High-temperature heating control method suitable for oil mist environment

Abstract

The invention provides a high-temperature heating control method suitable for an oil mist environment, and belongs to the technical field of industrial heating control. Geometric parameters of a spiral groove in the surface of an outer sleeve are optimized through a spiral flow guide constraint equation to form a high-speed shearing airflow layer to prevent oil mist from being attached, dynamic error compensation is achieved in cooperation with an oil mist concentration sensor, and the power and airflow distribution of an electric heating element are adjusted in combination with a composite PID control algorithm and a pulsating air supply control mode. When the oil mist concentration exceeds a safety threshold value, a graded nitrogen supplement adjusting mechanism is started to reduce the oxygen concentration, and the technical problem that the temperature measurement precision in the high-temperature heating process is seriously reduced due to thermocouple surface condensation attachment in the oil mist environment is solved.

Description

Technical Field

[0001] This invention belongs to the field of industrial heating control technology, and more specifically, relates to a high-temperature heating control method suitable for oil mist environments. Background Technology

[0002] In industrial high-temperature heating, particularly in metal heat treatment and surface coating processes, workpiece surfaces often retain residual organic oil films or lubricants. During the heating process within a sealed heating chamber, these organic substances volatilize, creating an oil mist environment. Traditional high-temperature heating control systems use ordinary thermocouples for temperature monitoring and adjust the heating power using a PID control algorithm to achieve temperature control. This method achieves good control accuracy in clean atmospheres. However, in oil mist environments, because the temperature sensor surface temperature is lower than the surrounding gas temperature, oil mist particles condense on the thermocouple surface, forming an oil film. This oil film has low thermal conductivity, creating thermal resistance between the sensor and the measured medium. This causes the thermocouple measurement value to deviate from the true temperature, with measurement errors reaching ±10°C or even higher. This systematic measurement deviation severely affects the accuracy of the temperature control algorithm, making precise control of the heating process difficult. In other words, existing technologies suffer from a significant technical problem: condensation and adhesion on the thermocouple surface in oil mist environments lead to a severe decrease in temperature measurement accuracy during high-temperature heating. Summary of the Invention

[0003] In view of this, the present invention provides a high-temperature heating control method suitable for oil mist environments, which can solve the technical problem in the prior art where condensation and adhesion on the surface of thermocouples in oil mist environments leads to a serious decrease in temperature measurement accuracy during the high-temperature heating process.

[0004] This invention is implemented as follows: This invention provides a high-temperature heating control method suitable for oil mist environments, including establishing a nitrogen circulation heating system and controlling the oxygen concentration in a sealed heating chamber; uniformly arranging adjustable-angle airflow nozzles around the sealed heating chamber; installing a double-tube self-cleaning thermocouple assembly including a first K-type thermocouple and a second K-type thermocouple; uniformly arranging redundant temperature measuring points in the sealed heating chamber; starting the heating system and collecting real-time measured values ​​of oil mist concentration; real-time collecting the workpiece surface temperature and nitrogen medium temperature and calculating the temperature deviation value; adjusting the variable frequency fan speed according to the ratio of the temperature deviation value to the target set temperature; and adjusting the oil mist concentration according to the measured values ​​of the nitrogen medium and the workpiece surface temperature and nitrogen medium temperature. The measured oil mist concentration is used to dynamically compensate for the temperature readings of the first and second type K thermocouples to obtain the compensated workpiece surface temperature and the compensated nitrogen medium temperature. The median value of the redundant temperature measurement data is used as the final temperature feedback value. A composite PID control algorithm is used to adjust the power of the electric heating element. A pulsed air supply control mode is activated, and the steady-state temperature stratification phenomenon in the sealed heating chamber is broken by adjusting the flow ratio of the adjustable angle airflow nozzle. When the measured oil mist concentration exceeds the preset safety threshold, the first-level nitrogen replenishment adjustment mechanism is activated. When the measured oil mist concentration exceeds the critical value, the second-level oxygen concentration monitoring and compensation algorithm is activated.

[0005] The dual-tube self-cleaning thermocouple assembly includes an outer tube, an inner tube, a thermocouple core wire, and a sealing joint. The outer tube has a continuous spiral groove on its surface. The gap between the inner tube and the outer tube is filled with argon gas to form a thermal barrier layer. The thermocouple core wire passes through the inner tube and extends out to the temperature measuring end.

[0006] The geometric parameters of the continuous spiral groove satisfy the spiral flow constraint equation, which is expressed as follows: the spiral groove pitch divided by the outer tube diameter equals 2.5 times the groove depth divided by the groove width.

[0007] The spiral flow constraint equation optimizes the spiral flow velocity distribution of the airflow on the outer tube surface, causing the nitrogen flow velocity to generate a local acceleration effect in the groove region, forming a high-speed shear flow layer. The high-speed shear flow layer continuously scours the surface of the outer tube to prevent the deposition of oil mist particles. At the same time, the centrifugal force generated by the spiral flow throws the attached oil mist droplets off the surface.

[0008] The outer tube has a diameter of [10, 20] mm, a groove depth of 1 mm, a groove width of 2 mm, and the pitch of the spiral groove is calculated from the outer tube diameter, groove depth, and groove width according to the spiral flow constraint equation.

[0009] The temperature deviation value is the difference between the workpiece surface temperature and the nitrogen medium temperature. When the ratio of the temperature deviation value to the target set temperature is ∈ [0.02, 0.08], the current variable frequency fan speed is kept constant and the temperature change is continuously monitored.

[0010] Specifically, when the ratio of the temperature deviation value to the target set temperature is ∈ (0.08, 0.15], the variable frequency fan speed is dynamically adjusted to between 60% and 90% of the rated speed based on the temperature deviation value.

[0011] The calculation formula for the dynamic error compensation is expressed as follows: the compensated workpiece surface temperature equals the workpiece surface temperature minus the error correction term, where the error correction term equals the measured oil mist concentration minus the upper limit of the oil mist concentration sensor's range (500). The ratio is multiplied by a temperature correction factor of 0.8 and then multiplied by a reference temperature deviation of 5℃.

[0012] The calculation method for the compensated nitrogen medium temperature is the same as the calculation method for the compensated workpiece surface temperature, except that the workpiece surface temperature is replaced by the nitrogen medium temperature.

[0013] The final temperature feedback value is the median of five temperature values: the temperature measured at four redundant temperature measurement points and the compensated workpiece surface temperature. This final temperature feedback value is used for closed-loop control of the composite PID control algorithm.

[0014] In the composite PID control algorithm, the proportional coefficient is equal to 0.2 plus the ratio of the temperature control deviation to 100℃ multiplied by the coefficient range difference of 1.8; the integral time constant is equal to 1 second plus the ratio of the temperature control deviation to 100℃ multiplied by the time range difference of 19 seconds; and the derivative time constant is equal to 1 second plus the ratio of the temperature control deviation to 100℃ multiplied by the time range difference of 19 seconds.

[0015] The pulsed air supply control mode is an operating mode in which the speed of the variable frequency fan fluctuates periodically according to a sine law. The speed change formula is expressed as the real-time speed equals the average speed multiplied by 1 plus 0.2 times the sine function. The independent variable of the sine function is the product of twice the pi, the frequency of 1 Hz, and time.

[0016] The flow rate adjustment is an operation that adjusts the air flow rate of each adjustable angle airflow nozzle according to the temperature field distribution in the sealed heating chamber. The flow rate adjustment is achieved by an electric regulating valve installed on the air inlet pipe of each adjustable angle airflow nozzle.

[0017] The first-stage nitrogen replenishment adjustment mechanism is activated when the measured oil mist concentration exceeds a preset safety threshold of 120. The control strategy involves increasing the nitrogen injection flow rate to dilute the oil mist concentration and reduce the oxygen concentration. The nitrogen injection flow rate is increased to 150% of the normal flow rate, while the variable frequency fan operation mode is switched to protection mode and the speed is reduced to 40% of the rated speed.

[0018] The second-level oxygen concentration monitoring and compensation algorithm is used when the measured oil mist concentration reaches a critical value of 200. The control strategy forcibly reduces the oxygen concentration in the sealed heating chamber. The oxygen concentration sensor monitors the oxygen content in real time. When the oxygen concentration is higher than the preset value, the nitrogen rapid injection valve is automatically opened and the exhaust valve is closed, so that the oxygen concentration drops rapidly to below 5%.

[0019] The nitrogen circulation heating system separates nitrogen with a purity of over 99.5% from compressed air through a nitrogen-oxygen separation device and injects it into a sealed heating chamber, controlling the oxygen concentration in the sealed heating chamber to be below 8%. Eight adjustable-angle airflow nozzles are evenly arranged around the sealed heating chamber, with the adjustable-angle airflow nozzles spaced at 45-degree intervals.

