EP0614045A1

EP0614045A1 - Method and device for feeding air-gas-mixtures at various flow rates to a premix burner

Info

Publication number: EP0614045A1
Application number: EP94200531A
Authority: EP
Inventors: Giorgio Scanferla
Original assignee: Sit La Precisa SpA
Current assignee: Sit La Precisa SpA
Priority date: 1993-03-05
Filing date: 1994-03-02
Publication date: 1994-09-07
Also published as: ITMI930436A1; ITMI930436A0; IT1271987B

Abstract

Method and device for feeding air/gas mixtures at varying flow rates to a premixed burner (2). The air and fuel gas flows are supplied separately into a tubular body (3) respectively from a blower (5) and a conduit (6) connected to the gas distribution network. The air flow rate is controlled by diverting an aliquot thereof toward a bypass conduit (15) of the burner (2).

This control, a shutter (17) slidable in a distribution chamber (10) and interposed between the blower (5) and the tubular body (3), allows the thermal power delivered by the burner (2) to be controlled in a simple and effective way.

Description

This invention relates to a method for feeding air/gas mixtures at varying flow rates to a so-called fully premixed burner in a heating apparatus which includes a circuit for disposing of the flue gas downstream from the burner, the method comprising the steps of:
conveying an air flow into a tubular body by means of a blower;
conveying a separate flow of a fuel gas into said tubular body through a respective supply conduit;
controlling the rate of said gas flow by throttling said conduit;
mixing said air and gas flows together through a mixing path formed in said tubular body, and delivering the resultant mixture to the burner over a supply conduit.
More particularly, the invention concerns a method for feeding air/gas mixtures to premix burners employed in heating apparatus for civil applications, such as flash boilers or water heaters. The invention is also directed to a device for implementing the method.
The term "blower" will be used throughout the description here below and the appended claims to indicate any air supply arrangement -- including a centrifugal fan -- of the type wherein, for each change in the delivery flow rate there corresponds an oppositely directed change in the air delivery pressure.
In the field of heating apparatus for civil applications, a well-recognized demand is, on the one side, that their thermal power should be controllable in a flexible manner, and on the other side, that their burners be made ever more compact in size and efficient combustion-wise.
Of the above demands, the former is of paramount importance in this case, as a controlled delivery of thermal power according to the user's own requirements should be ensured as fully as possible.
To fill this basic demand, so-called "blast" burners have been mainly employed in this field wherein the combustion air is partly premixed with the fuel gas downstream from the burner gas delivery nozzles (primary air), and partly supplied downstream from the burner to complete the combustion (secondary air).
Blast burners, while providing for the desired control of thermal power, do require to be operated with considerable excess air over the stoichiometric amount in order to ensure as complete a gas combustion as possible.
The use of excess air involves, however, a twofold drawback: on the one side, it is in practice impossible to reduce the burner size due to the low flame density that this would bring about, and on the other side, objectionable gases, specifically nitrogen oxides, are released in significant amounts.
In an attempt at obviating such drawbacks, the use of so-called fully premixed burners has been proposed which are fed air/gas mixtures including all the air required for complete combustion.
The function of feeding burners of this type with air/gas mixtures having the desired characteristics is usually served by devices wherein separate flows of air and fuel gas from a centrifugal fan and the gas distribution network, for example, are mixed together in metered amounts which can be varied according to the thermal power delivered by the burner.
In accordance with continued teaching by the prior art, the flow rates of the air and gas fed to the burner are controlled by changing the centrifugal fan rpm and throttling the gas supply conduit connected to the external distribution network, respectively.
According to a first methodology, the fuel gas supply conduit is throttled using valve means driven to conform with changes in the air flow rate by a device which comprises a diaphragm and a complex lever arrangement for mechanical amplification.
