Standards for process calculations of heat exchangers – please ask wiseboy to come in and help clarify this!
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This post was last edited by csldg on 2013-5-16 at 23:17. When performing calculations related to pumps, pipes, tanks, etc., there are relevant standards that contain standard formulas. I have been studying heat exchangers recently; however, although many books discuss the methods for calculating heat transfer coefficients and pressure drops, none of the standards specify which formula should be used for these calculations. We know that there are many methods for calculating the heat transfer coefficient, and different formulas yield different results. Which formula should be used in engineering design? I checked the tema standards, and they don’t mention any process formulas; maybe they do exist, but I don’t know where to find those standards. Could everyone let me know, or what methods do you use in your design?
Take a look at the heat exchanger design manual: http://bbs.hcbbs.com/viewthread.php?tid=4633&highlight=%C8%C8%BD%BB%BB%BB%C6%F7 http://bbs.hcbbs.com/viewthread.php?tid=157252
For shell-and-tube heat exchangers, there are not many calculation methods. For example, the heat transfer coefficient is generally calculated inside a tube using a formula with an exponent of -2/3 and 0.14, based on the internal heat transfer factor; of course, various formulas from chemical engineering principles can also be used, each with its own applicable conditions. For the shell side, the BELL method or the KERN method is typically employed. The pressure drop is calculated using the BELL method or the KERN method. As for the differences between the various formulas, they won’t be significant, staying within the limits permitted by engineering standards (20%)
That’s great~~ Hehe, thank you all……
Thank you to the two people above for their advice. I have a problem that I need help with: It’s a condenser in which the hot material flows through the shell side; the feed rate is 900 kg/h at a temperature of 280 degrees, and it is condensed to 38 degrees. The dew point of the condensable components can be considered constant at 41 degrees, while 160 kg/h corresponds to the non-condensable components. My question is: 1. How to calculate the average temperature difference: I read the help documentation for the HTFS software; if multi-component condensation is selected, the software calculates the logarithmic average temperature differences for the cooling, condensation, and cooling-down processes separately, and then computes a weighted average. May I ask, is this also the way calculations are done in actual engineering design? 2. How to calculate the heat transfer coefficient: The software allows the calculation of the heat transfer coefficient in the shell side to be based on both condensation phase change heat transfer and dry gas heat transfer. For engineering design, which method should we adopt? 3. How to calculate the heat transfer area: Based on the above two questions, if a weighted average temperature difference is used, does it mean that A = Q/(U*Tm) can be applied, where A represents the total area, Tm is the weighted average temperature difference, and U is the overall heat transfer coefficient calculated for the heat transfer of the phase change (or dry gas)? Or should I calculate separately the temperature drop difference minus the heat transfer coefficient for cooling, the condensation temperature drop difference minus the heat transfer coefficient for condensation, and the cooling temperature drop difference minus the heat transfer coefficient for cooling? After determining the heat transfer areas for these three processes, should I then find the sum of these three areas? When I was reading Coulson & Richardson’s Chemical Engineering, Vol. 6, there was some unclear wording in this section; perhaps it’s due to my limited English skills, as I couldn’t quite understand it. Please give me some advice, thank you!
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On the 3rd floor, while reading about the Kern method and the Bell method as described in Coulson & Richardson’s Chemical Engineering, I noticed that there was a significant difference in the calculation results obtained using these methods. Also, you say that a 20% error is acceptable in engineering; if such a large error is deemed acceptable, then I think there’s no need for any calculations at all – one can simply look up the values in experience charts, as the errors are likely to fall within that range. Additionally, I performed rating simulations for the same hydrogen cooler using TASC and Hetran, and noticed that the results were almost identical, but there were a few differences worth noting: 1. The shell-side flow velocity in Hetran was 12 m/s, while TASC indicated a flow velocity of 18.5 m/s. However, the shell-side heat transfer coefficients they calculated were quite consistent. 2. Hetran calculated a Reynolds number of 58,000, from which it can be deduced that the hydraulic radius used is 26 mm. The actual size is OD; the tube spacing is 38.1 mm, with a 90-degree square arrangement. The radius calculated using the Kern method should be 47 mm, which is quite different from the result obtained by Hetran. I wonder how Hetran calculates the heat transfer coefficient? 3. I calculated using the Kern method manually, and the results were approximately 30% lower than those obtained with software, but the flow rates were the same as those from TASC. It seems that Hetran and TASC use different methods to calculate the heat transfer coefficient, but the results are the same. The flow velocity in the kern method is the same as that in TASC; the hydraulic radius is much larger in Hetran’s method. It’s not clear what method Hetran uses to calculate the flow velocity, the Reynolds number, and the heat transfer coefficient. What do you think of this issue?
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