[0020] This invention addresses the problem of nitrogen circulation causing localized acceleration in the grooved region by machining continuous spiral grooves on the surface of the thermocouple outer tube and optimizing the groove geometry using a spiral flow constraint equation. This creates a high-speed shear airflow layer, with a velocity 1.8 to 2.2 times that of the main airflow, continuously scouring the outer tube surface and preventing oil mist particle deposition. Simultaneously, the centrifugal force generated by the spiral flow dislodging any attached oil mist droplets from the surface. Through this pneumatic self-cleaning mechanism, the oil film thickness on the thermocouple surface is controlled below 5 micrometers, reducing the temperature measurement error caused by oil mist condensation from ±10°C to within ±2°C. This ensures accurate input data for the dynamic error compensation algorithm, enabling the composite PID control to achieve precise power regulation based on reliable temperature feedback. In summary, this invention solves the technical problem mentioned in the background art where condensation and adhesion on the thermocouple surface in an oil mist environment leads to a severe decrease in temperature measurement accuracy during high-temperature heating. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention.

[0022] Figure 2 This is a schematic diagram of the structure of the double-tube self-cleaning thermocouple assembly in the embodiment.

[0023] Figure 3 This is a schematic diagram of the overall layout of the sealed heating chamber in the embodiment.

[0024] Figure 4 This is a block diagram of the temperature monitoring and control system in the embodiment.

[0025] Figure 5 This is a graph showing the fan speed variation under the pulsed air supply control mode in the embodiment.

[0026] Figure 6 The graph shows the temperature and oil mist concentration changes during the heating process in the embodiment. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0028] like Figure 1 The diagram shown is a flowchart of a high-temperature heating control method suitable for oil mist environments provided by the present invention. This method includes the following steps:

[0029] S01. Establish a nitrogen circulation heating system. Separate nitrogen with a purity of over 99.5% from compressed air through a nitrogen-oxygen separation device and inject it into a sealed heating chamber. Control the oxygen concentration in the sealed heating chamber to be below 8%. Arrange 8 adjustable angle airflow nozzles evenly around the sealed heating chamber. The adjustable angle airflow nozzles are spaced at 45-degree intervals.

[0030] S02. Install a double-tube self-cleaning thermocouple assembly, wherein the first K-type thermocouple is used to monitor the surface temperature of the workpiece, the second K-type thermocouple is used to monitor the temperature of the nitrogen medium, and four redundant temperature measuring points are evenly arranged in the sealed heating chamber for monitoring the temperature field distribution.

[0031] S03. Start the heating system. The electric heating element is powered on and heats up. The variable frequency fan drives the nitrogen gas to circulate. The oil mist concentration sensor collects the oil mist particulate matter concentration value in the sealed heating chamber in real time and records it as the measured oil mist concentration value. The collection time interval is 2 seconds.

[0032] S04. Real-time acquisition of the workpiece surface temperature measured by the first K-type thermocouple and the nitrogen medium temperature measured by the second K-type thermocouple, calculation of the temperature deviation between the workpiece surface temperature and the nitrogen medium temperature, and when the ratio of the temperature deviation to the target set temperature is within the normal range [0.02, 0.08], maintain the current variable frequency fan speed unchanged and continuously monitor temperature changes.

[0033] S05. When the ratio of the temperature deviation value to the target set temperature is within the adjustable range (0.08, 0.15), the variable frequency fan speed is dynamically adjusted to 60% to 90% of the rated speed according to the temperature deviation value to achieve gradient adjustment of the convection intensity.

[0034] S06. Based on the measured value of oil mist concentration measured by the oil mist concentration sensor, perform dynamic error compensation on the temperature measurement values ​​of the first K-type thermocouple and the second K-type thermocouple, calculate the compensated workpiece surface temperature and the compensated nitrogen medium temperature, and use the median value of the measurement data of the four redundant temperature measurement points as the final temperature feedback value.

[0035] S07. The power of the electric heating element is adjusted by a composite PID control algorithm. The PID control parameters are adjusted in real time according to the temperature control deviation between the compensated workpiece surface temperature and the target set temperature. The control cycle is 0.5 seconds.

[0036] S08. Start the pulsed air supply control mode, so that the speed of the variable frequency fan changes periodically at a frequency of 1 Hz, with a change range of ±20% of the average speed. By adjusting the flow ratio of 8 adjustable angle airflow nozzles, the steady-state temperature stratification phenomenon in the sealed heating chamber is broken, and the surface temperature uniformity of the workpiece is controlled within ±3℃.

[0037] S09. When the measured value of oil mist concentration collected by the oil mist concentration sensor exceeds the preset safety threshold of 120 mg / m³, the first-level nitrogen replenishment and adjustment mechanism is immediately activated and the nitrogen injection flow rate is increased to 150% of the normal flow rate. At the same time, the variable frequency fan operation mode is switched to protection mode and the speed is reduced to 40% of the rated speed.

[0038] S10. When the measured value of oil mist concentration continues to rise and exceeds the critical value of 200 mg / m³, the second-level oxygen concentration monitoring and compensation algorithm is activated to forcibly reduce the oxygen concentration in the sealed heating chamber to below 5%, and triggers the audible and visual alarm device to notify the operator to take emergency measures.

[0039] The nitrogen-oxygen separation unit is a device that uses the pressure swing adsorption principle to separate nitrogen from compressed air. It mainly consists of an air compressor, a refrigerated dryer, an adsorption tower group, a nitrogen buffer tank, and a control system. Gas separation is achieved through the selective adsorption of oxygen and nitrogen by molecular sieves.

[0040] The sealed heating chamber is a stainless steel box structure with an insulation layer, the chamber wall is 8 mm thick, the inner surface is coated with a high-temperature resistant ceramic coating, the top is equipped with an observation window and gas inlet and outlet, and the bottom is equipped with a workpiece support bracket.

[0041] The dual-tube self-cleaning thermocouple assembly includes an outer tube, an inner tube, a thermocouple core wire, and a sealing joint. The outer tube has a continuous spiral groove on its surface. The gap between the inner tube and the outer tube is filled with argon gas to form a thermal barrier layer. The thermocouple core wire passes through the inner tube and extends out to the temperature measuring end. The geometric parameters of the continuous spiral groove satisfy the spiral flow constraint equation. The spiral flow constraint equation solves the technical problem of increased temperature measurement error caused by oil mist condensation and adhesion on the thermocouple surface.

[0042] Among them, the continuous spiral groove is a spiral groove that is continuously distributed along the axial direction on the surface of the outer tube. It is used to guide high-speed nitrogen gas to flush the thermocouple surface along the spiral path to prevent oil mist adhesion and condensation.

[0043] The spiral flow guidance constraint equation is used to determine the geometric parameter configuration relationship of the continuous spiral grooves to achieve the optimal airflow self-cleaning effect. The inputs include the outer casing diameter, groove depth, and groove width, and the output is the pitch of the spiral groove. The spiral flow guidance constraint equation is expressed as: the pitch of the spiral groove. The unit is millimeters divided by the outer tube diameter. The unit is millimeters, which equals 2.5 times the groove depth. The unit is millimeters divided by the groove width. The unit is millimeters, that is The left side of the equation is the dimensionless ratio of pitch to outer tube diameter, and the right side of the equation is the dimensionless ratio of groove depth to groove width multiplied by a coefficient of 2.5. Both sides of the equation are dimensionless values.

[0044] The outer tube diameter is the outer diameter of the outer tube of the double-tube self-cleaning thermocouple assembly, in millimeters, ranging from 10 mm to 20 mm.

[0045] The groove depth is the vertical depth of the continuous spiral groove from the surface of the outer sleeve inward, in millimeters, and is set to 1 millimeter.

[0046] The groove width is the width of the continuous spiral groove along the circumference of the outer sleeve, in millimeters, and is set to 2 millimeters.

[0047] The pitch of the spiral groove is the axial distance that the continuous spiral groove advances in one revolution along the outer sleeve, in millimeters. It is calculated from the outer sleeve diameter, groove depth, and groove width according to the spiral flow constraint equation.

[0048] The spiral flow constraint equation optimizes the spiral flow velocity distribution of the airflow on the outer tube surface, causing the nitrogen flow velocity to generate a local acceleration effect in the groove region, forming a high-speed shear flow layer. The airflow velocity of the high-speed shear flow layer reaches 1.8 to 2.2 times that of the main airflow velocity. The high-speed shear flow layer continuously scours the outer tube surface to prevent oil mist particles from depositing. At the same time, the centrifugal force generated by the spiral flow throws the attached oil mist droplets off the surface, keeping the oil film thickness on the thermocouple surface below 5 micrometers. This reduces the temperature measurement error caused by oil mist condensation from ±10℃ to within ±2℃, ensuring the accuracy of the input data for the dynamic error compensation algorithm.

[0049] The adjustable-angle airflow nozzle is a conical nozzle that can adjust the airflow direction. The nozzle outlet diameter is 12 mm. It is installed on a rotatable universal joint and the adjustment angle range is ±45 degrees in the vertical direction and 360 degrees in the horizontal direction.

