Precombustion air preheating system for tube-type heating furnace

Abstract

The application provides a tubular heating furnace front combustion-supporting air preheating system, and relates to the technical field of tubular heating furnaces.The tubular heating furnace front combustion-supporting air preheating system comprises a water supply main pipe, a water return main pipe, a water supply branch pipe, a water return branch pipe and a plurality of preheater units.Each preheater unit is composed of two corresponding preheaters in series connection.The water inlet of the first preheater is connected with the water supply branch pipe, the water outlet of the first preheater is connected with the water inlet of the second preheater, and the water outlet of the second preheater is connected with the water return branch pipe.The plurality of preheater units are connected in parallel connection on the water supply branch pipe and the water return branch pipe.Through the design of series connection within groups and parallel connection between groups, the amount of hot water flowing through each preheater can be increased.

Description

Technical Field

[0001] This application relates to the field of tubular heating furnace technology, and in particular to a preheating system for combustion air in a tubular heating furnace. Background Technology

[0002] In the petrochemical industry, the heat loads of tubular furnaces used in hydrocracking, ethylene dichloride (EDC) cracking, ethylene cracking, styrene and propane dehydrogenation (PDH), and hydrogen production conversion are typically in the megawatt (MW) range or higher. Therefore, these tubular furnaces use a large number of burners, generally arranged in several rows, with multiple burners arranged at the bottom of each furnace. To improve the thermal efficiency of tubular furnaces, the industry standard practice is to install a preheater at the air inlet of each burner, utilizing waste heat from plant condensate, wastewater, or low-pressure steam to preheat the combustion air. Currently, the hot water inlet and outlet of these preheaters are connected in parallel to the hot water branch lines on each burner tube row; therefore, the widely adopted existing technology is a parallel connection system. This parallel design aims to maintain stable pressure in the preheater pipelines.

[0003] However, the existing pure parallel scheme has the following significant drawbacks: (1) Insufficient hot water distribution, resulting in low heat exchange efficiency: Due to the large number of preheaters, the amount of hot water distributed to each preheater is limited. Under the premise that the design heat load of the preheater is fixed, the insufficient hot water can only meet the heat exchange demand by reducing its outlet water temperature. The reduction of the outlet water temperature directly leads to a decrease in the average temperature difference (i.e., temperature and pressure) between the cold and hot fluids in the preheater. According to the principle of heat transfer, in order to achieve the same design heat load, the reduction of temperature and pressure must require an increase in the heat transfer area of ​​the preheater. (2) High material cost and difficult space layout: The increase in heat transfer area means an increase in the volume of the preheater and the amount of material used. This not only increases the manufacturing cost, but also, due to the extremely limited space at the bottom of the heating furnace, the excessively large volume of the preheater and the dense pipes will bring great difficulties to the on-site layout, and may even lead to the inability to install, thereby affecting the overall efficiency of the heating furnace and increasing fuel consumption. (3) High system operating energy consumption: In order to compensate for the increased air flow resistance, the induced draft fan of the heating furnace needs to consume more electricity, which increases the operating cost of the system. Therefore, there is an urgent need in this field for a new type of preheating air for tubular furnaces, which can effectively increase the amount of hot water flowing through a single preheater while ensuring stable pipeline pressure and uniform temperature field, thereby overcoming the above-mentioned defects and achieving the goal of reducing the volume of the preheater and reducing material and energy costs. Summary of the Invention

[0004] The purpose of this application is to provide a preheating system for combustion air in a tubular furnace, in order to solve the problem in the prior art of how to effectively increase the amount of hot water flowing through a single preheater while ensuring stable pipeline pressure and uniform temperature field, thereby overcoming the defects of parallel connection.

[0005] To solve the above-mentioned technical problems, in a first aspect, this application provides a preheating system for combustion air in a tubular furnace, comprising: a water supply main pipe, a water return main pipe, a water supply branch pipe, a water return branch pipe, and multiple preheater units;

[0006] Each preheater unit consists of two preheaters connected in series. The inlet of the first preheater is connected to the water supply branch pipe, the outlet of the first preheater is connected to the inlet of the second preheater, and the outlet of the second preheater is connected to the return water branch pipe.

[0007] Multiple sets of the preheater units are connected in parallel to the water supply branch pipe and the water return branch pipe.

[0008] Optionally, at least one of the preheater units in the plurality of preheater units further includes: a bypass pipe disposed between the inlet of the first preheater and the inlet of the second preheater in the same group of preheater units, wherein a shut-off valve is provided on the bypass pipe for adjusting the flow rate of hot water flowing into the second preheater to ensure that the outlet air temperature of the first preheater and the second preheater is the same.

