Combustion apparatus

Abstract

To correct an excess air ratio of an air-fuel mixture to a proper value even when the excess air ratio is higher than the proper value in a combustion apparatus that maintains the air excess ratio at the proper value by controlling gas amount adjustment means so that a measurement flame resistance value R becomes a flame resistance value (target resistance value) Rm on a reference flame resistance characteristic line corresponding to an air-fuel mixture supply amount.SOLUTION: When a measurement flame resistance value R exceeds an upper limit of a predetermined allowable range including a target resistance value Rm, fuel gas amount increase control is performed to reduce the measurement flame resistance value R to a value Rml lower than the target resistance value Rm. Next, fuel gas amount reduction control is performed until the measurement flame resistance value R increases to the target resistance value Rm.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a combustion 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 combustion device, there are provided gas amount adjusting means for adjusting the fuel gas supply amount, flame resistance measuring means for measuring the flame resistance value which is the electric resistance value of the flame formed by the combustion of the mixture ejected from the burner, and control means. The control means stores, as a reference flame resistance characteristic line, a characteristic line representing the relationship between the flame resistance value and the mixture supply amount when the air excess ratio (primary air amount / stoichiometric air amount) of the mixture is a predetermined appropriate value. The control means is configured to perform control to adjust the fuel gas supply amount by the gas amount adjusting means so that the flame resistance value measured by the flame resistance measuring means becomes the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated from the fan rotation speed (see, for example, Patent Document 1).

[0003] Here, 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 air excess ratio of the mixture is maintained at an appropriate value by adjusting the fuel gas supply amount based on the reference flame resistance characteristic line, and the occurrence of combustion failure can be prevented.

[0004] By the way, the appropriate value of the air excess ratio of the mixture is generally set to about 1.30. However, in order to suppress the generation of NOx as much as possible, it is preferable to set the air excess ratio of the mixture to be higher than 1.30 and exceed 1.40.

[0005] However, in such a region with a high air excess ratio, the following phenomena have been found. That is, as shown in Fig. 4, when comparing the change characteristic line a of the flame resistance value when reducing the fuel gas supply amount from a state with a low air excess ratio to increase the air excess ratio, and the change characteristic line b of the flame resistance value when increasing the fuel gas supply amount from a state with a high air excess ratio to decrease the air excess ratio, in the region with a high air excess ratio, the change characteristic line b has a higher flame resistance value than the change characteristic line a, and hysteresis occurs between the two. This is considered to be because when reducing the air excess ratio from a state with a high air excess ratio, the flame becomes in a lifted state and the combustion surface temperature becomes low in the state with a high air excess ratio, and the flame tends to elongate even in the process of reducing the air excess ratio, resulting in an increase in the flame resistance.

[0006] As a result, when setting the flame resistance value when the air excess ratio reaches, for example, 1.47 as the target resistance value Rm based on the change characteristic line a of the flame resistance value when increasing the air excess ratio from a state with a low air excess ratio, when correcting from a state where the air excess ratio is higher than the appropriate value, the air excess ratio will drop to about 1.45 when the flame resistance value reaches the target resistance value Rm. Therefore, it becomes impossible to correct the air excess ratio to the appropriate value λm of 1.47.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

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 combustion device capable of correcting the air excess ratio to an appropriate value even from a state where the air excess ratio of the air-fuel mixture is higher than the appropriate value.

Means for Solving the Problems

[0009] In order to solve the above problems, the present invention provides a combustion device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising: gas amount adjustment means for adjusting the fuel gas supply amount; flame resistance measurement means for measuring a flame resistance value, which is the electrical resistance value of a flame formed by the combustion of the mixture ejected from the burner; and control means. The control means performs a mixture amount calculation process for calculating the mixture supply amount from the fan rotation speed, stores a characteristic line representing the relationship between the flame resistance value and the mixture supply amount when the air excess ratio of the mixture is a predetermined appropriate value as a reference flame resistance characteristic line, and performs feedback control to adjust the fuel gas supply amount by the gas amount adjustment means so that the flame resistance value measured by the flame resistance measurement means becomes the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process. In this configuration, in the feedback control from a state where the flame resistance value measured by the flame resistance measurement means is higher than the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process, first, an increase control is performed to increase the fuel gas supply amount by the gas amount adjustment means so that the flame resistance value measured by the flame resistance measurement means becomes lower than the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process. After that, a decrease control is performed to decrease the fuel gas supply amount by the gas amount adjustment means so that the flame resistance value measured by the flame resistance measurement means becomes the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process. This is a characteristic feature of the present invention.

