DE69116842T2 - Wall-mounted gas boiler with premix burner, especially for home use - Google Patents

Abstract

This invention relates to a wall-mounted gas boiler, particularly useful for domestic employment, of the type comprising a pre-mix burner (13), a heat exchanger (12), a circuit (8) for allowing the combustion air to circulate, means (5) for causing said air to circulate, a chamber (18) for mixing air with the gas, in which chamber an opening (19) is provided, upstream of said mixing chamber (18) in the circuit (8) for the circulation of air, said opening being closed by valve means (20) and allowing the dilution air to flow towards the openings (21) without becoming mixed with the gas at the point corresponding to the mixing chamber (18), means (22, 23 and 24) being provided for changing the amount of gas introduced in proportion to the flowrate of the air flowing through the same. The boiler according to this invention can also be provided with a single-piece container (2) having a pressurized inner chamber (9), inside which both the burner (13) and the heat exchanger (12) are arranged, and an outer depressurized chamber (8) through which the air sucked in by the fan (5) flows, the outflow of smokes from the inner chamber (9) and the inflow of air into the outer chamber (8) occurring through an upper coaxial pipe (14); a closure member (3) of the container, said member being provided with a double seal for closing both the inner chamber (9) and the outer chamber (8) tightly; and a lower Archimedean screw (4) inside which the fan (5) is housed. <IMAGE>

Description

The invention relates to a wall-mounted gas boiler with a premix burner, in particular for domestic use.

In particular, this invention relates to a boiler of the type mentioned above, the features of which are such that the technology of premix burners is advantageously applicable to wall-mounted boilers for domestic use.

As is known, fanless burners are mainly used in the technology of wall-mounted gas boilers, i.e. burners in which the flame is fed with gas, with the supply of primary air, these components being mixed before entering the burner, and secondary air present around the flame.

The essential feature of this type of boiler is that, since air is available in abundance, optimal combustion conditions are present in a wide variety of situations.

However, such boilers are characterized by combustion residues that pollute the environment and by low power density.

In fact, it is well known that carbon dioxide, water, oxygen, nitrogen and NOx are formed during combustion.

The NOx oxides are composed of NO + NO₂, and such compounds, when dispersed in the atmosphere, react with water to form nitric acid and thus cause the well-known acid rain.

The formation of NOx takes place in the very core of the flame, at very high temperatures, so that the formation of NOx in fanless burners is very high, forming a very large flame so that a high temperature is reached.

Another type of boiler already known is that which provides a premix (or pre-mixed) burner, this type of boiler being used in particular in the case of high powers, since the technology involved is highly developed and advanced.

Such burner types only burn gas with the addition of primary air, with these components being mixed upstream of the burner.

JP-A-58085016 describes a heating device provided with a premixing device for regulating the air/gas ratio, the regulation being achieved by two different sensors 21 and 23.

It is obvious that a gas/air ratio of always optimal value and associated with this a constantly intimate mixture is necessary.

Precisely because of such requirements, this technology is only advisable for use in large-scale or high-performance systems.

However, these premix burners have technical features that make them interesting for use in households.

In fact, combustion takes place through the formation of a series of small flames, so that the temperature reached in one of them is averaged and does not reach the values of the fanless burner types.

This means a combustion with very low NOx formation, in addition to a higher power density (a value of about 1/10 with respect to the fanless boilers).

Consequently, a much less polluting combustion is obtained compared to the combustion obtained in plants equipped with fanless burners.

The structure of the premix burners also has, in addition to the problem associated with the sophisticated technology involved, the problem of noise generated by vibrations.

Such a disadvantage has certainly not contributed to the use of this type of burner in the field of wall-mounted boilers for domestic use.

In view of the above, the applicant has developed a wall-mounted boiler for domestic use, which includes a premix burner capable of operating in a modulated manner and which has a very compact structure.

Furthermore, the boiler according to the invention is designed in such a way that it does not generate any noise.

