Help: The concept of heat exchange
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Hello everyone, I have a question. During the heat exchange process, if the heat exchange area is sufficient, but the cold fluid stays in the heat exchanger for a short time (the cold fluid flows outside the tubes, and the height of the tube bundle is low), can the cold fluid be heated to the temperature specified in the calculations? In other words, given that the heat exchange area and the driving force are sufficient, is time a requirement for heat exchange to take place?
This is related to the heat transfer efficiency of the heat exchanger you use, as well as the temperature difference between the hot and cold fluids; it also depends on the properties of the hot and cold fluids themselves (the value of their heat transfer coefficients).
The original poster had better review the knowledge of chemical engineering principles and heat transfer; it’s impossible to explain this in just one or two sentences
As long as the designed heat exchange area and heat transfer coefficient are followed, the requirements can be met. Regarding the time issue you mentioned: once the flow rate and the conditions in the tube side and shell side are fixed, the time is determined. I suggest you take a look at the principles of chemical engineering
Time is represented by the heat transfer coefficient; for example, the unit of K is kcal/(h·m^2·°C) or W/(m^2·°C).
Generally, for well-designed heat exchangers, this issue has already been taken into account, and it will be specified in the technical parameters. The poster mentioned that the heat exchange area is sufficient, which means there is enough surface area for heat exchange within the heat exchanger; as a result, the viscous resistance to fluid flow increases, and the speed of the fluid flow naturally decreases. Moreover, in order to ensure heat exchange efficiency, there is a limit to the amount of driving force that can be applied to the fluid; otherwise, phase changes may occur in the fluid due to local pressure drops during the heat exchange process. In conventional heat exchangers, phase changes are detrimental to heat exchange and are therefore avoided as much as possible. So the concern expressed by the original poster should not be a problem. If the original poster is merely using it, following the technical specifications provided by the manufacturer will suffice; however, if they are involved in the design process, it’s not so simple to explain clearly – please refer to the relevant technical documentation.
Thank you all. In vertical counterflow heat transfer, cold air flows outside the tubes while hot flue gas flows inside them. If the speed of the cold air is high, meaning that the time for heat transfer with the hot flue gas is very short, will the air be heated to the specified temperature immediately? (From 100 degrees to 300 degrees)