【Weekly Topic】2011.10.30~11.06 Key points for selecting heat exchangers?

1. Heat load and flow rate； 2. Properties of fluids ； 3. Allowable ranges for temperature, pressure, and pressure drop ； 4. Requirements for cleaning and maintenance ； 5. Requirements for the equipment’s structure, materials, dimensions, and weight.

This post was last edited by sisism on 10-31-2011 at 08:38. 1. Applications: heating, cooling, condensation, evaporation, etc. 2. Material: selected based on the different materials being handled; sometimes special alloys or non-ferrous metals are required, and it’s important to consider the corrosiveness of the materials. 3. Temperature and pressure. 4. Type: plate-type, shell-and-tube type, coiled tube type, jacketed type, spiral type, etc., are chosen depending on the process, medium, and application; many new types of heat exchangers are currently being introduced. 5. Depending on the layout, vertical, horizontal, with supports or brackets, etc., can be chosen. 6. Versatility: a heat exchanger type with high versatility should be selected based on the existing heat exchangers in the factory. 7. It’s also important to choose a reputable manufacturer of heat exchangers; usually, professional heat exchanger manufacturers are preferred, but sometimes ordinary pressure vessel manufacturers are chosen due to cost considerations, with suppliers for the plates and tubes specified to ensure the quality of the steel. 8. Heat exchange efficiency: this aspect should be considered from the stage of design and selection, as it plays a crucial role in saving energy and reducing consumption. Why are hidden replies not allowed again? It’s really confusing!

1. Temperature difference 2. Medium viscosity 3. Whether chemical reactions can occur during heat exchange with the medium 4. Corrosivity

There are many types of heat exchangers, and each type has a specific range of applications. A heat exchanger that performs well in one application may see a significant change in its heat transfer efficiency and performance when used in another application. Therefore, it is important to choose the correct type of heat exchanger for specific circumstances. There are various factors to consider when selecting a heat exchanger, mainly including: 1) Heat load and flow rate; 2) Properties of the fluid; 3) Temperature, pressure, and allowable pressure drop ranges; 4) Requirements for cleaning and maintenance; 5) Equipment structure, materials, dimensions, and weight; 6) Price, safety in use, and lifespan. In addition to these factors, considerations such as structural strength, source of materials, manufacturing conditions, sealing performance, and safety should also be taken into account when choosing a heat exchanger. All of these are often interdependent and influence one another, and are resolved through design optimization. Depending on different process conditions and operating scenarios, we sometimes use special types of heat exchangers or special heat exchange tubes in order to reduce costs. Therefore, the process conditions and mechanical design requirements should be taken into comprehensive consideration to properly select an appropriate heat exchanger type in order to effectively reduce energy consumption in the process. For engineering and technical personnel, when designing heat exchangers, sufficient attention should be paid to the rational selection of design types, efficient operation, and cost reduction. When necessary, technical and economic indicators must be analyzed through calculations, as well as investment and operating costs compared, so that the design can achieve the best possible outcome under the given conditions.

There are various factors to consider when selecting a heat exchanger, mainly including: 1) the heat load and flow rate; 2) the properties of the fluid; 3) temperature, pressure, and the allowable pressure drop range; 4) requirements regarding cleaning and maintenance; 5) the equipment’s structure, materials, dimensions, and weight; 6) price, safety in use, and lifespan. In addition to these factors, when choosing a heat exchanger, attention should also be paid to aspects such as structural strength, source of materials, processing conditions, sealing performance, and safety. All of these are often interdependent and influence one another, and are resolved through design optimization. Depending on different process conditions and operating scenarios, we sometimes use special types of heat exchangers or special heat exchange tubes in order to reduce costs. Therefore, the process conditions and mechanical design requirements should be taken into comprehensive consideration to properly select an appropriate heat exchanger type in order to effectively reduce energy consumption in the process. For engineering and technical personnel, when designing heat exchangers, sufficient attention should be paid to the rational selection of design types, efficient operation, and cost reduction. When necessary, technical and economic indicators must be analyzed through calculations, as well as investment and operating costs compared, so that the design can achieve the best possible outcome under the given conditions.

