Premixing apparatus and method thereof

Abstract

A method of controlling a premixing apparatus that mixes fuel gas with air and supplies thus obtained air-fuel mixture, through a fan to a burner, includes at a time of changing over to a small-capacity state in which a first ventilation resistance in an air supply passage is increased and also in which a second ventilation resistance in a gas supply passage is increased, rotating a butterfly valve down to a closing side stop angle; and at a time of changing over to a large-capacity state in which the first ventilation resistance in the air supply passage is decreased and also in which the second ventilation resistance in the gas supply passage is decreased, rotating the butterfly valve up to an open-side stop angle.

Claims

1. A method of controlling a premixing apparatus that mixes fuel gas with air and supplies thus obtained air-fuel mixture, through a fan, to a burner, the premixing apparatus being so arranged that a downstream end of a gas supply passage having interposed therein a flow control valve for supplying fuel gas is connected to a gas suction section disposed in an air supply passage on an upstream side of the fan, the premixing apparatus comprising: an air resistance changeover means for changing over, between high and low, a first ventilation resistance in a first section of the air supply passage wherein the first section is on an upstream side of the gas suction section; and a gas resistance changeover means for changing over, between high and low, a second ventilation resistance in a second section of the gas supply passage wherein the second section is on the downstream side of the flow control valve, the air resistance changeover means being constituted by a butterfly valve disposed in the first section of the air supply passage so as to be rotatable into an opened posture parallel with a longitudinal direction of the air supply passage or a closed posture at right angles to the longitudinal direction of the air supply passage, and the gas resistance changeover means being constituted by a changeover valve disposed in the gas supply passage in an openable and closable manner, and the premixing apparatus further comprising an interlocking mechanism to open or close the changeover valve in interlocking relationship with a rotation of the butterfly valve into the opened posture or into the closed posture, wherein the interlocking mechanism is arranged to maintain the changeover valve in a fully-closed state when the butterfly valve lies in a predetermined first rotation angle range inclusive of the closed posture, and to maintain the changeover valve in a fully-opened state when the butterfly valve lies in a predetermined second rotation angle range inclusive of the opened posture; wherein the method of controlling the premixing apparatus comprises: a step of setting the rotation angle of the butterfly valve within the first rotation angle range to a closing-side stop angle of the butterfly valve at which an excess air ratio of the air-fuel mixture becomes a predetermined value; a step of setting the rotation angle of the butterfly valve within the second rotation angle range to an open-side stop angle of the butterfly valve at which an excess air ratio of the air-fuel mixture becomes a predetermined value; at a time of changing over to a small-capacity state in which the first ventilation resistance in the air supply passage is increased relative to a large capacity state and also in which the second ventilation resistance in the gas supply passage is increased relative to the large capacity state, a step of rotating the butterfly valve down to the closing side stop angle; and at a time of changing over to a large-capacity state in which the first ventilation resistance in the air supply passage is decreased relative to a small capacity state and also in which the second ventilation resistance in the gas supply passage is decreased relative to the small capacity state, a step of rotating the butterfly valve up to the open-side stop angle.

2. The method of controlling the premixing apparatus according to claim 1, further comprising a step of performing an ignition operation for the burner in a state in which the butterfly valve is rotated to the closed posture.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a side view, partly shown in section, of a premixing apparatus according to an embodiment of this invention.

(2) FIG. 2 is a plan view partly cut away along the line II-II in FIG. 1.

(3) FIG. 3 is a sectional view partly cut away along the line III-III in FIG. 1.

(4) FIG. 4 is a sectional view partly cut away along the line IV-IV in FIG. 1.

(5) FIG. 5 is a graph showing the change in excess air ratio of air-fuel mixture accompanied by the change in the rotation angle of the butterfly valve.

