Premixing device

Abstract

To provide a premixing device comprising a variable throttle valve 74 which is driven by a zero governor and a stepping motor 743 provided in a gas supply side and detection means 747 which detects movement of a valve body 742 of the variable throttle valve in which the detection means comprises a detection switch 7471 which is turned on when the valve body of the variable throttle valve enters a first stroke range at a closing side and is turned off when the valve body enters a second stroke range at an opening side in, so as to accurately detect a failure of the variable throttle valve.SOLUTION: After an origin is determined by moving a valve body 742 of a variable throttle valve 74 to a stroke end position in a closing direction, the valve body 742 is moved in an opening direction and an abnormality is determined in a case where the number of steps of a stepping motor 743 at a time point when a detection switch 7471 is switched from ON to OFF by the movement is deviated by a predetermined value or more from a reference step number required for moving from an origin position to entering a second stroke range.SELECTED DRAWING: Figure 2

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 technology]

[0002] Conventionally, as a premixing device of this kind, Patent Document 1 discloses a device including a zero governor that adjusts the secondary gas pressure to atmospheric pressure and is disposed in a gas supply passage whose downstream end is connected to a gas suction section disposed in a portion of the air supply passage upstream of the fan, a variable throttle valve that is driven by a stepping motor and disposed in a portion of the gas supply passage downstream of the zero governor, and a control means that controls the fan and the variable throttle valve. Here, the amount of fuel gas supplied varies depending on the pressure difference between the atmospheric pressure, which is the secondary gas pressure, and the negative pressure in the air supply passage. Since the negative pressure in the air supply passage varies depending on the fan rotation speed, the amount of fuel gas supplied varies in proportion to the fan rotation speed, i.e., the amount of air supplied. Also, the ratio of the amount of fuel gas supplied to the amount of air supplied varies depending on the opening degree of the variable throttle valve. By setting the opening degree of the variable throttle valve to a predetermined reference opening degree according to the type of gas used, the excess air ratio of the mixture becomes a predetermined appropriate value.

[0003] Furthermore, in the device described in Patent Document 1, the variable throttle valve is provided with detection means for detecting movement of the valve disc of the variable throttle valve. The detection means has a detection switch that turns on when the valve disc of the variable throttle valve moves in the closing direction and enters a first stroke range between a predetermined first position and a stroke end position in the closing direction, and turns off when the valve disc of the variable throttle valve moves in the opening direction and enters a second stroke range between a predetermined second position displaced in the opening direction from the first position and the stroke end position in the opening direction. The control means is configured to, during pre-burner operation in which the fan is driven without supplying fuel gas and which is performed before the burner is ignited, first perform a first operation of moving the valve element of the variable throttle valve to the stroke end position in the closing direction to return the stepping motor to its origin, and if the detection switch is not on at the completion of the first operation, determine that there is an abnormality, and if the detection switch is on, perform a second operation of moving the valve element of the variable throttle valve in the opening direction to a predetermined position within the second stroke range, and if the detection switch is not off at the completion of the second operation, determine that there is an abnormality, and perform ignition control only when the detection switch is off.

[0004] However, this does not provide sufficient accuracy in detecting failures in the variable throttle valve. Here, when the engine is not in operation and the fan is not driven, there is no need to shorten the waiting time until ignition, unlike when the engine is in pre-burner operation. Therefore, it is desirable to be able to accurately detect failures in the variable throttle valve when the engine is not in operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2022-150178 A Summary of the Invention [Problem to be solved by the invention]

