Premixing device

Abstract

To enable a premixing device which mixes air and fuel gas and supplies a burner 1 with the fuel-air mixture via a fan 5, to suppress fuel gas release when a main valve 72 is in a faulty state of being unable to open during a post purge operation, the premixing device comprising a gas supply passage 7 whose downstream end is connected to a gas suction unit 61 provided on an air supply passage 6 disposed upstream of the fan 5, and the main valve 72, a zero governor 73 and a variable throttle valve 74 attached thereto in that order from the upstream side, the premixing device carrying out control for regulating the opening of the variable throttle valve 74 so that the air excess ratio of the fuel-gas mixture calculated from the flame current detected by a flame rod 9 exposed to the flame of the burner 1, becomes a suitable one.SOLUTION: Determining that a main valve 72 is in a faulty state of being unable to open when a flame rod 9 detects a flame current during a process of carrying out a post purge operation, the opening of a variable throttle valve 74 is controlled to attain the minimum opening which is set so as to extinguish the burner 1.SELECTED DRAWING: Figure 1

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, a known premixing device of this type includes an air supply passage upstream of a fan, a gas supply passage whose downstream end is connected to a gas suction port provided in the air supply passage, a main valve, and a zero governor that adjusts secondary gas pressure to atmospheric pressure, which are disposed in the gas supply passage from upstream to downstream (see, for example, Patent Document 1). The amount of fuel gas supplied varies depending on the pressure difference between atmospheric pressure, which is the secondary gas pressure, and the negative pressure acting on the gas suction port. Because the negative pressure acting on the gas suction port 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. Therefore, by controlling the fan rotation speed according to the required combustion amount, an amount of air-fuel mixture corresponding to the required combustion amount is supplied to the burner, and the excess air ratio of the air-fuel mixture (primary air amount / stoichiometric air amount) remains constant.

[0003] However, even if the same type of gas is used as the fuel gas, the calorific value (Wobbe index) of the fuel gas may vary over time. In the above-mentioned conventional example, even if the calorific value of the fuel gas fluctuates, the ratio of the fuel gas supply rate to the air supply rate remains constant, so fluctuations in the calorific value of the fuel gas cause fluctuations in the excess air ratio of the air-fuel mixture, resulting in poor combustion.

[0004] Therefore, a premixing device has been known which includes a variable throttle valve disposed in a portion of the gas supply passage downstream of a zero governor, a flame rod exposed to the burner flame, and a control means, and which controls the opening of the variable throttle valve using the control means so that the excess air ratio of the air-fuel mixture calculated based on the flame current detected by the flame rod becomes a predetermined value (see, for example, Patent Document 2).

[0005] In the above-described conventional premixing device, even if an open valve failure (a failure that causes the valve to remain open) occurs, fuel gas is not drawn in unless the fan is driven, so fuel gas is not released when the fan is stopped. However, when a combustion stop command is issued and post-purge operation is performed in which the fan is driven with the main valve closed, fuel gas will be released if an open valve failure occurs in the main valve. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-179447 [Patent Document 2] Patent Publication No. 2021-25722 Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above, an object of the present invention is to provide a premixing device that can suppress the release of fuel gas during post-purge operation when an opening failure of the main valve occurs. [Means for solving the problem]

[0008] 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, and that includes an air supply passage upstream of the fan, a gas supply passage whose downstream end is connected to a gas suction section provided in the air supply passage, a main valve, a zero governor that adjusts secondary gas pressure to atmospheric pressure, and a variable throttle valve, which are provided in this order from the upstream side in the gas supply passage, and also includes a flame rod exposed to the flame of the burner, and control means, and the control means controls the excess air ratio of the mixture calculated based on a flame current detected by the flame rod. In a configuration configured to perform control to adjust the opening of the variable throttle valve to an appropriate value, the control means is characterized in that when a combustion stop command is issued and post-purge operation is performed in which the fan is driven with the main valve closed, if the flame rod does not detect a flame current, the control means maintains the opening of the variable throttle valve at the opening immediately before the combustion stop command was issued, but when the flame rod detects a flame current, it determines that an opening failure of the main valve has occurred and controls the opening of the variable throttle valve to the minimum opening set so that the burner is extinguished.

[0009] During post-purge operation when the main valve has an open-circuit malfunction, fuel gas is sucked in and combustion in the burner continues, causing the flame rod to detect a flame current. According to the present invention, if the flame rod detects a flame current during post-purge operation, the variable throttle valve is throttled to its minimum opening, extinguishing the burner and suppressing the release of fuel gas. Furthermore, according to the present invention, if the main valve does not have an open-circuit malfunction and the flame rod does not detect a flame current during post-purge operation, the variable throttle valve is maintained at the opening just before the combustion stop command was issued. Therefore, at the next ignition, it is not necessary to adjust the variable throttle valve opening based on the excess air ratio of the air-fuel mixture, enabling a quick ignition operation. Furthermore, the variable throttle valve and the flame rod, which are provided to maintain the excess air ratio of the air-fuel mixture at an appropriate value, can be used as a means for preventing gas release in the event of a main valve open-circuit malfunction, eliminating the need for additional components and providing cost advantages.

