Dear friends,
as I know for two reasons you must prepare bank statement : for embassy and university, with fund only for visa process you must bring with your document bank statement to embassy , in the previous summer while i decided to prepare it some of my friends told me it is better to give the letter from private bank! i do not know the exact reason but 95% i sure the letter from Sepah bank is not accepted but Meli bank and other maybe accepted, also it is not necessary bank keep your money many days, if you ask from bank that only report how much you have in your account . I got my letter from sama bank, the main building is in enghelab between valiasr and hafez, it takes time only 2 days.
and 5$ costs
good luck