PREMIXING APPARATUS

Abstract

In a premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan, a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the passage, which is located on an upstream side from the gas suction portion, and is rotatable to at least an open attitude and a closed attitude. At least one ventilation opening through which air flows from an upstream side to a downstream side of the valve at the closed attitude is formed in the valve, and a size of the valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the valve and a peripheral wall surface of the passage is substantially closed.

Description

TECHNICAL FIELD

The invention relates to a premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan.

BACKGROUND ART

Conventionally, as a premixing apparatus of this type, there has been known the premixing apparatus in which a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the air supply passage, which is located on an upstream side from the gas suction portion (see Patent Document No. 1, for example). In the aforementioned premixing apparatus, provided that a rotational axis line along a predetermined radial direction of the butterfly valve serves as a center, the butterfly valve is configured to be rotatable to at least two attitudes: an open attitude parallel to a longitudinal direction of the air supply passage and a closed attitude orthogonal to the longitudinal direction of the air supply passage. Accordingly, when the butterfly valve is rotated to the closed attitude, air flows through a clearance between an outer peripheral edge of the butterfly valve and a peripheral wall surface of the air supply passage.

Incidentally, when the butterfly valve is rotated to the closed attitude, air colliding with a surface of the butterfly valve, which faces an upstream side of the air supply passage, has a large velocity component directed outward in the radial direction and flows into the clearance between the outer peripheral edge of the butterfly valve and the peripheral wall surface of the air supply passage. Due to the large velocity component directed outward in the radial direction, dust in the air collides strongly with the peripheral wall surface of the air supply passage and tends to stick to the peripheral wall surface. Therefore, the dust sicks to and accumulates on the peripheral wall surface of the air supply passage in a relatively short period, and when the butterfly valve is rotated to the closed attitude, the aforementioned clearance is clogged. As a result, the amount of air flowing into the clearance is reduced, preventing the necessary amount of air from being maintained.

REFERENCE

Patent Document No. 1: JP2021-025722 A

SUMMARY OF INVENTION

Technical Problems

In light of the aforementioned problem, the invention provides a premixing apparatus that can prevent not-maintaining of the necessary amount of air over a long period when the butterfly valve is rotated to the closed attitude.

Solution to Problems

In order to solve the aforementioned problem, the invention presupposes a premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan, wherein a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the air supply passage, which is located on an upstream side from the gas suction portion, and wherein, provided that a rotational axis line along a predetermined radial direction of the butterfly valve serves as a center, the butterfly valve is configured to be rotatable to at least two attitudes: an open attitude parallel to a longitudinal direction of the air supply passage and a closed attitude orthogonal to the longitudinal direction of the air supply passage. In the premixing apparatus, at least one ventilation opening through which air flows from an upstream side to a downstream side of the butterfly valve at the closed attitude is formed in the butterfly valve, and a size of the butterfly valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the butterfly valve and a peripheral wall surface of the air supply passage is substantially closed, and air flows only through the ventilation opening.

According to the invention, when the butterfly valve is rotated to the closed attitude, air does not have a large velocity component directed outward in the radial direction and passes through the ventilation opening. Therefore, sticking of dust to a peripheral edge of the ventilation opening is suppressed, making clogging of the ventilation opening unlikely. Additionally, since the clearance between the outer peripheral edge of the butterfly valve and the peripheral wall surface of the air supply passage is substantially closed originally at the closed attitude of the butterfly valve, the amount of air is not reduced due to the clogging in the clearance. Accordingly, when the butterfly valve is rotated to the closed attitude, not-maintaining of the necessary amount of air over an extended period can be prevented over a long period.

Additionally, in the invention, it is desirable that multiple ventilation openings are formed, spaced in a circumferential direction of the butterfly valve. This can avoid hindrance of fuel-gas suction in the gas suction portion, which is caused by air drifting to only a part of the air supply passage on the downstream side of the butterfly valve when the butterfly valve is rotated to the closed attitude.