[0010] According to the present invention, when correcting from a state where the flame resistance value measured by the flame resistance measurement means is higher than the flame resistance value (target resistance value) on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process, that is, from a state where the air excess ratio of the mixture is higher than the appropriate value, the air excess ratio is made lower than the appropriate value by the increase control of the fuel gas supply amount, and then the decrease control of the fuel gas supply amount is performed. In the decrease control of the fuel gas supply amount, the flame resistance value changes along the above-described change characteristic line a. Therefore, by setting the target resistance value based on this change characteristic line a, it is possible to correct the air excess ratio to the appropriate value even from a state where the air excess ratio of the mixture is higher than the appropriate value.

[0011] Furthermore, in the above-described increment control, since it is not necessary to match the flame resistance value to the target resistance value, the fuel gas supply amount may be changed rapidly. On the other hand, in the above-described decrement control, it is necessary to slowly change the fuel gas supply amount in order to accurately match the flame resistance value to the target resistance value. Therefore, in the present invention, it is desirable to make the change rate of the fuel gas supply amount in the above-described increment control faster than the change rate of the fuel gas supply amount in the above-described decrement control. According to this, it is possible to shorten the time required for correction from a state where the air excess ratio of the air-fuel mixture is high and ensure the accuracy of the correction.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0013] The combustion device according to the embodiment of the present invention shown in FIG. 1 is a heat source machine including a fully premixed burner 1, a combustion chamber 2 surrounding 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.

[0014] In this combustion device, fuel gas is mixed with air, and the mixture is supplied to the burner 1 via the fan 5. The combustion device includes an air supply passage 6 upstream of the fan 5 and a gas supply passage 7 for supplying fuel gas. 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 of the air supply passage 6 adjacent to the upstream side of the gas suction portion 61 where a butterfly valve 62 described later is disposed is provided. The 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 as gas amount adjusting means are interposed in order from the upstream side.

[0015] 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 in the air supply passage 6. Here, the negative pressure in the air supply passage 6 changes according to the rotational speed of the fan 5. Therefore, the supply amount of 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 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 corresponding to the gas type to be used, the air excess ratio of the mixture becomes a predetermined appropriate value λm. Then, the controller 8 as control means controls the rotational speed of the fan 5 according to the required combustion amount (the combustion amount necessary for discharging hot water at the set hot water temperature), so that a mixture having an appropriate air excess ratio and an amount corresponding to the required combustion amount is supplied to the burner 1. In this embodiment, in order to significantly suppress the generation of NOx, the appropriate value λm of the air excess ratio is set to a relatively high value, for example, 1.47.

[0016] Further, in order to prevent exhaust problems caused by wind intrusion into the exhaust pipe 4, that is, to ensure the wind resistance performance, the lower limit rotational speed of the fan 5 cannot be set too low. And when the required combustion amount becomes equal to or less than a predetermined threshold 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.

[0017] Therefore, in a portion of the air supply passage 6 upstream of the gas suction portion 61, in order to switch the ventilation resistance of this portion in two steps of large and small, a butterfly valve 62 which is an air resistance switching means switched between a closed posture shown by a solid line in FIG. 1 and an open posture shown by a virtual line by a motor (not shown) is arranged. And when the required combustion amount becomes equal to or less than the above threshold by the controller 8, the butterfly valve 62 is set to the closed posture, the ventilation resistance of the air supply passage 6 is increased, and the rotational speed of the fan 5 is not made lower than the lower limit rotational speed, so that an amount of air corresponding to the required combustion amount equal to or less than the threshold can be supplied.