The principles to be followed in responding to 1# sun-rock are as follows: 1. Heat transfer efficiency: Heat should be able to be transferred effectively from one fluid to another; that is, high heat transfer efficiency means that more heat can be transferred per unit of heat transfer surface. Under a certain heat load, that is, when a specific amount of heat needs to be transferred per hour, the higher the heat transfer efficiency (usually expressed by the heat transfer coefficient), the smaller the required heat transfer area. This comparison is naturally made under the same temperature difference for heat transfer. The heat transfer efficiency varies greatly among different structural designs. 2. Structure of the heat exchanger: It should be capable of meeting the specified process operating conditions, operate safely and reliably, be leak-proof, easy to clean and maintain, have low fluid resistance, and have a long service life. Heat exchangers used in modern industry often require replacement, cleaning, and maintenance. Prolonged downtime can result in economic losses that may be greater than the cost of the heat exchanger itself; a properly designed structure can ensure a longer operating time, which is highly beneficial for overall cost analysis. 3. Terminal temperature difference: The terminal temperature difference of heat transfer equipment is usually determined by the requirements of the process. When the final temperature of the heat exchanger can be chosen, this value has a significant impact on whether the heat exchanger is economically viable. Since it affects the heat transfer efficiency of the equipment, various factors should be considered when making this choice. The terminal temperature difference generally falls within the following ranges: the temperature difference at the hot side should be above 20 degrees; when water or other cooling media are used, the temperature difference at the cold side can be lower, but it should generally not be less than 5 degrees; the minimum temperature difference for air coolers is also 20 degrees; when condensing fluids containing inert components, the outlet temperature of the coolant should be at least 5 degrees lower than the dew point temperature of the condensed portion. 4. Appropriate flow rate: Increasing the flow rate to enhance turbulence can improve heat transfer efficiency and also reduce fouling, thereby extending the equipment’s service life. However, too high a flow rate can cause erosion of the heat transfer equipment and lead to oscillations, affecting its service life. 5. Pressure drop: The pressure drop of a heat exchanger is related to its size and operating costs. 6. Overall heat transfer coefficient: If the heat transfer film coefficients on both sides of the heat transfer surface differ significantly, the side with the lower coefficient becomes the key factor determining heat transfer. The heat transfer limitations on the side with the lower coefficient restrict energy transfer to the other side; therefore, efforts should be made to increase the heat transfer film coefficient on that side. Common methods to overcome these heat transfer limitations include: (1) Reducing the cross-sectional area of the channels, using baffle plates or inserts to increase the flow rate and enhance turbulence; (2) Installing fins, which not only increase turbulence but also expand the heat transfer surface area; (3) Using techniques to enhance heat transfer, such as adding grooves or porosity to the surface, which can result in relatively high heat transfer film coefficients in processes involving phase changes such as condensation and boiling. 7. Fouling coefficient: After a period of use, fouling on the walls of heat exchangers is a common issue. Including a safety factor of 10%-30% in the design takes into account potential future fouling that could reduce heat transfer efficiency. During design, efforts should be made to minimize fouling, as fouling is closely related to the properties of the working medium, operating temperature, and flow rate. 8. Structural standards: When designing heat exchangers, standard designs, forms, and materials should be preferred to avoid using special mechanical specifications, thereby reducing costs. This also makes maintenance and component replacement easier. Of course, for processes involving highly corrosive media, additional thickness is often required to prevent premature wear and frequent maintenance. 9. Adaptation to local conditions: When selecting and designing equipment, the primary consideration should be its suitability for production, so as to maximize the return on investment. Secondly, the technology should be advanced to achieve maximum economic benefits. These two aspects need to be balanced appropriately. This ensures that the products produced are of high quality. However, highly advanced and automated equipment is suitable for large-scale continuous production. In situations where production volumes are small, such equipment may not be fully utilized due to insufficient load, and it is usually expensive to maintain, making it less suitable from an overall economic perspective.