Meanwhile, in the premixing apparatus, generally, in the air supply passage, a venturi portion, which is located on a downstream side from the portion in which the butterfly valve is disposed and has a smaller diameter than the portion in which the butterfly valve is disposed, is provided, and the gas suction portion is provided to surround the venturi portion. In this case, it is desirable that each of the ventilation openings is configured so that a part of each of the ventilation openings expands outward in the radial direction from a plane to which the venturi portion is projected upstream when the butterfly valve is rotated to the closed attitude. According to this, a ratio of air flowing near a peripheral surface of the venturi portion to air passing through each of the ventilation openings is increased. Therefore, a proportional relationship between a flow velocity of air near the peripheral surface of the venturi portion and the supply amount of air becomes easily maintained. Here, a negative pressure acting on the gas suction portion surrounding the venturi portion is approximately proportional to the flow velocity of air near the peripheral surface of the venturi portion. As a result, a ratio between the supply amount of the fuel gas supplied through the gas suction portion and the supply amount of air, i.e., an air excessive ratio of the air-fuel mixture, can be kept constant.

Additionally, it is desirable that each of the ventilation openings is circular in shape that has the shortest peripheral edge length for a same opening area. According to this, the clogging of each of the ventilation openings can be suppressed by shortening the length of the peripheral edge to which dust sticks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut side view showing a premixing apparatus as an embodiment of the invention.

FIG. 2 is a cut plane view cut along a II-II line indicated in FIG. 1.

FIG. 3 is a perspective view of a butterfly valve provided with the premixing apparatus of the embodiment.

DESCRIPTION OF EMBODIMENTS

A combustion apparatus shown in FIG. 1 is a heat source machine that includes a burner 1 preforming totally primary combustion, a combustion box 2 surrounding a combustion space of an air-fuel mixture ejected from a combustion surface 1a 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 is discharged outside through an exhaust pipe 4 connected to an end portion of the combustion box 2 after heating the heat exchanger 3. Additionally, a fuel gas is mixed with air, and the air-fuel mixture is supplied to the burner 1 through a fan 5, by a premixing apparatus A that is an embodiment of the invention.

The premixing apparatus A includes the fan 5, an air supply passage 6 located on an upstream side of the fan 5, and a gas supply passage 7 supplying the fuel gas. A downstream end of the gas supply passage 7 is connected to a gas suction portion 61 provided in the air supply passage 6. A butterfly valve 8, as mentioned below, is provided in a portion of the air supply passage 6, which is located on an upstream side from the gas suction portion 61. Additionally, a venturi portion 62, which is located on a downstream side from the portion in which the butterfly valve 8 is disposed and has a smaller diameter than the portion in which the butterfly valve 8 is disposed, is provided in the air supply passage 6. A portion of the air supply passage 6, which is adjacent to a downstream side of the venturi portion 62, is surrounded by a cylindrical portion 63 that has a larger diameter than the venturi portion 62. A downstream end portion of the venturi portion 62 is inserted into an upstream end portion of the cylindrical portion 63 with an annular clearance, and the gas suction portion 61 is configured by the annular clearance. Accordingly, the gas suction portion 61 is provided to surround the venturi portion 62.

A gas chamber 71 that communicates with the gas suction portion 61 is provided in a downstream end of the gas supply passage 7 to surround the cylindrical portion 63. A master valve 72, a zero governor 73 regulating a secondary pressure to atmospheric pressure, and a variable throttle valve 74 are interposed in the gas supply passage 7 in subsequence from an upstream side.

The amount of the fuel gas supplied through the gas suction portion 61 varies according to a differential pressure between the secondary pressure, equal to atmospheric pressure, and a negative pressure acting on the gas suction portion 61. Here, the negative pressure acting on the gas suction portion 61 varies with a rotation speed of the fan 5. Therefore, the supply amount of the fuel gas varies proportionally with the rotation speed of the fan 5, i.e., the supply volume of air. Additionally, a ratio between the supply amount of the fuel gas and the supply amount of air is varied by an opening degree of the variable throttle valve 74. An air excessive ratio of the air-fuel mixture can be adjusted to a predetermined appropriate value (e.g., 1.3) by regulating the opening degree of the variable throttle valve 74 to a predetermined standard opening degree according to a kind of gas being used. The air-fuel mixture, with the air excessive ratio regulated to the predetermined appropriate value and the amount corresponding to a required combustion amount (a combustion amount needed to discharge hot water at a set temperature), can be supplied to the burner 1 by controlling the rotation speed of the fan 5 according to the required combustion amount.

