Dew Point and Psychometric Application

Instrument air is used for pneumatic equipment power and it supplied from the compressor and at outlet side should be treatment or drying it by dehumidifier.

Psychometric Chart.
The first two processes, vie sensible heating & cooling involve only a change in the dry bulb temperature, whereas the process if humidifying and dehumidifying involve a change in the specific humidity.

Thus when the state of the air moves from O to A or O to B, there is no change in the moisture content of the air. If the state changes from O to C or to D, the DBT (dry bulb temperature) remains constant. However most practical moisture-transfer process involves a change in temperature as well. The last four fundamental processes listed above involve both changes in temperature as well as in humidity.


We shall now consider calculations for process involving change in temperature and humidity.


Adiabatic Dehumidifier.
Adiabatic dehumidifying is based on the principle of absorption, vie capillary action. The vapor which is condensed at surface of the adsorbent is drawn into capillaries, thereby reducing the vapor pressure at surface causing a pressure gradient, and hence a mass transfer from the passing air stream to the adsorbing surface. As the capillaries get filled with water, the attraction decrease and the rate of humidification falls of.

Thermodynamically, an adsorption process is the opposite of the adiabatic saturation process as shown. As the air passing over the adsorbing surface, water vapor flows to surface through the air film, condense and release its latent heat which raises the adsorbent and air temperature. Thus the heat of condensation supplies the sensible heat for heating of air. The process is the reverse of adiabatic saturation process.
In actually practice, however, the process is accompanied with the release of heat called the heat of adsorption. This heat with adsorbents, such as silica gel and activated alumina is very large. Thus the sensible heat gain of air exceeds the loss of latent heat and process line 1-2 lies above the constant WBT (wet bulb temperature) line.

It is to be noted that after adsorption, the material becomes saturated and has to be reactivated by heating as in the case of hygroscopic solutions.

Non Adiabatic Dehumidifier.
When a moist air stream is cooed at constant mixture pressure to a temperature below its dew point temperature, some condensation of the water vapor initially present would occur. Moist air enter at stare 1 and flows across a cooling coil trough which a refrigerant or chilled water circulates.

Some of the water vapor initially present in the moist air condenses and saturated moist air mixture exits the dehumidifier section at state 2 and the heating process at state 3.

Arora P, REFRIGERATION AND AIR CONDITIONING, Tata McGraw-Hill Publishing Company Ltd. New Delhi 1984
Moran & Shapiro FUNDAMENTAL OF ENGINEERING THERMODYNAMIC John Wiley & Sons New York 1988