Air preheaters make a considerable contribution to the improved overall efficiency of fossil-fuel-fired power plants. In this study we used a combination of fluid dynamics and a newly developed three-dimensional numerical model for heat transfer as the basis for a theoretical analysis of a rotary air preheater. The model enables studies of the flue-gas flow through the preheater and the adjoining channels as well as the regenerative heat transfer and the resulting temperature distribution in the matrix of the preheater. Special attention was focused on the influences of leakages on the flue-gas parameters in the preheater. The numerical analysis and the experimental results showed an obvious dependence of the flue-gas parameters on various sea] settings. Based on the results a method for online monitoring of the tightness of the radial seals is proposed. (c) 2006 Elsevier Ltd. All rights reserved.