Thanks Ray.
This approach is very similar to what I wanted to do, and I starting from this assumptions I would not use an expansion joint with such a small thermal difference.
Also because on shell side we have 6 bar, so an EJ might cause bigger problems, instead of helping.
Anyway, talking about this approach to define MMT, I think I will only rely on it when the fluids on both sides have similar thermal coefficients.
And also in that case, convective efficiency will always be unknown to me.
I think I will have to ask to our process dept more and more..
Thank you, bye!