AN IMPROVED AIR-GAS MIXER DEVICE

Abstract: An air-gas mixer device (1, 100), in particular for gas burners (2) with forced ventilation, is described and comprises an air-gas mixing chamber (12) for producing a combustible mixture to be delivered to the burner, in which the chamber (12) is supplied by a first flow of a mixture of primary air and of gas and by a second flow of secondary air through a first duct (5) and a second duct (6), respectively. The device comprises first means for choking the first duct (5) and/or the second duct (6) so as selectively to choke the quantity of the mixture of primary air and gas and/or of secondary air supplied to the mixing chamber (12).

Description: An improved air-gas mixer device DESCRIPTION Technical field The present invention relates to an improved air-gas mixer device according to the preamble to the main claim. Background art As is well known, devices of this type are used in boilers and similar apparatus mainly for domestic use, comprising gas burners, particularly with forced ventilation. In known devices, the metering of the combustible mixture delivered by the injection of gas into an air flow is regulated in dependence on the loss in pressure detected at a constriction point of the duct through which the flows of air and gas of the mixture are admitted. Moreover, the homogeneity of the air and gas mixture is generally improved by a fan located downstream of the said duct. The main limitations which may be encountered in these known devices are connected with imprecision in the metering of the gas delivered as well as with the relative narrowness of the effective range of modulation of the flow-rate delivered. Another limitation which may be encountered in known devices is connected with resonance phenomena triggered by the flame front in the burner. The resulting noise due to the propagation of sound waves out of the apparatus in which the device is installed may be quite annoying to the user. Disclosure of the invention The problem upon which the present invention is based is that of providing an improved air-gas mixer device which is designed structurally and functionally to prevent all of the problems indicated with reference to the prior art mentioned. This problem is solved by the invention by means of an air-gas mixer device formed in accordance with the appended claims . Brief description of drawings The characteristics and the advantages of the invention will become clearer from the detailed description of three preferred embodiments thereof, described below by way of non-limiting example with reference to the appended drawings, in which: Figure 1 is a partially-sectioned, schematic, side elevational view of apparatus equipped with an air-gas mixer device formed in accordance with the present invention, - Figure 2 is a perspective view showing a detail of the air-gas mixer device according to the invention, on an enlarged scale, - Figure 3 is a perspective view of a variant of the detail of the air-gas mixer device of Figure 2, - Figure 4 is a partially-sectioned, schematic side elevational view of apparatus equipped with a second variant of the air-gas mixer device of Figure 1, - Figure 5 is a perspective view showing a detail of the air-gas mixer device of Figure 4, on an enlarged scale, and - Figure 6 is a partially-sectioned, perspective view of a further detail of the air-gas mixer device of Figure 5. Best mode for carrying out the invention With initial reference to Figure 1, an air-gas mixer device formed in accordance with the invention, particularly for supplying a burner 2 with forced ventilation, is generally indicated 1. The device 1 comprises a tubular, cylindrical body 3 of axis X, having a cylindrical wall 4 and opposed axial ends 3a, 3b. Inside the tubular element 3 and coaxial therewith is a first, converging duct 5 which extends from the end 3a of the body 3 as far as the opposite end 3b, and by means of which the tubular body 3 is divided into the first, converging duct 5 and a second duct 6, the latter being defined by the cylindrical wall 4 and by an outer lateral surface 7 of the first, converging duct 5. A third duct 13 for the admission of gas into the converging duct 5 is formed coaxially with and inside the converging duct 5. The duct 13 is mounted in the duct 5 by means of a plurality of radial connecting bridge elements 14 (Figure 2) . The gas is supplied into the third duct 13 by means of a valve unit 17, connected to an external gas-supply mains, at a constant pressure equal to atmospheric pressure. The direction of the delivery flow of gas supplied by the valve unit 17 is indicated by the arrow G in Figure 1. The converging duct 5 is the seat of a primary air flow directed axially through the duct 5 in the direction indicated by the arrow F of Figure 1, from the end 3a towards the opposite end 3b of the tubular body 3. This primary air flow is mixed, in the region of the output section of the first duct 5, with the gas