Shell and Tube Heat Exchanger

Tubular (also known as shell and tube) heat exchanger is the most typical wall heat exchanger, which has a long history in industrial application, and still occupies a dominant position in all heat exchangers. The tubular heat exchanger is mainly composed of shell, tube bundle, tube plate and seal. The shell is mostly circular, and parallel tube bundle is installed inside, and both ends of the tube bundle are fixed on the tube plate.

Two kinds of fluids for heat transfer in the tube heat exchanger, one flowing in the tube, its stroke is called the tube; One flows outside the tube, and its travel is called the shell path. The wall of the tube bundle is the heat transfer surface. In order to improve the heat transfer coefficient of the fluid outside the tube, a certain number of transverse baffle plates are usually installed in the shell. Baffles can not only prevent the fluid short circuit, increase the fluid speed, but also force the fluid to flow through the tube bundle several times according to the specified path, so that the degree of turbulence is greatly increased. Commonly used baffles have two kinds of round shape and disc shape, the former is more widely used. Each time the fluid passes through the tube bundle in the tube is called a tube pass, and each time it passes through the shell is called a shell pass. In order to increase the fluid velocity in the pipe, appropriate partitions can be arranged in both ends of the head to evenly separate all the pipes into several groups. In this way, the fluid can only pass through part of the pipe and return to the tube bundle several times at a time, which is called multi-pipe. Similarly, in order to increase the flow rate outside the tube, a longitudinal baffle can be installed in the housing to make the fluid pass through the housing space several times, called multiple shell passes. In the tube heat exchanger, the temperature of the shell and tube bundle is also different due to the different fluid temperature inside and outside the tube.

The fixed tube-plate heat exchanger is to weld the two ends of the tube-plate directly with the shell. It is mainly composed of shell, tube plate, tube bundle, seal and other main components. The tube bundle is arranged in the shell, and the tube bundle is fixed on the tube plate by welding, expansion or expansion welding at both ends, and the outer circumference of the tube plate and the flange of the head are fastened by bolts. The fixed tubeplate heat exchanger has the advantages of simple structure, low cost, easy manufacture and convenient tube cleaning and maintenance, but the shell cleaning is difficult and the temperature difference stress exists after the tube bundle is manufactured. When there is a large temperature difference between the heat exchange tube and the shell, the shell should also be provided with an expansion joint.

One end of the tube plate of the floating head heat exchanger is fixed between the shell and the tube box, and the other end of the tube plate can move freely in the shell, that is, the thermal expansion of the shell and bundle can be free. Therefore, there is no temperature difference stress between the tube bundle and the shell. Generally, the floating head can be disassembled, and the tube bundle can be freely extracted and loaded. This structure of the floating head heat exchanger can be used in conditions where there is a large temperature difference between the bundle and the shell. The cleaning and maintenance of the tube bundle and the shell is more convenient, but its structure is relatively complex, and the requirements for sealing are also relatively high.

The U-shaped tube heat exchanger is a U-shaped heat exchange pipe, and both ends are fixed on the same tube plate. Because the shell and the heat exchange tube are separated, the heat exchange tube bundle can be freely expanded and expanded, and there will be no temperature difference stress due to the temperature difference of the medium. U-tube heat exchanger has only one tube plate, no floating head, the structure is relatively simple. The tube bundle can be freely extracted and loaded, easy to clean, with the advantages of floating head heat exchanger, but because the heat exchange tube is made into a U-shaped bend with different radius, the outermost heat exchange tube can be replaced after damage, and other tubes can only be blocked. At the same time, compared with the fixed tube-plate heat exchanger, because the heat exchange tube is limited by the bending radius, there is a gap in the central part of the tube bundle, and the fluid is easy to short-circuit, which affects the heat transfer effect.