Abstract: A bed of particulate material 6 is supported by an air distribution bottom 5. Under the distribution bottom 5 exist one or more compartments 7, which each being supplied with cooling air from a fan installation 8. The distribution bottom 5 is sectionalised in a number of smaller areas 9. Each smaller distribution area 9 is connected to the compartment 7 by ducts 10, 11 and 12. The one duct 10 does have a fixed orifice area. The ducts 11 and 12 do have floaters 13A/13B end stop 14A/14B and bottom support 15A/15B. It is hereby obtained that the total pressure loss across the air distribution bottom can be reduced, and so that the flow of the treatment air through the material bed is distributed in a desirable manner across the entire air distribution bottom regardless of the composition of the material bed and the distribution thereon, and optimal heat exchange efficiency is obtained.