[0009] Optionally, the shut-off valve on the bypass pipeline has a specified opening degree under normal operating conditions;

[0010] The calculation formula used for the specified opening degree is based on the principle of thermal balance and satisfies the following relationship: Q1=Q2;

[0011] Where Q1 is the heat exchange capacity of the first preheater, in kJ / h, Q2 is the heat exchange capacity of the second preheater, in kJ / h, and Q1 = 3.6 × K1 × H1 × Δt1, Q2 = 3.6 × K2 × H2 × Δt2;

[0012] Where K1 is the heat transfer coefficient of the first preheater, in W / m3.℃, and H1 is the area of ​​the heat transfer surface of the first preheater, in m². 2 Δt1 represents the temperature and pressure of the first preheater, and K2 represents the heat transfer coefficient of the second preheater, in W / m³. 3 .℃, H2 is the area of ​​the heat transfer surface of the second preheater, in m². 2Δt2 represents the temperature and pressure of the second preheater. When the heat transfer surface areas of the two preheaters are the same, the heat load of the first and second preheaters is balanced by controlling the opening degree K1×Δt1=K2×Δt2.

[0013] Optionally, the preset formula for calculating the opening degree is:

[0014] a = 1 - (Δt2 / Δt1) β ;

[0015] Where a is the specified opening degree, Δt1 is the temperature and pressure of the first preheater, Δt2 is the temperature and pressure of the second preheater, and β is a numerical value. The magnitude of the numerical value is determined by the relationship between the heat transfer coefficient and the flow rate. The introduction of β makes the specified opening degree a reflect the influence of the flow rate change on the heat transfer balance, and the upper limit of the opening degree range is 28%.

[0016] Optionally, the first preheater and the second preheater have the same structural parameters, the same hot water side parameters, and the same air preheating parameters. The structural parameters include the area of ​​the heat transfer surface, the hot water side parameters include the hot water side flow rate and pressure drop, and the air preheating parameters include the air side inlet temperature and the air side outlet temperature.

[0017] Optionally, the tubular furnace preheating system for combustion air further includes an induced draft fan, which provides flue gas flow power to the tubular furnace to which the preheating system for combustion air belongs, so as to make the air-side resistance of the multiple preheater units consistent.

[0018] Optionally, the tubular furnace preheating system for combustion air also includes a main pipe flow control element installed on the main water supply pipe and / or a branch pipe flow control element installed on the branch water supply pipe, for adjusting and evenly distributing the hot water flow to multiple preheater units.

[0019] Optionally, the ratio between the total pressure drop of a single preheater unit and the total pressure drop of the system is within a preset pressure drop range, and the lower limit of the preset pressure drop range is 0.8.

[0020] Optionally, the preheater unit further includes an online self-cleaning device, which includes a purging pipe and a purging nozzle. The purging nozzle is a fan-shaped atomizing nozzle with a spray angle of 60° to 120° and is arranged in a double-layer opposite manner for online purging of the heat exchange elements of the preheater.

[0021] Optionally, the tubular furnace includes at least one of the following: hydrocracking, EDC cracking, ethylene cracking, styrene, PDH propane dehydrogenation, and hydrogen production conversion furnace.

[0022] This application provides a preheating system for combustion air in a tubular furnace, comprising: a water supply main pipe, a water return main pipe, a water supply branch pipe, a water return branch pipe, and multiple sets of preheater units; each set of preheater units consists of two preheaters connected in series, the inlet of the first preheater is connected to the water supply branch pipe, the outlet of the first preheater is connected to the inlet of the second preheater, and the outlet of the second preheater is connected to the water return branch pipe; multiple sets of preheater units are connected in parallel to the water supply branch pipe and the water return branch pipe.

[0023] This application has the following advantages: In a purely parallel system, the total flow rate is evenly distributed among all preheaters. Therefore, with a total flow rate of Q and a total number of preheaters of 2N, each preheater in a purely parallel system receives a flow rate of Q / 2N. However, in this application, the preheaters are connected in series in pairs to form a preheater unit and then connected in parallel. The flow rate distribution changes as follows: the number of groups becomes N, and the flow rate of each group is Q / N. Because of the series connection within each group, the flow rate through each preheater is the group flow rate Q / N. In this comparison, the flow rate of each preheater is doubled compared to the prior art.

[0024] Furthermore, by installing a bypass pipe with a shut-off valve between the inlets of the first and second preheaters in the same preheater unit, and precisely controlling the specified opening degree of the shut-off valve based on the principle of heat balance and a calculation formula, the flow rate of hot water through the second preheater is precisely regulated. This effectively compensates for the problem of the low inlet water temperature of the second preheater caused by the series connection, ultimately ensuring that the outlet air temperature of the two preheaters remains consistent, guaranteeing the uniformity of the furnace temperature field of the tubular heating furnace, and improving the overall thermal efficiency and operational stability of the system. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This application provides a schematic diagram of the structure of a pre-heating system for a tubular furnace with pre-positioned combustion air.

