Abstract: This paper deals with a computing method for the design of the commercial rotary driers. The method has been proven in certain commercial rotary driers for the drying of different complex fertilizers. In the study of the heat transfer for particles, to be dried, a volume distribution frequency function of particle groups is introduced for the estimation of the interactions among particles during heat transfer. Parameters of this function are determined by employing a parameter estimation technique. Data collected in a pilot rotary drier serve for the estimation of the mentioned parameters. The suspension time of a particle in a rotary drier is defined to be equal to the drying time of the particle. To obtain the suspension time, mathematical methods, Monte Carlo method and Markov chain inclusive, are applied to simulate particle group transfer in the rotary drier.