Question 13.10: Sensible Cooling Process on Psychrometric Chart Moist air is......

First, we must determine whether moisture condenses. In terms of psychrometric variables, we need to determine whether the final temperature is above or below the dew point. By referring to the SI psychrometric chart (Figure 13.7), we observe that the dew point temperature T_{dp} = 19°C. Therefore, moisture does not condense, and this is a sensible cooling process with no latent heat removal. The problem can be solved using the properties of moist air as plotted in Figure 13.10 shown by points 1–2.

Second, we note that the humidity ratio will not change since no moisture condensation has occurred. Therefore, the humidity ratio at the end of the process will be identical to that at the start, 0.014  kg_{w}/kg_{a}.
Third, the relative humidity will change since the relative ability of the cooler air to hold moisture is less than that of the warm air at the start of the process. Since this process is a constant humidityratio process, it is plotted as a horizontal line on
the psychrometric chart. One moves to the left from the initial point until the intersection with the T_{db} = 30°C vertical line. At this point, we read a relative humidity value of 53%.
Comments
The heat removed from the air during this process can be read from the psychrometric chart by finding the enthalpy at the two endpoints.
Alternatively, the online HCB software can be used to provide more accurate results.