09-08-26**Question——(Participation rewards available)
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This post was last edited by jia717 on 2009-8-25 23:15. Discussion topic: Generally, in order to increase the heat transfer temperature difference of a heat exchanger, it is designed with counterflow flow. So, what kind of heat exchanger can be designed in a purely counterflow configuration? This question was provided by member jrwyc; the provider should pay close attention to it and provide guidance. The purpose of this* topic is to help everyone review some basic theoretical knowledge, gain new insights by reviewing what has been learned before, discuss and gain a deeper understanding of standards or specifications, as well as address specific issues encountered in practice. This enables us to learn together and improve together; we welcome all friends to participate actively. You can also provide us with good *questions; we will offer rewards to those who provide them. For more details, please refer to the top post in the heat exchange section, titled “Special Thread for Collecting Questions on Heat Exchange Area Technology”, with the link: http://bbs.hcbbs.com/thread-526242-1-1.html. Note: 1. For fill-in-the-blank, multiple-choice, and true/false questions, please reply using the hidden format. Rewards will be given to those who answer correctly; those who do not use the hidden format or revise their replies will not receive any rewards. Please cooperate actively. 2. For multiple accounts under the same IP responding to participate, only the first response is valid.
This is a good question. Pure counterflow configurations for shell-and-tube heat exchangers are very rare; the main reason for this is that baffle plates are installed in order to increase the flow velocity between the tubes, which results in a mixed-flow pattern. So I believe that only in the case of no baffle can true countercurrent flow be established. There is another case: during condensation, since there is no difference between co-current and counter-current flow, it can also be considered as counter-current flow. For other types of heat exchangers, such as plate and spiral plate types, they can be designed in a pure counterflow configuration.
For plate types, spiral plate types can be configured for pure counterflow; likewise, the baffle rod type in shell-and-tube types also functions as a system for pure counterflow
Plate-fin heat exchangers are mostly in pure counterflow configuration
It seems to me that heat exchangers almost always use counterflow contact.
This question was an essay question from the examiner’s exam in 2004. There was another question in the original version, which asked to draw a simple diagram and indicate the flow direction of the medium. Since some images aren’t easy to upload online, the requirements weren’t specified.
Pure counterflow has not been discussed much; for heat exchangers with phase changes, such as reboilers, it can probably be considered pure counterflow! On the 3rd floor, it was said that “the main reason is to increase the flow velocity between the pipes, and that’s why baffle plates are installed.” I think this explanation is incorrect! The purpose of installing baffle plates is to improve the heat transfer coefficient outside the tubes.
This post was last edited by astrodog on 2009-8-26 at 14:02. For two-flow heat exchangers, under the condition of identical inlet parameters, a counterflow configuration provides better heat transfer performance and lower losses compared to a coflow configuration. However, for multi-stream heat exchangers, the diversity of multi-fluid and multi-channel arrangements makes the coordination among fields such as the temperature field, velocity field, and temperature difference field more complex, thereby having a more severe impact on the heat transfer performance of the entire heat exchanger. Therefore, pure counterflow is relatively rare.
It seems unlikely for a heat exchanger to achieve true pure counterflow; it can only be said to be close to pure counterflow, such as in shell-and-tube heat exchangers and spiral plate heat exchangers
Setting it to counterflow mainly increases the logarithmic mean temperature difference, that is, the driving force for heat transfer.