"Gas-heating apparatus with pre-mix burner, particularly boiler for domestic use" ^'

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TEXT OF THE DESCRIPTION

The present invention relates to gas-heating apparatuses and has been developed with particular reference to boilers for domestic use equipped with a pre-mix burner.

Gas boilers for domestic use of a more traditional type use atmospheric burners, i.e., burners in which the flame is supplied by combustible gas and primary air, mixed in the burner, and by secondary air that is present around the flame. If on the one hand said boilers guarantee good conditions of combustion in an extremely wide range of situations, on the other hand they present the drawback of having a low power density and, above all, of causing a high emission in the environment of pollutant residue of combustion, in particular NO _x .

For the above reasons, in boilers of more recent construction the use of pre- mix burners has spread, which provides for a combustion only of gas and primary air, which are mixed upstream of the burner. In brief, the combustion air and the combustible gas are pre-mixed in such a way said that both the theoretical air

(stoichiometric value) and the excess air (necessary for bringing about complete combustion) are already contained in the mixture that reaches a particular micro- fine perforation of the burner cap .

Pre-mix burners are of more complex construction as compared to atmospheric burners and presuppose, for their proper operation, an air-gas ratio and a corresponding mixing that are always optimal. On the other hand, however, pre-mix burners enable a higher power density to be obtained as compared to atmospheric burners and, above all, a less pollutant combustion, with an extremely low production of NO _x . hi domestic boilers the air-gas mixing system of a pre-mix burner typically comprises a valve, for control of gas intake, and a fan, having the function of sucking in the combustion air and generating at the same time a differential pressure that constitutes a control signal for the aforesaid valve in order to dose the gas in the right amount. Basically, the mixing system is conceived in such a way that the amount of gas supplied via the aforesaid valve is proportional to the flow rate of combustion air sucked in by the fan. In some cases, the air-gas mixture is formed directly in the fan, whilst in other cases the air-gas mixture is formed in a mixing chamber set downstream of the fan.

In general terms, a pre-mix system should be conceived for generating at input to the fan a high reduction in pressure (or high negative pressure) in relation to the flow rate of air, present low head losses, and have a good stability of the pressure signal created., Known boilers with pre-mix burners respond in a more or less satisfactory way to the first three characteristics referred to, but present the drawback of being practically insensitive to a possible obstruction of the fume exhaust from the boiler or else of the inlet for the combustion air in the boiler.

In known boilers an obstruction of the fume exhaust can be detected only with some delay, following upon progressive decay in time of the performance of the boiler. A partial obstruction of the exhaust, for example, causes a certain stagnation of .the combustion fumes in the chamber in which the pre-mix burner is located, thus bringing about a reduction in the thermal power of the boiler. In said circumstance the control system of the boiler must issue a command for a marked increase in the speed of the fan, in an attempt to increase the heat load i.e., the flow of air, and hence of gas, to the burner). In effect, the boiler continues to operate apparently in a correct way, but in actual fact with a running speed of the fan higher than what it should be. The situation evidently worsens with the passage of time, as the degree of obstruction of the exhaust increases on account of the combustion deposits, up to the point in which, not even with the fan running at the maximum speed is the supply of air-gas mixture obtained sufficient to maintain the burner active.

Substantially similar problems arise in the case of an obstruction of the path for supply of the combustion air. A purpose of the present invention is to solve the aforesaid drawback, and in particular to provide a heating apparatus of the type referred to above capable of discriminating effectively whether a variation of flow rate of the combustion air is due to a desired variation of the heat load or a desired variation in the speed of the fan) or else to an obstruction present in the fume exhaust or in the air-supply path. Another purpose of the present invention is to provide a heating apparatus of the type referred to above in which, in the case of an obstruction in the fume exhaust or in the air-supply path, the aforesaid control valve interrupts in a relatively fast way the inflow of gas into the mixing system.

A further purpose of the present invention is to provide a system and a method for detection of an obstruction in the fume exhaust or in the path for

supply of the combustion air of a heating apparatus of the type referred to above that allows for the aforesaid purposes to be achieved.