[0018] However, simply closing the butterfly valve 62 and increasing the ventilation resistance of the air supply passage 6 increases the negative pressure acting on the gas suction portion 61, causes the supply amount of the fuel gas to become excessive, and the air excess ratio λ of the air-fuel mixture supplied to the burner 1 falls below the appropriate value λm. Therefore, when the required combustion amount is equal to or less than the threshold, the butterfly valve 62 is set to the closed posture 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, so that the ventilation resistance of the portion of the gas supply passage 7 downstream of the zero governor 73 is increased to a low-capacity state, and an air-fuel mixture having an appropriate air excess ratio and corresponding to the required combustion amount equal to or less than the threshold is supplied to the burner 1. When the required combustion amount exceeds the threshold, the butterfly valve 62 is set to the open posture 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, so that the ventilation resistance of the portion of the gas supply passage 7 downstream of the zero governor 73 is decreased to a high-capacity state, and an air-fuel mixture having an appropriate air excess ratio and corresponding to the required combustion amount exceeding the threshold is supplied to the burner 1.

[0019] Incidentally, 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 this case, if the ratio of the fuel gas supply amount to the air supply amount is constant, the excess air ratio of the air-fuel mixture will vary due to the variation in the calorific value of the fuel gas, resulting in combustion failure.

[0020] Here, the flame resistance value, which is the electrical resistance value of the flame formed by the combustion of the air-fuel mixture ejected from burner 1, has a correlation with the excess air ratio of the air-fuel mixture. Also, when the supply amount of the air-fuel mixture to burner 1, that is, the combustion amount increases, the amount of ions in the flame increases and the flame resistance value decreases. RL in Fig. 2 shows the reference flame resistance characteristic line representing the relationship between the flame resistance value and the air-fuel mixture supply amount when the excess air ratio of the air-fuel mixture is 1.47, which is a predetermined appropriate value λm. The reference flame resistance characteristic line RL is stored in the memory of controller 8. Note that the flame resistance value on the reference flame resistance characteristic line RL is the flame resistance value when the excess air ratio is changed from a state where it is less than or equal to the appropriate value λm to the appropriate value λm.

[0021] In this embodiment, flame resistance measuring means 9 composed of a flame rod facing the combustion surface 1a of burner 1 is provided, and the flame resistance value is measured from the frame current flowing through the flame rod and the applied voltage to the flame rod. Then, in order to correct the excess air ratio, controller 8 performs an air-fuel mixture amount calculation process for calculating the air-fuel mixture supply amount from the rotation speed of fan 5 (hereinafter referred to as the fan rotation speed), and feedback control is performed to adjust the fuel gas supply amount by variable throttle valve 74 so that the flame resistance value measured by flame resistance measuring means 9 becomes the flame resistance value (target resistance value Rm) on the reference flame resistance characteristic line RL that matches the air-fuel mixture supply amount calculated in the air-fuel mixture amount calculation process.

[0022] The following will describe this feedback control in detail with reference to FIG. 3. First, in STEP1, it is determined whether the flame resistance value measured by the flame resistance measuring means 9 (hereinafter referred to as the measured flame resistance value) R is within a predetermined allowable range including the target resistance value Rm. If it is outside the allowable range, it proceeds to STEP2, and it is determined whether the measured flame resistance value R is below the lower limit of the allowable range. When it is determined as "YES" in STEP2, that is, when the air excess ratio is smaller than the appropriate value λm and the measured flame resistance value R is lower than the lower limit of the allowable range, it proceeds to STEP3, and a reduction control is performed to reduce the opening degree of the variable throttle valve 74 and reduce the fuel gas supply amount until it is determined in STEP4 that the measured flame resistance value R has reached the target resistance value Rm.

