Tubular burner

Details Tubular burner Tubular burner

2001-10-22

2002-10-08

Yeung, James C. (Department: 3743)

Combustion

Mixer and flame holder

C431S326000, C431S328000

Reexamination Certificate

active

06461152

ABSTRACT:

BACKGROUND OF INVENTION
This invention relates to a tubular burner, and more specifically to a tubular hollow burner of desired cross-section being axially long compared to the dimensions of the cross-section. The invention also relates to a distributor disposed internally of the hollow burner which ensures good mixing of the gas and air in the combustible fluid urged through the burner and as near to an even flame profile as possible on the flame strip of said burner.
Although the following description related almost exclusively to fully pre-mixed cylindrical tubular burners, it is to be mentioned that the burner of the present invention may be used for non pre-mixed and normally aspirated applications and furthermore the cross-sectional shape of the burner is unimportant The invention should accordingly not be considered as limited by the following description. It should also be mentioned that the word tubular as used herein describing burners is to be taken to mean any burner having an outer surface which defines an internal cavity in which distribution means can be disposed.
Pre-mixed burners are so-called because the fuel, usually gas (under denominations for reference gases and test gases identified in European Standard EN 297), and a fan supplied quantity of air exceeding the stoichiometrically correct amount of air for the specific gas type (superstoichiometric) are mixed to produce a combustible mixture which subsequently is passed through the burner and ignited to produce a burner flame that, in the case of heating the water in a boiler, is applied to a heat exchanger of the boiler. The term pre-mixed arises therefore because of the mixing of the fuel and air before the combustible mixture passes through the flame strip.
There are other types of burners which operate in a mode in which a sub-stoichiometric amount of primary combustion air is mixed with the fuel before the flame strip, secondary air required for completing the combustion process being induced into the flame after ignition of the gas/primary air mixture. These other burners are known as partially pre-mixed burners. The present invention may be applicable to such burners, but its best application is to the fully pre-mixed type, as partially premixed burners are limited by the relatively high levels of nitrogen oxides (NOx) they generate during the combustion process and as such, these burners are diminishing in popularity.
Fully pre-mixed burners tend to be high intensity burners in which high volumes of gas/air mixture are forced through a relatively small area (in plan) burner, and specifically through the ports in the flame strip to give a compact, high intensity flame which sits on or near the surface of said flame strip.
The high volume of the gas/air flow being urged through the small port area of the burner flame strip means there is provided a high “port-loading” on each individual burner port. The fact that a compressible mixture flows through the burner at a certain velocity means that any instabilities created on ignition of said mixture are amplified and can ultimately develop a common frequency which constructively harmonizes with the natural frequency of the boiler system to generate a phenomenon called combustion resonance which is manifested in noise. Noise of any audible volume or frequency is unacceptable for pre-mixed burner applications.
The boiler system comprises the combustion chamber, the heat exchanger which will occupy a predetermined position within said combustion chamber, and a flue attached to said chamber to vent the exhaust gases of combustion. Any variance of these parameters will influence the harmonics of the system e.g. varying the flue length will change the back pressure on the combustion chamber.
The combustion resonance is manifested as three distinct types of resonance:
1. A low frequency (125 to 200 Hz) rumble on ignition; This is believed to be due to flame instabilities caused by poor gas/air mixing, bad gas/air mixture distribution and poorly timed ignition, such being associated with the burner appliance design factors of upstream mixing of gas and air, position of igniter etc.
2. A higher frequency (250 to 315 Hz) resonance on ignition at volumes up to 95 dB; Under standard repeat ignition conditions the flame ignites and thermally fluctuates initially as it stabilizes near the port. The differential pressures and temperatures created initially in the system exacerbate this instability creating a range of oscillating and fluctuating frequencies of flame vibration, some of which may harmonize and thus be amplified at one or more of the natural resonance frequencies bands of the system. However, once the system has been operational for approximately a minute, these instabilities dissipate and the resonance fades out.
3. A continuous high frequency resonance can develop once the flames have stabilized. This can arise from instabilities caused by ignition resonance and which are continuously excited by virtue of the gas/air flow movements within the system during operation, or by the inherent excitations developed by virtue of the burner design.
Currently, conventional metal burners of the type considered herein having ports integrally formed on an outer surface thereof cannot offer the “turndown” range (the range over which the flame is stable on or proximate the surface of the burner as the gas/air flow is gradually reduced) demanded by manufacturers of modern appliances, and therefore ceramic materials are often used in such applications. Additionally, the risk of “flashback” at low port loadings where the flame burns on or very close to the flame strip must be eliminated and this has heretofore proved difficult.
At much higher port loadings, the flame can lift away from the flame strip as a result of the increased and rapid volumetric throughput of gas through the ports, and flame instabilities can thus result.
The stability of the flame on the burner flame strip is dependent on the open area of said flame strip (port area per unit area of the flame strip), the surface area “land” surrounding each port, i.e. the length of the ports, the pattern of ports, the profile of the flame strip surface, and the efficacy of any distribution means disposed behind the flame strip internally of the burner. A stable flame requires sufficient “land” to anchor itself thermally to the port, but this requirement compromises the total open area of the plaque and hence increases the port loading, with the attendant disadvantages of lift-off mentioned above. There is therefore a trade off between these two parameters.
A further consideration in modern burner design is the recently introduced requirements for reducing emissions of noxious gases such as NOx (Nitrogen Oxide) and CO (Carbon Monoxide) below predetermined limits.
U.S. Pat. No. 5,743,727 to Rodgers describes a burner of similar construction to that with which the invention is concerned. Specifically, a cylindrical tubular burner is described having an outer cylindrical tube body with a blind or capped end and a plurality of perforations provided over a small sector of the body along the length of said tube. This perforated sector forms the flame strip of the burner and internal distribution of the combustible gas mixture underneath the flame strip to obtain an even flame profile thereon is achieved by means of an elongate distributor of similar cross-sectional shape but of smaller size so that the distributor can be slid within the body and sealed to the blind end or cap thereof.
Injection means is provided at the open end of the distributor and air may be drawn thereinto along with the gas so that a combustible mixture (which may at that stage not be evenly or thoroughly mixed) passes through the hollow distributor. In accordance with the invention of the U.S. patent, a plurality of outlets are provided on the underside of the distributor approximately 180° from the sector over which the perforations on the burner body are provided so that the combustible gas is mixed thoroughly with air before being urged through sa

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