Another purpose of the present invention is to provide a pre-mix system for a pre-mix burner designed to detect autonomously a possible obstruction in the fume exhaust or in the path for supply of the combustion air of a heating apparatus of the type referred to above.

A further purpose of the present invention is to provide a device for delivery of gas to the suction mouth. of a centrifugal fan that allows for the aforesaid purposes to be achieved. Yet a further purpose of the present invention is to provide a heating apparatus of the type referred to above that is efficient in operation, has compact dimensions, and is simple and economically advantageous to produce from the industrial standpoint.

One or more of the above purposes are achieved, according to the invention, by a heating apparatus, a pre-mix system, a detection system and method, a boiler, and an intake device having the characteristics recalled in the attached claims, which form an integral part of the descriptive content of the present patent application.

Further purposes, characteristics and advantages of the invention will emerge from the ensuing description with reference to the annexed plate of drawings, which is provided purely by way of non-limiting example and in which:

- Figure 1 is a perspective, partial, and schematic view of a heating apparatus according to the invention;

- Figure 2 is a schematic diagram of the apparatus of Figure 1; - Figure 3 is an exploded view of a fan assembly of a heating apparatus according to the invention;

- Figure 4 is a perspective view, in partial cross section, of the fan assembly of Figure 3;

- Figure 5 is a perspective view of the fan assembly of Figures 3 and 4; and - Figure 6 is a perspective view of a centrifugal fan forming part of the fan assembly of Figures 3, 4 and 5.

In Figure 1, the reference number 1 designates as a whole a wall-mounted gas boiler with pre-mix burner, of a domestic type, built according to the teachings of the present invention. The boiler 1 comprises a case 2, defining a chamber 3, partially housed in

which is at least one heat exchanger 4, of a conception in itself known. In the case exemplified, the heat exchanger 4 is of the type comprising a casing set within which is at least one tube wound to form a spiral. Positioned in a central area of the spiral formed by the aforesaid tube is a pre-mix burner, of a conception in itself known, designated as a whole by 5 in Figure 2 and visible only partially in Figure 1. The aforesaid tube of the heat exchanger 4 is connected to a water- heating system, via intake and delivery pipes, one of which is designated by 6 in Figure 1. A pump, designated by 7, causes the water to circulate in the aforesaid system, with modalities in themselves known. The boiler comprises a respective control system designated by 8, which is also of a conception that is as a whole known.

The burner 4 is supplied via a pre-mix system, which comprises a fan assembly 10 and a control valve for controlling supply of combustible gas 20, connected to a network for supply of the gas itself, designated by 21 in Figure 2. As may be seen in Figures 3 and 4, the assembly 10 comprises a centrifugal fan 11, of a type in itself known, having a casing 12 defining an inlet or suction mouth 13 and an outlet mouth 14. Mounted within the casing 12 is an impeller 15, made, for example, of plastic material, with substantially radial blades 15 a. The impeller 15 is preferably of the half-closed type, i.e., having blades 15a that rise from a disk-shaped element 15b. In the example, moreover present within the casing 12 is a fixed disk-shaped element 15c, not belonging to the impeller 15, having a central passage that gives out onto the suction mouth 13. The impeller 15 is fitted to the shaft 16a of an electric motor 16.

In the embodiment of the invention illustrated in the drawings, the assembly 10 further comprises a device for delivery of gas to the suction mouth 13 of the centrifugal fan 11, said device comprising a diaphragm or baffle plate 17 and, preferably, a manifold element 18. As will emerge clearly hereinafter, the diaphragm 17 and the manifold element 18 form part of a system for detecting, during operation of the fan 11 and in a predefined region close to the suction mouth 13, an increase in pressure, or else a flow of air leaving said suction mouth 13.