[0050] The redundant temperature measurement points are multiple auxiliary temperature monitoring positions set around the workpiece in the sealed heating chamber, located 50 mm away from the workpiece surface on the top, bottom, left and right sides of the workpiece, respectively.

[0051] Among them, the oil mist concentration sensor is a sensor that uses the laser scattering principle to measure the mass concentration of oil mist particles in the air, with a measurement range of 0 to 500. The response time is less than 3 seconds, and the output signal is a current signal of 4 to 20 milliamps.

[0052] The measured value of oil mist concentration is the mass concentration of oil mist particles in the sealed heating chamber measured by the oil mist concentration sensor at the time of data acquisition, and the unit is _____. .

[0053] The temperature deviation value is the difference between the workpiece surface temperature and the nitrogen medium temperature, expressed in °C. This temperature deviation value reflects the heat transfer state between the workpiece and the heating medium.

[0054] The normal range [0.02, 0.08] is an empirical value, determined as follows: Statistical analysis was conducted on heating experimental data of 100 sets of workpieces with different materials and sizes. When the ratio of the temperature deviation value to the target set temperature was less than 0.02, the convective heat transfer efficiency between the workpiece and the nitrogen medium was insufficient, resulting in a decrease in the heating rate of more than 30%. When the ratio was greater than 0.08, the excessive temperature gradient between the workpiece surface and interior caused thermal stress concentration, increasing the risk of workpiece cracking to more than 15%. Therefore, the lower limit of the normal range was set to 0.02 to correspond to the critical point of sufficient convective heat transfer, and the upper limit of the normal range was set to 0.08 to correspond to the critical point of thermal stress safety. The normal range is a closed interval, indicating that the system is still in a stable operating state when the ratio reaches the boundary value.

[0055] The adjustable range (0.08, 0.15) is an empirical value, and its determination process is as follows: Based on the upper limit of the normal range of 0.08 as the lower limit of the adjustable range, the upper limit of the adjustable range is determined through temperature field simulation calculation and experimental verification. When the ratio of the temperature deviation value to the target set temperature exceeds 0.15, even if the speed of the variable frequency fan is increased to 100% of the rated speed, it is still impossible to effectively improve the temperature uniformity, and the airflow disturbance in the sealed heating chamber is too strong, causing the surface temperature fluctuation of the workpiece to exceed ±5℃. Therefore, the upper limit of the adjustable range is set to 0.15, which corresponds to the limit value of the fan adjustment capability. The lower limit of the adjustable range adopts an open interval to indicate that the adjustment mechanism needs to be activated when the ratio just exceeds 0.08, and the upper limit adopts a closed interval to indicate that the adjustment mechanism is still effective when the ratio reaches 0.15.

[0056] The process of determining the variable frequency fan speed to 60% to 90% of the rated speed is as follows: A mapping relationship is established between the temperature deviation value and the optimal convection intensity. When the ratio of the temperature deviation value to the target set temperature is within the adjustable range (0.08, 0.15), the uniformity of the workpiece surface temperature and the heating efficiency at different speeds are experimentally measured. The results show that when the speed is below 60% of the rated speed, the convection intensity is insufficient to significantly improve the temperature distribution. When the speed is above 90% of the rated speed, the airflow velocity is too fast, causing workpiece surface temperature oscillation and increasing energy consumption by more than 40%. Therefore, the lower limit of the speed adjustment range is determined to be 60% of the rated speed, corresponding to the minimum effective convection intensity, and the upper limit is 90% of the rated speed, corresponding to the balance point between economy and effect. The speed adjustment uses a linear interpolation method to calculate the target speed based on the relative position of the temperature deviation value within the adjustable range.

[0057] The target set temperature is the target temperature value that needs to be reached and maintained during the workpiece heating process, in °C, and the range is from 200 °C to 500 °C.

[0058] The calculation formula for dynamic error compensation is expressed as: the surface temperature of the workpiece after compensation. The unit is ℃, which is equal to the surface temperature of the workpiece. The unit is ℃ minus the error correction term, where the error correction term equals the measured oil mist concentration. Units are With the upper limit of the oil mist concentration sensor range of 500 The ratio multiplied by the temperature correction factor of 0.8 and then multiplied by the reference temperature deviation of 5℃, i.e. Temperature of nitrogen medium after compensation The calculation method, expressed in °C, is the same as the calculation method for the compensated workpiece surface temperature. Replace with nitrogen medium temperature The unit is ℃.

[0059] The compensated workpiece surface temperature is the corrected workpiece surface temperature value obtained after dynamic error compensation calculation, in °C. The compensated workpiece surface temperature is used for temperature feedback in the composite PID control algorithm.

[0060] The compensated nitrogen medium temperature is the corrected nitrogen medium temperature value obtained after dynamic error compensation calculation, and the unit is ℃.

[0061] The final temperature feedback value is the median of five temperature values, measured at four redundant temperature measurement points and the compensated workpiece surface temperature, in °C. This final temperature feedback value is used for closed-loop control of the composite PID control algorithm.

[0062] The temperature control deviation is the difference between the target set temperature and the compensated workpiece surface temperature, in °C. The temperature control deviation is used for real-time adjustment of the PID control parameters in the composite PID control algorithm.

[0063] The formula for calculating the proportional coefficient in the composite PID control algorithm is expressed as: proportional coefficient Equals 0.2 plus temperature control deviation The ratio of the unit temperature (℃) to 100℃ multiplied by the coefficient range difference of 1.8, i.e. Integration time constant The formula for calculating the time constant in seconds is expressed as: the integral time constant equals 1 second plus the temperature control deviation. The unit is the ratio of ℃ to 100℃ multiplied by the time range difference of 19 seconds, i.e. Differential time constant The formula for calculating the time constant in seconds is: the differential time constant equals 1 second plus the temperature control deviation. The unit is the ratio of ℃ to 100℃ multiplied by the time range difference of 19 seconds, i.e. .

[0064] The PID control parameters include proportional coefficient, integral time constant, and derivative time constant, which are used to adjust the output power of the electric heating element.

[0065] Among them, the pulsed air supply control mode is an operating mode in which the speed of the variable frequency fan fluctuates periodically according to a sine law. The formula for the speed change is expressed as: real-time speed The unit is revolutions per minute, which equals the average rotational speed. The unit is revolutions per minute multiplied by 1 and then by 0.2 times the sine function. The independent variables of the sine function are twice pi, the frequency of 1 Hz, and time. The product in units of seconds, i.e. .

[0066] The average speed is the average speed of the variable frequency fan under the pulsed airflow control mode, and the unit is 1000 rpm. The value range is 600. Up to 1200 .

[0067] Among them, the flow ratio adjustment is the operation of adjusting the air flow of each adjustable angle airflow nozzle according to the temperature field distribution in the sealed heating chamber. The flow adjustment is achieved by an electric regulating valve installed on the air inlet pipe of each adjustable angle airflow nozzle. The opening range of the electric regulating valve is 10% to 100%.

[0068] The electric heating element adopts a corrugated heating tube structure with a stainless steel wall and a surface coating. - The composite ceramic coating has a thickness of 75 micrometers, a rated power of 5 kilowatts, and an operating voltage of 380 volts.

[0069] Among them, the surface temperature uniformity of the workpiece is the difference between the maximum and minimum temperatures at each measurement point on the workpiece surface, in °C, and is calculated using the temperature data from redundant temperature measurement points and the first type K thermocouple.

[0070] The first-stage nitrogen replenishment regulation mechanism is a control strategy that increases the nitrogen injection flow rate to dilute the oil mist concentration and reduce the oxygen concentration when the measured value of the oil mist concentration exceeds the preset safety threshold. The flow rate is increased by adjusting the mass flow controller on the nitrogen supply pipeline.

[0071] The normal flow rate refers to the nitrogen injection flow rate of the sealed heating chamber under normal operating conditions, with a value range of 50. Up to 100 .

[0072] The second-level oxygen concentration monitoring and compensation algorithm is a control strategy that forcibly reduces the oxygen concentration in the sealed heating chamber when the measured value of oil mist concentration reaches the critical value. It monitors the oxygen content in real time through an oxygen concentration sensor. When the oxygen concentration is higher than the preset value, it automatically opens the nitrogen rapid injection valve and closes the exhaust valve, so that the oxygen concentration drops rapidly to a safe range.

[0073] Among them, the oxygen concentration sensor is a sensor that uses electrochemical principles to measure the volume fraction of oxygen in a gas. The measurement range is 0 to 25%, the response time is less than 15 seconds, and the output signal is a current signal of 4 to 20 mA.

[0074] The specific implementation methods of the above steps are described in detail below.