flow emerging from the third duct 13. A radial fan, indicated 8, is driven by drive means 8b, is provided downstream of the tubular body 3, in the direction of the air and/or gas flow, and is arranged with its intake 8a facing the end 3b for the output of the air and gas mixture. The fan 8, by which the primary air and gas flow is drawn in, is enclosed in a box-like body 9 the cover 10 of which is integral with the tubular body 3. Preferably, the cover 10 is formed integrally with the tubular body 3, the converging duct 5, and the third duct 13, and is produced by pressure die-casting. A mixing chamber 12 defined inside the box-like body 9 between the fan 8 and the tubular body 3 is supplied by the first converging duct 5 and by the second duct 6, as described in greater detail below. According to a principal characteristic of the invention, the cylindrical wall 4 of the tubular body 3 has a first plurality of through-holes 18 which are arranged at a predetermined angular distance apart and through which a secondary air flow, drawn in by the fan 8 in the direction of the arrow F 2 of Figure 1, is admitted to the second duct 6. The mixing chamber 12 is thus supplied by the secondary air flow as well as by the above-mentioned flow of the gas and primary air mixture . The holes 18, of which there are three in the embodiment described, are preferably aligned circumferentially and spaced apart at regular intervals. The mixer device 1 also comprises first means for choking the holes 18, including a first shutter means 19. The choking achieved by this means is adjustable, as described in detail below. The first shutter means 19 has a first sleeve 20 fitted coaxially on the tubular body 3 and having opposed axial ends 20a, 20b. A first shoulder 21, by means of which the first shutter means 19 is in abutment with a second shoulder 23 formed on the cylindrical wall 4 of the tubular body 3, extends from the end 20b. An annular seat 28 is also formed in the shoulder 23, in particular in its surface which is in contact with the shoulder 21 of the first shutter means 19, and a seal 29, for example of the "O-ring" type, is housed therein. The first sleeve 20 has a second plurality of through- holes 22, preferably of similar shape and size to the holes 18 in the cylindrical wall 4 and extending in positions corresponding to those of the holes 18 when the shutter means 19 is fitted on the tubular body 3. The first choking means are operated by an electric motor 25, preferably a stepper motor, by which the shutter means 19 is rotated about the axis X by first drive- transmission means 26, for example, of the crown-wheel and pinion coupling type. This rotation is facilitated by lubricants interposed between the contacting surfaces of the cylindrical wall 4 of the tubular body 3 and of the first sleeve 20. The relative angular distances between the holes 18, and hence correspondingly between the holes 22, is such that the holes 22 for the flow of secondary air into the mixing chamber are movable continuously, by the rotation of the shutter means 19, between a first, fully-closed operative position, in which the holes 18 are closed by portions of the first sleeve 20 interposed circumferentially between the holes 22, and a second, fully-open operative position, that is, a position in which the holes 18 and 22 are superimposed. The flow-rate of secondary air flowing into the second duct 6, and hence into the mixing chamber 12, can thus be modulated between a minimum value substantially equal to zero and a maximum value . A variant of the invention, not shown, also provides for the above-mentioned choking to be preset, that is, for the first shutter means 19 to be locked relative to the tubular body 3 with a preselected angular positioning, without the provision of any operating actuator. In this case, the holes 18 are choked in a manner predefined at the calibration stage to ensure the flow of a preselected flow- rate of secondary air into the duct 6, in dependence on the type of burner 2 fitted. In a further variant, not shown, in which the choking is also preset and the shutter means is locked in a preselected angular position relative to the tubular body 3 , a set of shutter means 19 is provided, each shutter means 19 comprising respective calibrated holes 22 of predetermined cross-section, preferably smaller than the cross-sections of the holes 18. The respective shutter means 19 of the set can thus be mounted in the tubular body 3 alternatively, in dependence on the flow-rate of secondary air to be delivered to the mixing chamber. The electric motor 25 is connected electrically to an electronic control card 27 of the burner 2 and is operatively controlled thereby. Signals correlated with parameters detected by a plurality of sensors, for example, a combustion sensor 30 or a temperature sensor 31, are sent to the control card 27. The motor 8b of the fan 8 is also subject to electronic control by the card 27. In a second preferred