Accordingly, the invention comprises a wall gas boiler, particularly suitable for domestic use, of the type comprising a premix burner, a heat exchanger, a circuit for transporting the combustion air, a conveyor or fan for circulating the air, a chamber for mixing air with the gas and an opening upstream of the mixing chamber in the air circulation circuit, the boiler being characterized in that the opening is provided with valve members and allows the dilution air to flow downstream of the burner without being mixed with the gas present in the mixing chamber, the opening being of such a size that when it is completely closed by the valve, the burner operates at maximum power, whereas when the opening is completely open, the burner operates at minimum power, the valve members being opened depending on the water temperature in the heat exchanger. or closed, means being provided at the point corresponding to the mixing chamber to vary the quantity of gas introduced in proportion to the flow rate of the air passing through it, and the burner being provided with holes on the sides to allow the passage of air arriving from the opening immediately above the burner itself.

The valve elements are preferably formed from a throttle valve which is controlled by a stepper motor.

The devices used to vary the amount of gas introduced into the mixing chamber may, according to the invention, comprise two static pressure measuring tubes, each at the locations corresponding to the outlet of the mixing chamber and a reduction in cross-section downstream of the chamber and in front of the burner, the static pressure measuring tubes being connected to the valve for the introduction of gas.

Mixing in the mixing chamber is preferably carried out using the countercurrent method.

The invention will now be described in the following according to some preferred embodiments, with particular reference to the accompanying drawings, in which:

Figure 1 is a front view of the boiler according to the invention, with the closure element removed;

Figure 2 is a vertical transverse cross-sectional view of the boiler according to the invention; and

Figure 3 is a plan view of the Archimedes screw according to the invention.

With reference to all the drawings attached hereto, the boiler according to the invention is realized by means of an external container 1 composed of just three parts, i.e. a die-cast aluminum casing 2, a cover 3 and a lower Archimedean screw 4 which houses the fan 5.

The structure realized between the housing 2 and the cover 3 comprises two different sealing closures 6 and 7 (see in particular Figure 2) so as to define two zones, respectively an inner zone 8 which is under pressure and to create an external zone 9 which is subjected to external pressure.

The exchanger 12 and the premix burner 13 are provided inside the inner zone 8 so that they can slide on the guides 10 and 11. The outflow of smoke from the zone 8 and the inflow of air sucked in by the fan 5 take place through the coaxial pipe 14. The container 1 designed in this way reduces the probability of noise caused by vibration to a minimum.

Furthermore, the presence of a double seal prevents any escape of unburned gas from the inner zone 8 from being dispersed into the environment, so that the unburned gases are dispersed within the zone 9 and return to the circulation of the part brought to normal pressure.

A small window 15 made of a transparent material is provided at the location corresponding to the burner 13 for observing the flame.

The external air enters from the outer tube of the coaxial tube 14 and flows through the lateral passages of the zone 9 while being sucked by the fan 5 and reaches the Archimedes screw 4 which has a mixing chamber 16 for mixing the air with the gas.

The gas, in turn, is introduced into the chamber 16 through the nozzle 17 in countercurrent to the air in order to obtain an optimal mixture.

The gas-air mixture flows from the mixing chamber 16 to the burner 13, passing through a throttled region 18.

A hole 19, provided with a throttle valve 20, is provided in the path of the air, upstream of the mixing chamber 16, so that air can be transported directly into the zone 8. The air flows directly above the burner 13 through the side holes 21, as will be described below, without participating in the combustion.

Two static pressure pipes 22, 23 connected to the gas valve 24 are provided at the outlet of the mixing chamber 16 at the location corresponding to the throttled zone, the gas valve 24 in turn being connected to the nozzle 17. The fan 5 is provided with a motor 25.

The exchanger 12 is composed of two rows of alternating tubes 26 and 27, which are connected to the inlet 28 and to the outlet 29 for the water circulation through the system.

The fins in the smoke passage are laterally shaped to transport the external smoke towards the tubes 27 and are provided with holes 30 below the tubes 27 for dissipating heat to prevent them from absorbing an excess amount of heat.

The design and arrangement of the burner 13 and the exchanger 12 selected during the implementation allow a very compact boiler, which can be designed both horizontally and vertically so that this boiler can be installed in a standard hanging housing.

Regarding the description of the operation of the boiler according to the invention, it should be noted that the embodiment shown in the figure has a system for modulating power.

