Comparison of common heat exchanger types

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Comparison of Common Heat Exchanger Types I. Plate Heat Exchangers 1. Features: (1) Small size, low floor space requirement； Compact structure, small size, low floor space requirement, and light weight. Approximately 250 m2 of heat transfer area is provided per m3 of volume, and the floor space required is only 1/5 to 1/10 that of a shell-and-tube heat exchanger. It is particularly suitable for situations such as technological upgrades in the renovation of old factories, where existing equipment is made full use of to overcome space constraints. (2) High heat transfer efficiency ； It generally reaches 4,000–6,000 W/m2·°C (with water-water as the medium), which is 2–3 times higher than the heat transfer coefficient of shell-and-tube exchangers under the same flow rate. (3) Flexible assembly ； It is easy and quick to disassemble, clean, replace the gaskets, or replace the plates. (4) Low metal consumption and light weight; the heat transfer plates are thin, resulting in less metal usage – approximately 10 kg of metal is required per square meter of heating area. It is only 1/3 to 1/4 of that of a tubular heater. (5) Low heat loss ； (6) Easy to disassemble, clean, and maintain ； Strong corrosion resistance and long service life. (7) The material to be heated stays in the heater for a short time, resulting in few internal dead zones. (8) It offers great operational flexibility and a wide range of applications; the number of plates can be increased or decreased as needed to alter the heating area, or to change the operating conditions. (9) Plate heat exchangers have little fouling accumulation because of the strong turbulence of the fluid inside them and the smooth surfaces of the plates, which is why they are also known as “self-cleaning heat exchangers”. (10) The disadvantages of plate heat exchangers are that they have a long sealing perimeter, which makes leakage more likely; their operating temperature can only be below 150°C, they can withstand only low pressure differences, and their processing capacity is limited. Once scaling occurs on the plates, it is necessary to disassemble them for cleaning. 2. Sealing materials: ★ Nitrile rubber (N): Operating temperature ≤ 110°C; ★ Ethylene propylene diene monomer rubber (E): Operating temperature ≤ 150°C; ★ Silicone rubber (C): Operating temperature ≤ 230°C. 3. The corrugations on the plates serve two important functions: (1) Improving heat transfer efficiency. (2) Improve the stiffness and compressive strength of the plates. 4. Main parameters: heat exchange area per plate (m2), maximum installed area (m2), channel diameter (mm), maximum material flow rate (t/h), flow rate per channel (kg/s), heat transfer coefficient k value (W/m2·K), as well as the dimensions of the frame, etc. 5. When selecting a plate heat exchanger, the following points should be taken into consideration: (1) The heating area should not be based on that of the original tubular heat exchanger; generally, it only needs to be 30% to 50% of the latter’s area. (2) Pay attention to the channel diameter of the equipment, namely the diameter of the angular orifices and connection pipes, as it determines the maximum flow rate of the material. If a larger channel diameter is chosen, the number of plates can be increased later if needed to boost the material handling capacity. An insufficient diameter will limit the flow rate of the material. Especially when low-pressure steam is used as the heat source, an insufficient pipe diameter can result in inadequate steam supply, severely reducing the performance of the heat exchanger. (3) The spacing between the plates (gap width) is also important; for most plate heat exchangers, this value is 5–6 mm. It can be used with liquids as well as with water vapor under slight positive pressure (vapor pressure of 0.03–0.1 MPa). For some products, this value is less than 4 mm, and they can only be used with liquids or steam at high pressure ； If low-pressure steam is used, the insufficient channel area limits the amount of steam that can enter, which in turn reduces the performance of the heat exchanger. 6. Usage method of plate heat exchangers (1) Installation of plate heat exchangers: The connection pipelines of plate heat exchangers need to be properly handled. To prevent the weight of the piping and the tensile or compressive forces resulting from thermal expansion and contraction from acting on its connection flanges, 90° elbows should be installed on the connection piping of the heat exchanger. There should be a drain valve before its steam inlet valve; before starting up, the water and debris accumulated in the pipes must be removed to prevent water hammer and the entry of contaminants during startup. The material pipe should be equipped with bypasses and valves, and a bottom drain pipe should be installed at the lowest point. There should also be joints for pressure testing; before use, the channels on both sides should be tested for pressure separately. Thermometers and pressure