[0075] The specific implementation of step S01 is as follows: First, the air compressor is started to compress the outside air to 0.6 MPa and then sent to a freeze dryer for dehydration treatment, reducing the dew point to below -40°C. Then, the compressed air enters an adsorption tower group filled with carbon molecular sieves. Utilizing the selective adsorption characteristic of pressure swing adsorption (PSA), where oxygen molecules are preferentially adsorbed by the molecular sieve while nitrogen molecules penetrate, oxygen and nitrogen are separated. The resulting high-purity nitrogen gas, with a purity of over 99.5%, is then pressure-stabilized in a nitrogen buffer tank and passed through a mass flow controller at a rate of 50... Up to 100 The nitrogen flow rate is injected into the sealed heating chamber. At the same time, an oxygen concentration sensor is installed in the sealed heating chamber to monitor the oxygen content in real time. The nitrogen injection flow rate and the opening of the exhaust valve are adjusted by the control system to keep the oxygen concentration in the chamber stable below 8%. Eight adjustable angle airflow nozzles are evenly installed at 45 degrees around the sealed heating chamber. Each nozzle has an outlet diameter of 12 mm and is installed on a universal joint to achieve angle adjustment of ±45 degrees in the vertical direction and 360 degrees in the horizontal direction. The purpose of this step is to establish a low oxygen concentration nitrogen circulation heating environment to prevent oil mist from oxidizing and burning at high temperature, while achieving uniform temperature field distribution through multi-directional airflow nozzles.

[0076] The specific implementation of step S02 is as follows: A double-tube self-cleaning thermocouple assembly is installed at the temperature measurement position within a sealed heating chamber. This assembly consists of an outer tube, an inner tube, a thermocouple core wire, and a sealing connector. The outer tube has a diameter of 10 mm to 20 mm and its surface is machined with continuous spiral grooves along the axial direction. The groove depth is 1 mm and the width is 2 mm. The pitch of the spiral grooves is calculated and determined based on the spiral flow constraint equation. This equation establishes a linear relationship between the ratio of pitch to outer tube diameter and the ratio of groove depth to groove width, with a proportionality coefficient of 2.5. This geometric parameter configuration causes local acceleration in the groove region when high-speed nitrogen flows along the spiral path. A high-speed shear airflow layer, reaching 1.8 to 2.2 times the main airflow velocity, continuously washes the surface of the outer sleeve, preventing oil mist deposition. Argon gas is filled between the inner and outer sleeves to form a thermal barrier layer, reducing heat conduction loss. The first K-type thermocouple core wire extends through the inner sleeve and is used to monitor the workpiece surface temperature. The second K-type thermocouple core wire has the same structure and is used to monitor the nitrogen medium temperature. At the same time, four redundant temperature measurement points are arranged above, below, left, and right of the workpiece surface at a distance of 50 mm. The purpose of this step is to prevent temperature measurement errors caused by oil mist condensation and adhesion through a self-cleaning structure and to establish a multi-point temperature monitoring system to improve the reliability of temperature measurement.

[0077] The specific implementation of step S03 is as follows: the control system supplies power to a corrugated heating element with a rated power of 5 kilowatts and an operating voltage of 380 volts to start heating it up. The heating element wall is made of stainless steel with a surface coating. - The composite ceramic coating is 75 micrometers thick to improve high-temperature resistance and corrosion resistance. Simultaneously, a variable frequency fan drives nitrogen circulation within the sealed heating chamber to achieve heat convection transfer. An oil mist concentration sensor based on laser scattering principle is installed on the inner wall of the chamber, with a measurement range of 0 to 500. With a response time of less than 3 seconds, the sensor collects the mass concentration of oil mist particles in the sealed heating chamber in real time at 2-second intervals and outputs it to the control system via a 4 to 20 mA current signal, which is recorded as the measured value of oil mist concentration. The purpose of this step is to start the entire heating system and establish real-time monitoring of oil mist concentration to provide a data basis for subsequent dynamic error compensation.

[0078] The specific implementation of step S04 is as follows: The control system reads the workpiece surface temperature measured by the first K-type thermocouple and the nitrogen medium temperature measured by the second K-type thermocouple in real time, calculates the temperature deviation value by subtracting the nitrogen medium temperature from the workpiece surface temperature, and then calculates the ratio of the temperature deviation value to the target set temperature. The target set temperature is set in the range of 200℃ to 500℃ according to the workpiece material and heating process requirements. When the calculated ratio is within the normal range of 0.02 to 0.08, it indicates that the convective heat transfer between the workpiece and the nitrogen medium is sufficient and the thermal stress is at a safe level. At this time, the control system maintains the current variable frequency fan speed unchanged and continues to monitor the temperature change with a control cycle of 0.5 seconds. The purpose of this step is to assess the heat transfer state by monitoring the temperature difference between the workpiece and the heating medium and to determine whether the fan speed adjustment mechanism needs to be activated. The lower limit of the normal range of 0.02 corresponds to the critical point of sufficient convective heat transfer. Below this value, the heating rate decreases. The upper limit of 0.08 corresponds to the critical point of thermal stress safety. Above this value, the risk of workpiece cracking increases.

[0079] The specific implementation of step S05 is as follows: when the ratio of the temperature deviation value to the target set temperature exceeds 0.08 but does not exceed 0.15, it enters the adjustable range. The control system calculates the target speed using a linear interpolation method based on the relative position of this ratio within the adjustable range, and dynamically adjusts the variable frequency fan speed to between 60% and 90% of the rated speed. The lower limit of the speed adjustment range of 60% corresponds to the minimum effective convection intensity, and the upper limit of 90% corresponds to the balance point between economy and effect. By enhancing the nitrogen circulation flow intensity, the convective heat transfer coefficient between the workpiece surface and the nitrogen medium is improved, thereby achieving gradient adjustment of the temperature deviation. The purpose of this step is to improve the temperature distribution by adjusting the convection intensity when the temperature deviation exceeds the normal range but is still within the adjustable range, thus preventing the temperature deviation from further expanding. The upper limit of the adjustable range of 0.15 corresponds to the limit value of the fan adjustment capability. If this value is exceeded, even if the speed is increased to the maximum speed, the temperature uniformity cannot be effectively improved.

[0080] The specific implementation of step S06 is as follows: based on the physical mechanism that the adhesion of oil mist on the thermocouple surface increases thermal resistance, leading to a higher measured temperature value, the control system reads the measured value of oil mist concentration from the oil mist concentration sensor and calculates the difference between this value and the sensor's upper range limit of 500. The ratio is calculated, then multiplied by a temperature correction factor of 0.8 and a reference temperature deviation of 5℃ to obtain the error correction term. The workpiece surface temperature measured by the first K-type thermocouple is subtracted from the error correction term to obtain the compensated workpiece surface temperature. The same method is used to compensate the nitrogen medium temperature measured by the second K-type thermocouple to obtain the compensated nitrogen medium temperature. At the same time, the measurement data of 4 redundant temperature measurement points are read. The five temperature values, including the temperatures of these 4 redundant temperature measurement points and the compensated workpiece surface temperature, are sorted and the median value is taken as the final temperature feedback value and input into the subsequent control algorithm. The purpose of this step is to eliminate the influence of oil mist adhesion on the accuracy of temperature measurement through dynamic error compensation algorithm and to filter out abnormal measurement points by using the median value of multi-point temperature measurement data to improve the robustness of temperature feedback.

[0081] The specific implementation of step S07 is as follows: The control system uses a composite proportional-integral-derivative (PID) control algorithm to adjust the output power of the electric heating element. First, the temperature control deviation is obtained by subtracting the compensated workpiece surface temperature from the target set temperature. Then, the PID control parameters are calculated in real time based on the temperature control deviation. The input for calculating the proportional coefficient is the proportional coefficient value within the range of 0.2 to 2.0 of the temperature control deviation output. The inputs for calculating the integral time constant and the derivative time constant are the time constant values ​​within the range of 1 to 20 seconds of the temperature control deviation output. All three parameters increase with the increase of the temperature control deviation, establishing an adaptive adjustment mechanism. The control algorithm calculates the power output command of the electric heating element based on the calculated PID control parameters and the temperature control deviation with a control cycle of 0.5 seconds. The power adjustment is achieved by adjusting the duty cycle of the heating element through a solid-state relay. The purpose of this step is to achieve fast and accurate temperature tracking response through the adaptive PID control algorithm. When the temperature deviation is large, the control parameters are increased to improve the response speed, and when the temperature deviation is small, the control parameters are decreased to improve steady-state accuracy.

