Ceramic hollow plate and air preheater using the same

Abstract

The application discloses a ceramic hollow plate air preheater with an inter-plate wall type, air and flue gas respectively pass in and out of the hollow ceramic plate, high-density air-tight ceramic fiber sealing gaskets seal the two ends of the plate and the inter-plate, the plate in is similar to ceramic honeycomb, and the plate out is similar to plate heat exchanger. 2 / m 3 The application has the high specific surface area of ceramic honeycomb regenerator up to 1000m 2 / m 3 order of magnitude, does not need to reverse and has high-temperature air preheating up to 1400 DEG C or above, retains the high heat energy utilization efficiency of regenerative combustion, and gets rid of the frequent reversing defects of regenerative combustion, i.e. reversing valve, rotating mechanism and other dynamic seals, and transmits flue gas heat to air in a very small space and temperature difference.

Description

Technical Field

[0001] This application relates to an air-to-air heat exchanger, and more particularly to a ceramic hollow plate and an air preheater using the ceramic hollow plate. Background Technology

[0002] Ceramic honeycomb regenerators have rapidly replaced ceramic spheres as a key component of regenerative combustion systems over the past 30 years. Their development is rapid, and the honeycomb structure offers advantages such as light weight, large specific surface area, regularity, resistance to breakage, and high heat transfer coefficient, leading to increasingly wider applications. Regenerative combustion, due to its extremely high overall heat transfer efficiency, is widely used in billet heating furnaces and VOC regenerative thermal oxidizers (RTOs), becoming almost the largest market in the combustion industry. Even in traditional boilers, a large number of tubular air preheaters have been replaced with more material-efficient and smaller regenerative rotary air preheaters. However, both rotary and reversing valve regenerative heat exchangers suffer from dynamic sealing issues, making them prone to leakage and damage. When VOCs are incinerated in a Regenerative Thermal Oxidizer (RTO), the VOCs are mixed with the air, leading to pipe flushing problems. In large thermal power units nationwide with a capacity of over 300MW, the average leakage rate of rotary air preheaters is around 15% due to the large pressure difference between primary air and flue gas (approximately 10,000 Pa). Even in rotary RTOs where the pressure difference between air and flue gas is less than 1,000 Pa, the leakage rate reaches around 2%. Since VOCs are contained in the exhaust gas in an RTO, leakage means untreated exhaust gas leaks into the flue gas, reducing treatment efficiency. Compared to ordinary metal-walled heat exchangers, ceramic honeycomb regenerators in RTOs offer two advantages: a significantly larger specific surface area and lower cost. Typically, the specific surface area of ​​a partition wall heat exchanger can only reach 200 square meters per cubic meter, while that of a honeycomb heat exchanger can reach an average of 800 square meters per cubic meter. Ordinary partition wall heat exchangers made of carbon steel can only be used below 350°C, and even heat-resistant stainless steel 310 can only reach 1150°C, while ordinary mullite and cordierite can reach above 1200°C.

[0003] As Mr. Lu Zhenwei stated on page 70 of "Organic Waste Gas Purification Technology" published by Chemical Industry Press in 2011, why has RTO become the mainstream technology? For conventional indirect heat exchangers to achieve extremely high preheating temperatures, it is impractical (or too expensive) both in terms of materials and structure; however, regenerative heat exchangers can preheat waste gas to extremely high temperatures, typically with ceramic regenerators reaching up to 1200℃. The thermal efficiency of regenerative heat exchangers is usually >95%, while the thermal efficiency of indirect heat exchangers is generally around 70%.

[0004] Is there a good way to incorporate the advantages of regenerative heat exchangers into indirect heat exchangers, while avoiding the disadvantages of regenerative combustion such as commutation and dynamic seal leakage? This question has been the dream of all engineers in the thermal energy field since regenerative combustion was introduced to China more than 30 years ago. This would represent a significant step forward in energy conservation and emission reduction in the combustion industry, and could even spur the development of new production processes. For example, carbon black and glass production require high-temperature air of around 1000℃; otherwise, more expensive pure oxygen would be necessary.

