Heat exchanger, in particular for preheating liquid manure

A heat exchanger for using a hot liquid to warm a cold substance that is fluid and foamy, containing solid particles and gas, the heat exchanger being characterized by the fact that it comprises a plurality of unit heat exchangers (1) connected in series, each constituted by a horizontal tube (2) having a heat conducting wall, in which the substance is caused to circulate from one end to the other, and a concentric tubular case (3) surrounding said tube (2) in which the hot liquid is caused to circulate from one end to the other in the opposite direction to the circulation of said substance, and that inside said tube (2) there is coaxially mounted a rotary shaft (4) carrying a helical brush (40) forming an Archimedes' screw thread that rubs against the inside wall of the tube (2) to ensure that the substance is transported therealong, wherein said rotary shaft (4) is mounted inside the tube (2) via a floating mount, the shaft being centered automatically within the tube by the helical brush (40) bearing against the inside wall of the tube (2), and the shaft (4) is rotated by means of a coupling member (42) which leaves the shaft with a certain amount of freedom to move radially.