TWI807507B - Method for controlling temperature of heating furnace - Google Patents

Abstract

A method for controlling temperature of a heating furnace includes: calculating an average temperature of plural temperatures respectively measured by plural thermocouples; calculating a response time based on a temperature difference between an adiabatic flame temperature and the average temperature; calculating a ratio according to the response time; calculating a compensated proportional gain by multiplying a preset proportional gain by the ratio; calculating a compensated integral gain by multiplying a preset integral gain by the ratio; and performing a proportional integral control on a control valve of a burner of the heating furnace according to the compensated proportional gain and the compensated integral gain, so as to adjust an opening degree of the control valve.

Description

Temperature control method of heating furnace

The present invention relates to a control method, and in particular to a temperature control method of a heating furnace.

At present, most of the temperature control methods of industrial heat treatment furnaces use the temperature in the furnace measured by thermocouples to perform proportional integral control through fixed value, segmental, fuzzy and other methods to realize the feedback control of temperature closed loop. In addition, when the temperature in the furnace belongs to the medium and high temperature range (for example, above 650°C), in order to protect the thermocouple from high temperature damage, the thermocouple will be covered with a high temperature resistant ceramic shell to protect it.

However, due to the "hysteretic response of the thermocouple", the temperature reading of the thermocouple or the thermocouple covered with a high-temperature resistant ceramic shell still needs a response time of about 40 seconds to 130 seconds to correctly reflect the current temperature in the furnace. As a result, temperature overshoots, step jumps, and oscillations are likely to occur when the temperature is raised and lowered, and the temperature is maintained in stages, resulting in temperature control oscillations and changes in the system response bandwidth, resulting in inaccurate temperature control and wasted energy consumption.

The object of the present invention is to propose a temperature control method for a heating furnace, including: calculating the average temperature of thermocouples of a plurality of temperature readings respectively measured by a plurality of thermocouples; calculating the response time according to the temperature difference between the adiabatic flame temperature and the average temperature of the thermocouples; calculating the adjustment factor according to the response time; Valve opening of the control valve.

In some embodiments, the input of the proportional-integral control is the command error between the command temperature and the average temperature of the thermocouple, and the output of the proportional-integral control is the control signal of the control valve.

In some embodiments, the above proportional-integral control equation is: v ( t ) = K _pe ( t ) + K _i ʃ e ( t ) dt where v ( t ) is the control signal, e ( t ) is the command error, K _p is the compensation proportional coefficient, and K _i is the compensation integral coefficient.

In some embodiments, the formula for calculating the response time according to the temperature difference is: the input is a temperature difference and the output is a quadratic polynomial of the response time.

In some embodiments, the formula for calculating the adjustment factor according to the response time is: the input is the response time and the output is a cubic polynomial of the adjustment factor.

In some embodiments, the adiabatic flame temperature is calculated from the air-fuel ratio, preheated air temperature, and gas heating value.

In some embodiments, each thermocouple is a type K thermocouple or a type R thermocouple.

In some embodiments, the coefficients of the quadratic term, the linear term and the constant term of the quadratic polynomial used to calculate the response time according to the temperature difference are adjusted according to the type of each thermocouple.

In some embodiments, each thermocouple is overcoated with a ceramic sheath.

In some embodiments, the quadratic coefficient, the linear coefficient and the constant coefficient of the quadratic polynomial used to calculate the response time according to the temperature difference are adjusted according to the material of the ceramic casing.

In order to make the above-mentioned features and advantages of the present invention more comprehensible, the following specific embodiments are described in detail together with the accompanying drawings.

100: heating furnace

120:PI controller

140: thermocouple

160: burner

200: Thermocouple Response Combination

300: thermocouple response feed-forward compensation

e ( t ): command error

F _adj : Adjustment magnification

S1-S6: steps

T _AF : adiabatic flame temperature

T _avg : average temperature of thermocouple

T _C : command temperature

T _D : temperature difference

t _R : response time

v ( t ): control signal

A better understanding of aspects of the present invention can be obtained from the following detailed description in conjunction with the accompanying drawings. It is to be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or decreased for clarity of discussion.

