Standard polypropolyene has similar thermal conductivity ranges as ABS and PLA, all between .1 and .25 W/m.K. They all suck as a heat exchanger material as they don't conduct heat well at all, but I understand why you're looking to use them. If you make the walls thin enough to maximize the heat exchange, I'd worry about how water tight the wall would be over time with thermal expansion and contraction. I'd think that the coroplast, with walls being extruded all at once solid, would have better long term results then something printed. But you don't say what is running through the exchanger, what pressure, temperatures, shape, layers, etc so there may be benfits where a printed exchanger's advantages outweight it's disadvantages (or at least uncertainties).