Discussion on the axial compressive stress problem of heat exchange tubes
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According to 5.3.2.2 in GB/T151, \"when the heat exchange tubes are subjected to axial compressive stress, a cladded composite tube sheet is recommended.\" The calculation of the axial stress in shell-and-tube heat exchange tubes is given by equations 7-20, 7-35, and 7-47. There are three different operating conditions, and in all of these cases, axial compressive stress is present. Can it be inferred from this that in shell-and-tube heat exchangers, explosive welding composite tube sheets cannot be used, and only surfacing-welded composite tube sheets are permissible?
With that conclusion of yours, the scope of impact is far too broad. All three formulas you mentioned pertain to U-tube heat exchangers; therefore, it can at most be said that U-tube heat exchangers are not suitable for use with explode composite plates. Additionally, this issue actually needs to be discussed categorically. Taking the heat exchange tube D25x2 as an example: 1. If there is negative pressure on the tube side, an explode composite plate is generally not used on that side ； 2. The pressure on both the tube and shell sides is positive. (1) When ps=0, the axial stress in the tube (hereinafter referred to as stress) is 2.4 pt, which is positive ； (2) When pt=0, the stress is -3.4 ps, which is negative ； (3) When forces act on both sides, the stress is 2.4 pt–3.4 ps; it is positive when pt/ps > 1.42, negative otherwise, and the negative value is always less than that when pt = 0. The positive values mentioned above are not discussed; only the absolute value of the largest possible negative value is considered. Heat exchangers that use explode composite tube sheets generally operate at not very high pressures; depending on the diameter of the heat exchanger, these pressures usually do not exceed 6.4 MPa, and the pressure on the tube side is typically higher than that on the shell side. At a shell-side pressure of 6.4 MPa, the maximum axial compressive stress (or the tearing force on the laminate at the junction of the tube and the tube sheet) is 21.8 MPa; under such conditions, it is indeed not appropriate to use explode composite tube sheets. But when the shell-side pressure is very low, using explode is also acceptable. 3. Positive for the tube shell and negative for the other, (1) ps=0, the same as (1) above ； (2) pt=0, stress is positive ； (3) Both the tube and the shell act simultaneously, with a positive effect. Therefore, in the case of positive shell and negative tube, the axial stress in the tube must be tensile, and there is no problem using explode compounding on the tube side.