The thesis investigated whether the efficiency of the air conditioning system in the current forest machine could be improved by simply modifying the ductwork. The core focus of the thesis was to analyze in detail two alternative air distribution boxes and the original straight outlet connection of the air conditioning unit from the manufacturer. Based on the analysis results and fluid mechanics theory, the optimal solution for transferring air from the air conditioning unit to the forest machine cab was designed. The theoretical background of fluid mechanics and CFD flow simulation was studied in the thesis using various sources and literature. The theoretical part of the work covers the key concepts and calculations of fluid mechanics using familiar mathematical formulas. In addition, CFD flow simulation and its theory was studied. Flow simulation played a significant role in the thesis. In the analysis section, it was found that pressure losses in the air distribution system are mainly caused by the blow nozzles, but pressure losses can be significantly affected by optimizing the ductwork. In the optimization section, a prototype was designed with lower pressure losses and was also more cost-effective to manufacture.