Thermal measuring and testing – Housing – support – or adjunct
Reexamination Certificate
2002-08-28
2004-03-09
Verbitsky, Gail (Department: 2859)
Thermal measuring and testing
Housing, support, or adjunct
C374S141000, C374S179000, C136S200000
Reexamination Certificate
active
06702458
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the means of installing a conventional thermocouple in the interior of the air-gas supply duct of a gas burner, in order for it to provide, at one and the same time:
a) the function of “cold safety” in the case of flame extinction,
b) the function of “hot safety” in the case of the injector catching fire, and this without any additional device,
c) a considerably increased operational service-life, thanks to the permanent cooling, by the “cool” air-gas mixture, of this thermocouple which is very highly heated at the level of its elements close to the combustion chamber.
The device is particularly intended for apparatuses, the combustion chamber of which operates in a confined space at high temperatures and nevertheless requires the thermocouple to be positioned at the level of said chamber.
DESCRIPTION OF THE RELATED ART
In the present state of the art, thermocouples are component parts of a well-known safety device, the external appearance of which is in the form of a metal sheath, called an external conductor, which terminates, on the side exposed to the heat, in a sensing point in a bulb which forms a sleeve subject to a casing and, at the other end, in an electrical connection to a safety valve.
Inside the bulb sleeve, a segment formed from a specific metal which differs from that of the sleeve of the bulb is welded at its end to the point of the latter. This weld is called a “hot weld”. This segment is extended, at its other end, by another weld to an insulated conductor wire enclosed in the external conductor tube. This second weld is called a “cold weld”. Finally, the internal conductor wire is connected to the outlet of the conductor tube, the connection to the safety valve. Internal conductor and external conductor are thus connected to an electromagnetic coil inside a safety valve. When the sensor point of the bulb is subjected to the heat of a burner, the temperature difference which is established between the hot weld and the cold weld generates a movement of the electrons, and the difference in potential created engenders a continuous micro-current, the electromotive force of which is capable of inducing, at the level of the solenoid of the coil of the safety valve, an electromagnetic field sufficient to maintain, in the attracted position against the electromagnet of said coil, a displaceable core which carries the opening-closing flap of the safety valve. If the sensor point of the thermocouple cools when the production of heat by the burner ceases when it is extinguished, intentionally or not, the difference in potential disappears, and therefore also the electromagnetic field, and the flap, withdrawn by a spring, returns into a closed position. In the present state of the art, this safety device by thermocouple is entirely satisfactory in numerous applications where the configuration of the combustion apparatuses of the gas permits the thermocouple to be positioned so that only the hot weld of the sensor point is exposed to the heat of a flame. This is principally the case in devices with pilot lights or combustion igniters not confined in an environment of excessive temperature. And if the temperature, solely at the sensor point, does not exceed 600° C., the service-life of the thermocouple remains within the limits of a normal service-life and does not pose any special problems other than reasonable maintenance when the wear on the sensor point exposed to the oxidising combustion of a naked flame finally destroys the hot weld, thereby preventing the generation of the desired electromotive force.
The situation is not as satisfactory, or even acceptable, in other applications, and in particular new applications, where it is not possible to limit the exposure to heat of the only sensor point of the thermocouple and to contain, moreover, this exposure to heat within a temperature gradient which does not exceed 600° C. at the maximum. This is the case, by way of a non-limiting example, with the new combustion chambers of “high temperature” infrared radiators with metal refractory grilles where the desired configuration necessitates the introduction of the sensor point into a confined combustion chamber to attain temperatures reaching more or less 950° C.
In these conditions, not only is the sensor point of the bulb subjected to a temperature exceeding its own limits of 600° C. for its service-life, but also the casing to which this bulb is secured is subjected to an unacceptable temperature by conduction. It is inside and at the level of this casing that the “cold” weld is situated. This cold weld, raised to too high a temperature, affects the value of the difference in potential with the “hot” weld but, above all, subjects the segment between these 2 welds, which is formed from a specific alloy, to inexorable destruction by material loss according to a geometry of “pencil points” opposed by the summit (in the image of electric arc electrodes), this deterioration continuing until there is a definitive break in the segment at the level of the 2 opposite points.
To this is immediately added, upstream of the casing, the attack on the external conductor which becomes porous and particularly sensitive to oxidation and to the corrosive action of the burnt gases released by the neighbouring combustion.
In these temperature and proximity conditions, the service-life of a conventional thermocouple is reduced very substantially. The efforts used to extend this service-life consist substantially of retarding these destructive effects:
d) reinforcing the volume of the hot weld,
e) treating the surface with a deposit of nickel on the casing and on the portion of the external conductor close to the hot area.
No matter what is done, it must be admitted that these efforts for improvement only relate to the effects and not to the cause.
With reference to hot safety devices, in case of the injector catching fire, known electrical cut-off devices follow the “mechanical” concept, in that this break occurs by a fuse blowing or by opening a thermostatic contact. These devices obviously lead to extra cost with, moreover, the disadvantage of a “pirate” shunt always possible by an unaware or careless user.
SUMMARY OF THE INVENTION
Therefore, the object of the present invention is:
for cold safety:
f) to treat the cause of the precocious deterioration of thermocouples when these are placed, of necessity, in the very centre of combustion chambers which are raised to a high temperature, reaching more or less 950° C.;
for hot safety:
g) to take advantage of the consequences induced by the installation means used in order to treat this cause of deterioration, also to permit the thermocouple to provide the function of hot safety without a “mechanical” electrical cut-off device.
And hence to obtain, overall, with a conventional thermocouple, long use at minimum cost, since it is used to provide, at one and the same time, cold and hot safety functions on apparatuses having a confined combustion chamber which have the advantage of generating high temperatures. These high temperatures are particularly desirable for infrared light emitters in order to obtain electromagnetic wavelengths of between 1.5 and 4 micrometers.
More precisely, the invention comprises a device for installing a thermocouple in the interior of the air-gas duct of a gas burner so as to ensure the functions both of cold safety and of hot safety, said thermocouple comprising an external conductor, which terminates in a sensor point in a bulb forming a sleeve subject to a casing, characterised in that the downstream portion of the external conductor of the thermocouple, on the sensor point side, penetrates to the interior of the inlet duct for the air-gas mixture, in that the sensor point at the end of the bulb is supported on the internal surface of the contact zone of the perforated wall of a diffusion chamber, and in that the installation device comprises a small plate secured transversely to the outlet section of the air-gas duct, said small plate having an appropriately dim
Centre d'Etude et de Realisations D'Equipment et de Ma
Young & Thompson
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