gauges should be installed at each connection port. When heated with steam, the temperature of the condensed water should be only slightly lower than that of the steam (based on saturation conditions); if it drops too much, it indicates that there is water accumulation inside the device, which significantly reduces its heat transfer efficiency. The heat exchanger should be equipped with proper steam and condensate drainage systems. The multiple plates of a plate heat exchanger are pressed together to form a single unit by the end caps on both sides and various bolts. One of the end caps is fixed to the frame and used to connect the pipelines, while the other end cap can move along guide rails during installation and removal. When the material flows in a single direction only, all four pipes for the inlet and outlet of the hot and cold fluids are connected to a fixed end cap, which makes it the most convenient for management and installation. At this point, large circular holes are made in all four corners of each plate, running through from one end to the other. An elastic gasket is firmly attached to the same side of each plate. After tightening the bolt, the thickness of the compressed gasket equals the height of the protrusion of the plate’s ridges; at this point, the protruding ends of the plate and the gasket are on the same plane and in close contact with each other. The gasket thickness must be accurate. If the gasket is too thick, the corrugations of the plates cannot make contact with each other, and they will deform under pressure ； If the gasket is not thick enough, tightening the bolts will cause the tops of the corrugations in the plate to press together, resulting in small dents that can lead to leakage over time. The plates of the Swedish AM-20 type have gaskets whose thickness after compression is 5.4 mm. Before installation, the gasket and the plate must first be bonded together to form a single unit. The plate is provided with corresponding indentations at the locations for the device gaskets; once the gaskets are placed there, they are bonded using the appropriate adhesives and bonding methods depending on the material of the gaskets (some gaskets and adhesives require heating to harden). This approach is troublesome when gaskets need to be replaced on existing equipment, and leaks can occur if not handled properly. Plate heat exchangers manufactured by foreign manufacturers in recent years can be fixed using a \"clip on\" method instead of adhesives; the areas on the plates where gaskets are installed feature a special structure, allowing small clips to be used to secure the gaskets in the grooves. This method is simple and easy to implement, and it is also straightforward to remove and replace the old gaskets. This is more suitable for applications where gaskets tend to age quickly. (2) Installation and removal of plate heat exchangers: Newly purchased heat exchangers are already assembled as a single unit, allowing for overall installation. It should not be disassembled unless necessary. The disassembly and reinstallation of plate heat exchangers is a delicate task that must be carried out by experienced personnel following specific procedures, to ensure good sealing after installation and proper operation. Incorrect disassembly, assembly, and installation can lead to poor sealing, resulting in deformation and damage to the plates, which is difficult to repair. Before disassembling the heat exchanger, it is necessary to measure the original thickness of the plate pack using a steel ruler; measurements should be taken at the top, bottom, left, and right corners of the equipment, with the results recorded. This thickness should be restored as much as possible during reinstallation. If changing the number of plates, the correct total thickness must be calculated first. For example, using 80 am‑20 plates with a nominal thickness of: 80×(5.4+0.8) = 496 mm. The difference between the thickness after installation and compression and this nominal value should be less than 1%; therefore, in the above case, the thickness should be between 491 and 501 mm. Plate heat exchangers are usually pressed together into a single unit using 6 to 12 bolts. When installing or removing them, these bolts should be tightened or loosened evenly and uniformly; it is absolutely not allowed for there to be uneven tension. When removing the loosening bolts, start by loosening the bolts in the middle, and then move on to the corners. At first, 1–2 circles each time, then more, repeating several times until it is completely loose. It is required that during the relaxation process, the total thickness of the plate at the four corners be measured, with a left-right deviation of no more than 10 mm and an up-down deviation of no more than 25 mm. When tightening the bolts, tighten the bolts at the four corners first, then those in the middle, doing this in small steps and repeating the process multiple times. It is required that the asymmetrical deviation in the total thickness of the plate assembly during tightening also does not exceed