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!
 A: You're basically describing a car radiator. I can't see any reason why you'd need a more sophisticated heat exchanger.
A: If you have hot water that you want to cool with air (especially ambient air), you would want to use a car radiator, as John Rennie pointed out.  For a more industrial approach, you will often have a hot process stream (e.g., hydrocarbon) that you want to cool with cooling water.  The cooling water is in a closed loop, so it comes from the cooling tower at approximately 80 deg F, goes through the heat exchanger, and goes back to the cooling tower at approximately 110 deg F, where the heat is expelled to the atmosphere.  Since your employer has to pay for "make up" water (to make up for what evaporates, and for what is "blown down" to get rid of contaminants), you want to minimize how much water you purchase and you don't want a lot of water puddles all over the place, leading to the closed-loop circulation.  On the process side of the heat exchanger, the hydrocarbon is either an intermediate substance or your product, and it is very flammable and/or explosive, so you don't want to get sloppy with this either.  Due to these constraints, where you don't want to waste either water or process fluid, and you don't want either fluid to mix with the other, you would use something like a double pipe heat exchanger.