[0082] The specific implementation of step S08 is as follows: The pulsed air supply control mode is activated, causing the variable frequency fan speed to fluctuate periodically according to a sinusoidal law. The speed change frequency is set to 1 Hz, with a change range of ±20% of the average speed. The average speed is set within the range of 600 rpm to 1200 rpm based on the current heating stage. By periodically changing the airflow speed, the steady-state temperature stratification phenomenon originally formed in the sealed heating chamber is broken, enhancing the mixing uniformity of the temperature field. Simultaneously, the control system judges the non-uniform areas of the temperature field in the chamber based on the temperature distribution of four redundant temperature measuring points. By adjusting the opening of the electric regulating valve on the air inlet pipe of the adjustable angle airflow nozzle at the corresponding position within the range of 10% to 100%, the flow ratio is adjusted, directing more airflow to areas with lower temperatures and reducing the flow to areas with higher temperatures. The purpose of this step is to achieve workpiece surface temperature uniformity control within ±3℃ through the synergistic effect of pulsed air supply and flow ratio adjustment. The periodic disturbance of pulsed air supply avoids the solidification and stratification of the temperature field under constant wind speed, while the flow ratio adjustment specifically compensates for the local non-uniformity of the temperature field.

[0083] The specific implementation of step S09 is that the control system continuously monitors the measured value of oil mist concentration collected by the oil mist concentration sensor and compares it with a preset safety threshold of 120. By comparison, when the measured value of oil mist concentration exceeds the threshold, it indicates that the oil mist accumulation rate in the sealed heating chamber exceeds the nitrogen dilution rate, posing a safety hazard. The control system immediately activates the first-stage nitrogen replenishment and adjustment mechanism by increasing the set value of the mass flow controller to increase the nitrogen injection flow rate from the normal flow rate to 150% of the normal flow rate to accelerate the oil mist dilution rate and further reduce the oxygen concentration. At the same time, the variable frequency fan operation mode is switched to protection mode, and the speed is reduced to 40% of the rated speed to reduce airflow disturbance and prevent the rapid diffusion of oil mist in the chamber. The purpose of this step is to quickly reduce the oil mist concentration and maintain the continued operation of the heating process by combining the measures of increasing the nitrogen flow rate and reducing the fan speed when the oil mist concentration exceeds the safety threshold but has not yet reached the critical danger level.

[0084] The specific implementation of step S10 is as follows: when the measured value of oil mist concentration continues to rise and exceeds the critical value of 200... This indicates that the first-stage regulation mechanism has failed to effectively control oil mist accumulation. The control system immediately activates the second-stage oxygen concentration monitoring and compensation algorithm. This algorithm uses an oxygen concentration sensor based on electrochemical principles to monitor the oxygen volume fraction in the sealed heating chamber in real time. The sensor's measurement range is 0 to 25%, and the response time is less than 15 seconds. When the detected oxygen concentration is higher than 5%, the control system automatically opens the nitrogen rapid injection valve and closes the exhaust valve, allowing nitrogen to be injected into the chamber at maximum flow rate to rapidly replace the oxygen-containing gas. By forcibly reducing the oxygen concentration to below 5%, the conditions for oil mist oxidation and combustion are eliminated. At the same time, the audible and visual alarm device is triggered to issue an alarm signal to the operator, notifying them that the oil mist concentration in the sealed heating chamber has reached a critical dangerous level and that emergency measures such as stopping the machine, checking, and cleaning the source of the oil mist are required. The purpose of this step is to ensure system safety and prevent oil mist from undergoing oxidation or combustion accidents in a high-temperature, low-oxygen environment by forcibly reducing oxygen and using an alarm mechanism when the oil mist concentration reaches a critical value.

[0085] It should be noted that the first key technical concept of this invention is the design of a double-tube self-cleaning thermocouple assembly combined with a spiral flow constraint equation. By processing continuous spiral grooves on the surface of the thermocouple outer tube and optimizing the airflow velocity distribution based on the quantitative relationship between the ratio of the pitch to the outer tube diameter and the ratio of the groove depth to the width, nitrogen gas generates a local acceleration effect in the spiral groove region, forming a high-speed shear airflow layer. This airflow layer continuously scours the thermocouple surface, preventing oil mist particles from depositing and using the centrifugal force generated by the spiral flow to throw off the attached oil mist droplets. Compared with traditional straight-tube thermocouples that rely solely on natural convection and cannot effectively prevent oil mist adhesion leading to the continuous accumulation of temperature measurement errors, this self-cleaning structure actively removes oil mist through fluid dynamics principles, controlling the oil film thickness on the thermocouple surface to the micrometer level, thereby significantly reducing the temperature measurement error caused by oil mist condensation and providing accurate basic data for subsequent dynamic error compensation algorithms. The second key technical approach is a temperature feedback mechanism based on a dynamic error compensation algorithm for oil mist concentration combined with multi-point redundant temperature measurement. This algorithm calculates an error correction term based on the ratio of the measured oil mist concentration to the upper limit of the sensor range to compensate the thermocouple temperature measurement value in real time. At the same time, it takes the median value of the compensated workpiece surface temperature and the data from four redundant temperature measurement points as the final temperature feedback. Compared with the traditional method that ignores the influence of oil mist on temperature measurement or uses a coarse processing method with a fixed compensation value, this dynamic compensation mechanism establishes a quantitative correlation between oil mist concentration and temperature measurement error and eliminates interference from abnormal measurement points through median value filtering. This fundamentally solves the technical problem of inaccurate temperature measurement in an oil mist environment leading to the accumulation of control deviations. The third key technical approach is a synergistic temperature uniformity control strategy combining pulsed airflow control and flow rate adjustment. This strategy breaks steady-state temperature stratification by periodically fluctuating the variable frequency fan speed according to a sinusoidal law, while simultaneously adjusting the flow rate of each nozzle based on multi-point temperature distribution to achieve directional airflow compensation. Compared to traditional constant-speed heating methods, which easily form solidified temperature gradients and struggle to eliminate local hot spots, this synergistic strategy utilizes the temporal effect of periodic disturbances and the spatial adjustment of flow rate to achieve dynamic temperature field equilibrium. Through active control in both spatiotemporal dimensions, it significantly improves the surface temperature uniformity of the workpiece. The synergistic effect of these three key technical approaches lies in the fact that the double-tube self-cleaning structure ensures accurate temperature measurement, providing reliable input for dynamic error compensation; the precise temperature feedback output by the dynamic error compensation algorithm supports the effective operation of the adaptive control algorithm; and the pulsed airflow and flow rate adjustment achieve precise uniformity control based on accurate temperature field distribution information. These three elements form a complete technical chain from accurate measurement to precise control to uniform heating. Compared to traditional methods where independent design of each link lacks system synergy, resulting in limited overall performance, this invention achieves high-precision, high-uniformity, and reliable control of the high-temperature heating process in an oil mist environment through deep coupling of multiple technical paths.

[0086] It should be noted that this invention also solves the following technical problem: the problem of poor surface temperature uniformity of workpieces caused by steady-state temperature stratification within a sealed heating chamber. In traditional sealed chamber heating systems, due to natural convection and thermal stratification, the temperature in the upper part of the chamber is higher than that in the lower part, forming a stable temperature gradient distribution. This results in uneven heating of different parts of the workpiece, affecting the heating quality. This invention activates a pulsed air supply control mode, causing the variable frequency fan speed to fluctuate periodically at a frequency of 1 Hz according to a sinusoidal law, with a variation range of ±20% of the average speed. This unsteady airflow disturbance breaks the original steady-state temperature stratification structure within the chamber. Combined with the flow rate adjustment of eight adjustable-angle airflow nozzles, and the dynamic adjustment of the airflow rate and angle of each nozzle based on the temperature field distribution monitored by redundant temperature measurement points, a complex three-dimensional turbulent mixing flow field is formed in the nitrogen medium within the chamber. This enhances heat exchange between different areas, controls the surface temperature uniformity of the workpiece within ±3°C, and significantly improves the temperature consistency of the heating process.

[0087] Specifically, the principle of this invention is as follows: The invention solves this technical problem by establishing a quantitative relationship between the outer tube diameter, groove depth, groove width, and spiral groove pitch through a spiral flow constraint equation. This equation ensures that the geometric configuration of the spiral groove can induce a stable spiral flow field during nitrogen circulation. When nitrogen flows through the groove, the Bernoulli effect caused by the change in the flow channel cross-section locally increases the flow velocity. The shear stress exerted by the high-speed airflow on the thermocouple surface exceeds the adhesion force of the oil mist droplets. Simultaneously, the spiral flow generates radial centrifugal acceleration, which throws oil mist particles and droplets outward, preventing them from remaining and accumulating on the sensor surface. In the double-tube structure, argon gas is filled between the outer and inner tubes to form a thermal barrier layer, reducing the impact of outer tube temperature fluctuations on the internal thermocouple core wire and improving the stability of the temperature measurement response. Combined with a dynamic error compensation algorithm, the residual temperature measurement deviation is corrected based on the measured oil mist concentration, ultimately achieving high-precision temperature measurement in an oil mist environment and providing a reliable feedback signal basis for composite PID control.

[0088] The following provides a specific embodiment 1 of the present invention, and the specific implementation of each step in this embodiment 1 is described in detail below.

[0089] In this embodiment, the specific implementation of step S01 is the same as described above, and will not be repeated in detail here.

