Dry and wet state conversion undisturbed switching control method

Abstract

The invention provides an undisturbed switching control method for dry and wet state switching, relates to the technical field of thermotechnical automatic control, and aims to solve the problems that in the wet and dry state switching process of a direct current furnace of an existing unit, the running condition of a starting thermodynamic system is very complex, manual control easily causes large disturbance to the system, and the system is not stable. And particularly, improper control of the water-coal ratio causes frequent switching between a dry state and a wet state and severe fluctuation of unit parameters, so that normal operation of a unit is influenced. S2, model structure definition: a coordination control system is a three-input three-output multivariable system, controlled variables are determined to be a unit load N, a main steam pressure P and an intermediate point temperature T respectively, and controlled variables are determined to be a coal feed quantity B, a control valve opening degree mu and a feed water flow W respectively; and S3, transmission model parameter identification: identifying the load, the main steam pressure and the intermediate point temperature by adopting a particle swarm optimization (PSO) method based on the on-site operation historical data of the thermal power plant.

Description

Technical Field

[0001] The invention relates to the technical field of thermal automatic control, and in particular to a disturbance-free switching control method for dry-wet state conversion. Background Art

[0002] As the parameters of the unit increase, the complexity of the controlled object and the control requirements increase, and more problems arise. First, for traditional drum boilers, people tend to ignore some minor factors, so that the subcritical unit coordinated control system can be simplified into a dual-input dual-output controlled object. For the coordinated control system of supercritical units, the disturbance of the feed water flow rate will also have a great impact on the actual power and main steam pressure of the unit. Therefore, in the coordinated control system of supercritical units, the feed water flow rate and the intermediate point temperature (or enthalpy value) should also be used as the control quantity and controlled quantity of the coordinated control system, which makes the parameters of the already complex multivariable system more and the coupling stronger. Secondly, the supercritical unit does not have the drum link of the subcritical unit, and can only use a direct current furnace, which increases the circulation speed of water and water vapor in the unit, which requires the control system to be faster and more real-time; and due to the lack of heat storage link, this requires the control system to more strictly ensure the mass balance and energy balance of the unit. At the same time, the coordinated control system also has unfavorable factors such as dynamic nonlinearity, time variation, large time delay, and uncertain interference, which bring great challenges to the design and optimization of its control system. In the past, the traditional PID controller obtained by manual trial and error and parameter tuning will hardly guarantee the normal operation of the unit. Even if it is barely put into use, it will be difficult to apply when the dynamic characteristics of the unit change significantly. Therefore, there is an urgent need for an advanced control algorithm that can stably and effectively control this complex industrial process.

[0003] The DC boiler starting system maintains the minimum flow of the boiler during boiler starting, low-load operation of the unit (steam flow is lower than the minimum flow required by the furnace) and shutdown process to ensure the safety of the furnace water-cooled wall, and at the same time meet the steam flow requirements during the start-up, shutdown and low-load operation of the unit. The DC boiler starting system is divided into two types of starting systems: with a boiler water circulation pump and without a boiler water circulation pump. Among them, the starting system without a boiler water circulation pump has a simple structure, and the feed water passes through the feed water pump to the high-pressure heater, water supply operating valve, economizer, water-cooled wall, steam-water separator, water storage tank, overflow valve and other systems to form a feed water circulation loop. The starting system with a boiler water circulation pump is relatively complex, with more equipment and strong coupling, but it can greatly reduce the working fluid and heat loss during the unit startup, and greatly reduce the thermal shock to the boiler during the hot start of the unit. Therefore, it is particularly important to design a dry-wet state conversion disturbance-free switching control method to solve the above technical problems. Summary of the invention

[0004] The purpose of the present invention is to solve the problem in the prior art that during the wet-dry state switching process of the DC furnace of the existing unit, the operating conditions of the starting thermal system are very complicated, and manual control is likely to cause large disturbances to the system, especially improper control of the water-coal ratio will lead to frequent switching between dry and wet states and drastic fluctuations in unit parameters, thereby affecting the normal operation of the unit, and a disturbance-free switching control method for dry-wet state conversion is proposed.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a disturbance-free switching control method for dry-wet state conversion, through the method of mechanism modeling and data identification, a coordinated control model of an ultra-supercritical unit is established, including: S1, data selection and preprocessing: S2, model structure definition: the coordinated control system is a multivariable system with three inputs and three outputs, and the controlled quantities are determined to be the unit load N, main steam pressure P, and intermediate point temperature T, and the controlled quantities are respectively the coal feed amount B, valve opening μ, and feed water flow W, S3, transfer model parameter identification: the particle swarm (PSO) optimization method based on the historical data of on-site operation of the thermal power plant is used to identify the load, main steam pressure, and intermediate point temperature.

