Process Design

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I. Process Design 1. Create a flowchart. 2. Calculate the heat transfer capacity Q of the heat exchanger based on the production task. 3. Select the heat carrier and determine its flow rate. 4. Determine the flow paths of the cold and hot fluids. 5. Calculate the qualitative temperature to determine the physical property data of the fluid (density, specific heat, thermal conductivity, etc.). 6. Initially calculate 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. Conduct a preliminary design of the heat exchanger dimensions based on the initially calculated heat exchange area. This includes pipe diameter, pipe length, number of pipes, number of tube passes, pipe arrangement pattern, and shell inner diameter (which needs to be rounded). 9. Calculate K. 10. Verify the average temperature difference D. 11. Verify the heat transfer amount, requiring a margin of 15–25%. 12. Calculation of pressure drops in the tube side and shell side. II. Mechanical Design 1. Determination of the housing diameter and calculation of the housing wall thickness. 2. Selection of heat exchanger head. 3. Selection of heat exchanger flanges. 4. Determination of the tube sheet dimensions. 5. Calculation of pipe pulling force. 6. Selection and calculation of baffle plates. 7. Calculation of temperature difference stress. 8. Takeover, selection of takeover flanges, and hole reinforcement, etc. 9. Draw diagrams of the main components. III. Prepare a summary table of calculation results. IV. Draw the assembly diagram of the heat exchanger. V. Specify technical requirements. VI. Write the design description. Section 2: Process design of shell-and-tube heat exchangers I. Determination of the final temperature of heat exchange The final temperature of heat exchange has a significant impact on the heat transfer efficiency and heat transfer rate of the heat exchanger. In counterflow heat exchange, when the final temperature at the fluid outlet is close to the initial temperature of the hot fluid at the inlet, the heat utilization efficiency is high, but the heat transfer intensity is minimal, and the required heat transfer area is largest. To reasonably determine the medium temperature and the final temperature of heat exchange, the following data can be referred to: 1. The temperature difference at the hot end (large temperature difference) should be no less than 20°C. 2. The cold-end temperature difference (small temperature difference) shall be no less than 5°C. 3. In a cooler or condenser, the initial temperature of the coolant should be higher than the freezing point of the fluid to be cooled ； For the condensation of gases containing non-condensable gases, the final temperature of the coolant must be 5°C below the dew point of the gas to be condensed. II. Calculation of average temperature difference: Initially, when performing design calculations, the average temperature difference Dtｍ is determined by treating the heat exchange process as a counterflow process first. 1. For counterflow or co-current heat transfer processes, the average temperature difference can be calculated using equation (2–1): (2—1) Where, and represent the temperature differences at the large end and the small end, respectively. At that time, the arithmetic mean could be used. 2. For heat transfer processes with cross-flow or recirculating flow, if there is no phase change, a temperature difference correction must be applied, that is, calculation is carried out using equation (2-2). In equation (2‑2), ΔT_avg is the average temperature difference calculated on a counterflow basis, and the correction factor can be found using the relevant illustrations in textbooks on chemical engineering principles, depending on the specific conditions of the heat exchanger. The general requirement is >0.8; otherwise, a multi-shell design or the use of multiple heat exchangers in series should be adopted. III. Determination of the overall heat transfer coefficient K: The reference area for calculating the value of K is, conventionally, the external surface area of the tube. When the baseline conditions of the design object (equipment type, Reynolds number Re, fluid properties, etc.) are the same as or similar to those of a production facility with a known K value, the empirical data of that known facility’s K value can be used as the K value for the own design. Table 2-1 shows the approximate range of K values