thermodynamics - Double Pipe Heat Exchanger

I have a metal plate and I need to transfer heat out of the metal plate to the atmosphere. Hence, I can use a double pipe heat exchanger which contains 2 pipes. But what if I just use one pipe and let water flow through it. I can still bring the heat out right? Why is there a need for 2 pipes instead of just one pipe? My main point is a heat exchanger is used to transfer heat but a heat exchanger means having like two pipes separated by a metal wall. Why not just use one pipe and take the heat out?? Why have 2 pipes where in one pipe hot water flows in and turns cold at the end of the pipe and in another pipe cold water flows in and turns hot at the end of the pipe?

Thanks a lot in advance!