The paper deals with the results of experimental investigations at high temperature processes in disperse systems. The heat transfer was determined by a model reaction viz. the thermical cracking of calcium carbonate. A short description of experiments and a survey on the modelling and calculating is given. As a result a correlation between the Nu- and the Re-number was found. This correlation is in accordance with the heat transfer by spheres concerning the exponent of Re-number. An important difference however exists in the coefficient of Re-number in consequence of the high density of heat transfer. A quotient of the heat conductivities for the gas stream and the particles surface equalized the results.