for common shell-and-tube heat exchangers. The approximate value of K is selected from Table 2-1, which lists the empirical values of the overall heat transfer coefficient K for shell-and-tube heat exchangers. Cold fluid, Hot fluid, Overall heat transfer coefficient W/m2·℃: Water, Water, 850–1700; Water, Gas, 17–280; Water, Organic solvents, 280–850; Water, Light oil, 340–910; Water, Heavy oil, 60–280. Organic solvents, Organic solvents, 115–340. Water, Water vapor condensation, 1420–4250; Gas, Water vapor condensation, 30–300; Water, Low-boiling-point hydrocarbons condensation, 455–1140; Water boiling, Water vapor condensation, 2000–4250; Light oil boiling, Water vapor, 455–1020. The value of K is calculated using Equation (2-3). (2–3) In the formula: a – heat transfer coefficient, W/m2.℃ ； R – dirt thermal resistance, m2·℃/W ； δ – Wall thickness of the tube, mm ； λ – Thermal conductivity of the tube wall, W/m.℃ ； The subscripts i, o, m denote inside the tube, outside the tube, and average, respectively. At that time, it was approximated as a flat wall case; that is, when calculating the K value using equation (2-3), the fouling thermal resistance usually adopted empirical values, and the typical range for such fouling thermal resistances can be found in the relevant sections of \"Principles of Chemical Engineering\". In the formula, the heat transfer coefficient a is often calculated using relevant empirical formulas in the design of shell-and-tube heat exchangers. Some of the empirical relationships commonly used for calculating a are presented in \"Principles of Chemical Engineering\", and these can be selected for use during design. IV. Determination of the heat transfer area A: In engineering practice, the sum of the external surface areas of all the tubes in the tube bundle of a shell-and-tube heat exchanger is often considered as the heat transfer area, which is calculated using equations (2-4) and (2-5). (2‑4) (2‑5) In the equations: – Heat transfer coefficient based on the external surface, W/m2·℃ – Outer diameter of the tube, m ； L – effective length of each tube, m ； n – Total number of tubes. The effective length of a tube refers to its actual length minus the portion occupied by the tube sheets and baffles. The total number of pipes refers to the rounded number of pipes minus the number of tie rods. V. Determination of key process dimensions Once the heat transfer area has been determined, the design process enters the preliminary design stage for the heat exchanger dimensions, which includes the following: 1. Selection of tubes. Using tubes with a smaller diameter can increase the convective heat transfer coefficient of the fluid, as well as enlarge the heat transfer area per unit volume of the equipment; this results in a more compact design and less metal usage per unit of heat transfer area. However, such tubes are more difficult to manufacture, tend to scale up easily, and are not easy to clean, making them suitable for use with fluids that are relatively clean. Large-diameter pipes are used for fluids with high viscosity or a tendency to form scale. Seamless steel pipes are commonly used for shell-and-tube heat exchangers in our country, with specifications given as outer diameter × wall thickness. The common specifications for heat exchange tubes are φ19×2, φ25×2.5, and φ38×3. When selecting pipes, consideration should be given to the convenience of cleaning and the rational use of the piping material, as well as the matching between pipe length and diameter. The standard lengths for domestic pipe production are generally: 1.5, 2, 2.5, 3, 4.5, 5, 6, 7.5, 9, 12 meters, etc. The ratio of the length of the heat exchange tubes to the shell diameter in heat exchangers is generally between 6 and 10; for vertical heat exchangers, this ratio is preferably between 4 and 6. Shell side and shell side pressure drop: The pressure drop of the fluid within the heat exchanger is primarily determined by the operating pressure of the system, which in turn is provided by the pumping equipment. The greater the fluid resistance loss (pressure drop) in the heat exchanger, the higher the power required of the pumping equipment, and thus the higher the energy consumption. In heat transfer without phase change, the higher the fluid flow rate, the greater the heat transfer intensity; this allows for a reduction in the required heat transfer area, resulting in more compact equipment and lower manufacturing costs. It