Pre-mixing device

Abstract

To prevent the occurrence of a temporary combustion failure when an air excess rate of an air-fuel mixture is increased more than an appropriate value due to the reduction of a fuel gas supply quantity smaller than a quantity corresponding to a fan rotation number caused by the lowering of primary gas pressure, in a premixing device for mixing a fuel gas into air, and supplying the air-fuel mixture to a burner via a fan, in which a zero governor and a flow rate regulation valve are interposed in a gas supply path, and which performs control for adjusting an opening of the flow rate regulation valve so that the air excess rate of the air-fuel mixture reaches the appropriate value.SOLUTION: When control for increasing an opening Gθ of a flow rate regulation valve larger than a prescribed reference opening Gθn is performed, a lower limit of a fan rotation number Nf which is needed to increase an opening of the flow rate regulation valve is calculated and stored as a first threshold YNf1. When the fan rotation number Nf is succeedingly increased to the first threshold YNf1 or larger, the opening Gθ of the flow rate regulation valve is instantaneously set to an opening which is increased more than the reference opening Gθn by an opening amount which is obtained by multiplying a deviation from the first threshold YNf1 of the fan rotation number Nf by a prescribed coefficient K.SELECTED DRAWING: Figure 4

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 a fan, 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, and a zero governor interposed in the gas supply passage for regulating the secondary gas pressure to atmospheric pressure. 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 in the air supply passage. And since the negative pressure in the air supply passage changes according to the rotational speed of the fan, the supply amount of the fuel gas changes in proportion to the rotational speed of the fan, that is, the supply amount of air. Therefore, by controlling the rotational speed of the fan according to the required combustion amount, a mixture of an amount corresponding to the required combustion amount is supplied to the burner, and the air excess ratio (primary air amount / air amount of the theoretical air-fuel ratio) of the mixture becomes constant.

[0003] However, in foreign 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, due to the variation in the calorific value of the fuel gas, the air excess ratio of the mixture varies, resulting in combustion failure.

[0004] Therefore, conventionally, according to Patent Document 1, there is also known a premixing device including a flow rate regulating valve 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 adjust the opening degree of the flow rate regulating valve so that the air excess ratio of the mixture detected by the air excess ratio detecting means becomes a predetermined appropriate value. According to this, 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 opening degree of the flow rate regulating valve, and the occurrence of combustion failure can be prevented.

[0005] In addition, in the case of the device described in Patent Document 1, a butterfly valve is provided in the portion of the air supply passage upstream of the gas suction portion, and control is performed to switch the combustion capacity to at least two levels of large and small by changing the opening degrees of the butterfly valve and the flow rate adjustment valve. That is, when the required combustion amount is relatively small, the opening degree of the butterfly valve is set to a predetermined small-capacity opening degree on the closed side, and the opening degree of the flow rate adjustment valve is set to a relatively small predetermined small-capacity opening degree, thereby switching the combustion capacity to a small capacity so that an air-fuel mixture corresponding to a required combustion amount with an appropriate air excess ratio and a relatively small value is supplied to the burner. When the required combustion amount is relatively large, the opening degree of the butterfly valve is set to a predetermined large-capacity opening degree on the open side, and the opening degree of the flow rate adjustment valve is set to a relatively large predetermined large-capacity opening degree, thereby switching the combustion capacity to a large capacity so that an air-fuel mixture corresponding to a required combustion amount with an appropriate air excess ratio and a relatively large value is supplied to the burner.

[0006] By the way, when the primary gas pressure becomes low due to the installation status of the gas pipe or the like, when the rotational speed of the fan is increased according to the required combustion amount, the supply amount of the fuel gas becomes less than the amount corresponding to the rotational speed of the fan, and the air excess ratio of the air-fuel mixture increases from the appropriate value. In this case, in the conventional example described above, control (feedback control) is performed to increase the opening degree of the flow rate adjustment valve, so that the supply amount of the fuel gas increases and the air excess ratio of the air-fuel mixture is returned to the appropriate value.

[0007] However, in this case, it takes time for the air excess ratio of the air-fuel mixture to return to the appropriate value by changing the opening degree of the flow rate adjustment valve. And there is a risk of temporarily causing combustion failure before the air excess ratio is returned to the appropriate value.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] In view of the above points, an object of the present invention is to provide a premixing device capable of preventing the occurrence of temporary combustion failure when the primary gas pressure decreases.