[0006] In view of the above, an object of the present invention is to provide a premixing device that is capable of detecting a failure of a variable throttle valve with high accuracy when the device is not in operation and the fan is not driven. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides a premixing device which mixes fuel gas with air and supplies the mixture to a burner via a fan, the premixing device comprising: a zero governor which adjusts secondary gas pressure to atmospheric pressure and is provided in a gas supply passage whose downstream end is connected to a gas suction section provided in a portion of the air supply passage upstream of the fan; a variable throttle valve which is driven by a stepping motor and is provided in a portion of the gas supply passage downstream of the zero governor; and control means which controls the fan and the variable throttle valve, the variable throttle valve being provided with detection means which detects movement of a valve disc of the variable throttle valve, the detection means being turned on when the valve disc of the variable throttle valve moves in the closing direction and enters a first stroke range between a predetermined first position and a stroke end position in the closing direction, and a control means which detects movement of the valve disc of the variable throttle valve when the valve disc of the variable throttle valve moves in the opening direction and enters a first stroke range between a predetermined first position and a stroke end position in the closing direction. and a detection switch that turns off when the valve enters a second stroke range between a predetermined second position displaced in the opening direction from the first position and a stroke end position in the opening direction, the control means sequentially executes a first operation of moving the valve element of the variable throttle valve to a stroke end position in the closing direction to return the stepping motor to its origin and a second operation of moving the valve element of the variable throttle valve in the opening direction to a predetermined position within the second stroke range during non-operation when the fan is not being driven, and determines that an abnormality has occurred if the number of steps of the stepping motor at the time when the detection switch is switched from on to off in the second operation deviates by a predetermined value or more from a reference number of steps required to move from the origin position to the second position. Note that in the present invention, the number of steps of the stepping motor means a value obtained by counting the origin position of the stepping motor as zero, the number of rotation steps of the stepping motor in the valve opening direction as a positive number, and the number of rotation steps of the stepping motor in the valve closing direction as a negative number.

[0008] According to the present invention, if a malfunction occurs in which the valve disc of the variable throttle valve locks before the stroke end position in the closing direction, this malfunction can be detected. That is, if the valve disc of the variable throttle valve locks before the stroke end position in the closing direction during the first operation, the stepping motor is returned to the origin at this locked position. Therefore, the number of steps of the stepping motor at the time when the detection switch switches from on to off in the second operation will be a different value from the reference number of steps required to move from the origin position to the second position. Therefore, according to the present invention, if a malfunction occurs in which the valve disc of the variable throttle valve locks before the stroke end position in the closing direction, it is determined that an abnormality has occurred, and this malfunction can be detected.

[0009] In the present invention, the control means is preferably configured to sequentially execute a third operation of moving the valve element of the variable throttle valve in the closing direction until the detection switch switches from off to on, a fourth operation of moving the valve element of the variable throttle valve in the opening direction to a stroke end position in the opening direction, and a fifth operation of moving the valve element of the variable throttle valve in the closing direction until the detection switch switches from off to on, and determine that an abnormality exists if the difference between the number of steps of the stepping motor at the time of completion of the third operation and the number of steps of the stepping motor at the time of completion of the fifth operation is equal to or greater than a predetermined value. According to this, if a malfunction occurs in which the valve element of the variable throttle valve locks before the stroke end position in the opening direction, this malfunction can be detected. That is, if the valve element of the variable throttle valve locks before the stroke end position in the opening direction during the fourth operation, the valve element does not move in the opening direction thereafter, but the number of steps of the stepping motor is increased until the fourth operation is completed, when it is determined in control that the valve element has reached the stroke end position in the opening direction. Then, the number of steps of the stepping motor at the completion of the fifth action becomes greater than the number of steps of the stepping motor at the completion of the third action by the number of steps added after the valve body locks in the fourth action. Therefore, according to the above control, if a malfunction occurs in which the valve body of the variable throttle valve locks before the stroke end position in the opening direction, it is determined that an abnormality has occurred, and this malfunction can be detected.

[0010] Furthermore, in the present invention, it is preferable that the control means is configured to perform control such that it determines that an abnormality has occurred if the detection switch is off at the completion of the first action, or if the detection switch does not switch from off to on even if at least one of the third action and the fifth action continues for a predetermined time. According to this, if a movement failure of the valve body of the variable throttle valve in the closing direction occurs, it is determined that an abnormality has occurred, and this failure can be detected.

