Secondary heat exchanger for gas-fired furnaces
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- GD MIDEA HEATING & VENTILATING EQUIP CO LTD
- Filing Date
- 2024-12-10
- Publication Date
- 2026-06-12
Smart Images
Figure CN122192025A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of thermal energy conversion technology, and in particular to a two-stage heat exchanger for a gas-fired furnace. Background Technology
[0002] In high-efficiency gas-fired boilers, a two-stage heat exchange system is typically used to improve energy utilization. The first-stage heat exchanger usually employs a tubular or shell-and-tube design, achieving initial heat transfer through direct contact between the combustion gases and the heat exchange medium. However, even with a single-stage heat exchanger, it's difficult to completely recover all the combustion heat; a portion remains unutilized. Directly releasing this underutilized heat into the environment not only wastes energy but also increases the environmental burden. Therefore, efficiently recovering this excess heat is crucial for improving overall thermal efficiency. Furthermore, existing heat exchanger materials suffer from poor corrosion resistance, which not only limits the operating temperature but also increases maintenance costs and shortens lifespan due to corrosion, ultimately affecting the long-term stable operation and energy efficiency of the entire system. Summary of the Invention
[0003] To address at least one of the aforementioned technical problems, this application provides a secondary heat exchanger for a gas-fired furnace. The secondary heat exchanger has the advantages of compact structure, simple manufacturing process, excellent corrosion resistance, and high heat transfer efficiency. It can effectively recover the residual heat that the primary heat exchanger failed to utilize, significantly improve the overall thermal efficiency and energy utilization rate of the high-efficiency gas-fired furnace, and extend the service life of the equipment.
[0004] In a first aspect, this application provides a two-stage heat exchanger for a gas-fired furnace, comprising:
[0005] Multiple stainless steel tubes are used to contain the heat transfer medium;
[0006] Multiple aluminum fins are disposed on the outer surface of the stainless steel tube, and the stainless steel tube and the aluminum fins are riveted together by tube expansion to enhance the heat exchange between the heat transfer medium inside the stainless steel tube and the outside.
[0007] End plates are installed at both ends of the stainless steel pipe along its length. The two ends of the stainless steel pipe are riveted to the end plates by expansion joints to fix the stainless steel pipe.
[0008] By adopting the above technical solution, this application significantly improves the heat exchange efficiency between the heat transfer medium and the external environment by using multiple stainless steel tubes to accommodate the heat transfer medium and setting multiple aluminum fins on their outer surface, and strengthening the connection between the stainless steel tubes and aluminum fins through tube expansion and riveting technology. Furthermore, the tube expansion and riveting sealing structure between the two ends of the stainless steel tubes and the end plates not only ensures the structural stability and sealing of the equipment and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through these optimized designs, this application can effectively recover the unused residual heat from the primary heat exchanger, further improving overall thermal efficiency, reducing energy waste, and lowering operating costs, thereby significantly improving the energy utilization rate and environmental friendliness of the high-efficiency gas furnace.
[0009] Optionally, the stainless steel tube is provided with baffles to increase the flow resistance of the heat transfer medium and thus enhance heat exchange.
[0010] By adopting the above technical solution, the baffle strip increases the flow resistance of the gas in the pipe and prolongs the residence time of the gas in the pipe, thereby improving the heat exchange effect and further enhancing the thermal efficiency.
[0011] Optionally, the spoiler strip has a spiral structure.
[0012] By adopting the above technical solution, the spiral-shaped turbulence strip can more effectively change the gas flow path, increase the degree of turbulence, and improve the heat transfer coefficient, thereby further enhancing the heat exchange effect.
[0013] Optionally, the stainless steel tube and the spoiler are made of BSS186N stainless steel.
[0014] By adopting the above technical solutions, BSS186N stainless steel material has excellent corrosion resistance, meets relevant standard requirements, ensures product reliability, and is particularly suitable for treating condensate containing high concentrations of acidic substances. It can effectively extend the service life of equipment and ensure long-term stable operation in high temperature and corrosive environments.
