Flame sensors and methods of sensing flame

Details Flame sensors and methods of sensing flame Flame sensors and methods of sensing flame

1994-02-14

1995-11-07

Hannaher, Constantine

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

25033915, 250342, 250344, 250345, G01J 538

Patent

active

054649803

DESCRIPTION:

BRIEF SUMMARY
The invention relates to flame sensors and methods of sensing flame.
According to one aspect of the invention, there is provided a flame sensor comprising a cell having a window transmissive of radiation emitted by a flame and containing gas absorptive of selected radiation in the emission spectrum of the flame, and detection means responsive to heating of the gas caused by absorption of the selected radiation, wherein the gas comprises a mixture containing at least one isotope of carbon dioxide selected from .sup.13 C.sup.16 O.sub.2 and .sup.13 C.sup.18 O.sub.2.
In a preferred embodiment of the invention the mixture may also contain carbon dioxide .sup.12 C.sup.16 O.sub.2 and or nitrous oxide.
The detection means may detect directly a change of temperature of the fluid caused by said heating and may, for example, comprise a sensitive thermocouple or thermopile.
Alternatively, the detection means may detect a change of pressure of the fluid caused by said heating and may comprise a pressure-sensitive displacement transducer such as a capacitive condenser microphone.
According to a further aspect of the invention there is provided a flame sensor comprising a cell having a window transmissive of radiation emitted by a flame and containing a fluid absorptive of selected radiation in the emission spectrum of the flame, and detection means responsive to heating of the fluid caused by absorption of the selected radiation, wherein the detection means comprises a flexible membrane having a light-reflective surface, an optical fibre for transmitting a light beam onto said light-reflective surface and means for detecting a change of intensity of the reflected light beam caused by displacement of the membrane.
Flame sensors and methods of sensing flame in accordance with the invention are now described, by way of example only, with reference to the accompanying drawings in which:
FIG. 1 shows one embodiment of a flame sensor according to the invention;
FIG. 2 shows the emission spectrum of a hydrocarbon flame and the transmissivity of infra-red radiation from the flame through four meters of air;
FIG. 3 shows another embodiment of a flame sensor according to the invention; and
FIG. 4 shows a yet further embodiment of the flame sensor; and
FIG. 5 shows a yet further embodiment of the invention.
Referring to FIG. 1, the flame sensor comprises a cell 10 having a window 11, made from potassium bromide for example, through which radiation emitted by a flame F can enter the interior of the cell.
The cell contains a fluid (a gas, vapour or liquid) chosen to be absorptive of selected radiation in the emission spectrum of the flame, such absorption causing heating of the fluid which is detected to give an indication that a flame has been sensed.
In a preferred embodiment of the invention, the cell contains carbon dioxide gas (i.e. .sup.12 C.sup.16 O.sub.2). As will be explained, this choice of gas renders the flame sensor responsive to hydrocarbon flames and flames of other carbon-containing fuels.
The broken curve shown in FIG. 2 represents part of the emission spectrum of a hydrocarbon flame. The flame is strongly emissive of infra-red radiation in the wavelength range from 4.17 .mu.m to 4.76 .mu.m, almost all the radiation emitted in this range being due to the presence of excited carbon dioxide molecules in the hot gases of the flame. An emission spectrum measured at higher resolution would reveal that the broken curve shown in FIG. 2 is, in fact, the envelope of a large number of narrow emission lines resulting from transitions between different vibrational energy states of the excited carbon dioxide molecules.
By contrast, the temperature of the carbon dioxide gas contained in cell 10 of the flame sensor is at or close to ambient temperature giving a relatively narrow absorption spectrum centred on 4.26 .mu.m.
Carbon dioxide molecules are also present in the atmosphere at a concentration of about 350 ppm. Atmospheric carbon dioxide has the beneficial effect that it completely absorbs and blocks infra-red radiation from

REFERENCES:
patent: 2989638 (1961-06-01), Laudon et al.
patent: 3566106 (1971-02-01), Fletcher
patent: 4163382 (1979-08-01), Amer

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