[0027] Figure 2 A schematic diagram of a preheater unit in a preheating system for a tubular furnace provided in this application embodiment. Detailed Implementation

[0028] Existing technologies, under conditions of numerous preheaters and large total heat loads, create an irreconcilable contradiction between the "limited total system flow" and the "large flow demand of a single preheater." To address this technical problem, this application designs a novel system connection method that significantly increases the effective flow through a single preheater without changing the total system flow. It moves beyond the binary thinking of "either series or parallel." Researchers recognized that simply connecting all preheaters in series leads to problems such as unstable pressure and excessively low terminal temperatures; while simply connecting all preheaters in parallel cannot solve the insufficient flow problem. Therefore, a hybrid approach must be found. The originally independent 2N preheaters are considered as a system composed of multiple "functional units." The core innovation lies in integrating two preheaters into a "preheater unit," within which the relationship between the two preheaters is transformed from "competing for flow (parallel)" to "sharing flow (series)." Intra-group series connection allows two preheaters in the same group to "share" a larger branch flow. The effluent from the first preheater becomes the influent to the second preheater, ensuring that the flow rates through both are exactly the same and doubled. The parallel connection method connects multiple "series units" in parallel on the main pipe and branch pipes, inheriting the advantages of parallel systems such as stable pressure, independent branch lines, and ease of allocation and management. This structural reorganization fundamentally changes the flow distribution. The flow rate per unit increases from Q / 2N to Q / N. This increase in flow rate directly improves the heat transfer coefficient, allowing for a reduction in the heat transfer area of ​​a single preheater while maintaining the same heat load, thus realizing the design principle of "trading performance for structure."

[0029] To enable those skilled in the art to better understand the present application, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are merely some embodiments of the present application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] Figure 1 This is a schematic diagram of a preheating system for combustion air in a tubular furnace, provided as an embodiment of this application. Figure 1 As shown, the preheating system for the tubular heater includes: a water supply main pipe 1, a water return main pipe 2, a water supply branch pipe 3, a water return branch pipe 4, and multiple preheater units; each preheater unit consists of two preheaters connected in series, the inlet 5 of the first preheater is connected to the water supply branch pipe 3, the outlet 6 of the first preheater is connected to the inlet 5 of the second preheater, and the outlet 6 of the second preheater is connected to the water return branch pipe 4; multiple preheater units are connected in parallel on the water supply branch pipe 3 and the water return branch pipe 4.

[0031] It should be understood that the aforementioned water supply main pipe 1 is also referred to as the circulating hot water supply main pipe, and the aforementioned return water main pipe 2 is also referred to as the circulating hot water return main pipe. Furthermore, to distinguish multiple preheater units, they can be differentiated using different symbols E-01, E-02, ..., E-0N. Wherein, N is the total number of preheater units; in this application, N is an integer greater than or equal to 1, and this embodiment does not specifically limit its value. Moreover, the first preheater in each group of preheater units can be referred to as the high-temperature section (stage 1) preheater, and the second preheater in each group of preheater units can be referred to as the low-temperature section (stage 2) preheater. To distinguish the preheater to which it belongs and its temperature range within the preheater unit, it can be represented by E-0iA or E-0iB. Here, E-0iA represents the first preheater in the i-th preheater unit of N preheater units, and E-0iB represents the second preheater in the i-th preheater unit of N preheater units.

[0032] Figure 1 Due to the small text size, this application has extracted a portion of the text to clearly show the structure of each preheater unit. Figure 1 Part of the structure, as Figure 2 .pass Figure 2 It can be clearly seen that the first preheater E-01A and the second preheater E-01B in the first group of preheater units E-01 are connected in series.

[0033] This application's embodiment treats the originally independent 2N preheaters as a system composed of multiple "functional units." Its core innovation lies in integrating two preheaters into a single "preheater unit," transforming their relationship from "competitive flow (parallel)" to "shared flow (series)." Intra-group series connection allows two preheaters in the same group to "share" a larger branch flow. The effluent from the first preheater becomes the influent to the second, ensuring that the flow rates through both are identical and doubled. Inter-group parallel connection connects multiple "series units" in parallel on the main pipe and branch pipes, inheriting the advantages of parallel systems: stable pressure, independent branches, and ease of allocation and management. This structural reorganization fundamentally changes the flow distribution. The single-unit flow rate increases from Q / 2N to Q / N. This increase in flow rate directly improves the heat transfer coefficient, allowing for a reduction in the heat transfer area of ​​a single preheater while maintaining the same heat load, achieving the design goal of "trading performance for structure."

