Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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AC RO~ND OF THE INVENTION
Field of the Invention
The present invention relates to combustibles detectors
generally and part cularly to differential thermocouple devices
modified to detect combustible gases.
Description of the Prior Art
Combustible gases detectors are known which utilize
electrically heated platinum filaments exhibiting resistance
changes in response to variable concentrations of combustible
gases. The resistan~e change of such devices i5 notoriously
s~all requiring their use in a Wheatstone bridge arrangement.
This arrangement increases the cost of tne device. An example of
such a device may be found in U.S. Patent No. 3,092,799 issued
June 4, 1963 to A.R. Baker.
Along with the required use of a Wheatstone bridge these
known devices have other shortcomings. A constant current power
supply or other regulated excitation source is required and the
devices exhibit drift rates which are too large to permit
industrially stable operation at low combustibles concentrations.
Part of the drift problem comes about because the nominal re-
sistance value of these sensors is the order of 1.00 ohm and the
presence of combustibles causes typical resistance increases to
1.04 ohms. Resistance changes of this magnitude are too close
in size to the kinds of changes experienced due to fluctuations
in contact resistance, electromigration, grain growth, etc.
Differential thermocouples are also known for use in thermal
analysis as evidenced by U.S. Patent No. 3,491,581 issued
January 27, 1970 to F.E. Roberts. However the applicant is un-
aware of any prior art device using such differential thermo-
couples coated with a catalyst and a non-catalyst to provide a
self-powered, inexpensive and stable combustible gases detector.
SUMMARY OF THE INVENTION
In accordance with ~he present invention a differential
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thermocouple combustibl~ gase~ detector is provided which is
inexpensive, compact, self-powered and stable. One thermocouple
junction of a thermocouple pair is coated with catalytic material
such as platinum to allow heated combustible gases such as
carbon monoxide to chemically react with the catalyst to liberate
heat and to raise the temperature of the coated thermocouple
junction above ambient in proportion to the concentration of the
combustible gases. The other thermocouple junction of the
thermocouple pair is coated with a non-catalytic material such
as a refractory cement to prevent any reaction at that junction
a~lowing that junction to only monitor ambient temperature. The
connection of the thermocouple pair in opposition allows the
ambient temperature signals sensed by both thermocouple junctions
to cancel leaving only the temperature increase over ambient
~ caused by the reaction of combustible gases with the catalyst on
;~ the catalyst coated thermocouple junction. The differential
thermocouple thus establishes a signal proportional to the con-
centration of combustible gases independent of the ambient
temperature.
- In one specific embodiment of the invention the catalyst
coated junction is formed by crimping a platinum tube around
the ~unction and then coating the tube with platinum paste to
; further increase the surface area o the catalyst. The non-
catalyst junction is formed by crimping a soft metal tube such
as a brass tube around the junction and coating the tube with
refractory cement.
In certain situations the combustible gases may have to be
heated to a threshold~temperature at which they will react with
the catalyst. A coil heater is then rnounted around the two
thermocouple junctions to heat the combustible gases. Since
sulfur is present in some mixtures containing combustible gases
the catalyst must be maintained above the dew point of sulfuric
acid to prevent condensation of the acid on the catalyst and the
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"poisoning" of ~he catalyst. In such situations the heater output
is raised to m~intain the ambient temperature of the junctions
at approximately 800F.
In view of th~ foregoing it will be seen that one aspect of
the present invention is to provide a differential thermocouple
combustible gases detector.
Another aspect of the present invention is to provide a
heated combustible gases detector which will insure that the
combustible gases are above the reaction threshold temperature
~; 10 and prevent acid condensation on the catalyst.
These and other aspects of the present invention will be
more fully understood upon consideration of the following
description of the preferred embodiment in conjunction with the
associated drawings.
BRIEF DESCRIPTION OF THE DR~WINGS
.
Fig. l is a side view of the combustible gases detector of
the present invention.
Fig. 2 is a bottom plan view of the Fiy. l detector.
Fig. 3 is an expanded cut-away side view of the thermocouple
~, 20 junctions of the Fig. l detector.
