The shell side velocity needs to be below critical velocity. TEMA also has guidelines for maximum recommended rho-v-squared. Your spreadsheet indicates shell side velocity of 1435 m/s with pressure drop of 54000 kPa which might indicate you need additional tweaks to your design. For your spreadsheet, perhaps first look to modify your design to constrain the pressure drop to a reasonable value since you should know the steam inlet pressure and the minimum outlet pressure.
Are you trying to not condense the steam, but only desuperheat it? I made this same mistake in my college senior project! The engineering answer is 'don't do that, it does not work'. The problem with that is the tube wall temperature will be close to the temperature of the water side and the steam WILL condense when it touches the cold metal. When you use steam for a heating medium in an exchanger, condense it fully! The real-life method to desuperheat steam is to directly inject water into the steam.
Edited by Pilesar, 21 November 2017 - 09:42 AM.