Yesterday, people consulted on wechat about the heat exchange mode of the secondary superheater of CDQ boiler and the reason for adopting the heat exchange mode. In fact, some people have asked similar questions some time ago. Why are some local sub economizers designed for forward flow heat exchange and some for reverse flow heat exchange? According to my understanding, the CDQ designed by China Japan United and Jigang international in some places adopts these two different heat exchange methods? Why is it designed like this? The reason lies in several ways of heat exchanger.
For the heat exchanger, three heat exchange modes are generally adopted, i.e. reverse flow, paraflow (parallel flow) and mixed arrangement.
For the same heat exchanger, the heat exchange area required for parallel flow is large, the temperature difference between the paraflow heat exchanger and the reheater is small, more heating surfaces are needed, the smoke temperature at the steam outlet is low, and the metal wall temperature of the heating surface is low. This arrangement is safe to work, but its economy is poor. It is generally used in the final stage (high temperature section) superheater or reheater with the highest steam temperature. In countercurrent arrangement, there is a large heat transfer temperature difference, which can save metal consumption. However, the steam outlet is just in a high area, and the metal wall temperature is high, which is not good for safety. This arrangement is generally used in the low temperature section (inlet stage) of the superheater or reheater to obtain a large heat transfer temperature difference without too high wall temperature. The mixed layout is a compromise of the two kinds of layout, which retains their advantages and overcomes their disadvantages to some extent.
Advantages and disadvantages of counter current and parallel current:
1. Under the condition of the same inlet and outlet temperature, the average temperature difference of counterflow is the largest, and the average temperature difference of concurrent flow is the smallest. According to the Fourier heat conduction law, the heat transfer area is smaller and more economical under the same heat load;
2. In counterflow, the highest temperature of the fluid is at one end of the heat exchanger, and the temperature difference in the whole length direction is large, and the wall temperature is not uniform enough, especially when the cold fluid meets high temperature and needs chemical reaction in the final heating stage, it can only flow in parallel, and the temperature in the whole direction is small and uniform;
3. There are many countercurrent used in the application, the advantages are: the same heat exchanger, the heat transfer driving force (temperature difference) is relatively large, the heat exchange area is saved in the design time, and the economy is better;
4. When a fluid phase changes, there is no inverse difference. Application: ① when the material is heat sensitive, parallel flow shall be used; ② for the material with large viscosity, parallel flow shall be used.
Therefore, the secondary superheater of CDQ boiler is arranged in a downstream way mainly from the perspective of safe operation.