Let say I have one solid box....I want to create inside the box, there is a heat exchanger. how can I start to design that.Is it needs some equation of the pressure drop or some step to design it??just wanna see the inflow and outflow of the stream through the heat exchanger....??anybody can help me..??thanx a lot

You will have to be clearer on what type of exchanger, what flow you will be modeling i.e. shell or tube side etc.