The means provided to change the gas flow rate according to the air flow rate have, however, significant drawbacks originating from their construction and operation.
A first of said drawbacks comes from the limitations to which the diapnragm provided for measuring the presence and magnitude of each variation in the air flow rate is subject; its dimensioning, in fact, disallows minor variations in the air flow rate to be "perceived", with consequent failure to adjust the gas flow rate.
In addition, the hysteresis phenomena of the diaphragm and the presence of unavoidable friction in the linkage of the mechanical amplification arrangement cause an adjustment error to occur in the flow rate of the gas being fed to the burner, which is almost impossible to recover and increases with time.
The presence of this error involves the need to operate at a continued excess of air under all the burner conditions of operation. This to avoid that, at minimum thermal power, the amount of air be less than required, with the attendant risk of the burner head suffering damage from resultant overheating.
In accordance with a second methodology proposed by the prior art, an attempt at obviating these drawbacks provides for of the air and gas flow rates to be controlled through an electronic control system capable of varying said flow rates independently and simultaneously by acting on the centrifugal fan rpm and the valve means for throttling the fuel gas supply conduit, respectively.
However, this methodology has been unsuccessful in practice because electronic control systems as such are inherently unreliable; that is, they are uncapable of ensuring, on the occurrence of any failure, that the burner be put off and/or brought to a state of steady operation.
The adoption of so-called failsafe arrangements including two electronic control systems in parallel to overcome this limitation, has found however, no practical applications due to the considerable cost increase that it entails.
It is on account of tnese compounded drawbacks that the use of fully premixed burners in heating apparatus for civil applications -- highly desirable by itself in view of their compact design and high flame density features -- has encountered heretofore notable limitations.
The underlying technical problem of this invention is, therefore, to provide a method which can allow air/gas mixtures to be fed at variable rates to a so-called fully premixed burner, and at the same time overcome the drawbacks with which the aforementioned prior art is beset.
This problem is solved according to the invention by a method as indicated being characterized in that the air flow rate conveyed into said tubular body is controlled by diverting an aliquot of said flow toward a bypass conduit of the burner, said conduit causing, upon air being flowed therethrough, a head loss which is substantially equivalent to that produced by the combination of the tubular body, the burner supply conduit, the burner, and said circuit.
Advantageously, and in accordance with a further feature of this invention, the air flow from the blower is diverted into the bypass conduit, with no appreciable changes in overall head loss and without changing the blower rpm, through a shutter which is guided slidably within an air distribution chamber interposed between the blower and the mixing path.
To implement the above method, this invention provides a device of a type comprising a tubular body having an air/gas mixing path defined therein which is in fluid communication with an air delivery blower and a fuel gas supply conduit, said tubular body having an outlet opening facing a burner supply conduit, and being characterized in that it further comprises, located upstream of said mixing path, an air distribution chamber provided with an air inlet passageway in fluid communication with said blower, and opposed air outlet passageways facing said mixing path and a burner bypass conduit, respectively, valve means being provided in said chamber to selectively divert predetermined amounts of air toward said tubular body and said bypass conduit, respectively.
The features and advantages of the invention will be more clearly apparent from the description of a preferred embodiment of a method for feeding air/gas mixtures according to the invention, given herein below with reference to the accompanying drawings which illustrate, by way of example and not of limitation, a device according to the invention.
In the drawings:

Figure 1 is a part-sectional perspective view of a device according to the invention;
Figure 2 is a bottom view of the device in Figure 1;
Figure 3 is an elevation view of some details of the device in Figure 1;
Figure 4 is a longitudinal section view of the device in Figure 1, showing it in a first condition of its operation;
Figure 5 is a longitudinal section of the device in Figure 1, showing it in a second condition of its operation.

With reference to the drawing figures, generally shown at 1 is a device for supplying air/fuel gas mixtures to a burner 2 of the so-called fully premixed type, in a heating apparatus such as a wall-mounted boiler, known per se and not shown.
Said boiler includes, downstream from the burner 2, a circuit 40 for disposing of the flue gas which comprises a chamber 41 wherein a gas/water heat exchanger 42 is carried to generate heated water for space heating purposes, for example, in fluid communication with a hood, known per se and not shown.
The circuit 40 is, in turn, connected to a chimney, also known per se and not shown, for exhausting the flue gas to the ambience.
The device 1 comprises a substantially L-shaped tubular body 3 having an air and fuel gas mixing path 4 defined on its interior. The latter is in fluid communication, at one end with the burner 2 through a conduit 9, and at the other end with an air distribution chamber 10 interposed between the tubular body 3 and a blower 5, e.g. a centrifugal fan, to which it is connected by a conduit 11.
Shown respectively at 7 and 8 are inlet openings for admitting air and fuel gas into the device 1, which are conventionally associated with the centrifugal fan 5 and with a conduit 6 connected to the external gas distribution network, and shown at 12 is an outlet opening for the air/gas mixture facing the burner 2.
The chamber 10 is advantageously made of a plastics material and comprises two juxtaposed shells 10a, 10b which are associated with each other in a manner known per se, such as by a plurality of screws 43 threaded into eyelets 44, in turn formed by the junction of mating portions 44a, 44b extending integrally from the shells.
According to a further aspect of the present invention, the chamber 10 is mounted releasably to the tubular body 3 by means of a plurality of screws, collectively indicated at 45 and threadably engaged in corresponding supporting lugs 46 formed integrally on the tubular body exterior.
In this way, the chamber 10 and tubular body 3 can be associated together at the most appropriate position for mounting the device 1 on the boiler.
Thus, for example, shown in Figure 3 in solid and broken lines, respectively, are alternative mounting positions of the chamber 10 on the tubular body 3.
Furthermore, the chamber 10 is advantageously a scroll pattern, so as to convey the air from the centrifugal fan 5 toward the mixing path 4 with a minimum of head loss.
The chamber 10 is provided, downstream from said air inlet opening 7, with opposed air outlet passageways 13, 14 leading to the mixing path 4 and a bypass conduit 15 of the burner 2, respectively.
The conduit 15 is, in turn, connected, in a manner known per se and not shown, to the chimney for disposing of the flue gas from the circuit 40 inclusive of the burner 2.
This conduit 15 may be provided, where desired, with one or more diaphragms effective to induce a head loss in the air passageway which is substantially equivalent to that induced by the combination of the tubular body 3, the conduit 9, the burner 2 and the circuit 40.
In accordance with a feature of this invention, valve means 16 are provided within the chamber 10 to selectively divert predetermined amounts of air toward the tubular body 3 and/or the bypass conduit 15, respectively.
In a preferred embodiment, the valve means 16 comprise a substantially disk-shaped shutter 17 which is guided slidably within the chamber 10 for adjustable movement between opposed positions whereat it respectively opens and closes each of the air outlet passageways 13 and 14.
For this purpose, the valve means 16 are provided with an actuator device 18 which comprises an electric step motor 19 housed within the shutter 17 and driven in a manner known per se, e.g. through a conventional control unit, not shown, from a thermostat schematically shown at 37.
In the accompanying drawing figures, a cable is schematically shown at 20 which connects the step motor 19 electrically to said unit.
The actuator device 18 further includes a threaded shaft 21 which is driven rotatively by the step motor 19 to threadably engage in a threaded hole 22 provided in a bush 23 extending axially in the air outlet passageway 14, where it is supported by three radial webs 24a, 24b and 24c forming angles of about 120 degrees between each other.
Indicated at 25 is a key, extending integrally from the shutter 17, which engages slidably in a respective mating seat 26 arranged to face it within the chamber 10.
In accordance with a further feature of this invention, the actuator device 18 for the shutter 17 also drives valve means 27 to open/close which are accommodated downstream from the fuel gas supply conduit 6 and lie in the mixing path 4.
Said valve means 27 comprise a substantially L-shaped valve body 28 wherethrough a fuel gas path 39 is defined which extends between opposed gas inlet 29 and gas outlet 30 passageways housing a nozzle 31 and a fuel gas distribution head 32, respectively,. in the tubular body 3. The nozzle 31 is outside threaded for screw engagement with the gas inlet passageway 29, in which it can be positioned in an adjustable manner.
The inlet end 33 of the nozzle 31, being located inside the valve body 28, forms a valve seat adapted to co-operate with a substantially cone-shaped shutter 34 actuated against a spring means 25 by the shutter, wherewith it is associated with the intermediary of a rod 36 mounted in gas-tight relationship slidably within the valve body 28.
