Pre-mixing device

Abstract

To provide a premixing device which can prevent a variable throttle valve from taking much time to be adjusted resulting from a change of a hysteresis amount at the adjustment of the variable throttle valve by performing control for adjusting an opening of the variable throttle valve which is interposed in a gas supply path so that an air excess rate of an air-fuel mixture reaches a proper value, in the premixing device for mixing a fuel gas into air, and supplying the air-fuel mixture to a burner via a fan.SOLUTION: When a detection air excess rate is displaced to one side from a pass range, for example, displaced to a larger side, a motor is repeatedly rotated by each unit angle in an opening increase direction up until the detection air excess rate reaches a proper value λm. When the detection air excess rate is displaced to the other side from the pass range, for example, displaced to a smaller side, the motor is rotated at a high speed in an opening reduction direction up to a first position P1 in which it is assumed that the air excess rate reaches a value λk which is displaced to the larger side from the pass range by a constant amount, after that, the motor is rotated at a high speed in the opening increase direction up to a second position P2 at this side of a target position Pm, and also after that, the motor is rotated in the opening increase direction by each unit angle.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan.

Background Art

[0002] Conventionally, as this type of premixing device, there is known one including an air supply passage on the upstream side of a fan, a gas supply passage having a downstream end connected to a gas suction portion provided in the air supply passage, and a zero governor interposed in the gas supply passage for regulating the secondary gas pressure to atmospheric pressure (see, for example, Patent Document 1). Here, the supply amount of the fuel gas changes according to the differential pressure between the atmospheric pressure which is the secondary gas pressure and the negative pressure acting on the gas suction portion. And since the negative pressure acting on the gas suction portion changes according to the fan rotation speed, the supply amount of the fuel gas changes in proportion to the fan rotation speed, that is, the supply amount of air. Therefore, by controlling the fan rotation speed according to the required combustion amount, a mixture in an amount corresponding to the required combustion amount is supplied to the burner, and the air excess ratio (primary air amount / stoichiometric air amount) of the mixture becomes constant.

[0003] By the way, in some countries, even when the same gas type is used as the fuel gas, the calorific value (Wobbe index) of the fuel gas may vary over time. In the conventional example described above, even if the calorific value of the fuel gas varies, the ratio of the supply amount of the fuel gas to the supply amount of air is constant. Therefore, the air excess ratio of the mixture varies due to the variation in the calorific value of the fuel gas, resulting in combustion failure.

[0004] Therefore, conventionally, there is also known a premixing device provided with a variable throttle valve driven by a stepping motor and an air excess ratio detection means for detecting the air excess ratio of the mixture, which is interposed in a portion of the gas supply passage on the downstream side of the zero governor, and performs control to adjust the opening degree of the variable throttle valve so that the air excess ratio of the mixture detected by the air excess ratio detection means becomes constant (see, for example, Patent Document 2).

[0005] Here, taking the rotation direction of the stepping motor that changes the opening degree of the variable throttle valve in one of the increase and decrease directions as the forward rotation direction, and the rotation direction of the stepping motor that changes the opening degree of the variable throttle valve in the other of the increase and decrease directions as the reverse rotation direction, for the forward rotation direction change characteristic line representing the relationship between the rotation position of the stepping motor and the opening degree of the variable throttle valve when the stepping motor is rotated in the forward rotation direction, the reverse rotation direction change characteristic line representing the relationship between the rotation position of the stepping motor and the opening degree of the variable throttle valve when the stepping motor is rotated in the reverse rotation direction is shifted in the reverse rotation direction by the amount of hysteresis due to the backlash of the drive mechanism between the stepping motor and the valve body of the variable throttle valve. And when it is necessary to change the opening degree of the variable throttle valve to, for example, the one side during the previous adjustment and change the opening degree of the variable throttle valve to the other side during this adjustment, even if the stepping motor is rotated in the reverse rotation direction, if the rotation angle is within the amount of hysteresis, the opening degree of the variable throttle valve will not change. Therefore, in order to shorten the time required for adjusting the opening degree of the variable throttle valve, it is necessary to quickly rotate the stepping motor in the reverse rotation direction by the amount of hysteresis.