[0006] Preferably, the selection and preprocessing of the S1 data includes S11, selecting data with a smooth start-stop process and a sufficiently large signal-to-noise ratio during the operation process according to a suitable sampling period.

[0007] Preferably, the selection and preprocessing of the S1 data includes S12, in which the initial conditions of the system operation are used as the "zero point" of the system input and output to perform zero initial value processing.

[0008] Preferably, the structure in S2 is defined as follows: .

[0009] Preferably, the structure definition in S2 also includes the following: .

[0010] Preferably, the supercritical unit coordinated control system is considered as a whole, and a control model is established for the three controlled quantities of unit power, main steam pressure and intermediate point temperature at the same time: The controller parameters are optimized using the overall optimization method (particle swarm algorithm) of multi-objective optimization problems to obtain the Pareto solution set of the problem.

[0011] Preferably, the "weighted sum of the integrated square errors (ISE) of all control loops" is used as a criterion to select a suitable optimal solution from the above solution set.

[0012] Preferably, the wet state to dry state in the unit coordinated control model is: When the boiler operating condition changes from a circulation state to a direct current state, the feed water control is switched from feed water flow and water tank water level control to intermediate point temperature (enthalpy value) control. The conversion process is mainly achieved by controlling the fuel amount and feed water flow, and the boiler circulating water pump is tripped by the low water tank water level protection control logic.

[0013] Preferably, the dry state to wet state transition in the unit coordinated control model is: When the boiler is converted from a wet state to a dry state and the interlock control function is activated, the boiler circulation pump is started by interlock after the water level in the water storage tank reaches the interlock protection value. At the same time, the boiler circulation pump outlet regulating valve is switched to automatic control mode, and the recirculation flow is set to the difference between the feed water flow and the main steam flow.

[0014] Preferably, the water level of the water storage tank and the feed water flow rate are ensured not to fluctuate greatly, and the unit is prevented from tripping. The control logic of the boiler conversion from wet state to dry state is optimized as follows: when the boiler is converted from wet state to dry state and the interlocking control function is put into operation, the water level of the water storage tank reaches the interlocking protection value, and the boiler circulation pump is interlocked to start, and the outlet regulating valve of the boiler circulation pump is switched to the automatic control mode, and the recirculation flow rate is set to the difference between the feed water flow rate and the main steam flow rate. As the unit load decreases, the DC boiler is gradually converted from wet state to dry state operation through the total flow closed-loop control circuit.

[0015] Compared with the prior art, the advantages and positive effects of the present invention are: 1. In the present invention, if there is no such trip control logic, when the water level overflow valve of the water storage tank is closed, the temperature of the middle point of the steam-water separator is greater than 6°C, and the water level of the water storage tank is lower than the normal value, the boiler circulating water pump will trip. When the water level of the water storage tank is lower than the normal value and the boiler circulating water pump stops running, the water level overflow valve of the water storage tank is closed, and the temperature of the middle point of the steam-water separator is greater than 6°C, the unit enters the direct current state operation wet state to dry state In order to ensure that the water level of the water storage tank and the feed water flow do not fluctuate greatly and avoid the unit tripping, the boiler wet state to dry state conversion control logic is optimized as follows: when the boiler is converted from the wet state to the dry state and the interlock control function is put into use, the water level of the water storage tank reaches the interlock protection value and the boiler circulating pump is interlocked and started, and the boiler circulating pump outlet regulating valve is switched to the automatic control mode at the same time, and the recirculation flow is set to the difference between the feed water flow and the main steam flow. As the load of the unit decreases, the total flow closed-loop control circuit is used to gradually switch the DC furnace from wet to dry operation, which solves the problem that during the switching process of the DC furnace between wet and dry states of the existing unit, the operating conditions of the starting thermal system are very complicated, and manual control is prone to cause large disturbances to the system. In particular, improper control of the water-coal ratio will lead to frequent switching between dry and wet states and drastic fluctuations in unit parameters, thereby affecting the normal operation of the unit.

