Heat exchanger consisting of a reservoir with upwardly directed tubes through which flow a liquid to be heated and fluidized bed particles for internally cleaning the tubes. Connected to the reservoir is an external downcomer (5) for collecting fluidized bed particles coming from the top box of the reservoir and to be transported back to the bottom box (7) of the reservoir. The connecting line (4, 2, 1, 3, 6) between downcomer (5) and bottom box (7) is designed as two sections (2, 3) with an expansion bellows (1) arranged between them. The bottom section (3) is connected via two or more connecting stubs (6A, 6B) with the bottom box (7) for the uniform distribution of the flow of fluidized bed particles.