the aforementioned value. When tightening or loosening bolts, a wrench of appropriate length should be used to ensure the correct torque is applied. For example, if the fastening bolt for the AM-20 model is M39, the specified length of the wrench is 550 mm. Plate heat exchangers supplied abroad often come equipped with a \"force-limiting wrench\" that prevents the tightening torque from exceeding a certain limit (the wrench slips automatically when too much torque is applied). Excessively long wrenches and high torque levels are harmful; they can compress the gasket too much, resulting in small dents forming on the tops of the sheet metal ridges. In a domestic sugar factory, plate heat exchangers suffered from perforations in multiple plates at that location, resulting in their scrapping. The tightening bolts of plate heat exchangers are often equipped with small flat bearings mounted at the bottom of the compression nuts, in order to reduce surface friction when tightening the bolts and ensure an appropriate tightening force. When tightening or loosening the bolt, the other end is constrained by the end cap and cannot rotate. After the heat exchanger is disassembled, the plates should be hung on the frame of the unit. If they need to be removed for cleaning, they should be placed on a smooth surface; they must not be placed on an uneven surface, nor should multiple plates be stacked together, to prevent the plates from bending or deforming. Each plate should be hung back in its original position immediately after cleaning. Hitting with a hammer is not allowed; hitting stainless steel sheets with a hammer can cause changes in their internal structure, thereby reducing their rust resistance. After checking that all plates and gaskets are in good condition, the bolts are tightened to restore them to their proper position. (3) Cleaning of plate heat exchangers: Due to the strong turbulence of the fluids inside them and the smooth surfaces of the plates, plate heat exchangers accumulate less fouling; hence they are also known as “self-cleaning heat exchangers”. If the material flow rate is increased regularly or the material is made to flow in the opposite direction during operation, some of the deposits inside it can be washed away, thereby extending the service life of the heat exchanger. Before opening the heat exchanger for cleaning, flushing it with a large amount of water can also remove some of the deposits inside it. The modern chemical and food industries are vigorously developing chemical cleaning methods for various containers and equipment in order to simplify the cleaning process, and the same is true for plate heat exchangers. In the food industry, \"automatic cleaning in-place\" (CIP) technology has been widely adopted in recent years. Without the need to disassemble or open the containers and equipment, they are cleaned using chemical agents and water, with the process being automatically controlled by a computer according to predefined programs, **which improves the efficiency of cleaning tasks. Research and improvements are also continuously being made to the formulations and usage methods of cleaning agents. The material of plate heat exchangers can resist the effects of various chemical agents, and their small internal volume makes them easy to clean chemically. The chemicals used depend on the type and composition of the scale; for organic deposits, 2% NaOH or an alkaline detergent solution can be used, while for oxide or carbonate deposits, 2% sodium polyphosphate or sodium tripolyphosphate, 5% ethylenediaminetetraacetic acid, or 0.7% nitric acid (based on the volume of concentrated nitric acid) can be employed. The temperature of use is 50°C, with a maximum of 70°C. After chemical cleaning, rinse thoroughly by pumping water repeatedly. Under favorable conditions, plate heat exchangers can remain unopened throughout the year. When it is indeed necessary to disassemble and clean it, proceed with care: rinse the plates one by one with water, wipe them with a soft cloth or brush, or use an appropriate cleaner. Do not use steel brushes or other hard tools to avoid scratching the surface of the plates. After cleaning, it should be reinstalled and tightened immediately. 　II. Shell-and-tube heat exchangers. The shell-and-tube heat exchanger is the most commonly used type of heat exchanger; it includes fixed-tube-sheet heat exchangers, U-tube shell-and-tube heat exchangers, heat exchangers with expansion joints, floating-head heat exchangers, segmented heat exchangers, sleeve heat exchangers, and others. Fixed tube sheet heat exchangers have advantages such as simple structure, light weight, and low cost; the drawback is that the tubes bend due to thermal expansion. The U-tube shell and tube heat exchanger overcomes this drawback by shaping the tubes into a “U” shape, with one end fixed and the other movable, thereby allowing the heat exchanger to remain unaffected by expansion. It has a simple structure and is lightweight; however, its disadvantages include the inability to clean it mechanically, difficulty in