[0034] Therefore, given that the load of this type of tubular heater is fixed and the number of burners cannot be reduced, and the amount of hot water used is also fixed, the only way to increase the outlet temperature of the preheater is to increase the flow rate of the hot water passing through the preheater. Thus, the inlet water flow of two preheaters can be combined into one, making the two preheaters connected in series. These two series preheaters are then connected in parallel with two other series preheaters on the supply branch pipe 3 and the return branch pipe 4. This achieves the goals of stabilizing the pressure in the parallel preheater pipeline, increasing the hot water flow rate, reducing the preheater volume, and reducing the amount of preheater materials used. The pre-air preheater of this type of tubular heater effectively solves the problems of stabilizing the preheater pipeline pressure, increasing the hot water flow rate, reducing the preheater volume, and reducing the amount of preheater materials used by using a series-parallel busbar supply and return water system.

[0035] In one possible embodiment, at least one of the multiple preheater units further includes: a bypass pipe disposed between the inlet of the first preheater and the inlet of the second preheater in the same preheater unit, wherein a shut-off valve is provided on the bypass pipe to regulate the flow rate of hot water flowing into the second preheater so as to ensure that the outlet air temperature of the first preheater and the second preheater is the same.

[0036] It should be understood that a bypass pipe is also referred to as a hot water bypass pipe. This application includes the following situations: at least one preheater unit connected in series with two preheaters has a hot water bypass pipe between its inlet pipes; the remaining preheater units connected in series with two preheaters do not have a hot water bypass pipe between their inlet pipes; or, all preheater units connected in series with two preheaters have a hot water bypass pipe between their inlet pipes; or, none of the preheater units connected in series with two preheaters have a hot water bypass pipe between their inlet pipes. For example, a bypass pipe exists between the hot water inlet of the high-temperature section (stage 1) preheater and the hot water inlet of the low-temperature section (stage 2) preheater in a preheater unit composed of two adjacent preheaters, and a shut-off valve is installed on the bypass pipe.

[0037] It should be noted that this application uses two preheaters, but three or more preheaters can also be designed. However, two preheaters provide the best performance. This application provides the following specific data:

[0038]

[0039] From Table 1 above, we can see that (1) the cold air inlet temperature is given, i.e., 60℃, and the hot air outlet temperature is the design requirement, i.e., 80℃. (2) When the bypass valve is not open, i.e., when the flow rates through the first, second, and third stage preheaters are equal, i.e., all are 1425.2 kg / h, the hot air outlet temperature of the first stage is 84℃, the hot air outlet temperature of the second stage is 80℃, and the hot air outlet temperature of the third stage is 76.5℃. Based on the above data analysis, we can draw the following conclusions: under the conditions of the same air flow rate, the same preheater structure design, and the same amount of hot water flowing through, the outlet temperature of the first stage preheater exceeds 4℃, the second stage meets the design requirements, and the outlet temperature of the third stage does not meet the standard. Therefore, the series structure can only use two stages at most, and it is not advisable to use a three-stage method. Furthermore, for the series preheaters with the same design structure, when the bypass valve is open or closed, the hot water flow rate is 1425.2 kg / h, the hot air outlet temperature of the first stage exceeds 4℃, the temperature of the second stage meets the standard, and the temperature of the third stage does not meet the standard.

[0040] It should be noted that the heat transfer calculation formula in this application is: Q = 3.6 × K × H × Δt, where: Q is the heat absorbed, kJ / h, and K is the heat transfer coefficient, with units of W / m. 3 .℃, H is the heat transfer surface area, in m² 2 Δt is the temperature-pressure, which is the average temperature difference between the two media involved in the heat exchange across the entire heated surface, and is expressed in °C. From the heat transfer calculation formula above, we can see that when two preheaters with the same structure (i.e., the same H-heating surface) and the same air flow rate are connected in series, the hot water exiting the first-stage preheater is the inlet hot water of the second-stage preheater. To ensure that the outlet hot air temperature of the second-stage preheater is consistent with that of the first-stage preheater, the heat transfer coefficient K of the second-stage preheater and the product of temperature and pressure Δt must be consistent with those of the first-stage preheater. Therefore, the shut-off valve between the hot water inlet pipes of the first and second-stage preheaters is kept at a certain opening degree to ensure that the heat transfer system K of the second-stage preheater and the product of temperature and pressure Δt are consistent with those of the first-stage preheater. This is achieved by slightly reducing the flow rate of the hot water medium entering the first-stage preheater and slightly increasing the flow rate of the hot water medium entering the second-stage preheater to change the outlet water temperature of the first-stage preheater, thus ensuring that the heat transfer system K of the second-stage preheater and the product of temperature and pressure Δt are consistent with those of the first-stage preheater, thereby ensuring that the outlet hot air temperatures of the two preheaters connected in series are completely consistent.