In the case exemplified, the diaphragm 17, made, for example, of plastic or metal material, is as a whole disk-shaped and has a plurality of ports or passages in its central area, which are designed to be set in a position corresponding to the suction mouth 13 of the fan 11. hi particular, designated by 17a are ports for the

combustion air, and designated by 17b is a port for the combustible gas, the latter having preferably a section of passage smaller than that of the ports 17a. In the example, the ports 17a have a substantially circular cross section, whilst the port 17b has a substantially semicircular cross section and is made in a peripheral position of the set of ports 17a, 17b.

The body of the manifold 18, made, for example, of plastic material, has a main part having an as a whole annular shape 18a, radially extending from which is a mouth or tubular connector 18b. As may be seen in Figure 4, the connector 18b communicates with an annular cavity or recess 18c defined in the part 18a. The part 18a moreover has a projection 18d, which extends towards the inside of its annular body.

The manifold 18 is designed to be fixed, for example via screws 19, to the casing 12 of the fan 11 with the diaphragm 17 set in between, in such a way that the annular part 18a will surround the suction mouth 13 of the fan or will at least circumscribe the central area of the diaphragm 17 in which the ports 17a are present. Set, instead, on top of the port 17b is the projection 18d of the manifold 18 (see Figure 4). In the example represented, then, the port 17b opens out onto a substantially peripheral point of the suction mouth 13 of the fan 11. The assembled condition of the fan assembly 10 may be seen in Figures 4 and 5. As emerges from Figure 4, given the presence of the recess 18c, the manifold 18 and the diaphragm 17 define between them a substantially annular chamber, designated by C, which is in communication with the port 17b. It should be noted that, in a possible variant embodiment, the manifold 18 can be shaped so as to define autonomously the annular chamber C, with a respective passage which is to give out onto the port 17b of the diaphragm 17.

The connector 18b is designed for connection to the first end of a conduit, represented by the pipe designated by 22 in Figure 1. The second end of said conduit or pipe 22 is connected to the control valve 20 for controlling supply of the combustible gas. Once again in Figure 1, designated by 30 is a pipe, which connects the outlet mouth 14 of the fan 11 to the burner 5, for supplying the latter with an air-gas mixture formed in the fan itself. Finally, designated by 31 is an outlet for discharge of the heat exchanger 4, forming part of a path for evacuation from the boiler of the combustion fumes produced by the burner 5. Along said path there can be installed known sensor means (not represented) for detecting the

combustion fumes.

The chamber 3 of the boiler 1 forms part of a path for supply of combustion air necessary for the formation of the aforesaid air-gas mixture. In the case exemplified in Figure 1, the inlet of the fan assembly 10 - or the set of the ports 17a situated in the area circumscribed by the manifold 18 - opens out directly onto the chamber 3 defined by the outer casing 2 of the boiler, which communicates with the external environment via purposely provided passages or conduits (not represented). In other possible embodiments, the inlet region of the fan assembly 10 can be connected to a purposely provided pipe for introducing air directly from outside the boiler.

The system comprising the fan assembly 10 and the valve 20, connected to one another by means of the pipe 22, is prearranged both for detecting variations in pressure that occur in a region close to the suction mouth 13 of the fan 11 and for causing the combustible gas to reach said region. The valve 20 is prearranged, in a way in itself known, for operating on the basis of a differential pressure, and for this purpose is equipped with means for detecting a reference pressure. In the working diagram of Figure 2, said means are represented by a conduit 23, which enables the valve 20 to detect the aforesaid reference pressure in a point, designated by 24, which is set along the air-supply path, upstream of the fan assembly 10.

Purely by way of example, the valve 20 can be of the type having a detection chamber divided into two half-chambers by an elastic membrane, subject to the reaction of a spring and connected to a shutter element set on the path of the gas, upstream of the pipe 22. A first half-chamber is connected to the pipe 22, whilst the second half-chamber is connected to the pipe 23.

