Could an expert help me understand what this message means?
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When I’m performing dynamic simulations using HYSYS, such a message appears in the Trace Window: \"Heat Exchanger E-100-2 has an excessively low calculated Ft Factor. This value is limited to 0.2 (internal value) as the applicable range of the correlation formula has been exceeded; this situation may also lead to dynamics instability. If this message continues to appear during dynamic simulations, consider reducing the UA value. This message will appear only once per session for this unit operation, unless Trace Verbose is enabled.\" Although I can look up the meaning of words I don’t know, I’m not experienced in the chemical engineering field and I’m also new to HYSYS, so I don’t quite understand it. Could someone with expertise please explain to me what this means exactly? What went wrong?
The heat exchanger calculation didn’t pass; I guess it’s because the F correction factor is too small. HYSYS doesn’t accept it. Check in GB151 what factors are related to the F correction factor, and adjust the calculation data so that HYSYS can approve it.
Reply to 1# sbdsbdhh: Change the pipeline value to 1; I guess you’re using the default value of 2
Reply to 3# sybase666: Hello, thank you very much for your help. I did as you suggested and changed Tube Passes per Shell to 1, and indeed that message no longer appeared. But I don’t quite understand why this change is necessary. The document states that setting Tube Passes per Shell to 1 is intended to simulate Counter Current Flow, but I’m not sure what kind of flow pattern constitutes counter current flow. Although that prompt is no longer available, some variables in the model still exhibit oscillatory behavior under dynamic conditions. There are several other heat exchangers in the model, and I’m not sure whether the Tube Passes per Shell value for those heat exchangers needs to be adjusted. Therefore, I would like to ask you how to determine this value.
Reply to 4# sbdsbdhh: When calculating heat exchangers, we usually use the logarithmic mean temperature difference (you can check relevant books on heat exchangers); in the case of simple counterflow, the logarithmic mean temperature difference can be calculated directly using a formula. When there are multiple passes, the logarithmic mean temperature difference needs to be corrected using Ft. As for what value Ft should take, I remember that a Japanese author wrote a book on heat exchanger design that covered this topic; it was quite good. In HYSYS, why is Ft calculated as 0.2? I think it might be related to the settings for the process stream and the temperatures of the cooling or heating streams; I haven’t paid too much attention to it specifically. But switching to a single pipeline shouldn’t be the cause of instability in your model; the instability must be caused by other issues.
Reply to 5# sybase666: Thank you very much for your answer. I really don’t know much about heat exchangers. Just now I changed the tube side of all the heat exchangers to 1, and that message no longer appeared. Then I tried changing the integration step size in HYSYS to 0.1, and a new warning appeared: E-100-2 Warning: SinglePhasePHFlash failing: Cp
Reply to 6# sbdsbdhh: It is generally not recommended to change the step size used in calculations; using the default value is sufficient.