-In this study, flame structures, velocity distributions and exhaust gas temperatures were investigated numerically for four different boiler design of a liquid fuelled boiler. Keeping the outer dimensions constant at boiler design, two parameters were investigated: the effect of the smoke tube diameter and numbers and the structure of the flame tube. Calculations were performed at two dimensional axisymmetric conditions and Fluent software was used as the computational fluid dynamics program. In all calculations, the standard k-ε model was used for modeling the turbulent flow, the species transport model was used for modeling the combustion, and the pressurized swirl atomizer was used for modeling the injection. The calculations showed that decreasing the smoke tube diameters by doubling their numbers the exhaust gas temperatures decreased and the efficiency increased importantly. But by increasing the smoke tube numbers from two to three the decrease in exhaust gas temperature is negligibly small. Also, narrowing the flame tube and changing the flow of the combustion gases in flame tube have positive effect on the heat transfer.