A new heat transfer enhanced tube-the inside and outside spirally triangle finned tube with small spiral angles (IOSTF tube)-was developed and manufactured for improving the performance of high-pressure preheaters. The triangle flutes with small spiral angle on the outside surface of the IOSTF tube perform like the vertically fluted tube, and the triangle flutes with small spiral angle on the inside surface of the IOSTF tube perform like the spirally fluted tube. The experiments show that the total heat transfer coefficient of the vertical IOSTF tube is 63-95 per cent larger than that of the smooth tube with only a slight increase in the inside flowing friction and the field results show that a 43 per cent increase in the total heat transfer coefficient of the high-pressure preheater with the IOSTF tubes can be obtained.