Abstract: Heat transfer equipment is provided having a chamber adapted to contain a bed of free-flowing granular material and a plurality of spirally wound cylindrical tube coils nested together which extend concentrically around a centrally positioned tube collector. Each tube coil is wound on a cylindrical surface having an axis coinciding with the longitudinal axis of the chamber. The tubes at least at the top end of the chamber emerge from the coil and remain on the cylindrical surface as they extend in a direction substantially parallel to the longitudinal axis. The tubes then bend about 90.degree. and extend radially toward the longitudinal axis and connect to the centrally positioned tube collector. In the radial run, the tubes from different tube coils overlap one another on the top of the coils and extend radially over substantially the same outer surface of the tube coils. A retaining device adapted to be removed in sections is positioned below the tubes in the chamber.