JPH05248631A - Method for controlling furnace temperature - Google Patents

Abstract

PURPOSE:To enable controlling the furnace temperature in a wide range by providing a continuous combustion type control system for bringing the furnace temperature to a target point in a manner of feedback control and an intermittent combustion type control system both as means enabling large substantial changes in calorific value per unit time at the burner. CONSTITUTION:In a heat-treating furnace equipped with a radiant tube 1 having a burner 3 at one end inside the furnace 2, a logic circuit 13, on the one hand, gives a command to adjust the valve opening to an air flow-controlling valve 7 in accordance with quantitative control from a controller 12 and, on the other hand, adjusts the valve opening of a fuel flow-controlling valve 8 through a diaphragm 9 which is displaced in proportion to the flow rate of the air for the combustion. Simultaneously the deviation of the furnace temperature detected by a thermocouple 11 from the target temperature is inputted to a controller 12 to control the furnace temperature in a manner of feedback control. When the furnace temperature approaches the target point and the command for the valve opening has fallen below a low flow cut point, the control of the combustion is shifted to intermittent combustion type control whereby the air flow-controlling valve 7 and a low flow cut valve 10 are given commands for opening the valves at regular intervals of time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a furnace temperature control method for controlling the combustion of a burner and stably controlling the temperature inside the furnace to a target temperature in an industrial furnace using a burner as a heat source.

[0002]

2. Description of the Related Art In a heat treatment furnace provided with a radiant tube burner, or in an industrial furnace using a burner as a heat source such as a direct-fired heating furnace, a conventional furnace is used to control the temperature inside the furnace to a target temperature. Detects the temperature inside the furnace from the PI and determines the supply amount of combustion air, fuel, etc., to be sent to the burner.
A method of performing feedback control by operating a flow rate control valve via a controller such as a D controller is adopted.

However, the range of calorific value that can stably change the combustion of the burner by throttling the flow control valve is 1: 3.
However, if the flow control valve is throttled below that, combustion failure may occur and an accident may occur.

Therefore, in order to expand the range of change, thinning control has been conventionally performed. This thinning control is
When multiple burners are provided, divide them into, for example, two systems, and when the calorific value is small, burn the burner of one system only and the burner of the other system is stopped, and a large calorific value is required. Is to burn both burners.

[0005]

However, when the burner is divided into a plurality of systems as described above, the equipment such as the solenoid valve, the control valve, the operation switch, and the electric circuit needs to be several times in proportion to the number of divisions. However, there is a problem in that the cost will increase because of the increase.

Further, the controller such as the PID controller outputs a manipulated variable having three elements of a proportional term, an integral term and a differential term of the deviation between the detected temperature and the target temperature, and as is well known, it is a furnace. Although it is intended to quickly and stably converge the internal temperature to the target temperature, in the actual furnace operation, the coefficient of each of the above-mentioned terms is, for example, the temperature when switching between heating and cooling during soaking. It was difficult to make adjustments such as a difference between the value for reducing the fluctuation of the deviation and the value for improving the transient response when making the furnace temperature follow the change of the target temperature value, and the performance was limited. Also, when switching between heating and cooling, the integral term was sometimes eliminated to improve the switching response, so the original function of the integral term cannot be obtained in the section that involves switching between heating and cooling (at the time of soaking) and control performance is improved. Was not good.

[0007]

SUMMARY OF THE INVENTION A furnace temperature control method of the present invention is intended to solve the above-mentioned problems and is to control the supply amount of combustion air, fuel, etc. for detecting the temperature inside the furnace and sending it to the burner. A continuous combustion control system that controls the furnace temperature to the target temperature without stopping the combustion of the burner by operating the flow rate control valve via the An intermittent combustion type control system that shifts when the supply amount of air, fuel, etc. falls below a low flow rate cut point, and the intermittent combustion type control system is provided from the controller every predetermined cycle. The burner is burned for a time length corresponding to the required operation amount to be output, and when the low flow cut valve for fuel of the burner in the intermittent combustion control system is closed, the combustion air supplied to the burner is amount It is characterized in that so as to reduce or close simultaneously.

[0008]

Since the range of change in the amount of heat generation can be widened without using a plurality of systems, the performance of the control system can be improved at low cost. Further, by reducing the supply of unnecessary fuel air to the burner when the low flow cut valve is closed in the intermittent combustion control system, heat loss at that time is reduced.

[0009]

Embodiments of the present invention will now be described with reference to the drawings.
In FIG. 1, reference numeral 1 is a radiant tube provided in the furnace body 2, and a burner 3 is provided at one end thereof. Reference numeral 4 is an air supply pipe for supplying combustion air to the burner 3, and 5 is a gas supply pipe for supplying fuel to the burner 3.

The air supply pipe 4 is provided with a flow rate adjusting valve 7 whose opening can be adjusted by a control motor 6. 8
Is a flow rate control valve provided in the gas supply pipe 5, and the flow rate control valve 8 has its opening adjusted by a diaphragm 9 operated by the differential pressure between the gas supply pipe 4 and the gas supply pipe 5, The amount of fuel flowing is proportional to the amount of air flowing through the air supply pipe 4, and the air-fuel ratio in the burner 3 is always kept at a desired value. Reference numeral 10 is a low flow rate cut valve provided in the gas supply pipe 5.