[0006] However, the amount of hysteresis has individual differences and further changes over time. Therefore, before the rotation angle of the stepping motor in the reverse rotation direction reaches a predetermined amount of hysteresis, the opening degree of the variable throttle valve may start to change to the other side, and when the rotation angle of the stepping motor in the reverse rotation direction reaches a predetermined amount of hysteresis, the opening degree of the variable throttle valve may change excessively to the other side. In this case, it is necessary to rotate the stepping motor in the forward rotation direction to change the opening degree of the variable throttle valve to the one side. And when the stepping motor is quickly rotated in the forward rotation direction by a predetermined amount of hysteresis, the opening degree of the variable throttle valve may change excessively to the one side, and it may be necessary to change the opening degree of the variable throttle valve to the other side again. As a result, the change of the opening degree of the variable throttle valve between the other side and the one side may be repeated, and it may take a long time to adjust the variable throttle valve.

Prior Art Documents

Patent Documents

[0007] Patent Document 1 Japanese Unexamined Patent Application Publication No. 2018-179447 Patent Document 2 Japanese Unexamined Patent Application Publication No. 2021-25722 Summary of the Invention Problems to be Solved by the Invention

[0008] In view of the above points, an object of the present invention is to provide a premixing device capable of preventing the adjustment of a variable throttle valve from taking time due to a change in the amount of hysteresis. Means for Solving the Problems

[0009] In order to solve the above problems, the present invention provides a premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising an air supply passage on the upstream side of the fan, a gas supply passage having a downstream end connected to a gas suction portion provided in the air supply passage, a zero governor interposed in the gas supply passage for regulating the secondary gas pressure to atmospheric pressure, a variable throttle valve driven by a stepping motor interposed in a portion of the gas supply passage on the downstream side of the zero governor, an air excess ratio detecting means for detecting the air excess ratio of the mixture, and a control means. The control means performs control to vary the rotational speed of the fan according to the required combustion amount, and performs control to adjust the opening degree of the variable throttle valve so that the air excess ratio of the mixture detected by the air excess ratio detecting means becomes a predetermined appropriate value. In this configuration, the rotational direction of the stepping motor for changing the opening degree of the variable throttle valve in one of increasing and decreasing is defined as the forward rotation direction, and the rotational direction of the stepping motor for changing the opening degree of the variable throttle valve in the other of increasing and decreasing is defined as the reverse rotation direction. When the air excess ratio of the mixture detected by the air excess ratio detecting means is outside a predetermined pass range including the appropriate value and the value of the opening degree of the variable throttle valve is such that it becomes the appropriate value by changing the opening degree of the variable throttle valve in the one direction, the control means determines whether the air excess ratio of the mixture detected by the air excess ratio detecting means becomes the appropriate value in a state where the stepping motor is rotated in the forward rotation direction by a predetermined unit angle, and repeats the control until the air excess ratio of the mixture detected by the air excess ratio detecting means becomes the appropriate value. When the air excess ratio of the mixture detected by the air excess ratio detecting means is outside the pass range and the value of the opening degree of the variable throttle valve is such that it becomes the appropriate value by changing the opening degree of the variable throttle valve in the other direction, based on the reverse-direction change characteristic line representing the relationship between the rotational position of the stepping motor when the stepping motor is rotated in the reverse rotation direction and the opening degree of the variable throttle valve, the stepping motor is rotated in the reverse rotation direction until it reaches a predetermined first reference rotational position assumed to be an adjustment reference value that deviates by a certain amount in the change direction when the air excess ratio of the mixture changes from the pass range and the opening degree of the variable throttle valve is changed in the other direction. Next, based on the forward-direction change characteristic line representing the relationship between the rotational position of the stepping motor and the opening degree of the variable throttle valve when the stepping motor is rotated in the forward rotation direction,The stepping motor is rotated in the forward rotation direction from the first reference rotation position to the second reference rotation position a predetermined angle before in the forward rotation direction with respect to the target rotation position at which the air-fuel ratio of the air-fuel mixture is assumed to be an appropriate value, and then, in a state where the stepping motor is rotated by a predetermined unit angle in the forward rotation direction, it is determined whether or not the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means has become an appropriate value, and the control is repeated until the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means becomes an appropriate value. It is characterized by being configured to perform such control.

