Thats what a jacketed pipe is. For example you have a liquid in the inner pipe which you keep hot by pumping steam through the larger outer pipe.
It's exactly that.
Ive got to make a mix of 3"x4" and 4"x5". That's a 3" inner pipe with 4" jacket and so on.
It's for a chocolate factory, so you have chocolate (or product) going through the inner pipe and hot water or steam running through the outer keeping the chocolate molten... Simples