Once a technician has a grasp of the basic structure of the the psychrometric chart (understanding that there are six separate sets of lines as we discussed in previous segments here), the next important concept to know is that the chart can be used to evaluate the performance of a comfort cooling system in regard to the removal of sensible and latent heat.

Sensible heat, since it is simply defined as heat that can be measured, relates to the change in the temperature of an air sample. Latent heat, since it is defined as heat that brings about a change in state but not a change in temperature, is related to change in the level of water vapor in an air sample.

One simple way to get a grasp on the concept of the removal of these two kinds of heat and how the psychrometric chart can illustrate the level of a performance of a system relative to comfort via both temperature and relative humidity is to consider something known as State Point, shown in Figure One.

Figure One

Figure One

The two temperatures plotted in this example are 80-degrees dry bulb (red line) and 67-degrees  wet bulb (blue line), and the point at which the two lines converge is just above the 50% relative humidity line.

The idea to keep in mind here is that once this baseline is established, the psychrometric chart can be used to illustrate the amount of sensible and latent heat that is being removed in a given situation. If the level of sensible heat removal is too high and the removal of latent heat is too low, that won’t result in maximum comfort. (See Figure Two below)

Figure Two

Figure Two

In this example we’ve achieved a significant drop in dry bulb temperature all the way from 80-degrees down to 60-degrees, and from the state point of 67-degrees wet bulb down to 56.3-degrees, but what we haven’t accomplished is a balanced removal of heat. This is indicated by the fact that our relative humidity is now 80%.

In Figure Thee, though, we have a more balanced removal of heat that will result in a more comfortable situation.

Figure Three

Figure Three

In this example we have only dropped the dry bulb temperature from 80-degrees down to 75-degrees, and the wet bulb temperature from 67-degrees down to 61-degrees, but in the process, we have accomplished a more balanced removal of heat because our relative humidity is now at 45%.


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