Premixing apparatus

Abstract

In a premixing apparatus in which a butterfly valve is provided on an upstream side of a downstream end of a gas suction part to which the gas supply passage is connected, a failure to secure the necessary amount of air over a long period of time when the butterfly valve is brought into a closed position can be prevented. A ventilation hole, through which air flows from the upstream side to the downstream side of the butterfly valve when the butterfly valve is in the closed position, is formed in the butterfly valve. In addition, a protrusion extending in a thickness direction of the butterfly valve is provided, on each side surface of the butterfly valve, at a downstream part of a periphery of the ventilation hole when the butterfly valve is in an open position.

Description

TECHNICAL FIELD

[0001] The invention relates to a premixing apparatus that mixes fuel gas with air and supplies an air-fuel mixture to a burner through a fan.BACKGROUND ART

[0002] Conventionally, as a premixing apparatus of this kind, there has been known the premixing apparatus in which a downstream end of a gas supply passage is connected to a gas suction part that is disposed in an air supply passage positioned on an upstream side of the fan, and a butterfly valve is provided at a part of the air supply passage positioned on the upstream side of the gas suction part (for example, Patent Document 1). In this premixing apparatus, the butterfly valve is pivotable, about a pivot-shaft line extending along a predetermined diametrical direction of the butterfly valve, between at least two positions: an open position in which the butterfly valve is oriented parallel to a longitudinal direction of the air supply passage, a closed position in which the butterfly valve is oriented orthogonal to the longitudinal direction of the air supply passage. When the butterfly valve is brought into the closed position, air flows through a clearance defined between an outer circumferential edge of the butterfly valve and a circumferential wall surface of the air supply passage.

[0003] Meanwhile, when the butterfly valve is brought into the closed position, air that collides with a side surface facing the upstream side of the air supply passage flows through the clearance defined between the outer circumferential edge of the butterfly valve and the circumferential surface of the air supply passage with a large velocity component directed outward in the radial direction. Then, due to the large velocity component directed outward in the radial direction, dust contained in air strongly collides with the circumferential surface of the air supply passage and deposit and accumulate on the circumferential surface easily. Therefore, the dust deposits and accumulates on the circumferential surface of the air supply passage within a relatively short period, and when the butterfly valve is brought into the closed position, the clearance defined between the outer circumferential edge of the butterfly valve and the circumferential surface of the air supply passage tends to be clogged. As a result, the amount of air flowing through the clearance decreases, making it difficult to secure the necessary amount of air.

[0004] In view of these circumstances, the applicant previously proposed a butterfly valve disclosed in Japanese Patent Application No. 2022-141136, in which a ventilation hole is formed to allow air to pass from the upstream side to the downstream side of the butterfly valve when it is in the closed position. According to the butterfly valve described above, when the butterfly valve is brought into the closed position, air flows through the ventilation hole, thereby preventing a decrease in the amount of air caused by clogging of the clearance defined between the outer circumferential edge of the butterfly valve and the circumferential surface of the air supply passage.

[0005] However, even in the butterfly valve proposed in the Japanese Patent Application mentioned above, it was found that the following problem occurs: when the butterfly valve is brought into the open position, part of air strongly collides with a downstream portion of the circumferential surface of the ventilation hole when the butterfly valve is in the open position, causing the dust to deposit and accumulate on this portion. Therefore, an opening area of the ventilation hole decreases, so that when the butterfly valve is brought into the closed position, the necessary amount of air cannot be secured.PRIOR ARTPatent Document

[0006] Patent Document 1: JP2021-25722ASUMMARY OF INVENTIONProblem That the Invention is to be Solved

[0007] The invention has been made in view of the point mentioned above, and addresses a problem of providing a premixing apparatus that can prevent a failure to secure the necessary amount of air to be secured over a long period when the butterfly valve is brought into the closed position.Means for Solving the Problem