[0011] In the present invention, it is preferable that the control means is configured to perform control such that it determines that an abnormality has occurred when the detection switch is on at least one of the time points when the second action and the time point when the fourth action is completed. In this way, if a movement failure occurs in the valve body of the variable throttle valve in the opening direction, it is determined that an abnormality has occurred, and this failure can be detected. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is an explanatory diagram showing a premixing device according to an embodiment of the present invention. [Diagram 2] Cross-sectional view taken along line II-II in Figure 1. [Diagram 3] FIG. 2 is an exploded perspective view of a main portion of a variable throttle valve provided in the premixing device of the embodiment. [Figure 4] FIG. 4 is a flow diagram showing the contents of a failure detection control of the variable throttle valve by a control unit in the premixing device of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The combustion device shown in Fig. 1 is a heat source machine including a full primary combustion burner 1, a combustion housing 2 that encloses 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 housing 2. An upstream water supply pipe 31 and a downstream hot water outlet pipe 32 are connected to the heat exchanger 3. The combustion gas generated by the combustion of the air-fuel mixture heats the heat exchanger 3 and is then discharged to the outside through an exhaust pipe 4 connected to the end of the combustion housing 2. In addition, a premixing device A according to an embodiment of the present invention mixes fuel gas into air and supplies the mixture to the burner 1 through a fan 5.

[0014] The premixing device A includes an air supply passage 6 on the upstream side of the fan 5, a gas supply passage 7 for supplying fuel gas, and a controller 8 as a control means. The downstream end of the gas supply passage 7 is connected to a gas suction section 61 provided in the air supply passage 6. A venturi section 63 having a smaller diameter than a section in which a butterfly valve 62 (described later) is arranged is provided in a portion of the air supply passage 6 adjacent to the upstream side of the gas suction section 61. A portion of the air supply passage 6 adjacent to the downstream side of the venturi section 63 is surrounded by a cylindrical section 64 having a larger diameter than the venturi section 63. The downstream end of the venturi section 63 is inserted into the upstream end of the cylindrical section 64 with an annular gap, and this gap constitutes the gas suction section 61. A gas chamber 71 communicating with the gas suction section 61 is provided at the downstream end of the gas supply passage 7 so as to surround the cylindrical section 64. Further, in the gas supply passage 7, a main valve 72, a zero governor 73 that adjusts the secondary gas pressure to atmospheric pressure, and a variable throttle valve 74 are interposed in this order from the upstream side.

[0015] The amount of fuel gas supplied through the gas suction unit 61 changes according to the pressure difference between the atmospheric pressure, which is the secondary gas pressure, and the negative pressure in the air supply passage 6. Here, the negative pressure in the air supply passage 6 changes according to the rotation speed of the fan 5. Therefore, the amount of fuel gas supplied changes in proportion to the rotation speed of the fan 5, that is, the amount of air supplied. In addition, the ratio of the amount of fuel gas supplied to the amount of air supplied 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 type of gas used, the excess air ratio of the mixture becomes a predetermined appropriate value (for example, 1.3). Then, by controlling the rotation speed of the fan 5 according to the required combustion amount (the amount of combustion necessary to discharge hot water at the set hot water temperature), the mixture is supplied to the burner 1 in an amount according to the required combustion amount with an appropriate excess air ratio.

[0016] In order to prevent exhaust failure caused by wind entering the exhaust pipe 4, that is, to ensure wind resistance, the lower limit rotation speed of the fan 5 cannot be set too low. When the required combustion amount falls below a predetermined value corresponding to the lower limit rotation speed of the fan 5, it becomes impossible to supply the amount of air corresponding to the required combustion amount.

[0017] Therefore, in order to switch the ventilation resistance of the portion of the air supply passage 6 upstream of the gas suction portion 61 between large and small levels, a butterfly valve 62 is arranged which can be switched by a motor 621 (see FIG. 2) between a closed position shown by a solid line in FIG. 1 and an open position shown by a virtual line. When the required combustion amount falls below the above-mentioned predetermined value, the butterfly valve 62 is switched to the closed position to increase the ventilation resistance of the air supply passage 6, so that an amount of air corresponding to the required combustion amount below the predetermined value can be supplied without lowering the rotation speed of the fan 5 below the lower limit rotation speed.

