1. Sensible Heating:-
By sensible heating, we mean adding heat to air whereby the entire heat added goes to raise the temperature of the air. It is obvious that in such a process there is no change in the moisture content of the air.
In other word, during sensible heating process the air retains a constant moisture content and accordingly, its condition will move on a horizontal line corresponding to its constant moisture content. Since heat is being added during such process, its enthalpy also rises. Therefore, during the heating process the wet bulb temperature of the air will also rise, because as we have already seen, it is the wet bulb temperature lines which are identified as constant enthalpy lines.
2. Addition of Moisture:-
Likewise, if moisture is somehow or the other added to the air without adding any sensible heat, the process would be represented by a vertical line corresponding to its dry bulb temperature. In this case also,since the moisture added carries with the latent heat of vaporization of water, the heat content of the air also rises and hence it wet bulb temperature also rises.
3. Heating and Humidifying :-
If heat is added so that part of it goes to raise the temperature and the remaining part goes to vaporize water and add it to the air, such a process is called heating and humidifying.
4. Cooling and Dehumidifying :-
Cooling and dehumidifying is just the reverse of heating and humidifying. On a psychrometric chart such a process will also be represented in the same manner as for heating and humidifying, the only difference being the arrows representing the direction of movement of conditions would be just reverse.
5. Evaporative Cooling:-
Evaporative cooling is the process by which air is simply subjected to a spray of re-circulated water just as in the experiment described earlier, the only difference being, we do not provide an infinite number of spray banks as in the experiment. The chamber with the banks of spray is called an Air Washer. Air so subjected would of course tend to get saturated and change out at a temperature equal to its wet bulb temperature.
However, since we do not provide adequate number of spray banks to completely humidify, the air comes out not a 100% humidity but somewhat lower than that. Needless to say, since this process is adiabatic, the air has constant enthalpy throughout the process and hence its condition moves along the line representing its wet bulb temperature.
Psychometry as Applied to Air-Conditioning:-
It now remains for us to study psychometry as applied to air conditioning process. We will only see for the present what the heat load form is like and also the various sections into which it is divided. It is only after you understand this that you will be in better position to understand psychometry as applied to air conditioning.
When a space is maintained at a temperature below the atmospheric temperature surrounding the space, then there is a transfer of heat from outside into the conditioned area, which tends to raise the inside temperature unless this heat is removed as fast as it enters this space. Then you have heat or any other appliances which may be in the space. All such heat which are either transmitted into the room or generated from within due to occupants and appliances which which tend to raise the inside temperature are termed as room sensible heat.
In this manner, the occupants within the room also release moisture from their body into the room. There may be other sources inside the conditioned area which add up more moisture into the atmosphere. If the space has not only to be maintained at a particular temperature, but also to be held within certain limits of relative humidity, then it is necessary that such moisture gain inside the room should also be removed just as rapidly.
By removal of moisture what we mean is condensing this moisture from the air and discarding it outside. For condensing the moisture, to remove the latent heat of vaporization of water.
In heat load, there is one more source which contributes to the room sensible and room latent heat loads. This is infiltration of fresh air into the conditioned space and bypass of certain amount of fresh air that is normally taken into the system through the air handling apparatus.
The form is designed so that the room sensible heat, latent heat and the additional load due to outside air, not forming part of room load are all calculated separately.
Term “Bypass” means, the air which is not coming in contact with the cooling coil. For removing sensible heat and latent heat at the same rate at which they are being gained within the conditioned space, conditioned air is admitted within this space at a predetermined temperature and humidity condition such that this air would absorb the room sensible and room latent heat loads and in the process attain a final condition which is exactly equal to the condition to be maintained in the room.
Problem becomes bit more complicated because in every cooling coil there is always a small percentage of the total cfm which escapes totally untreated. When outside air taken into the system bypasses the coil, it will tend to raise the room temperature and humidity conditions above the desired level. It is, therefore, necessary to take into consideration the effect of bypass right at the time of making the heat load calculations.