[0008] In order to solve the problem described above, the invention provides a premixing apparatus in which fuel gas is mixed with air and an air-fuel mixture is supplied to a burner through a fan, comprising: a gas supply passage, having a downstream end connected to a gas suction part that is disposed in an air supply passage positioned on an upstream side of the fan; a butterfly valve, provided in a portion of the air supply passage positioned on the upstream side of the gas suction part, the butterfly valve being pivotable, about a pivot-shaft line extending along a predetermined diametrical direction of the butterfly valve, between at least two positions: an open position in which the butterfly valve is oriented parallel to a longitudinal direction of the air supply passage; and a closed position in which the butterfly valve is oriented orthogonal to the longitudinal direction of the air supply passage; and a ventilation hole, formed in the butterfly valve, through which air flows from the upstream side to the downstream side of the butterfly valve when the butterfly valve is in the closed position, wherein a protrusion extending outward in a thickness direction of the butterfly valve is provided, on each side surface of the butterfly valve that faces the upstream side or the downstream side when the butterfly valve is in the closed position, at a location corresponding to a portion of a periphery of the ventilation hole that becomes the upstream side when the butterfly valve is in the open position.

[0009] According to the invention, when the butterfly valve is in the open position, the protrusion serves as a wind-preventing wall, preventing air from colliding with a downstream part of a circumferential surface of the ventilation hole when the butterfly valve is in the open position. Therefore, a decrease in an opening area of the ventilation hole, resulting from dust deposition and accumulation on that portion of the circumferential surface of the ventilation hole, can be suppressed. Accordingly, when the butterfly valve is brought into the closed position, a failure to secure the necessary amount of air caused by the decrease in the opening area of the ventilation hole can be prevented.

[0010] In the invention, it is desirable that a part of a circumferential surface of the ventilation hole, which becomes the downstream side when the butterfly valve is in the open position, is formed as an inclined surface slopping toward the downstream side. According to this, even if the dust deposits on the downstream part of the circumferential surface of the ventilation hole when the butterfly valve is in the open position, the effective opening area of the ventilation hole does not decrease. Accordingly, when the butterfly valve is brought into the closed position, the failure to secure the necessary amount of air can be prevented more effectively.

[0011] In addition, generally, at a central part of each side face along the pivot-shaft line, a raised portion projecting the most raised in the thickness direction is provided. Furthermore, the ventilation hole is formed in each part of the butterfly valve, located on the upstream side and downstream side of the raised portion when the butterfly valve is in the open position. In this case, it is desirable that the protrusion provided around the ventilation hole, formed in a part of the butterfly valve, located on the upstream side of the raised portion when the butterfly valve is in the open position, has extension parts extending to the vicinity of the raised portion, so that, when the butterfly valve is in the open position, the extension parts surround the downstream part of the periphery of the ventilation hole from both sides in the longitudinal direction of the pivot-shaft line of the butterfly valve. According to this, when the butterfly valve is brought into the open position, air is prevented from flowing around in the direction of the pivot-shaft line and colliding with a part of the side surface of the butterfly valve that is positioned between the raised portion and the ventilation hole. Accordingly, the decrease in the effective opening area of the ventilation hole caused by the deposition of the dust due to the collision of air with the part mentioned above can be effectively suppressed.

[0012] In addition, each side surface of the butterfly valve is generally inclined so that the thickness of the butterfly valve gradually decreases with increasing distance from the raised portion. Consequently, when the valve is in the open position, the dust tends to deposit on a half of each side surface of the butterfly valve that is positioned on the upstream side (an upstream half of each side surface of the butterfly valve). Thus, when the dust is deposited near the outer circumferential edge of each side surface of the butterfly valve, pivoting the butterfly valve from the open position to the closed position may cause the dust to be trapped between the outer circumferential edge and the circumferential wall surface of the air supply passage, potentially preventing the butterfly valve from achieving the closed position. Then, in the invention, it is desirable that a half of each side surface of the butterfly valve, which is located on the upstream side when the butterfly valve is in the open position, is arcuately inclined such that an inclination angle in the thickness direction of the butterfly valve gradually becomes more gentle toward regions farther from the raised portion. According to this, when the butterfly valve is brought into the open position, it becomes difficult for the dust to deposit around the outer circumferential edge of the upstream half of each side surface of the butterfly valve. Therefore, occurrence of the trouble mentioned above can be suppressed.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a cut side sectional view showing a premixing apparatus according to an embodiment of the invention.

