Note: Real exam questions from 2012, Knowledge section, afternoon session, question 51
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51. Regarding the design of shell-and-tube heat exchangers and instrumented processes, which of the following statements is correct? A: For normal cold liquids, flow is from bottom to top; for hot fluids, flow is from top to bottom. B: When steam is used as the heat source, it should be introduced from above while the condensate is discharged from below. C If both media involved in heat exchange are liquids, counterflow is more advantageous than co-flow. D Except for U-tube heat exchangers, media that tend to cause scaling or are corrosive should flow through the shell side. AB is correct, D is incorrect. Is C okay? According to the analysis above, counterflow is superior to co-flow; therefore, heat exchangers in production are mostly operated in counterflow mode. However, under certain production process requirements, if there are restrictions on the temperature of the fluid – for example, it is specified that a cold fluid must not exceed a certain temperature when heated, or that a hot fluid must not drop below a certain temperature when cooled – then co-current operation is preferable. ”
C is wrong; the exception should be cases where special requirements exist regarding the manufacturing process! What is stated in the exam must be absolute – avoid any options that are ambiguous!
Personally, I think C is correct; the examiner is testing the issue of the magnitude of the temperature difference.
Although that is the case, these are the exact words from page 387 of Tian Da Fu* Tutorial, word for word. So it’s better to choose ABC