The limit value of the Nusselt number for pure heat conduction was numerically determined for different particle shapes. In the case of nonspherical particles, substantial errors can result in the calculation of the heating time when the Nusselt number of the sphere Nu = 2 is applied and the sieve diameter is used as the characteristic dimension. That a uniform description of the Nusselt number is possible for all considered particle shapes is demonstrated. However, this Nusselt number, determined using the sieve diameter, is then not constant, but depends exponentially on the ratio of the Sauter diameter to the sieve diameter. (C) 2001 Elsevier Science Ltd. All rights reserved.