[0010] According to the present invention, when the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means deviates from the qualified range to the side where the opening degree of the variable throttle valve becomes an appropriate value by changing the opening degree of the variable throttle valve to one side, while determining whether or not the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means is at the appropriate value, the stepping motor is repeatedly rotated in the forward rotation direction by a predetermined unit angle, whereby the variable throttle valve can be adjusted in a relatively short time so that the air-fuel ratio of the air-fuel mixture becomes an appropriate value. Further, when the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means deviates from the qualified range to the side where the opening degree of the variable throttle valve becomes an appropriate value by changing the opening degree of the variable throttle valve to the other side, the stepping motor is rotated in the reverse rotation direction to the first reference rotation position and then rotated in the forward rotation direction to the second reference rotation position, so that even if the rotation angle corresponding to the amount of hysteresis in the reverse rotation direction changes due to the change in the amount of hysteresis, this change is offset by the change in the rotation angle corresponding to the amount of hysteresis in the forward rotation direction. Therefore, when the stepping motor is rotated in the forward rotation direction to the second reference rotation position, the opening degree of the variable throttle valve becomes the opening degree corresponding to the second reference rotation position defined by the change characteristic line in the forward rotation direction without being affected by the change in the amount of hysteresis, and the opening degree does not change excessively to one side. Accordingly, even when the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means deviates from the qualified range to the side where the opening degree of the variable throttle valve becomes an appropriate value by changing the opening degree of the variable throttle valve to the other side, the stepping motor is quickly rotated between the first reference rotation position and the second reference rotation position, and then, while determining whether or not the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means is at the appropriate value, the stepping motor is repeatedly rotated in the forward rotation direction from the second reference rotation position by a predetermined unit angle, whereby the variable throttle valve can be adjusted in a relatively short time so that the air-fuel ratio of the air-fuel mixture becomes an appropriate value. That is, it is possible to prevent the adjustment of the variable throttle valve from taking a long time due to the change in the amount of hysteresis.

[0011] Further, in the present invention, the control means detects the air excess ratio of the air-fuel mixture by the air excess ratio detection means in a state where the stepping motor is rotated in the reverse direction to the first reference rotation position. When the detected air excess ratio of the air-fuel mixture does not reach the adjustment reference value, it is desirable to be configured to perform control to correct the first reference rotation position based on the change characteristic line in the reverse direction so that the air excess ratio of the air-fuel mixture reaches the adjustment reference value. According to this, when the stepping motor is rotated to the first reference rotation position, the opening degree of the variable throttle valve changes due to the change in the hysteresis amount. Even if the air excess ratio of the air-fuel mixture deviates from the adjustment reference value, by correcting the first reference rotation position, next, when the stepping motor is rotated to the first reference rotation position, the air excess ratio of the air-fuel mixture reaches the adjustment reference value, and it is possible to prevent the combustion state of the burner from deteriorating excessively.

[0012] By the way, the air excess ratio detection means can be composed of a flame rod that detects the frame current value and is inserted into the flame generated by the burner. The frame current value is maximized when the air excess ratio of the air-fuel mixture is 1.0, and decreases as the air excess ratio increases or decreases from 1.0. When the change of the opening degree of the variable throttle valve in the other direction is a change that decreases the opening degree, when the air excess ratio of the air-fuel mixture is less than 1.0 and gas-rich, when the stepping motor is rotated in the reverse direction to the first reference rotation position to decrease the opening degree of the variable throttle valve, the air excess ratio of the air-fuel mixture increases toward 1.0, and the frame current value rises. When the change of the opening degree of the variable throttle valve in the other direction is a change that increases the opening degree, when the air excess ratio of the air-fuel mixture is less than 1.0 and gas-rich, when the stepping motor is rotated in the reverse direction to the first reference rotation position to increase the opening degree of the variable throttle valve, the air excess ratio of the air-fuel mixture further decreases, and the frame current value decreases.