Incidentally, in order to prevent exhaust failure due to wind intrusion into the exhaust pipe 4, in other words, in order to ensure wind-resistant performance, a lower limit of the rotation speed of the fan 5 cannot be to set too low. In a case where the required combustion amount becomes the predetermined value corresponding to the lower limit of the rotation speed of the fan 5 or less, air of the amount corresponding to the required combustion amount cannot be supplied.

Then, the butterfly valve 8 is disposed in a portion of the air supply passage 6, which is located on the upstream side from the gas suction portion 61, to switch ventilation resistance of the aforementioned portion of the air supply passage 6 to two stages: large and small. Provided that a rotational axis line 8a along a predetermined radial direction of the butterfly valve 8 serves as a center, the butterfly valve 8 is rotated by a motor, not shown, to an open attitude parallel to a longitudinal direction of the air supply passage 6, as indicated by an imaginary line in FIG. 1, and to a closed attitude orthogonal to the longitudinal direction of the air supply passage 6, as indicated by a solid line in FIG. 1. In the case where the required combustion amount becomes the aforementioned predetermined value or less, the butterfly valve 8 is rotated to the closed attitude, thereby increasing the ventilation resistance of the air supply passage 6. This allows air of the amount corresponding to the combustion amount of the predetermined value or less to be supplied without lowering the rotation speed of the fan 5 to the lower limit or below.

Meanwhile, simply rotating the butterfly valve 8 to the closed attitude and increasing the ventilation resistance will increase the negative pressure acted on the gas suction portion 61. This results in an excessive supply amount of the fuel gas, and the air excess ratio of the air-fuel mixture supplied to the burner 1 is lowered below the appropriate value. Therefore, in a case where the required combustion amount is relatively small, a combustion capacity is switched to a small capacity by rotating the butterfly valve 8 to the closed attitude and regulating the opening degree of the variable throttle valve 74 to a preset small-capacity standard opening degree such that the air excessive ratio of the air-fuel mixture becomes the appropriate value at the closed attitude of the butterfly valve 8. This enables the air-fuel mixture, of which the air excessive ratio is the appropriate value and the amount corresponds to the relatively small combustion amount, to be supplied to the burner 1. Additionally, in a case where the required combustion amount is relatively large, the combustion capacity is switched to a large capacity by rotating the butterfly valve 8 to the open attitude and regulating the opening degree of the variable throttle valve 74 to a preset large-capacity standard opening degree such that the air excessive ratio of the air-fuel mixture becomes the appropriate value at the open attitude of the butterfly valve 8. This enables the air-fuel mixture, of which the air excessive ratio is the appropriate value and the amount corresponds to the relatively large combustion amount, to be supplied to the burner 1.

Now, referring also to FIGS. 2, 3, the butterfly valve 8 will be described more in detail. A rotary shaft 81, which is connected to the motor and conforms with the rotational axis line 8a, is connected to the butterfly valve 8. At least one ventilation opening 82 through which air flows from an upstream side to a downstream side of the butterfly valve 8 is formed in the butterfly valve 8. A size of the butterfly valve 8 is set so that, at the closed attitude, a clearance between an outer peripheral edge 8b of the butterfly valve 8 and a peripheral wall surface 6a of the air supply passage 6 is substantially closed, and air flows only through the ventilation opening 82. Specifically, only a sliding clearance necessary for rotating the butterfly valve 8 is created between the outer peripheral edge 8b and the peripheral wall surface 6a at the closed attitude of the butterfly valve 8, with no clearance for air flow. This is definition of the aforementioned phrase, “a clearance is substantially closed”.

According to the embodiment, when the butterfly valve 8 is rotated to the closed attitude, air does not have a large velocity component directed outward in the radial direction and passes through the ventilation opening 82. Therefore, sticking of dust to a peripheral edge of the ventilation opening 82 is suppressed, making clogging of the ventilation opening 82 unlikely. Additionally, since the clearance between the outer peripheral edge 8b of the butterfly valve 8 and the peripheral wall surface 6a of the air supply passage 6 is substantially closed originally at the closed attitude of the butterfly valve 8, the amount of air is not reduced due to the clogging in the clearance. Accordingly, when the butterfly valve 8 is rotated to the closed attitude, not-maintaining the necessary amount of air can be prevented over a long period.