embodiment shown in Figure 3 , in which details similar to those of the previous embodiment are distinguished by the same reference numerals, a set of stator blades 15 is provided in the annular space between the third duct 13 and the first duct 5. The set of blades 15 comprises a plurality of blades 16 with preselected twist, extending radially from the duct 13 and fixed, at their opposite ends, to the wall 7 of the first duct 5. The blades 16 also have the function of structurally connecting the duct 5 to the duct 13. The blades 16 are stationary and have a curvature of their twist profiles such as to impart a rotational component to the primary air-flow entering the first, converging duct 5. A variant, not shown, on the other hand, has a set of stator blades similar to the set of blades 15, exclusively in the second duct 6 and, in this case, the blades are fixed to the wall 7 of the converging duct 5 at one end and to the cylindrical wall 4 of the tubular body 3 at the other end. In a further variant, also not shown, sets of stator blades are provided both in the first duct 5 and in the second duct 6. In the second embodiment described, and in its variants, the fan is arranged to have the same direction of rotation as the rotation imparted by the set of stator blades 15. In this case, by virtue of the fact that the direction of rotation of the fan is the same as that of the rotational component imparted to the flow of the gas and primary air mixture and/or to the flow of secondary air, the above-mentioned mixture and/or the secondary air is admitted to the fan 8 with very low pressure losses . In a third preferred embodiment shown in Figures 4 to 6, in which details similar to those of the previous embodiment are distinguished by the same reference numerals, the air-gas mixer device of the invention is generally indicated 100. In this device, the third duct 13 for the admission of gas to the mixing chamber 12 comprises, at one end 13b, a closure base 13a and a cylindrical wall 13d in which a third plurality of through-holes 32 is formed, at the end 13b. These holes 32, of which there are two in the embodiment shown in Figure 6, are aligned circumferentially and uniformly spaced in the cylindrical wall 13d. The device 100 also comprises second choking means including a second shutter means 33 of similar structure to the first shutter means 19 described above, for choking the third plurality of holes 32 in an adjustable manner. The second shutter means 33 comprises a second sleeve 34 fitted coaxially on the third duct 13. A fourth plurality of radial through-holes 35 is also formed in the second sleeve 34 with an arrangement such as to choke the third ' plurality of holes 32 by angular rotation in the same manner in which the first plurality of holes 18 is choked by the sleeve 20. The gas is admitted to the mixing chamber 12 through the holes 35 in a substantially radial direction relative to the axis X. A connecting arm 36 extends from the second shutter means 33 and is connected, at one of its ends 36a, to the first sleeve 20 in order to fix the second shutter means 33 for rotation with the first shutter means 19. The fixing- together of the two shutter means defines a choking ratio between the holes 18 and the holes 32 by which a ratio is fixed between the flow-rate of secondary air and the flow- rate of gas admitted to the mixing chamber 12. The device of the invention operates as follows . The operation of the fan 8 draws primary air and secondary air into the first, converging duct 5 and into the second duct 6, respectively. The primary air flow passes through the converging duct 5 and is mixed with the gas which emerges from the third duct 13 and the metering of which depends on the signal of the pressure existing in the constricted section of the first duct 5 and on any rotation imparted to the primary air-flow. The mixture formed is then admitted to the mixing chamber 12. The choking of the holes 18 may be further modified, if desired, according to the power demanded from the burner 2, or in dependence on the signals sent by the combustion and/or temperature sensors 30, 31 to the electronic control card. In this case, a starting signal for the electric motor 25 is generated by the card 27. The shutter means 19 is thus rotated by the drive-transmission means 26 to the angular position corresponding to the preselected choking of the holes 18 and consequently to the desired value of the flow- rate of secondary air in the duct 6. In the mixing chamber 12, the secondary air is mixed with the gas and air mixture and the resulting combustible mixture is then delivered to the burner 2. If the shutter means 19 is locked relative to the tubular body 3 in accordance with the above-mentioned variant, the choking of the holes 18 is preselected at the installation and calibration stage in dependence on the type of boiler installed and on the characteristics of the fan, in order for the flow-rate of secondary air to