Why does the cold exchange equipment need to enter the cold flow first and then the hot flow?

Used to control the metal wall temperature of the heat exchange element to prevent the heat exchange element from exceeding its designed metal wall temperature. However, what LZ said is not applicable to all working conditions. When the heat exchange equipment is low-temperature equipment, the hot flow should be introduced first, and then the cold flow should be introduced. Same reason as above.

We usually don’t have such problems in the chemical industry, unless the temperature of the hot fluid is too high and exceeds the service limit of the equipment, and the usage method is stipulated in the design, but this is rare! On the 1st floor, in what information or occasions did you encounter such a provision?

The purpose of advancing the cold flow and then entering the hot flow is to gradually heat up the heat exchanger and prevent it from heating too fast, causing leakage due to uneven thermal expansion of the heat exchanger elements.

I agree with what the 4th floor said, it is indeed the case!

The purpose of advancing the cold flow and then entering the hot flow is to gradually heat up the heat exchanger and prevent it from heating too fast, causing leakage due to uneven thermal expansion of the heat exchanger elements.

This is an operational requirement. For example, the cooler must first pass in cooling water, and then the reboiler must first cool the material and then pass the steam. Cold exchange equipment with high pressure must be "heat-tightened" after being put into use to prevent leakage! When designing the cold exchange equipment, the average tube temperature and the average process temperature must be provided, which are both working conditions. Gating hot materials is not allowed from a process and equipment perspective!

One is the need for equipment, because the design temperatures of the hot and cold flow sides are different to prevent the cold flow temperature from being too high. The other is the process requirements. Generally, cold exchange equipment is used for cooling. If the hot fluid is passed through first, the hot fluid will enter the process, endangering the safety of the equipment in the entire process system, such as the compressor. If the temperature of the intercooler does not drop, it will cause a series of problems in the compressor, which may eventually lead to interlocking shutdown due to excessive temperature, causing losses to production.

First let me explain something: When the heat exchanger is put into use, the cold flow is switched on first and then the hot flow is switched. When it is deactivated, the hot flow is switched on first and then the cold flow is switched on. This is not only a design factor that needs to be considered, but also a process operation and safety factor. 1. If the heat exchanger first supplies cold flow, the equipment can be synchronized to the cold flow medium temperature first, then the hot flow temperature is supplied, and then the cold flow temperature is raised to the hot flow temperature, which can reduce the temperature gradient. It is of positive significance to avoid equipment leakage or equipment damage caused by excessive temperature rise. 2. The most important thing is that the temperature difference between the hot and cold flow is too large, and phase changes or reactions may occur during the process. It is necessary to turn on the cold flow first and then change the hot flow when it is put into use. When it is stopped, it is necessary to cut off the hot flow first and then the cold flow. This is just like boiling water with a kettle at home. You can't turn on the stove first and add water. An accident that occurred at a refinery affiliated to PetroChina last year was an error when putting the heat exchanger into operation. The hot flow was put in first, and then the cold flow. The hot flow was steam, and the cold flow was an easily vaporized medium. After the order was wrong, the cold flow instantly vaporized, causing the heat exchanger to overpressure, which ultimately caused a major accident. 3. For heat exchangers with too large pressure difference between the tube and shell sides, it should be considered during design that if the shell side is a high-pressure medium and has a cold flow, it may cause secondary extrusion of the floating head gasket when it is first put into use. After being put into use, secondary leakage of the floating head is easy to occur, especially for heat exchangers with larger diameters, this problem becomes more prominent. Our plant's heavy oil catalyzed slurry steam generator has this problem. The shell side is filled with 5MPa raw water and the tube side is catalyzed oil slurry. It often leaks during commissioning and use. When people analyze the reasons for leakage, there are many reasons, but the secondary extrusion of the gasket is also one of the factors. There is no good solution yet. Consider preparing to install disc springs on floating head bolts and pipe box bolts to deal with the relative reduction in bolt pre-tightening force caused by operating fluctuations and secondary extrusion of gaskets.

Prevent excessive temperature difference from causing excessive temperature difference stress, such as at the tube sheet, which should be avoided. At the same time, it is also basically based on the counter-heat transfer method to improve heat transfer efficiency.