Additionally, in the embodiment, multiple ventilation openings 82 are formed, spaced in a circumferential direction of the butterfly valve 8. Specifically, four ventilation openings 82 are arranged, each separated by 90°. This can avoid hindrance of fuel-gas suction in the gas suction portion 61, which is caused by air drifting to only a part of the air supply passage 6 on the downstream side of the butterfly valve 8 when the butterfly valve 8 is rotated to the closed attitude.

Meanwhile, for a same opening area, a shape with the shortest peripheral edge length is circular. Therefore, each of the ventilation openings 82 is circular in shape. According to this, the clogging of each of the ventilation openings 82 can be suppressed by shortening the length of the peripheral edge to which dust sticks.

Furthermore, each of the ventilation openings 82 is configured so that a part of each of the ventilation openings 82 expands outward in the radial direction from a plane to which the venturi portion 62 is projected upstream (a plane conforming with a circle indicated by a symbol 62 in FIG. 2) when the butterfly valve 8 is rotated to the closed attitude. According to this, a ratio of air flowing near a peripheral surface of the venturi portion 62 to air passing through each of the ventilation openings 82 is increased. Therefore, a proportional relationship between a flow velocity of air near the peripheral surface of the venturi portion 62 and the supply amount of air becomes easily maintained. Here, the negative pressure acting on the gas suction portion 61 surrounding the venturi portion 62 is approximately proportional to the flow velocity of air near the peripheral surface of the venturi portion 62. As a result, the ratio of the amount between the fuel gas supplied through the gas suction portion 61 and the supply amount of air, i.e., the air excessive ratio of the air-fuel mixture, can be kept constant.

The embodiment of the invention is explained referring to the figures in the above. On the other hand, the invention is not restricted to the aforementioned embodiment. For example, in the embodiment, while the butterfly valve 8 is configured to be rotatable to the two attitudes: the open attitude and the closed attitude, the butterfly valve 8 may be configured to be rotatable to three or more attitudes including the open and closed attitudes. Additionally, it may be possible to form a single ventilation opening, with an area equal to the total opening area of the four ventilation openings 82 in the embodiment, at a central portion of the butterfly valve 8. Alternatively, multiple ventilation openings 82 with a number other than four could be formed. In the case where the number of the ventilation openings 82 is increased, each of the ventilation openings 82 may be configured so that all parts of each of the ventilation openings 82 expand outward in the radial direction from the plane to which the venturi portion 62 is projected upstream.

EXPLANATION OF SYMBOLS

• • A Premixing apparatus
  • 1 Burner
  • 5 Fan
  • 6 Air supply passage
  • 6 a Peripheral wall surface
  • 61 Gas suction portion
  • 62 Venturi portion
  • 7 Gas supply passage
  • 8 Butterfly valve
  • 8 a Rotational axis line
  • 8 b Outer peripheral edge
  • 82 Ventilation opening

Claims

1. A premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan, wherein a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the air supply passage, which is located on an upstream side from the gas suction portion, andwherein, provided that a rotational axis line along a predetermined radial direction of the butterfly valve serves as a center, the butterfly valve is configured to be rotatable to at least two attitudes: an open attitude parallel to a longitudinal direction of the air supply passage and a closed attitude orthogonal to the longitudinal direction of the air supply passage,wherein:at least one ventilation opening through which air flows from an upstream side to a downstream side of the butterfly valve at the closed attitude is formed in the butterfly valve, and a size of the butterfly valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the butterfly valve and a peripheral wall surface of the air supply passage is substantially closed, and air flows only through the ventilation opening.

2. The premixing apparatus as claimed in claim 1, wherein multiple ventilation openings are formed, spaced in a circumferential direction of the butterfly valve.

3. The premixing apparatus as claimed in claim 2, wherein, in the air supply passage, a venturi portion, which is located on a downstream side from the portion in which the butterfly valve is disposed and has a smaller diameter than the portion in which the butterfly valve is disposed, is provided, and the gas suction portion is provided to surround the venturi portion, wherein:each of the ventilation openings is configured so that a part of each of the ventilation openings expands outward in the radial direction from a plane to which the venturi portion is projected upstream when the butterfly valve is rotated to the closed attitude.

4. The premixing apparatus as claimed in claim 1, wherein each of the ventilation openings is circular in shape.