Waste heat recovery system and process for holding furnace of aluminum alloy hub low-pressure casting machine

Abstract

According to the waste heat recovery system and process for the holding furnace of the aluminum alloy hub low-pressure casting machine, the system comprises a sealed heat collection storage tank, a heat-conducting medium circulation loop, a full-oil-free vortex air compressor, a preheating exchanger and a control system. High-temperature gas in the furnace is efficiently collected through the full-oil-free vortex air compressor, waste heat dissipated from the furnace top is absorbed through the preheating exchanger to heat compressed air, the compressed air is supplemented to the sealed heat collection storage tank, and then the hot compressed air in the sealed heat collection storage tank is introduced into the heat preservation furnace for molten metal heat preservation and die casting operation. The electric energy consumption required for heating the compressed air can be obviously reduced, the energy utilization efficiency is improved, the production cost is reduced, and the working environment of a workshop is improved.

Description

Technical Field

[0001] This invention relates to the field of industrial energy-saving technology, specifically to a waste heat recovery system and process for the holding furnace of a low-pressure casting machine for aluminum alloy wheels. Background Technology

[0002] Automotive aluminum alloy wheels are generally produced using low-pressure casting. The core equipment of this process is the low-pressure casting machine and its supporting holding furnace. The holding furnace contains high-temperature molten aluminum alloy (usually maintained at 700-750℃), and high-temperature compressed air is introduced to apply pressure to the surface of the molten aluminum alloy, causing the molten aluminum alloy to fill along the riser pipe and mold. The molten aluminum alloy then enters the forming mold of the low-pressure casting machine to be die-cast into the aluminum alloy wheel. One die-casting cycle (i.e., the cycle of forming one aluminum alloy wheel) requires the high-temperature aluminum alloy molten metal to go through four stages: rising, filling, holding pressure, and depressurization to complete the casting process of one aluminum alloy wheel.

[0003] In the actual production of automotive aluminum alloy wheels, the surface temperature of the holding furnace wall is extremely high, continuously releasing a large amount of heat into the surrounding environment. This not only causes significant energy waste and increases production costs but also leads to increased ambient temperature in the casting workshop, worsening the working environment for workers. Currently, most manufacturers do not effectively recover and utilize this waste heat from the surface of the holding furnace, typically relying solely on the workshop ventilation system to directly vent it outdoors, resulting in low energy efficiency. Furthermore, during the processes of liquid lifting, mold filling, and pressure holding, compressed air needs to be injected into the holding furnace. Currently, the industry practice is to use an air compressor to compress room-temperature air into the holding furnace, while the hot compressed air discharged from the holding furnace during the depressurization process is directly released into the atmosphere. Injecting room-temperature compressed air into the holding furnace causes the aluminum liquid temperature to drop too quickly, affecting the quality of the cast aluminum alloy wheels; directly releasing the hot compressed air into the atmosphere during depressurization wastes energy. Summary of the Invention

[0004] The purpose of this invention is to address the deficiencies of the existing technology by providing a waste heat recovery system and process for the holding furnace of a low-pressure casting machine for aluminum alloy wheels. This system can effectively utilize the waste heat on the surface of the holding furnace, thereby reducing energy consumption and costs, and also effectively improving the environment of the casting workshop.

[0005] The technical solution adopted by this invention to achieve the above objectives is as follows: a waste heat recovery system for a holding furnace in a low-pressure casting machine for aluminum alloy wheels, comprising a sealed heat collection tank, a heat transfer medium circulation loop, an oil-free scroll air compressor, a preheating exchanger, and a control system. The sealed heat collection tank includes a gas storage tank and a gas tank insulation layer wrapped around the outer surface of the gas storage tank. An air inlet is provided on the lower side wall of the sealed heat collection tank, and an air outlet is provided on the upper side wall. The preheating exchanger is installed on the outer surface of the top wall of the holding furnace. Above the top wall of the holding furnace, there is also a structure capable of covering the top wall of the holding furnace and the preheating exchanger. The heat exchanger has a sealed heat collection cover, and the heat transfer medium circulation loop includes a compressed air inlet pipe, an insulated air pipe I, an insulated air pipe II, and an insulated air pipe III. The compressed air inlet pipe is connected to the inlet end of the preheating exchanger. The outlet end of the preheating exchanger is connected to the inlet of the sealed heat collection tank through the insulated air pipe I. The outlet of the sealed heat collection tank is connected to the inner cavity of the heat preservation furnace through the insulated air pipe II. The inner cavity of the heat preservation furnace is connected to the inlet of the sealed heat collection tank through the insulated air pipe III. An oil-free scroll air compressor is connected in series in the insulated air pipe III. The oil-free scroll air compressor is connected to the control system through a control circuit.

[0006] A further technical solution of the present invention is: the heat-conducting medium circulation loop further includes proportional valve I, proportional valve II and proportional valve III, proportional valve I is connected in series in the compressed air inlet pipe, proportional valve II is connected in series in the heat-insulating air pipe II, proportional valve III is connected in series in the heat-insulating air pipe III, and proportional valve I, proportional valve II and proportional valve III are respectively connected to the control system through a control circuit.

[0007] A further technical solution of the present invention is as follows: a control cabinet is provided on one side of the low-pressure casting machine, the control system is set in the control cabinet, a temperature sensor and a pressure sensor I are respectively provided inside the sealed heat collection tank, and a pressure sensor II is provided inside the connection end between the heat preservation gas pipe II and the heat preservation furnace. The temperature sensor, pressure sensor I and pressure sensor II are respectively connected to the control system through circuits.

[0008] A further technical solution of the present invention is: the preheating exchanger is formed by welding multiple steel pipes, the steel pipes of the preheating exchanger are evenly distributed around the liquid riser on the top wall of the heat preservation furnace, the air inlet end of the preheating exchanger passes through the sealed heat collection cover and is connected to the compressed air inlet pipe, and the air outlet end of the preheating exchanger passes through the sealed heat collection cover and is connected to the heat preservation air pipe I.

[0009] Another technical solution adopted by the present invention to achieve the above objectives is: a waste heat recovery process for the holding furnace of a low-pressure casting machine for aluminum alloy wheels, using the above-mentioned waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheels, including the following steps: (i) the riser pipe fills the mold of the low-pressure casting machine with high-temperature aluminum alloy molten liquid. When a die casting cycle enters the depressurization stage from the pressure holding stage, the control system controls the proportional valve III to open, and at the same time, the control system controls the oil-free scroll air compressor to work. The high-temperature gas in the furnace chamber of the holding furnace is discharged from the holding gas pipe III during the depressurization stage and is extracted and transported to the sealed heat collection tank by the oil-free scroll air compressor; (ii) at the beginning of the next die casting cycle, the control system controls the proportional valve II to open, and the high-temperature and high-pressure gas in the sealed heat collection tank is discharged from the furnace chamber. Gas is discharged from the outlet and flows through the heat-insulating gas pipe II into the heat-insulating furnace for pressurization, so that the liquid riser pipe fills the mold of the low-pressure casting machine with high-temperature aluminum alloy melt, and the pressure in the heat-insulating furnace is continuously maintained in a pressure-holding state until the high-temperature aluminum alloy melt completes the liquid rise, mold filling and pressure holding process of one die casting cycle; (iii) When the compressed air in the sealed heat-collecting tank is lower than the set value after use, the control system controls the proportional valve I to open, and the compressed air enters the preheating exchanger through the compressed air inlet pipe through the inlet end. The compressed air absorbs the heat between the top wall of the heat-insulating furnace and the sealed heat-collecting cover in the preheating exchanger. The compressed air with the increased temperature after passing through the preheating exchanger enters the sealed heat-collecting tank through the outlet end and through the heat-insulating gas pipe III to replenish the missing pressure.

[0010] The waste heat recovery system and process of the present invention for the holding furnace of the low-pressure casting machine for aluminum alloy wheel hubs have the following beneficial effects: 1. High efficiency and energy saving: This invention directly targets the largest heat loss source of the heat preservation furnace (heat dissipation from the furnace wall) and recovers it. The preheating exchanger is set on the outer surface of the top wall of the heat preservation furnace and covered by a sealed heat collection cover. A heat collection duct is formed between the sealed heat collection cover and the top wall of the heat preservation furnace. The preheating exchanger can fully absorb the heat in the heat collection duct and use the originally wasted heat energy to preheat the compressed air required for the production process. This can save the electricity required for heating compressed air and significantly reduce the overall energy consumption. 2. High return on investment: The system structure of this invention is relatively simple, can be modified from existing equipment, has low initial investment cost, significant energy-saving effect, and short investment payback period; 3. Stable and reliable: The compressed air of this invention absorbs heat through a preheating exchanger, adopting an indirect heat exchange method. The heat transfer medium circuit and the compressed air system are independent of each other, which does not affect the stability and safety of the original die-casting process. The intelligent control system ensures that the system operates automatically and stably. 4. Environmental Improvement: By absorbing and utilizing the heat from the furnace wall, this invention effectively reduces the radiant heat around the furnace, improves the thermal environment of the foundry workshop, enhances the comfort of the working environment, and embodies the concept of green manufacturing. 5. High versatility: The system and process of this invention are not only applicable to low-pressure casting and heat preservation furnaces for automobile wheel hubs, but can also be extended to other similar industrial kilns that require heat preservation and have large surface heat dissipation.

[0011] The waste heat recovery system and process for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs according to the present invention will be further described below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs according to the present invention. Figure 2 This is a schematic diagram of a preheating exchanger installed on the top wall of an insulated furnace. Figure 3 This is an enlarged view of the bend connection of the preheating exchanger; Reference numerals: 1-Insulation furnace, 2-High-temperature aluminum alloy melt, 3-Lift pipe, 4-Preheat exchanger, 5-Sealed heat collection cover, 6-Aluminum alloy hub, 7-Mold, 8-Proportional valve I, 9-Compressed air inlet pipe, 10-Control cabinet, 11-Insulated air pipe II, 12-Proportional valve II, 13-Outlet, 14-Sealed heat collection tank, 15-Insulation layer of air tank, 16-Air tank, 17-Temperature sensor, 18-Pressure sensor I, 19-Inlet, 20-Proportional valve III, 21-Insulated air pipe III, 22-Oil-free scroll air compressor, 23-Insulated air pipe I, 24-Pressure sensor II, 25-Inlet end, 26-Outlet end. Detailed Implementation

[0013] like Figure 1 , Figure 2 As shown, the present invention relates to a waste heat recovery system for a heat preservation furnace in a low-pressure casting machine for aluminum alloy wheel hubs, comprising a sealed heat collection tank, a heat transfer medium circulation loop, an oil-free scroll air compressor, a preheating exchanger, and a control system.

[0014] like Figure 1 As shown, the sealed thermal storage tank includes a gas storage tank and a gas tank insulation layer wrapped around the outer surface of the gas storage tank. The gas tank insulation layer is made of insulation material and completely covers the outer surface of the gas storage tank to prevent heat loss from the tank. An air inlet is provided on the lower side wall of the sealed thermal storage tank, and an air outlet is provided on the upper side wall. Figure 1 , Figure 2 As shown, the preheating exchanger is installed on the outer surface of the top wall of the holding furnace. The preheating exchanger is formed by welding multiple steel pipes together, with the pipes connected at bends by two 90-degree bend joints with intermediate inserted pipes. Figure 3As shown, the steel pipes of the preheating exchanger are evenly distributed around the riser pipes on the top wall of the holding furnace. A sealed heat-collecting hood is also installed above the top wall of the holding furnace to cover both the furnace top and the preheating exchanger. This hood is made of insulation material, such as ceramic fiber blankets, which can be laid on the inner wall. A heat-collecting air duct is formed between the sealed heat-collecting hood and the top wall of the holding furnace. The total length of the preheating exchanger's steel pipes is 10 meters. The winding and meandering nature of the pipes effectively increases the contact between the pipes and the heat-collecting air duct, allowing the preheating exchanger to fully absorb heat from the air duct and facilitating the absorption of heat by the cool compressed air inside the pipes.

[0015] like Figure 1 As shown, the heat transfer medium circulation loop includes a compressed air inlet pipe, insulated pipe I, insulated pipe II, and insulated pipe III. The compressed air inlet pipe is connected to the inlet end of the preheating exchanger, and the other end of the compressed air inlet pipe is connected to the outlet pipe of the air compressor (not shown in the figure). The outlet end of the preheating exchanger is connected to the inlet of the sealed heat collection tank through insulated pipe I. Since the preheating exchanger is covered by a sealed heat collection cover, the inlet end of the preheating exchanger passes through the sealed heat collection cover and connects to the compressed air inlet pipe, and the outlet end of the preheating exchanger passes through the sealed heat collection cover and connects to insulated pipe I. The outlet of the sealed heat collection tank is connected to the inner cavity of the heat preservation furnace through insulated pipe II, and the inner cavity of the heat preservation furnace is connected to the inlet of the sealed heat collection tank through insulated pipe III. An oil-free scroll air compressor is connected in series in insulated pipe III. The oil-free scroll air compressor is connected to the control system through a control circuit, and the control system controls the operation of the oil-free scroll air compressor. The heat transfer medium circulation loop also includes proportional valves I, II, and III. Proportional valve I is connected in series in the compressed air inlet pipe, proportional valve II is connected in series in the insulated air pipe II, and proportional valve III is connected in series in the insulated air pipe III. Proportional valves I, II, and III are each connected to the control system via control circuits, and the control system controls the opening and closing of proportional valves I, II, and III respectively. A control cabinet is located on one side of the low-pressure casting machine, and the control system is housed in the control cabinet. The control system is a PLC programmable logic controller, which is existing equipment and will not be described in detail here.

[0016] A temperature sensor and pressure sensor I are respectively installed inside the sealed solar collector tank. The temperature sensor is used to collect the temperature value inside the sealed solar collector tank, and pressure sensor I is used to collect the pressure value inside the sealed solar collector tank. A pressure sensor II is installed inside the connection end between the insulated gas pipe II and the insulated furnace. Pressure sensor II is used to collect the pressure value inside the insulated furnace. The temperature sensor, pressure sensor I, and pressure sensor II are respectively connected to the control system through circuits. The signals collected by the temperature sensor, pressure sensor I, and pressure sensor II can be transmitted to the control system in real time. The control system can issue control commands to proportional valve I, proportional valve II, proportional valve III, and the oil-free scroll air compressor based on the collected signals.

[0017] This invention relates to a waste heat recovery process for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs. Employing the aforementioned waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs, the process includes the following steps: (i) The riser pipe fills the mold of the low-pressure casting machine with high-temperature molten aluminum alloy. During a die-casting cycle, as the pressure holding stage transitions to the pressure relief stage, the control system opens proportional valve III. Simultaneously, the control system operates the oil-free scroll air compressor. High-temperature gas from the furnace chamber is discharged through insulation pipe III during the pressure relief stage and extracted by the oil-free scroll air compressor, transported to a sealed heat collection tank. The gas temperature at the furnace outlet and insulation pipe III is 328℃~350℃. Recovering the depressurized compressed air saves a significant amount of energy used for heating compressed air. The opening time of proportional valve III is controlled in real-time by the control system based on the pressure value collected by pressure sensor II. When the pressure value reaches the minimum pressure value set for pressure relief (set in the control system according to the size and material of the cast aluminum alloy wheel hub), the control system closes proportional valve III and stops the oil-free scroll air compressor. The processes of rising liquid, filling mold, holding pressure, and depressurizing during the casting of aluminum alloy wheel hubs using a low-pressure casting machine are existing technologies and will not be described in detail here. Figure 1 The direction indicated by the middle arrow is the direction of compressed air flow.