[0016] 2. In the present invention, during use, the control method of the present invention is used to reduce losses, improve energy conversion and utilization efficiency, and reduce the production and processing costs of enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The object structure of the unit coordinated control system of the present invention; Figure 2 The control logic of the present invention that the wet state automatically changes to the dry state and reduces the water flow rate; Figure 3 The coal feeding amount control logic is increased for the automatic conversion from wet state to dry state in the present invention. DETAILED DESCRIPTION

[0018] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0019] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0020] See also Figure 1 - Figure 3 The present invention collects the operating data of the unit under different working conditions and establishes the control object model by using big data technology. Based on the model, the advanced control and intelligent optimization algorithms are used to optimize the control parameters, and the disturbance-free switching control strategy of the coordinated control system of the ultra-supercritical unit during the dry / wet state conversion process is proposed. The overall steps are as follows: (1) Collect the DCS data of the unit and establish the control object model through the method of mechanism modeling + data identification; (2) Use the PSO multi-objective optimization algorithm to optimize the controller parameters (3) Improve the control logic to realize the disturbance-free switching control strategy of the coordinated control system of the ultra-supercritical unit during the dry / wet state conversion process.

[0021] Establishment of the ultra-supercritical unit model of the present invention: According to the static characteristics analysis of supercritical units, the coordinated control system of supercritical units is a multivariable system with three inputs and three outputs. Its controlled quantities are unit load N, main steam pressure P, and intermediate point temperature T, and its controlled quantities are coal feed B, valve opening μ, and feed water flow W. Therefore, the object transfer function model structure of the coordinated control system of supercritical units is as follows Its structural diagram is as follows Figure 1shown.

[0022] According to previous experience, thermal system objects are generally self-balanced objects, and their model order is generally within the third order. Therefore, in summary, the simplified model set of supercritical unit objects selected during system identification can be written as follows: Where: k——gain coefficient Where: k——gain coefficient T1, T2——Inertia time constant h——reverse response coefficient According to the dynamic characteristics analysis of supercritical units, when the coal feed rate increases stepwise, the responses of the unit load, main steam pressure and intermediate point temperature all increase positively, and there is no reverse response process. Therefore, it can be known that the reverse response coefficient h in GNB, GPB and GTB is equal to zero.

[0023] When the throttle opening increases in a step, the response of the unit load is to increase first and then decrease to near the initial value, so it can be seen that there is a reverse response process, that is, the reverse response coefficient h in GNμ is not zero. The response of the main steam pressure increases negatively, and there is no reverse response process, that is, the reverse response coefficient in GPμ is equal to zero. The response of the intermediate point temperature is to decrease first and then increase to near the initial value, and there is a reverse response process, that is, the reverse response coefficient h in GTμ is not zero.

[0024] When the feedwater flow rate increases stepwise, the response process of the unit load and main steam pressure is similar, both of which increase first and then decrease to near the initial value, that is, there is a reverse response process, so the reverse response coefficient h in GNW and GPW is not zero. The response process of the intermediate point temperature is negative increase and there is no reverse response process, that is, the reverse response coefficient h in GTW is equal to zero. Combined with the above research on the static and dynamic characteristics of the mechanism of the coordinated control system of supercritical units, the linear dynamic mathematical model structure of the coordinated control system object of the supercritical unit is determined as follows: The selection of on-site historical data follows the following principles: (1) The starting and ending conditions should be as stable as possible; (2) The data during the process of operating condition changes must have a sufficiently large signal-to-noise ratio; (3) The sampling period should be appropriate.