[0041] Therefore, in one possible embodiment, the shut-off valve on the bypass pipe has a specified opening degree under normal operating conditions; the calculation formula used for the specified opening degree is based on the principle of thermal balance and satisfies the following relationship: Q1=Q2;

[0042] Where Q1 is the heat exchange capacity of the first preheater, in kJ / h, and Q2 is the heat exchange capacity of the second preheater, in kJ / h, and Q1 = 3.6 × K1 × H1 × Δt1, Q2 = 3.6 × K2 × H2 × Δt2; where K1 is the heat transfer coefficient of the first preheater, in W / m3.℃, and H1 is the area of ​​the heat transfer surface of the first preheater, in m². 2 Δt1 represents the temperature and pressure of the first preheater, and K2 represents the heat transfer coefficient of the second preheater, in W / m³. 3 .℃, H2 is the area of ​​the heat transfer surface of the second preheater, in m². 2 Δt2 represents the temperature and pressure of the second preheater. When the heat transfer surface area of ​​the two preheaters is the same, the heat load of the first and second preheaters is balanced by controlling the opening degree K1×Δt1=K2×Δt2.

[0043] In this embodiment, the use of a bypass pipe ensures that the combustion air temperature, flow rate, and differential pressure at the air inlet and outlet of the high-temperature section (stage 1) preheater and the low-temperature section (stage 2) preheater are completely consistent in a preheater unit consisting of two adjacent preheaters. Therefore, by setting up a bypass pipe and accurately calculating the opening of the shut-off valve based on the principle of heat balance, this embodiment can dynamically adjust the hot water flow rate entering the second preheater, effectively compensating for insufficient heat exchange caused by its lower inlet water temperature. This ensures that the temperature, flow rate, and pressure drop parameters of the combustion air at the outlet of the two preheaters connected in series are completely consistent, ultimately guaranteeing a uniform and stable temperature field in the tubular furnace, significantly improving heating efficiency and system operational reliability.

[0044] In one possible embodiment, the preset opening degree calculation formula is:

[0045] a = 1 - (Δt2 / Δt1) β ;

[0046] Where 'a' is the specified opening degree, Δt1 is the temperature and pressure of the first preheater, Δt2 is the temperature and pressure of the second preheater, and β is a numerical value. The value is determined by the relationship between the heat transfer coefficient and the flow rate. For example, its value is 1.25. The introduction of β makes the specified opening degree 'a' reflect the influence of the flow rate change on the heat transfer balance, and the upper limit of the opening degree range is 28%.

[0047] For example, in heat exchanger design, the heat transfer coefficient K is closely related to the fluid velocity (or flow rate G) flowing through the heat exchanger. Since the temperature-pressure Δt and the heat transfer coefficient K are interrelated under the premise of fixed heat transfer capacity and heat transfer area, the above relationship can be transferred to the relationship between the opening degree and the temperature-pressure ratio. In order to establish a direct functional relationship between the opening degree α and the temperature-pressure ratio Δt2 / Δt1, the above power law relationship needs to be transformed. β is defined as the reciprocal of the exponent n, the specific value of n is determined by the fluid flow state (laminar or turbulent) and the heat transfer surface structure. For the common turbulent heat transfer condition in preheaters outside finned tubes, after a lot of engineering practice and experimental verification, the exponent n value of the heat transfer coefficient and the flow rate is usually about 0.8, β=1 / n=1.25. This value has been proven to most accurately reflect the actual impact of flow rate changes on the heat transfer balance under this type of condition, so that the calculation result of the opening degree calculation formula is in high agreement with the actual engineering situation. The value of β is not arbitrarily set, but is derived from heat transfer theory and verified by engineering practice in specific application scenarios (such as turbulent heat transfer in preheaters), ensuring the theoretical correctness and practical reliability of the opening calculation formula.

[0048] The fundamental advantage of this application's embodiments lies in the flow multiplication effect brought about by the "intra-group series connection". If the bypass valve opening is too large, most of the high-temperature water will directly "short-circuit" away through the bypass pipe, with only a small amount of water flowing through the first preheater. This essentially destroys the series structure, degenerating it into a distorted system dominated by bypass, thus losing the core significance of increasing flow rate through series connection. The opening degree of the shut-off valve in the bypass pipe is kept within 28%, ensuring that even if the bypass valve is fully open (under the most extreme adjustment requirements), more than 70% of the mainstream high-temperature water must still flow through the first preheater. This fundamentally guarantees that the series mode is always the main path for the working fluid flow, thereby maintaining the core value of this application in increasing the flow rate of a single unit. In addition, the amount of water flowing through the first preheater is a prerequisite for its effective heat exchange. If the bypass flow is too large, resulting in a small water flow rate in the first preheater, its heat transfer coefficient will drop sharply (the heat transfer coefficient is positively correlated with the flow rate), which may not be able to provide enough heat to the air, and it may also experience local overheating or uneven heat exchange. The 28% opening limit is the minimum flow rate required to ensure the normal and efficient operation of the first preheater. Therefore, opening the bypass valve is equivalent to adding a parallel branch to the main line, altering the local resistance characteristics. Limiting the opening to a relatively small range (≤28%) keeps this hydraulic disturbance within acceptable limits, preventing excessive impact on the flow distribution of the entire parallel branch and maintaining overall system pressure stability.