[0005] CN201335629Y discloses a structure for a ceramic heat exchanger. The main heat exchange unit is honeycomb ceramic, and the end caps are similar to those formed by stacking hollow plates with intermediate support. The honeycomb ceramic and the end caps are bonded together and then sintered. For example, mullite has a linear expansion coefficient of 5.3×10^-6℃, and the thermal expansion at 1000℃ reaches 5.3mm / m, making cracking and air leakage inevitable. After 10 years of verification, it has been largely phased out. CN201210013840 discloses a ceramic corrugated plate heat exchanger, but due to the difficulty in solving the expansion sealing problem and the cross-flow, the heat exchange temperature difference is small, so it is rarely used. CN201110051620 describes a honeycomb perforated heat exchanger that also uses a similar end cap to CN201335629Y to separate the hot and cold fluids at both ends of the honeycomb structure. The three patents mentioned above are quite representative, representing the desire of engineers in the field of thermal energy to design a partitioned heat exchanger by drawing on the highest technology of regenerative heat exchangers—honeycomb ceramics—to achieve the goals of low price, small size, high heat exchange efficiency, low air leakage, and high temperature resistance. Summary of the Invention

[0006] Technical Problem: The purpose of this application is to overcome the shortcomings of existing regenerative heat exchanger and indirect heat exchanger technologies, and to provide a practical, easy-to-manufacture, low-cost, and reliable ceramic honeycomb indirect air preheater structure. This structure combines the advantages of high specific surface area, low cost, and high temperature resistance of regenerative ceramic honeycomb with the high reliability of indirect heat exchangers due to the absence of moving parts. This application is closest to existing plate heat exchangers and regenerative ceramic honeycomb heat exchangers. To aid reader understanding, the terminology of plate heat exchangers and ceramic honeycomb heat exchangers is used extensively in this document.

[0007] Technical solution.

[0008] A type of ceramic hollow plate.

[0009] This application employs a hollow panel (also known as a polycarbonate sheet or endurance sheet) structure. The ceramic hollow panel has the shape of a flat plate with multiple holes in the center, and is made of ceramic, specifically alumina, mullite, cordierite, clay, silicon carbide, aluminum titanate, zircon, etc. The processing is similar to that of hollow panels, polycarbonate sheets, and honeycomb ceramics, involving extruding a mud-like clay slurry through a mold, followed by cutting, drying, and sintering. The hollow panel in this application is very similar to the heat exchanger plates of a plate heat exchanger; they can be considered as a single, integrally manufactured assembly of two pairs of heat exchanger plates, which are then pressed together using conventional methods with sealing gaskets. Because the ceramic hollow panel is made of ceramic material, it lacks the strength and stamping performance of conventional metal plates. Therefore, its manufacturing process is similar to that of honeycomb ceramics and PC hollow panels, thus exhibiting both similarities and differences from existing plate heat exchangers and honeycomb ceramics. The specific surface area of ​​hollow ceramic panels is almost comparable to that of honeycomb ceramics, but their structural performance and thermal expansion properties are superior. Decades of use in polycarbonate sheets in building materials have proven their excellent mechanical and thermodynamic properties. Furthermore, their heat transfer coefficient is higher than that of current square and hexagonal honeycomb ceramics, while their flow resistance is lower. Because the hydraulic diameters of the flow channels inside and outside the hollow panel differ from those in a honeycomb structure—the interior of the hollow panel resembles a honeycomb structure, while the exterior is rectangular with a larger hydraulic diameter—the flow resistance outside the hollow panel is lower. Calculations show that, under normal circumstances, the flow channels inside and outside the hollow panel are mostly in laminar flow. Figure 5 As shown, the larger the hydraulic diameter, the lower the resistance. Below 10 mm, laminar flow resistance is higher than turbulent flow resistance. As the hydraulic diameter decreases, the resistance curve increases more steeply.

[0010] Figure 5 The calculated wind speed is fixed at 5 m / s. The horizontal axis represents the pipe's inner diameter (mm); the vertical axis represents the friction loss over a 1-meter length (Pa). The heat transfer coefficient of laminar flow inside the pipe is independent of the Reynolds number and depends only on the shape of the flow channel. However, the heat transfer coefficient outside the plate, or inside a rectangular orifice within the plate, depends on the aspect ratio b / a of the rectangle, such as... Figure 6 As shown, the aspect ratio of the rectangle between the two plates is close to infinite, more than twice that of a conventional square honeycomb structure. Therefore, the preferred shape of the holes in this application is rectangular. Since the heat transfer coefficient increases with the aspect ratio in the rectangular flow channel, and considering the actual situation of this application, the long side of the rectangular hole in the hollow plate is parallel to the upper and lower surfaces of the hollow plate, the short side is perpendicular to the upper and lower surfaces, and the aspect ratio 2≤b / a≤8.

