The state of a given atmosphere is represented by a point on the chart, known as the status point. If any two of the three commonly available characteristics DBT, WBT and RH are known, the others can be read from the chart. Psychrometric processes, ie, changes in the condition of the atmosphere, can be represented by the movement of this status point in the following ways:
Heating or cooling:
The addition or removal of heat, without any change in the moisture content (AH), resulting in the change in DBT. The status point will move horizontally to the left (cooling) or to the right (heating).
Note that while the AH does not change, the change in temperature means the relative humidity (RH) changes. It increases if the temperature lowers and vice versa.
Dehumidification by cooling:
If, as a result of cooling, the point moving towards the left reaches the saturation line, some condensation will start. The DBT corresponding to this point is referred to as the dew-point temperature of the original atmosphere. If there is further cooling, the status point will move along the saturation line and condensation will occur. The reduction in the vertical ordinate (on the AH scale) represents the amount of moisture precipitatied, ie, condensed out. This process will reduce the absolute humidity, but will always end with 100% RH.
Adiabatic humidification (evaporative cooling):
If moisture is evaporated into an air volume without any heat input or removal (this is the meaning of the term 'adiabatic'), the latent heat of evaporation is taken from the atmosphere. The sensible heat content - thus the DBT - is reduced, but the latent heat content is increased. The status point moves up and to the left, along a WBT line. This is the process involved in evaporative cooling.
Note that by this process, the relative humidity is increased. It increases only until it hits the saturation line, at which it becomes 100%. Beyond it there is no decrease in sensible temperature. This is the reason why during hot and humid months, evaporative cooling is ineffective and uncomfortable.
Adiabatic dehumidification (by sorbents):
If the air is passed through a chemical sorbent material (eg, silica gel), some of the moisture is removed and the latent heat of evaporation is released. There will be an increase in sensible heat content, thus in the system (ie, if the process is adiabatic), the state point will move down and towards the right along an enthalpy line.
This process, in effect is the reverse of the previous one.
If two air streams are mixed, having:
1 and m2,
-mass flow rates m
-dry bulb temperatures t1and t2,
-enthalpies H1and H2,
the result will be:
1t1 + m2t2 = [m1 + m2]t3,
m1H1 + m2H2 = [m1 + m2]H3
3 = (m1t1 + m2t2) / m1 + m2
H3 = (m1H1 + m2H2) / m1 + m2
The psychrometric chart can be used to establish the value of t3 and H3. The two state points are connected by a straight line, which is then divided in inverse proportions of m1 and m2. If the mass flow rate m1 is the greater, the resulting point P will represent the state of the combined air stream.
NOTE : This psychrometric process is more relevant for air conditioning calculations. For architectural purposes, suffice to understand the mixing principle involved.
Now that we understand the psychrometric chart and psychrometric processes, next we will learn how to plot climatic data and what it means.