PROCESS FOR THE IGNITION OF A BURNER

Abstract: A process for the ignition of a burner (l), in particular for a boiler or a heating appliance said burner comprising a gas supply (4), an air supply provided with a fan (ll) and a burner plate (7) provided with openings. An electronic ignition element (8) being arranged which is connected to a control device (9) for putting burner into operation and igniting the air-­ gas mixture. During the ignition phase of the burner the air/fuel ration is lowered and after ignition of the mixture the ratio is raised to the operating value of the burner. The control device (9) being connected to regulating elements (ll,l2) which are disposed in the air inlet and/or fuel inlet for controlling the air/fuel ratio.

Description: The present invention relates to a method of igniting a burner in a burner assembly having also a fuel supply and a fan supplying combustion air to the burner, said fuel and air forming a mixture having an air/fuel ratio, as well as a controllable ignition device, where the fan and/or the fuel supply are energized to bring about a lower than normal air/fuel ratio at which the mixture is ignited by the ignition device and after ignition has been obtained the air/fuel ratio is raised to a normal operating value. Such a method is known from Patent Abstract of Japan, vol. 9, No. 11 (M-351) (1734), 18th January and JP-A-59 158 914. According to this known method - which is intended to ignite the burner of an oil burner - the burner fan motor initially generates an air flow rate lower than the normal air flow so that consequently when the ignition device is energized the air/fuel ratio is lower than normal. After ignition is obtained the air/fuel ratio is gradually increased to the normal operating value. In gas burners the ideal air/gas ratio on which ignition occurs greatly depends upon many parameters, such as the characteristics of the particular type of gas, ambient temperature, and temperature of the burner assembly - a cold burner assembly has different ignition characteristics than a warm one - and also the temperature of the supplied combustion air. The invention aims to provide the method of the above mentioned kind which results into a sure and safe ignition of a gas burner. The method according to the invention is characterized in that for igniting a gas burner assembly with a perforated burner plate the air/fuel ratio of the mixture supplied to the burner is gradually lowered from a value above the maximum value at which under the prevailing circumstances ignition occurs until ignition is obtained. According to the invention one starts with an air/fuel ratio of the mixture supplied to the burner which has a higher value than the one which ensures ignition. By gradually lowering the air/fuel ratio one automatically reaches the point at which under the prevailing circumstances ignition will take place and, after the burning has been ignited, the air/fuel ratio is raised to the normal operating value. In this way the circumstances prevailing at a particular moment cannot influence the ignition phase: the burner is always ignited surely and safely. It is pointed out that means for varying the air factor are known per se. However, this has hitherto taken place as a function of the load of the burner, in order to ensure the optimum possible combustion. The invention is explained in greater detail with reference to the drawing, in which: Fig. 1 shows a schematic illustration of a burner with which the process according to the invention can be used; Fig. 2 shows a graph in which the change in the air factor is plotted against time. In Fig. 1 the process according to the invention is explained with reference to a burner 1 with complete premixing of combustion air. The process according to the invention can, however, also be used with an atmospheric burner, where only the secondary combustion air is supplied by means of a fan. As can be seen from Fig. 1, the burner 1 comprises a mixing chamber 2, which is connected on one side to an air inlet 3 and a fuel inlet 4 for the infeed of an air-gas mixture. The burner 1 is disposed inside a combustion chamber 5 of a boiler or heating appliance, in which combustion chamber is disposed a heat exchanger 6 for heating of a medium. The mixing chamber 2 is shut off at the side facing away from the air and fuel inlet by a burner plate 7 which is situated in the combustion chamber 5 and is provided with openings through which the air-gas mixture flows into the combustion chamber. Disposed some distance away from the burner plate inside the combustion chamber 5 is an ignition element 8 which is connected to a control device 9. Near the burner plate 7 in the combustion chamber 5 there is also a detector for monitoring the combustion of the air-gas mixture, said detector also being connected to the control device 9. The air inlet 3 is provided with a fan 11, which is connected to the control device 9, while the fuel inlet 4 is provided with a regulating valve 12, which is also connected to the control device 9. Finally, one or more room thermostats 13 are connected to the control device. When the thermostats 13 call for heat, the control element 9 will start the fan and open the regulating valve 12, so that in the mixing chamber 2 of the burner 1 an air-gas mixture is formed and flows through the openings in the burner plate 7 into the combustion chamber 5. The electronic ignition element 8 will be actuated by the control element 9 for ignition of the air-gas mixture. If the heating appliance is cold, the air fed in through the inlet 3 will meet with little resistance, so that the fan 11 has a greater output. This results in a high air factor, which makes the air-gas mixture difficult to ignite. As a result of the lower temperature of the air, the density of the air is greater, so that relatively more air is displaced by the fan, which further increases the above-mentioned effect of a high air factor. In order to ensure the ignition of the burner, the speed of the fan 11 is reduced and/or the fuel supply to the burner increased during the ignition phase, so that during the ignition the air factor is reduced, and the above-mentioned influences are removed, so that the mixture has the correct air-gas ratio which is ideal for the ignition. The ignition of the mixture is monitored by the detector 10, which then gives off a signal to the control element 9, following which the air factor increases to the normal operating value of the burner. The process according to the invention can also be used with an atmospheric burner where the secondary combustion air is supplied by means of a fan. The secondary combustion air supplied by the fan makes ignition difficult through the "blowing away" of the air-gas mixture emerging through the burner plate. The combustible mixture is converted by the inflowing secondary air into a narrow cloud, the correct position of which is difficult to determine beforehand. This means that there is a risk of the ignition element lying outside this cloud. The high speeds and the eddies occurring also make ignition difficult. By reducing the speed of the fan during the ignition phase, a broad "cloud" of combustible air-gas mixture is produced near the burner plate, so that it is ensured that the ignition element lies within this cloud. It will be clear that the means for reducing the air factor during the ignition phase can vary from those which are shown in the drawing. For example, the air inlet 3, can also have a variable resistance element, by means of which the quantity of infed air during the ignition phase can be reduced. The fan could be shut off entirely during the ignition. Fig. 2 is a graph showing the curve of the air factor, where the ignition area for a gas A and for a gas B are indicated. It can be seen from this figure that different gases can have different ideal ignition areas and it is possible by means of the process according to the invention to ignite the burner irrespective of the type of gas used.

Claims: A method for igniting a burner in a burner assembly having also a fuel supply and a fan supplying combustion air to the burner, said fuel and air forming a mixture having an air/fuel ratio, as well as a controllable ignition device, where the fan and/or the fuel supply are energized to bring about a lower than normal air/fuel ratio at which the mixture is ignited by the ignition device and after ignition has been obtained the air/fuel ratio is raised to a normal operating value, characterized in that for igniting a gas burner assembly with a perforated burner plate the air/fuel ratio of the mixture supplied to the burner is gradually lowered from a value above the maximum value at which, under the prevailing circumstances, ignition occurs until ignition is obtained.

Inventors: VLOON, PAUL
Applicants: NEDERLANDSE INDUSTRIELE MAATSCHAPPIJ  NEFIT B.V.
Application year: 1986
Main classification: F23N1
IPC codes: F23N1