[0011] Taking a hollow plate with a thickness of 5mm and a wall thickness of 1mm as an example, the laminar heat transfer coefficient inside the plate is approximately 50w / m². 2 k, thermal resistance Ri=0.02, approximately 80W / m outside the plate. 2Given a thermal resistance Ro = 0.0125 and a mullite thermal conductivity of 4 W / mK, the wall's thermal resistance is approximately Rδ = 0.001 / 4 = 0.00025. Therefore, the thermal resistance of the ceramic wall is an order of magnitude smaller than that of the convection side. Therefore, the main considerations for the materials used in hollow core panels are high temperature resistance, corrosion resistance, and structural strength. Common materials include silicon carbide and ceramics. For ceramics, existing alumina ceramics can be used, or cheaper materials similar to ceramic honeycomb, such as mullite, cordierite, and clay, can be used. A ceramic-like material is sufficient; a fully ceramic material is not necessary. Although the thermal conductivity of 95% alumina ceramic Al2O3 reaches 22 W / mK, almost equivalent to that of stainless steel, it is expensive. Cordierite has a thermal conductivity of only about 2 W / mK, while mullite reaches 4-5 W / mK. Its advantage is its low price. Cordierite and mullite ceramic honeycomb have been proven in regenerative combustion to maintain a long service life at around 1200℃ / 1450℃. Moreover, when using this application, the temperature of each part of the heat exchanger remains constant, unlike the temperature of the heat storage body in regenerative combustion, which usually fluctuates by about 60-90℃ within a 90-second reversal cycle. Therefore, the ceramic body has a longer service life when using this application.

[0012] Compared to steel, which has a specific gravity of 7.8, ceramics typically have a specific gravity of 2.4. For heat exchange plates of the same wall thickness, ceramics weigh only one-third of steel. Mullite ceramics are priced similarly to carbon steel; therefore, with a reasonable structure, ceramic air preheaters have a cost advantage over steel air preheaters. In the low-temperature section, water vapor and acid vapor in the flue gas condense on the honeycomb ceramic surface of this application. Due to the acid resistance of ceramics, corrosion is not as severe as in traditional low-temperature air preheaters. Therefore, this application represents an ideal method for recovering low-temperature waste heat and desulfurizing flue gas, easily enabling condensing boilers and improving boiler efficiency, even exceeding 100%.

[0013] An air preheater that uses a ceramic hollow plate.

[0014] The air preheater of this application adopts a similar method to a plate heat exchanger, in which a series of ceramic hollow plates are stacked together, and sealing gaskets are used between the ceramic hollow plates to support and seal the flow channels.

[0015] Internal flow: The channels in the middle of the hollow plate are through-holes, allowing gas to pass through. External flow: The flow channels formed between the ceramic hollow plates and the sealing gaskets constitute the external flow.

[0016] The function of gaskets is similar to that of gaskets in plate heat exchangers, primarily separating the air and flue gas flow paths to prevent mixing. Gaskets also provide support for the hollow plates. Gasket materials include silicon carbide, ceramics, ceramic fiber boards, rubber, asbestos boards, graphite, graphite nickel wire packing, asbestos packing, and metals. Ceramic fiber boards, such as aluminosilicate fiber boards, become high-density rigid ceramic fiber boards with reduced permeability after adding aggregates, while still retaining some flexibility and high-temperature resistance. They perform excellently as gaskets in high-temperature applications. Gaskets made of airtight, high-density rigid ceramic fiber boards can operate continuously at temperatures above 1200℃. The thickness of the gasket is 1~20mm, preferably half the thickness of the hollow plate used.

[0017] To concisely illustrate the working principle of the ceramic hollow plate heat exchanger, the number of holes in the hollow plate in the simplified diagram of this application is chosen to be moderate, sufficient to express the structure and principle of the hollow plate without being overly complex. The simplified diagram shows fewer holes, but the actual number is certainly more. The holes in the ceramic hollow plate of this application are typically rectangular, hexagonal, circular, or triangular, with a size, or hydraulic diameter, usually between 1 and 12 mm, preferably between 2 and 6 mm, and a wall thickness between 0.5 and 2 mm. The thickness of the ceramic hollow plate in this application is between 2 and 20 mm, preferably between 3 and 8 mm.

