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Bellows Heat Exchanger Design Standard



Abstract: Bellows heat exchanger is a kind of high efficiency shell-and-tube heat exchanger. It is necessary to promote its strength and safety in order to popularize its application range. This paper introduces the method and main contents of the design of the strength of the bellows heat exchanger, summarizes the problems in the operation of the bellows heat exchanger, and analyzes the design and calculation examples to illustrate the design and selection of the bellows heat exchanger. For the design, manufacture, safety monitoring and use of the corrugated pipe heat exchanger.
1 Overview
The early 90s of the 20th century, China developed a highly efficient heat exchanger components - corrugated tube. The corrugated tube is composed of two parts: bellows and joint.
With the bellows instead of smooth straight tube made of corrugated pipe heat exchanger, the heat transfer efficiency can be increased by 2 to 4 times, this efficient heat exchanger also has easy to scale, the temperature difference is small and so on. However, since the bellows is made of thin-walled (5 to 10 mm) smooth tube, the stress state is complicated after molding. The strength and stiffness of the tube bundle are very different from the smooth tube, resulting in the heat exchanger tube and tube plate stress analysis difficult. As the tube bundle and tube sheet strength problem is not resolved, making this highly efficient equipment, the scope of application is limited, but also security risks. In order to make this kind of energy efficient products can be applied to a wide range of industrial fields such as petroleum and chemical industry, it is necessary to study the strength of bellows heat exchangers and establish corresponding design standards.
The work was incorporated in 2002. Boiler and Pressure Vessel Standardization Technical Committee project, by the Shenyang Institute of Chemical Technology and Shenyang Special Equipment Testing and Research Institute (the original Shenyang City, the prosecution) and other units responsible for the drafting of the bellows heat exchanger design method For the GB151 "shell and tube heat exchanger," the appendix, (spiral plate heat exchanger) and in 2004 with the "Austrian stainless steel bellows heat exchanger design standard case" (hereinafter referred to as "standard case") formal Promulgated.
In this paper, the main contents of the standard case are introduced, and the problems in the operation of the bellows heat exchanger are summarized and summarized. The design and selection of the bellows heat exchanger are pointed out by the design example. problem.
2, the standard case brief
Corrugated pipe heat exchanger is also a kind of shell and tube heat exchanger, its overall design, manufacture, inspection is in line with the requirements of GB151, but because of changes in tube stiffness, making its associated compression components design calculation is different. Based on the theoretical analysis and experimental research, the standard case is based on the principle of safety, reliability, economic rationality and advanced technology, which stipulates the design and calculation method of the tube bundle and tube plate of the bellows heat exchanger as the supplement of GB151.
2-1 Scope of application
The heat exchanger type is the same as GB151. Design pressure PN ≤ 40MPa, considering the corrugated tube is made of thin-walled tube processing, forming a larger residual stress, in order to avoid stress corrosion and fatigue damage, the working pressure can not be too high. Standard case provides two kinds of heat transfer tube nominal diameter, that is, crest / trough diameter of 32 / 25mm and 42 / 33mm, the tube diameter of 25mm and 33mm, which is currently used in the project two specifications. The spacing of the baffle plate is based on engineering experience, and its large spacing is about 2/3 of the large spacing of 5953 in GB151, and does not take into account the fluid induced vibration.
Considering the shape of bellows and the actual use of ten years, the standard case only applies to the corrugated pipe material is austenitic stainless steel.
Design of Corrugated Heat Transfer Tubes
After the smooth tube is formed into a bellows through the expansion, the bearing capacity of the bellows is much lower than that of the corresponding light pipe (ie, the tube) due to the existence of residual stress and local stress concentration. The axial stiffness and stability of the corresponding bellows are also greatly reduced. In the third case of the standard case, the formula for the allowable internal pressure, allowable external pressure, axial stiffness and critical pressure of the bellows is given. These formulas are based on experimental and theoretical analysis.
2-2-1 Permissible internal pressure of corrugated heat transfer tubes
For the various parts of the bellows stress detailed calculation, is still being studied. The stress distribution in the internal pressure has been tested by a large number of tests by the stress test method. At the same time, the actual value of the burst pressure is close to the theoretical value calculated by the medium diameter formula, and the measured value Greater than the theoretical value.