Means for Solving the Problems

[0010] In order to solve the above problems, the present invention is a premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising a fan, 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 flow rate regulating valve 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 flow rate regulating 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, when the control means increases the rotational speed of the fan according to the required combustion amount and the air excess ratio of the mixture detected by the air excess ratio detecting means increases more than the appropriate value and the control is performed to increase the opening degree of the flow rate regulating valve more than a predetermined reference opening degree set so that the air excess ratio of the mixture becomes the appropriate value, the lower limit of the rotational speed of the fan at which an increase in the opening degree of the flow rate regulating valve is required is calculated as a first threshold value and stored. When the rotational speed of the fan next increases to the first threshold value or more, the control is configured to immediately increase the opening degree of the flow rate regulating valve by an opening degree obtained by multiplying the deviation from the first threshold value of the rotational speed of the fan by a predetermined coefficient from the reference opening degree.

[0011] According to the present invention, when the air excess ratio of the mixture increases more than the appropriate value when the rotational speed of the fan is increased to the first threshold value or more due to a decrease in the primary gas pressure, once the first threshold value is calculated and stored, when the rotational speed of the fan increases to the first threshold value or more, the air excess ratio of the mixture can be immediately returned to the appropriate value by the above-described control. Therefore, it is possible to prevent the occurrence of temporary combustion failure when the primary gas pressure decreases.

[0012] Further, in the present invention, when a butterfly valve is provided in a portion of the air supply passage upstream of the gas suction portion as in the above conventional example, the control means increases the rotational speed of the fan to a value exceeding a predetermined second threshold value greater than the first threshold value according to the required combustion amount. When the air excess ratio of the air-fuel mixture detected by the air excess ratio detecting means increases beyond the appropriate value and the opening degree of the flow rate regulating valve is increased to the maximum opening degree but the air excess ratio does not reach the appropriate value, the opening degree of the butterfly valve is decreased to a predetermined opening degree at which the air excess ratio becomes the appropriate value, and this predetermined opening degree is stored. Also, with the opening degree of the flow rate regulating valve in the state of the maximum opening degree, the rotational speed of the fan is increased beyond the reference rotational speed corresponding to the required combustion amount until the combustion amount of the burner reaches the required combustion amount. The increase amount of the rotational speed of the fan at this time with respect to the reference rotational speed is stored as a rotational speed correction value. When the rotational speed of the fan next increases to a value exceeding the second threshold value, immediately, the opening degree of the flow rate regulating valve is set to the maximum opening degree, the opening degree of the butterfly valve is set to the predetermined opening degree, and further, control is performed to increase the rotational speed of the fan according to the rotational speed correction value. It is desirable to configure it in this way.

[0013] According to this, when the rotational speed of the fan is increased to a value exceeding the second threshold value, due to the decrease in the primary gas pressure, even if the opening degree of the flow rate regulating valve is increased to the maximum opening degree and the air excess ratio of the air-fuel mixture does not reach the appropriate value, once the predetermined opening degree of the butterfly valve and the rotational speed correction value are stored, when the rotational speed of the fan next increases to a value exceeding the second threshold value, by the above-described control, immediately, the air excess ratio of the air-fuel mixture can be returned to the appropriate value and the combustion amount of the burner can be made the required combustion amount. Therefore, it is possible to prevent the occurrence of temporary combustion failure and insufficient combustion amount in the high rotational speed region of the fan when the primary gas pressure becomes low.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Mode 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 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. Further, by the premixing device A of the embodiment of the present invention, fuel gas is mixed with air, and the air-fuel mixture is supplied to the burner 1 via a fan 5.

[0016] The premixing device A includes a fan 5, an air supply passage 6 on the upstream side 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. Then, the downstream end of the venturi portion 63 is inserted into the upstream end of the cylindrical portion 64 with an annular gap, 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.

[0017] 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 flow control valve 74 are interposed in order from the upstream side. Further, the premixing device A includes a controller 8 composed of a microcomputer which is a control means for controlling the fan 5, the main valve 72, the flow control valve 74, and a butterfly valve 62 described later.

