The first and most important step in evaporator design is a thorough and complete heat and mass balance. Once you have chosen or calculated the overall heat transfer coefficients you can calculate the heating surface area required, from the formula:
A = Q / k / ΔTL
It is imporant to then calculate the specific evaporation rate and compare the calculated values against values likely to be achieved in real life.
The next step is to choose a distance between tube plates (about 2 metres is reasonable) along with a tube diameter and tube pitch (spacing between tubes) and a downtake diameter (this is chosen so that the total cross sectional area of the bore of the tubes is about 15 times the cross sectional area of the downtake). From this data the minimum number of tubes required can be calculated. Armed with the number of tubes and tube pitch you can work out the diameter of the tube plate (either graphically or by calculation)
The final step is to calculate the sizes of the various inlet and outlet nozzles using flow velocity as the main criterion.