[0018] Beneficial Effects: 1. This application combines the advantages of both regenerative honeycomb ceramic heat exchangers and indirect heat exchangers, offering high heat exchange efficiency, small size, and low price, while avoiding the dynamic sealing problems and large air leakage associated with regenerative heat exchangers. 2. The temperature field within the regenerator in this application is stable, similar to that of a conventional indirect heat exchanger, whereas the temperature within the honeycomb ceramic body of a regenerative heat exchanger alternates once per commutation cycle. Therefore, the hollow plate in this application has a longer service life. 3. Because the main regenerator material of this heat exchanger is ceramic, the amount of metal used is small, making it more suitable for cooling flue gas containing corrosive gases and for condensing flue gas cooling. For example, the exhaust temperature of a gas-fired boiler can be reduced to around 60℃, enabling a condensing boiler, improving boiler efficiency, and simultaneously achieving flue gas dewhitening. 4. Due to the ceramic material of the hollow plate, the heat exchange specific surface area is huge, and the walls are thin. Therefore, the cost of this application is low, less than half that of existing tubular air preheaters, and the volume is about one-quarter that of existing tubular air preheaters. 5. Due to the use of ceramic heat exchangers, the heat exchange in this application largely retains the characteristics of high-temperature air combustion in regenerative combustion. The air preheating temperature can reach over 1400℃. When the hollow plate and sealing gasket are made of alumina ceramic and ceramic fiber, the air preheating temperature can reach 1600℃.

[0019] Attached image description.

[0020] Figure 1 A type of ceramic hollow plate.

[0021] Figure 2 A basic unit for a ceramic hollow plate.

[0022] Figure 3 A ceramic hollow plate heat exchanger inner core.

[0023] Figure 4 An air preheater that uses a ceramic hollow plate.

[0024] Figure 5 The relationship between pipe inner diameter and flow resistance.

[0025] Figure 6 The relationship between the aspect ratio of a rectangular cross-section pipe and the Nusselt number (Nu) for laminar heat transfer.

[0026] 1-Hollow plate, 2-Sealing gasket, 3-Support bridge, 4-Heat exchanger shell.

[0027] Detailed implementation method.

[0028] The embodiments of this application will be further described below with reference to the accompanying drawings.

[0029] Example 1: As Figure 1 , Figure 2 , Figure 3 and Figure 4 The diagram shows a ceramic hollow plate and an air preheater. In this application, the air flows through the outer shell of the tubes, and the flue gas flows through the inner tubes. Conventional shell-and-tube heat exchangers also have a configuration where air flows through the tubes and flue gas flows through the shell; this application also possesses this functionality. Furthermore, counter-current and co-current flow are both common practices in heat exchangers. Although this application has illustrated the flow direction in the diagram, the accompanying drawings do not limit the application of conventional heat exchangers such as counter-current and co-current flow.

[0030] Figure 1 This application describes a ceramic hollow plate. Existing hollow plates are typically made of polyester fiber. This application borrows from existing hollow plate technology, replacing the material with common ceramic materials used in regenerative honeycomb ceramics. The shape and size of the holes are based on common hollow plate holes and regenerative honeycomb ceramic holes, typically rectangular, hexagonal, circular, or triangular. The hole size, or hydraulic diameter, is usually between 1 and 20 mm, preferably between 2 and 10 mm. The wall thickness is between 0.3 and 2 mm, with the outer wall thickness usually greater than the inner wall thickness between the plates. Preferably, the aspect ratio (b / a) of the rectangular holes in the ceramic hollow plate is ≥2, and the long side is parallel to the top and bottom surfaces of the ceramic hollow plate. The thickness of the ceramic hollow plate is between 3 and 20 mm, preferably between 4 and 8 mm. Although the hollow plate resembles a honeycomb structure and appears to be a single-layer honeycomb, for heat exchangers, the hollow plate solves the problem of leakage between hot and cold heat exchange gases, representing a significant technological revolution.

[0031] The processing of ceramic hollow plates is similar to that of ceramic honeycomb structures. A clay slurry is extruded using a mold, then cut, dried, and sintered. Having patterns on the upper and lower surfaces similar to those in plate heat exchangers has the advantage of increasing the heat transfer coefficient between the plates, but the disadvantage is difficulty in sealing the plate ends. When the clay is freshly extruded from the mold, it is feasible to press patterns onto the outer surface because the wet clay is soft; this can be done by referring to the patterns used in plate heat exchangers. Figure 1 A ceramic hollow plate has an oblique opening at the left end, which can reduce the possibility of inlet blockage. This is often encountered in honeycomb ceramic heat storage bodies. In this application, there is no flow channel between the plates at the pipe head, and the oblique opening can flush most of the debris into the plate space by the airflow.

