Note: Chemistry 2014 – Professional Knowledge 24
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Which of the following statements is correct? A. Since the heat transfer coefficient of plate heat exchangers is higher than that of shell and tube heat exchangers, less energy is required to achieve the same heating effect. B. Finned plate heat exchangers can be used in high-pressure conditions. C. The presence of non-condensable gases has no impact on the steam condensation heat transfer coefficient. D. Under the same operating conditions, changing a shell and tube heat exchanger from a single-pass design to a multi-pass design necessarily increases the overall heat transfer rate. ----------------------------------- My colleague answered B, and I have some doubts. A and C are definitely talking nonsense; there’s no need to discuss that. The debate centers on whether finned tube heat exchangers can be used in high-pressure conditions – this is similar to shell-and-tube heat exchangers, as is well known. But it’s unclear for finned plate heat exchangers. I don’t think the pressure in a plate heat exchanger can be very high. It seems fine to read D. With single-to-double variation and u doubling, K must be large. What are everyone's opinions?
Even with fins added to enhance heat transfer, this does not change its fundamental nature as a plate heat exchanger, and its pressure resistance remains low. When it comes to D, I really don’t dare to choose between those two characters. However, from the presence or absence of phase change to the inside and outside of the tube, even if the flow pattern changes, it indeed seems to increase in size.
Plate fin type B is not suitable for high pressure; incorrect. D is correct.
A. Since the heat transfer coefficient of plate heat exchangers is higher than that of shell and tube heat exchangers, less energy is required to achieve the same heating effect. B. Finned plate heat exchangers can be used in high-pressure conditions. C. The presence of non-condensable gases has no impact on the steam condensation heat transfer coefficient. D. Under the same operating conditions, changing a shell and tube heat exchanger from a single-pass design to a multi-pass design necessarily increases the overall heat transfer rate. I don’t think there is a correct option for this question. A: Plate heat exchangers generally have higher efficiency than shell-and-tube heat exchangers, but the second part of the statement isn’t easy to understand; to achieve the same purpose, the amount of heat transferred should be the same ； B. Plate heat exchangers are generally not used in high-pressure conditions; the maximum operating pressure stated in textbooks is 3 MPa, although studies indicate that pressures of up to 9 MPa can be achieved ； C. The presence of non-condensable gases causes the convective heat transfer coefficient to drop sharply ； D: counterflow in single pass, cross-flow in multiple passes. The counterflow logarithmic mean temperature difference is greater than that of cross-flow; therefore, the overall heat transfer rate for multiple passes might be lower than that for a single pass
This post was last edited by Higee on 2015-6-26 at 13:53. The operating pressure of a plate heat exchanger should not be too high, as this can lead to leaks. When changing from a single-pass to a double-pass configuration, K increases, but the average temperature difference decreases. Based on the formula Q=KAΔtm, it’s not appropriate to assert anything definite in this regard. However, from the perspective of the number of heat transfer units and efficiency, an increase in K results in an increase in NTU=KA/mcp; with CR remaining constant, the heat transfer efficiency e increases, and thus the total heat transfer rate Q=e*mcp*(temperature difference between the two fluids entering the exchanger) also increases. Therefore, the answer to this question is D
This post was last edited by yaojin911 on 2015-6-26 23:26. Regarding D, on page P225 of Chen Minheng’s version of \"Principles of Chemical Engineering, Volume 1\": \"When the flow rate is constant, the more tubes or shells there are, the greater the heat transfer coefficient, which is beneficial for heat transfer.\" ” There’s not much to say about option B; page 336 of the manual states that the maximum operating pressure for plate exchangers is 2.8 MPa, which clearly makes it unsuitable for high-pressure conditions. AC, needless to say.
I thought I was so smart; it turns out it’s because the boss writes well...
I saw that one fellow diver answered B, and I thought it was incorrect; but seeing that everyone here shares the same opinion as me, I feel relieved.