[0018] 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 Nf of the fan 5. Therefore, the supply amount of the fuel gas changes in proportion to the rotational speed Nf 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 Gθ of the flow rate control valve 74. By setting the opening degree Gθ of the flow rate control valve 74 to a predetermined reference opening degree according to the type of gas used, the air excess ratio λ of the air-fuel mixture becomes a predetermined appropriate value Yλ (for example, 1.3). Then, by controlling the rotational speed Nf of the fan 5 according to the required combustion amount Qd (the combustion amount required to discharge hot water at the set hot water temperature), an air-fuel mixture with an air excess ratio λ of the appropriate value Yλ and an amount corresponding to the required combustion amount Qd is supplied to the burner 1.

[0019] In addition, in order not to cause poor exhaust due to the intrusion of wind into the exhaust pipe 4, that is, in order to ensure the wind resistance performance, the lower limit rotational speed of the fan 5 cannot be set too low. When the required combustion amount Qd becomes equal to or 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 Qd.

[0020] 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 two levels of large and small, a butterfly valve 62 that can be switched between an open position shown by a solid line in FIG. 1 and a closed position shown by a virtual line by a motor (not shown) is disposed. When the required combustion amount Qd becomes equal to or less than the above-described predetermined value, the butterfly valve 62 is set to the closed position to increase the ventilation resistance of the air supply passage 6, and the rotation speed Nf of the fan 5 is not reduced to the lower limit rotation speed or less, so that an amount of air corresponding to a combustion amount equal to or less than the predetermined value can be supplied. However, simply closing the butterfly valve 62 to increase the ventilation resistance of the air supply passage 6 increases the negative pressure in the air supply passage 6, resulting in an excessive supply amount of fuel gas and causing the air excess ratio λ of the air-fuel mixture supplied to the burner 1 to fall below the appropriate value Yλ. Therefore, when the required combustion amount Qd is relatively small, the butterfly valve 62 is closed, and the opening degree Gθ of the flow rate adjustment valve 74 is set to a relatively small small-capacity reference opening degree preset so that the air excess ratio of the air-fuel mixture becomes an appropriate value with the butterfly valve 62 in the closed position. In the small-capacity state, an air-fuel mixture corresponding to a relatively small combustion amount with the air excess ratio λ being the appropriate value Yλ is supplied to the burner 1. When the required combustion amount Qd is relatively large, the butterfly valve 62 is opened, and the opening degree Gθ of the flow rate adjustment valve 74 is set to a relatively large large-capacity reference opening degree preset so that the air excess ratio of the air-fuel mixture becomes an appropriate value with the butterfly valve 62 in the open position. In the large-capacity state, an air-fuel mixture corresponding to a relatively large combustion amount with the air excess ratio λ being the appropriate value Yλ is supplied to the burner 1. The relationship between the rotation speed Nf of the fan 5 and the supply amount of the air-fuel mixture, that is, the combustion amount Q of the burner 1 is as shown by the characteristic line L in FIG. 2 in the small-capacity state and as shown by the characteristic line H in FIG. 2 in the large-capacity state.

[0021] Incidentally, even when the same gas type is used as the fuel gas, the calorific value (Wobbe index) of the fuel gas may vary with time. In this case, if the ratio of the supply amount of the fuel gas to the supply amount of the air is constant, the variation in the calorific value of the fuel gas causes the air excess ratio λ of the air-fuel mixture to vary, resulting in combustion failure.

[0022] Therefore, an air-fuel ratio detection means 9 for detecting the air-fuel ratio λ of the air-fuel mixture is provided. In the present embodiment, the air-fuel ratio detection means 9 is constituted by a frame rod provided facing the combustion surface 1a of the burner 1, and the air-fuel ratio λ of the air-fuel mixture is detected from the frame current flowing through the frame rod. Incidentally, since the flame approaches or moves away from the combustion surface 1a according to the air-fuel ratio λ of the air-fuel mixture, the back surface temperature of the combustion surface 1a changes according to the air-fuel ratio λ of the air-fuel mixture. Therefore, it is also possible to constitute the air-fuel ratio detection means 9 by a temperature sensor for detecting the back surface temperature of the combustion surface 1a.