[0023] When it is determined as "NO" in STEP2, that is, when the air excess ratio is larger than the appropriate value and the measured flame resistance value R is higher than the upper limit of the allowable range, it proceeds to STEP5, and an increase control is performed to increase the opening degree of the variable throttle valve 74 and increase the fuel gas supply amount until the measured flame resistance value R decreases to a predetermined value Rml (for example, 80% of Rm) lower than the target resistance value Rm in STEP6. Next, it proceeds to STEP3, and a reduction control is performed to reduce the opening degree of the variable throttle valve 74 and reduce the fuel gas supply amount until it is determined in STEP4 that the measured flame resistance value R has reached the target resistance value Rm.

[0024] Here, when an increase control of the fuel gas supply amount is performed from a state where the air excess ratio is larger than the appropriate value λm and the flame resistance value exceeds the target resistance value Rm, as described above, the flame resistance value decreases along the change characteristic line b in FIG. 4. And when the flame resistance value has decreased to the target resistance value Rm, the air excess ratio becomes smaller than the appropriate value λm. On the contrary, according to the above-described feedback control, after performing the increase control until the flame resistance value is below the target resistance value Rm, a reduction control is performed to increase the flame resistance value along the change characteristic line a in FIG. 4. Therefore, if the flame resistance value is increased to the target resistance value Rm by the reduction control, the air excess ratio will be the appropriate value λm.

[0025] Incidentally, in the increment control in STEP5, since it is not necessary to match the flame resistance value to the target resistance value Rm, the fuel gas supply amount may be changed quickly. On the other hand, in the decrement control in STEP3, it is necessary to slowly change the fuel gas supply amount in order to accurately match the flame resistance value to the target resistance value Rm. Therefore, the change rate of the fuel gas supply amount in the increment control in STEP5 is made faster than the change rate of the fuel gas supply amount in the decrement control in STEP3. According to this, it is possible to shorten the time required for correction from a state where the air excess ratio of the air-fuel mixture is high and ensure the accuracy of correction.

[0026] 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, in the above embodiment, a zero governor 73 and a variable throttle valve 74 as a gas amount adjusting means are provided in the gas supply passage 7, but it is also possible to provide a proportional valve instead of the zero governor, and in this case, the proportional valve may be also used as the gas amount adjusting means.

Explanation of Signs

[0027] 1... Burner, 5... Fan, 74... Variable throttle valve (gas amount adjusting means), 8... Controller (control means), 9... Flame resistance measuring means, R... Flame resistance value measured by the flame resistance measuring means, RL... Reference flame resistance characteristic line, Rm... Target resistance value (flame resistance value on the reference flame resistance characteristic line corresponding to the air-fuel mixture supply amount).

Claims

1. A combustion device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising gas amount adjusting means for adjusting the fuel gas supply amount, flame resistance measuring means for measuring the flame resistance value, which is the electrical resistance value of the flame formed by the combustion of the mixture ejected from the burner, and control means. The control means performs a mixture amount calculation process for calculating the mixture supply amount from the fan rotation speed, stores a characteristic line representing the relationship between the flame resistance value and the mixture supply amount when the air excess ratio of the mixture is a predetermined appropriate value as a reference flame resistance characteristic line, and the control means is configured to perform feedback control to adjust the fuel gas supply amount by the gas amount adjusting means so that the flame resistance value measured by the flame resistance measuring means becomes the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process. In the feedback control from a state where the flame resistance value measured by the flame resistance measuring means is higher than the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process, the control means first performs an increasing control to increase the fuel gas supply amount by the gas amount adjusting means so that the flame resistance value measured by the flame resistance measuring means becomes lower than the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process, and then performs a decreasing control to decrease the fuel gas supply amount by the gas amount adjusting means so that the flame resistance value measured by the flame resistance measuring means becomes the flame resistance value on the reference flame resistance characteristic line that matches the mixture supply amount calculated by the mixture amount calculation process. The combustion device is characterized by this configuration.

2. The combustion device according to claim 1, characterized in that the change speed of the fuel gas supply amount in the increasing control is made faster than the change speed of the fuel gas supply amount in the decreasing control.

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