[0018] However, simply increasing the air flow resistance of the air supply passage 6 by closing the butterfly valve 62 increases the negative pressure in the air supply passage 6, causing the amount of fuel gas supplied to be excessive, and causing the excess air ratio of the mixture supplied to the burner 1 to fall below the appropriate value. Therefore, when the required combustion amount is relatively small, the butterfly valve 62 is closed and the opening of the variable throttle valve 74 is set to a predetermined small capacity opening smaller than the reference opening, thereby switching the combustion capacity to small capacity, so that the amount of the mixture corresponding to the relatively small required combustion amount and the appropriate excess air ratio is supplied to the burner 1, and when the required combustion amount is relatively large, the butterfly valve 62 is opened and the opening of the variable throttle valve 74 is set to the large capacity opening, which is the reference opening, thereby switching the combustion capacity to large capacity, so that the amount of the mixture corresponding to the relatively large required combustion amount and the appropriate excess air ratio is supplied to the burner 1.

[0019] However, even if the same type of gas is used as the fuel gas, the calorific value of the fuel gas (Wobbe index) may vary over time. In this case, if the ratio of the fuel gas supply to the air supply is constant, the fluctuation in the calorific value of the fuel gas will cause the excess air ratio of the mixture to fluctuate, resulting in poor combustion.

[0020] Therefore, an excess air ratio detection means 9 is provided to detect the excess air ratio of the mixture. In this embodiment, the excess air ratio detection means 9 is configured with a flame rod provided facing the combustion surface 1a of the burner 1. The excess air ratio of the mixture is detected from the flame current flowing through the flame rod. Since the flame approaches or moves away from the combustion surface 1a depending on the excess air ratio of the mixture, the back surface temperature of the combustion surface 1a changes depending on the excess air ratio of the mixture. Therefore, the excess air ratio detection means 9 can also be configured with a temperature sensor that detects the back surface temperature of the combustion surface 1a.

[0021] The controller 8 performs feedback control to adjust the opening of the variable throttle valve 74 so that the excess air ratio of the mixture detected by the excess air ratio detection means 9 becomes constant, i.e., so that it is kept at a predetermined appropriate value. Specifically, when the excess air ratio of the mixture decreases due to an increase in the heat value of the fuel gas, the opening of the variable throttle valve 74 is decreased to decrease the ratio of the supply amount of fuel gas to the supply amount of air so that the excess air ratio becomes an appropriate value, and when the excess air ratio of the mixture increases due to a decrease in the heat value of the fuel gas, the opening of the variable throttle valve 74 is increased to increase the ratio of the supply amount of fuel gas to the supply amount of air so that the excess air ratio becomes an appropriate value. In this way, even if the heat value of the fuel gas fluctuates, the excess air ratio of the mixture can be kept at an appropriate value, and poor combustion can be prevented.

[0022] Next, the structure of the variable throttle valve 74 will be described. With reference to Fig. 2 and Fig. 3, the variable throttle valve 74 is electrically operated and includes a valve body 742 that moves toward and away from a valve seat 7411 in a valve housing 741 in the axial direction, a stepping motor 743, and a motion conversion mechanism that moves the valve body 742 in the axial direction by the rotation of the stepping motor 743. A primary gas chamber 7412 having a gas inlet 7412a and a secondary gas chamber 7413 having a gas outlet 7413a communicating with the gas chamber 71 are provided in the valve housing 741. A member 744 on which a valve seat 7411 that separates the primary gas chamber 7412 and the secondary gas chamber 7413 is mounted.

[0023] The motion conversion mechanism includes a cam pin 7421 fixed to the valve body 742, a guide tube 745 having an axially extending long hole 7451 formed therein with which the cam pin 7421 slidably engages, and a cylindrical cam body 746 having a spiral cam portion 7461 with which the cam pin 7421 engages through the long hole 7451. A shaft portion having a non-circular cross section of a connector 7432 connected to an output shaft 7431 of a stepping motor 743 is fitted into the end of the guide tube 745, so that the guide tube 745 rotates with the rotation of the stepping motor 743.

[0024] The valve body 742 has a needle portion 742a that can be inserted into a valve hole 7411a formed in the valve seat 7411, and also has a tube portion 7422 that is inserted into the guide tube 745. A cam pin 7421 is provided at the end of the tube portion 7422 so as to protrude radially outward. The base end of the cam pin 7421 is slidably engaged with an elongated hole 7451 formed in the guide tube 745. Therefore, the valve body 742 is connected to the guide tube 745 so as to be movable in the axial direction and to rotate together with the guide tube 745.

