09-03-19 Topic - How to determine the heat transfer coefficient of mixed media in heat exchangers? (Rewards for participation)
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discuss: What is the problem of determining the heat transfer coefficient of mixed media in heat exchangers? When designing heat exchangers, there are many cases where the medium composition is not single, and some are accompanied by phase changes. In this case, it is more difficult to obtain the k value. Note: This topic is provided by zzuyj members. Please pay attention to the provider in time and give a summary or correct answer within 24 hours. If you have good topics, you can also provide them to us. See the "Special Post for Collection of Daily and Monthly Topics" in the top post at the top of the forum, link: http://bbs.hcbbs.com/thread-335484-1-1.html , you will get prizes if you participate, and you can also participate in the selection at the end of the month, with generous rewards.
Viscosity, specific heat capacity, and thermal conductivity in Nusselt correlation. The key lies in the thermal conductivity, which can be calculated based on the relevant calculation formulas in the fluid flow chapter of the first volume of "Principles of Chemical Engineering" published by Tianjin University. Or based on chemical engineering thermodynamics calculations.
The situation mentioned by LZ is generally to calculate the average molecular weight of the gas mixed medium, find the density, and find the physical parameters according to the proportion of each component. After getting a comprehensive value, it can be regarded as a single gas, and then the calculation is OK.
In fact, there is one of the most convenient methods---export from professional heat exchange software! The accuracy of this data should be greater than the first two methods, and it is convenient. In fact, when doing heat exchanger design calculations, we no longer do calculations by hand. Designed with software, the unit uses genuine TASC5.01, so the heat transfer coefficient of the medium is not calculated and is directly derived. Of course, I think this kind of theoretical calculation and engineering estimation skills are still very necessary for engineering designers. Looking back on research, I have never studied theoretical calculations, and they rely too much on software. This post was last edited by znpmg on 2009-3-19 07:13 ]
It is indeed difficult to find a suitable empirical correlation for the theoretical calculation of mixed media. Manual calculation is very difficult, and it is even more difficult for two-phase flows involving phase changes. Programming is used to calculate, that is, most software is used to calculate directly. The principle is generally to consider the changes in the physical properties of the heat exchange process in each section, and finally calculate the heat transfer coefficient integrally. Only in this way will the relative error be smaller. However, whether using hand calculations or software, it is inseparable from the most basic principles of convection, heat conduction, mass transfer, gas-liquid balance, etc.
In school, we still use formulas to calculate units and use software to calculate them. It is too complicated to calculate by hand. It is recommended to use TASC5.01.