[0023] Then, the air-fuel ratio λ of the air-fuel mixture detected by the air-fuel ratio detection means 9 is input to the controller 8, and the controller 8 feedback-controls the flow rate control valve 74 so that this air-fuel ratio λ becomes constant, that is, so that it is maintained at a predetermined appropriate value Yλ. Specifically, when the air-fuel ratio λ of the air-fuel mixture decreases due to an increase in the calorific value of the fuel gas, the opening degree Gθ of the flow rate control valve 74 is decreased from the reference opening degree (the small-capacity reference opening degree in the small-capacity state and the large-capacity reference opening degree in the large-capacity state), and the ratio of the supply amount of the fuel gas to the supply amount of the air is decreased so that the air-fuel ratio λ becomes the appropriate value Yλ. Also, when the air-fuel ratio λ of the air-fuel mixture increases due to a decrease in the calorific value of the fuel gas, the opening degree Gθ of the flow rate control valve 74 is increased from the reference opening degree, and the ratio of the supply amount of the fuel gas to the supply amount of the air is increased so that the air-fuel ratio λ becomes the appropriate value Yλ. According to this, even if the calorific value of the fuel gas fluctuates, the air-fuel ratio λ of the air-fuel mixture is maintained at the appropriate value Yλ, and the occurrence of combustion failure can be prevented.

[0024] Also, when the primary gas pressure decreases due to the installation status of the gas piping or the like, when the rotational speed Nf of the fan 5 is increased according to the required combustion amount Qd in the high-capacity state, the supply amount of the fuel gas becomes less than the amount corresponding to the rotational speed Nf of the fan 5, and the air excess ratio λ of the air-fuel mixture increases beyond the appropriate value Yλ. In this case, the opening degree Gθ of the flow rate control valve 74 is increased beyond the high-capacity reference opening degree Gθn by feedback control, the supply amount of the fuel gas increases, and the air excess ratio λ of the air-fuel mixture is returned to the appropriate value Yλ. However, in such feedback control of the flow rate control valve 74, it takes time for the air excess ratio λ of the air-fuel mixture to be returned to the appropriate value Yλ, and during that time, there is a risk of temporary combustion failure.

[0025] Here, when the primary gas pressure decreases, the relationship between the rotational speed Nf of the fan 5 in the high-capacity state and the opening degree Gθ of the flow rate control valve 74 at which the air excess ratio λ of the air-fuel mixture becomes the appropriate value Yλ is as shown in FIG. 3. That is, until the rotational speed Nf of the fan 5 becomes equal to or higher than a predetermined first threshold value YNf1, the supply amount of the fuel gas is maintained at the amount corresponding to the rotational speed Nf of the fan 5, and the opening degree Gθ of the flow rate control valve 74 is maintained at the high-capacity reference opening degree Gθn. However, when the rotational speed Nf of the fan 5 becomes equal to or higher than the first threshold value YNf1, the supply amount of the fuel gas becomes less than the amount corresponding to the rotational speed Nf of the fan 5, and the opening degree Gθ of the flow rate control valve 74 increases from the high-capacity reference opening degree Gθn at a predetermined slope as the rotational speed Nf of the fan 5 increases. That is, in the region where Nf ≧ YNf1, the following equation, Gθ = Gθn + K(Nf - YNf1)…(1) holds. Note that the coefficient K in equation (1) is a value specific to each model of the premixing device A and can be obtained experimentally.

[0026] Further, when the opening degree Gθ of the flow rate control valve 74 is increased according to the formula (1), when the rotational speed Nf of the fan 5 reaches a predetermined second threshold value YNf2 that is greater than the first threshold value YNf1, the opening degree Gθ of the flow rate control valve 74 reaches the maximum opening degree Gθmax. And when the rotational speed Nf of the fan 5 increases to the second threshold value YNf2 or more due to the increase in the required combustion amount Qd, the air excess ratio λ of the air-fuel mixture increases due to the insufficient supply amount of the fuel gas and becomes greater than the appropriate value Yλ. In this case, by decreasing the opening degree Aθ of the butterfly valve 62 from the fully open opening degree which is the opening degree in the opening posture of the butterfly valve 62, the supply amount of air can be decreased, and the air excess ratio λ of the air-fuel mixture can be returned to the appropriate value Yλ. Incidentally, as it is, the combustion amount Q of the burner 1 does not reach the required combustion amount Qd due to the insufficient supply amount of the fuel gas. However, by increasing the rotational speed Nf of the fan 5 to be higher than the reference rotational speed Nfn (the rotational speed corresponding to the characteristic line of H in FIG. 2) corresponding to the required combustion amount Qd, the combustion amount Q of the burner 1 can be increased to the required combustion amount Qd.