In a preferred embodiment of the invention, the rod 36 extends from the shutter 17 on the opposite side from the shaft 21, wherewith it lies substantially coaxial.
In accordance with a further feature of this invention, the shutter 34 may be given such a shape, e.g. by selecting a suitable flare angle for the cone, as to allow the flow rate of the fuel gas conveyed along the mixing path 4 to be varied at will and as expected.
In other words, by an appropriate configuration of the shutter 34, it becomes possible to both retain a constant air/gas ratio in the mixture delivered to the burner 2 and vary this ratio according to a predetermined law, e.g. as determined experimentally to suit the characteristics of the individual burner.
For optimum distribution of the gas through the mixing path 4 defined in the tubular body 3, the head 32 is given a substantially truncated cone shape and has three radial supporting webs, collectively designated 38, arranged at an offset angle of 120 degrees from one another and co-operating with the valve body 28, in abutting relationship therewith, to define three annular outlet slots for the fuel gas from the valve means 27.
An implementation of the inventive method will now be described with reference to the device just described and Figures 4 and 5 of the drawings.
In a starting condition as shown in Figure 4, where the burner 2 is delivering its full thermal power, for example, the device 1 effects the mixing of air from the centrifugal fan 5 with the gas from the conduit 6 at the distribution head 32 along the mixing path 4.
In this condition, the shutter 17 occupies a first position where it shuts off the air outlet passageway 14 to the bypass conduit 15, thereby directing the air flow from the centrifugal fan 5 fully along the mixing path 4, i.e. to the burner 2.
In this condition, the shutter 34 in turn occupies a position where the fuel gas delivery nozzle 31 is opened wide.
When the thermal power delivered by the burner 2 requires to be decreased the thermostat 37 will, through the step motor 19, drive the shutter 17 to open the bypass conduit 15.
Advantageously, the invention provides for the shutter 17 to be positioned inside the chamber 10, in an adjustable manner by rotating the shaft 21 in the bush 22 to correspondingly traverse the shutter 17, whose sliding movement will be guided by the key 25.
The shutter 17 displacement will simply divert an aliquot of the air flow from the centrifugal fan 5 toward the bypass conduit 15, with no significant change in overall head loss downstream of the chamber 10, that is without causing any significant alteration of the fan conditions of operation.
By virtue of the circuits located downstream from the chamber (bypass conduit 15, on the one side, and tubular body 3, conduit 9, burner 2 and circuit 40, on the other side) being substantially equivalent, the air flow rate to the burner can advantageously be controlled by just changing the shutter 17 setting within the chamber 10.
In accordance with a further feature of this invention, each displacement of the shutter 17 within the chamber 10 results in a corresponding displacement of the shutter 34 toward the inlet 33 of the nozzle 31 to throttle down the passage therethrough and concurrently control the fuel gas flow rate.
By selecting a suitable profile shape for the shutter 54, moreover, a change in the air/gas ratio can also be produced, as explained above and according to a law preliminarly established to suit the burner 2 characteristics.
During the setting step for the thermal power delivered by the burner 2, the thermostat 37 will position the shutters 17 and 34 such that the burner 2 is supplied the appropriate air/gas mixture for optimum combustion.
Such a condition of operation is depicted in Figure 5, for example.
The manifold advantages to be obtained with this invention can be appreciated from the foregoing description.
A first major advantage is that a so-called fully premixed burner can be supplied air/gas mixtures at varying flow rates in an extremely simple and reliable manner.
Since the air flow rate to the burner is controlled by diverting an aliquot of it toward a bypass conduit, there exists no need to adjust the rpm of the centrifugal fan 5, thus preventing the latter from supplying an air flow at continually changing pressures and rates.
Since the head losses downstream from the distribution chamber 10 are substantially equivalent both through the circuit including the burner 2 and the bypass conduit 15, the air flow rate to the burner will only be dependent on the position occupied by the shutter 17 within the chamber 10.
By selecting a suitable profile shape for the cone shutter 34 and linking the latter directly to the movements of the shutter 17, the possibility is ensured of changing the air/gas ratio according to a preliminarly established law to suit the burner 2 characteristics.
Thus, the invention allows, for example, a near-stoichiometric air/gas ratio to be selected at maximum power of the burner 2, and a higher (excess air) air/gas ratio at minimum thermal power, to best exploit the potential of the fully premixed burner.
In addition, the delivery of "cool" air to the chimney through the bypass conduit 15 has further advantages in that it increases the draft in boilers of the so-called natural draft type, and lowers the dew point of the flue gas, thereby attenuating the risk of condensate forming on the chimney interior.
The inventive device is, moreover, a so-called inherently safe type, that is, in the event of a failure, the device operation would enter a condition of steady state operation because of the air and gas being always mixed in precisely balanced ratii from the stoichiometric standpoint.
As a not least advantage, the invention provides a device which is simple, can be adapted at the assembly stage for a wide range of different boiler designs, is reliable in operation, and includes elements which can be mass produced at a low cost using conventional techniques in the industry.