[0014] FIG. 2 is a perspective view showing a butterfly valve provided with the premixing apparatus of the embodiment when the butterfly valve is in an open position.

[0015] FIG. 3 is a cross-sectional view taken along a III-III line of FIG. 1.

[0016] FIG. 4 is a cross-sectional view taken along a IV-IV line of FIG. 1.MODES FOR CARRYING OUT THE INVENTION

[0017] A combustion apparatus illustrated in FIG. 1 is a heat source machine provided with a totally primary-aerated burner 1, a combustion box 2 surrounding a combustion space of an air-fuel mixture to be ejected from a combustion surface la of the burner 1, and a heat exchanger 3 disposed within the combustion box 2. A combustion gas generated by combustion of the air-fuel mixture transfers heat to the heat exchanger 3 and is subsequently exhausted through an exhaustion pipe 4 connected to one end of the combustion box 2. By means of a premixing apparatus according to an embodiment of the invention, a fuel gas is mixed with air, and the resulting air-fuel mixture is supplied to the burner 1 through a fan 5.

[0018] The premixing apparatus A is provided with the fan 5, an air supply passage 6 positioned on an upstream side of the fan 5, and a gas supply passage 7 for supplying the fuel gas. A downstream end of the gas supply passage 7 is connected to a gas suction part 61 disposed in the air supply passage 6. A butterfly valve 8, as mentioned below, is provided on the upstream side of the gas suction part 61 in the air supply passage 6. In addition, a venturi part 62, having a smaller diameter than that of a part in which the butterfly valve 8 is disposed, is positioned on the downstream side of the part in which the butterfly valve 8 is disposed within the air supply passage 6. A part of the air supply passage 6 adjacent to the downstream side of the venturi part 62 is surrounded by a cylindrical part 63 having a larger diameter than that of the venturi part 62. Furthermore, a downstream end portion of the venturi part 62 is inserted into an upstream end portion of the cylindrical part 63 via an annular clearance, which defines the gas suction part 61.

[0019] A gas chamber 71 communicating with the gas suction part 61 is provided at a downstream end of the gas supply passage 7 so as to surround the cylindrical part 63. In addition, a master valve 72, a zero governor 73 that regulates a secondary pressure to atmospheric pressure, and a variable throttle valve 74 are interposed in sequence, from the upstream side, in the gas supply passage 7.

[0020] The amount of fuel gas supplied through the gas suction part 61 varies in accordance with a differential pressure between the atmospheric pressure, serving as the secondary pressure, and a negative pressure acting on the gas suction part 61. Here, the negative pressure acting on the gas suction part 61 varies depending on the rotational speed of the fan 5. Therefore, the amount of fuel gas supplied is proportional to the rotational speed of the fan 5, that is, to the amount of air supplied. In addition, a ratio between the amounts of fuel gas and air supplied changes depending on an opening degree of the variable throttle valve 74. An air excess ratio of the air-fuel mixture is set to a predetermined appropriate value (for example, 1.3) by adjusting the opening degree of the variable throttle valve 74 to a predetermined standard opening degree corresponding to the type of gas used. Furthermore, the air-fuel mixture having the appropriate air excess ratio and an amount corresponding to the required combustion amount (i.e., the combustion amount necessary to discharge hot water at a set temperature) is supplied to the burner 1 by controlling the rotational speed of the fan 5 in accordance with the required combustion amount.

[0021] It should be noted that, in order to prevent poor exhaustion due to intrusion of wind into the exhaust pipe 4, that is, to ensure wind-resistant property, a lower limit of the rotational speed of the fan 5 cannot be set too low. When the required combustion amount is smaller than a predetermined value corresponding to the lower limit of the rotational speed of the fan 5, the amount of air corresponding to the required combustion amount cannot be supplied.