[0015] Optionally, the ends of the stainless steel pipe do not need to be flared, and there is no need to weld it to other pipes.
[0016] By adopting the above technical solution, the two ends of the stainless steel pipe do not need to be flared or welded, which simplifies the manufacturing process, reduces production costs, reduces potential leakage risks, and improves the reliability and safety of the equipment.
[0017] Optionally, the aluminum fins have a flange of at least 3 mm at the edge.
[0018] By adopting the above technical solution, the flanged design at the edge of the aluminum fins not only increases the contact area between the fins and the stainless steel tube, ensuring tight contact between the two and thus improving heat transfer efficiency, but also enhances the mechanical strength of the aluminum fins, improving structural stability and durability, preventing deformation under high temperature and pressure environments, and further improving the overall performance and reliability of the secondary heat exchanger. In addition, this design simplifies the manufacturing process, reduces production costs, and ensures long-term stable operation of the equipment.
[0019] Optionally, the thickness of the aluminum fins is 0.1-0.3 mm.
[0020] By adopting the above technical solution, the thickness of the aluminum fins is in the range of 0.1-0.3 mm, which not only ensures sufficient strength and durability, but also achieves lightweight design, improves heat conduction efficiency, and reduces material costs.
[0021] Optionally, the stainless steel tube has a diameter of 9-15 mm and a length of 350-1000 mm.
[0022] By adopting the above technical solutions, the diameter and length of the stainless steel tubes are designed within a reasonable range, which can adapt to high-efficiency gas furnace products of different specifications, ensuring the versatility and flexibility of the heat exchanger, while optimizing the heat exchange effect.
[0023] Optionally, the end plate is made of 316L stainless steel.
[0024] By adopting the above technical solution, the end plates are made of 316L stainless steel, which fully utilizes the excellent corrosion resistance and high-temperature strength of 316L stainless steel, ensuring long-term stable operation of the end plates in high-temperature and corrosive environments and extending the service life of the equipment. At the same time, 316L stainless steel has good mechanical and processing properties, meeting various requirements during the manufacturing and installation of the end plates, ensuring the reliability and safety of the entire secondary heat exchanger.
[0025] Secondly, this application provides a high-efficiency gas furnace heat exchanger, including a primary heat exchanger and a secondary heat exchanger provided in this application, wherein the secondary heat exchanger is disposed after the primary heat exchanger.
[0026] By adopting the above technical solution, this application can effectively recover the residual heat that the primary heat exchanger fails to fully utilize, further improving the overall thermal efficiency. Specifically, the primary heat exchanger initially transfers the heat of the combustion gas through a tubular or shell-and-tube design, while the secondary heat exchanger significantly enhances the heat exchange efficiency between the heat transfer medium and the external environment through a combination design of multiple stainless steel tubes and aluminum fins, as well as tube expansion and riveting technology. Furthermore, the optimized design of the secondary heat exchanger, such as the flanged edges of the aluminum fins, the internal baffles of the stainless steel tubes, and the tube expansion and riveting sealing structure of the end plates, not only improves the stability and sealing of the structure and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through the synergistic effect of this two-stage heat exchange system, this application significantly improves the overall thermal efficiency and energy utilization rate of the high-efficiency gas furnace, reduces energy waste, lowers operating costs, and enhances the environmental friendliness and economy of the system.
[0027] In summary, the present invention includes at least one of the following beneficial technical effects.
[0028] 1. This application significantly improves the heat exchange efficiency between the heat transfer medium and the external environment by employing multiple stainless steel tubes to contain the heat transfer medium and setting multiple aluminum fins on their outer surface. The connection between the stainless steel tubes and the aluminum fins is strengthened through tube expansion and riveting technology. Furthermore, the tube expansion and riveting sealing structure between the two ends of the stainless steel tubes and the end plates not only ensures the structural stability and sealing of the equipment and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through these optimized designs, this application can effectively recover the unused residual heat from the primary heat exchanger, further improving overall thermal efficiency, reducing energy waste, and lowering operating costs, thereby significantly enhancing the energy utilization rate and environmental friendliness of the high-efficiency gas furnace.
