Application analysis of stainless steel tube in condenser

The application of stainless steel to condenser tubes has been around since the 1960s. At present, more than 60% of condenser in the United States use stainless steel tubes. The used length is 243.84 million meters, and more than 96% of the pipes installed on the condenser are still in use. Among European countries, companies such as Germany and France began to use stainless steel tubes as condenser tubes in the 1970s.

Feasibility analysis of stainless steel tube:

Technical analysis

1. Heat transfer performance: For the condenser of the generator set, when the cooling tube material is replaced by stainless steel instead of navy brass, the heat transfer change is: the heat release conditions on both sides of the heat transfer tube do not change, that is, the heat release form and release on both sides The thermal coefficient is the same, only the change in the thermal resistance of the tube wall caused by the material and wall thickness of the heat transfer tube, but the effect of the wall thickness is relatively small.

The wall thickness affects only 2% of the total thermal resistance, and the material has a relatively large effect. According to the HEI standard, the material heat transfer coefficient of navy brass is 1.01 (tube φ25×1), while the material heat transfer coefficient of stainless steel is 0.89 (tube φ25×0. 6) Therefore, it can be seen that the heat transfer coefficient of stainless steel pipes of the same specification is about 11% lower than that of copper pipes. However, because the total heat transfer coefficient and the speed are square root, when the cooling water speed of the stainless steel tube is increased from 2m/s of the copper tube to 2.3m/s, the total heat transfer coefficient can be increased by 7%, and the stainless steel tube The surface gloss and the cleaning coefficient can also be increased to 0.9, so that the total heat transfer coefficient of the two is basically close.

The overall heat transfer coefficient of stainless steel tubes is not necessarily worse than that of copper tubes, and stainless steel tubes are more superior after a period of operation. The reason is that the contaminated and fouled cooling pipe will have an adverse effect on heat transfer. The adiabatic phenomenon caused by corrosion and fouling can reduce the heat transfer coefficient by up to 50%.

2. Tube bundle vibration problem: The strength and elastic modulus of stainless steel are higher than those of copper. But because the original tube bundle seismic calculation is based on the natural frequency of the tube to avoid the resonance caused by the steam turbine at 3000 rpm, the distance between the partitions is relatively large. And now the tube vibration calculation is based on the empirical formula of the US HEI standard on preventing steam flow induced vibration, that is, the condition that the steam turbine exhaust reaches the speed of sound to determine the maximum span value of the condenser cooling tube to ensure that the condenser cooling tube is not Due to vibration and damage, the calculation result is about 900mm, so depending on the distance of the original condenser diaphragm, whether it is necessary to add a small diaphragm.
3. Improve corrosion resistance. The erosion and corrosion resistance of stainless steel is far better than that of copper tubes, but austenitic stainless steel has poor corrosion resistance to chloride ions. Different stainless steel pipes can be selected according to the content of chloride ions in the cooling water. When the content of chloride ions in the cooling water is low, TP304 and TP304L can be used, and when the content of chloride ions in the water is high, TP316L, TP321, etc. can be used.
4. The tube is connected to the tube plate. If the tube sheet is made of stainless steel composite plate, then the tube and tube sheet can be expanded and sealed to ensure that the condenser is leak-free, and the service life can reach 30 to 40 years. If the tube is replaced only, the tube and tube sheet are expanded. In order to ensure that the connection does not leak, sealant or tube sheet can be grooved.
5. Manufacturing process of stainless steel tube. Stainless steel pipe has higher strength and elastic modulus than other materials, especially copper pipe, with good plasticity and high elongation.

Economic analysis:

1. In the past, everyone felt that the use of stainless steel tubes as condensers was more expensive. But this is not the case. Due to technological development, stainless steel pipes in developed countries are now welded pipes with seams. First, the stainless steel strip is made, and then it is formed by roll forming, welding, heat treatment, straightening, cutting, air pressure inspection, non-destructive testing, stress relief treatment, and there is an automatic production line for the whole process of packaging and leaving the factory. In this way, the quality of welded pipes can fully meet the quality standards of seamless pipes, and the wall thickness of seamed stainless steel pipes is more uniform than that of seamless stainless steel pipes, which ensures the quality of pipe expansion. The price of seamed stainless steel pipes is higher than that of seamless stainless steel pipes. Cheap. In addition, the strength and elastic modulus of stainless steel pipes are higher than copper pipes, so thin-walled pipes can be used for stainless steel pipes. In the case of the same area, the weight is lighter, and the prices of the two are basically similar.
2. When the condenser tube and tube sheet of the stainless steel tube are expanded and sealed, a welding process needs to be added, so the installation cost is slightly increased.
3. It can prevent leakage, which is beneficial for maintaining the vacuum of the shell side of the condenser and keeping the condensed water clean of the shell side, reducing maintenance work and avoiding the loss caused by the shutdown of the condenser due to leakage.
4. The service life is increased 3-4 times of copper tube