replacing the tubes, and low heat transfer capacity per unit volume and per unit mass. It is suitable for applications with large temperature differences and where the fluid inside the tubes is relatively clean. Heat exchangers with expansion joints can address the issue of expansion; thanks to their design featuring expansion joints, they are suitable for fluids with large temperature differences as well as high-pressure fluids. Since the joints can be removed for cleaning, they can be used with fluids that tend to form scale. However, they are not suitable for low-pressure gases, and their drawback is their complex manufacturing process. In a floating-head shell-and-tube heat exchanger, the floating head is not connected to the shell and can move freely, which not only solves the problem of thermal expansion but also facilitates cleaning; during maintenance, the tube core can be removed. For fixed-tube-sheet, shell-and-tube, and double-pipe heat exchangers, the heat transfer area is approximately 30–40 m² per 1 m³ of shell volume. For U-tube shell and tube heat exchangers and floating-head heat exchangers, when the volume of each shell is 1 m3, their heat transfer area is around 70 m2. III. Spiral Plate Heat Exchangers Spiral plate heat exchangers are heat exchangers composed of spiral-shaped heat transfer plates. It has better heat transfer performance than shell-and-tube heat exchangers, is compact in structure, easy to manufacture, and convenient to install. 　　The structure of a spiral plate heat exchanger includes components such as spiral heat transfer plates, partitions, cover plates, spacing posts, and connection pipes, and its design varies depending on the type. All types of spiral plate heat exchangers consist of two steel plates about 2–6 mm thick, which are rolled together to form a pair of concentric spiral flow channels separated from each other. Cold and hot fluids flow alternately using the spiral plate as the heat transfer surface. 　　Depending on the flow pattern of the fluid within the flow channel and the operating conditions, spiral plate heat exchangers can be divided into three structural types: I, II, and III, as shown in Figure 5-20. Type I: The two fluids flow in a spiral pattern on both sides of the spiral channel. Typically, cold fluid flows from the periphery toward the center and is discharged there, while hot fluid flows from the center toward the periphery and is discharged there, enabling efficient counterflow heat transfer; this approach is often used in liquid-liquid heat exchange. Since both sides of the channel are fully welded and sealed, the Type I structure is a non-detachable design. Type II: In this configuration, one fluid flows in a spiral pattern within a spiral-shaped channel, while another fluid flows axially in a spiral channel on the other side. Therefore, the two ends of the axial flow channel are open, while the two ends of the spiral flow channel are sealed. This type is suitable for situations where there is a large difference in flow rates between the two fluids, and is commonly used in condensers, gas coolers, etc. Type III: In this configuration, one fluid flows in a spiral pattern within a spiral-shaped channel, while the other fluid moves in a combination of axial and spiral flow within another spiral channel. This type is suitable for the condensation cooling of steam; the steam first enters the axial flow section where it condenses, and after its volume decreases, it moves to the spiral flow channel for further cooling. 　　As can be seen from the above structure, due to the centrifugal force that generates secondary circulation as the fluid flows between the spiral plates, along with the perturbing effect of the spaced columns, turbulence is induced at relatively low Reynolds numbers (Re=1400–1800). Therefore, a higher flow velocity is permissible in the heat exchanger (2 m/s for liquids and 20 m/s for gases), resulting in a high heat transfer coefficient. Due to the high flow velocity of the fluid and its flow within a spiral-shaped channel, once dirt accumulates in certain areas of the flow path, the cross-sectional area there decreases, which in turn increases the local flow velocity of the fluid at that location. This makes it easier to wash away the dirt, providing a certain self-cleaning effect; such design is suitable for handling suspensions and fluids with high viscosity. Due to the long flow channel and the possibility of counterflow heat transfer, it facilitates precise control of the outlet temperature of the fluid and helps in recovering low-temperature thermal energy; in a pure counterflow situation, the temperature difference at the outlets of the two fluids is as low as 3°C. 　　