Hm yes might have something to do with heat exchanger efficiency, like there is a limit based on flow rate, temperature differential, etc. Unless maybe you want to go to finned tube, that seems like it would be a pain to clean. Maybe it also leaves the water in the middle of the flow stream cold while the water on the outside is warm, there is some kind of tube that has like these 'swirly' things in it to mix the liquid as it flows and I forget the name completely now, I know I have seen it in petrochemical applications.