Annotation real questions 2013 knowledge morning questions 32
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(2013-1) 32. Which of the following statements about the characteristics of heat exchangers is correct? A. The higher the fluid flow rate, the smaller the required heat transfer area. B. The higher the cold fluid outlet temperature, the greater the average driving force. C. The logarithmic average driving force is always smaller than the arithmetic average driving force. D. Counterflow operation is definitely better than parallel flow. A and C both seem to be correct.
I think option A is wrong. Compared with natural convection, a high flow rate increases the heat transfer coefficient. If the flow rate is too fast, the heat transfer coefficient of the fluid is not the main factor. At this time, the fouling thermal resistance or film coefficient will play a key role. For example, if the fluid flows instantaneously at an extremely high speed, it is estimated that the heat loss is very small and there will be no heat transfer.
Yeah, that makes sense. I should choose C. Thank you.
Option D is too absolute. Tianda textbook 165 is also the original words: Except for some processes where co-current operation is appropriate when the temperature of the fluid is limited, the heat exchanger should use counter-current operation as much as possible. This sentence means that in some cases, parallel flow has advantages over countercurrent.