For chemical reaction engineering, a particle size of d(p) less-than-or-equal-to 1 mm is favourable. According to the two-phase theory of fluidization, the emulsion phase of a bubbling fluidized bed remains more or less in the state of minimum fluidization with Pe less-than-or-equal-to 10 for d(p) less-than-or-equal-to 1 mm. For particle-to-gas heat transfer in the emulsion phase, two different situations are of interest : first, the heat transfer from a single particle; and second, the heating-up of a gas penetrating into a particle array. It is shown that a simple theoretical model, namely that of the equivalent pipe diameter, accurately describes both situations without any empirical scaling factor.