Thread Content
1. Process design 1. Make a flow diagram. 2. Calculate the heat exchange heat Q of the heat exchanger according to the production task. 3. Select the heat carrier and find the flow rate of the heat carrier. 4. Determine the flow paths of cold and hot fluids. 5. Calculate the qualitative temperature and determine the physical property data of the fluid (density, specific heat, thermal conductivity, etc.). 6. Initial calculation of the average heat transfer temperature difference. 7. Select or estimate the K value based on experience or field data, and initially calculate the required heat transfer area. 8. Carry out preliminary design of the size of the heat exchanger based on the initially calculated heat exchange area. Including pipe diameter, pipe length, number of pipes, number of pipe passes, pipe arrangement, shell inner diameter (rounding required), etc. 9. Accounting K. 10. Check the average temperature difference D. 11. When checking the heat transfer, a margin of 15-25% is required. 12. Calculation of pressure drop on tube side and shell side. 2. Mechanical design 1. Determination of shell diameter and calculation of shell wall thickness. 2. Heat exchanger head selection. 3. Heat exchanger flange selection. 4. The tube sheet size is determined. 5. Calculation of pipe pull-off force. 6. Selection and calculation of baffles. 7. Calculation of temperature difference stress. 8. Selection of nozzle, nozzle flange and hole reinforcement, etc. 9. Draw diagrams of main components. 3. Prepare a summary table of calculation results. 4. Draw the assembly diagram of the heat exchanger. 5. Put forward technical requirements. 6. Prepare the design instructions. Section 2. Process design of tubular heat exchanger 1. Determination of the final heat exchange temperature. The final heat exchange temperature has a great influence on the heat transfer efficiency and heat transfer intensity of the heat exchanger. In countercurrent heat exchange, when the final temperature of the fluid outlet is close to the initial temperature of the hot fluid inlet, the heat utilization rate is high, but the heat transfer intensity is the smallest and the required heat transfer area is the largest. In order to reasonably determine the medium temperature and heat exchange final temperature, the following data can be referred to: 1. The temperature difference at the hot end (large temperature difference) is not less than 20℃. 2. The cold end temperature difference (small temperature difference) is not less than 5℃. 3. In the cooler or condenser, the initial temperature of the coolant should be higher than the freezing point of the fluid being cooled. ； For condensation containing non-condensable gases, the final temperature of the coolant is required to be lower than 5°C below the dew point of the condensed gas. 2. Calculation of average temperature difference When initially calculating the average temperature difference Dtm during design, the heat exchange process is first considered as a countercurrent process. 1. For the countercurrent or parallel flow heat exchange process, the average temperature difference can be calculated according to formula (2-1): (2-1) In the formula, , are the temperature difference of the big end and the temperature difference of the small end respectively. At that time, the arithmetic mean was available. 2. For the cross-flow or baffled heat exchange process, if there is no phase change, the temperature difference needs to be corrected, that is, calculated using formula (2-2). (2-2) where is the average temperature difference calculated based on counter-flow. The correction coefficient can be found out from the relevant illustrations in the chemical engineering principles textbook according to the different conditions of the heat exchanger. The general requirement is >0.8, otherwise multiple shell passes should be used or multiple heat exchangers should be used in series. 3. Determination of the total heat transfer coefficient K. Calculate the base area for the K value. * The external area of the commonly used pipe. When the basic conditions of the design object (equipment type, Reynolds number Re, fluid physical properties, etc.) are the same or similar to a production equipment with a known K value, the empirical data of the K value of the known equipment can be used as the K value of your own design. Table 2-1 shows the approximate range of K values for common tube and tube heat exchangers. Select the approximate K value from Table 2-1. Table 2-1 Empirical value of the total heat transfer coefficient K in the tube heat exchanger. Cold fluid hot fluid total heat transfer coefficient W/m2.℃ Water water 850-1700 Water gas 17-280 Water organic solvent 280-850 Water light oil 340-910 Water heavy oil 60-280 Organic solvent Organic solvent 115-340 Water vapor condensation 1420-4250 Gas vapor condensation 30-300 Water low boiling point hydrocarbon condensation 455-1140 Water boiling water vapor condensation 2000-4250 Light oil boiling water vapor 455-1020 Use formula (2-3) to calculate the K value. (2-3) In the formula: a - heat transfer coefficient, W/m2.℃ ； R－dirt thermal resistance, m2.℃/W ； δ - pipe wall thickness, mm ； λ - Thermal conductivity of pipe wall, W/m.