Combustion – Porous – capillary – particulate or sievelike flame holder,... – Means supplying fuel for passage through the flame holding...
Patent
1984-07-03
1986-08-26
Green, Randall L.
Combustion
Porous, capillary, particulate or sievelike flame holder,...
Means supplying fuel for passage through the flame holding...
F23D 1412
Patent
active
046080120
DESCRIPTION:
BRIEF SUMMARY
This invention relates to gas burners utilising a heat radiant burner element made of finely porous ceramic material, known as ceramic foam, through the pores of which a combustible mixture of gas and air, or oxygen, is passed to emerge and burn at a surface of the element.
Ceramic foam is made by impregnating a precursor matrix of a reticulated polyurethane foam, or like combustible foam material, with an aqueous ceramic slip or slurry, drying and firing the impregnated material so as to burn out the combustible matrix and leave a porous ceramic structure corresponding to a lining or coating of the cellular structure of the original polyurethane or other matrix. By selection of the precursor foam matrix and ceramic impregnant, the porosity of the ceramic foam can be determined and graded in terms of the number of pores per linear unit, for example pores per linear 25 mm or per linear inch.
Gas does not pass easily through the small pores of ceramic foam and previous proposals to use such material for radiant gas burner elements have involved special structures, for example of relatively coarse and fine porous layers, or the use of air or gas and air mixture under applied pressure instead of ordinary supply pressure.
The present invention provides a self-aerating gas burner utilising simply ceramic foam material as a radiant burner element, mounted on a box base, and only the supply pressure of gas, mains or bottled, injected through a gas jet to induce flow of air into the box base to mix with the gas and pass through the burner element.
According to the invention, a self-aerating radiant gas burner assembly comprises a box base mixing chamber having an air inlet into which is directed a gas injector jet to induce flow of air through the inlet, the chamber being surmounted by a radiant burner element of ceramic foam material, the bore diameter of the gas injector jet being between 0.5 and 2.0 mm inclusive, the nominal porosity of the ceramic foam material being between 15 and 40 pores per linear 25 mm inclusive, the thickness of the burner foam material being between 8 and 30 mm inclusive and the dimensions within these ranges being selected for a specified gas and pressure range with the relationship that the lower the gas pressure the larger the jet size.
The polyurethane or like precursor matrix foams, by the use of which are made the ceramic foam materials used in the burners of the present invention, are supplied by the manufacturers with a nominal porosity stated in pores per linear unit. In practice, it has been found that there is a variable tolerance factor which may be as much as .+-.5 pores per linear 25 mm. This is due to the inexact nature of the precursor foam which is, of course, carried through to the resulting ceramic foam material. It must therefore be understood that the porosity values given in this specification are nominal values subject to manufacturing tolerances.
The porosity of the ceramic foam material used in the gas burners of the present invention is the most critical feature for satisfactory performance. When ceramic foam materials of a porosity of 10 pores per linear 25 mm are used, it is not possible to get the required combination of stable combustion with acceptable radiant output because it has been found that the burner lights back, that is to say the flame front travels back from the outer face of the burner element to the inner surface towards the burner base. When ceramic foam materials of a porosity of 45 pores per linear 25 mm are used, the pore size is too small to pass a sufficient quantity of gas/air mixture to provide stable combustion and there is excessive back pressure in the mixing chamber, preventing sufficient air from being induced to provide the correct proportion for stable combustion.
Whilst we have found that ceramic foam materials with porosities in the range 15 to 40 pores per linear 25 mm can be used to manufacture satisfactory self-aerating gas burners, the best results have been obtained with a porosity of about 30 pores per linear 25 mm.
The t
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patent: 3954387 (1976-05-01), Cooper
patent: 4413976 (1983-11-01), Scherer
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Green Randall L.
Hinds William R.
Morgan Thermic Limited
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