[0029] 2. The baffle strip increases the flow resistance of the gas in the pipe and prolongs the residence time of the gas in the pipe, thereby improving the heat exchange effect and further enhancing the thermal efficiency.
[0030] 3. The spiral-shaped turbulence strips can more effectively change the gas flow path, increase the degree of turbulence, improve the heat transfer coefficient, and thus further enhance the heat exchange effect.
[0031] 4. BSS186N stainless steel has excellent corrosion resistance, meets relevant standard requirements, ensures product reliability, and is particularly suitable for treating condensate containing high concentrations of acidic substances. It can effectively extend the service life of equipment and ensure long-term stable operation in high temperature and corrosive environments.
[0032] 5. This application effectively recovers the residual heat that the primary heat exchanger fails to fully utilize, further improving overall thermal efficiency. Specifically, the primary heat exchanger initially transfers heat from the combustion gases through a tubular or shell-and-tube design, while the secondary heat exchanger significantly enhances the heat exchange efficiency between the heat transfer medium and the external environment through a combination of multiple stainless steel tubes and aluminum fins, as well as tube expansion and riveting technology. Furthermore, the optimized design of the secondary heat exchanger, such as the flanged edges of the aluminum fins, the internal baffles of the stainless steel tubes, and the tube expansion and riveting sealing structure of the end plates, not only improves the stability and sealing of the structure and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through the synergistic effect of this two-stage heat exchange system, this application significantly improves the overall thermal efficiency and energy utilization rate of the high-efficiency gas furnace, reduces energy waste, lowers operating costs, and enhances the environmental friendliness and economic efficiency of the system. Attached Figure Description
[0033] Figure 1 This is a structural diagram of the secondary heat exchanger of the gas-fired boiler in Example 1;
[0034] Figure 2 This is a structural diagram of aluminum fins;
[0035] Figure 3 This is a partial structural diagram of the aluminum fins;
[0036] Attached reference numerals: 1- Stainless steel spoiler; 2- Stainless steel tube; 3- Stainless steel left end plate; 4- Aluminum fins; 5- Stainless steel right end plate. Detailed Implementation
[0037] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.
[0038] In high-efficiency gas-fired boilers, a two-stage heat exchange system is typically used to improve energy utilization. The first-stage heat exchanger usually employs a tubular or shell-and-tube design, achieving initial heat transfer through direct contact between the combustion gases and the heat exchange medium. However, even with a single-stage heat exchanger, it is difficult to completely recover all the combustion heat; a portion of the heat remains unutilized. Directly releasing this underutilized heat into the environment not only wastes energy but also increases the environmental burden. Therefore, efficiently recovering this excess heat is a key issue in improving overall thermal efficiency.
[0039] Therefore, in order to solve the problem of energy waste caused by the incomplete and inefficient utilization of heat in the primary heat exchanger in the existing technology, the applicant...
[0040] This application provides a secondary heat exchanger for a gas-fired boiler, comprising: multiple stainless steel tubes for containing a heat transfer medium; multiple aluminum fins disposed on the outer surface of the stainless steel tubes, the stainless steel tubes and the aluminum fins being connected by tube expansion riveting to enhance heat exchange between the heat transfer medium inside the stainless steel tubes and the outside environment; and end plates disposed at both ends of the stainless steel tubes along their length, the ends of the stainless steel tubes being connected to the end plates by tube expansion riveting to fix the stainless steel tubes. This application significantly improves the heat exchange efficiency between the heat transfer medium and the outside environment by using multiple stainless steel tubes to contain the heat transfer medium and disposing of multiple aluminum fins on their outer surface, and strengthening the connection between the stainless steel tubes and the aluminum fins through tube expansion riveting technology. Furthermore, the tube expansion riveting sealing structure between the ends of the stainless steel tubes and the end plates not only ensures the structural stability and sealing of the equipment and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through these optimized designs, this application can effectively recover the residual heat that was not utilized in the primary heat exchanger, further improving overall thermal efficiency, reducing energy waste, and lowering operating costs, thereby significantly improving the energy utilization rate and environmental friendliness of the high-efficiency gas-fired boiler.