℃ ； The subscripts i, o, and m represent inside the tube, outside the tube, and average, respectively. At that time, the calculation was approximately based on a flat wall, that is,: When calculating the K value using equation (2-3), the dirt thermal resistance usually uses empirical values. The general range of commonly used dirt thermal resistance can be found in the relevant content of "Principles of Chemical Engineering". The heat supply coefficient a in the formula is often calculated using relevant empirical value formulas in the design of tube-and-tube heat exchangers. Some commonly used empirical correlations for calculating a in engineering have been introduced in "Principles of Chemical Engineering" and should be selected during design. 4. Determination of heat transfer area A. In engineering, the sum of the external areas of all tubes in the tube bundle in the tube bundle is often regarded as the heat transfer area, and is calculated by equations (2-4) and (2-5). (2-4) (2-5) In the formula: - Heat transfer coefficient based on outer surface, W/m2.℃ - Pipe outer diameter, m ； L - the effective length of each pipe, m ； n - the total number of pipes. The effective length of the pipe refers to the actual length of the pipe minus the portion occupied by the tube sheet and baffle. The total number of tubes refers to the number of tubes after rounding minus the number of tie rods. 5. Determination of the main process dimensions. After the heat transfer area is determined, the design work enters the preliminary design stage of the heat exchanger size, including the following contents: 1. Selection of pipes. Choosing smaller diameter tubes can improve the convection heat transfer coefficient of the fluid and increase the heat transfer area per unit volume of the equipment. The equipment is more compact and consumes less metal per unit heat transfer area. However, it is troublesome to manufacture. Small tubes are easy to scale and difficult to clean, so they can be used for cleaner fluids. Large diameter pipes are used for fluids that are more viscous or prone to scaling. my country's tube-type heat exchangers often use seamless steel pipes, the specifications are outer diameter × wall thickness, commonly used heat exchange tube specifications: φ19×2, φ25×2.5, φ38×3. The selection of pipes should consider the convenience of cleaning work and the rational use of pipe materials. At the same time, the coordination of pipe length and pipe diameter should also be considered. Domestic pipe production specifications, the length is generally: 1.5, 2, 2.5, 3, 4.5, 5, 6, 7.5, 9, 12ｍ, etc. The ratio of the heat exchange tube length to the shell diameter of the heat exchanger is generally 6-10. For vertical heat exchangers, the ratio is 4-6. The pressure drop between the shell side and the shell side, and the pressure drop of the fluid in the heat exchanger are mainly determined by the operating pressure of the system, and the operating pressure of the system is provided by the transportation equipment. The greater the fluid resistance loss (pressure drop) in the heat exchanger, the greater the power of the conveying equipment required and the higher the energy consumption. For heat exchange without phase change, the higher the fluid flow rate, the greater the heat exchange intensity, which can reduce the heat exchange area, make the equipment compact, reduce production costs, and help suppress the generation of dirt. However, if the flow rate is too high, there are also disadvantages. The pressure drop increases, the pump power increases, and the erosion of the heat transfer tubes is intensified. Therefore, in the design of the heat exchanger, there is the problem of selecting an appropriate flow rate and controlling a reasonable pressure drop. General experience: for liquids, the pressure drop should be controlled between 0.01 and 0.1MPa, and for gases, the pressure drop should be controlled between 0.001 and 0.01MPa. Table 2-2 lists the empirical data of reasonable pressure drop under different operating conditions of the heat exchanger for design reference. Table 2-2 Selection of reasonable pressure drop for tube heat exchangers Heat exchanger operating conditions Negative pressure operation Low pressure operation Medium pressure operation (including pumping liquid) Higher pressure operation P0.17 Operating pressure (MPa absolute pressure) P=0～0.1 P=0.1～0.17 P＝0.17～1.1 P=1.1～3.1 P=3.1～8.2 Reasonable pressure drop (MPa) DP＝P/10 DP＝ｐ/2 DP=0.035 △ｐ=0.035～0.18 △ｐ=0.07～0.25 2. Determination of the total number of tubes n. For the determined heat transfer area, after selecting the pipe diameter and pipe length, the required number of pipes n can be found, which is calculated by formula (2-6). (2-6) where - heat transfer area, ； －Pipe outer diameter, m ； L - effective length of each pipe, m ； The calculated tube n is rounded 3 and the tube pass number m is determined. According to the number n of tubes, the flow rate of the fluid in the tube can be calculated by formula (2-7). (2-7) where vs - pipe side fluid volume flow rate, - pipe inner diameter, m ； n - number of tubes. If the flow rate is very small compared to the required appropriate flow rate, multi-tube passes need to be used. The number of tube passes m can be calculated according to formula (2-8). m=u/ (2-8) Where - the flow velocity in the pipe calculated using the number of pipes n, m/s ； u-required suitable flow rate, m/s ； The appropriate flow rate u in formula (2-8) should be selected based on the commonly used flow rate range in tube heat exchangers. Please refer to the relevant content of "Principles of Chemical Engineering". It is generally required to work under turbulent flow (except for high-viscosity fluids). The corresponding Re value is 5×103 for liquids and - for gases. When dividing the passes, the number of tubes in each pass should be roughly equal. The number of tube passes commonly used in production is 1, 2, 4, 6. 