DESCRIPTION OF EMBODIMENTS

(6) With reference to FIG. 1, reference numeral 1 denotes a burner which is made up of a totally aerated combustion type burner (also called “all primary air burner”) and the like having a combustion surface 1a in which the air-fuel mixture is ejected and combusted. The burner 1 has connected thereto a fan 2 and, by means of a premixing apparatus A according to an embodiment of this invention, the air is mixed with fuel gas and the air-fuel mixture thus obtained is supplied, through a fan 2, to a burner 11.

(7) The premixing apparatus A is provided with an air supply passage 3 on an upstream side of the fan 2, and a gas supply passage 4 to supply the fuel gas. In an upstream section of the gas supply passage 4, there are interposed a gate valve 5, and a flow control valve 6 which is made up of a proportional valve or a zero governor, as shown in FIG. 4. Further, the downstream end of the gas supply passage 4 is connected to a gas suction section 31 which is disposed in the air supply passage 3.

(8) In addition, the premixing apparatus A is provided with: an air resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the air supply passage 3 which is on an upstream side of the gas suction section 31; and a gas resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the gas supply passage 4 which is on the downstream side of the flow control valve 6. When the required combustion amount has fallen below a predetermined value, a small-capacity state is attained in which the ventilation resistance in the air supply passage 3 is made high by the air resistance changeover means and also in which the ventilation resistance in the gas supply passage 4 is made high by the gas resistance changeover means. It is thus so arranged that the amount of air and fuel gas can be supplied depending on the required combustion amount below the required value. Further, when the required combustion amount has exceeded the predetermined value, a large-capacity state is attained in which the ventilation resistance in the air supply passage 3 is made low by the air resistance changeover means, and also in which the ventilation resistance in the gas supply passage 4 is made low by the gas resistance changeover means.

(9) In that portion of the air supply passage 3 which lies on an upstream side of the gas suction section 31, there is provided an inner tube 33 with a clearance to the inner circumferential wall surface 32 of the air supply passage 3. By the clearance between the inner circumferential wall surface 32 of the air supply passage 3 and the outer peripheral surface of the inner tube 33, there is constituted a subsidiary passage 3b which is in parallel with the main passage 3a inside the inner tube 33. In the flange section 33a on a downstream end (upper end in FIGS. 1 and 3) of the inner tube 33, there are formed a plurality of arcuate through holes 33b which serve as outlets to the subsidiary passage 3b.

(10) Inside the inner tube 33, there is disposed a butterfly valve 7 which is made up of a disc so as to be rotatable about a shaft 71. This butterfly valve 7 constitutes the air resistance changeover means. A shaft 71 of the butterfly valve 7 has connected thereto an actuator 72 such as a stepping motor and the like. At the time of changing over to the small-capacity state, the butterfly valve 7 is rotated, by the operation of the actuator 72, from the open posture side, which is in parallel with the longitudinal direction of the air supply passage 3, as shown by imaginary lines in FIG. 3 to a closed posture side, which is at right angles to the longitudinal direction of the air supply passage 3, as shown by solid lines in FIGS. 1 through 3. Further, at the time of changing over to the large-capacity state, the butterfly valve 7 is rotated, by the operation of the actuator 72, from the closed posture side to the open posture side. When the butterfly valve 7 is rotated to the closed posture, the main passage 3a is almost fully closed and the space through which the air flows is substantially limited to the subsidiary passage 3b, and the ventilation resistance in the air supply passage 3 becomes high.

(11) That section of the air supply passage 3 which is adjacent to the upstream side of the gas suction section 31 is provided with a Venturi section 34 which is smaller in diameter than that section of the air supply passage 3 which has disposed therein the inner tube 33. That section of the air supply passage 3 which is adjacent to the downstream side of the Venturi section 34, is enclosed by a cylindrical section 35 which is larger in diameter than the Venturi section 34. Then, the downstream end section of the Venturi section 34 is inserted, while leaving an annular clearance, into an upstream end section. It is thus so arranged that this clearance constitutes a gas suction section 31 which is in communication with the gas chamber 41 by this clearance. In addition, the inner circumferential wall surface of the air supply passage 3 between the subsidiary passage 3b and the Venturi section 34 is formed into a tapered surface 36 with a smaller diameter toward the Venturi section 34.