[0027] Taking the above into consideration, in the present embodiment, in the high-capacity state, the controller 8 performs the control shown in FIG. 4. Hereinafter, this control will be described. When switched to the high-capacity state, first, in STEP1, it is determined whether or not the first flag F1 is reset to "0". The first flag F1 is reset to "0" in the initial state. Therefore, proceeding from STEP1 to STEP2, the rotational speed Nf of the fan 5 is set to the reference rotational speed Nfn corresponding to the required combustion amount Qd, and normal control is performed to adjust the opening degree Gθ of the flow rate control valve 74 so that the air excess ratio λ of the air-fuel mixture detected by the air excess ratio detection means 9 becomes the appropriate value Yλ.

[0028] Next, proceed to STEP3 and determine whether the opening degree Gθ of the flow rate control valve 74 is greater than the opening degree obtained by adding the upper limit value α of the adjustment range due to the change in the calorific value of the fuel gas to the large-capacity reference opening degree Gθn. When Gθ > Gθn + α, it is the case where, when the rotational speed Nf of the fan 5 is increased according to the required combustion amount Qd, the supply amount of the fuel gas becomes less than the amount corresponding to the rotational speed Nf of the fan 5 due to the decrease in the primary gas pressure, and the air excess ratio λ of the air-fuel mixture increases beyond the appropriate value Yλ. Therefore, if Gθ > Gθn + α, proceed to STEP4 and calculate the first threshold value YNf1, which is the lower limit of the rotational speed Nf of the fan 5 for which an increase in the opening degree of the flow rate control valve 74 is required, from the following equation derived from Equation (1): YNf1 = Nf - (Gθ - Gθn) / K…(2) Then, calculate and store it, and calculate the second threshold value YNf2 from the following equation: YNf2 = YNf1 + (Gθmax - Gθn) / K…(3) And calculate and store it.

[0029] Next, after setting the first flag F1 to "1" in STEP5, return to STEP1. In this case, since it is determined as "NO" in STEP1, proceed to STEP6 and determine whether the rotational speed of the fan 5, that is, the reference rotational speed Nfn corresponding to the required combustion amount Qd at that time, has increased to be equal to or greater than the first threshold value YNf1. If Nfn < YNf1, proceed to STEP2 and perform normal control. On the other hand, if Nfn ≥ YNf1, proceed to STEP7 and determine whether the reference rotational speed Nfn corresponding to the required combustion amount Qd is equal to or less than the second threshold value YNf2. And if Nfn ≤ YNf2, proceed to STEP8 and increase the opening degree Gθ of the flow rate control valve 74 by an amount greater than the large-capacity reference opening degree Gθn by an amount obtained by multiplying the deviation from the first threshold value YNf1 of the rotational speed Nf(=Nfn) of the fan 5 by a predetermined coefficient K according to the above Equation (1), and set the rotational speed Nf of the fan 5 to the reference rotational speed Nfn corresponding to the required combustion amount Qd.

[0030] According to the above control, when the rotational speed Nf of the fan 5 increases to be equal to or higher than the first threshold value YNf1, if the air-fuel ratio λ of the air-fuel mixture increases beyond the appropriate value Yλ due to the decrease in the primary gas pressure, once the first threshold value YNf1 is calculated and stored, when the rotational speed Nf of the fan 5 increases to be equal to or higher than the first threshold value YNf1, the opening degree Gθ of the flow rate control valve 74 is adjusted to an opening degree that can immediately return the air-fuel ratio λ of the air-fuel mixture to the appropriate value Yλ without relying on feedback control. Therefore, it is possible to prevent the occurrence of temporary combustion failure when the primary gas pressure becomes low.

