Based on sources I referred, I got to know that a double pipe heat exchanger operates in a way where hot water flows in a pipe and comes out as cold water and cold water flows into another pipe and comes out as hot water. I dont understand the logic of flowing hot water to remove heat. for example if have a hot metal plate and I would like to take the heat away. why do I have to flow hot water in one of the pipes? Because in most examples, that is how a double pipe heat exchanger is shown to be:
One pipe: hot water in, cold water out Another pipe: Cold water in, hot water out
How will flowing hot water in one of the pipes help?
Thanks in advance!