4. Determine the arrangement of tubes and the distance between tubes. The principle of arranging the tubes on the tube plate is: The tubes are evenly distributed throughout the cross-section of the heat exchanger, are compactly arranged, and have a reasonable structural design, which is convenient for manufacturing and suitable for the characteristics of the fluid. The arrangement is usually two types: equilateral triangle and square. Concentric circle arrangement and combined arrangement are also used. In some multi-pass tube heat exchangers, the arrangement within the passes is generally an equilateral triangle, but a square arrangement is often used between passes, which is very beneficial for the installation of partitions. At this time, the arrangement on the entire tube plate is called a combined arrangement. For multi-tube pass heat exchangers, the split-pass longitudinal partitions occupy part of the area of the tube plate, and the actual number of tubes is less than the theoretical number. The actual number of tubes should be obtained from the tube plate layout during design. When arranging tubes, the tube spacing should be determined first. When deciding the distance between tubes, you should first consider the strength of the tube sheet and the method required to clean the surface of the tubes. The size is also related to the way the tubes are fixed on the tube sheet. A large amount of practice has proved that the empirical value of the minimum tube spacing is: Welding expansion method, generally takes (1.3~1.5). The distance between the center of the outermost tube of the tube bundle and the inner surface of the shell is not less than. 5. Calculation of shell. The inner diameter of the tube heat exchanger shell should be equal to or slightly larger (for floating head heat exchangers) than the diameter of the tube plate, which can be calculated by equation (2-9). Di＝a（b－1）＋2L (2－9) In the formula, Di－the inner diameter of the shell, mm ； a - tube spacing, mm ； b - the number of tubes on the diagonal of the outermost hexagon ； L - The distance from the center of the outermost tube to the inner wall of the shell, generally L=(1~1.5), mm ； If the pipe is divided, Di=f+2L How to determine f value: It can be obtained by looking up a table or using a diagram. When the number of tubes n and the distance between tubes a are known, start to arrange them in an equilateral triangle until n tubes are arranged, and then count the number of tubes on the diagonal. The calculated shell diameter Di should be rounded to within the standard size series of the container. Section 3 Mechanical Design of Tube Heat Exchangers There are many types of tube heat exchangers in chemical enterprises, such as plate type, sleeve type, volute type, and tube type. Among them, although the tube heat exchanger is not as good as the plate heat exchanger in terms of thermal efficiency, compactness, metal consumption, etc., it has the characteristics of solid structure, high reliability, strong adaptability, and wide range of materials. Therefore, it has become the main structural form of heat exchangers in petroleum and chemical production, especially high temperature, high pressure, and large heat exchangers. Tube heat exchangers mainly include fixed tube plate heat exchangers, floating head heat exchangers, filler heat exchangers and U-shaped tube heat exchangers. Among them, fixed tube plate heat exchangers are the most commonly used due to their simple structure and low cost. Mechanical design of tube and tube heat exchangers includes: 1. Determination of shell diameter and calculation of shell wall thickness. 2. Heat exchanger head selection. 3. Pressure vessel flange selection. 4. The tube sheet size is determined. 5. Calculation of pipe pull-off force. 6. Selection and calculation of baffles. 7. Calculation of temperature difference stress. 8. Selection of nozzle, nozzle flange and hole reinforcement, etc. 9. Draw main component diagrams and assembly diagrams. Described below: 1. Determination of shell diameter and calculation of shell wall thickness. 1. Known conditions: The type of media, temperature, pressure, temperature difference between shell and wall, and heat exchange area on the tube side and shell side are known from the process design. 