[0041] In some embodiments, the stainless steel tube is provided with baffles to increase the flow resistance of the heat transfer medium, thereby enhancing heat exchange. The baffles increase the flow resistance of the gas in the tube and prolong the residence time of the gas in the tube, thereby improving the heat exchange effect and further enhancing the thermal efficiency.
[0042] In some embodiments, the turbulence strips are spiral in shape. The spiral structure of the turbulence strips can more effectively change the flow path of the gas, increase the degree of turbulence, improve the heat transfer coefficient, and thus further enhance the heat exchange effect.
[0043] When selecting suitable stainless steel materials for stainless steel pipes and baffles, the applicant discovered that although AL29-4C stainless steel imported from the United States possesses good corrosion resistance and meets relevant standards, it suffers from high costs and the risk of supply disruptions. Therefore, through experimentation and research, the inventors of this application concluded that BSS186N stainless steel not only has corrosion resistance similar to AL29-4C stainless steel but also exhibits better elongation, thus better meeting the requirements of secondary heat exchangers in gas-fired boilers operating in high-temperature and corrosive environments, while simultaneously reducing costs and supply chain risks.
[0044] In some embodiments, the stainless steel pipes and baffles are made of BSS186N stainless steel. BSS 186N stainless steel has excellent corrosion resistance, meets relevant standard requirements, ensures product reliability, and is particularly suitable for treating condensate containing high concentrations of acidic substances, effectively extending the service life of the equipment and ensuring long-term stable operation in high-temperature and corrosive environments.
[0045] In some embodiments, the ends of the stainless steel pipe do not need to be flared or welded to other pipes. Eliminating the need for flaring and welding at the ends simplifies the manufacturing process, reduces production costs, and also reduces the potential risk of leakage, thereby improving the reliability and safety of the equipment.
[0046] In some embodiments, the aluminum fins have a flange of at least 3 mm at their edges. This flanged design not only increases the contact area between the fins and the stainless steel tube, ensuring tight contact and thus improving heat transfer efficiency, but also enhances the mechanical strength of the aluminum fins, improving structural stability and durability, preventing deformation under high temperature and pressure conditions, and further improving the overall performance and reliability of the secondary heat exchanger. Furthermore, this design simplifies the manufacturing process, reduces production costs, and ensures long-term stable operation of the equipment.
[0047] In some embodiments, the thickness of the aluminum fins is 0.1-0.3 mm. A thickness of 0.1-0.3 mm ensures sufficient strength and durability while achieving a lightweight design, improving heat transfer efficiency, and reducing material costs.
[0048] In some embodiments, the stainless steel tube has a diameter of 9-15 mm and a length of 350-1000 mm. The diameter and length of the stainless steel tube are designed within a reasonable range to accommodate different specifications of high-efficiency gas furnace products, ensuring the versatility and flexibility of the heat exchanger while optimizing the heat exchange effect.
[0049] In some embodiments, the end plates are made of 316L stainless steel. The use of 316L stainless steel in the end plates fully utilizes the excellent corrosion resistance and high-temperature strength of 316L stainless steel, ensuring long-term stable operation of the end plates in high-temperature and corrosive environments and extending the service life of the equipment. Simultaneously, 316L stainless steel possesses good mechanical and processing properties, meeting various requirements during the manufacturing and installation of the end plates, ensuring the reliability and safety of the entire secondary heat exchanger.