[0010] In the present invention, the minimum opening of the variable throttle valve is preferably a fully closed opening that shuts off the flow of fuel gas, thereby reliably preventing the release of fuel gas during post-purge operation in the event of an open valve failure in the main valve. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an explanatory diagram showing a premixing device according to an embodiment of the present invention and a combustion device equipped with a burner to which an air-fuel mixture is supplied from the premixing device; [Figure 2] FIG. 4 is a flowchart showing the control executed by the control unit of the premixing device according to the embodiment when a combustion stop command is issued. DETAILED DESCRIPTION OF THE INVENTION

[0012] The combustion device shown in Fig. 1 is a heat source machine comprising a full primary combustion burner 1, a combustion housing 2 that encloses the combustion space for the air-fuel mixture ejected from the combustion surface 1a of the burner 1, and a heat exchanger 3 disposed within the combustion housing 2. The combustion gas generated by the combustion of the air-fuel mixture heats the heat exchanger 3 and is then discharged to the outside via 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 the mixture is supplied to the burner 1 via a fan 5 and a mixture supply passage 51 downstream of the fan.

[0013] The premixing 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 serving as 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 section 61 provided in the air supply passage 6. A venturi section 63 having a smaller diameter than the section in which 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 section 61. A cylindrical section 64 having a larger diameter than the venturi section 63 is enclosed in a portion of the air supply passage 6 adjacent to the downstream side of 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 therebetween, and this gap forms 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.

[0014] The amount of fuel gas supplied through the gas suction port 61 varies depending on the pressure difference between atmospheric pressure, which is the secondary gas pressure, and the negative pressure acting on the gas suction port 61. Here, the negative pressure acting on the gas suction port 61 varies depending on the rotation speed of the fan 5. Therefore, the amount of fuel gas supplied varies in proportion to the rotation speed of the fan 5, i.e., the amount of air supplied. The ratio of the amount of fuel gas supplied to the amount of air supplied varies depending on the opening of the variable throttle valve 74 (the amount of gap between the valve body and the valve seat, not shown). By setting the opening of the variable throttle valve 74 to a predetermined standard opening depending on the type of gas used, the excess air ratio of the air-fuel mixture becomes a predetermined appropriate value (e.g., 1.3). Then, by controlling the rotation speed of the fan 5 depending on the required combustion amount (the amount of combustion required to dispense hot water at a set temperature), the amount of air-fuel mixture corresponding to the required combustion amount at the appropriate excess air ratio is supplied to the burner 1.

[0015] In order to prevent poor exhaust due to wind entering the exhaust stack 4, i.e., to ensure wind resistance, the lower limit rotation speed of the fan 5 cannot be set too low. If the required combustion rate falls below a predetermined value corresponding to the lower limit rotation speed of the fan 5, it will no longer be possible to supply the amount of air corresponding to the required combustion rate.

[0016] Therefore, a butterfly valve 62 that can be switched between a closed position shown by solid lines in FIG. 1 and an open position shown by phantom lines is provided in the portion of the air supply passage 6 upstream of the gas suction section 61 to switch the air flow resistance of that portion between large and small. When the required combustion amount falls below the predetermined value, the butterfly valve 62 is switched to the closed position to increase the air flow 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. However, simply switching the butterfly valve 62 to the closed position to increase the air flow resistance of the air supply passage 6 increases the negative pressure in the air supply passage 6, causing an excessive amount of fuel gas to be supplied, 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 to increase the air flow resistance of the air supply passage 6, and the variable throttle valve 74 is narrowed from the standard opening by a predetermined opening, thereby increasing the air flow resistance of the portion of the gas supply passage 7 downstream of the zero governor 73, establishing a small capacity state in which the excess air ratio is at an appropriate value and an amount of mixture corresponding to the relatively small required combustion amount is supplied to the burner 1. When the required combustion amount is relatively large, the butterfly valve 62 is opened to decrease the air flow resistance of the air supply passage 6, and the variable throttle valve 74 is opened to the standard opening, thereby decreasing the air flow resistance of the portion of the gas supply passage 7 downstream of the zero governor 73, establishing a large capacity state in which the excess air ratio is at an appropriate value and an amount of mixture corresponding to the relatively large required combustion amount is supplied to the burner 1.

[0017] Furthermore, even if the same type of fuel gas is used, 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.