In static conditions, with the fan 11 inactive, the pressures in the two aforesaid half-chambers are in equilibrium, in so far as they correspond to the pressures existing at the port 17b of the diaphragm 17 and at the point 24 of the circuit for supply of the combustion air, respectively. Instead, with the fan 11 active, the pressures present at the point 24 and at the port 17b, and hence in the two half-chambers, are different: given the action of suction of the air at the mouth 13 of the fan 11, the first half-chamber, connected to the port 17b via the pipe 22 and the manifold 18, will be at a pressure lower than that of the second half- chamber, connected to the pipe 23. In said condition, the aforesaid membrane bends so as to reduce the volume of the first half-chamber at a lower pressure,

thus controlling opening of the shutter element of the valve 20. As may be readily appreciated, the amount of bending of the membrane is proportional to the differential pressure: the greater the difference in pressure between the two half- chambers, the greater the bending of the membrane and the degree of opening of the shutter element.

Basically, therefore, when the burner 5 and the fan 11 are inactive, the pressure in the two points 24 and 17b of the circuit for supply of the combustion air is substantially the same: the shutter element of the valve 20 is closed, and the gas is hence not supplied to the conduit 22. When the burner is active, in the course of normal operation of the boiler, the fan 11 sucks air into the casing 12 so that, in the region of the suction mouth 13 — onto which the port 17b gives out — there occurs a drop in pressure. As has been explained, this causes onset of a differential pressure, on the basis of which the shutter element of the valve 20 is opened to a greater or lesser extent: the gas can hence flow into the pipe 22 in depression, passing then into the chamber C, to be subsequently injected, via the port 17b, into the suction mouth 13 of the fan 11. On the other hand, the combustion air is sucked within the casing 12 of the fan 11 via the ports 17a alone of the diaphragm 17. The air and the gas mix intimately within the casing 12. The air-gas mixture thus formed comes out of the casing 12 through the outlet mouth 14 of the fan 11 and then reaches, via the conduit 30, the burner 5. The normal principle of operation is illustrated in Figure 5 by the arrows 40, 50 and 60, which indicate, respectively, the flow of the combustible gas, the flow of the combustion air, and the flow of the air-gas mixture.

As has already been mentioned, the amount of gas injected via the port 17b is proportional to the differential pressure, and hence to the speed of rotation of the fan 11 : by varying the speed of the fan 11, the flow rate of combustion air taken in varies, and hence the flow rate of gas varies proportionally, as consequently does the heat load for the boiler.

In the preferred embodiment of the invention, the pre-mix system of the boiler is prearranged for detecting directly, in a rapid way, a possible obstruction, even only a partial one, of the path for supply of the combustion air or else of the path for discharge of the fumes from the boiler. For said purpose the system is conceived in such a way that, when the flow rate of air introduced by the fan into the boiler varies on account of an obstruction, at the port 17b a counter-pressure (or in any case an increase in pressure) will be generated, instead of a negative

pressure (or in any case a drop in pressure).

The inventors have ascertained that, in a boiler of the type in question, in the case of an obstruction downstream or else upstream of a centrifugal fan with impeller that rotates at a relatively high speed with respect to the flow rate of air introduced, a condition of anomalous operation arises for the fan itself. In particular, in the aforesaid conditions, the high speed of the impeller generates turbulence in the area of the suction mouth of the fan, which gives rise both to a flow of air that penetrates into the inlet mouth and to a flow of air that comes out from said inlet mouth. Said concept is exemplified in Figure 6, where the arrow IN indicates the aforesaid flow entering the mouth 13, whilst the arrow OUT indicates the aforesaid flow coming out of the mouth 13.

On the basis of said observation, the diaphragm 17 assumes the specific purpose of dividing into sections the suction mouth 13 of the fan 11, identifying one or more ports 17a at which, during operation of the fan, a drop in pressure or an inflow into the mouth 13 occurs, and at least one port 17b, at which, in the presence of an obstruction upstream or downstream of the fan, an increase in pressure or an outflow from the mouth 13 occurs.

The presence of a number of air-inlet ports 17a having a reduced section is preferable in order to maintain a more stable pressure at the section or port 17b. Operation of the system is in any case guaranteed even if the ports 17a are replaced with just one opening with larger total section.