[0025] Since the transfer function refers to the ratio of the Laplace transform of the output and input of the system under zero initial conditions, it is assumed that the input, output and derivatives of the system under zero initial conditions are all zero. It can be seen that the identification data must be processed with zero initial values, otherwise it is equivalent to assuming that the values ​​of all system parameters under the initial conditions are at the actual zero value point. Obviously, this does not conform to the actual situation of most production processes. The "zero point" of the unprocessed input and output data u(k) and y(k) collected may be arbitrary. When the system data acquisition starts at a certain initial condition of the system operation, it is assumed that this initial condition is the "zero point" of the system input and output. The data after zero initial value processing is: Where: N——Number of zero initial point data u* (k) System input data after zero initial value processing y* (k) System output data after zero initial value processing Identify the unit load: The unit load N is identified. At this time, the three input fuel quantity B, the throttle opening μ, and the feed water flow W act on the unit load N at the same time. Its structure is as follows: N=GNB ·B+GNμ ·μ+GNW ·W (1) The particle swarm optimization (PSO) method based on the historical operation data of the thermal power plant is used to identify the supercritical units.

[0026] Combined with the field historical data, the particle swarm algorithm is used to optimize the 12 parameters of the given model structure GNB, GNμ and GNW. Based on experience and the relatively reasonable parameters obtained initially, the parameter optimization range is gradually narrowed. At this time, the 12 parameters in the identification formula are: k11, T111, T112, τ11, k12, T121, h12, k13, T131, T132, h13 Set the upper and lower limits and PSO optimization algorithm parameters respectively, and establish the corresponding optimization objectives. The corresponding mathematical model can be obtained.

[0027] Identification of main steam pressure: The main steam pressure P is identified. At this time, the three input fuel quantity B, the throttle opening μ, and the feed water flow W act on the main steam pressure P at the same time. Its structure is as follows: P=GPB ·B+GPμ ·μ+GPW ·W (3) Combined with the field historical data, the particle swarm algorithm is used to optimize the 11 parameters in the given model structure GPB, GPμ and GPW. Based on experience and the relatively reasonable parameters obtained initially, the parameter optimization range is gradually narrowed. At this time, the 11 parameters in the identification formula are k21, T211, T212, τ21, k22, T221, h22, k23, T231, T232, h23 Set the upper and lower limits and PSO optimization algorithm parameters respectively, and establish the corresponding optimization objectives. The optimization objectives are: The corresponding mathematical model can be obtained.

[0028] Identification of the midpoint temperature: The middle point temperature T is identified. At this time, the three input fuel quantity B, the valve opening μ, and the water flow rate W act on the middle point temperature T at the same time. Its structure is as follows: T=GTB ·B+GTμ ·μ+GTW ·W (5) Combined with the field historical data, the particle swarm algorithm is used to optimize the 11 parameters in the given model structure GTB, GTμ and GTW. Based on experience and the relatively reasonable parameters obtained initially, the parameter optimization range is gradually narrowed. At this time, the 11 parameters in the identification formula are k31, T311, T312, τ31, k32, T321, h32, k33, T331, T332, h33 Set the upper and lower limits and PSO optimization algorithm parameters respectively, and establish the corresponding optimization objectives. The corresponding mathematical model can be obtained.

[0029] Control strategy based on advanced control algorithms: When the coordinated control system of the supercritical unit is viewed as a whole, and the three controlled quantities of unit power, main steam pressure, and intermediate point temperature are optimized at the same time, an overall optimal solution is obtained. That is, the Pareto solution set of the problem is first obtained by using the overall optimization method of the multi-objective optimization problem, and then a suitable optimal solution is selected from the obtained Pareto solution set according to certain rules or criteria. It is worth pointing out that in the process of controller parameter optimization, the selection of the objective function is very important, because different objective functions represent different requirements for control quality, and finally different objective functions may lead to different results of controller parameter optimization. The objective function selected by the present invention when optimizing the coordinated control system of the supercritical unit is the weighted sum of the square integral of the error (ISE) of all control loops. As shown in the following formula: Where: e1 is the input of the unit load controller, e2 is the input of the main steam pressure controller, e3 is the input of the intermediate point temperature controller, w1, w2, w3 are the weighted coefficients of each control loop respectively.