[0090] The specific implementation of step S02 is as follows: a double-tube self-cleaning thermocouple assembly is installed at the temperature measurement position within a sealed heating chamber. The outer tube surface of this assembly is machined with continuous spiral grooves. The geometric parameters of the spiral grooves are determined by a spiral flow constraint equation. This constraint equation establishes a quantitative relationship between the ratio of the pitch to the outer tube diameter and the ratio of the groove depth to the groove width. The formula is expressed as follows:

[0091] ;

[0092] In the formula, The pitch of the spiral groove, in mm, represents the axial distance that the continuous spiral groove travels in one revolution along the outer sleeve. The outer diameter is in mm, representing the outer diameter of the outer tube of the double-tube self-cleaning thermocouple assembly, ranging from 10 mm to 20 mm. The groove depth, in mm, represents the vertical depth of the continuous spiral groove from the surface of the outer tube inward, and is 1 mm in value. The groove width, in mm, represents the width of the continuous spiral groove along the circumference of the outer sleeve, and is set to 2 mm. The parameter is obtained as follows: Selected based on thermocouple installation space and strength requirements. and The local acceleration effect in the groove region was determined through airflow velocity field simulation optimization. The above formula is used to calculate the spiral flow constraint equation. By optimizing the geometric configuration of the spiral grooves, the high-speed nitrogen gas flows along the spiral path and generates a local acceleration effect in the groove region, forming a high-speed shear airflow layer with an airflow velocity of 1.8 to 2.2 times that of the main airflow velocity. This airflow layer continuously scours the surface of the outer tube to prevent the deposition of oil mist particles. At the same time, the centrifugal force generated by the spiral flow throws the attached oil mist droplets off the surface, keeping the oil film thickness on the thermocouple surface below 5 μm. This reduces the temperature measurement error caused by oil mist condensation from ±10℃ to within ±2℃.

[0093] The specific implementation method of step S03 is the same as described above, and will not be repeated in detail here.

[0094] The specific implementation of step S04 is as follows: the control system reads the workpiece surface temperature measured by the first K-type thermocouple and the nitrogen medium temperature measured by the second K-type thermocouple in real time, and the formula for calculating the temperature deviation value is expressed as follows:

[0095] ;

[0096] In the formula, This is the temperature deviation value, in °C, which reflects the heat transfer state between the workpiece and the heating medium. The surface temperature of the workpiece is measured in °C by a type K thermocouple. The temperature of the nitrogen medium, in °C, is obtained by measuring a type K thermocouple. The parameter acquisition method is as follows: and All readings are performed in real time by the control system via a thermocouple signal acquisition module at 0.5-second intervals. The ratio of the temperature deviation to the target set temperature is then calculated using the following formula:

[0097] ;

[0098] In the formula, This is the temperature deviation ratio, dimensionless, used to determine whether the heat transfer state is within the normal range; Set the target temperature in degrees Celsius (°C), with a range of 200°C to 500°C, depending on the workpiece material and heating process requirements. When the temperature is within the normal range of 0.02 to 0.08, the current variable frequency fan speed is kept constant and the temperature change is continuously monitored. The lower limit of the normal range of 0.02 corresponds to the critical point of sufficient convective heat transfer, and the upper limit of 0.08 corresponds to the critical point of thermal stress safety.

[0099] The specific implementation of step S05 is as follows: when the temperature deviation ratio When the ratio is greater than 0.08 and less than or equal to 0.15 within the adjustable range, the target speed is calculated using a linear interpolation method based on the relative position of this ratio within the adjustable range. The speed adjustment formula is expressed as follows:

[0100] ;

[0101] In the formula, The target rotational speed, in units of This indicates the required speed value for the variable frequency fan; Rated speed, unit: The value represents the rated operating speed of the variable frequency fan; 0.6 in parentheses is the lower limit coefficient of the speed adjustment range, dimensionless, corresponding to 60% of the rated speed; 0.3 is the upper and lower limit difference coefficient of the speed adjustment range, dimensionless, corresponding to a 30% change in the rated speed; 0.08 is the lower limit of the adjustable range, dimensionless; 0.15 is the upper limit of the adjustable range, dimensionless; the 1 in the numerator and denominator in parentheses is used for dimensionless processing. The parameter acquisition method is as follows: Determined by the parameters on the fan nameplate. Calculated from step S04, The speed regulation is executed by the frequency converter control system after calculation using the above formula. By increasing the intensity of nitrogen circulation flow, the convective heat transfer coefficient between the workpiece surface and the nitrogen medium is improved, thereby achieving gradient regulation of temperature deviation.

[0102] The specific implementation of step S06 is as follows: based on the physical mechanism that the adhesion of oil mist on the thermocouple surface increases thermal resistance and leads to an overestimation of the temperature reading, the control system performs dynamic error compensation on the thermocouple temperature reading based on the measured oil mist concentration. The formula for calculating the workpiece surface temperature after compensation is expressed as follows:

[0103] ;

[0104] In the formula, The corrected workpiece surface temperature is expressed in °C and represents the corrected workpiece surface temperature value obtained after dynamic error compensation calculation. This is the measured value of oil mist concentration, in units of... The value is obtained by the oil mist concentration sensor at the time of data acquisition; 500 is the upper limit of the oil mist concentration sensor's range, in units of... ; 0.8 is the temperature correction factor, dimensionless, with an empirical value of 0.8; 5 is the reference temperature deviation, in °C, representing the maximum temperature measurement error caused when the oil mist concentration reaches the upper limit of the range. The parameter acquisition method is as follows: Measured by the first type K thermocouple, Real-time data was collected by an oil mist concentration sensor at 2-second intervals. A temperature correction factor of 0.8 and a reference temperature deviation of 5°C were determined through experimental calibration. The experimental steps included: Step 1, measuring the oil film thickness on the thermocouple surface under different oil mist concentrations and calculating the corresponding thermal resistance increment; Step 2, calculating the temperature measurement error based on the thermal resistance increment and fitting the temperature correction factor and the reference temperature deviation. The formula for calculating the compensated nitrogen medium temperature is as follows:

[0105] ;

[0106] In the formula, The temperature of the nitrogen medium after compensation is expressed in °C. The nitrogen medium temperature, in °C, is obtained by measuring with a second type K thermocouple. Simultaneously, measurement data from four redundant temperature measurement points are read. The median of these five temperature values ​​(the temperatures from these four redundant points and the compensated workpiece surface temperature) is taken as the final temperature feedback value, as expressed in the formula below:

[0107] ;

[0108] In the formula, The final temperature feedback value, in °C, is used for closed-loop control of the composite proportional-integral-derivative control algorithm. This represents the median-value function; The function's purpose is to arrange the five input temperature values ​​in ascending order, and then select the third value in the middle position as the output result. This function can effectively filter out the interference of abnormal measurement values ​​and improve the robustness of temperature feedback. , , , The measured temperatures, in °C, are obtained from four redundant temperature measurement points located above, below, to the left and right of the workpiece.

[0109] The specific implementation of step S07 is as follows: the control system uses a compound proportional-integral-derivative control algorithm to adjust the output power of the electric heating element. First, the temperature control deviation is calculated, and the formula is expressed as follows:

[0110] ;

[0111] In the formula, This represents the temperature control deviation, expressed in °C, used for real-time adjustment of control parameters in the composite proportional-integral-derivative (CIDR) control algorithm. The CIDR control parameters are then calculated in real-time based on the temperature control deviation. The formula for calculating the proportional coefficient is as follows:

[0112] ;

[0113] In the formula, is the proportionality constant, dimensionless, ranging from 0.2 to 2.0; 0.2 is the reference value for the proportionality constant, dimensionless; 100 is the normalized reference value for the temperature deviation, in °C; 1.8 is the difference in the range of the proportionality constant, dimensionless. The formula for calculating the integral time constant is as follows:

[0114] ;

[0115] In the formula, The integral time constant is expressed in seconds (s), ranging from 1 s to 20 s; 1 is the baseline value of the integral time constant, also in seconds; 19 represents the range difference of the integral time constant, in seconds. The formula for calculating the differential time constant is as follows:

[0116] ;

[0117] In the formula, 1 represents the differential time constant, in seconds, ranging from 1 s to 20 s; 1 represents the baseline value of the differential time constant, in seconds; and 19 represents the range difference of the differential time constant, in seconds. The parameters are obtained as follows: The settings are determined by the operator according to the process requirements. Calculated from step S06, Calculated using the above formula , , All parameters are calculated in real time using the above formulas and input into the proportional-integral-derivative (PID) control algorithm, with a control cycle of 0.5 seconds. This adaptive PID control algorithm establishes a linear relationship between the control parameters and the temperature control deviation. When the temperature deviation is large, the control parameters are increased to improve the response speed, and when the temperature deviation is small, the control parameters are decreased to improve the steady-state accuracy, thereby achieving a fast and accurate temperature tracking response.