　Basic types of shell and tube heat exchangers: (1) Fixed tube sheet heat exchanger – It has a simple and compact structure, with the tube sheet often serving as the flange as well. It is suitable for situations where the temperature difference between the tube side and the shell side is small, or where the temperature difference is large but the pressure is not high; also suitable for cases where the medium on the shell side is clean, or where scaling is present but can be removed through chemical cleaning. (2) Floating-head heat exchangers: The tube sheet at one end of the tube bundle can move freely, thus being free from stress caused by temperature differences. However, their structure is complex; sealing the internal floating head is difficult, there are many forged components involved, and the cost is high. During maintenance, only the tube bundle needs to be replaced; this approach is suitable for situations where there is a large temperature difference between the tube side and the shell side, but the operating pressure does not exceed 10 MPa. The downside is that it is necessary to remove the tube bundle. (3) U-tube heat exchanger: The tube bundle can expand and contract freely; there is only one tube sheet, resulting in fewer sealing surfaces. The core can be extracted for maintenance and replacement. It is suitable for applications with a large temperature difference between the tube and shell sides, as well as high temperature and high pressure conditions. The shell side requires core removal for cleaning; the medium inside the tubes is clean, or although scaling may occur, it can be removed through chemical cleaning. (4) Fill-and-seal heat exchanger: The tube bundle can expand and contract freely; both the shell side and the tube side can be disassembled for cleaning. It has a simple structure and is suitable for applications with large temperature differences between the tube side and the shell side. It has poor pressure resistance, temperature resistance, and sealing capabilities; the operating pressure should not exceed 40 MPa. It is not suitable for handling volatile, flammable, explosive, toxic, or valuable media. (5) Pot-type reboiler: The upper part of the shell serves as a vapor space, equivalent to 1 theoretical tray. The heat source is provided by a floating head or U-tube bundle. It is suitable for situations where there is a large temperature difference between the tube side and the shell side, with no restrictions on pressure; the space at the bottom of the tower is limited, the vaporization rate ranges from 30% to 80%, and the liquid phase of the reboiler’s process medium is required to be used as a product or to meet high separation standards, although installation space is limited. When used as a steam generator, lower requirements are placed on the quality of the steam. The heat exchange tubes are the heat transfer elements in shell-and-tube heat exchangers; using high-efficiency heat transfer elements is the most direct and effective way to improve the heat transfer performance of such exchangers. The following types of heat exchange tubes are in use domestically. (1) Threaded tube: Also known as low-fin tube, it is made by rolling plain tubes and is suitable for heat exchange in single-phase flow applications where the external thermal resistance of the tube is 1.5 times or more than the internal thermal resistance, as well as for materials with high viscosity such as residue oil and wax oil that are prone to corrosion and scaling. (2) T-fin tubes: Used in external boiling, they can effectively reduce the nucleation points for bubbles; as a result, the boiling heat transfer coefficient increases by 1.6 to 3.3 times. They are ideal tubes for use in evaporators and reboilers. (3) Surface porous tube: This type of tube features a porous metal coating formed on the surface of the optical tube; the numerous small pores in this coating create many vaporization centers, thereby enhancing boiling heat transfer. (4) Helical grooved tubes can enhance heat transfer between the fluids flowing inside the tube. In these tubes, the fluid near the wall flows in a helical pattern, while another portion of the fluid moves in an axial vortex. The former flow helps to thin the boundary layer, whereas the latter flow causes the boundary layer to separate and increases fluid turbulence; as a result, the heat transfer coefficient increases by 1.3 to 1.7 times, but the pressure drop increases by 1.7 to 2.5 times. (5) Bellows: These are thin-walled stainless steel bellows formed by extrusion; the inner and outer surfaces of these bellows contribute to improved heat transfer. However, bellows exchangers have a limited pressure resistance, the tube center spacing is large with few tubes, and it is difficult to control short circuits in the shell side. 　Special structures of shell-and-tube heat exchangers (1) Dual-pass structure: A longitudinal partition is placed between the tubes of the heat exchanger; sealing gaskets prevent fluid leakage from occurring between the partition and the inner wall of the shell, thereby creating two separate flow passes. Applicable scenarios: ① When the flow rate in the tube side is high while that in the shell side is low, this structure can double the flow velocity, increasing the heat transfer coefficient by 1.2 to 1 time. ②When the temperatures of hot and cold fluids intersect, multiple single-pass heat exchangers are required to achieve heat transfer; whereas a single double-pass heat exchanger not only enables heat transfer but also allows for a larger temperature difference for heat transfer. 2) Spiral baffle heat exchanger: The spiral baffles prevent dead zones and backmixing, resulting in a relatively low pressure drop. When logistics pass through such a structured heat exchanger, there is a significant radial variation in temperature; therefore, it is not suitable for applications that require high thermal efficiency. (3) Double tube sheet structure: An additional tube sheet is added to the tube sheet of the conventional structure, resulting in a double tube sheet structure that is used to collect leaking fluids and prevent mixing of the fluids from the two streams. (4) High-temperature and high-pressure sealing structure ① Metal gasket (octagonal gasket or elliptical gasket). This structure is simple to manufacture and provides reliable sealing, but for large-diameter, high-pressure hydrogenation heat exchangers, it requires a large amount of metal, the metal gaskets are difficult to fabricate, and the sealing is not reliable. This structure is suitable for operating conditions with a pressure of 6–9 MPa and a diameter of less than 1000 mm. ②Threaded locking ring structure. Compared with the steel washer sealing structure, its advantages are good sealing reliability and lower metal consumption. However, there are many machined components, the structure is complex, the design calculations are cumbersome, the cost is high; it is not possible to completely prevent internal leakage of the medium between the tube side and the shell side, and disassembly for maintenance is rather complicated. The structure is shown in Figure 1. ③The hermetically sealed cover type structure possesses many of the advantages of the threaded locking ring design; the difference is that the sealing of its tube box section is achieved through hermetic welding applied to the outer perimeter of the cover. ④Ω-ring sealing structure. A new type of sealing structure for high-pressure heat exchangers, whose advantages include lower pre-tightening loads and operating loads on the main bolts, which in turn reduces the size and weight of the equipment’s flanges and main bolts; it facilitates disassembly for maintenance, provides reliable sealing; it is simple to manufacture, has a low cost, and offers a wide range of applicable diameters, pressures, and temperatures. Under the same conditions, the fixed tube sheet heat exchanger has the most compact structure; the U-tube and floating head heat exchangers are comparable to each other. Fixed-tube-sheet heat exchangers are the most economical, while floating-head heat exchangers are less so. If the operating conditions permit, the preferred order of heat exchangers is fixed-tube sheet type, U-tube type, and floating-head type.