[0030] For actual units in thermal power plants, the differential coefficient in the PID controller is often too large and the differential effect is too strong. That is, when the actual signals of the unit load N, main steam pressure P and intermediate point temperature T have high-frequency interference, or jitter occurs for some reason, the controller's reaction will be very strong, which can easily cause the entire system to swing or even become unstable. Therefore, in actual applications and simulation studies, in most cases, it is necessary to abandon the differential term in the PID controller and only retain the proportional and integral effects, that is, all three controllers use PI controllers, and their structures are shown in the following formula: The particle swarm optimization (PSO) algorithm is used to optimize the three controller parameters simultaneously so that the square integral of the error of all loops The weighted sum of is the smallest, where the weighted coefficients of each loop are 1, and the controller parameters can be finally obtained. Control strategy for disturbance-free switching of dry-wet state conversion: 1. Dry-wet state conversion: During the unit startup process, in order to prevent the boiler heating surface temperature from overheating, the economizer flow setting value is set to be much higher than the designed boiler minimum flow. In this way, the steam flow can meet the requirement of a larger boiler circulation flow when converted to dry state; if the unit is converted to dry state by increasing the coal feed, the load of the converted unit is high and difficult to operate.

[0031] When the unit is de-loaded, the boiler startup system will gradually separate liquid water from the steam-water separator as the load decreases. The liquid water will then enter the water storage tank. As the water level in the water storage tank increases, the boiler startup circulation pump will be started, the boiler circulation pump outlet regulating valve will be slowly opened, and the circulation flow rate will be adjusted to put the unit into wet operation.

[0032] (II) Automatic dry-wet state conversion control When the boiler operating condition changes from the circulation state to the direct current state, the feed water control switches from the feed water flow rate and the water level control of the water storage tank to the intermediate point temperature (enthalpy value) control. The conversion process is mainly achieved by controlling the fuel amount and the feed water flow rate. (1) See Figure 2, the steam-driven feedwater pump is put into automatic control mode, and the feedwater flow rate is reduced to the flow rate corresponding to the current steam flow rate (greater than the minimum feedwater flow rate instruction of the boiler); the feedwater flow rate is reduced by 50t / h every 40s, and the feedwater flow rate is gradually reduced to the target value; when the deviation between the feedwater flow rate and the instruction exceeds 10s or the main parameters of the unit (unit load, main steam pressure, main steam temperature, etc.) fluctuate greatly, the reduction of the feedwater flow rate is suspended, and the feedwater flow rate is continued after the feedwater flow rate is adjusted to the target value or the fluctuation of the unit parameters caused by the feedwater flow adjustment is stable; when the temperature of the middle point of the steam-water separator is greater than 5℃ or after the total fuel trip (MFT), the feedwater flow rate is reduced; when the boiler reaches a dry state, the enthalpy value or temperature of the middle point of the steam-water separator is put into automatic control mode, and the set value offset of its enthalpy value or temperature is set to 0, and the PID controller adjusts the temperature of the middle point of the steam-water separator according to the corresponding temperature under different loads.

[0033] (2) See Figure 3 , put the capacity wind damper and fuel main control of the running coal mill into automatic control mode, increase the coal supply, and increase the coal supply by 5t / h every 120s until the temperature of the mid-point of the steam-water separator is greater than 5℃; when the main parameters of the unit (unit load, main steam pressure, main steam temperature, etc.) fluctuate greatly, stop increasing the coal supply to the furnace, and continue to increase the coal supply after the unit parameter fluctuation caused by the adjustment of the coal supply stabilizes, and the maximum coal supply is increased to 1.3 times the coal supply before the increase; put the enthalpy value or temperature of the mid-point of the steam-water separator into automatic control mode, and set its enthalpy value or temperature set value offset to 0, and the PID controller adjusts the temperature of the mid-point of the steam-water separator according to the corresponding temperature under different loads.

[0034] During the automatic conversion of the unit from wet state to dry state, the boiler circulating water pump should be tripped through the low water level protection control logic of the water storage tank. If there is no such trip control logic, the boiler circulating water pump will be tripped when the water level overflow valve of the water storage tank is closed, the temperature of the middle point of the steam-water separator is greater than 6°C, and the water level of the water storage tank is lower than the normal value. When the water level of the water storage tank is lower than the normal value and the boiler circulating water pump stops running, the water level overflow valve of the water storage tank is closed, and the temperature of the middle point of the steam-water separator is greater than 6°C, the unit enters the direct current state operation. In order to ensure that the water level of the water storage tank and the feed water flow do not fluctuate greatly and avoid unit tripping, the control logic of the boiler wet state to dry state conversion is optimized as follows: when the boiler is converted from wet state to dry state and the interlock control function is put into use, the water level of the water storage tank reaches the interlock protection value and the boiler circulating pump is started interlocked, and the outlet regulating valve of the boiler circulating pump is switched to the automatic control mode, and the recirculation flow is set to the difference between the feed water flow and the main steam flow. As the unit load decreases, the DC boiler is gradually switched from wet to dry operation through the total flow closed-loop control circuit.