Fig. 4 is a graphical representation of the output signal
variation of the Fig. l detector in response to CO concentration
in a gas mixture.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, Figs. 1 and 2 disclose a
combustible gases detector assembly 10 mounted to a block 12 by
screw~ 14. The block 12 is designed to allow flue gases which
include air diluted combustible gases to flow through the bloek
12 from an inlet 16 to an outlet 18 so as to come in contact with ;
the detector assembly 10. The block 12 may be a part of a
complete gas sampling analyzing system also including an oxygen
detector as is described in U.S. Patent No. 3,960,500 issued
June 1, 1976 which provides for recirculation of flue gases from
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a duct and back thereto. Further details of such a gas sampling
analyzing system are available in the mentioned patent and the
reader is referred thereto fox any further required clarification.
The detector assembly 10 includes a metal block 20 having
mounting flanges 22 through which the screws 14 sealably mount
the detector assembly lO to the block 12. Thin wall tubing 24 is
pressed onto the block 20 on both ends of the flanges 22 to ex
tend beyond the ends of the metal block 20 and to provide a
protected space 26 at both ends of the metal block 20. Two pairs
of electrical leads 28 and 30 are extended through the block 20 to
b~ electrically isolated therefrom and to have ends extending into
the spaces 26. The electrical leads 28 and 30 are brass with gold
flash; however, nickel has also been found to be a suitable
material.
The heart of the detector assembly lO is a differential
thermocouple assembly 32 connected across the electrical leads 30.
As may be best seen in Fig. 3, the differential thermocouple
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assembly 32, a first thermocouple junction 34 r and a second
thermocouple junction 36 is provided by overlapping the ends of
Alumel* wires 38 and 40 with a common connecting wire 42 of ~
Chromel*. As is known, the thermocouple junctions 34 and 36 will ~`
induce opposing millivoltage signals depending on the temper-
atures of the respective junctions 34 and 36. These millivoltage
signals will be transmitted along the electrical leads 30 by
virtue of the connection of the thermocouple assembly 32 thereto
to a voltmeter 44 connected to the opposite ends of the elect-
rical leads 30. To have the thermocouple assembly 32 act as a
combustible gases detector, the respective thermocouple junctions
34 and 36 must be coated with catalytic material and non-catalytic
material. To accomplish this, as well as allow for ease of
! manufacture, the thermocouple junction 34 is formed by threading
the ends of the Chromel wire 38 and one end of the Alumel wire 42
into a platinum tube 46 and crimping the platinum tube 46 onto
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the ends of the mentioned wires. Platinum is a known catalytic
material and is used in the preferred embodiment, although other
known material such as palladium could also have been used. To
further increase tlle surface area of the catalyst, platinum paste
48 is coated around the platinum tube 46 and the assembly baked
at 1000F to solidify the paste and to form a catalytic thermo-
couple junction~
The thermocouple junction 36 is formed into a non-catalytic
thermocouple junction by threading the ends of the Chromel wire
40 and the common ~lumel wire 42 into a brass ~ube 50 and crimping
t~e tube 50 onto the thermocouple junction 36 to retain it there-
to. To further insure the non-catalytic aspects of the thermo-
couple junction 36, the brass tube 50 is coated with a resistor
cement 52 which is known as Sauereisen* No. 7 resistor cement
commercially available and being composed of aluminum oxide and
silicon oxide among other elements. Coating the brass tube 50
with resistor cement matches the thermal mass of the non-catalytic
junction with that of the catalytic junction to equali~e the
transient thermal response of the two junctions.
The ability of the described catalytic and non-catalytic
~' coated thermocouple junctions to operate as a combustible gases
detector rests on the fact that combustibles such as carbon mon-
oxide and methane will react with oxygen in the presence of a
heated catalyst such as platinum to orm carbon dioxide and
liberate heat to the catalyst according to the following chemical
~ equations;
:! 2C0 ~ 0 Pt 2C0 + heat
, 2 --~ 2
CH4 ~ 202 ~ C02 1 2H20 ~ heat
The catalyst must be heated to a threshold temperature at which
the above reactions take place. For platinum this threshold
temperature is around 400F.