[0049] This design ensures that the bypass valve's function is precisely defined as "fine-tuning" and "compensation." While maintaining the effectiveness of the main series flow, it diverts a small portion of high-temperature water to compensate for insufficient inlet water temperature in the second preheater, thus finely balancing the outlet air temperature of both units. It does not overshadow the core series architecture. Furthermore, by setting a clear upper limit, a safe operating boundary is provided for system commissioning and operation. Operators or automatic control systems can adjust within this range without worrying about malfunctions causing the core operating mode to fail, greatly improving the system's fault tolerance and operational reliability. Within a smaller opening range, valve adjustment is typically more sensitive and linear. This means that even a relatively small change in opening can significantly affect the mixed inlet water temperature of the second preheater. This makes temperature equalization adjustment faster and more efficient, avoiding the drawbacks of coarse, non-linear adjustments over large opening ranges.

[0050] In one possible embodiment, the first preheater and the second preheater have the same structural parameters, the same hot water side parameters, and the same air preheating parameters. The structural parameters include the area of ​​the heat transfer surface, the hot water side parameters include the hot water side flow rate and pressure drop, and the air preheating parameters include the air side inlet temperature and the air side outlet temperature.

[0051] According to this design, the embodiments of this application achieve high system integration and optimal performance by setting the two preheaters connected in series as standardized units with identical structural parameters, hot water side parameters, and air preheating parameters. Its core advantages are: First, it greatly simplifies the design, manufacturing, installation, and maintenance processes. Component standardization reduces production costs and spare parts inventory pressure, improving the convenience and economy of engineering implementation. Second, identical structure and operating parameters lay a solid foundation for achieving precise thermal balance, ensuring that the two preheaters theoretically possess completely identical thermal characteristics. This allows subsequent flow fine-tuning via bypass valves to be performed on a predictable and efficient baseline, avoiding uncontrollable variables introduced by differences in the equipment itself. This results in more precise and easier achievement of consistent outlet air temperature, ensuring absolute uniformity of the furnace temperature field. Finally, this symmetrical design endows the system with excellent coordination and stability, not only optimizing overall heat exchange efficiency but also significantly enhancing system reliability and maintainability, reflecting a combination of engineering practicality and high-performance requirements.

[0052] In one possible embodiment, the preheating system for the tubular furnace also includes an induced draft fan, which provides flue gas flow power to the tubular furnace to which the preheating system for the tubular furnace belongs, so as to make the air-side resistance of the multiple preheater units consistent.

[0053] This application's embodiment introduces an induced draft fan to provide unified flue gas flow power for the entire system. Its core advantage lies in achieving active balancing of air-side resistance and flow rate across multiple preheater units, thereby significantly improving the system's overall integrity, stability, and energy efficiency. Specifically, the series-parallel structure of this application reduces the preheater volume and optimizes the flow channels, thus lowering its air-side resistance. Furthermore, the unified power supply from a single induced draft fan ensures highly consistent inlet air pressure across the parallel preheater units, effectively eliminating "air grabbing" or uneven flow rates caused by differences in pipe layout, equipment manufacturing tolerances, or ash accumulation. This allows the airflow through each preheater to be precisely controlled within the design value, ensuring that all burners receive highly uniform combustion air in terms of temperature and flow rate. Ultimately, this lays a solid foundation for ensuring the extreme uniformity of the furnace temperature field. Meanwhile, through system optimization rather than partial compromise, this design reduces the number of power equipment and total power consumption by using a centralized fan, thereby reducing the resistance of the preheater itself. This not only simplifies the system structure and reduces initial investment and maintenance costs, but also fundamentally achieves the intensive and minimal operation of energy consumption, thus achieving multiple goals of equipment compactness, performance optimization, and operational economy.

[0054] In one possible embodiment, the preheating system for the tubular furnace also includes a main pipe flow control element installed on the main water supply pipe 1 and / or a branch pipe flow control element installed on the branch water supply pipe 3, for adjusting and evenly distributing the hot water flow to multiple preheater units.

[0055] This application embodiment constructs a refined active flow distribution system by setting graded flow control elements on the water supply main pipe and / or water supply branch pipes. Its core advantage lies in solving the problem of flow balance among multiple parallel preheater units from the source. Specifically, the main pipe flow control element is responsible for macroscopic total flow regulation and pressure stabilization, while the flow control elements on each branch pipe perform microscopic precise control for each parallel branch. This graded control strategy can effectively compensate for inherent flow deviations caused by differences in pipe path length, local resistance, or installation errors, ensuring that the exact same hot water flow is distributed to each "series unit". This precise flow distribution is the cornerstone of maintaining the thermal balance of the entire system—it allows all parallel branches to operate under the same starting conditions, thereby ensuring that each series preheater can obtain the designed amount of hot water to take advantage of its flow multiplication, and ultimately ensuring that the combustion air at the inlet of all burners is heated to the exact same temperature, providing the most fundamental guarantee for forming a highly uniform furnace temperature field. This design transforms the advantages of the series-parallel structure in this application from "possibility" to "stable and reliable practical effect," greatly improving the system's adaptability, reliability, and overall thermal efficiency.