Claims

1. A disturbance-free switching control method for dry-wet state conversion, which establishes a unit coordinated control model through mechanism modeling and data identification methods, and is characterized in that: include: S1. Data selection and preprocessing: S2. Model structure definition: The coordinated control system is a multivariable system with three inputs and three outputs. The controlled quantities are determined to be the unit load N, the main steam pressure P, and the intermediate point temperature T. The controlled quantities are the coal feed B, the regulating valve opening μ, and the feed water flow W. S3. Transfer model parameter identification: The particle swarm optimization (PSO) method based on the historical data of on-site operation of the thermal power plant is used to identify the load, main steam pressure and intermediate point temperature.

2. A dry-wet state conversion disturbance-free switching control method according to claim 1, characterized in that: The selection and preprocessing of the S1 data include S11, according to a suitable sampling period, selecting data with a smooth start-stop process and a sufficiently large signal-to-noise ratio during the operation process.

3. A dry-wet state conversion disturbance-free switching control method according to claim 1, characterized in that: The selection and preprocessing of the S1 data include S12, which performs zero initial value processing by using the initial conditions of the system operation as the "zero point" of the system input and output.

4. The dry-wet state conversion disturbance-free switching control method according to claim 1 is characterized in that: The structure of S2 is defined as follows: 。 5. The dry-wet state conversion disturbance-free switching control method according to claim 1 is characterized in that: The structure definition in S2 also includes the following: 。 6. A dry-wet state conversion disturbance-free switching control method according to claim 1, characterized in that: The above-mentioned coordinated control system of the unit is considered as a whole, and the control model is established for the three controlled quantities of unit power, main steam pressure and intermediate point temperature at the same time: The controller parameters are optimized using the overall optimization method (particle swarm algorithm) of multi-objective optimization problems to obtain the Pareto solution set of the problem.

7. A dry-wet state conversion disturbance-free switching control method according to claim 6, characterized in that: The "weighted sum of the squared integral errors (ISE) of all control loops" is used as a criterion to select a suitable optimal solution from the above solution set.

8. A dry-wet state conversion disturbance-free switching control method according to claim 6, characterized in that: The wet state to dry state transition in the unit coordinated control model is: When the boiler operating condition changes from a circulation state to a direct current state, the feed water control is switched from feed water flow and water tank water level control to intermediate point temperature (enthalpy value) control. The conversion process is mainly achieved by controlling the fuel amount and feed water flow, and the boiler circulating water pump is tripped by the low water tank water level protection control logic.

9. A dry-wet state conversion disturbance-free switching control method according to claim 6, characterized in that: The dry state to wet state transition in the unit coordinated control model is: When the boiler is converted from a wet state to a dry state and the interlock control function is activated, the boiler circulation pump is started by interlock after the water level in the water storage tank reaches the interlock protection value. At the same time, the boiler circulation pump outlet regulating valve is switched to automatic control mode, and the recirculation flow is set to the difference between the feed water flow and the main steam flow.

10. The dry-wet state conversion disturbance-free switching control method according to claim 1 is characterized in that: The water level in the water storage tank and the feed water flow rate do not fluctuate significantly to avoid unit tripping, and the control logic of the boiler conversion from wet state to dry state is optimized as follows: when the boiler is converted from wet state to dry state and the interlocking control function is put into use, the water level in the water storage tank reaches the interlocking protection value and the boiler circulation pump is started by interlocking, and the outlet regulating valve of the boiler circulation pump is switched to the automatic control mode, and the recirculation flow rate is set to the difference between the feed water flow rate and the main steam flow rate. As the unit load decreases, the total flow closed-loop control circuit is used to gradually convert the DC boiler from wet state to dry state operation.

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