2012 Day 1 Morning Question 70
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70. After being dried at high temperatures, the materials have reached the specified moisture content; in order to package them, they must be cooled further. However, when selecting the cooling temperature, it is necessary to prevent the materials from absorbing moisture and becoming damp again after cooling. The water vapor partial pressure in the air at the dryer outlet has been measured to be 5.10 kPa. Please determine which of the following cooling temperatures will not cause rehumidification and are therefore suitable for use A: 40℃, saturated water vapor pressure of 7.375 kPa; B: 35℃, saturated water vapor pressure of 5.623 kPa; C: 30℃, saturated water vapor pressure of 4.242 kPa; D: 25℃, saturated water vapor pressure of 3.167 kPa. The forum’s answer is AB, but I think it should be CD. Since the partial pressure of water vapor at the outlet is 5.1 kPa, the saturated vapor pressures of both A and B are greater than 5.1. In other words, under the AB operating condition, the dry gas can still absorb water without reaching saturation, and rehumidification will occur. Therefore, CD must be chosen.
Refer to this post: http://bbs.hcbbs.com/thread-1232133-1-1.html
Using the concept of relative humidity to explain: if the relative humidity is less than 1, it means that the steam still has the ability to absorb water; if it is equal to 1, it means it is saturated and can no longer absorb water. Any excess water must then be absorbed by the material itself. Therefore, AB is correct