The tubeside mean metal temperature is determined for the calculation of the differential thermal expansion.
There are various published methods that determine Outside Film Heat Transfer Coefficients. The effects of the baffles, lane and window leakages are used to correct the heat transfer coefficient so obtained.
You need to calculate film heat transfer coefficients (i.e. hi, ho, hw, hfo, hfio) and overall heat transfer coefficient Uo.
Next step to determine the mean metal wall temperature is determine the temperature profile. Th, Twh, Twc, Tw. The fractional temperature drop remains equal to the fractional contribution to the overall heat transfer coefficient.
The mean metal temp is the temperature at half the thickness of the tube.
Now you are ready with the parameter that is to be used for thermal expansion.
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Best luck...