Considering a boiler of dimensions suitable for operation between a minimum output of 7 000 kcal/hour and a maximum output of 20 000 kcal/hour, the boiler starts to operate when it is switched on by the user and is set to supply water at 70 ºC, with the valve 20 in the position that the hole 19 is completely closed.

Gas and the air sucked in by the fan 5 are mixed in the chamber 16 and then flow to the burner 13.

In this state and until the set temperature is almost reached, the boiler operates at maximum power.

As previously mentioned, premix burners must ensure a constant optimal mixture regulation: appropriately a gas-air mixture with a 30% excess of air quantity with respect to the stoichiometric ratio.

The two static pressure measuring tubes 22 and 23 serve this purpose.

In fact, the value of Δp detected by them based on the change in the cross section is proportional to the value of the flow velocity of the fluid (gas-air) that is flowing, even if the value of p always remains positive.

This value Δp, which is read from the valve 24, determines the amount of gas that is introduced into the mixing chamber 16.

When the water temperature value comes close to the value set by the consumer, it is necessary to regulate the boiler output.

This could also be done by reducing the speed of the fan to reduce the air quantity. Such a reduction is read from the static pressure measuring tubes 22 and 23 so that there is also a reduction in the gas quantity. Consequently, the burner's output is reduced.

However, such a modulation system is hampered by the disadvantage linked to the fact that the dimensions of the exchanger 12 are designed for 20,000 kcal/hour - smoke or exhaust gases would be cooled too much, which would lead to the occurrence of condensation, with all the problems that this entails.

Accordingly, the boiler according to the invention has been designed so that its output is modulated without condensation occurring.

This is possible thanks to the hole 19 and the throttle valve 20 which are provided in the air flow, upstream of the mixing chamber 16.

The hole 19 has dimensions such that when it is completely closed by the valve 19, the maximum power of the burner is used, whereas when the hole is completely open, the burner has the minimum power.

When it is necessary to reduce the boiler output, the reduction being determined on the basis of the water temperature, an electronic device causes the valve 20 to open by means of a small stepper motor.

Part of the air, instead of being transported into the chamber 16, is transported above the burner 13 through the small holes 21 directly into the zone 8.

In this way, the value of Δp is reduced because the flow velocity of the air through the throttled zone 18 is reduced, so that the transported gas is also reduced and the boiler output is thus reduced.

At the same time, air flowing through hole 19, called "dilution air", dilutes the smoke or exhaust gases to prevent them from condensing when they come into contact with exchanger 12.

It can be observed that the dilution air does not participate in any way in the combustion, so that the burner 13 retains its premixing characteristics unchanged.

Claims (4)

1. Wall-mounted gas boiler, particularly for domestic use, and of the type comprising a premix burner (13), a heat exchanger (12), a circuit (14, 8, 4) for conducting the combustion air, conveying means (5) for circulating this air, a chamber (16) for mixing air with gas and an opening (19) arranged upstream of the mixing chamber (16) in the air circulation circuit, the boiler being characterized in that the opening (19) is provided with valve members (20) and allows the dilution air to flow downstream of the burner without mixing with the gas present in the mixing chamber (16), this opening (19) being of such a size that when it is completely closed by the valve (20), the burner operates at maximum power and when the opening (19) is completely open, the burner operates at minimum power, the valve elements (20) being opened or closed depending on the water temperature in the heat exchanger (12), and that a device (24) is provided at the location corresponding to the mixing chamber (16) in order to adjust the amount of gas supplied in accordance with the amount of air flowing through it and that the burner is provided with holes (21) on the side which allow the passage of the air flowing out of the opening (19) immediately above the burner (13) itself. 2. Wall gas heating boiler according to claim 1, characterized in that the valve member (20) is formed from a throttle valve controlled by a stepper motor. 3. Wall gas heating boiler according to one of the preceding claims, characterized in that the device (24) for adjusting the amount of gas supplied to the mixing chamber contains two static pressure pipes which are respectively connected to the Outlet of the mixing chamber (16) and at the point corresponding to a taper (18) of the cross-section, which is realized downstream of the chamber (16) in front of the burner and wherein the aforementioned static pressure pipes are connected to the gas inlet valve (26). 4. Wall gas heating boiler according to one of the preceding claims, characterized in that the mixing in the mixing chamber (16) takes place according to the countercurrent principle.