(12) The gas supply passage 4 is provided with a valve chamber 81 which is positioned on an upstream side of that gas chamber 41 on a downstream end of the gas supply passage 4 which is in communication with the gas suction section 31, the valve chamber being in parallel with a passage section 42 normally communicated with the gas chamber 41. Inside the valve chamber 81, there is provided a changeover valve 8 for opening or closing a valve hole 83 which is in communication with the passage section 42, the valve hole being formed in a valve seat 82 at the lower end of the valve chamber 81. The gas resistance changeover means is constituted by this changeover valve 8. When the changeover valve 8 is brought to a fully-closed state in which the changeover valve 8 is seated on the valve seat 82, thereby blocking the valve hole 83, the flow of the gas through the valve chamber 81 is shut off, and the ventilation resistance in the gas supply passage 4 increases.

(13) The changeover valve 8 is operated to be opened or closed through an interlocking mechanism 9 accompanied by the rotation of the butterfly valve 7. This interlocking mechanism 9 is constituted, as shown in FIGS. 1 and 4, by: a cam 91 which is coupled to the shaft 71 of the butterfly valve 7; and a rod 92 which extends upward for connection to the changeover valve 8 and a lower end of which comes into contact with the cam 91. As a result of rotation of the butterfly valve 7, the rod 92 moves up and down through the cam 91, whereby the changeover valve 8 is operated to be opened or closed. In other words, when the butterfly valve 7 is rotated to the opened posture side, the cam 91 pushes up the rod 92. The changeover valve 8 moves up against the urging force of a valve spring 84, i.e., is operated to be opened. When the butterfly valve 8 is rotated to the closed posture side, the upward force of the rod 92 by the cam 91 is released. The changeover valve 8 moves downward by the urging force of the valve spring 84, i.e., is operated to be closed.

(14) The interlocking mechanism 9 is arranged as follows, i.e., when the butterfly valve 7 lies within a predetermined first rotation angle range (e.g., 0° through 15°, where the angle of the butterfly valve 7 at the closed posture is defined to be 0°, and the angle of the butterfly valve 7 is at the opened posture is defined to be 90°), the changeover valve 8 is maintained at the fully-closed state, and when the butterfly valve 7 lies within a predetermined second rotation angle range (e.g., 75° through 90°), the changeover valve 8 is maintained at the fully-opened state. More specifically, it is so arranged that, when the butterfly valve 7 lies within the first rotation angle range, the rod 92 is free from contact with the cam 91 and that, when the butterfly valve 7 is rotated beyond the first rotation angle range into the opened side, the cam 91 comes into contact with the rod 92, and the rod 92 starts an upward movement. Further, an arrangement is made that, when the butterfly valve 7 has rotated to the border angle on the closed side of the second rotation angle range, the changeover valve 8 will become a fully-opened state and, even if the butterfly valve 7 thereafter rotates to the opened-posture side and consequently the changeover valve 8 further moves upward, the ventilation resistance in the gas supply passage 4 will be maintained at the value of the fully-opened state.

(15) FIG. 5 shows the relationship between the rotation angle of the butterfly valve 7 and the excess air ratio of the air-fuel mixture. In case the butterfly valve 7 is rotated from the closed posture to the opened posture, the changeover valve 8 is maintained in the fully-closed state within the first rotation angle range, and only the rotation angle of the butterfly valve 7 will be changed. Therefore, accompanied by the increase in the rotation angle of the butterfly valve 7, the ventilation resistance in the air supply passage 3 will decrease, and the excess air ratio of the air-fuel mixture will gradually increase. In case the butterfly valve 7 has rotated to the opening side beyond the first rotation angle range, the changeover valve 8 will be started to be subjected to opening operation, and the excess air ratio of the air-fuel mixture will gradually decrease. Thereafter, at the time when the butterfly valve 7 has been rotated to the intermediate angle between the closed posture and the opened posture, the excess air ratio of the air fuel mixture becomes minimum. When, on the other hand, the butterfly valve 7 lies within the second rotation angle range, the changeover valve 8 will be maintained in the fully-opened state, and only the rotation angle of the butterfly valve 7 changes. Therefore, accompanied by the increase in the rotation angle of the butterfly valve 7, the excess air ratio of the air-fuel mixture will gradually increase.

