Air Electric Heater Tube Sheet Structure Design and Tube Sheet Strength Calculation
Analyze the temperature condition of a tube sheet on an air electric heater, and reasonably design the tube sheet structure and calculate the tube sheet strength. The design pressure of this air electric heater is MPa 2.0, the design temperature is 440°C, the medium is air, the electric heating power is 1200KW, the main pressure-bearing element material is 16Mn, Q345R.
The tube sheet of the air electric heater is equivalent to the tube sheet of a U-tube heat exchanger (without tube box and baffle). The tube hole diameter on the tube sheet is Φ22.5 mm, and a sleeve with specification Ф22×2.5 mm is welded in each tube hole. The diameter of the U-tube electric heating element is Φ16 mm. The electric heating element extends out of the sleeve and enters the wiring compartment at the top of the equipment. A sealing weld is used between the electric heating element and the outer end face of the sleeve. The section 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 of hole arrangement is equal. The U-tube electric heating elements span across the tube holes of these two adjacent 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 installed in the center hole.
According to the design pressure of the vessel P=2.0MPa, design temperature t=440°C, the material of the vessel flange is 16Mn forging. According to NB/T47020~47027-2012 "Pressure Vessel Flanges, Gaskets, and Fasteners", the vessel flange selects 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 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, 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 at 150°C enters the vessel from the upper nozzle and flows downward, is heated by the electric heating elements, the temperature rises, and after heating, the temperature rises to 420°C, and flows out from the lower nozzle of the vessel. During this process, the temperature of the gas gradually increases. The gas just entering the upper part of the vessel has not yet been heated, and the temperature of the tube sheet and vessel flange in contact with the gas is only about 150°C. Moreover, there are 2 layers of baffles below the tube sheet for heat insulation. The temperature of the shell gradually increases from 150°C to 420°C from the gas inlet end to the gas outlet end. Therefore, it is unreasonable to set the design temperature of the tube sheet the same as that of 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 reduced, and it is unnecessary to take the design temperature of the vessel shell. HG/T 20580-2011 "Steel Chemical Vessel Design Basis Regulations" Clause 5.0.1, Item 4 stipulates that when the metal temperatures of different parts of the vessel under working conditions are different, the design temperature 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°C. Therefore, the actual temperature of the tube sheet is approximately 180°C. Considering a certain design margin, it is safe to take the design temperature of the tube sheet as 200°C. The vessel flange is in the same situation as the tube sheet, and its position is also in the upper part of the low-temperature gas inlet. The actual temperature will not exceed 200°C, so it should be safe to take the design temperature of the vessel flange as 200°C. When the design temperature of the vessel flange is taken as 200°C, 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. Recalculate the above conditions and parameters using SW6 calculation software, and the design thickness of the tube sheet is δ=88 mm. Through the above analysis and calculation, after setting the design temperatures of the shell, tube sheet, and vessel flange separately according to the working conditions of the electric heater, 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 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 of the equipment.
For pressure vessels such as electric heaters, because the temperature of the medium gradually increases from the inlet to the outlet, various parts of the shell
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