Enter the actual metal temperature for the tubes under a realistic operating condition.
It is important, especially when evaluating fixed tubesheets without expansion joints, that you enter accurate values for metal temperatures for each operating condition. You may have to run the analysis more than once to check several metal temperature cases. Frequently the metal temperatures will be less severe than the design temperatures, due to thermal resistances. For example, if the shellside fluid has a good heat transfer coefficient and the tubeside fluid has a relatively poor heat transfer coefficient, then the tube temperature will be quite close to the shell temperature.
Do not forget to evaluate the condition of shellside or tubeside loss of fluid. Especially
for shellside loss of fluid, this design condition may govern the exchanger design.
For a fixed tubesheet, you can instruct the program, to evaluate multiple load cases.
Refer to TEMA standard, section T-4 (8th Ed.) for guidance to compute the Mean Metal Temperatures.