Reference numeral 11 is a thermocouple for detecting the temperature in the furnace body 2. In this control system, however, the deviation between the furnace temperature detected by the thermocouple 11 and the target temperature is calculated, and the deviation is input to the controller 12 such as a PID controller. The controller 12 calculates the magnitude of the deviation, the temporal displacement, the temporal accumulation, that is, the proportional value, the derivative value, and the integral value, and the sum is output to the logic circuit 13 as the manipulated variable.

In the logic circuit 13, the opening degree is commanded to the flow rate control valve 7 according to the operation amount received from the controller 12. Therefore, the combustion air is supplied to the burner 3 through the air supply pipe 4 in accordance with the opening degree of the flow rate adjusting valve 7, and the diaphragm 9 adjusts the opening degree of the flow rate adjusting valve 8 according to the flow rate of the combustion air. The fuel gas is supplied and burned in the burner 3 to heat the inside of the furnace body 2 via the radiant tube 1.

The opening of the flow rate control valve 7 instructed by the logic circuit 13 in such a continuous combustion control system is controlled by the controller 12.
It changes according to the manipulated variable output. That is, as illustrated in FIG. 2, when the opening degree of the flow rate control valve 7 commanded by the logic circuit 13 is equal to or higher than a predetermined low flow rate cut point, the low flow rate cut valve 10 is in the open state and the logic circuit 13 The commanded opening of the flow rate adjusting valve 7 is increased in proportion to the operation amount output from the controller 12, and the opening degrees of the flow rate adjusting valve 7 and the flow rate adjusting valve 8 are adjusted. The calorific value is adjusted, and the furnace temperature detected by the thermocouple 11 at that time is fed back, so that the furnace temperature is automatically controlled to the target temperature.

On the other hand, when the temperature inside the furnace is close to the target temperature, the manipulated variable output from the controller 12 becomes small, and the opening degree commanded to the flow rate control valve 7 falls below the low flow rate cut point, intermittent combustion type Move to control system. In this intermittent combustion type control system, the logic circuit 13 issues an open command to the flow rate control valve 7 and the low flow rate cut valve 10 at every predetermined constant period t as shown in FIG. The length m of the open command, that is, the combustion duration of the burner 3 is proportional to the operation amount output from the controller 12 as shown in FIG.
As shown in (B) and (C), the length is gradually increased. The opening of the flow rate control valve 7 at this time is maintained slightly above the low flow rate cut point.

In this way, the burner 3 is burned at a constant cycle t, and the combustion duration m is controlled in proportion to the manipulated variable, whereby the calorific value within that time is controlled and the furnace temperature is brought to the target temperature. Can be maintained. When the low flow rate cut point or more is reached, the intermittent combustion control system is returned to the original continuous combustion control system, and the logic circuit 13 issues the opening command to the flow control valve 7 again.

It should be noted that if the cycle t is set short, the fluctuation range of the temperature in the furnace can be made smaller, but if it is set too short, the opening and closing frequency of the flow rate control valve 7 and the low flow rate cut valve 10 becomes unfavorable and its durability becomes a problem. Therefore, about 2 to 5 minutes is desirable for practical use, in which case the temperature fluctuation range in the furnace during soaking is 2 ° C.
It was possible to suppress the fluctuation within the range, and a significant improvement was recognized compared to the conventional fluctuation range (7 ° C or higher). Since the optimum value of the cycle t is determined in consideration of the temperature characteristics of the furnace, the durability of the valve, etc., it may fall outside the range of 2 to 5 minutes.

If the flow control valve 7 is left open when the low flow cut valve 10 is closed, only the combustion air is supplied to the burner 3 as it is, and the combustion air circulates in the radiant tube 1. Since the furnace body 2 is cooled and heat is lost because it is rotated, in the present invention, the low flow cut valve 10 is closed and at the same time the flow control valve 7 is closed or throttled to reduce the combustion air supplied to the burner 3. Since it is closed, the furnace body 2 is not cooled through the radiant tube 1 and unnecessary heat release is eliminated, which is beneficial for energy saving.

[0018]

As described above, the furnace temperature control method of the present invention is
By providing a continuous combustion type control system and an intermittent combustion type control system, the calorific value of the burner per unit time can be changed substantially substantially, and the temperature inside the furnace can be adjusted over a wide range. .. In addition, the furnace temperature can be maintained at the target temperature with high accuracy, the control performance is improved, and there is an effect of saving energy.

[Brief description of drawings]

FIG. 1 is a block diagram of a furnace temperature control method according to the present invention.

FIG. 2 is a diagram showing a relationship between an operation amount and a flow control valve opening.

FIG. 3 is a diagram showing the relationship between the manipulated variable and the burner combustion duration.

FIG. 4 is a timing chart of burner combustion.

[Explanation of symbols]

 2 furnace body 3 burner 4 air supply pipe 5 gas supply pipe 7 flow control valve 8 flow control valve 10 low flow cut valve 11 thermocouple 12 controller 13 logic circuit

Claims (1)

[Claims] 1. A furnace temperature is detected without stopping combustion of the burner by controlling the supply amount of combustion air, fuel, etc., which detects the furnace temperature and supplies it to the burner, via a controller. A continuous combustion control system that performs feedback control to the target temperature, and an intermittent combustion that shifts when the supply amount of combustion air, fuel, etc. to the burner falls below the low flow cut point during the operation of the above continuous combustion control system. The intermittent combustion control system is configured to burn the burner for a predetermined period and for a time length corresponding to a required operation amount output from the controller, thereby controlling the intermittent combustion control. A furnace temperature control method characterized in that when the low flow cut valve for fuel of the burner is closed in the system, the amount of combustion air supplied to the burner is simultaneously reduced or closed.