[ Fig. 1 ] is a flowchart of a temperature control method of a heating furnace according to an embodiment of the present invention.

[ Fig. 2 ] is a schematic diagram of a temperature control method of a heating furnace according to an embodiment of the present invention.

Embodiments of the invention are discussed in detail below. However, it is understandable Yes, the embodiments provide many applicable concepts that can be implemented in a wide variety of specific contexts. The discussed and disclosed embodiments are for illustration only, and are not intended to limit the scope of the present invention.

FIG. 1 is a flowchart of a temperature control method of a heating furnace according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a temperature control method of a heating furnace according to an embodiment of the present invention.

In step S1 , the thermocouple average temperature T _avg of the plurality of temperature readings respectively measured by the plurality of thermocouples 140 is calculated. The thermocouple average temperature T _avg is the average of the multiple temperature readings of the multiple thermocouples 140 . Wherein, a plurality of thermocouples 140 are respectively fixedly arranged in a plurality of different positions of the heating furnace 100 to measure air temperatures in a plurality of different positions of the heating furnace 100 .

In the embodiment of the present invention, the thermocouple 140 is a K type thermocouple, but the present invention is not limited thereto, and the thermocouple may also be an R type (R type) thermocouple or other types of thermocouples.

In the embodiment of the present invention, the thermocouple 140 is covered with a ceramic sheath, but the present invention is not limited thereto, and the thermocouple may not be covered with a ceramic sheath, depending on the temperature range of the heating furnace 100 . For example, if the furnace temperature of the heating furnace 100 belongs to the medium-high temperature range (for example, above 650° C.), the thermocouple 140 is covered with a ceramic sheath. In an embodiment of the present invention, the material of the ceramic shell is PTO (PbTiO ₃ ) or SUS310.

In step S2, the response time t _R is calculated according to the temperature difference T _D between the adiabatic flame temperature T _AF and the thermocouple average temperature T _avg .

In an embodiment of the present invention, the adiabatic flame temperature T _AF is a function of the air-fuel ratio, the temperature of the preheated air and the calorific value of the gas. In other words, the adiabatic flame temperature T _AF is calculated according to the air-fuel ratio, preheated air temperature and gas heating value. Specifically, the adiabatic flame temperature T _AF is obtained by calculating the enthalpy according to the air-fuel ratio, preheated air temperature and gas calorific value through the chemical equilibrium formula of thermal reaction (CH ₄ +2O ₂ →CO ₂ +2H ₂ O) and looking up the table. Wherein, the air-fuel ratio is the ratio between the air flow measured by the air flow meter and the gas flow measured by the gas flow meter. Wherein, the temperature of the preheated air can be inversely obtained from the outlet temperature of the heating furnace 100 . Wherein, the heating value of gas is a known fixed value.

In step S2, firstly, subtract the average thermocouple temperature T _avg obtained in step S1 from the adiabatic flame temperature T _AF to obtain the temperature difference T _D , that is, temperature difference T _D = adiabatic flame temperature T _AF - average thermocouple temperature T _avg .

Next, the temperature difference T _D is substituted into a thermocouple response combination 200 to calculate the response time t _R . In an embodiment of the present invention, the thermocouple response combination 200 is a quadratic polynomial whose input is the temperature difference T _D and whose output is the response time t _R , that is, the response time t _R =a*T _D ² +b*T _D +c, wherein a, b, and c are the quadratic term coefficient, the first term coefficient and the constant term coefficient of the quadratic polynomial, respectively. In other words, the formula for calculating the response time t _R according to the temperature difference T _D is: the input is a quadratic polynomial of the temperature difference T _D and the output is the response time t _R .

Specifically, the quadratic term coefficient a, the first term coefficient b, and the constant term coefficient c of the quadratic polynomial that calculates the response time t _R according to the temperature difference T _D are the most suitable parameters obtained by polynomial fitting (Polynomial Fitting) using multiple historical data (multiple historical temperature differences and corresponding multiple historical response times).

