Water enters the upper tubes.
As it passes through the tubes, turbulence of the water is maintained, even at low water flow, due to the internal rifling of the tubing. Refrigerant liquid enters via the lower refrigerant tube and is forced to follow a long spiral flow path. This flow path is created by the tight tolerance between the copper tubing and the steel side walls. A small amount of refrigerant travels in an axial flow caused by the pressure drop between spiral layers. This flow strips stagnant refrigerant from the finned tubing.