[0022] Therefore, the butterfly valve 8 is disposed in a part of the air supply passage positioned on the upstream side of the gas suction part 61, in order to switch the ventilation resistance of that part between two stages: large and small. The butterfly valve 8 is pivotable, about a pivot-shaft line 8a extending along a predetermined diametrical direction of the butterfly valve 8, between two positions: an open position, shown by an imaginary line in FIG. 1, in which the butterfly valve 8 is oriented parallel to the longitudinal direction of the air supply passage 6; and a closed position, shown by a solid line in FIG. 1, in which the butterfly valve 8 is oriented orthogonal to the longitudinal direction of the air supply passage 6. A pivot shaft 81, which is driven by a motor (not shown) and corresponds to the pivot-shaft line 8a described above, is connected to the butterfly valve 8. When the required combustion amount is smaller than the predetermined value mentioned above, the butterfly valve 8 is brought to the closed position to increase the ventilation resistance. This enables the amount of air corresponding to the combustion amount smaller than the predetermined value to be supplied without lowering the rotational speed of the fan 5 below the lower limit of the rotational speed.

[0023] It should be noted that, when the butterfly valve 8 is in the closed position, since only the ventilation resistance of the air supply passage 6 increases, the negative pressure acting on the gas suction part 61 increases, thereby causing the supply amount of the fuel gas to be excessively large, as a result, air excess ratio of the fuel-gas mixture supplied to the burner 1 lowers than the appropriate value. Therefore, when the required combustion amount is relatively small, the butterfly valve 8 is brought into the closed position, and in addition, the opening degree of the variable throttle valve 74 is set to a small-capacity standard opening which is predetermined and relatively small, so that the air excess ratio of the air-fuel mixture becomes the appropriate value in the state where the butterfly valve 8 is in the closed position, whereby the combustion capacity switches to a small capacity. Accordingly, the amount of the air-fuel mixture, having the appropriate air excess ratio and corresponding to the relatively small combustion amount, is supplied to the burner 1. In addition, when the required combustion amount is relatively large, the butterfly valve 8 is brought into the open position, and in addition, the opening degree of the variable throttle valve 74 is set to a large-capacity standard opening which is predetermined and relatively large, so that the air excess ratio of the air-fuel mixture becomes the appropriate value in the state where the butterfly valve 8 is in the open position, whereby the combustion capacity switches to a large capacity. Accordingly, the amount of the air-fuel mixture, having the appropriate air excess ratio and corresponding to the relatively large combustion amount, is supplied to the burner 1.

[0024] Now, with reference also to FIGS. 2 to 4, the butterfly valve 8 will be described in more detail. Four ventilation holes 82, through which air passes from the upstream side to the downstream side of the butterfly valve 8 in the closed position, are formed at intervals of 90° in the circumferential direction. It should be noted that, at a central part along the pivot-shaft line 8a of each side face 83 of the butterfly valve 8 that faces the upstream side or the downstream side when the butterfly valve 8 is in the closed position, a raised portion 83a, which projects the most in the thickness direction, is provided in order to form a shaft hole 81a for fitting the pivot shaft 81 in the butterfly valve 8. Two ventilation holes 82 are formed in each part of the butterfly valve 8 located on the upstream and the downstream side of the raised part 83a when the butterfly valve 8 is in the open position. In accordance with the formation of the ventilation holes 82 in the butterfly valve 8 as described above, when the butterfly valve 8 is brought into the closed position, air flows through the ventilation holes 82. Therefore, a decrease in the amount of air caused by clogging of a clearance between an outer circumferential edge of the butterfly valve 8 and a circumferential wall surface of the air supply passage 6 can be prevented.

[0025] It should be noted that, when the butterfly valve 8 is in the open position, a part of air strongly collides with a downstream part (an upper side part in FIGS. 2, 3) of a circumferential surface of each ventilation hole 82 when the butterfly valve 8 is in the open position, causing dust to deposit and accumulate on that part. In addition, when the dust is deposited and accumulated, since an opening area of each ventilation hole 82 decreases, when the butterfly valve 8 is brought into the closed position, the necessary amount of air cannot be secured.

