Compact heat exchanger

A heat exchanger for a dense fluidized bed, said heat exchanger comprising at least one first group of tubes extending in a vertical plane and forming a first sheet of tubes for connecting a first inlet fluid manifold disposed at the bottom and on one side of a case to an outlet manifold disposed in a central top portion of the case, and at least one second group of tubes extending in said vertical plane and forming a second sheet of tubes for connecting a second inlet fluid manifold disposed at the bottom and on one side of the case to the outlet manifold, wherein each of the first and second sheets of tubes comprises first tubes which extend substantially horizontally and parallel to one another in the bottom portion of the case, second tubes which are connected via multiple bifurcations to the first tubes and which extend substantially vertically and parallel to one another between the bottom portion of the case and the middle portion of the case, and bent third tubes which are connected via multiple bifurcations to the second tubes and which extend parallel to one another between the middle portion of the case and the top portion of the case where they are connected to the outlet manifold, the third tubes of the first sheet of tubes and the third tubes of the second sheet of tubes being disposed to form a channel along which the solids can flow, which channel has an O-shaped cross-section, and wherein said first and second sheets of tubes are stiffened at their bottoms by continuous fins disposed between the adjacent tubes.