Claims

A method for feeding air/gas mixtures at varying flow rates to a so-called fully premixed burner (2) in a heating apparatus which includes a circuit (40) for disposing of the flue gas downstream from the burner (2), the method comprising the steps of:
conveying an air flow into a tubular body (3) by means of a blower (5);
conveying a separate flow of a fuel gas into said tubular body (3) through a supply conduit (6) in fluid communication with said body (3);
controlling the rate of said gas flow by throttling said conduit (6);
mixing said air and gas flows together through a mixing path (4) formed in said tubular body (3), and delivering the resultant mixture to the burner (2) over a respective supply conduit (9),
characterized in that the air flow rate conveyed into said tubular body (3) is controlled by diverting an aliquot of said flow toward a bypass conduit (15) of the burner (2), said conduit (15) causing, upon air being flowed therethrough, a head loss which is substantially equivalent to that produced by the combination of the tubular body (3), the burner supply conduit (9), the burner (2), and said circuit (40)
A method according to Claim 1, characterized in that said air flow is diverted by means of a first shutter (17) being slidable in a guided manner within an air distribution chamber (10) in fluid communication with said bypass conduit (15), said chamber (10) being interposed between said blower (5) and said tubular body (3)
A method according to Claim 2, characterized in that said shutter (17) can be positioned in an adjustable manner within said chamber (10) according to the demand for thermal power placed on the burner (2).
A method according to Claim 2, characterized in that said gas flow is controlled simultaneously as said air flow by throttling said gas supply conduit (6) by means of a second shutter (34) arranged to perform traversing movements along with said first shutter (17) whereby it is driven toward and away from said conduit (6).
A method according to Claim 4, characterized in that said gas flow is controlled, using said second shutter (34), simultaneously as the air/gas ratio of the mixture being supplied to said burner (2).
A device for feeding air/gas mixtures at varying flow rates to a so-called fully premixed burner (2), being of a type which comprises a tubular body (3) having an air/gas mixing path (4) defined therein which is in fluid communication with an air delivery blower (5) and a fuel gas supply conduit (6), said tubular body (3) having an outlet opening (12) facing a burner (2) supply conduit (9), and being characterized in that it further comprises, located upstream of said mixing path (4), an air distribution chamber (10) provided with an air inlet opening (7) in fluid communication with said blower (5), and opposed air outlet passageway (13,14) facing said mixing path (4) and a burner (2) bypass conduit (15), respectively, first valve means (16) being provided in said chamber (10) to selectively divert predetermined air flow rates toward said tubular body (3) and said bypass conduit (15), respectively.
A device according to Claim 6, characterized in that said air distribution chamber (10) is substantially a scroll pattern.
A device according to Claim 6, characterized in that said air distribution chamber (10) and said tubular body (3) are releasably associable with each other at predetermined positions.
A device according to Claim 6, characterized in that said chamber (10) comprises two shell halves (10a,10b) releasably associated with each other.
A device according to Claim 6, characterized in that said valve means (16) comprise a shutter (17) slidable in a guided manner within said chamber (10) where it can be positioned adjustably between opposed positions to respectively open and close each of said air outlet passageways (13,14).
A device according to Claim 10, characterized in that said shutter (17) includes respective motive means (19) for its adjustable positioning within said chamber (10).
A device according to Claim 11, characterized in that said motive means (19) are driven by a thermostat (37).
A device according to Claim 6, cnaracterized in that it comprises second valve means (27) for throttling said gas supply conduit (6), being linked operatively to said first valve means (16).
A device according to Claims 10 and 13, characterized in that said second valve means (27) comprise a second shutter (34) arranged to perform traversing movements along with said first shutter (17) whereby it is driven, against respective spring means (35), toward and away from said gas supply conduit (6).
A device according to Claim 14, characterized in that said second shutter (34) is substantially cone-like in shape.
A device according to Claim 14, characterized in that said second shutter (34) has a profile shape such that, upon said gas supply conduit (6) being throttled, air/gas ratii will be set which vary with the position setting of said first shutter (17).
A device according to Claim 13, characterized in that said second valve means (27) for throttling said gas supply conduit (6) lie at least partially in said mixing path (4).
A heating apparatus of a type incorporating a burner (2) of the so-called fully premixed type. characterized in that it comprises an air/gas mixture feeding device according to any of Claims 6-17.