[0018] Here, the flame current value detected by flame rod 9, which is provided for flame detection and exposed to the flame of burner 1, is maximum when the excess air ratio of the air-fuel mixture is 1.0, and decreases as the excess air ratio increases or decreases from 1.0. Therefore, the excess air ratio of the air-fuel mixture can be calculated based on the flame current value detected by flame rod 9. Therefore, the controller 8 controls the opening of variable throttle valve 74 so that the excess air ratio of the air-fuel mixture calculated based on the flame current value becomes an appropriate value.

[0019] Furthermore, when the water supply is stopped and a combustion stop command is issued, the controller 8 performs post-purge operation control shown in Figure 2. In this control, first, in STEP 1, the main valve 72 is closed while the fan 5 continues to operate. If an open valve malfunction occurs in the main valve 72, fuel gas is drawn in from the gas suction section 61 by the suction force of the fan 5, and the air-fuel mixture is supplied to the burner 1. Combustion in the burner 1 continues, and the flame rod 9 detects a flame current. Therefore, in STEP 2, it is determined whether the flame current value Ifr detected by the flame rod 9 is equal to or less than a predetermined threshold YI, which is the criterion for extinction. If Ifr ≤ YI, it is then determined in STEP 3 whether a certain time has elapsed. When the certain time has elapsed, the fan 5 is stopped in STEP 4, and post-purge operation is terminated.

[0020] On the other hand, if Ifr>YI, it is determined that an opening malfunction has occurred in the main valve 72, and the process proceeds to STEP 5, where the variable throttle valve 74 is narrowed to the minimum opening set to extinguish the burner 1. Specifically, the minimum opening is set so that the valve disc abuts against the valve seat and the gas supply rate falls below the flammability limit, extinguishing the burner 1. Thereafter, the fan 5 is stopped in STEP 6, and an error message is displayed in STEP 7 to indicate that an opening malfunction has occurred in the main valve 72.

[0021] According to the above control, if the flame rod 9 detects a flame current during post-purge operation (if Ifr > YI), the variable throttle valve 74 is throttled to its minimum opening, extinguishing the burner 1 and suppressing the release of fuel gas. On the other hand, if the main valve 72 does not have an open-circuit malfunction and the flame rod 9 does not detect a flame current during post-purge operation (if Ifr ≦ YI), no control is performed to change the opening of the variable throttle valve 74. Therefore, the opening of the variable throttle valve 74 is maintained at the opening immediately before the combustion stop command was issued. Therefore, at the next ignition, it is not necessary to adjust the opening of the variable throttle valve 74 based on the excess air ratio of the air-fuel mixture, enabling a quick ignition operation. Furthermore, the variable throttle valve 74 and the flame rod 9, which are provided to maintain the excess air ratio of the air-fuel mixture at an appropriate value, can be used as a means for preventing gas release in the event of an open-circuit malfunction of the main valve 72, which is cost-effective.

[0022] Furthermore, the variable throttle valve 74 may have a seal member such as rubber attached to either the valve disc or the valve seat. In this case, the valve disc abuts against the valve seat via the seal member rather than metal-to-metal, so the variable throttle valve 74 can be throttled down to the fully closed position that blocks the flow of fuel gas. Then, in STEP 5, it is desirable to throttle the variable throttle valve 74 down to the fully closed position, which is the minimum opening. This reliably prevents fuel gas from being released during post-purge operation when an opening failure has occurred in the main valve 72.

[0023] Although the embodiment of the present invention has been described above with reference to the drawings, the present invention is not limited to this and can be implemented with various modifications within the scope of the invention. [Explanation of symbols]

[0024] A...premixing device, 1...burner, 5...fan, 6...air supply passage, 61...gas suction section, 7...gas supply passage, 72...main valve, 73...zero governor, 74...variable throttle valve, 8...controller (control means), 9...flame rod.

Claims

1. A premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan, the device comprising: an air supply passage upstream of the fan; a gas supply passage having a downstream end connected to a gas suction section provided in the air supply passage; a main valve, a zero governor that adjusts secondary gas pressure to atmospheric pressure, and a variable throttle valve, which are provided in this order from the upstream side in the gas supply passage; a flame rod exposed to the burner flame; and control means, the control means being configured to adjust the opening of the variable throttle valve so that the excess air ratio of the mixture calculated based on the flame current detected by the flame rod becomes an appropriate value; The control means is configured so that, when a combustion stop command is issued and post-purge operation is performed in which the fan is driven with the main valve closed, if the flame rod does not detect a flame current, the control means maintains the opening of the variable throttle valve at the opening position immediately before the combustion stop command was issued, but if the flame rod detects a flame current, it determines that an opening failure of the main valve has occurred and controls the opening of the variable throttle valve to the minimum opening position set so that the burner is extinguished.

2. 2. The premixing device according to claim 1, wherein the minimum opening of the variable throttle valve is a fully closed opening that blocks the flow of fuel gas.

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