Abstract: A heat exchanger comprises two fluidization chambers separately supplied with fluidization gas. A heat transfer medium in the form of particulate material is supplied to the first chamber which is in communication with the second chamber. The second chamber has an outlet for discharging the heat transfer medium and contains a plurality of heat exchange tubes immersed in the fluidized bed therein. The particles of the heat transfer medium have average dimensions between 50 and 3,000 micrometers, and the maximum speed of the fluidizing gas in the second chamber is between 1.5 times and 10 times the minimum speed of fluidization of the particles of the heat transfer medium. Adjacent heat exchange tubes are arranged to be spaced by a horizontal distance which is between 1.4 times and 2.1 times the external diameter of these tubes. Additionally, the tubes have a vertical spacing between 1.4 and 2.1 times the external diameter of these tubes.