The present study addresses the problem of the scale-up/down of dense gas fluidized beds that may interact with their air-supply system (restricted here to the plenum and the distributor). An abundant literature about scaling issues yielded various sets of dimensionless numbers to be matched in order to ensure the invariance of the bed dynamics at different scales. All these sets embed the first indispensable requirement that is the geometrical similarity between scaled beds. Despite its self-evident character, this letter highlights important and new considerations about the applicability of this requirement for the design of the distributor and the plenum. For the distributor, it is shown that the geometrical similarity introduced by some authors is not always relevant and should only be used under particular conditions. For the scaling of the plenum, to our knowledge, nothing is mentioned in the literature. The present work shows that the 'by-default' geometrical scaling of the plenum is completely irrelevant. A practical rule to determine the plenum size at different scales is given so that similarity of the dynamic interaction between the bed and its air-supply system is ensured. (C) 2009 Elsevier Ltd. All rights reserved.