also helps to prevent the formation of fouling. However, too high a flow rate has its disadvantages as well: it increases pressure drop, raises pump power requirements, and exacerbates erosion of the heat transfer tubes. Therefore, in the design of heat exchangers, there is the issue of selecting an appropriate flow rate and controlling a reasonable pressure drop. As a general rule, for liquids, the pressure drop should be kept between 0.01 and 0.1 MPa, while for gases it should be maintained between 0.001 and 0.01 MPa. Table 2-2 provides empirical data on the appropriate pressure drop under various operating conditions of the heat exchanger, for reference in design. Table 2-2 Selection of the appropriate pressure drop for shell-and-tube heat exchangers. Operation conditions of the heat exchanger: Operation under negative pressure, Operation at low pressure, Operation at medium pressure (including liquid transfer via pumps), Operation at relatively high pressure. Operating pressure (absolute MPa): P=0～0.1, P=0.1～0.17, P=0.17～1.1, P=1.1～3.1, P=3.1～8.2. Appropriate pressure drop (MPa): DP=P/10, DP=p/2, DP=0.035; Δp=0.035～0.18, Δp=0.07～0.25. 2. Determination of the total number of tubes n. For a given heat transfer area, once the pipe diameter and length are selected, the required number of pipes n can be determined using equation (2–6). In equation (2‑6), – is the heat transfer area ； －Outer diameter of pipe, m ； L – Effective length of each pipe, in m ； Round off the calculated tube value n. 3. Determine the number of tube sides m. The flow velocity of the fluid inside the pipe can be calculated based on the number of pipes n, using equation (2-7). (2–7) In the equation, vs is the volumetric flow rate of the fluid in the pipe, and d is the inner diameter of the pipe, in meters ； n – number of tubes. If the flow rate is much lower than the desired optimal flow rate, a multi-pass design is necessary; the number of passes m can be calculated using equation (2–8). m = u / (2 – 8), where u is the flow velocity inside the pipe, calculated using the number of pipes n, in m/s ； u – required appropriate flow velocity, m/s ； The appropriate flow rate u in equation (2-8) must be selected based on the range of flow rates commonly used in shell-and-tube heat exchangers; refer to the relevant sections in \"Principles of Chemical Engineering\" for details. It is generally required that operation take place in a turbulent state (except for fluids with high viscosity). The corresponding Re value is 5×103 for liquids, and – for gases. When dividing the stages, the number of tubes in each stage should be roughly equal; the commonly used numbers of tube stages in production are 1, 2, 4, and 6. 4. Determination of the pipe arrangement and pipe spacing. The principle for arranging the tubes on the tube sheet is: the tubes are evenly distributed across the entire cross-section of the heat exchanger, arranged compactly, with a rational structural design that facilitates manufacturing and takes into account the properties of the fluid. Their arrangement patterns are usually either equilateral triangles or squares; concentric circular arrangements and combined arrangements are also used. In some multi-pass shell and tube heat exchangers, the tubes are generally arranged in an equilateral triangle pattern within each pass, but a square arrangement is commonly used between passes; this is advantageous for the installation of the partition plates. In such cases, the overall arrangement on the entire tube sheet is referred to as a combined arrangement. In multi-pass heat exchangers, the longitudinal partitions between the passes occupy a portion of the area on the tube sheet, resulting in a lower actual number of tubes compared to the theoretical value. The actual number of tubes should be determined through the tube sheet layout diagram during design. When arranging the pipes, the pipe spacing should be determined first. When determining the tube spacing, the strength of the tube sheet and the methods required for cleaning the outside of the tubes should be considered first; its size is also related to the way in which the tubes are fixed to the tube sheet. Numerous practices have shown that the empirical value for the minimum tube spacing is: for welding and expansion bonding methods, a value of (1.3–1.5) is generally used; moreover, the distance between the center of the outermost tube in the tube bundle and the inner surface of the shell should be no less than a certain value. 