Heat exchanger with internal stirrer

A positive displacement apparatus for efficiently exchanging heat between a fluid and a heat exchanger. A moving comb-shaped blade is used to stir the fluid through a heat exchanger having fins located internally thereto adapted to receive the blade and to intermesh with the tines which are part of the blade in such a manner that the blade can pass through the heat exchanger while the amount of fluid backflow behind the blade in the direction opposite to the direction of movement thereof is kept low when compared to that driven by the blade. The blade simultaneously pulls the fluid behind it through the heat exchanger. The efficiency of the apparatus is related to the amount of fluid leaking past the blade while it is traversing the heat exchanger.