Estimate the heat transfer area, initially select the heat exchanger model to determine the flow paths for the two fluids within the heat exchanger. (1) Calculate the heat transfer amount based on the heat transfer task ； (2) Determine the temperatures of the fluid at both ends of the heat exchanger, calculate the qualitative temperature, and determine the fluid properties ； (3) Determine the type of heat exchanger based on the temperature difference between the two fluids ； (4) Calculate the average temperature difference, and determine the number of shell passes or adjust the final temperature of the heating or cooling medium in accordance with the principle that the temperature difference correction factor should be no less than 0.8 ； (5) Select the overall heat transfer coefficient based on the empirical range of the overall heat transfer coefficient or the actual production conditions ； (6) Estimate the heat transfer area using the overall heat transfer rate equation, and determine the basic dimensions of the heat exchanger or select the equipment specifications according to series standards. When calculating the overall heat transfer coefficient and the heat transfer area, the actual heat transfer area of the heat exchanger used should be about 10%–25% larger than the area required for the calculations.

1. First, select the type of heat exchanger based on production requirements and suitability, while taking into account factors such as energy consumption, initial investment, and pressure drop. 2. Then select the material for the heat exchanger based on the properties of the medium. 3. Finally, the process requirements involve calculating the heat exchange area and interface dimensions ； When considering practical applications, appropriate design coefficients such as heat loss and fouling coefficients should be selected.

1 Determine the process conditions, such as pressure drop, flow rate, pressure, temperature, etc., and determine the flow direction of the medium. 2 Initially select the appropriate type of heat exchanger based on factors such as heat load, pressure, temperature, flow rate, and the properties of the material. 3 Consider equipment size, cleaning requirements, and material requirements ； The requirements for sealing performance, safety, etc., are determined. 4 Determine the parameters of the heat exchanger: tube length, wall thickness, baffle type, etc. 5 Shell-and-tube heat exchangers have strong adaptability, low cost, and wide applications, so they can be considered as a preferred choice.