There are several types of shell-and-tube heat exchangers which can compensate or reduce the thermal stress properly, so as to avoid more losses. In fact, the thermal stress damage of tube heat exchangers is due to the thermal expansion and cooling of the fluid in the tube when the temperature difference between the fluid in the tube and the outside is large. The shrinkage principle leads to the change of the adhesion degree of each fittings in shell-and-tube heat exchanger and the damage to fittings or shell.
Because the temperature of the fluid inside and outside the tube is different, the temperature of the shell and tube bundle of the heat exchanger is also different. If the two temperatures are very different, great thermal stresses will occur in the heat exchanger, which will cause the tube to bend, break, or pull off the tube sheet. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50 C, appropriate compensation measures should be taken to eliminate or reduce the thermal stress. According to the compensation measures adopted, shell-and-tube heat exchangers can be divided into the following main types:
1. Floating-head heat exchanger can cope with thermal stress effectively.
The tube sheet at one end of heat transfer tube of floating head heat exchanger can float freely to eliminate thermal stress. Floating-head heat exchanger is widely used in shell-and-tube heat exchangers. The tube sheet at one end of the tube bundle can float freely, eliminating the thermal stress completely, and the whole tube bundle can be pulled out from the shell for easy mechanical cleaning and maintenance. The principle of stress relief is to release the tube sheet of a section of the heat transfer tube bundle and allow it to float freely in a certain space to eliminate the thermal stress. The heat transfer tube bundles of floating head heat exchanger can be extracted from the shell, which is convenient for cleaning and maintenance. However, due to the complex structure, the price of floating head heat exchanger is higher.
2. Fixed tubesheet heat exchanger bundles are conducive to thermal stress compensation.
Fixed tubesheet heat exchanger heat transfer tube sheet is directly connected with the shell to compensate for thermal stress. The tubesheet at both ends of the tube bundle is connected with the shell. The fixed tubesheet heat exchanger is a simple tubesheet heat exchanger. The tubesheet at both ends of the heat transfer tube bundle is directly connected with the shell. The shell is equipped with a stress compensation ring, which can reduce the thermal stress when the temperature difference inside the fixed tubesheet heat exchanger is large. Fixed tubesheet heat exchanger has less thermal stress compensation, and can not adapt to the work of larger temperature difference. When the temperature difference is slightly larger and the pressure of shell side is not too high, elastic compensation rings can be installed on the shell to reduce the thermal stress.
3. Cleaning of U-tube heat exchanger is complex but can completely eliminate the influence of thermal stress.
The partition of U-tube heat exchanger is divided into two parts of inlet and outlet to eliminate thermal stress. The heat transfer tube bundles are U-shaped bending. The two ends of the bundles are fixed on the upper and lower parts of the same tube plate. Then the bundles are divided into two parts: the inlet and the outlet by the baffle in the tube box, which completely eliminates the influence of thermal stress on the bundles. U-tube heat exchanger has simple structure and convenient application, but it is difficult to disassemble and clean. With the help of the partition in the pipe box, it is divided into two chambers, the inlet and outlet. The heat exchanger completely eliminates the thermal stress, and its structure is simpler than that of the floating head type, but the pipe side is not easy to clean.