The main disadvantage of spiral plate heat exchangers is that the operating pressure and temperature cannot be too high; they generally can only operate at pressures below 2.0 MPa and temperatures below 300–400°C, in addition to having a relatively high flow resistance. In addition, there are also problems with difficult inspection and maintenance. IV. Chemical cleaning of plate heat exchangers Plate heat exchangers were first proposed in the 19th century, and began to be used in the milk industry for sterilization in the early 1920s. Due to the unique features of plate heat exchangers in terms of both manufacturing and operation, they developed rapidly once they were successfully adopted in industry. Early plate heat exchangers were mostly used in the food industry; today, they are employed in various sectors such as food processing, brewing, chemicals, machinery, metallurgy, power generation, and transportation. 1. Introduction to plate heat exchangers: A plate heat exchanger is a type of heat exchanger in which thin metal sheets (with a thickness of generally around 0.5–1.5 mm) are pressed into plates with specific wave patterns, and these plates are then stacked together. The working fluid flows through the narrow and tortuous channels formed by the two plates. The hot and cold fluids pass through their respective channels in sequence, separated by a plate, through which heat exchange takes place. A plate heat exchanger is mainly composed of plates, gaskets, frames, compression mechanisms, etc. 1) The plates are the main component of a plate heat exchanger, that is, the heat exchange surface. A sheet is a single piece formed by stamping thin metal sheets, and it generally consists of the following parts. (1) The plate body portion, which is the main heat exchange surface. (2) Liquid inlet and outlet holes (corner holes), which serve as pathways for the flow of fluid and also function as manifolds. (3) The inlet and outlet guide section, which is located between the corner holes and the main body of the plate, serves primarily to ensure uniform flow of the fluid between the plates; it also plays a role in heat exchange. (4) Sealing groove, located around the plate, is used to accommodate sealing washers in order to prevent the working fluid from leaking outward and from leaking between the internal components. (5) Positioning holes and hooks, used to ensure the correct relative positioning and fixation of the plates during assembly. Based on the different patterns of the plates, plate heat exchangers can be classified into 4 categories: counterflow type, corrugated flow type, mesh type, and turbulence-type. The main materials currently used for plates are austenitic stainless steel, titanium and titanium alloys, nickel and nickel alloys, etc. 2) The gasket is an important auxiliary component in plate heat exchangers, primarily serving to ensure sealing when fluids flow between the plates ； The cross-sectional dimensions of the washer often affect the operating size of the plate gap, thereby influencing its heat dissipation capacity and resistance characteristics. The material of the gasket is an extremely critical factor; it has now become an important element affecting the range of applications for plate heat exchangers. When selecting a gasket material, considerations must be given to its temperature resistance, pressure resistance, chemical stability, as well as its elasticity. Commonly used materials at present include nitrile rubber, EPDM rubber, fluororubber, silicone rubber, neoprene rubber, polytetrafluoroethylene, and composite gaskets. 3) The frame serves to hold the plates in place, and it also functions as a pressing device. 2. Characteristics of plate heat exchangers 2.1 Advantages of plate heat exchangers 1) High heat transfer coefficient. Plate heat exchangers have small flow channels; their plates are wave-shaped, resulting in a complex cross-sectional shape that causes the flow direction and velocity of the fluid to change continuously. This increases turbulence in the fluid, enabling turbulent flow even at low velocities and thus yielding a high heat transfer coefficient. It is particularly suitable for liquid-liquid heat transfer and heat transfer between fluids with high viscosity. 2) High adaptability. The required heat transfer area can be achieved by increasing or decreasing the number of plates. A heat exchanger can be divided into several units, allowing for heating or cooling between multiple fluids simultaneously. 3) Compact structure, small size, and low consumption of materials. The heat transfer area per cubic meter of volume can reach 250 m2, and only about 15 kg of metal is required per square meter of heat transfer surface. 4) Its high heat transfer coefficient and low metal consumption enable an effective heat transfer efficiency of over 85%–90%. 5) Easy to disassemble, clean, and repair. 6) Low dirt coefficient. Due to strong flow disturbances, dirt does not settle easily ； The plates used are of high quality, with little corrosion, all of which result in a low fouling coefficient. 7) Plate heat exchangers mainly use metal sheets, so the cost of raw materials is lower than that of pipes made from the same metal. 2.2 Disadvantages of plate heat exchangers: 1) Poor sealing performance, prone to leakage. The washers need to be replaced frequently, which is troublesome. 2) The operating pressure is subject to certain limitations, generally not exceeding 1 MPa. 3) The operating temperature is limited by the heat resistance of the washer material. 