[0013] Therefore, when the air excess ratio detection means is constituted by a frame rod, when the control means rotates the stepping motor in the reverse direction to the first reference rotation position, if the change of the opening degree of the variable throttle valve to the other direction is a change that decreases the opening degree, when the frame current value detected by the frame rod increases, and if the change of the opening degree of the variable throttle valve to the other direction is a change that increases the opening degree, when the frame current value detected by the frame rod decreases, it is preferably configured to perform an error stop that determines that it is abnormal and stops the combustion of the burner. According to this, it is possible to perform an error stop when the air excess ratio of the air-fuel mixture is less than 1.0, which is safe.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0015] The combustion device shown in FIG. 1 is a heat source machine including a fully premixed burner 1, a combustion chamber 2 that surrounds the combustion space of the air-fuel mixture ejected from the combustion surface 1a of the burner 1, and a heat exchanger 3 disposed in the combustion chamber 2. The combustion gas generated by the combustion of the air-fuel mixture is discharged to the outside through an exhaust pipe 4 connected to the end of the combustion chamber 2 after heating the heat exchanger 3. Further, by the premixing device A according to the embodiment of the present invention, fuel gas is mixed with air, and the air-fuel mixture is supplied to the burner 1 through a fan 5 and a mixture supply passage 51 on the downstream side thereof.

[0016] The pre-mixing device A includes an air supply passage 6 upstream of the fan 5, a gas supply passage 7 for supplying fuel gas, and a controller 8 which is a control means for controlling the fan 5, a butterfly valve 62, a main valve 72, and a variable throttle valve 74 described later. The downstream end of the gas supply passage 7 is connected to a gas suction portion 61 provided in the air supply passage 6. A venturi portion 63 having a smaller diameter than the portion where the butterfly valve 62 is disposed is provided in a portion of the air supply passage 6 adjacent to the upstream side of the gas suction portion 61. A portion of the air supply passage 6 adjacent to the downstream side of the venturi portion 63 is surrounded by a cylindrical portion 64 having a larger diameter than the venturi portion 63. The downstream end of the venturi portion 63 is inserted into the upstream end of the cylindrical portion 64 with an annular gap therebetween, and the gas suction portion 61 is constituted by this gap. A gas chamber 71 communicating with the gas suction portion 61 is provided at the downstream end of the gas supply passage 7 so as to surround the cylindrical portion 64. Further, in the gas supply passage 7, a main valve 72, a zero governor 73 for regulating the secondary gas pressure to atmospheric pressure, and a variable throttle valve 74 driven by a stepping motor 74a are interposed in order from the upstream side.

[0017] The amount of fuel gas supplied through the gas suction portion 61 changes according to the differential pressure between the atmospheric pressure which is the secondary gas pressure and the negative pressure acting on the gas suction portion 61. Here, the negative pressure acting on the gas suction portion 61 changes according to the rotational speed of the fan 5. Therefore, the supply amount of the fuel gas changes in proportion to the rotational speed of the fan 5, that is, the supply amount of air. Further, the ratio between the supply amount of the fuel gas and the supply amount of air changes according to the opening degree of the variable throttle valve 74. By setting the opening degree of the variable throttle valve 74 to a predetermined reference opening degree according to the gas type used, the air excess ratio of the air-fuel mixture becomes a predetermined appropriate value (for example, 1.3). Then, by controlling the rotational speed of the fan 5 according to the required combustion amount (the combustion amount required to discharge hot water at a set hot water temperature), an air-fuel mixture with an appropriate air excess ratio and an amount corresponding to the required combustion amount is supplied to the burner 1.

[0018] Further, in order to prevent exhaust problems due to wind intrusion into the exhaust pipe 4, that is, to ensure wind resistance performance, the lower limit rotational speed of the fan 5 cannot be set too low. And when the required combustion amount becomes less than a predetermined value corresponding to the lower limit rotational speed of the fan 5, it becomes impossible to supply an amount of air corresponding to the required combustion amount.