In the embodiment of the present invention, the quadratic coefficient a, the first-order coefficient b and the constant coefficient c of the quadratic polynomial used to calculate the response time t _R according to the temperature difference T _D are adjusted according to the type of each thermocouple. In other words, depending on the type of thermocouple, the coefficient a of the quadratic term, the coefficient b of the first term, and the coefficient c of the constant term of the quadratic polynomial will also be different.

In an embodiment of the present invention, the quadratic coefficient a, the first-order coefficient b, and the constant coefficient c of the quadratic polynomial used to calculate the response time t _R according to the temperature difference T _D are adjusted according to the material of the ceramic shell. In other words, the materials of the ceramic casing are different, and the coefficient a of the quadratic term, the coefficient b of the first term and the coefficient c of the constant term of the quadratic polynomial will also be different.

In step S3, the adjustment factor F _adj is calculated according to the response time t _R . Substitute the response time t _R into a thermocouple response feed-forward compensation 300 to calculate the adjustment factor F _adj . In an embodiment of the present invention, the input of the thermocouple response feedforward compensation 300 is the response time t _R and the output is a cubic polynomial of the adjustment factor F _adj , that is, the adjustment factor F _adj = d*t _R ³ +e*t _R ² +f*t _R , wherein d, e, and f are the cubic coefficient, the quadratic coefficient, and the first-order coefficient of the cubic polynomial, respectively. In other words, the formula for calculating the adjustment magnification F _adj according to the response time t _R is: the input is the response time t _R and the output is a cubic polynomial of the adjustment magnification F _adj .

Specifically, the cubic coefficient d, the quadratic coefficient e, and the first-order coefficient f of the cubic polynomial to calculate the adjustment factor F _adj according to the response time t _R are the most suitable parameters obtained by polynomial fitting using multiple historical data (multiple historical response times and the multiple corresponding adjustment factors).

In step S4 , calculate the compensation proportional coefficient K _p of multiplying the default proportional coefficient by the adjustment factor F _adj , that is, the compensation proportional factor K _p =preset proportional factor*adjustment factor F _adj . Wherein, the preset proportional coefficient is a preset proportional gain of the proportional integral (PI) controller 120 .

In step S5 , calculate the compensation integral coefficient K _i of multiplying the preset integral coefficient by the adjustment factor F _adj , that is, the compensation integral factor K _i =preset integral factor*adjustment factor F _adj . Wherein, the preset integral gain is a preset integral gain of the PI controller 120 .

In step S6, the PI controller 120 performs proportional-integral (PI) control on the control valve of the burner 160 of the heating furnace 100 according to the compensation proportional coefficient K _p and the compensation integral coefficient K _i , so as to adjust the valve opening of the control valve of the burner 160 of the heating furnace 100.

First, subtract the average thermocouple temperature T _avg obtained in step S1 from the command temperature T _C to obtain the command error e ( t ), that is, command error e ( t ) = command temperature T _C - average thermocouple temperature T _avg .

Next, the PI controller 120 obtains the control signal v (t) of the control valve of the burner 160 of the heating furnace 100 based on the compensation proportional coefficient K _p and the compensation integral coefficient Ki according to the command error e ( t ₎ , so that the control signal v( t ) of the control valve of the burner 160 of the heating furnace 100 can be determined based on the command error e ( t ) .

In an embodiment of the present invention, the input of the proportional-integral (PI) control performed by the PI controller 120 in step S6 is the command error e ( t ) between the commanded temperature T _C and the average temperature T _avg of the thermocouple, and the output of the proportional-integral control is the control signal v ( t ) of the control valve of the burner 160 of the heating furnace 100.

In an embodiment of the present invention, the equation of the proportional-integral (PI) control performed by the PI controller 120 in step S6 is: v ( t ) = K _pe ( t ) + K _i ʃ e ( t ) dt (1) wherein, v ( t ) is the control signal, e ( t ) is the command error, K _p is the compensation proportional coefficient, and K _i is the compensation integral coefficient. For example, v ( t ) is the voltage control signal of the control valve of the burner 160 of the heating furnace 100. For example, when the voltage control signal is 10 volts, it means that the valve opening of the control valve of the burner 160 of the heating furnace 100 is 100%. For example, when the voltage control signal is 0 volts, it means that the valve opening of the control valve of the burner 160 of the heating furnace 100 is 0%.

