Tube Sheet Structure Design and Strength Calculation of Air Electric Heater
Analyze the temperature conditions of the tube sheet on an air electric heater, and reasonably design the tube sheet structure and perform strength calculation. The design pressure of this air electric heater isMPa 2.0, the design temperature is440℃, the medium is air, the electric heating power is1200KW, and the main pressure-bearing component materials are16Mn andQ345R.
The tube sheet of the air electric heater is equivalent to the tube sheet of a U-tube heat exchanger (without channel and partition plate). The tube hole diameter on the tube sheet is Φ22.5 mm. A sleeve with specification Ф22×2.5 mm is welded in each tube hole. The U-tube electric heating element has a diameter of Φ16 mm. The heating element extends out of the sleeve and enters the wiring compartment at the top of the equipment. The heating element and the outer end face of the sleeve are sealed by welding. The section of the heating element from the wiring end to the heat insulation baffle below the tube sheet is the non-heating zone. The tube holes on the tube sheet are distributed in concentric circles. From the outermost circle inward, the number of tube holes on each adjacent two circles is equal. The U-tube electric heating elements span across the tube holes of the adjacent two circles. The innermost circle has6 tube holes, into which3 U-tube electric heating elements of different lengths are inserted in a quincunx pattern. A thermocouple well is assembled in the center hole.
According to the design pressure of the vesselP=2.0MPa, design temperaturet=440℃, the material of the vessel flange is16Mn forging. According toNB/T47020~47027-2012 "Pressure Vessel Flanges, Gaskets, and Fasteners", the vessel flange is selected as a long-neck flange with nominal pressurePN4.0, flange outer diameter Φ1215 mm, stud specificationM30, quantity48, stud material35CrMoA, and gasket is a spiral wound gasket. Using the above conditions and parameters, the thickness of the tube sheet is calculated with calculation software, and the design thickness is obtained as δ=124 mm (where the sum of the widths of all openings on the dangerous radial section of the flat cover is Σb=420mm).
From the above calculation results, it can be seen that if the tube sheet is calculated according to the design temperature of the vessel, the design thickness of the tube sheet is relatively large. However, the gas at150℃ enters the vessel from the upper nozzle and flows downward, is heated by the electric heating elements, and its temperature rises. After heating, the temperature rises to420℃, and it flows out from the lower nozzle. During this process, the gas temperature gradually increases. The gas just entering the upper part of the vessel has not been heated yet, and the temperature of the tube sheet and vessel flange in contact with the gas is only about150℃. Moreover, there are2 layers of baffles below the tube sheet for heat insulation. The temperature of the shell from the gas inlet end to the gas outlet end gradually increases from150℃ to420℃. Therefore, it is unreasonable to design the tube sheet with the same design temperature as the vessel shell. Based on the structure and operating conditions of this electric heater, we believe that the design temperature of the tube sheet can be lowered, and it is unnecessary to adopt the design temperature of the vessel shell.HG/T 20580-2011 "Code for Design of Steel Chemical Vessels - Basis" Clause5.0.1, Item4, stipulates that when the metal temperatures of different parts of a vessel differ under working conditions, the design temperatures of each part can be set separately. Considering the heat conduction of the shell and the heat transfer of the electric heating elements, the temperature of the tube sheet is higher than the gas temperature, and this temperature difference is estimated to be around30℃. Therefore, the actual temperature of the tube sheet is approximately180℃. Considering a certain design margin, it is safe to take the design temperature of the tube sheet as200℃. The vessel flange is in the same situation as the tube sheet; its position is also above the low-temperature gas inlet, and the actual temperature will not exceed200℃. Therefore, it should be safe to take the design temperature of the vessel flange as200℃. When the design temperature of the vessel flange is taken as200℃, the nominal pressure of the flange can be selected asPN2.5, with flange outer diameter Φ1195 mm, stud specificationM27, quantity36, and stud material40MnB. Recalculate with the above conditions and parameters using theSW6 calculation software, and the design thickness of the tube sheet is obtained as δ=88 mm. Through the above analysis and calculation, according to the working conditions of the electric heater, after separately setting the design temperatures of the shell, tube sheet, and vessel flange, the design thickness of the tube sheet is reduced from124 mm to88 mm, significantly reducing the thickness of the tube sheet and lowering material consumption. Of course, if different design temperatures are set for different pressure-bearing components on a vessel, the main pressure-bearing components corresponding to different design temperatures should be listed in the design data table on the general arrangement drawing.
For pressure vessels such as electric heaters, because the temperature of the medium gradually increases from the inlet to the outlet, the various parts of the shell
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