[0019] Therefore, in the portion of the air supply passage 6 upstream of the gas suction portion 61, a butterfly valve 62 that can be switched between a closed position shown by a solid line and an open position shown by a virtual line in FIG. 1 is arranged to switch the ventilation resistance of this portion in two steps of large and small. And when the required combustion amount becomes less than the above-mentioned predetermined value, the butterfly valve 62 is set to the closed position to increase the ventilation resistance of the air supply passage 6, so that an amount of air corresponding to a required combustion amount below a predetermined value can be supplied without reducing the rotational speed of the fan 5 below the lower limit rotational speed. However, just closing the butterfly valve 62 and increasing the ventilation resistance of the air supply passage 6 only increases the negative pressure in the air supply passage 6, resulting in an excessive fuel gas supply amount and the air excess ratio of the air-fuel mixture supplied to the burner 1 falling below the appropriate value. Therefore, when the required combustion amount is relatively small, the butterfly valve 62 is set to the closed position to increase the ventilation resistance of the air supply passage 6, and at the same time, the variable throttle valve 74 is throttled by a predetermined opening from the reference opening to increase the ventilation resistance of the portion of the gas supply passage 7 downstream of the zero governor 73 to a low-capacity state, so that an air-fuel mixture corresponding to a relatively small required combustion amount with an appropriate air excess ratio is supplied to the burner 1. When the required combustion amount is relatively large, the butterfly valve 62 is set to the open position to decrease the ventilation resistance of the air supply passage 6, and at the same time, the variable throttle valve 74 is opened to the reference opening to decrease the ventilation resistance of the portion of the gas supply passage 7 downstream of the zero governor 73 to a high-capacity state, so that an air-fuel mixture corresponding to a relatively large required combustion amount with an appropriate air excess ratio is supplied to the burner 1.

[0020] Also, even when the same gas type is used as the fuel gas, the calorific value (Wobbe index) of the fuel gas may vary over time. In this case, if the ratio of the fuel gas supply amount to the air supply amount is constant, the air excess ratio of the air-fuel mixture will vary due to the variation in the calorific value of the fuel gas, resulting in combustion problems.

[0021] Therefore, an air-fuel ratio detection means for detecting the air-fuel ratio of the air-fuel mixture is provided. In the present embodiment, the air-fuel ratio detection means is constituted by a frame rod 9 that detects a frame current value and is inserted into the flame generated by the burner 1. As shown in FIG. 2, the frame current value becomes maximum when the air-fuel ratio of the air-fuel mixture is 1.0, and decreases as the air-fuel ratio increases or decreases from 1.0. Therefore, the air-fuel ratio of the air-fuel mixture can be detected based on the frame current value detected by the frame rod 9. Then, a detection signal of the air-fuel ratio is input to the controller 8, and control is performed to adjust the opening degree of the variable throttle valve 74 by the controller 8 so that the detected air-fuel ratio becomes an appropriate value.

[0022] Hereinafter, the control for adjusting the opening degree of the variable throttle valve 74 will be described. Before that, regarding the change characteristics representing the relationship between the rotational position of the stepping motor 74a that drives the variable throttle valve 74 and the opening degree of the variable throttle valve 74, the rotational direction of the stepping motor 74a that changes the opening degree of the variable throttle valve 74 in the increasing direction is defined as the forward rotation direction, and the rotational direction of the stepping motor 74a that changes the opening degree of the variable throttle valve 74 in the decreasing direction is defined as the reverse rotation direction for explanation. The change characteristic line in the forward rotation direction representing the relationship between the rotational position of the stepping motor 74a and the opening degree of the variable throttle valve 74 when the stepping motor 74a is rotated in the forward rotation direction is the line indicated by La in FIG. 3, and the change characteristic line in the reverse rotation direction representing the relationship between the rotational position of the stepping motor 74a and the opening degree of the variable throttle valve 74 when the stepping motor 74a is rotated in the reverse rotation direction is the line indicated by Lb in FIG. 3 that is shifted in the reverse rotation direction by the amount of hysteresis hs due to the backlash of the drive mechanism between the stepping motor 74a and the valve body of the variable throttle valve 74 with respect to the change characteristic line in the forward rotation direction. Also, the air-fuel ratio of the air-fuel mixture decreases as the opening degree of the variable throttle valve 74 increases, and increases as the opening degree of the variable throttle valve 74 decreases. In the example shown in FIG. 3, the opening degree of the variable throttle valve 74 when the air-fuel ratio of the air-fuel mixture becomes the appropriate value λm is θm. Note that the numerical values for specifying the change characteristic lines La and Lb including the slopes of the change characteristic lines La and Lb in the forward rotation direction and the reverse rotation direction are stored in the controller 8.

