An afternoon case study on evaporation calculations
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The problem states that the boiling point increase is 9 degrees; knowing the temperature of the secondary steam, it’s possible to calculate the heat load. The heat transfer coefficients for both the inside and outside of the tubes are given, so it’s possible to determine the heat exchange area. The issue is that the temperature of the liquid exiting the system is unknown – how can it be calculated?
It should be simple addition and subtraction; this problem seems easy, but there’s a trick in it that has probably caused many people to fail. . . .
The boiling point of the solution is 81+9=90
I’m also 90; I’m so nervous, and with such little time available, there’s no time to think carefully. I took a look at the question: the boiling point of a solution can only be related to the temperature of its vapor. When a pure liquid boils, the temperature of the gas and the liquid is the same. In the case of a solution, the gas should still maintain the same evaporation temperature, but since the concentration of the solution changes, the boiling point rises. I thought for five seconds, then did it; and luckily there was an answer, so I still had time to think carefully about the details
The book states that the effective temperature difference = t + boiling point elevation value
To include this temperature rise, I added it
The boiling point of the finished solution is the temperature of the secondary steam plus the boiling point elevation. This increased value already takes into account the effects of three factors.