity or the relative humidity of an air – water vapour mixture can be measured in principle with the help of a device called the adiabatic saturator (see figure35.2).

The air – water vapour mixture flows steadily into the device. The or of the incoming mixture has to be determined.

The air – water vapour mixture leaves the adiabatic saturator as saturated mixture. Let the device be insulated so that there is no energy loss.

Since the unsaturated air – water vapour mixture is sweeping over a layer of liquid water, some water evaporates. The energy needed for the evaporation comes from the air mixture. Hence, the air – water vapour mixture leaves the adiabatic saturator at a temperature lower than that of the entering air. As the air leaving the adiabatic saturator is in equilibrium with the liquid water, the temperature of the liquid water is equal to the temperature of the saturated air – water vapour mixture.

Mass balance for air

(35.16)

Mass balance for water

(35.17)

Energy balance

(35.18)

Dividing (35.17) by

	(35.19)

Dividing (35.18) by

or

(35.20)

The quantity

Another term, has to be estimated properly.

Here refer to the enthalpy of saturated water vapour at temperature and refers to the enthalpy of the saturated liquid at temperature or ) water pool temperature at steady state is same as )

Therefore

(35.21)

enthalpy of the saturated water vapour at state 1

enthalpy of the saturated liquid water at temperature or ,

does not depend on the temp at which the liquid water enters the device (make-up water temperature)

The adiabatic saturation temperature depends only on the conditions of the entering fluid.

Finally

(35.22)