[0023] Referring to FIG. 4, the control of the opening adjustment of the variable throttle valve 74 will be described. In this control, first, in STEP1, it is determined whether or not the air excess ratio of the air-fuel mixture (hereinafter referred to as the detected air excess ratio) λs detected based on the detected frame current value by the frame rod 9 falls within a predetermined acceptable range including the appropriate value λm, that is, whether λml ≤ λs ≤ λmu, where λml and λmu are the lower and upper limits of the acceptable range, respectively. If it is outside the acceptable range, proceed to STEP2 and determine whether the detected air excess ratio λs is a value at which the opening of the variable throttle valve 74 changes in the decreasing direction to become the appropriate value λm, that is, whether it is below the lower limit λml of the acceptable range. Then, if the current rotational position of the stepping motor 74a and the opening of the variable throttle valve 74 are Pnow and θnow in FIG. 3, respectively, and the detected air excess ratio λs is below the lower limit λml of the acceptable range, proceed to STEP3.

[0024] In STEP3, based on the change characteristic line La in the forward rotation direction, the target rotational position Pm at which the air excess ratio of the air-fuel mixture is assumed to become the appropriate value λm is calculated by the following formula: Pm = Pnow - (λm - λs) / slope of the change characteristic line And a second reference rotational position P2 is set at a position a predetermined amount before the target rotational position Pm in the forward rotation direction, for example, at a position 10% before the rotational angle from the fully closed position to the target rotational position Pm. In addition, a predetermined first reference rotational position P1 at which the adjustment reference value λk (for example, 1.45) at which the air excess ratio of the air-fuel mixture deviates by a certain amount in the increasing direction, that is, the change direction when the opening of the variable throttle valve 74 changes in the decreasing direction from the acceptable range, is assumed to be reached is preset based on the change characteristic line Lb in the reverse rotation direction.

[0025] When the processing in STEP3 is completed, proceed to STEP4 and start high-speed rotation of the stepping motor 74a in the reverse direction. Next, in STEP5, determine whether the frame current value detected by the frame rod 9 has decreased. If the frame current value has decreased, in STEP6, determine whether the rotational position of the stepping motor 74a has reached the first reference rotational position P1, and repeat the processing of STEP4 to 6 until the first reference rotational position P1 is reached. Here, when the air excess ratio of the air-fuel mixture is less than 1.0 and gas-rich, when the stepping motor 74a is rotated in the reverse direction to the first reference rotational position P1 to decrease the opening degree of the variable throttle valve 74, the air excess ratio of the air-fuel mixture increases toward 1.0, and the frame current value rises. In this case, in STEP5, if it is determined that the frame current value has increased, proceed to STEP7, determine that it is abnormal, and an error stop is performed to stop the combustion of the burner. Therefore, it is possible to prevent the combustion from continuing in a state where the air excess ratio of the air-fuel mixture is less than 1.0, which is safe.

[0026] When the stepping motor 74a is rotated at high speed in the reverse direction to the first reference rotational position P1, proceed to STEP8 and determine whether the detected air excess ratio λs has reached the above adjustment reference value λk. If λs≠λk, proceed to STEP9, and based on the change characteristic line Lb in the reverse direction, adjust the first reference rotational position P1 so that the air excess ratio of the air-fuel mixture becomes the adjustment reference value λk according to the following formula: P1 = current P1 - (λk - λs) / slope of the change characteristic line Then, proceed to STEP10. If λs = λk, proceed directly from STEP8 to STEP10.