5. Calculation of the housing. The inner diameter of the shell of a tube heat exchanger should be equal to or slightly larger than (for floating-head heat exchangers) the diameter of the tube sheet, and can be calculated using equation (2-9). Di = a(b – 1) + 2L (2–9), where Di is the inner diameter of the shell, in mm ； a – Pipe spacing, mm ； b – 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 is taken as (1–1.5) mm ； If the pipe is divided into sections, then Di = f + 2L. The method for determining the value of f: it can be found by consulting tables or through graphical methods. Starting with the known numbers of tubes n and tube spacing a, arrange them in a regular triangle until n tubes have been placed, and then count the number of tubes on the diagonals. The calculated shell diameter Di must be rounded to one of the container’s standard size series. Section 3 Mechanical Design of Tubular Heat Exchangers In chemical enterprises, there are many types of tubular heat exchangers, such as plate-type, shell-and-tube type, volute type, and tubular type. Among them, although tubular heat exchangers are inferior to plate heat exchangers in terms of thermal efficiency, compactness, and metal consumption, they possess advantages such as a robust structure, high reliability, strong adaptability, and a wide range of available materials. As a result, they become the primary design choice for heat exchangers in petroleum and chemical manufacturing, especially those operating under high temperature and pressure conditions or on a large scale. Tube heat exchangers mainly include fixed-tube-sheet heat exchangers, floating-head heat exchangers, packed-bed heat exchangers, and U-tube heat exchangers. Among them, fixed-tube-sheet heat exchangers are the most widely used due to their simple structure and low cost. The mechanical design of a shell-and-tube heat exchanger includes: 1. Determination of the shell diameter and calculation of the shell wall thickness. 2. Selection of heat exchanger head. 3. Selection of pressure vessel flanges. 4. Determination of the tube sheet dimensions. 5. Calculation of pipe pulling force. 6. Selection and calculation of baffle plates. 7. Calculation of temperature difference stress. 8. Takeover, selection of takeover flanges, and hole reinforcement, etc. 9 Draw the diagrams of major components and assembly drawings. The details are as follows: 1. Determination of the shell diameter and calculation of the shell wall thickness. 1. Given conditions: From the process design, the types of media in the tube side and shell side, their temperatures and pressures, the temperature difference between the shell and the walls, as well as the heat exchange area are known. 2. Calculation: (1) Number of tubes n: Seamless steel pipes are commonly used in shell-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 choice of tube material depends on the type of medium; carbon steel can be used if the medium is non-corrosive, while stainless steel should be chosen when the medium is corrosive. The pipe length specifications are 1500, 2000, 2500, 3000, 4500, 5000, 6000, 7500, 9000, 12000 mm. n=A/(pdmL), where A is the heat exchange area (m2) ； L—Length of heat exchange tube in mm ； dm—average diameter of the pipe in mm. Because 4 or 6 tie rods need to be installed in the tube-type heat exchanger. Therefore, the actual number of heat exchange tubes is {n-4(6)}. (2) Determine the arrangement of the pipes and the pipe spacing. The arrangement of tubes generally adopts a regular triangular pattern within a row, while a regular square pattern is used between rows. The pipe spacing is selected based on the minimum pipe spacing. Minimum pipe spacing, outer diameter of pipes (mm): 14, 19, 25, 32, 38, 45, 57. Minimum pipe spacing (mm): 16, 25, 32, 40, 48, 57, 70. (3) Determination of the heat exchanger shell diameter: The formula for calculating the shell diameter is Di = a(b–1) + 2L, where Di represents the inner diameter of the heat exchanger ； a—pipe spacing ； b—Number of tubes on the diagonal of an equilateral triangle ； L—the distance from the center of the outermost tube to the edge of the shell wall. If the tube is divided into sections, then Di = f + 2L, where f is the center-to-center distance between the tubes at both ends of the same inner diameter of the housing, in mm ； Di, L as above. After calculating Di, it must also be rounded to a nominal diameter