2. Calculate (1) the number of tubes n: Seamless steel pipes are commonly used in tube and tube heat exchangers with the following specifications:: Carbon steel f19×2 f25×2.5 f32×3 f38×3 Stainless steel f19×2 f25×2 f32×2 f38×2.5 The selection of pipe material is based on the type of medium. If the medium is non-corrosive, choose carbon steel, and if the medium is corrosive, choose stainless steel. Pipe length specifications are 1500, 2000, 2500, 3000, 4500, 5000, 6000, 7500, 9000, 12000mm. n=A/(pdmL), where A—heat exchange area (m2) ； L—Heat exchange tube length mm ； dm—the average diameter of the pipe in mm. Because 4 or 6 tie rods need to be installed in the tube heat exchanger. Therefore, the actual number of heat exchange tubes is {n-4(6)}. (2) The arrangement of tubes and the distance between tubes are determined. The arrangement of pipes generally adopts an equilateral triangle arrangement within a process, and a square arrangement between processes. The tube spacing is selected based on the minimum tube spacing. Minimum tube spacing, tube outer diameter (mm) 14 19 25 32 38 45 57 Minimum tube spacing (mm) 16 25 32 40 48 57 70 (3) Determination of heat exchanger shell diameter Shell diameter calculation formula: When an equilateral triangle arrangement is adopted, it is Di=a(b-1)+2L, where Di—inner diameter of the heat exchanger ； a—tube spacing ； b—the number of tubes on the diagonal of the equilateral triangle ； L—The distance from the center of the outermost tube to the edge of the shell wall. If the pipe is divided, Di = f + 2L. In the formula, f - the distance between the centers of the two ends of the same inner diameter of the shell in mm. ； Di and L are the same as above. After calculating the diameter, it must be rounded to the nominal diameter series. (4) Calculation of the wall thickness of the heat exchanger shell The calculated wall thickness is S=PDi/(2σ] tΦ－P) where P——design pressure, MPa ； When P﹤0.6 MPa, take P=0.6 MPa ； Di—casing inner diameter, mm ； Φ—Weld coefficient, select Φ=0.85-1.0 according to the weld condition ； σ] t—allowable stress of the shell material at the design temperature, MPa. The material selection principles are the same as those for pipes. After calculating S, the steel plate thickness negative deviation C1 must be selected according to the steel plate thickness negative deviation table. ； Select the corrosion allowance C2 according to the corrosion situation, C2=KaB, where Ka is the corrosion rate (mm/a), and B is the design life of the container. When the corrosion rate of the material is 0.05~0.1mm/a, C2=1~2mm for single-sided corrosion and C2=2~4mm for double-sided corrosion. When the corrosion rate of the material is less than or equal to 0.05mm/a, C2=1mm for single-sided corrosion and C2=2mm for double-sided corrosion. For stainless steel, C2=0 can be taken when the medium is extremely corrosive. Finally, S+C1+C2 is rounded into the steel plate thickness series, so the total thickness Sn=S+C1+C1+C', C'—rounded value. 2. Heat exchanger head selection Various head types are available, but the most commonly used is the standard oval head, and there are currently standard series. Please check the JB-1154-73 standard when using it. See Appendix 1. 3. Selection of container flange 1. Material: Determined based on the medium in contact with the container and the temperature and pressure conditions. 2. Flange type: There are three types of container flanges to choose from, namely type A flat welding flange, type B flat welding flange and long neck butt welding flange. Its standard number is JB4700~4707-92, see Appendix 2. 4. Determine the tube plate size. Use a fixed heat exchanger tube plate, which also serves as a flange. It is recommended to adopt the relevant content in the "Steel Tube Fixed Tube Sheet Heat Exchanger Structural Design Manual". See Appendix 3. 5. Calculation of pull-off force The definition of pull-off force is the force exerted on the expansion joint perimeter of the pipe per square meter. For joints where the pipe and the tube sheet are welded, experiments have shown that the strength of the joint is higher than the strength of the pipe itself and the metal, and the pull-off force is not enough to cause damage to the joint; but for joints where the pipe and the tube sheet are expanded, the pull-off force may cause damage to the joint and sealing, or cause the pipe to pull off. In order to ensure a firm connection between the pipe end and the tube sheet and good sealing performance, the pull-off force must be checked. 1. Under operating conditions, the temperature difference axial force in the tube or shell is F=at(tt-to)-as(ts-to)] /+1/EsAs] In the formula, At, As--cross-sectional area of heat exchanger tube and shell wall ； at—pipe linear expansion coefficient 1/℃ ； as—shell material linear expansion coefficient 1/℃ ； to—temperature during installation ℃ ； tt—temperature in operating conditions ℃. The temperature difference stress in the tube and shell is: st=F/At ； ss=F/As 2. Under the operating pressure, the force on the expansion joint per square meter Qq=Pf/(pdoL) where P={pipe side pressure Pt or shell side pressure Ps} whichever is greater f=0.866a2-p/4, triangular arrangement = a2-p/4, square arrangement, a--pipe spacing 3. The force Qq on the expansion joint periphery of the pipe per square meter under the action of temperature difference stress: Qq=st.at／pdoL=st(-)/4doL where st—temperature difference stress in the pipe ； ａat—cross-sectional area of each pipe wall, mm2 ； , —Pipe outer and inner diameter mm. Qq and Qt may be in the same direction or in opposite directions.: q=Qq+Qt reverse direction: q=|Qq-Qt| direction determination principle: ① When Pt>;Ps, and tt>ts, then the same direction ② When Pt