[0027] Here, when the stepping motor 74a is rotated in the reverse direction from the Pnow position to the first reference rotation position P1, the opening degree of the variable throttle valve 74 does not change until the rotation angle from the start of rotation reaches the hysteresis amount hs. The hysteresis amount hs has individual differences and further changes over time. Therefore, the opening degree θ1 of the variable throttle valve 74 when the rotation position of the stepping motor 74a reaches the first reference rotation position P1 changes due to the change in the hysteresis amount, and the air excess ratio of the air-fuel mixture deviates from the adjustment reference value λk. By correcting the first reference rotation position P1 as described above, next, when the stepping motor 74a is rotated to the first reference rotation position P1, the air excess ratio of the air-fuel mixture becomes the adjustment reference value λk, and it is possible to prevent the combustion state of the burner 1 from deteriorating excessively.

[0028] In STEP10, the high-speed forward rotation of the stepping motor 74a in the forward rotation direction from the first reference rotation position P1 is started. Then, the high-speed forward rotation of the stepping motor 74a in the forward rotation direction is continued until it is determined in STEP11 that the rotation position of the stepping motor 74a has reached the second reference rotation position P2. Next, proceed to STEP12, rotate the stepping motor 74a in the forward rotation direction by a predetermined unit angle, and then proceed to STEP13 to determine whether the detected air excess ratio λs has become the appropriate value λm. Then, the processes of STEP12 and 13 are repeated until λs = λm. Also, when it is determined in STEP2 that the detected air excess ratio λs exceeds the upper limit λmu of the pass range, the processes of STEP12 and 13 are repeated until λs = λm.

[0029] Here, when the stepping motor 74a is rotated in the reverse direction to the first rotation reference position P1, even if the rotation angle corresponding to the hysteresis amount hs in the reverse direction changes due to the change in the hysteresis amount hs, this change is canceled out by the change in the rotation angle corresponding to the hysteresis amount hs in the forward direction when rotating in the forward direction to the second reference rotation position P2. Therefore, when the stepping motor 74a is rotated in the forward direction to the second reference rotation position P2, the opening degree of the variable throttle valve 74 becomes the opening degree θ2 corresponding to the second reference rotation position P2 defined by the forward change characteristic line La without being affected by the change in the hysteresis amount hs. Therefore, the opening degree of the variable throttle valve 74 does not change excessively in the increasing direction. Thereafter, while determining whether or not the detected air excess ratio λs has reached the appropriate value λm, the stepping motor 74a is repeatedly rotated in the forward direction by a predetermined unit angle from the second reference rotation position P2, so that the variable throttle valve 74 can be adjusted without taking much time so that the air excess ratio of the air-fuel mixture becomes the appropriate value λm. That is, it is possible to prevent the adjustment of the variable throttle valve 74 from taking time due to the change in the hysteresis amount hs.

[0030] As described above, the embodiments of the present invention have been described with reference to the drawings, but the present invention is not limited thereto. For example, the forward rotation direction and the reverse rotation direction of the stepping motor 74a may be, contrary to the above-described embodiment, the rotation direction for changing the opening degree of the variable throttle valve 74 in the decreasing direction and the rotation direction for changing it in the increasing direction, respectively. In this case, when the detected air excess ratio λs is out of the acceptable range and has a value such that the opening degree of the variable throttle valve 74 becomes the appropriate value λm by changing in the decreasing direction, that is, when it is below the lower limit λml of the acceptable range, proceed to STEP12, and when it exceeds the upper limit λmu of the acceptable range, proceed to STEP3. Also, in STEP5, it is determined whether or not the frame current value has increased, and when the frame current value has decreased, proceed to STEP7 to stop with an error.

[0031] Further, it is also possible to configure the air excess ratio detection means with something other than the frame rod 9 of the above embodiment. That is, since the flame approaches or moves away from the combustion surface 1a according to the air excess ratio of the air-fuel mixture, the temperature on the back surface of the combustion surface 1a changes according to the air excess ratio of the air-fuel mixture. Therefore, it is also possible to configure the air excess ratio detection means with a temperature sensor that detects the temperature on the back surface of the combustion surface 1a. Also, in the above embodiment, the butterfly valve 62 is provided in the portion of the air supply passage 6 upstream of the gas suction portion 61, but it is also possible to omit the butterfly valve.

