Dear sir, the sizes for nozzles N1 & N5 are taken considering maximum evaporation (worst case condition). For nozzle N6, if you consider to change the utility to hot oil or hot water (in case you change your feed or process conditions), then you need to have N6 nozzle same as N5. Hence we have considered 200 NB, 80 NB & 80 NB sizes for nozzles N1, N5 & N6