[0025] The cam body 746 is prevented from rotating relative to the valve housing 741. Specifically, a rib 7462 protruding from the outer peripheral surface of the cam body 746 engages with a groove 7414 formed on the inner peripheral surface of the valve housing 741, thereby preventing the cam body 746 from rotating relative to the valve housing 741. In addition, in order to reduce the number of parts, the cam body 746 is formed integrally with the member 744 on which the valve seat 7411 is formed. The cam portion 7461 provided on the cam body 746 is configured with a spiral inclined side against which the cam pin 7421 can abut. In addition, a spring member 7423 is provided to bias the cam pin 7421 toward the cam portion 7461. Then, by forward and reverse rotation of the stepping motor 743, the cam pin 7421 rotates and is guided by the cam portion 7461 to move in one axial direction and the other, and the valve body 742 moves in a closing direction approaching the valve seat 7411 and in an opening direction moving away from the valve seat 7411. Furthermore, the valve body 742 is movable between a stroke end position in the closing direction where the valve body 742 abuts against the valve seat 7411 and a stroke end position in the opening direction where a protrusion 7452 protruding from the tail end of the guide tube 745 abuts against a stopper portion (not shown) provided inside the valve housing 741, preventing further reverse rotation of the guide tube 745 (rotation in a direction that would move the valve body 742 in the opening direction).

[0026] The variable throttle valve 74 is also provided with a detection means 747 for detecting the movement of the valve body 742 of the variable throttle valve 74. The detection means 747 is composed of a detection switch 7471 consisting of a reed switch provided at a position facing the needle portion 742a on the outer surface of the secondary gas chamber 7413, and a magnet 7472 built into the needle portion 742a. ​​The detection switch 7471 is turned on by receiving a magnetic force from the magnet 7472 when the valve body 742 of the variable throttle valve 74 moves in the closing direction and enters a first stroke range between a predetermined first position and a stroke end position in the closing direction, and is turned off when the valve body 742 moves in the opening direction and enters a second stroke range between a predetermined second position displaced in the opening direction from the first position and the stroke end position in the opening direction.

[0027] During non-operation when the fan 5 is not driven, for example, after the burner 1 is turned off and after-purging is performed, the controller 8 performs the control shown in FIG. 4 to detect a failure of the variable throttle valve 74. In this control, first, in STEP 1, the first operation is performed to move the valve body 742 of the variable throttle valve 74 to the stroke end position in the closing direction to return the stepping motor 743 to the original position. Next, in STEP 2, it is determined whether the detection switch 7471 is on or not, and if it is on, the process proceeds to STEP 3 to start the second operation to move the valve body 742 in the opening direction to a predetermined position within the second stroke range. After starting the second operation, it is determined in STEP 4 whether the detection switch 7471 is off or not, and if it is off, the number of steps NS1 of the stepping motor 743 at the time when the detection switch 7471 is switched from on to off is stored in STEP 5. Next, in STEP 6, it is determined whether or not the valve body 742 has reached a predetermined position within the second stroke range based on the number of steps from the origin position of the stepping motor 743, and if it is determined that the valve body 742 has reached a predetermined position within the second stroke range, the second operation is completed.

[0028] When the second operation is completed, in STEP 7, it is determined whether the detection switch 7471 is off. If it is off, the process proceeds to STEP 8, where it is determined whether the difference between the reference number of steps NS0 required to move from the origin position to the second position and the above-mentioned number of steps NS1 is less than a predetermined value (for example, 200 steps). Here, if the valve body 742 locks before the stroke end position in the closing direction during the first operation, the stepping motor 743 is returned to the origin at this locked position. Therefore, the number of steps NS1 of the stepping motor 743 at the time when the detection switch 7471 switches from on to off in the second operation is a value different from the above-mentioned reference number of steps NS0. Therefore, if a malfunction occurs in which the valve body 742 locks at a position some distance before the stroke end position in the closing direction, the result in STEP 8 is determined as "NO" and the process proceeds to STEP 20, where it is determined that an abnormality has occurred. Therefore, this malfunction can be detected.

