A coaxial heat exchanger is a kind kind of tube heat exchanger, it is usually used as condenser or evaporator in a heat pump, marine air conditioner, swimming pool, water chiller etc. Mostly it is a water cold heat exchanger. The structure can be as round, rectangle, spiral, double helix and so on.
The coaxial heat exchanger is designed as a "tube in tube" structure. Usually the outer tube is made of steel/copper, and the inner tube can be titanium, copper, copper-nickel depending on the requirements of working condition. For the inner tube is has many spiral grooves on the surface, which will enlarge the connection surface and get more heat transferred.
How does the heat exchange in the coaxial heat exchanger?
Mostly water flow in the inner tube and refrigeration flow between the outer & inner tube(refrigeration loop). The temperature of refrigeration and water are different, so heat will transfer from one with high temperature to the other one. The mostly it is counterflow.