be optimal. Alternatively, in the further variant, optimal choking of the holes is achieved by the installation of a shutter means in which the cross-section of the holes is suitable for achieving the preselected secondary air flow rate . In the third embodiment, a rotation of the first shutter means 19 through a predetermined angle, brought about by the electric motor 27, causes a corresponding rotation of the second shutter means 33 through the same angle, by means of the connecting arm 36, so that both the secondary air flow and the flow of gas which are admitted to the mixing chamber 23 are choked by the same movement. The flow-rates of the two flows are thus varied whilst their ratio is kept constant. The invention thus achieves the objects proposed, affording many advantages over the prior art mentioned. A first advantage made clear by tests carried out by the Applicant is that the geometrical characteristics of the system, as well as the admission of a secondary air flow of the type described, which is separate from the primary flow of the air and gas mixture into the mixer device, clearly contribute to the damping of the sound waves emitted by the flame fronts which are generally produced in known devices of this type and which may be audible outside the boiler. A second advantage is that the combined regulation of the flow-rates of the secondary air flow and of the gas flow which can be achieved in the mixer device according to the invention, achieves a significant increase in the effective range of modulation of the flow-rate of the air-gas mixture delivered by the device. The continuously adjustable choking of the secondary air flow enables the fan constantly to be operated close to maximum-efficiency conditions corresponding to a predetermined C0 2 value and to the minimum value of CO and NO x emissions for each preselected flow-rate, even with variations in the power demanded from the burner. Not least, by virtue of the optimal regulation of the stoichiometric ratio of air and gas in the mixture, the pollutant emissions of the burner supplied by the mixer device of the invention are particularly low.

Claims: CLAIMS 1. An air-gas mixer device (1, 100), in particular for gas burners (2) with forced ventilation, comprising an air-gas mixing chamber (12) for producing a combustible mixture to be delivered to the burner, the chamber (12) being supplied by a first flow of a mixture of primary air and of gas and by a second flow of secondary air, through a first duct (5) and a second duct (6) , respectively, characterized in that it comprises first means for choking the first duct (5) and/or the second duct (6) so as selectively to choke the quantity of the mixture of primary air and gas and/or of secondary air supplied to the mixing chamber (12) . 2. A mixer device according to Claim 1 in which the second duct (6) comprises at least one through-hole (18) for the admission of the secondary air flow, the hole being capable of being choked by the first choking means . 3. A mixer device according to Claim 2 in which the second duct (6) comprises a first plurality of through-holes (18) which can be choked selectively. . A mixer device according to one or more of the preceding claims in which the choking is adjustable. 5. A mixer device according to one or more of the preceding claims in which the first duct (5) has a cross-section which decreases in the direction of the flow of the air and gas mixture and is coaxial with the second duct (6) . 6. A mixer device according to Claim 5 comprising a tubular body (3) , the first duct (5) being disposed inside the tubular body (3) and the second duct (6) being defined by the tubular body (3) and the first duct (5), the first plurality of holes (18) being provided in the tubular body (3) . 7. A mixer device according to Claim 6 in which the holes (18) of the first plurality are aligned circumferentially and spaced apart uniformly in the tubular body (3) . 8. A mixer device according to one or more of Claims 3 to 7 in which the first choking means comprise a first shutter means (19) movable between a first operative position in which the first plurality of holes (18) is fully closed and a second operative position in which it is fully open. 9. A device according to Claim 8 in which the first shutter means (19) comprises a first sleeve (20) fitted coaxially on the tubular body (3) and acting on the holes (18) . 10. A device according to Claim 9 in which the first sleeve (20) has a second plurality of holes (22) aligned circumferentially in positions corresponding to those of the first plurality of holes (18) when the first shutter means (19) is fitted on the tubular body (3) so that the first plurality of holes (18) can be choked by portions of the first sleeve (20) defined between respective adjacent holes (22) of the second plurality. 