In the case of an obstruction, with consequent reduction in the thermal power of the boiler, the control system 8 controls an increase in the speed of rotation of the impeller 15 of the fan 11, substantially as occurs according to the known art.

In the case of the boiler 1 according to the invention, however, when the impeller 15 reaches a relatively high threshold speed (in proportion to the flow rate admitted through the ports 17a) the condition of anomalous operation described above arises, with the result that air is sucked into the casing 11 of the fan through the ports 17a, whilst air is blown into the port 17b. In this way, within the chamber C of the manifold 18 and of the pipe 22, an increase in pressure or an over-pressure occurs, detected in a practically immediate way by the valve 20, which, consequently, interrupts the inflow of gas (in practice, the increase in pressure causes bending of the aforesaid elastic membrane, which brings about

closing of the open/close element of the valve): the control system 8 is prearranged for detecting closing of the valve 20 with the fan 11 active and consequently sets the boiler into a safety condition.

Basically, then, whereas in conditions of proper operation of the boiler the valve 20 detects a depression or negative pressure (i.e., a pressure lower than the reference pressure at the point 24), in the case of an obstruction said valve detects an over-pressure (i.e., a pressure higher than the reference pressure at the point

24).

Practical tests conducted by the inventors have made it possible to ascertain that the invention enables the pre-set purposes to be effectively achieved, in terms of precision and rapidity of detection of a condition of total or partial obstruction in the fume exhaust or in the path for supply of the combustion air to the boiler. The means used for achieving said purposes are moreover simple and economically advantageous to produce from an industrial standpoint. The invention has been described with reference to a possible configuration of the manifold 18 and of the baffle 17. The principle underlying the invention is, however, applicable to the case of any centrifugal fan for a boiler with pre-mix burner, via simple adaptations of the aforesaid components 17, 18, which depend upon the structure of the fan chosen for the practical embodiment. The invention has moreover been described with reference to a pre-mix system in which the combustible gas is injected at the suction mouth of a centrifugal fan and in which the function of detection of possible obstructions is performed by exploiting the pipe 22 for delivery of the gas. The invention may, however, be applied also to the case where the gas is introduced into a mixing chamber set downstream of the fan or else directly into the fan in a point different from its inlet mouth.

In said alternative embodiments, delivery of the gas to the pre-mix system occurs through a dedicated pipe, which stems from the control valve set downstream of the corresponding shutter element. On the other hand, the pipe 22, and possibly the diaphragm 17 and/or the manifold 18, are maintained, but for the sole purpose of detecting the differential pressure, in the course of normal operation of the boiler, and onset of the aforesaid over-pressure that brings about closing of the valve, in the case of an obstruction, as clarified above. hi the embodiments described above, the function of detection of the aforesaid increase in pressure is performed by exploiting the valve for delivery of

the gas 20. In an alternative embodiment, however, the invention can be implemented by rendering operative a pressure sensor in the predefined area where detection takes place (in the example given herein, at the port 17b). The signal originated by said sensor upon onset of the aforesaid over-pressure, possibly processed by means of an electronic transducer, is transmitted to the gas valve or to the control system of the boiler so as to perform the appropriate adjustments or set the boiler itself in a condition of safety. It is moreover evident that the invention can be implemented envisaging a flow-meter instead of a pressure sensor, given that, as has been explained above, the aforesaid condition of faulty operation of the fan gives rise to a flow of air leaving the suction mouth of the fan, i.e., a flow having a direction opposite to what occurs in conditions of normal operation of the boiler. The aforesaid pressure and flow sensors can be of any known type suitable for the function proposed herein.

Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what is described and illustrated herein purely by way of example, without thereby departing from the scope of the present invention.

The diaphragm 17 and the manifold 18, which, as has been said, form a device for delivery of gas to the suction mouth 13 of the centrifugal fan 11, can of course be integrated in a single component and, as has been said, have a shape different from the one represented ^' by way of example.