The temperature condition of the tube sheet on an air electric heater, reasonable design of tube sheet structure and strength calculation. The design pressure of this air electric heater is MPa 2.0, design temperature is 440℃, medium is air, electric heating power is 1200KW, main pressure-bearing components material 16Mn, Q345R.
The tube sheet of this air electric heater is equivalent to the tube sheet of a U-tube heat exchanger (without tube box and partition plate). The tube hole diameter on the tube sheet is Φ22.5 mm, and a section of sleeve with specification Φ22×2.5 mm is welded in each tube hole. The U-tube electric heating element diameter is Φ16 mm. The electric heating element extends out of the sleeve and enters the junction box at the top of the equipment. Sealing weld is used between the electric heating element and the outer end face of the sleeve. The section from the terminal end to the heat insulation baffle below the tube sheet is the non-heating zone. The tube holes on the tube sheet are arranged in concentric circles. From the outermost circle inward, the number of tube holes on each adjacent two circles of hole arrangement circles is equal. The U-tube electric heating elements span across the tube holes in these adjacent two circles. The innermost circle of 6 tube holes is cross-inserted with 3 U-tube electric heating elements of different lengths in a plum blossom pattern. A thermocouple sleeve is assembled in the center hole.
According to the design pressure P=2.0MPa and design temperature t=440℃ of the vessel, the material of the vessel flange is 16Mn forging. According to NB/T47020~47027-2012 "Pressure Vessel Flanges, Gaskets, Fasteners", the vessel flange is selected as a long-neck flange with nominal pressure PN4.0, flange outer diameter Φ1215 mm, stud specification M30, quantity 48 pieces, stud material 35CrMoA, gasket using 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 opening widths on the dangerous radial section of the flat cover is Σb=420mm).
From the above calculation results, 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 at 150℃ enters the vessel from the upper nozzle and flows downward, is heated by the electric heating elements, the temperature rises, and after heating ends, the temperature rises to 420℃, and 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. The temperature of the tube sheet and vessel flange in contact with the gas is only about 150℃, and there are two layers of baffles below the tube sheet for heat insulation. The temperature of the shell gradually increases from 150℃ at the gas inlet end to 420℃ at the gas outlet end. Therefore, it is unreasonable to design the tube sheet with the same design temperature as the vessel shell. According to the structure and operating conditions of this electric heater, we believe that the design temperature of the tube sheet can be reduced, and it is unnecessary to take the design temperature of the vessel shell. Clause 4 of Article 5.0.1 of HG/T 20580-2011 "Specification for Design Basis of Steel Chemical Vessels" stipulates that when the metal temperatures of various parts of the vessel under working conditions are different, 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 about 30℃. Therefore, the actual temperature of the tube sheet is about 180℃. Considering a certain design margin, it is safe to take the design temperature of the tube sheet as 200℃. 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 exceed 200℃, so it should be safe to take the design temperature of the vessel flange as 200℃. When the design temperature of the vessel flange is taken as 200℃, the nominal pressure of the flange can be selected as PN2.5, flange outer diameter Φ1195 mm, stud specification M27, quantity 36 pieces, stud material 40MnB. Using the above conditions and parameters, recalculate with SW6 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 setting the design temperatures of the shell, tube sheet, and vessel flange separately, the design thickness of the tube sheet is reduced from 124 mm to 88 mm, significantly thinning the tube sheet thickness and reducing material consumption. Of course, if different pressure-bearing components on a vessel are set with different design temperatures, 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 metal temperatures of various parts of the shell are different. Therefore, when performing strength calculations, if pressure-bearing components in low-temperature areas are calculated according to the design temperature of the vessel, the required thickness is larger. According to the working conditions, the possible metal temperature of the component can be determined separately, and its design temperature can be set, and then the required thickness of the component can be calculated.
The air electric heater described in this article has the tube sheet assembling electric heating elements in the low-temperature area, i.e., the low-temperature gas enters the electric heater shell from a position close to the tube sheet. If the temperature difference between gas inlet and outlet is large, setting the design temperatures of the tube sheet and shell separately can significantly reduce the design thickness of the tube sheet, which can not only reduce material consumption but also shorten the machining time of the tube sheet, especially the drilling time of tube holes.