[0026] Then, in the embodiment, a protrusion 84 extending outward in a thickness direction of the butterfly valve 8 is provided, on each side surface of the butterfly valve 8, at a location corresponding to a portion of a periphery of the butterfly valve 8 that becomes the upstream side (a lower side portion in FIGS. 2, 3) when the butterfly valve 8 is in the open position, more preciously, at a half portion of the periphery on the upstream side. According to this, when the butterfly valve 8 is in the open position, the protrusion 84 serves as a wind-preventing wall, preventing air from strongly colliding with a downstream part of a circumferential surface of each ventilation hole 82 when the butterfly valve 8 is in the open position. Therefore, a decrease in the opening area of each ventilation hole 82, resulting from dust deposition and accumulation on that part of the circumferential surface of each ventilation hole 82, can be suppressed. Accordingly, when the butterfly valve 8 is brought into the closed position, a failure to secure the necessary amount of air caused by the decrease in the opening area of each ventilation hole 82 can be prevented over a long period.

[0027] In addition, in the embodiment, the downstream part of the circumferential surface of each ventilation hole 82, when the butterfly valve 8 is in the open position, is formed as an inclined surface 82a sloping toward the downstream side. According to this, even if the dust deposits on the inclined surface 82a that becomes the downstream part of the circumferential surface of each ventilation hole 82 when the butterfly valve is in the open position, the effective opening area of each ventilation hole 82 does not decrease. Accordingly, when the butterfly valve 8 is brought into the closed position, the failure to secure the necessary amount of air can be prevented more effectively.

[0028] Furthermore, the protrusion 84 provided around each ventilation hole 82, formed in the part of the butterfly valve 8 located on the upstream side of the raised portion 83a when the butterfly valve 8 is in the open position, has extension parts 84a, 84a extending to the vicinity of the raised portion 83a, so that, when the butterfly valve 8 is in the open position, the extension parts 84, 84a surround the downstream part of the periphery of each ventilation hole 82 from both sides in the longitudinal direction of the pivot-shaft line 8a of the butterfly valve 8. According to this, when the butterfly valve 8 is brought into the open position, air is prevented from flowing around in the direction of the pivot-shaft line and colliding with a part of the side surface 83 of the butterfly valve 8 that is positioned between the raised portion 83a and each ventilation hole 82. Here, when air collides with and deposits on the part mentioned above of the side surface 83 of the butterfly valve 8, the dust gradually accumulates from that part to extend into each ventilation hole 82. As a result, the effective opening area of each ventilation hole 82 decreases. In contrast, in the embodiment, the deposition of the dust due to the collision of air with the part mentioned above of the side surface 83 of the butterfly valve 8 can be suppressed. Therefore, the decrease in the effective opening area of each ventilation hole 82 caused by the deposition of the dust due to the collision of air with the part mentioned above can be effectively suppressed.

[0029] Meanwhile, each side surface 83 of the butterfly valve 8 is inclined so that the thickness of the butterfly valve 8 gradually decreases with increasing distance from the raised portion 83a. Consequently, when the valve 8 is in the open position, the dust tends to deposit on an a half of each side surface 83 of the butterfly valve 8 that is positioned on the upstream side. Thus, when the dust is deposited near the outer circumferential edge of each side surface 83 of the butterfly valve 8, pivoting the butterfly valve 8 from the open position to the closed position may cause the dust to be trapped between the outer circumferential edge and the circumferential wall surface of the air supply passage 6, potentially preventing the butterfly valve 8 from achieving the closed position.

[0030] Then, in the embodiment, a half of each side surface 83 of the butterfly valve 8 that is positioned on the upstream side when the valve8 is in the open position (hereinafter, referred to as a upstream half) is, as shown in FIG. 4, arcuately inclined such that an inclination angle in the thickness direction of the butterfly valve 8 gradually becomes more gentle toward regions farther from the raised portion 83a. According to this, when the butterfly valve 8 is brought into the open position, it becomes difficult for the dust to deposit around the outer circumferential edge of the upstream half of each side surface 83 of the butterfly valve 8. Therefore, the failure to pivot the butterfly valve 8 to the closed position, which is caused by the dust trapped between the outer circumferential edge and the circumferential wall surface of the air supply passage 6, is effectively prevented.