To insure that the gases flowing through the catalytic and
non-catalytic coated thermocouple junctions are above the
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threshold tempe~ature of 400F a spiral heater 54 is wrapped
around the catalytic coa-ted thermocouple junction 34 and the non~
catalytic coated thermocouple junction 36. The heater 54 is
connected across t~e electrical leads 28 with the opposite ends
of the electrical leads 28 bein~ connected to a suitable power
source 56 which provides energy to the heater 54. Since sulfur
dioxide is present along with certain combustible gases in the
combustion of fossil fuels, a problem of poisoning the catalyst
48 may occur. Sulfur dioxide may react with hydrogen to form
sulfuric acid which could condense on the platinum paste and
cause the catalytic action to deteriorate. The applicant has
found that by setting the heater 54 to maintain a temperature of
approximately 800F in the ambient of the catalytic coated thermo-
couple 34, the thermocouple 34 is maintained at a temperature at
which the sensor 10 output is substantially linear. This
temperature is also above the normal dew point of sulfuric acid,
thereby preventing condensation of the acid onto the catalytic
surface and preventing deterioration of the catalyst thereby.
From the foregoing it will be now readily seen that the -~
thermocouple assembly 32 is ideally suited to monitor combustibles
in the flue gases entering the inlet 16 and exiting through the
outlet 18. Depending on how complete the combustion in the
combustion process will be, different amounts of combustible gases
such as carbon monoxide will be produced. The carbon monoxide
will react with the platinum on the catalytic coated thermocouple
junction 34 to produce carbon dioxide and liberate heat to the
catalytic coated thermocouple junction 34. The more carbon mon-
oxide is present in the flue gases, the more carbon dioxide will
be produced and the more heat will be liberated to the catalytic
coated thermocouple junction 34. The non-catalytic coated thermo-
couple junction 36 will not provide any kind of a reaction liber-
ating heat to the junction 36 and as such will only measure the
amhient temperature common to both the non-catalytic coated
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thermocouple junction 36 and the catalytic coated thermocouple
junction 34. Because of the electrical connection of the two
coated thermocouple junctions 34 and 36 and their sharing of a
common ambient temp~rature, the ambient temperature effect will
be cancelled out, leaviny only the temperature rise above ambient
produced at the catalytic coated thermocouple junction 34 as a
result of the chemical reaction occurring at the platinum sur-
face 48. Referring to Fig. 4, it may be seen that varying con-
centrations of carbon monoxide will produce a corresponding
variable millivoltage output at the voltmeter 44 as a result of
the variable heat liberated to the catalytic coated thermocouple
junction 34 dependent on the concentration of carbon monoxide in
~; the flue gases. The millivoltmeter 44 may be thus calibrated
according to the chart disclosed in Fig. 4 to provide a direct
readout of the percent concentration of combustible gases such as
carbon monoxide in the flue gases passing by the detector
assembly 10.
To insure that no other catalytic reaction occurs at various
elements of the detector assembly 10 which would influence the
measure of combustible gases by the detector assembly 10, special
care must be taken in the choice of metals used for forming the
thermocouple junctions or the heater. As a precaution, the
applicant has found it desirable to coat the heater 54 with
~ resistor cement such as Sauereisen. Similarly, there is a
; possibility that the chromel-alùmel thermocouple wires may be
attacked by sulfuric acid fumes. P.s a precaution from such attack
as well as to prevent any possible catalytic reaction on the wires,
such wires could also be coated with Sauereisen resistor cement.
It may also be desirable to use other materials for the thermo-
couple wires such as Platinel I and II, which are a well-known
platinum material thermocouple wire. The use of such Platinel
wires would very definitely require the coating of such wires
with resistor cement such as Sauereisen because of the catalytic
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action that wou~d occur on such wires.
Certain modifications and improvements will occur to those
skilled in the art upon reading this specification. It will be
understood that all such improvem~nts and modifications have
been deleted herein for the sake of conciseness and readability
but are properly within the scope of the following claims.
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