series. (4) Calculation of the wall thickness of the heat exchanger shell: The wall thickness is calculated using the formula S = PDi/(2σ]tΦ – P), where P is the design pressure, in MPa ； When P < 0.6 MPa, take P = 0.6 MPa ； Di—inner diameter of the shell, mm ； Φ—is the weld coefficient, which is selected as Φ=0.85-1.0 depending on the condition of the weld ； σ]t—allowable stress of the shell material at the design temperature, MPa. The principles for selecting the material are the same as those for selecting pipes. After calculating S, it is necessary to select the negative thickness deviation C1 for the steel plate based on the table of negative thickness deviations for steel plates ； The corrosion margin C2 is selected based on the degree of corrosion, with 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 0.05–0.1 mm/a, C2 is set to 1–2 mm for single-sided corrosion, and to 2–4 mm for double-sided corrosion. When the corrosion rate of the material is less than or equal to 0.05 mm/a, C2 is set to 1 mm for single-sided corrosion and 2 mm for double-sided corrosion. For stainless steel, when the corrosivity of the medium is extremely low, C2 can be set to 0. Finally, S+C1+C2 is rounded to the steel plate thickness series; thus, the total thickness Sn = S+C1+C1+C’, where C’ is the rounding value. II. Selection of heat exchanger end caps: All types of end caps can be used, but the standard elliptical end cap is the most commonly employed one, and standard series are already available. Refer to the JB-1154-73 standard when in use. See Appendix 1. III. Selection of container flanges 1. Material: Determined based on the medium in contact with the container, as well as temperature and pressure conditions. 2. Flange type: There are three types of container flanges available, namely Type A flat weld flange, Type B flat weld flange, and long-neck butt weld flange. Its standard numbers are JB4700–4707—92, see Appendix 2. IV. Determination of tube sheet dimensions: A fixed-type heat exchanger tube sheet is selected, which also serves as a flange. It is recommended to adopt the relevant content from the \"Structural Design Manual for Steel Shell and Tube Heat Exchangers with Fixed Tube Sheets\". See Appendix 3. V. Calculation of pulling force: The pulling force is defined as the force exerted per square meter around the fusion joint of the pipe. For joints where the tube and the tube sheet are connected by welding, experiments show that the strength of such joints is higher than that of the tube itself and the metal, and the pulling force is not sufficient to cause failure of the joint. However, for joints where the tube and the tube sheet are connected by expansion fitting, the pulling force can lead to damage at the joint site and to a loss of sealing performance, or it may cause the tube to be pulled apart. To ensure a firm connection between the tube end and the tube sheet as well as good sealing properties, it is necessary to conduct a verification of the pulling force. 1. Under operating conditions, the axial force due to the temperature difference in the tube or shell is given by the formula F = at(tt – to) – as(ts – to)] / [1 + EsAs]. Here, At and As represent the cross-sectional areas of the heat exchanger tubes and the shell walls respectively ； at—linear expansion coefficient of tubing 1/℃ ； as—linear expansion coefficient of the shell material 1/℃ ； to—temperature during installation ℃ ； tt—Temperature in operating condition, °C. The temperature difference stress in the tubes and casing is: st=F/At ； ss = F/As. 2. Under operating pressure, the force acting on each square meter of the expansion joint perimeter is given by Qq = Pf/(pdoL), where P is the greater of the pressure in the tube side or the pressure in the shell side; f = 0.866a2 – p/4. For a triangular arrangement, f = a2 – p/4; for a square arrangement, a represents the pipe spacing. 3. The force acting on each square meter of the expansion joint perimeter due to thermal stress is given by Qq = st·at/(pdoL) = st(–)/4doL, where st represents the thermal stress in the pipe ； at—cross-sectional area of the wall per tube, mm2 ； — Outer and inner diameter of the pipe in mm. Qq and Qt can be in the same direction or opposite directions. When they are in the same direction: q = Qq + Qt; when they are in opposite directions: q = |Qq – Qt|. Principles for determining the direction: ① When Pt > Ps and tt > ts, they are in the same direction. ② When Pt