plate type heat exchanger

What are the process and utility fluids, their temperatures and pressures?

What other materials are under consideration for these plates?

let say for example condensate water (100degree cel.)against normal water

pressure 5 bar

my doubt is why we are using ss material

Hot condensate is very aggressive. Were mild steel to be used for plates, the chances are that at those sort of thicknesses, it wouldn't last very long.

That type of heat exchanger starts at stainless steel, and the material quality goes upwards from there. Mild steel plates of that type are probably not available.

The exchanger type originates with, among others, the APV company, whose origins lie in milk processing. Mild steel and milk do not mix, for obvious reasons!