[0031] It should be noted that a half of each side surface 83 of the butterfly valve 8 that is positioned on the downstream side when the butterfly valve 8 is in the open position (hereinafter, referred to as a downstream half) is also arcuately inclined. However, since possibility of dust depositing on the downstream half is low, the downstream half may be inclined linearly.

[0032] In the above, the embodiment of the invention has been explained with reference to the figures; however, the invention is not limited to this embodiment. For example, in the embodiment described above, although the butterfly valve 8 is pivotable between two positions, namely the open position and the closed position, the butterfly valve 8 may alternatively be pivotable among three or more positions, including an intermediate position between the open position and the closed position. In addition, instead of the four ventilation holes of the embodiment, only one ventilation hole having a total opening area equal to that of the four ventilation holes may be formed at the central part of the butterfly valve 8, or a plurality of ventilation holes other than the four may be formed in the butterfly valve 8.EXPLANATION OF MARKS1 Burner

[0034] 5 Fan

[0035] 6 Air supply passage

[0036] 61 Gas suction part

[0037] 7 Gas supply passage

[0038] 8 Butterfly valve

[0039] 8a Pivot-shaft line

[0040] 82 Ventilation hole

[0041] 82a Inclined surface of downstream part of circumferential of ventilation hole

[0042] 83 Side surface of butterfly valve

[0043] 83a Raised portion

[0044] 84 Protrusion

[0045] 84a Extension part of protrusion

Claims

1. A premixing apparatus in which fuel gas is mixed with air and an air-fuel mixture is supplied to a burner through a fan, comprising:a gas supply passage, having a downstream end connected to a gas suction part that is disposed in an air supply passage positioned on an upstream side of the fan;a butterfly valve, provided in a portion of the air supply passage positioned on the upstream side of the gas suction part, the butterfly valve being pivotable, about a pivot-shaft line extending along a predetermined diametrical direction of the butterfly valve, between at least two positions: an open position in which the butterfly valve is oriented parallel to a longitudinal direction of the air supply passage; and a closed position in which the butterfly valve is oriented orthogonal to the longitudinal direction of the air supply passage; anda ventilation hole, formed in the butterfly valve, through which air flows from the upstream side to the downstream side of the butterfly valve when the butterfly valve is in the closed position,wherein a protrusion extending outward in a thickness direction of the butterfly valve is provided, on each side surface of the butterfly valve that faces the upstream side or the downstream side when the butterfly valve is in the closed position, at a location corresponding to a portion of a periphery of the ventilation hole that becomes the upstream side when the butterfly valve is in the open position.

2. The premixing apparatus according to claim 1, wherein a part of a circumferential surface of the ventilation hole, which becomes the downstream side when the butterfly valve is in the open position, is formed as an inclined surface slopping toward the downstream side.

3. The premixing apparatus according to claim 1, wherein at a central part of each side face along the pivot-shaft line, a raised portion projecting the most raised in the thickness direction is provided, and the ventilation hole is formed in each part of the butterfly valve, located on the upstream side and the downstream side of the raised portion when the butterfly valve is in the open position,wherein:the protrusion provided around the ventilation hole, formed in a part of the butterfly valve, located on the upstream side of the raised portion when the butterfly valve is in the open position, has extension parts extending to the vicinity of the raised portion, so that, when the butterfly valve is in the open position, the extension parts surround the downstream part of the periphery of the ventilation hole from both sides in the longitudinal direction of the pivot-shaft line of the butterfly valve.

4. The premixing apparatus according to claim 1, wherein at a central part of each side face along the pivot-shaft line, a raised portion projecting the most raised in the thickness direction is provided, and each side surface of the butterfly valve is inclined so that the thickness of the butterfly valve gradually decreases with increasing distance from the raised portion,wherein:a half of each side surface of the butterfly valve, which is located on the upstream side when the butterfly valve is in the open position, is arcuately inclined such that an inclination angle in the thickness direction of the butterfly valve gradually becomes more gentle toward regions farther from the raised portion.

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