Hello Marks
I think you are referring the mean metal temperature of the tubes. As you correctly point out, the tubes are subject to both the tubeside temperature and the shell side temperature. To further complicate matters, the fluid (shellside and tubeside) enter at one temperature, and exit at another temperatures. There are therefore 4 terminal temperatures involved.
I the case of the mean metal temperature of the tubes, one method is this; take the average of the 4 terminal temperatures. You can do this in the absence of more detailed information
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Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM