A two-stage heat energy recovery system and method for a natural gas direct-fired nonwoven fabric oven

Abstract

The application discloses a two-stage heat energy recovery system and method of a natural gas direct combustion non-woven fabric oven, relates to the technical field of non-woven material drying and finishing equipment, and forms a two-stage recovery cycle through a cloth surface preheating section, a main drying section, an air filtering section and an air preheating section; high-temperature tail gas first passes through a shell-and-tube heat exchanger to preheat combustion-supporting air in a first stage, is cooled, and then is introduced into the cloth surface preheating section to preheat non-woven fabric in a second stage; the air filtering section adopts a double-channel parallel structure, and filters can be replaced on line without shutdown. The application realizes step-by-step utilization of tail gas, the utilization rate of waste heat reaches 55%-70%, the single consumption of fuel gas is reduced by 15%-30%, the combustion efficiency is improved by preheating combustion-supporting air, the drying efficiency is improved by preheating a cloth surface, the problem of ash accumulation is solved by on-line filtering without shutdown, the application can be directly connected to existing ovens for modification, has strong adaptability, and is suitable for energy-saving upgrading of various non-woven fabric continuous drying production lines.

Description

Technical Field

[0001] This invention relates to the fields of nonwoven material drying technology and finishing equipment technology, specifically to a two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven, and particularly to a composite preheating structure based on the cascade utilization of exhaust gas waste heat and its energy synergistic regulation method. Background Technology

[0002] Currently, natural gas direct-fired ovens are widely used for hot air drying in nonwoven fabric production. These devices directly burn natural gas to heat air, creating high-temperature hot air that penetrates the fabric surface to evaporate moisture, resulting in relatively high thermal efficiency. However, traditional structures have significant energy efficiency bottlenecks: the exhaust gas temperature at the oven outlet is typically as high as 80-140℃, carrying a large amount of sensible heat and latent heat of water vapor condensation. Conventional systems only recover a portion of this heat through heat exchangers or discharge it directly without recovery, resulting in a waste heat utilization rate generally below 30%. When nonwoven fabric enters the oven, it is at room temperature (approximately 25℃) and generally carries 200%-400% of its own weight in moisture. A large amount of heat energy is required at the front end of the oven to raise the temperature to the initial drying temperature of around 80℃, resulting in concentrated heat load at the front end and low thermal efficiency in the heating zone. Furthermore, the combustion air entering the burner is not preheated, leading to a decrease in combustion temperature and weakened flame stability, while also increasing the gas consumption per unit area during nonwoven fabric production.

[0003] Existing technologies often employ a single heat exchange path, such as air preheating or fabric preheating alone, which makes it difficult to simultaneously consider the material's heat capacity characteristics and combustion kinetics requirements, thus failing to fully realize the potential for waste heat recovery. Therefore, a novel heat recovery system with a compact structure, strong thermal coupling, and the ability to simultaneously improve the fabric's thermal response speed and combustion efficiency is needed.

[0004] Chinese patent document (CN110926178A) discloses a hot air drying oven with waste heat recovery function. The core of the oven lies in its composition, which consists of a fiber introduction module, a heating zone module, a transition zone module, and a cooling zone module arranged sequentially from right to left, with each module interconnected. Waste heat recovery mechanisms are installed at corresponding positions on the front and rear sides of the fiber introduction module. These mechanisms pass through pipes sequentially through the lower parts of the heating, transition, and cooling zone modules, ultimately connecting with each module. When the nonwoven fiber web layer enters the fiber introduction module, it absorbs waste heat from other modules through the waste heat recovery mechanism, achieving preliminary preheating. This reduces energy consumption in the heating zone and improves overall energy efficiency. However, the drawback is that waste heat recovery is limited by the module layout. If the pipe design between modules is unreasonable (e.g., excessive length or too many bends), heat loss may increase, resulting in lower-than-expected recovery efficiency. Furthermore, the fixed module layout may limit the oven's adaptability to different process requirements; for example, the waste heat recovery effect may fluctuate when processing nonwoven fabrics of different thicknesses or materials. Dust accumulation and maintenance issues in pipelines: Dust or fiber debris may accumulate in waste heat recovery pipelines during long-term operation, especially when processing materials containing dust or easily detached fibers. Dust accumulation reduces heat transfer efficiency and can even fall onto the non-woven fabric being dried or clog the pipelines, affecting the normal operation of the drying oven. The patent does not explicitly mention pipeline cleaning or anti-clogging designs, which may increase later maintenance costs.

[0005] Chinese patent document (CN205128358U) proposes a closed-loop hot air circulation oven structure. This structure uses a fan to recirculate hot air from the oven through a filter before returning it to the heating zone, reducing the amount of fresh air needed and thus lowering the energy required to heat the fresh air. Its key feature is the addition of a return air proportional control valve and a temperature sensor to achieve a hybrid control of partial hot air circulation and partial fresh air replenishment, representing an internal hot air reuse solution. However, its energy-saving principle relies on reducing the volume of cold air to be heated, while the exhaust gas is directly discharged into the atmosphere without any heat exchange treatment. All the heat from the oven's exhaust gas is lost, and the reused hot air contains a large amount of moisture. If this moisture is not treated, it will continuously increase within the oven, leading to a decrease in drying efficiency.

[0006] Chinese patent document (CN106048950B) discloses a natural gas direct-fired heating device for a stenter. Its innovation lies in its "double-isolation chamber" structure: the outer layer is a cold air isolation chamber, and the inner layer is a high-temperature combustion chamber. An air curtain isolates the chamber to prevent high-temperature flue gas leakage and improves thermal airtightness. The burner employs a premixed structure, emphasizing flame uniformity and hot air distribution stability. Its technical focus is primarily on combustion safety, hot air uniformity control, and reducing exhaust gas generated during long-term heating in the oven, thus increasing equipment safety. However, the device has drawbacks. It requires a high-precision burner, isolation chambers, and a circulating fan, resulting in high equipment costs. Strict requirements are placed on oven structural modifications, and upgrading older equipment requires significant additional investment. It is suitable for large stenters used in continuous production but less adaptable to small-batch, multi-variety production scenarios. While some exhaust gas is reintroduced into the combustion system to reduce heat emissions, the exhaust gas is mostly water vapor and inert gases. The increase in inert gases reduces the peak flame temperature, affecting heat transfer efficiency. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven. By providing a heat recovery system for a natural gas direct-fired drying oven that achieves cascade utilization of heat energy, reduces overall energy consumption, shortens effective drying time, and improves thermal response consistency through the synergistic effect of preheating the combustion air and the nonwoven fabric body by the waste heat of the exhaust gas.

[0008] The objective of this invention is achieved through the following technical solution: a two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven, comprising a fabric preheating section, a main drying section, an air filtration section, an air preheating section, and several airflow conveying pipes; The fabric preheating section is equipped with an insulated preheating box. The bottom of the insulated preheating box has a drain and air vent for connecting to an external water treatment component. The pre-dehydrated non-woven fabric enters from one side of the insulated preheating box, and after preheating, exits from the other side of the insulated preheating box and enters the main drying section. The main drying section is equipped with a natural gas direct-fired hot air oven, which is located behind the insulated preheating box and is used to dry the preheated non-woven fabric. The bottom of the natural gas direct-fired hot air oven is equipped with a combustion air inlet for introducing combustion air to assist the burner in the natural gas direct-fired hot air oven. The top of the natural gas direct-fired hot air oven is equipped with a tail gas exhaust port, which is connected to the inlet of the air filter section through an airflow conveying pipe. The air filtration section is equipped with a dual-channel parallel pipeline structure to filter the exhaust gas discharged from the natural gas direct-fired hot air oven, and a dehumidification fan is installed on the airflow conveying pipeline between the exhaust gas outlet and the air filtration section. The air preheating section is equipped with a shell-and-tube heat exchanger. The shell-side inlet of the shell-and-tube heat exchanger serves as the heat source inlet and is connected to the outlet of the air filtration section via an airflow pipeline, thereby introducing the filtered exhaust gas into the shell side of the shell-and-tube heat exchanger. The tube-side inlet of the shell-and-tube heat exchanger serves as the cold source inlet for introducing outside air. The tube-side outlet of the shell-and-tube heat exchanger is connected to the combustion air inlet via an airflow pipeline. After heat exchange with the exhaust gas, the outside air enters the natural gas direct-fired hot air oven as combustion air, forming the first stage of heat recovery. The exhaust gas, which has been cooled down after heat exchange, is discharged from the shell-side outlet of the shell-and-tube heat exchanger and is connected to the reuse hot air inlet at the top of the insulated preheating box via an airflow pipeline to preheat the non-woven fabric, forming the second stage of heat recovery.

[0009] As a further technical solution, the top of the insulated preheating box adopts an "eight"-shaped structure and the bottom adopts a funnel-shaped structure. Infrared thermometers are installed at the inlet and outlet of the fabric surface of the insulated preheating box to monitor the temperature of the non-woven fabric. An air pressure sensor is installed inside the insulated preheating box to monitor the air pressure inside the insulated preheating box.

[0010] As a further technical solution, the heat-insulating preheating box is equipped with multiple sets of guide rollers parallel to the running direction of the fabric. Each set of guide rollers includes an upper guide roller and a lower guide roller located below the upper guide roller. The upper guide roller and the lower guide roller are staggered, so that the running trajectory of the fabric around the guide rollers after entering the heat-insulating preheating box is a continuous W shape. Each upper guide roller or each lower guide roller is driven by an independent motor, thereby adjusting the speed individually.

[0011] As a further technical solution, a liquid collection trough is set on the inner wall of the heat-insulating preheating box near the fabric inlet and outlet sides. The liquid collection trough is set along the width of the fabric surface, and the edge of the liquid collection trough along the width of the fabric surface extends beyond the width of the dried non-woven fabric. The highest position of the liquid collection trough in the vertical direction is higher than the position of the non-woven fabric, and the liquid collection trough adopts a structure that is higher in the middle and lower on both sides, so that the condensed water flows to both sides after flowing into the liquid collection trough.

[0012] As a further technical solution, the water treatment component includes a suction fan connected to a drain outlet, a water storage device connected to the outlet of the suction fan, and a water treatment device connected to the rear of the water storage device.

[0013] As a further technical solution, air pressure monitors are installed at both the inlet and outlet of the air filtration section, and a temperature and flow sensor is also installed at the inlet of the air filtration section. The air filtration section includes a first channel and a second channel arranged in parallel, and a pre-filter valve, a filter screen and a post-filter valve are installed on each of the two channels in sequence. The opening and closing of the corresponding channels are controlled by the pre-filter valve and the post-filter valve.

[0014] As a further technical solution, a preliminary filter device is installed on the tube-side inlet pipe of the shell-and-tube heat exchanger to perform preliminary filtration of the outside air.

[0015] As a further technical solution, the outer wall of the airflow conveying pipeline is covered with an insulation layer, which is made of one or more of the following: aluminum silicate fiber products, glass wool products, rock wool products, phenolic foam plastics, and aerogel composite insulation materials.

[0016] A two-stage heat recovery method for a natural gas direct-fired nonwoven fabric drying oven, employing the aforementioned two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven, includes the following steps: S1. The high-temperature exhaust gas discharged from the natural gas direct-fired hot air oven is sent to the air filtration section through the airflow conveying pipeline and the dehumidification fan to filter and remove fiber debris and dust. S2. The filtered high-temperature exhaust gas enters the shell side of the shell-and-tube heat exchanger and exchanges heat with the ambient temperature combustion air in the tube side to form waste heat exhaust gas. The combustion air is preheated and then sent to the combustion air inlet of the natural gas direct-fired hot air oven to achieve the first stage of heat energy recovery. S3. The waste heat exhaust gas after heat exchange and cooling is discharged from the shell side of the shell-and-tube heat exchanger and introduced into the heat-insulated preheating box. The waste heat exhaust gas and the initially dehydrated non-woven fabric undergo convective heat exchange to preheat the fabric surface and realize the second stage of heat energy recovery. S4. The low-temperature waste gas and condensate generated after the waste heat exhaust gas heat exchange are discharged through the drain and air outlet at the bottom of the insulated preheating box. The condensate is collected by the water treatment component and reused.

[0017] As a further technical solution, the temperature of the high-temperature exhaust gas is 80-140℃, and the moisture content is 10%-30%; the temperature of the preheated combustion air is 40-80℃, the temperature of the waste heat exhaust gas is 50-90℃, the temperature of the fabric surface after heat exchange is 35℃-60℃, and the temperature of the low-temperature exhaust gas is 40℃-65℃; in steps S1 to S4, the operating frequency of the dehumidification fan and the water treatment components is adjusted according to the actual production process.

[0018] Preferably, the fabric preheating section is located before the main drying section and is equipped with a support for the fabric preheating section.

[0019] Furthermore, the main function of the fabric preheating section is to preheat the fabric surface using the exhaust gas from the main drying section, which has undergone preliminary heat exchange in the shell-and-tube heat exchanger.

[0020] As a preferred option, the overall structure of the fabric preheating section is an insulated preheating box.

[0021] Preferably, the insulated preheating box of the fabric preheating section has a length of 1-4 meters, a width of 1-4 meters, and a height of 1-3 meters.

[0022] As a preferred option, the preheating chamber is equipped with multiple sets of guide rollers, a reusable hot air inlet is provided at the top of the preheating chamber, and a liquid drain and air outlet is provided at the bottom of the preheating chamber. The top of the preheating chamber has an "eight" shape design, and the bottom has a funnel shape design. Liquid collection and guide grooves are provided on the inner walls of the fabric inlet and outlet sides of the preheating chamber.

[0023] Preferably, the multiple sets of guide rollers inside the preheating chamber run parallel to the fabric surface. Furthermore, the guide rollers are arranged vertically, and the side view of the running trajectory of the fabric when it enters the preheating box is a continuous W shape.

[0024] Furthermore, the vertical spacing between the guide rollers is 0.5-2.5m.

[0025] Furthermore, the upper and lower guide rollers are evenly arranged with the same spacing, and the spacing between adjacent upper guide rollers or adjacent lower guide rollers is 20-40cm.

[0026] Furthermore, an upper guide roller plus the nearest lower guide roller is called a group, and the number of guide roller groups in the preheating box is set to 2-20 groups.

[0027] Preferably, each upper guide roller is driven by an independent motor or each lower guide roller is driven by an independent motor.

[0028] Preferably, the reclaimed hot air inlet at the top of the preheating chamber is located at the figure-eight shaped opening.

[0029] Furthermore, the recycled hot air inlet has the function of uniformly distributing airflow, ensuring that the recycled hot airflow is evenly distributed in the width direction.

[0030] Preferably, the angle between the "eight"-shaped inner wall at the top of the preheating chamber and the horizontal direction is 15°-45°.

[0031] Furthermore, the inner wall of the top, shaped like the number "8," is coated with a PTFE high-temperature resistant, waterproof, and oil-resistant coating. Its main function is to prevent moisture from condensing at the top during the fabric preheating process and then falling back onto the fabric surface, thus preventing uneven fabric humidity.

[0032] Preferably, liquid collection tanks are provided on the inner walls of the preheating box on the fabric inlet and outlet sides.

[0033] Furthermore, the liquid collection tank is positioned vertically higher than the nonwoven fabric, with the middle of the tank being higher and the sides lower. The condensed water flows into the collection tank and then moves to both sides of the tank.

[0034] Furthermore, the liquid collection tank is set along the width of the fabric surface, and the edge of the liquid collection tank in the width direction of the fabric surface needs to exceed the width of the dried nonwoven fabric, so as to guide the liquid to the bottom of the preheating box and prevent the liquid from flowing onto the nonwoven fabric.

[0035] Preferably, an exhaust and drainage outlet is provided at the funnel-shaped bottom of the preheating chamber.

[0036] Furthermore, the exhaust and drainage outlets are connected to a suction fan and a water storage device via pipes to promptly remove excess water vapor and condensed liquid.

[0037] Furthermore, the condensed liquid is collected by a water storage device and then treated by a water treatment facility, so that it can be recycled as process water.

[0038] Preferably, infrared thermometers for fabric surface are installed at the inlet and outlet of the preheating chamber to provide feedback on the temperature changes of the fabric surface before and after preheating.

[0039] Preferably, an internal pressure sensor is installed inside the preheating chamber to monitor the internal pressure.

[0040] Preferably, the main drying section follows the fabric preheating section.

[0041] Furthermore, the main drying section is a natural gas direct-fired hot air oven.

[0042] Preferably, the burner in the main drying section uses combustion air that has been preheated by the air preheating section to assist in the combustion of natural gas.

[0043] Furthermore, the temperature of the combustion air is increased to 40℃-80℃.

[0044] As a preferred option, the natural gas direct-fired hot air oven in the main drying section can monitor and display the moisture content of the hot air inside the oven in real time.

[0045] As a preferred option, the main drying section is equipped with an exhaust outlet.

[0046] Furthermore, the main function of the exhaust outlet is to discharge dried exhaust gas with temperature and a certain moisture content.

[0047] Furthermore, the temperature of the exhaust gas emitted from the exhaust port is 80℃-140℃. The moisture content of the emitted exhaust gas is 10%-30%.

[0048] As a preferred option, a dehumidification fan is installed at the exhaust outlet.

[0049] Furthermore, the air inlet and exhaust outlet of the dehumidifying fan are connected by a pipe.

[0050] Furthermore, the rated exhaust volume of the dehumidifying fan is 4500-20000 m3 / h.

[0051] As a preferred option, an air filter section is installed after the dehumidification fan.

[0052] Furthermore, the inlet of the air filtration section is connected to the outlet of the dehumidification fan via a duct.

[0053] Furthermore, the air filtration section features a dual-channel parallel design.

[0054] Furthermore, each channel of the air filtration section is sequentially equipped with a pre-filtration valve, a filter screen, and a post-filtration valve.

[0055] Furthermore, the pre-filter and post-filter valves control the opening or closing of the channel.

[0056] Preferably, the air filter is a high-temperature resistant and high-efficiency air filter.

[0057] As a preferred option, the high-temperature resistant and high-efficiency air filter has a box-type structure.

[0058] As a preferred option, the filter media for high-temperature resistant high-efficiency air filters is high-temperature resistant glass fiber filter paper or alumina fiber filter felt.

[0059] Preferably, an internal air pressure monitor is installed between the air filtration section and the dehumidification fan, as well as at the outlet of the air filtration section, to monitor the air pressure before and after the air filtration section, providing a basis for replacing the high-temperature resistant and high-efficiency air filter and adjusting the operating power of the dehumidification fan.

[0060] Preferably, during normal operation, only one of the parallel dual-channel air filtration units is activated. The other channel is activated only when the high-temperature, high-efficiency air filter needs replacement. This ensures that the high-temperature, high-efficiency air filter can be replaced while the equipment is running continuously.

[0061] Furthermore, the replacement procedure for the high-temperature resistant high-efficiency air filter in the air filtration section is as follows: Open the pre-filter and post-filter valves of the channel on the side where the high-temperature resistant high-efficiency air filter does not need replacement, allowing the channel to enter the normal state of filtering oven exhaust gas. Then, close the pre-filter and post-filter valves of the channel where the high-temperature resistant high-efficiency air filter needs replacement. After the channel is closed, remove the high-temperature resistant high-efficiency air filter that needs replacement and replace it with a new high-temperature resistant high-efficiency air filter to complete the replacement operation.

[0062] Preferably, an air preheating section is set after the air filtration section.

[0063] As a preferred option, the main working equipment in the air preheating section is a shell-and-tube heat exchanger.

[0064] Furthermore, the heat source inlet of the shell-and-tube heat exchanger is connected to the outlet of the air filter section, and the cold source inlet of the shell-and-tube heat exchanger is connected to the ambient temperature air that has undergone preliminary filtration.

[0065] Furthermore, the heat source gas of the shell-and-tube heat exchanger enters the shell side, and the outlet end of the heat source gas, which has cooled down after heat exchange, is connected to the fabric preheating section. The cold source gas of the shell-and-tube heat exchanger enters the tube side, and the cold source gas, which has heated up after heat exchange, is connected to the natural gas direct-fired burner in the main drying section, serving as a combustion-supporting gas to continuously supply natural gas for combustion.

[0066] Preferably, the shell-and-tube heat exchanger is made of 316L stainless steel or 2205 duplex steel to resist strong corrosion under conditions of high moisture content and high temperature.

[0067] Furthermore, the shell-side flow velocity range for shell-and-tube heat exchangers is 5-8 m / s. Increasing the velocity can improve heat exchange efficiency, but it is also necessary to avoid condensate erosion and corrosion under high-speed and high-humidity conditions.

[0068] Furthermore, the tube-side flow velocity of the shell-and-tube heat exchanger is 1.5-2 m / s to achieve better heat transfer efficiency.

[0069] Furthermore, regarding pressure drop control, the tube-side pressure drop is less than 0.03 MPa, and the shell-side pressure drop is less than 300 Pa. Increased pressure drop will lead to increased power consumption. The above selection comprehensively considers both safety and economy.

[0070] Furthermore, the total length of the shell-and-tube heat exchanger is set at 3-5 meters. Increasing the length will allow for a larger design area for the heat exchange module and improve heat exchange efficiency, but it will be limited by the floor space required.

[0071] Furthermore, the tube length to shell diameter ratio of the shell-and-tube heat exchanger is 4-6, which effectively prevents vibration and noise.

[0072] Furthermore, the heat exchange area of ​​the shell-and-tube heat exchanger is 30m². 2 -60m 2 .

[0073] Preferably, the outlet end of the heat source gas whose temperature has decreased after heat exchange in the tube side of the shell-and-tube heat exchanger is connected to the reclaimed hot air inlet at the top of the insulated box of the fabric preheating section via a pipe, and is used as the heat source gas for the fabric preheating of the insulated box.

[0074] As a preferred option, the airflow conveying pipelines of the two-stage heat recovery system of the above-mentioned natural gas direct-fired non-woven fabric drying oven are all designed with heat preservation to reduce the heat loss of the oven exhaust gas during the conveying process.

[0075] Furthermore, the insulation design involves wrapping the exhaust gas conveying pipe of the oven with heat-resistant insulation material.

[0076] Furthermore, the insulation material can be aluminum silicate fiber products, glass wool products, rock wool products, phenolic foam plastics, aerogel composite insulation materials, etc.

[0077] The beneficial effects of this invention are as follows: 1. By constructing a tiered utilization path of "exhaust gas → air preheating → fabric preheating," the sensible and latent heat recovery of the oven exhaust gas is maximized. The first stage of recovery preheats the combustion air to 40-80℃, improving combustion efficiency and flame stability. The second stage of recovery uses the cooled exhaust gas (still at 50-90℃) to preheat the nonwoven fabric, raising its temperature from room temperature to 35-60℃, significantly shortening the heating time in the main drying section, reducing the front-end heat load, and effectively improving the overall drying speed and production efficiency. The synergistic effect of the two-stage recovery reduces the system's fuel consumption by 15%-30% and increases the waste heat utilization rate to 55%-70%.

[0078] 2. Preheating the combustion air to 40-80℃ enhances flame rigidity and combustion temperature, reduces incomplete combustion and CO emissions, improves flame stability, and increases thermal radiation efficiency.

[0079] 3. The dual-channel parallel filtration structure allows for online filter replacement without shutting down the system, solving the problem of fiber dust accumulation and clogging, extending the life of the heat exchanger, and ensuring high equipment reliability without affecting production cycle. It effectively reduces the accumulation of scale inside the pipes of the two-stage heat recovery system, reducing the reduction in heat exchange efficiency caused by scale buildup inside the shell-and-tube heat exchanger, and effectively extending the service life of the shell-and-tube heat exchanger. At the same time, the air filtration section effectively prevents fiber debris or dust from accumulating on the nonwoven fabric again through the recycled hot air inlet of the two-stage heat recovery system pipes.

[0080] 4. The insulated chamber reduces heat loss during preheating; the figure-eight shaped top and funnel-shaped bottom facilitate even airflow distribution and rapid condensate drainage; the liquid collection and diversion channel effectively prevents condensate from dripping back onto the fabric surface. The liquid collection and diversion channel is set along the width of the fabric surface to ensure that condensate flowing down from the side walls is intercepted by the channel and directed to the bottom of the chamber, preventing any condensate from falling onto the fabric surface and ensuring product quality.

[0081] 5. The W-shaped fabric path significantly increases the residence time of the nonwoven fabric in the preheating chamber and the contact area with hot air, thereby improving the convective heat transfer efficiency; the guide rollers are driven by independent motors, which can precisely control the fabric tension and avoid fabric deformation or deviation caused by uneven tension.

[0082] 6. The drain and exhaust vents are connected to an external water treatment component, which can collect and reuse condensate, realizing the recycling of water resources.

[0083] 7. Infrared thermometers are installed at both the inlet and outlet of the insulated preheating chamber, and a pressure sensor is installed inside the chamber. The temperature and pressure sensors provide data support for system control, ensuring stable preheating effect under different working conditions.

[0084] 8. A preliminary filter is installed on the tube-side inlet pipe of the shell-and-tube heat exchanger to preliminarily filter the outside air and prevent impurities in the outside air from entering the natural gas direct-fired hot air oven.

[0085] 9. All airflow delivery pipelines in the system are designed with insulation to reduce heat loss during the delivery process. Attached Figure Description

[0086] Figure 1 This is a schematic diagram of the structure of the present invention.

[0087] Figure 2 This is a schematic diagram of the pipeline connection structure of the air filter section in this invention.

[0088] Figure 3 This is a schematic diagram of the fabric preheating section (insulated preheating box) in this invention.

[0089] Explanation of reference numerals in the attached drawings: 1. Insulated preheating chamber; 1-1. Guide roller; 1-2. Liquid collection tank; 1-3. Recycled hot air inlet; 1-4. Liquid drain and exhaust outlet; 1-5. Suction fan; 1-6. Water storage device; 1-7. Water treatment equipment; 2. Natural gas direct-fired hot air oven; 2. Tail gas exhaust outlet; 2-1. Combustion air inlet; 2-2. Burner; 2-3. Air filter section; 3. First channel; 3-1. Second channel; 3-2. Pre-filter valve; 3-3. Filter screen; 3-4. Post-filter valve; 3-5. Shell-and-tube heat exchanger; 4. Shell-side inlet; 4-1. Tube-side inlet; 4-2. Shell-side outlet; 4-3. Tube-side outlet; 4-4. Exhaust fan; 5. Air pressure monitor; 6. Preliminary filtration device; 7. Infrared thermometer; 8. Temperature and flow sensor; 9. Detailed Implementation

[0090] The present invention will now be described in detail with reference to the accompanying drawings: Example 1: As shown in the attached document Figures 1-3 As shown, this natural gas direct-fired nonwoven fabric drying oven has a two-stage heat recovery system, including a fabric preheating section, a main drying section, an air filtration section 3, an air preheating section, and several airflow conveying pipes.

[0091] Reference Appendix Figure 1 The fabric preheating section is located in front of the main drying section. An insulated preheating chamber 1 is installed in this section. Drainage and ventilation outlets 1-4 are located at the bottom of the insulated preheating chamber 1 for connecting to an external water treatment component. The water treatment component includes a suction fan 1-5 connected to the drainage and ventilation outlets 1-4. The outlet of the suction fan 1-5 is connected to a water storage device 1-6, and a water treatment device 1-7 is connected to the rear of the water storage device 1-6. The nonwoven fabric, after preliminary dehydration, enters from one side of the insulated preheating chamber 1, exits from the other side after preheating, and enters the main drying section.

[0092] Furthermore, such as Figure 3 As shown, the top of the heat-insulating preheating box 1 adopts an "eight"-shaped structure and the bottom adopts a funnel-shaped structure. Infrared thermometers 8 are installed at the inlet and outlet of the fabric surface of the heat-insulating preheating box 1 to monitor the temperature of the non-woven fabric. An air pressure sensor is installed inside the heat-insulating preheating box 1 to monitor the air pressure inside the heat-insulating preheating box 1.

[0093] The heat-insulating preheating chamber 1 is equipped with multiple sets of guide rollers 1-1 parallel to the fabric's running direction. Each set of guide rollers 1-1 includes an upper guide roller and a lower guide roller located below the upper guide roller, and the upper and lower guide rollers are staggered, so that the fabric's running trajectory around the guide rollers 1-1 after entering the heat-insulating preheating chamber 1 is a continuous W-shape. Preferably, each upper guide roller or each lower guide roller is driven by an independent motor, thereby allowing for individual speed adjustment.

[0094] A liquid collection trough 1-2 is provided on the inner wall of the heat-insulating preheating chamber 1 near the fabric inlet and outlet sides. The liquid collection trough 1-2 is arranged along the width direction of the fabric surface, and its edge extends beyond the width of the nonwoven fabric being dried, so as to guide the liquid to the bottom of the heat-insulating preheating chamber 1 and prevent the liquid from flowing onto the nonwoven fabric. Preferably, the highest point of the liquid collection trough 1-2 in the vertical direction is higher than the position of the nonwoven fabric, and the liquid collection trough 1-2 adopts a structure that is higher in the middle and lower on both sides, so that the condensed water flows to both sides after flowing into the liquid collection trough 1-2.

[0095] like Figure 1 As shown, the main drying section is equipped with a natural gas direct-fired hot air oven 2, which is located behind the insulated preheating box 1 and is used to dry the preheated non-woven fabric. The bottom of the natural gas direct-fired hot air oven 2 is equipped with a combustion air inlet 2-2, which is used to introduce combustion air to assist the burner 2-3 in the natural gas direct-fired hot air oven 2 to burn. The top of the natural gas direct-fired hot air oven 2 is equipped with a tail gas outlet 2-1, which is connected to the inlet end of the air filter section 3 through an airflow conveying pipe.

[0096] Reference Appendix Figure 1 , 2 The air filtration section 3 has a dual-channel parallel pipeline structure to filter the exhaust gas discharged from the natural gas direct-fired hot air oven 2, and a dehumidifying fan 5 is installed on the airflow conveying pipeline between the exhaust port 2-1 and the air filtration section 3. Preferably, a pressure monitor 6 is installed at both the inlet and outlet ends of the air filtration section 3, and a temperature and flow sensor 9 is also installed at the inlet end of the air filtration section 3. The air filtration section 3 includes a first channel 3-1 and a second channel 3-2 arranged in parallel, and each channel is sequentially equipped with a pre-filtration valve 3-3, a filter screen 3-4, and a post-filtration valve 3-5, which control the opening and closing of the corresponding channels.

[0097] Furthermore, such as Figure 1 As shown, the air preheating section is equipped with a shell-and-tube heat exchanger 4. The shell-side inlet 4-1 of the shell-and-tube heat exchanger 4 serves as a heat source inlet and is connected to the outlet end of the air filtration section 3 through an airflow pipeline, thereby introducing the filtered exhaust gas into the shell side of the shell-and-tube heat exchanger 4. The tube-side inlet 4-2 of the shell-and-tube heat exchanger 4 serves as a cold source inlet for introducing outside air. The tube-side outlet 4-4 of the shell-and-tube heat exchanger 4 is connected to the combustion air inlet 2-2 through an airflow pipeline. After heat exchange between the outside air and the exhaust gas, the outside air enters the natural gas direct-fired hot air oven 2 as combustion air, forming the first stage of heat recovery. The exhaust gas with a reduced temperature after heat exchange is discharged from the shell-side outlet 4-3 of the shell-and-tube heat exchanger 4 and is connected to the reuse hot air inlet 1-3 opened on the top of the insulated preheating box 1 through an airflow pipeline to preheat the non-woven fabric, forming the second stage of heat recovery.

[0098] Preferably, a preliminary filter device 7 is installed on the tube-side inlet 4-2 pipe of the shell-and-tube heat exchanger 4 to perform preliminary filtration of the outside air.

[0099] In this system, the outer walls of each airflow conveying pipe are covered with an insulation layer. Preferably, the insulation layer can be made of one or more of the following: aluminum silicate fiber products, glass wool products, rock wool products, phenolic foam plastics, and aerogel composite insulation materials.

[0100] Example 2: A two-stage heat recovery method for a natural gas direct-fired nonwoven fabric drying oven, employing the two-stage heat recovery system for the natural gas direct-fired nonwoven fabric drying oven described in Example 1, including the following steps: S1. The high-temperature exhaust gas (temperature 80-140℃, moisture content 10%-30%) discharged from the natural gas direct-fired hot air oven 2 is sent to the air filter section 3 through the airflow conveying pipeline and the dehumidification fan 5 to filter and remove fiber debris and dust. S2. The filtered high-temperature exhaust gas enters the shell side of the shell-and-tube heat exchanger 4 and exchanges heat with the ambient temperature (25°C) combustion air in the tube side to form waste heat exhaust gas at 50-90°C. The combustion air is preheated to 40-80°C and then sent to the combustion air inlet 2-2 of the natural gas direct-fired hot air oven 2 to achieve the first stage of heat energy recovery. S3. The waste heat exhaust gas after heat exchange and cooling is discharged from the shell side of the shell-and-tube heat exchanger 4 and introduced into the heat-insulated preheating box 1. The waste heat exhaust gas exchanges heat with the initially dehydrated non-woven fabric through convection, preheating the fabric surface. The temperature of the preheated fabric surface is 35℃-60℃, realizing the second stage of heat energy recovery. S4. The low-temperature exhaust gas (temperature 40℃-65℃) generated after the waste heat exhaust gas heat exchange and the condensate are discharged through the drain outlets 1-4 at the bottom of the insulated preheating box 1. The condensate is collected by the water treatment component and reused.

[0101] Furthermore, in the above steps, the operating frequency of the dehumidification fan 5 and the water treatment components (exhaust fans 1-5) is adjusted according to the actual production process (based on the parameters of the air pressure monitor 6, infrared thermometer 8, temperature and flow sensor 9, and other sensors).

[0102] Example 3: The following example has a width of 3.4 meters and a product weight of 80g / m². 2 Taking the modification of a spunlace nonwoven fabric production line with a production speed of 120m / min, and the introduction of a two-stage heat recovery system with a natural gas direct-fired nonwoven fabric drying oven, as an example, this paper will further explain the equipment configuration and process control. The actual operating results will also be described.

[0103] After hydroentangling and initial dehydration, the nonwoven fabric enters the fabric preheating section. This section uses an insulated preheating chamber measuring 4 meters long, 4 meters wide, and 3 meters high, equipped with 20 sets of guide rollers. The distance between adjacent upper and lower guide rollers is 20 cm, and the vertical spacing between the rollers is 2.5 m. Each lower guide roller is driven by an independent motor. The "8"-shaped inner wall at the top of the preheating chamber forms a 15° angle with the horizontal direction and is coated with a PTFE high-temperature resistant, waterproof, and oil-resistant coating. Liquid collection tanks are located on the inner walls of the fabric inlet and outlet sides of the preheating chamber. The funnel-shaped bottom of the preheating chamber has exhaust and drainage outlets connected to a suction fan and a water storage device. Infrared thermometers are installed at the fabric inlet and outlet of the preheating chamber, and a pressure sensor is installed inside the chamber.

[0104] Following the fabric preheating section is the main drying section. The exhaust gas temperature from the main drying section's outlet is 80℃-140℃, and the moisture content is 10%-30%. The rated exhaust volume of the dehumidification fan is 20,000 m³ / h. 3 / h, with the actual operating frequency controlled at around 60%.

[0105] An air filtration section is connected to the exhaust fan's outlet. This air filtration section is a dual-valve, dual-duct high-temperature resistant filtration device. Each duct has an independently controllable valve for opening and closing; during normal operation, one duct's valve is open while the other is closed. Inside the ducts of this dual-valve, dual-duct high-temperature resistant filtration device are box-type air filters using high-temperature resistant glass fiber filter paper as the filter media. Internal air pressure monitors are installed between the air filtration section and the exhaust fan, as well as at the air filtration section's outlet.

[0106] An air preheating section is connected to the air filter outlet. The air preheating section uses a shell-and-tube heat exchanger made of 2205 duplex stainless steel. The shell-side flow velocity of the shell-and-tube heat exchanger is controlled at 5-8 m / s, and the tube-side flow velocity is controlled at 1.5-2.0 m / s. Regarding pressure drop control, the tube-side pressure drop is less than 0.03 MPa, and the shell-side pressure drop is less than 300 Pa. The total length of the shell-and-tube heat exchanger is set at 5 meters, the tube length / shell diameter ratio is 6, and the heat exchange area is 60 m². 2 Oven exhaust gas at 80℃-140℃ and with a moisture content of 10%-30% enters the shell side of the shell-and-tube heat exchanger, where it exchanges heat with the oven combustion air in the tube side, raising the temperature of the oven combustion air to 40℃-80℃. Then, the oven burner uses the preheated combustion air mixed with natural gas to continuously burn and heat the gas inside the oven.

[0107] The oven exhaust gas, having undergone preliminary heat exchange in the shell-and-tube heat exchanger and cooled to 50℃-90℃, is transported to the fabric preheating section via the shell-side gas outlet. From the reclaimed hot air inlet at the top of the preheating section, it enters the insulated preheating chamber. Through convective heat exchange, it preheats the nonwoven fabric, which contains 200%-400% of its own weight in moisture, raising the fabric temperature from approximately 25℃ to 35℃-60℃. After heat exchange, the low-temperature airflow (40℃-65℃) and water vapor are discharged through the exhaust and drain outlets at the bottom of the insulated preheating chamber under the negative pressure of a suction fan. The condensed moisture is collected, and the remaining exhaust gas is discharged.

[0108] Based on the actual production process, the operating frequency of the dehumidification fan and the operating frequency of the suction fan connected to the exhaust and drain ports at the bottom of the insulated preheating chamber can be adjusted. This ensures the effective utilization of waste heat from the oven exhaust and the stable operation of the two-stage heat recovery system.

[0109] The temperature gradient after two-stage heat exchange is as follows: oven exhaust gas 80℃-140℃ → air preheating → exhaust gas 50℃-90℃ → fabric preheating → exhaust gas 40℃-65℃ exhaust. The waste heat utilization rate is in the range of 55%-70%.

[0110] It is understood that, for those skilled in the art, any equivalent substitutions or modifications to the technical solutions and inventive concepts of this invention should fall within the scope of protection of the appended claims.

Claims

1. A two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven, characterized in that: It includes a fabric preheating section, a main drying section, an air filtration section (3), an air preheating section, and several airflow conveying pipes; The fabric preheating section is equipped with an insulated preheating box (1). The bottom of the insulated preheating box (1) has a drain and air outlet (1-4) for connecting to an external water treatment component. The non-woven fabric that has been initially dehydrated enters from one side of the insulated preheating box (1), and after preheating, exits from the other side of the insulated preheating box (1) and enters the main drying section. The main drying section is equipped with a natural gas direct-fired hot air oven (2). The natural gas direct-fired hot air oven (2) is located behind the heat-insulated preheating box (1) and is used to dry the preheated non-woven fabric. The bottom of the natural gas direct-fired hot air oven (2) is equipped with a combustion air inlet (2-2) for introducing combustion air to assist the burner (2-3) in the natural gas direct-fired hot air oven (2) to burn. The top of the natural gas direct-fired hot air oven (2) is equipped with a tail gas outlet (2-1). The tail gas outlet (2-1) is connected to the inlet end of the air filter section (3) through an airflow conveying pipe. The air filtration section (3) is equipped with a dual-channel parallel pipeline structure for filtering the exhaust gas discharged from the natural gas direct-fired hot air oven (2), and a dehumidifying fan (5) is installed on the airflow conveying pipeline between the exhaust port (2-1) and the air filtration section (3). The air preheating section is equipped with a shell-and-tube heat exchanger (4). The shell-side inlet (4-1) of the shell-and-tube heat exchanger (4) serves as a heat source inlet and is connected to the outlet end of the air filtration section (3) through an airflow conveying pipe, thereby introducing the filtered exhaust gas into the shell side of the shell-and-tube heat exchanger (4). The tube-side inlet (4-2) of the shell-and-tube heat exchanger (4) serves as a cold source inlet for introducing outside air. The tube-side outlet (4-4) of the shell-and-tube heat exchanger (4) is connected to the combustion air inlet (2-2) through an airflow conveying pipe. After the outside air exchanges heat with the exhaust gas, it enters the natural gas direct-fired hot air oven (2) as combustion air, forming the first stage of heat recovery. The exhaust gas with a reduced temperature after heat exchange is discharged from the shell-side outlet (4-3) of the shell-and-tube heat exchanger (4) and is connected to the reuse hot air inlet (1-3) opened on the top of the heat-insulated preheating box (1) through an airflow conveying pipe to preheat the non-woven fabric, forming the second stage of heat recovery.

2. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The top of the heat-insulating preheating box (1) adopts an "eight" shaped structure and the bottom adopts a funnel-shaped structure. Infrared thermometers (8) are installed at the inlet and outlet of the fabric surface of the heat-insulating preheating box (1) to monitor the temperature of the non-woven fabric. An air pressure sensor is installed inside the heat-insulating preheating box (1) to monitor the air pressure inside the heat-insulating preheating box (1).

3. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The heat-insulating preheating box (1) is equipped with multiple sets of guide rollers (1-1) parallel to the running direction of the fabric surface. Each set of guide rollers (1-1) includes an upper guide roller and a lower guide roller located below the upper guide roller. The upper guide roller and the lower guide roller are staggered, so that the running trajectory of the fabric surface around the guide roller (1-1) after entering the heat-insulating preheating box (1) is a continuous W shape. Each upper guide roller or each lower guide roller is driven by an independent motor, so that the speed can be adjusted individually.

4. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The heat-insulating preheating box (1) has a liquid collection tank (1-2) on the inner wall near the fabric inlet and outlet sides. The liquid collection tank (1-2) is set along the width of the fabric surface, and the edge of the liquid collection tank (1-2) along the width of the fabric surface exceeds the width of the dried non-woven fabric. The highest position of the liquid collection tank (1-2) in the vertical direction is higher than the position of the non-woven fabric. The liquid collection tank (1-2) adopts a structure that is higher in the middle and lower on both sides, so that the condensed water flows to the liquid collection tank (1-2) and then flows to both sides of it.

5. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The water treatment component includes a suction fan (1-5) connected to a drain outlet (1-4), the outlet of the suction fan (1-5) being connected to a water storage device (1-6), and a water treatment device (1-7) being connected to the rear of the water storage device (1-6).

6. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The air filter section (3) is equipped with air pressure monitors (6) at both the inlet and outlet ends. A temperature and flow sensor (9) is also provided at the inlet end of the air filter section (3). The air filter section (3) includes a first channel (3-1) and a second channel (3-2) arranged in parallel. Each of the two channels is equipped with a pre-filter valve (3-3), a filter screen (3-4), and a post-filter valve (3-5) in sequence. The opening and closing of the corresponding channels are controlled by the pre-filter valve (3-3) and the post-filter valve (3-5).

7. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: A preliminary filter device (7) is installed on the tube-side inlet (4-2) pipe of the shell-and-tube heat exchanger (4) for preliminary filtration of the outside air.

8. The two-stage heat recovery system for a natural gas direct-fired nonwoven fabric drying oven according to claim 1, characterized in that: The outer wall of the airflow conveying pipeline is covered with an insulation layer, which is made of one or more of the following materials: aluminum silicate fiber products, glass wool products, rock wool products, phenolic foam plastics, and aerogel composite insulation materials.

9. A two-stage heat recovery method for a natural gas direct-fired nonwoven fabric drying oven, employing the two-stage heat recovery system of the natural gas direct-fired nonwoven fabric drying oven as described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1. The high-temperature exhaust gas discharged from the natural gas direct-fired hot air oven (2) is sent to the air filter section (3) through the air flow pipeline and the dehumidification fan (5) to filter and remove fiber debris and dust. S2. The filtered high-temperature exhaust gas enters the shell side of the shell-and-tube heat exchanger (4) and exchanges heat with the ambient temperature combustion air in the tube side to form waste heat exhaust gas. The combustion air is preheated and then sent to the combustion air inlet (2-2) of the natural gas direct-fired hot air oven (2) to achieve the first stage of heat energy recovery. S3. The waste heat exhaust gas after heat exchange and cooling is discharged from the shell side of the shell-and-tube heat exchanger (4) and introduced into the heat-insulated preheating box (1). The waste heat exhaust gas and the initially dehydrated non-woven fabric exchange heat through convection, preheating the fabric surface and realizing the second stage of heat energy recovery. S4. The low-temperature waste gas and condensate generated after the waste heat exhaust gas heat exchange are discharged through the drain and air outlet (1-4) at the bottom of the heat-insulated preheating box (1). The condensate is collected by the water treatment component and reused.

10. The two-stage heat recovery method for a natural gas direct-fired nonwoven fabric drying oven according to claim 9, characterized in that: The temperature of the high-temperature exhaust gas is 80-140℃, and the moisture content is 10%-30%; the temperature of the preheated combustion air is 40-80℃, the temperature of the waste heat exhaust gas is 50-90℃, the temperature of the fabric surface after heat exchange is 35℃-60℃, and the temperature of the low-temperature exhaust gas is 40℃-65℃; in steps S1 to S4, the operating frequency of the dehumidification fan (5) and the water treatment components is adjusted according to the actual production process.

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