[0118] The specific implementation of step S08 is to activate the pulsed air supply control mode so that the speed of the variable frequency fan fluctuates periodically according to a sine law. The formula for the speed change is as follows:

[0119] ;

[0120] In the formula, Real-time rotational speed, unit: This represents the instantaneous speed of the variable frequency fan in the pulsed airflow control mode; Average rotational speed, in units of This represents the average speed of the variable frequency fan in the pulsed airflow control mode, with a value range of 600. Up to 1200 0.2 is the speed fluctuation amplitude coefficient, which is dimensionless and represents the speed change amplitude as ±20% of the average speed. It is a sine function; Pi, with an empirical value of 3.14159; The rotational speed variation frequency is expressed in Hz and has a value of 1Hz. The time unit is seconds (s), starting from the moment the pulsed airflow control mode is activated. The parameter acquisition method is as follows: Based on the current heating stage and temperature control requirements. Determined based on the disturbance period required to break the temperature stratification. Provided by the control system's real-time clock. The speed is adjusted by the frequency converter after real-time calculation using the above formula. By periodically changing the airflow speed, the steady-state temperature stratification phenomenon originally formed in the sealed heating chamber is broken, enhancing the mixing uniformity of the temperature field. At the same time, the flow rate ratio of each adjustable angle airflow nozzle is adjusted according to the temperature distribution of the four redundant temperature measuring points, so as to control the surface temperature uniformity of the workpiece within ±3℃.

[0121] The specific implementation methods of steps S09-S10 are the same as those described above, and will not be repeated in detail here.

[0122] The principles and effects of the formulas and mathematical models involved in this invention are explained below. The spiral flow constraint equation establishes a linear relationship between the ratio of the spiral pitch to the outer tube diameter and the ratio of the groove depth to the groove width. Based on the velocity distribution law of spiral flow in fluid mechanics, it ensures that high-speed nitrogen gas generates a local acceleration effect in the spiral groove region, forming a high-speed shear gas flow layer. This gas flow layer actively removes oil mist adhering to the thermocouple surface through continuous scouring and centrifugal force, fundamentally solving the technical problem of continuous accumulation of thermocouple temperature measurement errors in an oil mist environment, and providing accurate basic data support for subsequent dynamic error compensation algorithms. The dynamic error compensation formula is based on the physical mechanism that oil mist adhesion increases thermal resistance, leading to overestimation of temperature. By establishing a quantitative correlation between oil mist concentration and temperature measurement error, it dynamically calculates the error correction term based on the measured oil mist concentration and performs real-time compensation for the thermocouple temperature measurement value. Compared with traditional methods that ignore the influence of oil mist or use a coarse processing method with a fixed compensation value, this dynamic compensation mechanism can track the impact of oil mist concentration changes on temperature measurement accuracy in real time and make precise corrections, significantly improving the accuracy of temperature measurement and the closed-loop feedback quality of the control system. The adaptive proportional-integral-derivative (PID) control algorithm establishes a linear mapping relationship between control parameters and temperature control deviation. Based on the influence mechanism of parameters on system response characteristics in PID theory, it realizes an adaptive control strategy that automatically adjusts control parameters according to temperature deviation. When the temperature deviation is large, the proportional coefficient and integral-derivative time constant are increased to improve system response speed and accelerate temperature tracking. When the temperature deviation is small, the control parameters are decreased to reduce system gain, improve steady-state control accuracy, and reduce temperature fluctuations, thereby achieving a dynamic balance between speed and stability throughout the heating process. The pulsed airflow speed variation formula is based on the periodic characteristics of a sine function. By making the fan speed fluctuate sinusoidally at a fixed frequency and amplitude, periodic airflow disturbances are generated to break the steady-state temperature stratification phenomenon formed by natural convection and temperature gradients in the sealed heating chamber. The periodic disturbances in the time domain enhance the mixing uniformity of the temperature field. Combined with flow rate ratio adjustment, local non-uniform regions of the temperature field are specifically compensated in the spatial domain. The synergistic effect of the spatiotemporal dimensions significantly improves the uniformity of the temperature distribution on the workpiece surface and controls temperature fluctuations within the process requirements.

[0123] To better understand and implement this invention, a specific application scenario is provided in Example 2: A technical team undertook a heat treatment task for a large precision forged workpiece. The workpiece was made of 42CrMo alloy steel and had a cylindrical structure with a diameter of 380mm and a height of 520mm. A large amount of cutting fluid and lubricating oil remained on the workpiece surface after machining, which would evaporate significantly during high-temperature heating, forming an oil mist environment. Traditional resistance furnace heating methods suffer from uneven temperature distribution and the oil mist affecting temperature measurement accuracy, making it difficult to guarantee the quality of the workpiece heat treatment. The technical team decided to use the nitrogen circulation heating control method of this invention to solve this technical problem.

[0124] The technical team first established a nitrogen circulation heating system, using a pressure swing adsorption (PSA) nitrogen-oxygen separator to separate high-purity nitrogen (99.7%) from compressed air. The nitrogen was then distributed at a rate of 75 kJ / kg using a mass flow controller. The airflow is injected into the sealed heating chamber. The sealed heating chamber is welded from 8mm thick 310S stainless steel plates, with internal dimensions of 1200mm × 1000mm × 900mm. The inner wall of the chamber is coated with a 150μm thick alumina ceramic coating to improve thermal insulation performance. An oxygen concentration sensor monitors the oxygen content inside the chamber in real time, maintaining it stably at a safe level of 6.8%. Eight adjustable-angle airflow nozzles are evenly arranged at 45-degree intervals around the sealed heating chamber. Each nozzle has an outlet diameter of 12mm and is mounted on a rotatable spherical universal joint, allowing for angle adjustment covering ±45 degrees vertically and 360 degrees horizontally.

[0125] like Figure 2 As shown, the technical team installed a double-tube self-cleaning thermocouple assembly for temperature monitoring. The first K-type thermocouple, with its measuring end 15mm from the workpiece surface, is used to monitor the workpiece surface temperature. The second K-type thermocouple is installed in the center of the airflow channel, 300mm from the workpiece, to monitor the nitrogen medium temperature. The outer tube of the thermocouple has a diameter of 16mm and is machined with continuous spiral grooves on its surface, with a groove depth of 1mm and a groove width of 2mm. Based on the spiral flow constraint equation, the pitch of the spiral grooves is calculated to be 20mm. Argon gas is filled between the outer tube and the inner tube to form a heat insulation layer, effectively reducing heat conduction errors. Four redundant temperature measuring points are also arranged around the workpiece in the sealed heating chamber, located 50mm above, below, to the left, and to the right of the workpiece surface, respectively, to monitor the temperature field distribution.

[0126] like Figure 3 As shown, after the heating system is started, all eight corrugated heating elements are simultaneously energized and heated. Each heating element has a rated power of 5kW and an operating voltage of 380V AC. The surface of the heating elements is coated with... - The composite ceramic coating, with a thickness of 75 μm, significantly improves oxidation resistance and thermal radiation efficiency. A variable frequency fan drives nitrogen gas to circulate within the sealed heating chamber, with the fan speed initially set at 900 r / min. An oil mist concentration sensor, installed 600 mm above the chamber sidewall, measures the oil mist particulate concentration in real time using laser scattering principles, with a data acquisition interval of 2 seconds. The oil mist concentration rises rapidly during the initial heating phase, reaching a peak of 68% after 180 seconds of heating. It then gradually decreased and stabilized at 45. about.

[0127] The technical team set the target temperature at 420℃ and collected temperature data from two thermocouples in real time during the heating process. The first K-type thermocouple measured the workpiece surface temperature at 388℃, and the second K-type thermocouple measured the nitrogen medium temperature at 372℃, with a temperature deviation of 16℃. The ratio of the temperature deviation to the target temperature was 0.038, which was within the normal range. The system maintained a constant variable frequency fan speed of 900 r / min and continuously monitored the temperature. As the heating process progressed, 480 seconds after the start of heating, the workpiece surface temperature rose to 405℃, and the nitrogen medium temperature reached 363℃, increasing the temperature deviation to 42℃. At this point, the ratio was 0.10, entering the adjustable range. The system dynamically adjusted the variable frequency fan speed based on the temperature deviation, using a linear interpolation method to calculate the target speed as 72% of the rated speed of 1200 r / min, i.e., 864 r / min, achieving gradient adjustment of the convection intensity.

[0128] like Figure 4 As shown, the oil mist concentration sensor continuously collects oil mist concentration data, and the measured value is 45. The system corrects the thermocouple temperature readings using a dynamic error compensation formula. After compensation, the workpiece surface temperature is 404.64℃, and the nitrogen medium temperature is 362.64℃. The measured temperatures at the four redundant temperature measurement points are 401℃, 406℃, 403℃, and 408℃. The median of these five temperature values ​​(404.64℃ for the compensated workpiece surface temperature) is 404℃, which is used as the final temperature feedback value input to the composite PID control algorithm.