[0032] Figure 2 A basic unit of a ceramic hollow plate includes two rectangular ceramic gaskets 2 of the same material bonded together at both ends of the hollow plate (component 1). The gaskets 2 serve to seal the surface, similar to the gaskets between the plates in a plate heat exchanger. Component 2 can be made of ceramic fiberboard or ceramic fiber mat, preferably high-density rigid ceramic fiberboard. If the hollow plate is long, for example, exceeding 500mm, a support bridge 3, cut from a hollow plate of the same material as the hollow plate, is bonded to the hollow plate 1 at the middle. The support bridge 3 is a short section of hollow plate used for support. The support bridge 3 can be omitted when the hollow plate is short, and its number can be increased when the hollow plate is long. Since the hollow plate occupies space due to its wall thickness, if the thickness of the supporting bridge is the same as the thickness of the hollow plate, the flow cross-sectional area outside the plate is generally 2 to 3 times that inside the plate. In order to reduce the volume and optimize the structure of the heat exchanger, the thickness of the sealing gasket and the supporting bridge between the plates should be reduced. Preferably, the thickness of the sealing gasket 2 and the supporting bridge 3 is half the thickness of the hollow plate 1.

[0033] Figure 3 A ceramic hollow plate heat exchanger core, which will Figure 2 The image shows a type of ceramic hollow plate where multiple basic units are stacked together, with another unit placed on top. Figure 1 The ceramic hollow plates shown form the core of a ceramic hollow plate heat exchanger. Multiple ceramic hollow plate basic units typically do not need to be bonded together as a whole. During maintenance, similar to plate heat exchangers, they can be disassembled piece by piece; if any is damaged, only the damaged heat exchange plate needs to be replaced.

[0034] Figure 4 An air preheater using ceramic hollow plates includes a heat exchanger shell (component 4). The shell, closest to the inner core, is typically made of high-density, airtight ceramic fiber board, surrounded by an insulating ceramic fiber blanket. A steel structure and an outer protective plate then follow. The steel structure should have a pressure mechanism that applies pressure to the overlapping ceramic hollow plates via the high-density, airtight ceramic fiber board, ensuring a tight seal between the hollow plates, similar to the pressure between plates in a plate heat exchanger.

Claims

1. A ceramic hollow plate and an air preheater using the ceramic hollow plate, characterized in that: The structure of the ceramic hollow plate is a flat plate with multiple holes inside, and the material is ceramic. The air preheater using the ceramic hollow plate includes 1-hollow plate, 2-sealing gasket, 4-heat exchanger shell. Multiple hollow plates (1) and sealing gaskets (2) are stacked together at intervals to form a plate heat exchanger. The inside of the plate is similar to a ceramic honeycomb, and the outside of the plate is similar to a plate heat exchanger. Air and flue gas pass through the inside and outside of the plate, respectively.

2. The ceramic hollow plate and the air preheater using the ceramic hollow plate according to claim 1, characterized in that: The ceramic material of the hollow plate is specifically one or more of the following: alumina, mullite, cordierite, clay, silicon carbide, aluminum titanate, zircon, silicon carbide, and silicon nitride.

3. The ceramic hollow plate and the air preheater using the ceramic hollow plate according to claim 1, characterized in that: The heat exchanger gasket is made of one or more of the following materials: ceramic fiber board, silicon carbide, ceramic, rubber, graphite, graphite nickel wire packing, asbestos, and metal.

4. The ceramic hollow plate and the air preheater using the ceramic hollow plate according to claim 1, characterized in that: The thickness of the sealing gasket of the heat exchanger is 0.5~6 mm, which is half the thickness of the hollow plate.

5. A ceramic hollow plate and an air preheater using the ceramic hollow plate according to claim 1, characterized in that: The hollow plate has a thickness of 3 to 20 mm and a wall thickness of 0.5 to 2 mm. The shape of the holes in the hollow plate is rectangular, equilateral triangle, circular, or regular hexagonal.

6. The ceramic hollow plate and the air preheater using the ceramic hollow plate according to claim 1, characterized in that: The hollow plate has rectangular holes with a length-to-width ratio of 2 ≤ b / a ≤ 8, and the long side is parallel to the top and bottom surfaces of the ceramic hollow plate.

7. A ceramic hollow plate and an air preheater using the ceramic hollow plate according to claim 1, characterized in that: The support bridge (3) of the heat exchanger is a hollow plate.

8. A ceramic hollow plate and an air preheater using the ceramic hollow plate according to claim 1, characterized in that: The hollow plate has a bevel at one end.

9. A ceramic hollow plate and an air preheater using the ceramic hollow plate according to claim 1, characterized in that: The heat exchanger, with its multiple hollow plates, can be disassembled into individual plates during assembly and maintenance, just like a plate heat exchanger.

10. A ceramic hollow plate and an air preheater using the ceramic hollow plate according to claim 1, characterized in that: The manufacturing process of the hollow board involves extruding mud-like clay slurry through a mold, followed by cutting, drying, and sintering.

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