Processing Heat Exchanger Tubes
1.1 Carbon steel pipes
SA-556Gr.C2 carbon steel heat exchanger tubes have been widely used in high-pressure heat exchanger tubes, and the processing technology is relatively mature. There is no problem in the welding and expansion of heat exchanger tubes and high-pressure tube sheets.
1.2 Low alloy steel
There is also some equipment using 16Mo3 heat exchanger tubes at home and abroad. Because the tube material contains Mo element, and its welding performance is slightly inferior to SA-556Gr.C2. However, the manufacturing technology of this material is mature. By improving the welding process, the material’s welding performance is improved. Materials such as SA213T11 are often used for low-alloy steel pipes and are not widely used in high-pressure heaters. In the ASME standard, materials such as SA213T11 have a lower allowable stress than carbon steel, but the material has better hardness and strength. In certain equipment, 15CrMo which is equivalent to SA-213T11 was used as the heat exchanger tube. The specification is 15.88mm X 2.6mm. When the heat exchanger tube was tested, it was found that it was difficult to complete the expansion joint. It can be seen from Table 1 that low alloy steel heat exchanger tubes can be applied in subcritical units, if it is used in supercritical and ultra-supercritical units, the pipe wall is thicker, which makes expansion difficult, so it should be selected carefully.
Table 1 Calculating the wall thickness of heat exchanger tubes in various materials (The outer diameter is 15.88.) Unit: mm
SA-556Gr.C2 carbon steel heat exchanger tubes have been widely used in high-pressure heat exchanger tubes, and the processing technology is relatively mature. There is no problem in the welding and expansion of heat exchanger tubes and high-pressure tube sheets.
1.2 Low alloy steel
There is also some equipment using 16Mo3 heat exchanger tubes at home and abroad. Because the tube material contains Mo element, and its welding performance is slightly inferior to SA-556Gr.C2. However, the manufacturing technology of this material is mature. By improving the welding process, the material’s welding performance is improved. Materials such as SA213T11 are often used for low-alloy steel pipes and are not widely used in high-pressure heaters. In the ASME standard, materials such as SA213T11 have a lower allowable stress than carbon steel, but the material has better hardness and strength. In certain equipment, 15CrMo which is equivalent to SA-213T11 was used as the heat exchanger tube. The specification is 15.88mm X 2.6mm. When the heat exchanger tube was tested, it was found that it was difficult to complete the expansion joint. It can be seen from Table 1 that low alloy steel heat exchanger tubes can be applied in subcritical units, if it is used in supercritical and ultra-supercritical units, the pipe wall is thicker, which makes expansion difficult, so it should be selected carefully.
Table 1 Calculating the wall thickness of heat exchanger tubes in various materials (The outer diameter is 15.88.) Unit: mm
| Materials | Wall thickness | ||
| Subcritical | Supercritical | Ultra supercritical | |
| SA-556 Gr. C2 | 2.1 (Considering minimum wall thickness) | 2.3 | 2.7 |
| 16Mo3 | 2.1 | 2.5 | 2.7 |
| SA-213T11 | 2.4 | 2.9 | 2.2 |
| SA-213T12 | 2.3 | 2.8 | 2.1 |
| SA-213T22 | 2.2 | 2.8 | 2.0 |
| SA-213TP304 | 2.3 | 2.8 | 2.1 |
| SA-213TP304N | 2.1 | 2.6 | 2.9 |
| SA-213TP304L | 2.6 | 2.2 | 2.6 |
| SA-213TP316 | 2.2 | 2.7 | 2.0 |
| SA-213TP316L | 2.6 | 2.2 | 2.6 |
| SA-803TP439 | 3.0 | 2.5 | 2.7 |
1.3 Austenitic stainless steel
Austenitic stainless steel tubes are often selected as heat exchanger tubes for low-pressure heaters. Austenitic stainless steel has good processing performance. Affected by the "sensitization" characteristics of austenitic stainless steel tubes, the overall heat treatment of the heater cannot be performed. The shell and the welding seam of the connecting pipe are heat treated separately. When the thickness of the shell is thick, there are problems such as uneven heating during local heat treatment.
1.4 Ferritic stainless steel
SA-803TP439 is a commonly used ferritic stainless steel, which is widely used in high-pressure heaters and separators of nuclear power plants, but the material has brittleness and the thickness of the material is limited. It is understood that the SA-803TP439 steel pipe produced by a steel plant in the United States has a maximum thickness of only 2.2 mm. After exceeding this thickness, the brittleness of the material will increase significantly. When the material thickness reaches 2.5 mm, its non-plastic transition temperature has reached 20°C, this material property limits the application of the tubing.
According to the allowable stress value given in the ASME standard, this material can only be used below 325°C. In the design scheme of the thermal power high-pressure heater, the design temperature of the high-pressure heater and the thickness of the heat exchanger tube exceeds the allowable value of the material. Therefore, this material is not suitable for thermal power high-pressure heaters.
2. Performance of heat exchanger tubes
The heat transfer tubes undertake the task of heat transfer in the high-pressure heater. Therefore, in the calculation of heat transfer, the heat transfer performance of the heat transfer tubes needs to be considered emphatically. The heat transfer properties of heat exchanger tubes made of different materials are different, and the thermal conductivity coefficients of each material at different temperatures are shown in Table 2.
Table 2 Thermal conductivity of each material at different temperatures
Unit: w/m2 •k
| Items | Temperature/°C | |||||
| 100 | 150 | 200 | 250 | 300 | 350 | |
| SA-556Gr. C2 | 58 | 55.9 | 53.6 | 51.4 | 49.2 | 47.0 |
| 16Mo3 | 58 | 55.9 | 53.6 | 51.4 | 49.2 | 47.0 |
| SA-213T11 | 40.6 | 40.4 | 40.1 | 39.5 | 38.7 | 37.8 |
| SA-213T12 | 40.6 | 40.4 | 40.1 | 39.5 | 38.7 | 37.8 |
| SA-213T22 | 36.9 | 37.1 | 37.2 | 37.1 | 36.7 | 36.2 |
| SA-213TP304 | 16.2 | 17.0 | 17.9 | 18.6 | 19.4 | 20.1 |
| SA-213TP304N | 16.2 | 17.0 | 17.9 | 18.6 | 19.4 | 20.1 |
| SA-213TP304L | 16.2 | 17.0 | 17.9 | 18.6 | 19.4 | 20.1 |
| SA-213TP316 | 15.4 | 16.1 | 16.8 | 17.6 | 18.3 | 19.0 |
| SA-213TP316L | 15.4 | 16.1 | 16.8 | 17.6 | 18.3 | 19.0 |
It can be seen from Table 1 that the thermal conductivity of SA-556Gr.C2 and 16Mo3 is the highest; the thermal conductivity of low alloy steel is second, and the thermal conductivity of stainless steel is the lowest. Regarding the material SA-803TP439, the thermal conductivity of the material is not given in the ASME material, but domestic heaters with heat exchanger tubes of this material already exist and have achieved satisfactory operating results.