[0129] The technical team employed a composite PID control algorithm to adjust the power of the electric heating element, with a control cycle set to 0.5 seconds. The temperature control deviation was calculated by subtracting the compensated workpiece surface temperature of 404.64℃ from the target set temperature of 420℃, resulting in 15.36℃. Based on the proportional coefficient calculation formula, the proportional coefficient was 0.477; based on the integral time constant calculation formula, the integral time constant was 3.92s; and based on the derivative time constant calculation formula, the derivative time constant was also 3.92s. The system adjusts the output power of the electric heating element in real time based on these PID control parameters to achieve precise temperature control.

[0130] like Figure 5As shown, the system activates a pulsed airflow control mode to break the steady-state temperature stratification within the chamber. The variable frequency fan speed varies periodically according to a sinusoidal law, with an average speed of 864 r / min, a variation frequency of 1 Hz, and a variation range of ±20% of the average speed. In pulsed airflow mode, the variable frequency fan speed fluctuates periodically between 691 r / min and 1037 r / min. The technical team dynamically adjusts the opening of the electric regulating valve installed on the air inlet pipe of each nozzle according to the temperature field distribution by adjusting the flow ratio of eight adjustable-angle airflow nozzles. The valve opening of the two nozzles above the workpiece is set to 85%, the valve opening of the four nozzles on the side of the workpiece is set to 70%, and the valve opening of the two nozzles below the workpiece is set to 95%. This differentiated flow ratio effectively improves temperature uniformity. After 15 minutes of pulsed airflow control, the surface temperature uniformity of the workpiece improved from the initial 8℃ to 2.5℃, achieving the control target within ±3℃.

[0131] During a certain stage of the heating process, due to the large-scale evaporation of residual oil on the workpiece surface, the measured value of the oil mist concentration collected by the oil mist concentration sensor suddenly rose to 135. It exceeded the preset safety threshold of 120. The system immediately activated the first-stage nitrogen replenishment regulation mechanism, and the mass flow controller increased the nitrogen injection flow rate from 75... Increased to 112.5 The flow rate was increased to 150% of the normal flow rate. Simultaneously, the inverter fan switched to protection mode, reducing its speed to 40% of the rated speed (480 rpm) to minimize airflow disturbance and prevent further oil mist diffusion. After 180 seconds of nitrogen dilution, the oil mist concentration gradually decreased to 85%. The system has returned to normal operation.

[0132] like Figure 6 As shown in Table 1, the temperature curve and oil mist concentration change curve during the heating process are shown in Table 1.

[0133] Table 1 Recording Table of Key Parameters for Heating Process

[0134]

[0135] After the entire heating process lasted 15 minutes, the surface temperature of the workpiece stabilized at 420℃, with temperature fluctuations controlled within ±1.8℃, meeting the requirements of the heat treatment process. The technical team successfully solved the problem of high-temperature heating control in an oil mist environment using the nitrogen circulation heating control method of this invention. The metallographic structure of the workpiece after heat treatment was uniform, and all mechanical properties met the requirements.

[0136] This invention represents a significant technological advancement over traditional resistance furnace heating methods. Traditional methods use air as the heating medium, which poses a risk of combustion and explosion in oil mist environments due to the mixing of oxygen and oil mist. This invention, however, uses a nitrogen circulation system to control the oxygen concentration below 8%, fundamentally eliminating this safety hazard. Traditional methods are prone to oil contamination on thermocouple surfaces, leading to increased temperature measurement errors. This invention employs a double-tube self-cleaning thermocouple assembly, using a spiral groove to guide high-speed nitrogen to scour the surface, combined with a dynamic error compensation algorithm, significantly improving temperature measurement accuracy. Traditional methods rely on single-point temperature measurement and fixed PID parameters for temperature control, making it difficult to adapt to complex heat transfer conditions in oil mist environments. This invention uses multi-point redundant temperature measurement and an adaptive PID control algorithm to achieve more precise temperature regulation. Traditional methods have simple airflow organization, easily leading to temperature stratification. This invention, through a pulsed airflow control mode and adjustable-angle airflow nozzle flow ratio adjustment, effectively breaks steady-state temperature stratification, achieving a uniform temperature field distribution.

[0137] It should be noted that the variables involved in this invention are explained in detail in Table 2 below.

[0138] Table 2 Variable Explanation Table

[0139]

[0140] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A high temperature heating control method suitable for use in an oil mist environment, characterized by, The process includes establishing a nitrogen circulation heating system and controlling the oxygen concentration within a sealed heating chamber; uniformly arranging adjustable-angle airflow nozzles around the sealed heating chamber; installing a double-tube self-cleaning thermocouple assembly, including a first K-type thermocouple and a second K-type thermocouple, and uniformly arranging redundant temperature measuring points within the sealed heating chamber; starting the heating system and collecting real-time measured values ​​of oil mist concentration; collecting real-time workpiece surface temperature and nitrogen medium temperature and calculating the temperature deviation value; adjusting the variable frequency fan speed based on the ratio of the temperature deviation value to the target set temperature; performing dynamic error compensation on the temperature values ​​measured by the first K-type thermocouple and the second K-type thermocouple based on the measured oil mist concentration value to obtain the compensated workpiece surface temperature and the compensated nitrogen medium temperature; and using the median value of the measurement data from the redundant temperature measuring points as the final temperature feedback value. The power of the electric heating element is adjusted by a composite PID control algorithm; the pulsed air supply control mode is activated and the steady-state temperature stratification phenomenon in the sealed heating chamber is broken by adjusting the flow ratio of the adjustable angle airflow nozzle; when the measured value of oil mist concentration exceeds the preset safety threshold, the first-level nitrogen replenishment adjustment mechanism is activated; when the measured value of oil mist concentration exceeds the critical value, the second-level oxygen concentration monitoring and compensation algorithm is activated.

2. The high temperature heating control method suitable for an oil mist environment according to claim 1, characterized by, The dual-tube self-cleaning thermocouple assembly includes an outer tube, an inner tube, a thermocouple core wire, and a sealing joint. The outer tube has a continuous spiral groove on its surface. The gap between the inner tube and the outer tube is filled with argon gas to form a thermal barrier layer. The thermocouple core wire passes through the inner tube and extends out to the temperature measuring end.

3. The high temperature heating control method suitable for an oil mist environment according to claim 2, characterized by, The geometric parameters of the continuous spiral groove satisfy the spiral flow constraint equation, which is expressed as follows: the spiral groove pitch divided by the outer tube diameter equals 2.5 times the groove depth divided by the groove width.

4. The high temperature heating control method suitable for an oil mist environment according to claim 3, characterized by, The spiral flow constraint equation optimizes the spiral flow velocity distribution of the airflow on the outer tube surface, causing the nitrogen flow velocity to generate a local acceleration effect in the groove region, forming a high-speed shear airflow layer. The high-speed shear airflow layer continuously scours the outer tube surface, preventing oil mist particles from depositing. At the same time, the centrifugal force generated by the spiral flow throws the already attached oil mist droplets off the surface.

5. The high-temperature heating control method suitable for oil mist environments according to claim 4, characterized in that, The outer tube has a diameter of [10, 20] mm, a groove depth of 1 mm, and a groove width of 2 mm. The pitch of the spiral groove is calculated from the outer tube diameter, groove depth, and groove width according to the spiral flow constraint equation.

6. The high-temperature heating control method for oil mist environments according to claim 5, characterized in that, The temperature deviation value is the difference between the workpiece surface temperature and the nitrogen medium temperature. When the ratio of the temperature deviation value to the target set temperature is ∈ [0.02, 0.08], the current variable frequency fan speed is kept constant and the temperature change is continuously monitored.

7. The high-temperature heating control method for oil mist environments according to claim 6, characterized in that, When the ratio of the temperature deviation value to the target set temperature is ∈ (0.08, 0.15], the variable frequency fan speed is dynamically adjusted to between 60% and 90% of the rated speed according to the temperature deviation value.

8. The high-temperature heating control method for oil mist environments according to claim 7, characterized in that, The calculation formula for the dynamic error compensation is expressed as follows: the compensated workpiece surface temperature equals the workpiece surface temperature minus the error correction term, where the error correction term equals the measured oil mist concentration minus the upper limit of the oil mist concentration sensor's range (500). The ratio is multiplied by a temperature correction factor of 0.8 and then multiplied by a reference temperature deviation of 5℃.

9. The high-temperature heating control method for oil mist environments according to claim 8, characterized in that, The calculation method for the compensated nitrogen medium temperature is the same as the calculation method for the compensated workpiece surface temperature, except that the workpiece surface temperature is replaced by the nitrogen medium temperature.

10. The high-temperature heating control method for oil mist environments according to claim 9, characterized in that, The final temperature feedback value is the median of five temperature values: the temperature measured at four redundant temperature measurement points and the compensated workpiece surface temperature. The final temperature feedback value is used for closed-loop control of the composite PID control algorithm.

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