★Specialized Examination for Chemical Engineering ★ — One question per day (2015-11-27) — Points are awarded for participation
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This post was last edited by zshiwei2008 on 2015-11-30 at 10:02. Points system for the daily question: Participation – 1~3 points; Correct answer – 5 points; In-depth analysis – 5~10 points. The answer is provided the next day. Due to limited capabilities, I am unable to provide more in-depth explanations for each question; therefore, on the second day, in addition to offering reference options, I will also remove the restriction on reading replies. I hope that all candidates can learn from one another and improve together! Daily Question (2015-11-27): 1. In a single-pass shell-and-tube heat exchanger, the fluid flows in a forced turbulent manner through the tube side. If the physical properties of the fluid remain unchanged and the flow rate is increased to twice its original value, the convective heat transfer coefficient will be approximately equal to what multiple of the original convective heat transfer coefficient? A. 1 B. 1.74 C. 2 D. 4 2. For a fluid with a constant mass flow rate that flows under forced turbulent conditions in the tube side of a single-pass shell and tube heat exchanger, if the flow rate and physical properties of the fluid remain unchanged, as well as the total number of tubes in the heat exchanger, then when this shell and tube heat exchanger is converted to a four-pass design, its convective heat transfer coefficient will be approximately a multiple of the original convective heat transfer coefficient multiplied by ( ). A. 1.00 B. 1.74 C. 3.03 D. 4 Answer: These two questions test the same concept: when a fluid flows turbulently in a circular pipe, it obeys the Dittus-Boelter relationship, namely Nu=Re^0.8Pr^n. This implies that the convective heat transfer coefficient is proportional to the velocity raised to the power of 0.8. Therefore, the answer to the first question is 20.8 = 1.74, and the answer to the second question is 40.8 = 3.03
For both questions in BC, the Dittus-Boelter correlation is used; the convective heat transfer coefficient α is proportional to u^0.8
1. When the flow rate doubles, the speed also doubles; 2*0.8 cubic meters equals 1.74. 2. Four passings are equivalent to a four-fold increase in flow velocity; 4*0.8 equals 3.03. The formula is as follows
B C Under other conditions remaining constant, the convective heat transfer coefficient is proportional to the 0.8th power of the flow velocity
For B and C, where forced turbulence occurs within the pipe, the heat transfer coefficient is proportional to the 0.8th power of the flow velocity; the values are 2^0.8=1.74 and 4^0.8=3.03 respectively
In a single-pass shell-and-tube heat exchanger, the fluid undergoes forced turbulence in the tube side. If the physical properties of the fluid remain unchanged and the flow rate is increased to twice its original value, its convective heat transfer coefficient will be approximately a multiple of the original convective heat transfer coefficient multiplied by (B). For a fluid with a constant mass flow rate that undergoes forced turbulence in the tube side of a single-pass shell and tube heat exchanger, if the flow rate and physical properties of the fluid remain unchanged, as well as the total number of tubes in the heat exchanger, then when this shell and tube heat exchanger is converted to a four-pass design, its convective heat transfer coefficient will be a multiple of the original convective heat transfer coefficient, and it will be close to (C).
1. B a1/a2 = (u1/u2) ^0.8 2. D