[0056] In one possible embodiment, the ratio between the total pressure drop of a single preheater unit and the total pressure drop of the system is within a preset pressure drop range, and the lower limit of the preset pressure drop range is 0.8.

[0057] This application embodiment fundamentally constructs a highly stable and self-balancing flow distribution system by forcibly distributing the vast majority of the system pressure drop (not less than 80%) to a single preheater unit. Its core principle lies in the fluid mechanics law of "resistance-dominated distribution"—when the majority of the resistance in a parallel system is concentrated within the functional units of each branch, rather than the main pipeline, the sensitivity of the flow rate of each branch to pressure fluctuations in the main pipeline or minor installation differences between them will be significantly reduced. In this design, this means that even if there are slight differences in the resistance of the main pipeline due to pipe layout, valve opening, or manufacturing tolerances, the impact of these external disturbances on the total flow distribution will be minimized because the preheater unit itself constitutes the undisputed main body of resistance, thus strongly ensuring that the hot water flow through each set of series and parallel units is almost completely uniform. This high degree of flow uniformity is the most fundamental prerequisite for the system to achieve its ultimate goal—ensuring a strictly uniform combustion air temperature at the inlet of all burners. Therefore, setting this pressure drop range is not only a parameter optimization, but also a strategic decision at the system architecture level. It enables the entire preheating system to obtain strong intrinsic stability, transforming the thermal performance advantages of the series-parallel structure of this application into a stable, reliable and replicable engineering reality.

[0058] In one possible embodiment, the preheater unit further includes an online self-cleaning device, which includes a purging pipe and a purging nozzle. The purging nozzle is a fan-shaped atomizing nozzle with a spray angle of 60° to 120° and is arranged in a double-layer opposite manner for online purging of the heat exchange elements of the preheater.

[0059] This application embodiment integrates an online self-cleaning device consisting of a purging pipe and a fan-shaped atomizing nozzle, and employs a wide spray angle of 60° to 120° combined with a double-layered, opposite arrangement to achieve continuous, efficient, and thorough cleaning and maintenance of the preheater heat exchange elements. Its core advantage lies in its ability to proactively prevent and remove accumulated dirt and impurities from the surface of the heat exchange elements without shutting down the system. The wide-angle coverage of the fan-shaped nozzles ensures that a single spray can clean a larger area, while the double-layered, opposite, staggered arrangement forms a three-dimensional, cross-shaped cleaning network, allowing ash to be simultaneously covered and effectively removed by purging flows at different angles, regardless of whether it adheres to the windward or leeward side of the heat exchange tube bundle or the gaps between tubes. This design fundamentally solves the key problems of traditional preheaters, such as increased heat transfer resistance, continuous decline in heat exchange efficiency, and increased system resistance caused by ash accumulation during long-term operation. It enables the equipment to maintain a high-efficiency heat exchange state under design conditions, thereby stabilizing the preheating temperature of the combustion air and ensuring the thermal efficiency of the heating furnace. At the same time, this online cleaning capability avoids production interruptions caused by manual ash removal during shutdown, significantly improving the continuity and reliability of the unit's operation, greatly reducing maintenance costs and safety risks, and providing key technical support for the high-load continuous production process requirements of the petrochemical industry.

[0060] In one possible embodiment, the tubular furnace includes at least one of hydrocracking, EDC cracking, ethylene cracking, styrene, PDH propane dehydrogenation, and hydrogen production conversion furnaces.

[0061] This application demonstrates the strong process adaptability and platform value of its preheating system by explicitly covering its application scope to mainstream tubular furnaces in hydrocracking, EDC cracking, ethylene cracking, styrene, PDH propane dehydrogenation, and hydrogen conversion. Its core advantage lies in creating a highly universal solution—while these furnaces process different materials and follow different processes, they all require a large number of bottom burners, face the same bottlenecks in waste heat recovery efficiency, and have requirements for furnace temperature field uniformity. This system, through its unique "intra-group series-inter-group parallel" core architecture, precisely identifies and solves this common problem across processes, enabling a single technical solution to seamlessly adapt to multiple key process units. This design greatly expands the market boundaries and application potential of the technology, builds a broad protection scope for patent layout, and provides significant economies of scale for the transformation of R&D results. For equipment manufacturers, this means standardization and serialization of products, significantly reducing R&D, design, and production costs. For end-users (petrochemical companies), it allows the adoption of the same mature and efficient technology across various units, reducing the complexity of procurement, installation, and maintenance, as well as spare parts inventory, thereby maximizing investment returns. This technological innovation ensures that this application will have the broadest possible industry impact.

