An improved air-gas mixer device Technical field
The present invention relates to an air-gas mixer device according to the preamble to the main claim. Technological background
As is well known, devices of this type are used in apparatus with gas burners, in particular, with forced ventilation, and generally comprise a valve unit for delivering the gas-flow into the smaller-sectioned region of a Venturi duct at '" a. pressure which is controlled in i. dependence on the pressure detected upstream of the Venturi duct. The degree of homogeneity of the air-gas mixture is improved by means of a fan which is normally positioned downstream of the Venturi duct . A mixer device having the above-mentioned characteristics is described, for example, in Italian patent No. 1288006. This device has the particular characteristic of enabling adequate homogeneity of the air-gas mixture to be achieved by the admission of the gas-flow to the Venturi duct in a predominantly axial direction, along the duct. Description of the invention
A main object of the present invention is to provide an air-gas mixer of the above-mentioned type which is designed structurally and functionally to improve the accuracy of the metering of the fuel gas in the air-gas mixture delivered to the burner.
Another object is to provide an air-gas mixer which can extend the range of modulation of the flow-rate delivered.
These and other objects which will be explained below are achieved by the invention by means of an air-gas mixer device formed in accordance with the appended claims .
Brief description of the drawings
The characteristics and the advantages of the invention will become clearer from the detailed description of a preferred embodiment thereof, described below by way of non- limiting example with reference to the appended drawings, in which:
- Figure 1 is a perspective view of an air-gas mixer device according to the present invention,
- Figure 2 is a longitudinal section through the mixer device of Figure 1, part of which is shown only schematically,
- Figure 3 is a section taken on the line III-III of Figure 2, and
- Figure 4 is a perspective view of a detail of the mixer device of the preceding drawings, on an enlarged scale.
Preferred embodiment of the invention
With reference initially to Figure 1, an air-gas mixer device, in particular for supplying a forced-ventilation gas burner (not shown) and formed in accordance with the present invention, is generally indicated 1.
The device 1 comprises a Venturi duct 2 of axis X, positioned coaxially inside a tubular duct 3 having opposed axial ends 3a, 3b.
The air-flow passing through the tubular duct 3 is directed axially relative to the duct, in the direction indicated by the arrow F of Figure 2, from the end 3a towards the opposite end 3b.
The mixer device 1 also comprises a valve unit 6, shown schematically in Figure 2, for delivering gas into the Venturi duct 2 at a pressure which is controlled in dependence on the air pressure detected inside the tubular
duct 3 upstream of the Venturi duct 2. The reduced pressure in the constriction of the Venturi duct 2 induces a corresponding pressure of the gas supplied through a hole 11 in the tubular duct 3. The arrow 9 indicates the admission of gas from the external supply mains to the valve unit 6, and the arrow 10 indicates the controlled delivery of gas to the hole 11 by the valve unit 6.
The Venturi duct 2, which is inside and coaxial with the tubular duct 3, has a double-cone configuration defined by an inner wall 2a of a hollow body 12 which is engaged in the tubular duct 3 in a leaktight manner and abuts a shoulder 3c on the side facing towards the end 3b.
The hollow body 12 comprises a first, converging, inlet portion 14, a second, intermediate portion 15 with a narrow cross-section, and a third, diverging, outlet portion 16. As is well known, the narrow cross-sectioned intermediate portion 15 is the portion in which the gas-flow passing through the Venturi duct has the fastest speed and the lowest pressure. In the region of the second and third portions 15, 16, the hollow body 12 has an external constriction defining, with the surface of the tubular duct 3, a gas-distribution chamber 17 which is in communication with the inlet hole 11. In the region of the narrow cross-sectioned portion 15, the hollow body 12 has a series of four ribs 18a, 18b, 18c, 18d defining a corresponding number of recesses 19a, 19b, 19c, 19d in the bases of which respective shoulders, indicated 20a, 20b, 20c, 20d are formed. The shoulders 20a, 20b, 20c, 20d are arranged substantially at right angles to the axis X of the Venturi duct 2 at the downstream end (with
reference to the direction of the arrow F) of the narrow- sectioned portion 15.
For each shoulder 20a-d, starting from the wall of the shoulder 20a-d, the inner wall of the hollow body 12, that is, the wall defining the Venturi duct 2, has a respective depression 21a-d with a cylindrical surface and an axis substantially parallel to the axis X of the Venturi duct 2. Owing to the taper of the third portion 16 of the Venturi duct, the depressions 21a-d have cross-sections which decrease progressively in the direction of the flow F.
A slot 22a-d opening in the respective shoulder 20a-d is formed at the base of each depression 21a-d, owing to its intersection with the corresponding recess 19a-d. The slots 22a-d define, with the respective depressions 21a-d, a corresponding number of intakes for admitting gas into the Venturi duct 2 from the distribution chamber 17. The gas- intakes open into the Venturi duct 2 in an equiangularly- spaced arrangement .
The slots 22a-d lie in a plane perpendicular to the axis X of the Venturi duct 2 and, owing to the positions of the depressions 21a-d, cause the orientations of the gas- intakes to have a predominant axial component .
According to a main characteristic of the device according to the invention, a distributor member, generally indicated 23, with stationary blading, is positioned upstream of the Venturi duct 2 in the region of the end 3a. The member 23 is mounted coaxially relative to the tubular duct and comprises a hub 24 from which a plurality of blades, all indicated 25, extend radially, the blades 25 having a predetermined twist, and being fixed to a skirt 26 at their ends remote from the hub 24.
The blades 25 are stationary and have a curvature of their twist profiles such as to impart a rotational component to the air-flow entering the Venturi duct 2. The rotational kinetic component imparted to the air-flow is such as to induce a greater reduction in pressure at the narrow-sectioned point 15 of the Venturi duct 2 where the injection of gas takes place, achieving greater accuracy in the regulation of the flow-rate of fuel gas injected through the slots 22a-d. The greater accuracy in the admission of the gas-flow also enables the effective range of modulation of the flow-rate of the air-gas mixture delivered by the device to be increased.
It should be noted that tests carried out by the Applicant have shown that the provision of distributor members with stationary blading in a mixer device with axial injection of gas into the Venturi duct leads to greater accuracy in the regulation of the metering of the gas and to a wider range of modulation of the flow-rate in comparison with conventional mixers with substantially radial admission of the gas to the Venturi duct .
It has also been noted that the rotational kinetic component imparted to the gas-flow entering the Venturi duct enables the detachment of the flow from the wall of the diverging outlet portion 16 to be delayed as far as possible or even completely prevented, advantageously reducing pressure losses in this region.
According to a further characteristic of the invention, a radial fan 28 is connected with its intake opening 28a at the outlet opening of the diverging portion 16 of the Venturi duct 2.
If the direction of rotation of the fan 28 is selected so as to be the same as the direction of the rotational component imparted to the flow of the air-gas mixture, the latter will enter the fan 28 with very low losses. If, on the other hand, the direction of rotation of the fan and of the rotational component of the flow are not the same, a greater amplification of the pressure signal controlling the valve unit is achieved in the region in which the gas is injected into the Venturi duct. The invention thus ' achieves the objects proposed, affording the advantages mentioned above with reference to the prior art cited.