How to solve Question 22 in the morning session of the 2016 real exam question

This post was last edited by Higee on 2017-6-7 23:58. This question is much more confusing than Question 18 from 2011; those interested can take a look at that question. At first glance, this problem seems to require using Q=KAΔtm to determine the heat transfer area, and then calculating the number of tubes using A=nπdl. This requires calculating K; based on the given conditions, the heat resistance due to condensation, the heat resistance of the tube walls, and the heat resistance caused by fouling can be ignored, i.e., Ki=αi. The question below is how to determine the convective heat transfer coefficient αi for the material inside the pipe; the problem specifies a range for Re that falls within the transitional flow and turbulent flow regions. This naturally leads one to think of the formula for calculating the convective heat transfer coefficient applicable to transitional flow and turbulence, namely the Gnielinski formula, which is particularly complex in form. Since this formula involves the Darcy friction coefficient, and the problem provides the value of pressure drop, it seems logical to use this approach for calculation. If this is the case, there are two problems: ① The Re calculation involves the unknown variable u; u must be present in f as well. Moreover, it is very difficult to determine u using the pressure drop formula in the logarithmic terms (a nonlinear equation that requires trial and error!) )② Pr, a quantity necessary for calculating the heat transfer coefficient α, cannot be determined as the material’s thermal conductivity λ is not provided in the problem! Therefore, this problem is essentially not a heat transfer issue, but rather a fluid flow issue. The solution process is as follows: The Reynolds number Re is calculated as Re = duρ/μ = 0.02×u×980/0.66e-3 = 29696.97u. The Darcy friction factor f can be determined using the Brashausen formula (since the roughness value is not given, the calculation is done for a smooth pipe). Thus, f = 0.3164/Re^0.25 = 0.3164/(13.127u^0.25). By substituting this value into the pressure drop formula Δp = f×(l/d)×ρu^2, the value of u is found to be 0.943 m/s. Using the energy balance equation Q = ρ×n×πd^2/4×u×cp×Δt, we have 2500 = 980×n×0.785×0.02^2×0.943×4.16×20; from this, n = 103.55 ≈ 104. The correct answer is C

Could you share the real exam questions from 2016?

This post was last edited by panyb15715 on 2017-6-8 at 17:23. I also downloaded it from this forum; it’s not complete – there are only a few questions scattered here. Do you need it? http://bbs.hcbbs.com/thread-1614712-1-1.html

2016 Real Exam Questions for the Chemical Engineering Major (Morning Part, Day 2)

Everyone is welcome to discuss this question further. I believe the key to solving it lies in establishing a relationship between λ and Re. The second poster simplified the situation by assuming a smooth pipe and used the formulas applicable to smooth pipes; however, the question does not state that such simplification is permissible. The third poster used formulas for rough pipes, but I was unable to find these formulas in either the textbooks on principles of chemical engineering or those used in professional exams related to chemical engineering. The most common method in principles of chemical engineering is to use friction coefficient charts (i.e., Moody’s friction coefficient chart), but this requires knowledge of the relative roughness, which is not provided in the question. According to page 176 of the association’s textbook, the absolute roughness of seamless steel pipes is 0.1–0.2 mm; by referring to the friction coefficient chart based on this value, it seems more reasonable for λ to be around 0.028, as calculated by the third poster. As mentioned on the 2nd floor, using the trial-and-error method to solve equations on the 3rd floor seems unreasonable; therefore, what is a more appropriate way to solve this problem? I hope for a detailed discussion on this.

Personally, I think the solution approach on the second floor is more reasonable; the Tianjin University textbook also specifies the range of Re values for which this friction coefficient formula should be applied, and that is likely what the question setter had in mind.

The last edit to this post was made by Higee on 2017-6-9 at 10:48. The formula for rough pipes in the third floor refers to one proposed by renowned Chinese chemical engineering experts such as Gu Yuzhen; for more details, see Equation 1-95 on page 47 of He Chaohong’s Volume 1 of Principles of Chemical Engineering

This question definitely requires the formula for smooth pipes; there’s no need to worry about it. The range is clearly specified, so the answer is D

Let’s discuss it further; the answer is a hydraulically smooth pipe, using Poiseuille’s empirical formula, and the calculated value is C