[0050] Secondly, this application provides a high-efficiency gas-fired boiler heat exchanger, including a primary heat exchanger and a secondary heat exchanger provided in this application, with the secondary heat exchanger positioned after the primary heat exchanger. This application can effectively recover the residual heat that the primary heat exchanger fails to fully utilize, further improving the overall thermal efficiency. Specifically, the primary heat exchanger initially transfers the heat of the combustion gases through a tubular or shell-and-tube design, while the secondary heat exchanger significantly enhances the heat exchange efficiency between the heat transfer medium and the external environment through a combination design of multiple stainless steel tubes and aluminum fins, as well as tube expansion and riveting technology. Furthermore, the optimized design of the secondary heat exchanger, such as the flanged edges of the aluminum fins, the baffles inside the stainless steel tubes, and the tube expansion and riveting sealing structure of the end plates, not only improves the stability and sealing of the structure and prevents condensate backflow, but also extends the service life of the equipment, simplifies the manufacturing process, and reduces production costs. Through the synergistic effect of this two-stage heat exchange system, this application significantly improves the overall thermal efficiency and energy utilization rate of the high-efficiency gas-fired boiler, reduces energy waste, lowers operating costs, and enhances the environmental friendliness and economic efficiency of the system.
[0051] The solution of this application will be described below with reference to the following specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments are all from commercially available products, and the devices or equipment used are all purchased from conventional market sales channels.
[0052] Example 1
[0053] This embodiment provides a two-stage heat exchanger for a gas-fired boiler; see structural diagram below. Figure 1 See the structural diagram of the aluminum fins. Figure 2 See the structural diagram of the aluminum fins in detail. Figure 3 .
[0054] The secondary heat exchanger includes: 33 stainless steel tubes 2, aluminum fins 4 fixed on the outer surface of the stainless steel tubes; the stainless steel tubes and aluminum fins are reinforced by tube expansion; a stainless steel right end plate 5; a stainless steel left end plate 3; the two ends of the stainless steel tubes are sealed to the end plates by tube expansion and riveting, and includes 66 tube expansion points.
[0055] Each stainless steel tube also features a spiral-shaped baffle 1 inside, all made of BSS186N stainless steel. The aluminum fins have a 3mm flange at the edge.
[0056] The stainless steel pipe in this embodiment is made of BSS186N stainless steel. The end plate in this embodiment is made of 316L stainless steel. The two ends of the stainless steel pipe 2 in this embodiment do not need to be flared, and welding to other pipes is not required.
[0057] The aluminum fins in this embodiment are 0.18 mm thick. The stainless steel tube in this embodiment has a diameter of 9.65 mm and a length of approximately 462 mm. The secondary heat exchanger is installed after the primary heat exchanger.
[0058] This embodiment provides a high-efficiency secondary heat exchanger for a gas-fired furnace, designed to significantly improve heat transfer efficiency, structural stability, and durability through a series of innovative designs. The heat exchanger consists of several stainless steel tubes made of BSS186N stainless steel, with appropriately thick aluminum fins fixed externally. The connection between the two is strengthened using tube expansion technology, ensuring good heat transfer performance and a tight contact surface, preventing condensate backflow. The two ends of the stainless steel tubes are sealed to end plates made of 316L stainless steel via tube expansion riveting, eliminating the need for flaring and welding. This simplifies the manufacturing process, reduces costs, and improves the safety and reliability of the equipment. Furthermore, spiral-shaped baffles, also made of BSS186N stainless steel, are added inside the stainless steel tubes. This design increases the flow resistance of the gas within the tubes, prolonging its residence time and significantly improving heat exchange efficiency. In addition, the spiral structure of the baffles optimizes the gas flow path, increases turbulence, improves the heat transfer coefficient, effectively avoids the generation of local hot spots, and ensures the uniformity and efficiency of the heat exchange process. Furthermore, the aluminum fins feature a 3mm flange at the edge, which not only increases the contact area between the fins and the stainless steel tube, improving heat transfer efficiency, but also enhances mechanical strength, ensuring structural stability and durability under high temperature and pressure conditions. In summary, this two-stage heat exchanger design achieves advantages in high thermal efficiency, long lifespan, low cost, and high reliability, making it suitable for various high-efficiency gas furnace products and significantly improving overall thermal efficiency and energy utilization.
[0059] Comparative Examples 1-3
[0060] Comparative Example 1
[0061] The difference between this comparative example and Example 1 is that the stainless steel tube in this comparative example is made of 316L stainless steel.
[0062] Comparative Example 2
[0063] The difference between this comparative example and Example 1 is that the stainless steel tube in this comparative example is made of 904L stainless steel.