[0029] If the answer in STEP 8 is "YES", the process proceeds to STEP 9, where the third operation is started to move the valve body 742 in the closing direction until the detection switch 7471 switches from off to on. When the third operation is started, in STEP 10, it is determined whether the detection switch 7471 is on or not, and if it is off, in STEP 11, it is determined whether a predetermined time (e.g., 7 seconds) has elapsed since the start of the third operation, and the process returns to STEP 9 to continue the third operation until the predetermined time has elapsed. If the detection switch 7471 is on, the process proceeds to STEP 12, where the number of steps NS2 of the stepping motor 743 at the time when the detection switch 7471 switches from off to on, i.e., at the time when the third operation is completed, is stored.

[0030] Next, the process proceeds to STEP 13, and the fourth operation is performed to move the valve body 742 in the opening direction to the stroke end position in the opening direction. After that, in STEP 14, it is determined whether the detection switch 7471 is off at the time of completion of the fourth operation. If it is off, the process proceeds to STEP 15, and the fifth operation is started to move the valve body 742 in the closing direction until the detection switch 7471 switches from off to on. When the fifth operation is started, it is determined in STEP 16 whether the detection switch 7471 is on, and if it is off, it is determined in STEP 17 whether a predetermined time has elapsed since the start of the fifth operation, and the process returns to STEP 15 to continue the fifth operation until the predetermined time has elapsed. If the detection switch 7471 is on, the process proceeds to STEP 18, and the number of steps NS3 of the stepping motor 743 at the time when the detection switch 7471 switches from off to on, that is, at the time when the fifth operation is completed, is stored.

[0031] Next, in STEP 19, it is determined whether or not the difference between the number of steps NS2 of the stepping motor 743 at the completion of the third operation and the number of steps NS3 of the stepping motor 743 at the completion of the fifth operation is less than a predetermined value. Here, if the valve body 742 locks before the stroke end position in the opening direction during the fourth operation, the valve body 742 does not move in the opening direction thereafter, but the number of steps of the stepping motor 743 is added until the fourth operation is completed when it is determined in terms of control that the valve body 742 has reached the stroke end position in the opening direction. Then, the number of steps NS3 of the stepping motor 743 at the completion of the fifth operation becomes larger than the number of steps NS2 of the stepping motor 743 at the completion of the third operation by the number of steps added after the valve body 742 locks in the fourth operation. Therefore, if a failure occurs in which the valve body 742 locks before the stroke end position in the opening direction, the determination in STEP 19 is "NO" and the process proceeds to STEP 20, where it is determined that there is an abnormality. Therefore, this failure can also be detected.

[0032] It is also possible to omit the fourth and fifth operations, move the valve body 742 to the stroke end position in the opening direction in the second operation, and then perform the third operation. When the difference between the number of steps of the stepping motor 743 at the time when the detection switch 7471 is switched from off to on in the third operation and the reference number of steps when the valve body 742 reaches the first position is equal to or greater than a predetermined value, it is determined that there is an abnormality. However, the detection switch 7471 made of a reed switch is temperature dependent, and the position at which it switches from on to off, i.e., the first position, changes depending on the temperature. Therefore, in the above method in which the fourth and fifth operations are omitted, even if the valve body 742 does not have a malfunction of locking before the stroke end position in the opening direction, the difference between the number of steps of the stepping motor 743 at the time when the detection switch 7471 is switched from off to on and the reference number of steps when the valve body 742 reaches the first position may become equal to or greater than a predetermined value due to the change in the first position depending on the temperature, and it may be erroneously determined that there is an abnormality. In contrast, in this embodiment, the presence or absence of an abnormality is determined by comparing the number of steps NS2 of the stepping motor 743 at the time when the detection switch 7471 is switched from off to on in the third operation with the number of steps NS2 of the stepping motor 743 at the time when the detection switch 7471 is switched from off to on in the fifth operation, so that it is possible to prevent erroneous determination due to a change in the first position according to temperature. Note that, in order to shorten the time required for the second operation, it is desirable to set the predetermined position, which is the position reached by the valve body 742 in the second operation, as close as possible to the second position within a range where the detection switch 7471 is reliably turned off.