11. A mixer device according to Claim 10 in which the first shutter means (19) can rotate about the axis common to the tubular body (3) and to the shutter means (19) so that the second plurality of holes (22) is movable continuously between the first, fully-closed operative position to the second, fully-open operative position in which the first plurality (18) and the second plurality (22) of holes are superimposed. 12. A mixer device according to one or more of Claims 8 to 11 in which the choking means comprise an electric drive motor (25) and drive-transmission means (26) interposed between the motor (25) and the shutter means (19) , the first shutter means (19) being rotated by the electric motor (25) by means of the drive-transmission means (26) . 13. A mixer device according to Claim 12 in which the electric motor (25) is a stepper motor. 1 . A mixer device according to Claim 12 or Claim 13 in which the electric motor (25) is operatively controlled by an electronic control card (27) and is operated in dependence on signals correlated with parameters detected by sensors (30, 31) and sent to the electronic control card (27) . 15. A mixer device according to one or more of the preceding claims, comprising a fan (8) located downstream of the first duct (5) and of the second duct (6) , with reference to the direction of the primary and secondary air-flows. 16. A mixer device according to Claim 15 in which the fan (8) is enclosed in a box-like body (9) and the mixing chamber (12) is defined inside the box-like body (9) . 17. A mixer device according to Claim 16 in which the boxlike body (9) comprises a cover (10) formed integrally with the first duct (5) and the second duct (6) . 18. A mixer device according to Claim 17" in which the cover (10) is produced by pressure die-casting. 19. A mixer device according to one or more of the preceding claims, comprising a set of stator blades (15) inside the first duct (5) for imparting a rotational component to the flow of the primary air and gas mixture entering the first duct (5) . 20. A mixer device according to one or more of the preceding claims, comprising a set of stator blades (15) inside the second duct (6) for imparting a rotational component to the secondary air flow entering the second duct. 21. A mixer device according to one or more of the preceding claims, comprising a third duct (13) for the admission of gas to the mixing chamber (12) . 22. A mixer device according to Claim 21 in which the third duct (13) is coaxial with and inside the first duct (5) . 23. A mixer device according to Claim 21 or Claim 22, comprising second choking means for choking the third duct (13) in order selectively and adjustably to choke the quantity of gas delivered to the mixing chamber (12) . 24. A mixer device according to one or more of Claims 21 to 23 in which the third duct (13) comprises a closure base (13a) at a free end (13b) , at least one through-hole (32) being provided in the region of the end (13b) . 25. A mixer device according to one or more of Claims 21 to 24 in which the third duct (13) comprises a third plurality of holes (32) which can be choked selectively. 26. A mixer device according to Claim 24 or Claim 25 in which the second choking means comprise a second shutter means (33) movable between a first operative position in which the third plurality of holes (32) is fully closed and a second operative position in which it is fully open. 27. A mixer device according to Claim 26 in which the second shutter means (33) is operatively connected to the first shutter means (19) so that the choking of the first plurality of holes (18) is correlated with the choking of the third plurality of holes (32) as a result of an angular movement of the first shutter means (19) between the first and second operative positions . 28. A mixer device according to Claim 26 or Claim 27 in which the second shutter means (33) comprises a second sleeve (34) fitted on the third duct (13) , the second sleeve (34) being rigidly connected to the first sleeve (20) . 29. A mixer device according to Claim 28, comprising at least one connecting arm (36) extending radially between the first and second sleeves (20, 34) so as to fix the second shutter means (33) for rotation with the first shutter means (19) . 30. A mixer device according to Claim 28 or Claim 29 in which a fourth plurality of holes (35) is provided in the second sleeve (34) , the holes (35) being aligned circumferentially and in positions corresponding to those of the third plurality of holes (32) when the second sleeve (34) is fitted on the third duct (13) so that the third plurality of holes (32) can be choked by portions of the second sleeve (34) defined between respective adjacent holes of the fourth plurality (35) .

Inventors: DORIGO ROBERTO, RIMONDO FILIBERTO, ZOCCHI RAFFAELE, DORIGO, ROBERTO, RIMONDO, FILIBERTO, ZOCCHI, RAFFAELE
Applicants: DORIGO ROBERTO, RIMONDO FILIBERTO, SIT LA PRECISA SPA, ZOCCHI RAFFAELE, DORIGO, ROBERTO, RIMONDO, FILIBERTO, SIT LA PRECISA S.P.A., ZOCCHI, RAFFAELE
Application year: 2001
Main classification: F23D14
IPC codes: F23D14