[0062] The foregoing provides a detailed description of a preheating system for combustion air in a tubular furnace. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of these embodiments are merely illustrative of the method and its core concepts. It should be noted that those skilled in the art can make various improvements and modifications to this application without departing from its principles, and these improvements and modifications also fall within the protection scope of this application.

Claims

1. A preheating system for combustion air in a tubular furnace, characterized in that, include: Water supply main pipe (1), water return main pipe (2), water supply branch pipe (3), water return branch pipe (4) and multiple preheater units; Each preheater unit consists of two preheaters connected in series. The inlet of the first preheater is connected to the water supply branch pipe (3), the outlet of the first preheater is connected to the inlet of the second preheater, and the outlet of the second preheater is connected to the return water branch pipe (4). Multiple sets of the preheater units are connected in parallel to the water supply branch pipe (3) and the return branch pipe (4); At least one of the preheater units in the plurality of preheater units further includes: a bypass pipe disposed between the inlet of the first preheater and the inlet of the second preheater in the same group of preheater units, wherein a shut-off valve is provided on the bypass pipe for adjusting the flow rate of hot water flowing into the second preheater to ensure that the outlet air temperature of the first preheater and the second preheater is the same. The first preheater and the second preheater have the same structural parameters, hot water side parameters, and air preheating parameters. The structural parameters include the area of ​​the heat transfer surface, the hot water side parameters include the hot water side flow rate and pressure drop, and the air preheating parameters include the air side inlet temperature and the air side outlet temperature. The tubular furnace preheating system also includes an induced draft fan, which provides flue gas flow power to the tubular furnace to which the tubular furnace preheating system belongs, so as to make the air-side resistance of the multiple preheater units consistent. The tubular heating furnace preheating system for combustion air also includes a main pipe flow control element installed on the main water supply pipe (1) and / or a branch pipe flow control element installed on the branch water supply pipe (3), for adjusting and evenly distributing the hot water flow to multiple preheater units.

2. The preheating system for combustion air in a tubular furnace according to claim 1, characterized in that, The shut-off valve on the bypass pipeline has a specified opening degree under normal operating conditions; The calculation formula used for the specified opening degree is based on the principle of thermal balance and satisfies the following relationship: Q1=Q2; Where Q1 is the heat exchange capacity of the first preheater, in kJ / h, Q2 is the heat exchange capacity of the second preheater, in kJ / h, and Q1 = 3.6 × K1 × H1 × Δt1, Q2 = 3.6 × K2 × H2 × Δt2; Where K1 is the heat transfer coefficient of the first preheater, in W / m3.℃, and H1 is the area of ​​the heat transfer surface of the first preheater, in m². 2 Δt1 represents the temperature and pressure of the first preheater, and K2 represents the heat transfer coefficient of the second preheater, in W / m³. 3 .℃, H2 is the area of ​​the heat transfer surface of the second preheater, in m². 2 Δt2 represents the temperature and pressure of the second preheater. When the heat transfer surface areas of the two preheaters are the same, the heat load of the first and second preheaters is balanced by controlling the opening degree K1×Δt1=K2×Δt2.

3. The preheating system for combustion air in a tubular furnace according to claim 2, characterized in that, The preset formula for calculating the opening degree is: a =1-(Δt2 / Δt1) β ; in, a To specify the opening degree, Δt1 represents the temperature and pressure of the first preheater, and Δt2 represents the temperature and pressure of the second preheater. β is a The value, the magnitude of which is determined by the relationship between the heat transfer coefficient and the flow rate, β The introduction of makes the specified opening degree a It reflects the impact of flow rate changes on heat transfer balance, and the upper limit of the specified opening range is 28%.

4. A preheating system for combustion air in a tubular furnace according to any one of claims 1 to 3, characterized in that, The ratio between the total pressure drop of a single preheater unit and the total pressure drop of the system is within a preset pressure drop range, and the lower limit of the preset pressure drop range is 0.

8.

5. A preheating system for combustion air in a tubular furnace according to any one of claims 1 to 3, characterized in that, The preheater unit also includes an online self-cleaning device, which includes a purging pipe and a purging nozzle. The purging nozzle is a fan-shaped atomizing nozzle with a spray angle of 60° to 120° and is arranged in a double-layer opposite manner for online purging of the heat exchange elements of the preheater.

6. The preheating system for combustion air in a tubular furnace according to claim 1, characterized in that, The tubular heater includes at least one of the following: hydrocracking, EDC cracking, ethylene cracking, styrene, PDH propane dehydrogenation, and hydrogen production conversion heater.

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