Abstract This article presents study of shell and tube economizer with centered trianglehole aluminums twisted tape for once-through boiler. The exhaust heat from once-through boiler the water heat tube have total surface area 3.55 m2, outside diameter 0.033 m, and total length 34 m. The twisted tape inside the water heat tube have twisted ratio, y/w = 3 and total area of triangle-hole is 0.078 m2. It was found that the designed economizer performed with 5 m/s feed speed, outer and inner tube’s heat transfer ratio at 441.53 and 280.41 kW, heat transfer coefficient at 5,422.46 and 23.29 W/m2oC, pressure drop at 28.31 and 5.98 Pa. The performance of the designed economizer was at 0.62, which save cost around 61,976.34 Baht/year and the payback was at 2.66 year period of investment. This condition is suitable for the boiler that generates 500 kg/h steam rate with the maximum pressure of 7 kg/cm2.