Common problems of heat exchanger units and how to deal with them, some common problems may often be ignored. The most common and easily overlooked problems in the operation of plate heat exchanger units are summarized, and the treatment methods are summarized, which has certain reference significance for the designers and users of plate heat exchanger units. Shell and Tube Heat Exchanger
Shell and tube heat exchanger (shell and tube heat exchanger) is also called tube heat exchanger. It is a partition heat exchanger with the wall surface of the tube bundle enclosed in the shell as the heat transfer surface. This kind of heat exchanger is relatively simple in structure and reliable in operation. It can be made of various structural materials (mainly metal materials), and can be used under high temperature and high pressure. It is the most widely used type at present. The structure consists of shells, heat transfer tube bundles, tube sheets, baffles (baffles) and tube boxes. The shell is mostly cylindrical, with tube bundles installed inside, and both ends of the tube bundles are fixed on the tube sheet. Two fluids, hot and cold, for heat exchange, one flows inside the tube, called the tube side fluid; the other flows outside the tube, called the shell side fluid. In order to improve the heat transfer coefficient of the fluid outside the tube, several baffles are usually installed in the shell. The baffle can increase the velocity of the fluid on the shell side, forcing the fluid to pass through the tube bundle several times laterally according to the prescribed path, and enhance the degree of fluid turbulence. The heat exchange tubes can be arranged in equilateral triangles or squares on the tube sheet. The equilateral triangle arrangement is compact, the fluid outside the tube has a high degree of turbulence, and the heat transfer coefficient is large; the square arrangement is convenient for cleaning outside the tube, and is suitable for fluids that are prone to scaling. Each time the fluid passes through the tube bundle is called a tube pass; each time it passes through the shell is called a shell pass. The picture shows the simplest single-shell-pass single-tube-pass heat exchanger, referred to as 1-1 type heat exchanger for short. In order to improve the fluid velocity in the pipe, partitions can be set in the pipe boxes at both ends to divide all the pipes into several groups. In this way, the fluid only passes through part of the tubes at a time, so it goes back and forth in the tube bundle many times, which is called multi-tube pass. Similarly, in order to increase the flow velocity outside the pipe, longitudinal baffles can also be installed in the shell to force the fluid to pass through the shell space multiple times, which is called multi-shell pass. Multi-tube pass and multi-shell pass can be used together. Type Because the temperature of the fluid inside and outside the tube is different, the temperature of the shell of the heat exchanger and the tube bundle are also different. If the two temperatures are very different, there will be a lot of thermal stress in the heat exchanger, causing the tubes 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 thermal stress.
The treatment method of the heat exchanger unit, according to the compensation measures adopted, the shell and tube heat exchanger can be divided into the following main types:
① The tube sheets at both ends of the tube bundle of the fixed tube sheet heat exchanger unit are integrated with the shell. The structure is simple, but it is only suitable for heat exchange operations when the temperature difference between the cold and hot fluids is not large, and the shell side does not need mechanical cleaning. When the temperature difference is slightly larger and the shell side pressure is not too high, an elastic compensation ring can be installed on the shell to reduce thermal stress.
②The tube sheet at one end of the tube bundle of the floating head heat exchanger can float freely, completely eliminating thermal stress; and the entire tube bundle can be pulled out from the shell, which is convenient for mechanical cleaning and maintenance. The floating head heat exchanger is widely used, but the structure is more complicated and the cost is higher.
③ Each heat exchange tube of the U-shaped tube heat exchanger is bent into a U shape, and the two ends are respectively fixed on the same tube plate in the upper and lower areas, and are divided into two chambers of inlet and outlet by means of the partition in the tube box. This kind of heat exchanger completely eliminates thermal stress, the structure is simpler than the floating head type, but the tube side is not easy to clean. Non-metallic material heat exchanger For heat exchange of strong corrosive fluids in chemical production, non-metallic materials such as ceramics, glass, PTFE, graphite and other non-metallic materials are used to make shell-and-tube heat exchangers. This type of heat exchanger has poor heat exchange performance and is only used in occasions with low pressure, low vibration and low temperature. Selection of runners For heat exchange of hot and cold fluids, the runners should be selected according to the following principles: 1. Unclean and fouling fluids should go through the tube, because it is easier to clean inside the tube; 2. Corrosive fluids should go through the tube to avoid tube bundles. Corrosion at the same time as the shell; ③ The fluid with high pressure should go through the tube to prevent the shell from being under pressure
④Saturated steam should go to the shell side, because the heat transfer coefficient of steam condensation has nothing to do with the flow rate, and the condensate is easy to discharge;
⑤ If the temperature difference between the two fluids is large, when selecting a fixed tube-plate heat exchanger unit, the fluid with a large heat transfer coefficient should go to the shell side to reduce thermal stress. When operating the intensified plate heat exchanger unit, when the heat transfer coefficients on both sides of the tube wall are very different (such as the heat transfer between liquid and gas with low viscosity), try to reduce the thermal resistance on the side with the low heat transfer coefficient. If the coefficient of heat transfer outside the tube is small, an externally threaded tube (low finned tube) can be used to increase the heat transfer area and fluid turbulence on the outside of the tube and reduce thermal resistance. If the heat transfer coefficient in the tube is small, additives such as twist iron and spiral coils can be set in the tube to enhance the disturbance in the tube and enhance the heat exchange. Of course, the flow resistance of the fluid will also increase at this time.