2014 Knowledge Morning Question 24
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24. Which of the following statements is correct? (A) Since plate heat exchangers have a higher heat transfer coefficient than shell and tube heat exchangers, less energy is required to achieve the same level of heating. (B) Finned plate heat exchangers can be used in high-pressure conditions. (C) The presence of non-condensable gases has no effect 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. Please explain each of these points. Another question: A liquid in a tank is heated by saturated steam inside a jacket. The inner diameter of the jacket is 2 m, the wall thickness is 2 mm, the material is carbon steel, and λm = 46.5 W/(m2·℃). The heat transfer coefficient for steam condensation is α = 11630 W/(m²·℃), and the heat transfer coefficient between the solution and the wall is α = 787 W/(m²·℃). If the thermal resistance due to fouling on both sides of the wall is ignored, what is the total heat transfer coefficient? (A) 654 (B) 784 (C) 6204 (D) 5814 I calculated 649.5, but there’s no answer!
For the first question, ACD is clearly wrong; A doesn’t even know what it’s talking about. As far as I remember, plate exchangers should not be used at high pressures. For the second question, the result is only 714; there is no answer.
Which of the following statements is correct? (C) The presence of non-condensable gases has no effect on the steam condensation heat transfer coefficient
24. 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 level of heating; the amount of heat should be the same. (B) Finned plate heat exchangers can be used in high-pressure conditions. High pressure generally refers to pressures greater than 10 MPa; pressures of a few MPa are not considered high pressure strictly speaking. (C) The presence of non-condensable gases has no effect on the steam condensation heat transfer coefficient. There must be an effect; otherwise, why would there be a non-condensable gas extractor connected to the surface cooler? (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. This is a measure to enhance heat transfer; by switching from a single pass to multiple passes, the flow velocity increases and the heat transfer coefficient per pass also increases, thereby raising the overall heat transfer coefficient. Another question: A liquid in a tank is heated by saturated steam inside a jacket. The inner diameter of the jacket is 2 m, the wall thickness is 2 mm, the material is carbon steel, and λm = 46.5 W/(m2·℃). The heat transfer coefficient for steam condensation is α = 11630 W/(m²·℃), and the heat transfer coefficient between the solution and the wall is α = 787 W/(m²·℃). If the thermal resistance due to fouling on both sides of the wall is ignored, what is the total heat transfer coefficient? (A) 654 (B) 784 (C) 6204 (D) 5814 1/(2.002/787/2+2*2.002/46.5/2002+1/11630)=784
11630>>787 The overall heat transfer coefficient is close to that of the one with a lower heat transfer coefficient, :) Select the answer quickly
It’s a shame that with this choice, the answer is wrong.
Question 24: The wording of the question isn’t very precise. Plate heat exchangers have a high heat transfer coefficient; under the same operating conditions, they require less heat exchange area. Their volume is much smaller than that of tube heat exchangers, resulting in lower heat loss. Indeed, less energy is required to achieve the same level of heating. However, the intention of the questioner is to get answers with the same amount of heat. B depends on how high voltage is defined. The impact of non-condensable gases is severe, without any doubt. D is not necessarily the case; it depends on who is exchanging heat with whom. If neither side undergoes a phase change, it is true that the heat transfer coefficient on the pipe side increases, but the average temperature difference decreases, and thus the overall heat transfer rate may not increase. The only optional answer is B. For the other question, ignore the wall thermal resistance and the jacket diameter, and treat it as a flat wall: 1/(1/787+1/11630) = 737. What is the thermal resistance of 2mm carbon steel? Taking this into account, the heat transfer coefficient will decrease slightly, but not by much. There is no correct answer to choose.
The original poster must have remembered the question wrong. In the version I saw, the wall thickness was 8mm, which is logical; there can’t be a heating tank with a wall thickness of only 2mm, right?
Bro, your formula doesn’t match your calculation result; moreover, the formula itself is incorrect. Please check it
This method of judgment is incorrect; only a preliminary decision can be made as to which one to choose between A and B
Answer: A. The question is incorrect; the wall thickness is 8 mm, and it is not the inner diameter of the jacket but rather the inner diameter of the container, which is 2 m. For this multiple-choice question, assuming that the inner and outer diameters are approximately equal, the calculation yields: 1 ÷ (1/787 + 0.008/46.5 + 1/11630) = 654