2-2-2 Corrugated Heat Exchanger Allowable External Pressure
Shell and tube heat exchanger tube at the same time to withstand the pressure of the tube and shell, so the stability of the tube to withstand external pressure should also be considered. Most of the bellows shape is a nod, that is, by the section and the spherical part of the composition. Straight part of the rigidity is much smaller than the peak part of the test also proved that the bellows by the external pressure when the straight part of the first instability. For the whole bellows can be regarded as the external pressure of the cylinder, the peak part of the equivalent of rigid components. In this way, the calculated length of the tube subjected to external pressure is greatly reduced, and in the standard case it is safe to take the large pitch in the single root tube as the calculated length. According to engineering practice experience and experimental research, the standard case cited GB150 in Chapter 6 of the external pressure cylinder design calculation method.
According to L / d1 and d1 / δt direct use of GB150 in Chapter 6 of the external pressure calculation diagram, find the A value and B value, you can get the bellows allowable external pressure, namely:
2-3 tube sheet design calculation
So far, most of the heat exchanger standards on the tube plate strength calculation, the basic are the tube plate as a uniform load, placed on the basis of elasticity and the tube hole weakened equivalent round plate to consider. The tube sheet design in GB151 is also based on this consideration. For the bellows heat exchanger, relative to the conventional (smooth tube) shell and tube heat exchanger is concerned, the super-major difference is the heat pipe stiffness problem. Based on this consideration, the parameters related to the axial stiffness of the pipe are introduced into the calculated values ​​of the corrugated pipe stiffness, and then the design calculation of the corrugated pipe heat exchanger tube plate is carried out directly by using the calculation formula of the corresponding tube plate type in GB151.
3, corrugated pipe heat exchanger common failure form and its causes analysis
(1) corrugated heat transfer tube trough or crest trough transition site thinning cracking. Corrugated pipe heat exchanger operation, the corrugated heat transfer tube trough and its vicinity of thinning cracking, resulting in internal leakage is the main form of its failure. Failure reason is in the shell baffle at the corrugated tube and the baffle pipe hole vibration vibration, bump, so that the corrugated pipe wall thinning, resulting in cracking leakage. Now some production plants use thickened baffle plate, so that the bellows peak and pipe hole contact to ensure that the tube and the hole gap is too small to prevent vibration friction; also in the baffle to the bellows plus sets. These are good measures to avoid and reduce the failure of this bellows form.
(2) corrugated tube collapse (circumferential instability). Corrugated tube instability occurs in the circumferential flat collapse, is another failure form of the bellows heat exchanger. This is mainly due to the thinner wall thickness of the bellows, generally below 1mm, its ability to resist external pressure instability is very low. In the design of the heat exchanger, the heat transfer capacity of the heat transfer tube is generally not checked and calculated. Therefore, when the shell pressure reaches and exceeds the critical pressure of the heat transfer tube itself, The The standard case specifies the calculation method for the permissible external pressure of the corrugated heat transfer tube.
(3) corrugated heat transfer tube axial bending deformation is too large (axial instability). The failure to produce this failure is due to the effect of pipe pressure and temperature stress. Bellows material is austenitic stainless steel, the linear expansion coefficient is much larger than the carbon steel, pipe and shell temperature in the same temperature can produce temperature stress, and then the axial stiffness of the bellows is very small, so when the tube When the pressure is larger or the wall temperature is higher than the shell wall temperature, the corrugated heat transfer tube is prone to excessive bending deformation. Standard case specified baffle no support span than the provisions of GB151 is small, is to consider the axial instability of the heat transfer tube.
(4) corrugated tube corrosion fracture and the overall embrittlement failure. This failure is mainly due to the corrosion of the media caused. Austenitic stainless steel ultra-easy to produce intergranular corrosion, when the bellows heat exchanger used in high chloride ions and hydrogen sulfide and other media, there have been corrugated tube corrosion fracture. In practice, some heat transfer tubes have been found to have an overall embrittlement phenomenon.
(5) the connection between the bellows and the thick-walled pipe joint. The corrugated tube is made up of bellows and both ends of the joint, and the weld quality at the joint is difficult to guarantee due to the difference in the welding process and the welding technology.