[0064] Comparative Example 3
[0065] The difference between this comparative example and Example 1 is that the comparative example uses imported AL29-4C stainless steel from the United States.
[0066] Experimental testing:
[0067] Mechanical property testing standard: ASTM A268 / A268M-10 (2016).
[0068] Experiment 1: The following tests were conducted on the BSS186N stainless steel and the American-imported AL29-4C stainless steel used in the embodiments of this application. The test results are shown in Table 1.
[0069] Table 1-
[0070]
[0071] Results Analysis: When selecting suitable stainless steel materials for the stainless steel pipes and baffles, the applicant found that although imported AL29-4C stainless steel products from the United States possess good corrosion resistance and meet relevant standards, they are costly and subject to supply chain risks. Therefore, through experimentation and research, the inventors of this application concluded that BSS186N stainless steel not only has corrosion resistance similar to AL29-4C stainless steel but also exhibits superior elongation at 35%, significantly higher than AL29-4C's 25%. This allows it to better adapt to complex working environments and improves the material's toughness and ductility. Furthermore, BSS186N stainless steel is lower in cost and has a more stable supply, eliminating the risk of supply shortages. This ensures the operational needs of the secondary heat exchanger for the gas furnace in high-temperature and corrosive environments while significantly reducing production costs and supply chain risks.
[0072] Experiment 2: The pitting corrosion resistance equivalent (PREN) of the BSS186N stainless steel material used in the embodiments of this application and the stainless steel materials in Comparative Examples 1-3 were tested. The relevant test structures are shown in Table 2.
[0073] Pitting corrosion resistance equivalent (PREN) = Cr% + 3.3Mo% + 16N%.
[0074] Table 2-
[0075]
[0076] Results Analysis: The pitting corrosion resistance equivalent (PREN) of the BSS186N stainless steel used in the embodiments of this application and the stainless steel materials in comparative groups 1-3 were tested. The results showed that the PREN value of BSS186N was 42.0, the PREN value of AL-29-4C was 42.5, and the PREN values of the other comparative groups were 25.2, 25.2, and 37.2, respectively. These data indicate that the BSS186N stainless steel material has high pitting corrosion resistance, close to that of AL-29-4C stainless steel, demonstrating good corrosion resistance.
[0077] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the principles of this application should be included within the protection scope of this application.
Claims
1. A two-stage heat exchanger for a gas-fired boiler, characterized in that, include: Multiple stainless steel tubes are used to contain the heat transfer medium; Multiple aluminum fins are disposed on the outer surface of the stainless steel tube, and the stainless steel tube and the aluminum fins are riveted together by tube expansion to enhance the heat exchange between the heat transfer medium inside the stainless steel tube and the outside. End plates are installed at both ends of the stainless steel pipe along its length. The two ends of the stainless steel pipe are riveted to the end plates by expansion joints to fix the stainless steel pipe.
2. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The stainless steel tube is equipped with baffles to increase the flow resistance of the heat transfer medium and thus enhance heat exchange.
3. The secondary heat exchanger for a gas-fired boiler according to claim 2, characterized in that, The baffle strip has a spiral structure.
4. The secondary heat exchanger for a gas-fired boiler according to any one of claims 1-3, characterized in that, The stainless steel pipe and the spoiler are made of BSS186N stainless steel.
5. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The stainless steel pipe does not need to be flared at both ends and does not need to be welded to other pipes.
6. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The aluminum fins have a flange of at least 3 mm at the edge.
7. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The thickness of the aluminum fins is 0.1-0.3 mm.
8. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The stainless steel tube has a diameter of 9-15 mm and a length of 350-1000 mm.
9. The secondary heat exchanger for a gas-fired boiler according to claim 1, characterized in that, The end plate is made of 316L stainless steel.
10. A high-efficiency gas-fired furnace heat exchanger, characterized in that, It includes a primary heat exchanger and a secondary heat exchanger according to any one of claims 1-9, wherein the secondary heat exchanger is disposed after the primary heat exchanger.