Specifically, the present invention calculates the response time t _R based on the temperature difference T _D between the adiabatic flame temperature T _AF and the thermocouple average temperature T _avg , thereby modulating the PI controller 120 accordingly, controlling the temperature by the response bandwidth, and controlling the response time of the system in advance, that is, realizing the purpose of controlling the PI controller 120 in advance by calculating the response time t _R .

In detail, the present invention can compensate the preset proportional coefficient and preset integral coefficient of the PI controller 120 according to different response times, so that the PI control performed by the PI controller 120 can achieve fast, ruthless and accurate effects. In other words, the present invention can calculate the current response time and then substitute the optimal proportional coefficient and the optimal integral coefficient into the PI controller 120 in advance to achieve the purpose of advanced compensation, which not only improves the accuracy of temperature control, but also saves energy consumption.

The features of several embodiments are outlined above, so those skilled in the art can better understand aspects of the present invention. Those skilled in the art should appreciate that they can easily use the present invention as a basis to design or modify other processes and structures to achieve the same goals and/or achieve the same advantages as the embodiments described herein. Those skilled in the art should also understand that these equivalent constructions do not depart from the spirit and scope of the present invention, and that they can make various changes, substitutions and alterations without departing from the spirit and scope of the present invention.

100 : furnace 120 : PI controller 140 : thermocouple 160 : burner 200 : thermocouple response combination 300 : thermocouple response feedforward compensation : command error F _adj : adjustment factor T _AF : adiabatic flame temperature T _avg : thermocouple average temperature T _C : command temperature T _D : temperature difference t _R : response time : control signal

Claims (10)

A temperature control method for a heating furnace, comprising: calculating the average temperature of a thermocouple of a plurality of temperature readings respectively measured by a plurality of thermocouples; calculating a response time according to a temperature difference between an adiabatic flame temperature and the average temperature of the thermocouple; calculating an adjustment factor according to the response time; calculating a preset proportional factor multiplied by a compensation proportional factor of the adjustment factor; calculating a preset integral factor multiplied by a compensation integral factor of the adjustment factor; A control valve of a burner of the furnace performs a proportional integral control to adjust a valve opening of the control valve. The temperature control method of the heating furnace as described in Claim 1, wherein the input of the proportional-integral control is a command error between a command temperature and the average temperature of the thermocouple, and the output of the proportional-integral control is a control signal of the control valve. The temperature control method of the heating furnace as described in Claim 2, wherein the equation of the proportional integral control is: v ( t ) = K _p e ( t ) + K _i ʃ e ( t ) dt where v ( t ) is the control signal, e ( t ) is the command error, K _p is the compensation proportional coefficient, and K _i is the compensation integral coefficient. The temperature control method for a heating furnace as described in Claim 1, wherein the formula for calculating the response time according to the temperature difference is: the input is the temperature difference and the output is a quadratic polynomial of the response time. The temperature control method of the heating furnace as described in Claim 1, wherein the formula for calculating the adjustment ratio according to the response time is: the input is the response time and the output is a cubic polynomial of the adjustment ratio. The temperature control method of a heating furnace as claimed in claim 1, wherein the adiabatic flame temperature is calculated according to an air-fuel ratio, a preheated air temperature and a gas calorific value. The temperature control method for a heating furnace as described in Claim 1, wherein each of the thermocouples is a K-type thermocouple or an R-type thermocouple. The temperature control method of the heating furnace as described in Claim 7, wherein the quadratic term coefficient, the first term coefficient and the constant term coefficient of the quadratic polynomial for calculating the response time according to the temperature difference are adjusted according to the types of each of the thermocouples. The temperature control method for a heating furnace as described in Claim 1, wherein each of the thermocouples is covered with a ceramic sheath. The temperature control method of the heating furnace as described in Claim 9, wherein the quadratic term coefficient, the first term coefficient and the constant term coefficient of the quadratic polynomial used to calculate the response time according to the temperature difference are adjusted according to the material of the ceramic shell.