[0031] In addition, the correct values of the first and second threshold values YNf1 and YNf2 may be different from the values calculated and stored in STEP4 due to changes in the primary gas pressure. In this case, in the process of STEP8, the air-fuel ratio λ of the air-fuel mixture does not become the appropriate value Yλ. Therefore, in STEP9 after the process of STEP8, it is determined whether or not the air-fuel ratio λ of the air-fuel mixture detected by the air-fuel ratio detection means 9 has become the appropriate value Yλ. If λ = Yλ, the process returns to STEP1 as it is, but if λ ≠ Yλ, in STEP10, the first and second flags F1 and F2 are reset to "0" and then the process returns to STEP1. According to this, when proceeding to STEP4 again, the first and second threshold values YNf1 and YNf2 are updated, and when the process of STEP8 is performed next, the air-fuel ratio λ of the air-fuel mixture becomes the appropriate value Yλ.

[0032] In STEP7, when it is determined that the reference rotational speed Nfn of the fan 5 corresponding to the required combustion amount Qd at that time has increased to a value exceeding the second threshold value YNf2, the process proceeds to STEP11, the opening degree Gθ of the flow rate control valve 74 is set to the maximum opening degree Gθmax, and then in STEP12, it is determined whether or not the second flag F2 has been reset to "0". The second flag F2 is reset to "0" in the initial state. Therefore, when the rotational speed Nf (= Nfn) of the fan 5 first increases to a value exceeding the second threshold value YNf2, the process proceeds from STEP12 to STEP13, and the opening degree Aθ of the butterfly valve 62 is decreased from the fully open degree to a predetermined opening degree YAθ at which the air-fuel ratio λ of the air-fuel mixture detected by the air-fuel ratio detection means 9 becomes the appropriate value Yλ, and this predetermined opening degree YAθ is stored.

[0033] Next, proceed to STEP14. Until the combustion amount Q of burner 1 reaches the required combustion amount Qd, that is, until the temperature of the hot water discharged rises to the set temperature, increase the rotation speed Nf of fan 5 to be higher than the reference rotation speed Nfn corresponding to the required combustion amount Qd. At this time, store the increase amount (= Nf - Nfn) of the rotation speed Nf of fan 5 with respect to the reference rotation speed Nfn as the rotation speed correction value ΔNf. The line indicated by H' in FIG. 2 becomes the characteristic line obtained by adding the rotation speed correction value ΔNf to the reference rotation speed Nfn.

[0034] When the process in STEP14 is completed, after setting the second flag F2 to "1" in STEP15, return to STEP1. Therefore, when the reference rotation speed Nfn of fan 5 corresponding to the required combustion amount Qd at that time increases to a value exceeding the second threshold value YNf2 next, and when proceeding to STEP12 after setting the opening degree Gθ of the flow rate control valve 74 to the maximum opening degree Gθmax in STEP9, it is determined as "NO". In this case, proceed to STEP16, set the opening degree Aθ of the butterfly valve 62 to the predetermined opening degree YAθ, and increase the rotation speed Nf of fan 5 according to the rotation speed correction value ΔNf, that is, increase the rotation speed Nf of fan 5 to the rotation speed obtained by adding the rotation speed correction value ΔNf to the reference rotation speed Nfn corresponding to the required combustion amount Qd.

[0035] According to this, when the rotation speed Nf of fan 5 is increased to a value exceeding the second threshold value YNf2, due to the decrease in the primary gas pressure, even if the opening degree Gθ of the flow rate control valve 74 is increased to the maximum opening degree, and the air excess ratio λ of the air-fuel mixture does not become the appropriate value Yλ, once the predetermined opening degree YAθ of the butterfly valve 62 and the rotation speed correction value ΔNf are memorized, when the rotation speed Nf (= Nfn) of fan 5 increases to a value exceeding the second threshold value YNf2, immediately, the air excess ratio λ of the air-fuel mixture can be returned to the appropriate value Yλ, and the combustion amount Q of burner 1 can be made the required combustion amount Qd. Therefore, it is possible to prevent the occurrence of temporary combustion failure and insufficient combustion amount in the high rotation speed region of fan 5 when the primary gas pressure becomes low.

