I assume that you have first stage stage condenser of conventional shell and tube exchanger after that the vapor is going to air fin cooler.In CDU OVHD circuit, two types of corrosion is expected i.e. HCL attack by presence of salt in the crude and sulphur attack.In your case since the tower top operating temperature is above 150 deg,c which is more than the HCL dew point hence acid attack is ruled out. However,due to high amount of sulphur in the stream along with moisture (stripping steam)heavy amount of scaling is expected. The designer has choosen to provide heat exchanger in the vapor circuit to have better heat recovery in the preheat train. However,it is to be noted that exchanging heat with mixed media(OVHDVapor +Moisture+ Condensation)into oil is not a wise decision. The amount heat recovery is very minimial being a vapor state.To solve the above problem it is better to go for a duplex grade SS tube which will solve both HCL + Suplhur attack. Ti may be the best alternative however due to hydring above 130Deg.C it is not recommended. regards, krish