Explanation of Signs

[0032] A... Premixing device, 1... Burner, 5... Fan, 6... Air supply passage, 61... Gas suction portion, 7... Gas supply passage, 73... Zero governor, 74... Variable throttle valve, 74a... Stepping motor, 8... Controller (control means), 9... Frame rod (air excess ratio detection means), La... Change characteristic line in the forward rotation direction, Lb... Change characteristic line in the reverse rotation direction, λm... Appropriate value of air excess ratio, λk... Adjustment reference value, P1... First reference rotation position, P2... Second reference rotation position, Pm... Target rotation position.

Claims

Claim 1 A premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising: an air supply passage upstream of the fan; a gas supply passage having a downstream end connected to a gas suction portion provided in the air supply passage; a zero governor interposed in the gas supply passage for regulating the secondary gas pressure to atmospheric pressure; a variable throttle valve driven by a stepping motor interposed in a portion of the gas supply passage downstream of the zero governor; an air excess ratio detecting means for detecting the air excess ratio of the mixture; and a control means, The control means is configured to perform control for varying the rotational speed of the fan according to the required combustion amount, and to perform control for adjusting the opening degree of the variable throttle valve so that the air excess ratio of the mixture detected by the air excess ratio detecting means becomes a predetermined appropriate value, The rotational direction of the stepping motor that changes the opening degree of the variable throttle valve in one of increase and decrease is defined as the forward rotation direction, and the rotational direction of the stepping motor that changes the opening degree of the variable throttle valve in the other of increase and decrease is defined as the reverse rotation direction, When the air-fuel ratio detected by the air-fuel ratio detection means is outside the predetermined acceptable range including the appropriate value and becomes a value such that the opening degree of the variable throttle valve is changed to the other side to reach the appropriate value, the control means rotates the stepping motor in the forward rotation direction by a predetermined unit angle, and determines whether or not the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means has reached the appropriate value, and repeats the control until the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means reaches the appropriate value. When the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means is outside the acceptable range and becomes a value such that the opening degree of the variable throttle valve is changed to the one side to reach the appropriate value, based on the reverse-direction change characteristic line representing the relationship between the rotation position of the stepping motor when the stepping motor is rotated in the reverse rotation direction and the opening degree of the variable throttle valve, the stepping motor is rotated in the reverse rotation direction until it reaches a predetermined first reference rotation position that is assumed to be an adjustment reference value that deviates by a certain amount in the changing direction when the air-fuel ratio of the air-fuel mixture is changed from the acceptable range to the other side of the opening degree of the variable throttle valve. Next, based on the forward-direction change characteristic line representing the relationship between the rotation position of the stepping motor when the stepping motor is rotated in the forward rotation direction and the opening degree of the variable throttle valve, the stepping motor is rotated in the forward rotation direction from the first reference rotation position to a second reference rotation position that is a predetermined angle before the target rotation position where the air-fuel ratio of the air-fuel mixture is assumed to reach the appropriate value. Thereafter, the control means rotates the stepping motor in the forward rotation direction by a predetermined unit angle, and determines whether or not the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means has reached the appropriate value, and repeats the control until the air-fuel ratio of the air-fuel mixture detected by the air-fuel ratio detection means reaches the appropriate value. A premixing device characterized by being configured as described above.

2. The control means detects the air-fuel ratio of the air-fuel mixture by the air-fuel ratio detection means while the stepping motor is rotated in the reverse rotation direction to the first reference rotation position. When the detected air-fuel ratio of the air-fuel mixture does not reach the adjustment reference value, the control means is configured to perform control to correct the first reference rotation position based on the reverse-direction change characteristic line so that the air-fuel ratio of the air-fuel mixture reaches the adjustment reference value. The premixing device according to claim 1, characterized in that.

3. The premixing device according to claim 1 or 2, wherein the air excess ratio detection means is composed of a flame rod that is inserted into the flame generated by the burner and detects a frame current value. When the control means rotates the stepping motor in the reverse direction to the first reference rotation position, if the change of the opening degree of the variable throttle valve to the other side is a change that decreases the opening degree, when the frame current value detected by the flame rod increases, and if the change of the opening degree of the variable throttle valve to the other side is a change that increases the opening degree, when the frame current value detected by the flame rod decreases, it is configured to perform an error stop that determines that it is abnormal and stops the combustion of the burner. The premixing device is characterized by this.

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