4) The flow channel is small, making it unsuitable for gas-to-gas heat exchange or steam condensation. 5) Prone to clogging; not suitable for fluids containing suspended solids. 6) The flow resistance is greater than that of the shell-and-tube type. Considering the characteristics of plate heat exchangers and drawing on the experience of other cleaning methods, chemical cleaning proves to be an effective approach for cleaning such exchangers. Chemical cleaning generally follows these steps. 3. Chemical cleaning of plate heat exchangers 3.1 Preparations before cleaning The preparations before cleaning mainly include the following tasks. 1) Check information such as the device’s activation date, usage records, maintenance records, and device drawings. 2) Understand clearly the materials used in the equipment. 3) Collect scale samples from the equipment and conduct analytical testing on them. 4) Determine the distribution of scale buildup within the equipment. 5) Conduct scale dissolution experiments and determine the formula and process of the cleaning agent through these experiments. 3.2 Determination of the cleaning process For plate heat exchangers that have not been disassembled, static or dynamic cleaning processes can be employed. 1) The static immersion cleaning process is shown in Figure 1. 2) The steps of the dynamic cyclic cleaning process are as follows. (1) Water flushing and system pressure testing. The purpose of water flushing and pressure testing is to remove dust, sediment, detached metal oxides, and other loose contaminants from the system, and to check for leaks at the temporary connections under simulated cleaning conditions. Figure 1 Block diagram of the static immersion cleaning process (2) Alkali washing. Alkaline washing is used to remove organic compounds and oils, as well as to soften the scale to make it easier to remove. Time: 10–24 hours; temperature requirement: 85°C; flow rate: below 0.3 m/s. (3) Rinse with water after alkali washing. The purpose is to remove the residual alkaline cleaning solution and to dislodge some impurities from the surface so that they can be carried away. (4) Acid cleaning. Acid wash solution is used to react with impurities such as scale to produce soluble substances. (5) Rinse with water after pickling. It is to remove residual acid and detached solid particles in order to proceed with subsequent processes. (6) Rinsing uses a rinsing solution to bind with the iron ions remaining in the system, and to remove the secondary rust formed during water rinsing, thereby reducing its concentration in preparation for passivation. (7) Neutralization and passivation. It involves using a passivator to form a passivation film on the metal surface, thereby preventing the metal of the equipment from rusting again. 3) For the disassembled plate heat exchangers, the following “circulation + immersion” cleaning process can be employed. (1) Manual processing. The goal is to minimize the amount of scale through manual efforts; this not only reduces the processing burden for the subsequent steps but also decreases the use of chemical agents, thereby easing the handling of waste liquids. For this process, nylon brushes, hair brushes, copper brushes, and small cleaning machines can be primarily used. (2) Cycling + soaking cleaning. Cleaning agents are used to dissolve and remove dirt, and forced circulation is employed to maintain a certain flow rate in order to achieve better cleaning results. (3) Manual processing. After cleaning, the sediment is manually treated. 4. Application example: A detachable plate heat exchanger used in an aluminum plant for the production of alumina had suffered severe scaling, which significantly affected the normal operation of the production process; it needed to be cleaned. The material of this heat exchanger was austenitic stainless steel. The original scale sample is grayish-white, and the scale layer is divided into an upper layer and a lower layer. The surface scale is mainly carbonate scale, which can be treated relatively easily ； The underlying scale is hard in texture and contains a large amount of acid-insoluble substances; it reacts extremely slowly with ordinary acid cleaning solutions. Analysis shows that this scale is primarily composed of highly dense alumina, along with other oxides as well as small amounts of magnetic iron oxide and carbonaceous material. Although it is thin, the scale is very hard and dense. Therefore, dealing with the underlying scale becomes the key to cleaning. It is difficult to achieve good cleaning results for bottom-scale using a single cleaning formula. Scale dissolution tests showed that the use of dilute H2SO4, cold HNO3, HF, and strong bases could not dissolve the scale; even after prolonged treatment, the scale did not become loose, with only tiny bubbles forming extremely slowly. Under heating conditions, a concentrated HNO3 can loosen and dissolve the scale sample. A12O3 + 6HNO3 → 2Al(NO3)3 + 3H2O; C + 4HNO3 → 4NO2 + CO2 + 2H2O; Fe2O3 + 6HNO3 → 2Fe(NO3)3 + 3H2O; FeO + 4HNO3 → Fe(NO3)3 + NO2 + 2H2O. The cleaning formula was determined through scale-removal tests. Since the equipment is made of stainless steel, repeated experiments led to the conclusion that nitric acid should be used as the main cleaning agent, as it helps to loosen and dissolve the dirt ； And cleaning aids such as accelerators, penetrants, and corrosion inhibitors are added. The cleaning formula is shown in Table 1. Drug name, quality fraction / %: Function description – Nitric acid: 25–30%; main cleaning agent: 10.5–1%; accelerator: an organic substance with an irritating odor; its aqueous solution is acidic. X21—3 penetrant is highly corrosive, has a pungent odor, and is a weak inorganic acid. It has a strong ability to dissolve iron oxide. Lan-8260.3 corrosion inhibitor – Although higher temperatures, greater concentrations, and longer exposure times yield better results, the corrosive nature of nitric acid requires certain restrictions on temperature, concentration, and time during cleaning, in order to reduce the workload associated with waste liquid treatment and disposal resulting from the use of this agent. 1) Cleaning method: Clean it in accordance with the cleaning procedure for the disassembled plate heat exchanger. (1) First, use manual brushing along with high-pressure water (with the pressure kept below 10 MPa to prevent deformation of the equipment) to remove the loose scale layer from the plates. (2) Prepare the cleaning solution in the cleaning tank; since the concentration of the cleaning liquid is high, the heating temperature should not be too high. Place the plates in the cleaning tank for cyclic + immersion cleaning; during cleaning, a combination of movement and stillness should be used, with stillness being the primary approach. The cleaning time should be kept at around 12 hours. Due to the special nature of plate heat exchangers, it is necessary to strictly control the flow rate of the acid cleaning solution during cleaning, in order to prevent excessive flow rates from undermining the corrosion-inhibiting effect of the inhibitor and causing corrosion of the equipment. (3) After the chemical cleaning is complete, the plates are removed and rinsed with a small cleaning machine, leaving the heat exchanger plates as bright as new. (4) Treatment of cleaning waste liquid. The discharge standards for treated cleaning wastewater should meet the third-level discharge standards specified in GB8978—1996, the \"Comprehensive Wastewater Discharge Standards\". Due to the high concentration of nitric acid used, the waste liquid is highly acidic and cannot be discharged directly; it can be treated by adding alkali for neutralization. In addition, lime powder (CaO) must be added to the waste liquid to treat other harmful ions. 2) Cleaning results (1) Corrosion condition. Based on the monitoring of the corrosion rate during cleaning, the cleaning corrosion rate is 0.97 g/m2·h. (2) Scale removal rate. The scale removal rate reaches 100%, and the heat exchanger plates become as bright as new after cleaning. Through multiple cleanings of the removable plate heat exchangers in aluminum plants, and by examining the cleaning results, it was found that cleaning these heat exchangers by disassembling them yields better results; the cleaning effect can be observed directly. Additionally, other cleaning methods can be used as supplements to reduce the use of chemical agents. However, disassembling a plate heat exchanger requires considerable time, labor, and resources. For non-demontable plate heat exchangers of types such as fully welded plate heat exchangers and wide-channel welded plate heat exchangers, when it is not possible to disassemble them, the fluid flow velocity inside the plate heat exchanger is quite high; therefore, during cleaning, the circulation speed must be strictly controlled at 0.5–0.6 m/s or less (the flow velocity within the plates). Prevent excessive flow rates from affecting the corrosion-inhibiting efficiency of the inhibitor and causing equipment corrosion. A stabilizer also needs to be added during cleaning to keep the particulate scale suspended in the cleaning solution, so that it can be carried out of the heat exchanger through the circulation of the cleaning solution. The cleaning of non-demontable plate heat exchangers is more suitable for scales that are easy to dissolve, such as carbonate scales ； The cleaning of detachable plate heat exchangers is mainly applicable to types of scale that are difficult to remove, such as material-related scale. V. Common types of heat exchangers: Spray-type heat exchanger – Figure 4-43; Fixed-tube-sheet heat exchanger with compensating rings – Figure 4-45; Floating-head heat exchanger – Figure 4-46; Jacketed heat exchanger; Spiral-plate heat exchanger – Type “I” structure (a), Type “II” structure (b), Type “III” structure (c). Feel free to speak if you have any questions! Email: ZHP1389@126.come ； Phone number: 15823077486. If we’re destined to meet, I’ll be able to get in touch with you and we can become good friends!