[0033] Also, if the detection switch 7471 is off at the time of completion of the first operation and the result of STEP 2 is "NO", or if the detection switch 7471 does not switch from off to on even after the third or fifth operation is continued for a predetermined time and the result of STEP 11 or STEP 17 is "YES", the process proceeds to STEP 20 and a determination is made that there is an abnormality. Therefore, even if a movement failure of the valve body 742 in the closing direction occurs, this failure can be detected. Furthermore, if the detection switch 7471 is on at the time of completion of the second or fourth operation and the result of STEP 7 or STEP 14 is "NO", the process proceeds to STEP 20 and a determination is made that there is an abnormality. Therefore, even if a movement failure of the valve body 742 in the opening direction occurs, this failure can be detected.

[0034] Although the embodiment of the present invention has been described above with reference to the drawings, the present invention is not limited thereto. For example, in the above embodiment, the detection switch 7471 is configured with a reed switch, but the detection switch can also be configured with a magnetic detection switch using a Hall element or an MR element, or a microswitch that cooperates with a dog that displaces in conjunction with the valve body 742. [Explanation of symbols]

[0035] A...premixing device, 1...burner, 5...fan, 6...air supply passage, 61...gas suction section, 7...gas supply passage, 73...zero governor, 74...variable throttle valve, 742...valve body, 743...stepping motor, 747...detection means, 7471...detection switch, 8...controller (control means).

Claims

1. A premixing device which mixes fuel gas into air and supplies the mixture to a burner via a fan, comprising: a zero governor which is provided in a gas supply passage whose downstream end is connected to a gas suction section provided in a portion of the air supply passage upstream of the fan, and which adjusts secondary gas pressure to atmospheric pressure; a variable throttle valve which is driven by a stepping motor and which is provided in a portion of the gas supply passage downstream of the zero governor; and control means which controls the fan and the variable throttle valve; A variable throttle valve is provided with detection means for detecting movement of the valve disc of the variable throttle valve, the detection means having a detection switch which turns on when the valve disc of the variable throttle valve moves in the closing direction and enters a first stroke range between a predetermined first position and a stroke end position in the closing direction, and turns off when the valve disc of the variable throttle valve moves in the opening direction and enters a second stroke range between a predetermined second position displaced in the opening direction from the first position and the stroke end position in the opening direction, The control means is configured to sequentially execute, during non-operational time when the fan is not being driven, a first operation of moving the valve element of the variable throttle valve to a stroke end position in the closing direction to return the stepping motor to its origin, and a second operation of moving the valve element of the variable throttle valve in the opening direction to a predetermined position within the second stroke range, and to perform control to determine that an abnormality has occurred if the number of steps of the stepping motor at the time when the detection switch is switched from on to off in the second operation deviates by more than a predetermined value from a reference number of steps required to move from the origin position to the second position.

2. 2. The premixing device according to claim 1, wherein the control means sequentially executes, after the second operation, a third operation of moving the valve element of the variable throttle valve in the closing direction until the detection switch is switched from off to on, a fourth operation of moving the valve element of the variable throttle valve in the opening direction to a stroke end position in the opening direction, and a fifth operation of moving the valve element of the variable throttle valve in the closing direction until the detection switch is switched from off to on, and determines that an abnormality has occurred when a difference between the number of steps of the stepping motor at the completion of the third operation and the number of steps of the stepping motor at the completion of the fifth operation is equal to or greater than a predetermined value.

3. The premixing device according to claim 2, characterized in that the control means is configured to perform control to determine that an abnormality has occurred if the detection switch is off at the time when the first operation is completed, or if the detection switch does not switch from off to on even after at least one of the third operation and the fifth operation is continued for a predetermined time.

4. The premixing device according to claim 2 or 3, characterized in that the control means is configured to perform control such that it determines that an abnormality has occurred when the detection switch is on at least one of the time points at which the second operation is completed and the time points at which the fourth operation is completed.

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