This diploma thesis deals with the calculation of pressure losses in the flue gas ducts. First part of this work describes properties of flue gas and flow dynamics. Part of the theoretical analysis was detailed study of pipeline elements with emphasis on the reasons for the existence of pressure loss in the flow, calculations of the resistance coefficients and possible methods of reducing the losses. In the practical part, a model of the pressure loss calculation of the existing flue gas line installed in the incineration plant was created. At the end of the thesis a proposal for optimization of this route was made with emphasis on the reduction of the overall pressure loss. By optimization the pressure drop of the specified route was reduced from 3581 Pa to 2647 Pa, i.e by 26% and the required power input of the fan was reduced from 225 kW to 166,5 kW.