[0036] When the process in STEP16 is completed, the process proceeds to STEP9 to determine whether the air-fuel ratio λ of the air-fuel mixture detected by the air-fuel ratio detection means 9 is equal to the appropriate value Yλ. If λ = Yλ, the process directly returns to STEP1. If λ ≠ Yλ, the first and second flags F1 and F2 are reset to "0" in STEP10, and then the process returns to STEP1. Therefore, if λ ≠ Yλ, when the process proceeds to STEP12 next, it is determined as "NO", and the process proceeds to STEP13 and 14, where the predetermined opening degree YAθ of the butterfly valve 62 and the rotational speed correction value ΔNf are updated.

[0037] 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, 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 62. In this case, by restricting the amount of water flowing through the heat exchanger 3, it is possible to prevent the required combustion amount Qd from increasing to an amount equal to or greater than the combustion amount corresponding to the second threshold value YNf2, and it is sufficient to not perform the processes below STEP11 in FIG. 4.

Explanation of Reference Numerals

[0038] A... Premixing device, 1... Burner, 5... Fan, 6... Air supply passage, 61... Gas suction portion, 62... Butterfly valve, 7... Gas supply passage, 73... Zero governor, 74... Flow control valve, 8... Controller (control means), 9... Air-fuel ratio detection means, Q... Combustion amount of the burner, Qd... Required combustion amount, λ... Air-fuel ratio of the air-fuel mixture, Yλ... Appropriate value, Nf... Rotational speed of the fan, YNf1... First threshold value, YNf2... Second threshold value, Nfn... Reference rotational speed, ΔNf... Rotational speed correction value, Gθ... Opening degree of the flow control valve, Gθn... Reference opening degree, Gθmax... Maximum opening degree, Aθ... Opening degree of the butterfly valve, YAθ... Predetermined opening degree.

Claims

1. A premixing device that mixes fuel gas with air and supplies the mixture to a burner via a fan, comprising: a fan; 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 flow rate regulating valve 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, wherein the control means is configured to perform control to vary the rotational speed of the fan according to the required combustion amount, and to perform control to adjust the opening degree of the flow rate regulating valve so that the air excess ratio of the mixture detected by the air excess ratio detecting means becomes a predetermined appropriate value, and when the control means increases the rotational speed of the fan according to the required combustion amount and the air excess ratio of the mixture detected by the air excess ratio detecting means increases beyond the appropriate value and the control is performed to increase the opening degree of the flow rate regulating valve beyond a predetermined reference opening degree set in advance so that the air excess ratio of the mixture becomes the appropriate value, the control means calculates and stores the lower limit of the rotational speed of the fan at which an increase in the opening degree of the flow rate regulating valve is required as a first threshold value, and when the rotational speed of the fan next increases to the first threshold value or more, immediately performs control to set the opening degree of the flow rate regulating valve to an opening degree increased from the reference opening degree by an opening degree obtained by multiplying the deviation of the rotational speed of the fan from the first threshold value by a predetermined coefficient. A premixing device characterized by this.

2. The premixing device according to claim 1, wherein a butterfly valve is provided in a portion of the air supply passage upstream of the gas suction portion. When the control means increases the rotational speed of the fan to a value exceeding a predetermined second threshold value greater than the first threshold value according to the required combustion amount, if the air excess ratio of the air-fuel mixture detected by the air excess ratio detection means increases beyond the appropriate value and the air excess ratio does not reach the appropriate value even when the opening degree of the flow rate control valve is increased to the maximum opening degree, the opening degree of the butterfly valve is decreased to a predetermined opening degree at which the air excess ratio becomes the appropriate value, and this predetermined opening degree is stored. Also, with the opening degree of the flow rate control valve in the maximum opening degree state, the rotational speed of the fan is increased beyond the reference rotational speed corresponding to the required combustion amount until the combustion amount of the burner reaches the required combustion amount. The increase amount of the rotational speed of the fan at this time with respect to the reference rotational speed is stored as a rotational speed correction value. When the rotational speed of the fan next increases to a value exceeding the second threshold value, immediately, the opening degree of the flow rate control valve is set to the maximum opening degree, the opening degree of the butterfly valve is set to the predetermined opening degree, and further, control is performed to increase the rotational speed of the fan according to the rotational speed correction value. A premixing device characterized by being configured as such.

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