(16) By the way, the apparatus is generally designed such that, in a state in which the butterfly valve 7 is rotated to the closed posture and to the opened posture, the excess air ratio of the air-fuel mixture becomes an appropriate value (an appropriate value, e.g., 1.3, for stable combustion after ignition). However, due to dimensional errors and the like of the air supply passage 3, in a state in which the butterfly valve 7 is rotated to the closed posture and to the opened posture, there are cases where the excess air ratio of the air-fuel mixture will deviate from the appropriate value.

(17) As a solution, in this invention, the apparatus is designed such that, in a state in which the butterfly valve 7 is rotated to an intermediate angle between the respective first and the second rotation angle ranges, the excess air ratio of the air-fuel mixture becomes an appropriate value. According to this arrangement, in a state in which the butterfly valve 7 has been rotated to the intermediate angle, even if the excess air ratio of the air-fuel mixture deviates from the appropriate value, the excess air ratio of the air-fuel mixture becomes the appropriate value when the butterfly valve 7 is rotated to any one of the angles within each of the first and the second rotation angle ranges. Then, in the inspection step of the apparatus, the butterfly valve 7 is rotated within the first rotation angle range to thereby find out the rotation angle of the butterfly valve 7 at which the excess air ratio of the air-fuel mixture becomes the appropriate value. Then, this rotation angle is set as the closing-side stop angle. At the time of changing over to the small-capacity state, the butterfly valve 7 is rotated to the opening-side stop angle and then stop there. Similarly, the butterfly valve 7 is rotated within the second rotation angle range to thereby find out the rotation angle of the butterfly valve 7 at which the excess air ratio of the air-fuel mixture becomes the appropriate value. Then, this rotation angle is set as the opening-side stop angle. At the time of changing over to the large-capacity state, the butterfly valve 7 is rotated to the opening-side stop angle and then stop there. According to this arrangement, in either of the small-capacity state and the large-capacity state, adjustments can be made without the necessity of replacing the parts so as to attain the appropriate value of the excess air ratio of the air-fuel mixture.

(18) By the way, in order to prevent the explosive ignition of the burner 1, the igniting operation of the burner 1 (in order for an ignition electrode, not illustrated, facing the ignition surface 1a, to cause sparks) must be performed in the small-capacity state. Here, it is at the time when the butterfly valve 7 is rotated to the closed posture that the excess air ratio of the air-fuel mixture becomes the lowest within the range of the first rotation angle. Therefore, the ignition operation of the burner 1 is arranged to be performed in a state in which the butterfly valve 7 is rotated to the closed posture. According to this arrangement, the ignition operation is performed in a state in which the excess air ratio of the air-fuel mixture is relatively low and in an easily ignitable gas-rich state. The occurrence of ignition failure can thus be prevented. After the ignition, the butterfly valve 7 is rotated to the closing-side stop angle (in case the required combustion amount is small, thereby making the state to a small-capacity state) or to the open-side stop angle (in case the required combustion amount is large, thereby making the state to a large-capacity state).

(19) Explanation has so far been made of embodiments of this invention with reference to the drawings. However, this invention shall not be limited to the above, but may be carried out by variously modifying the invention within a range not departing from the essence of this invention.

REFERENCE SIGNS LIST

(20) A premixing apparatus 1 burner 2 fan 3 air supply passage 31 gas suction section 4 gas supply passage 6 flow control valve 7 butterfly valve 8 changeover valve 9 interlocking mechanism