[0018] (ii) At the start of the next die casting cycle, the control system controls the proportional valve II to open, and the high-temperature and high-pressure gas in the sealed heat collection tank is discharged from the outlet and flows through the heat preservation pipe II into the heat preservation furnace for pressurization. The high-temperature compressed air pressurizes the surface of the high-temperature aluminum alloy melt in the heat preservation furnace, so that the riser pipe fills the mold of the low-pressure casting machine with high-temperature aluminum alloy melt, and keeps the pressure in the heat preservation furnace continuously in a pressure-holding state until the high-temperature aluminum alloy melt completes the riser, mold filling and pressure holding process of one die casting cycle. The entire process requires dynamic control of the compressed air flow and pressure. If there is a small amount of air leakage, the proportional valve II needs to be kept open to continuously replenish the heat preservation furnace with compressed air to ensure the continuous and stable pressure in the heat preservation furnace.

[0019] (III) When the compressed air pressure in the sealed heat collection tank is lower than the set value (the set value is set in the control system and can be set differently according to the aluminum alloy wheel hubs of different sizes and materials), the control system controls proportional valve I to open, and the air compressor connected to the compressed air inlet pipe to work. The compressed air enters the preheating exchanger through the compressed air inlet pipe. The room temperature compressed air absorbs heat between the top wall of the insulation furnace and the sealed heat collection cover in the preheating exchanger. According to the test, the temperature of the top wall of the insulation furnace can reach 120℃, and the temperature of the preheating exchanger pipe can reach 110℃. The compressed air with increased temperature after passing through the preheating exchanger then enters the sealed heat collection tank through the outlet and the insulation air pipe III to replenish the lost pressure. When the pressure value in the sealed heat collection tank reaches the set value, the control system controls proportional valve I to close.

[0020] After testing, the temperature of the molten aluminum inside the holding furnace was 680℃~710℃. The heat rise in the holding furnace caused the temperature of the furnace top to increase, reaching 120℃. The temperature of the preheating exchanger pipe reached 110℃. The gas temperature at the beginning of the holding furnace outlet recovery pipe (i.e., the connection end between the holding gas pipe III and the holding furnace) was 328℃~350℃. Without the need for additional heating, the gas temperature inside the sealed heat collection tank was always maintained at 46℃~65℃. During the liquid lifting, filling, and pressure holding processes, high-temperature compressed air was introduced into the holding furnace, which slowed down the temperature drop of the molten aluminum inside the holding furnace, ensuring the quality of the cast aluminum alloy wheel hub. Recovering the depressurized compressed air can save a lot of electricity for heating the compressed air.

[0021] By repeatedly cycling the above process steps to achieve waste heat recovery, power consumption and aluminum dross production can be reduced while filling and heating more molten aluminum. Trial use of the waste heat recovery system and process of this invention in the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs showed an average reduction of 30.05% in power consumption per ton of molten aluminum, a 2.92% reduction in power consumption per mold, and a 13.14% reduction in high aluminum dross content. Regarding production quality, statistics from one month of production showed a 95.23% pass rate when using the waste heat recovery system and process, compared to 93.4% without waste heat recovery (existing technology), representing an increase of 1.83%.

[0022] The system and process of this invention can effectively utilize the residual heat on the surface of the holding furnace, thereby reducing energy consumption and production costs, improving the environment of the foundry workshop, and enhancing product quality.

[0023] The above embodiments are merely preferred embodiments of the present invention. The structure of the present invention is not limited to the forms listed in the above embodiments. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheels, characterized in that, The system includes a sealed heat collection tank (14), a heat transfer medium circulation loop, an oil-free scroll air compressor (22), a preheating exchanger, and a control system. The sealed heat collection tank (14) includes a gas storage tank (16) and a gas tank insulation layer (15) wrapped around the outer surface of the gas storage tank (16). The lower side wall of the sealed heat collection tank (14) is provided with an air inlet (19), and the upper side wall of the sealed heat collection tank (14) is provided with an air outlet (13). The preheating exchanger is installed on the outer surface of the top wall of the heat preservation furnace (1). A sealed heat collection cover (5) that can cover the top wall of the heat preservation furnace (1) and the preheating exchanger is also provided above the top wall of the heat preservation furnace (1). The heat transfer medium circulation loop includes a compressed air inlet pipe (9) and an insulation pipe I. (23), Insulated air pipe II (11), Insulated air pipe III (21), Compressed air inlet pipe (9) is connected to the inlet end (25) of the preheating exchanger, the outlet end (26) of the preheating exchanger is connected to the inlet (19) of the sealed heat collection tank (14) through insulated air pipe I (23), the outlet (13) of the sealed heat collection tank (14) is connected to the inner cavity of the heat preservation furnace (1) through insulated air pipe II (11), the inner cavity of the heat preservation furnace (1) is connected to the inlet (19) of the sealed heat collection tank (14) through insulated air pipe III (21), and the oil-free scroll air compressor (22) is connected in series in insulated air pipe III (21). The oil-free scroll air compressor (22) is connected to the control system through the control circuit.

2. The waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs as described in claim 1, characterized in that, The heat transfer medium circulation loop also includes proportional valve I (8), proportional valve II (12) and proportional valve III (20). Proportional valve I (8) is connected in series in the compressed air inlet pipe (9), proportional valve II (12) is connected in series in the heat insulation pipe II (11), and proportional valve III (20) is connected in series in the heat insulation pipe III (21). Proportional valve I (8), proportional valve II (12) and proportional valve III (20) are respectively connected to the control system through the control circuit.

3. The waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs as described in claim 1, characterized in that, A control cabinet (10) is provided on one side of the low-pressure casting machine. The control system is located in the control cabinet (10). A temperature sensor (17) and a pressure sensor I (18) are respectively provided inside the sealed heat collection tank (14). A pressure sensor II (24) is provided inside the connection end between the heat preservation gas pipe II (11) and the heat preservation furnace (1). The temperature sensor (17), pressure sensor I (18) and pressure sensor II (24) are respectively connected to the control system through circuits.

4. The waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs as described in claim 1, characterized in that, The preheating exchanger is formed by welding multiple steel pipes. The steel pipes of the preheating exchanger are evenly distributed around the liquid riser (3) on the top wall of the heat preservation furnace (1). The air inlet (25) of the preheating exchanger passes through the sealed heat collection cover (5) and is connected to the compressed air inlet pipe (9). The air outlet (26) of the preheating exchanger passes through the sealed heat collection cover (5) and is connected to the heat preservation gas pipe I (23).

5. A waste heat recovery process for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs, employing the waste heat recovery system for the holding furnace of a low-pressure casting machine for aluminum alloy wheel hubs as described in any one of claims 1-4, characterized in that... Includes the following steps: (i) The riser pipe (3) fills the mold (7) of the low-pressure casting machine with high-temperature aluminum alloy molten liquid. When a die casting cycle enters the depressurization stage from the pressure holding stage, the control system controls the proportional valve III (20) to open. At the same time, the control system controls the oil-free scroll air compressor (22) to work. The high-temperature gas in the furnace chamber of the heat preservation furnace (1) is discharged from the heat preservation gas pipe III (21) during the depressurization stage and is extracted by the oil-free scroll air compressor (22) and transported to the sealed heat collection tank (14). (ii) When the next die casting cycle begins, the control system controls the proportional valve II (12) to open. The high-temperature and high-pressure gas in the sealed heat collection tank (14) is discharged from the outlet (13) and flows through the heat preservation gas pipe II (11) into the heat preservation furnace (1) for pressurization, so that the riser pipe (3) High-temperature aluminum alloy melt is injected into the mold (7) of the low-pressure casting machine, and the pressure inside the heat preservation furnace (1) is continuously maintained in a pressure-holding state until the high-temperature aluminum alloy melt completes the liquid raising, filling and pressure holding process of one die casting cycle; (iii) When the compressed air in the sealed heat collection tank (14) is lower than the set value after use, the control system controls the proportional valve I (8) to open, and the compressed air enters the preheating exchanger through the compressed air inlet pipe (9) and the inlet end (25). The compressed air absorbs the heat between the top wall of the heat preservation furnace (1) and the sealed heat collection cover (5) in the preheating exchanger. The compressed air with the increased temperature after passing through the preheating exchanger enters the sealed heat collection tank (14) through the outlet end (26) and the heat preservation air pipe III (21) to replenish the missing pressure.

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