How do you decide which fluid goes on the tube side vs. shell side?

Tube side is preferred for: corrosive fluids (easier/cheaper to use corrosion-resistant tubes), high-pressure fluids (tubes handle pressure better), fouling fluids (easier to clean straight tubes), and toxic fluids (lower leak risk). Shell side is preferred for: viscous fluids (baffles improve heat transfer), condensing vapors (larger flow area), low-pressure gases, and fluids with high flow rates. If both fluids have similar properties, place the higher-pressure fluid tube-side for economic design.