Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGRO~ND OF lllE INV~NTION
Field of Invention
. _ _
The present invention relates to an apparatws Eor measuring an
amount of gas. The present invention also relates to a standardization means
for such a gas measuring device. Typically, these gases are ~he exhaust
gases, emitted through stacks, produced as a result Qf combustion.
Prior Art
Gas measuring apparatus for monitoring the output Qf combustion at
the stack is well known, see for example United States Patent ~4,076,425
Julian Saltz February 28, 1978. Typically, these devices operate in harsh
environments and in locations that are not easily accessible. Some of the
problems, caused thereby, are: lamp aging, d~ift in electronics and dirt
build-up on the window. Thus, to vperate effectively, i.c. main~ain accuracy
and repeatability, these devices must have self-contained standardization
means.
Hereto~ore, one sta dardization means ls described in United States
Patent #3,836,237 Dale W. Egan, Septernber 17, 1974. That reference teaches,
inter alia, the use o air curtains to keep windows clean. However, despite
this practice of air curtains, dirt does build up on the window and must be
accounted for in the standardization process. United States Patents
#3,838,925 William S. Marks, October 1, 1974 and #4,076,425, Julian Saltz
February 28, 1978 teach the use of alternative cptical paths to correct for
lamp aging and drift in electronics. These references, however, do not
teach the correction of other factors, such as dirt on the windows.
SUMMARY OF TH~ INVFNTION
,
A gas measuring apparatus comprises a source capable of emitting
a beam of radiation. The beam is aligned to impinge a detector. A housing
means encloses the beam. The housing means has a plurality of apertures
permitting the gas to enter the housing means, to intercept the beam, and to exit
Erom the housing means. The apparatus further comprises a plurality of means for
closing the apertures and a means for purging said gas from the housing means.
According to the invention there is provided a gas measuring apparatus
with standardization means, comprising: a source, for coupling on one side of a
stack and capable of emitting a beam of radiation; a detector or coupling on the
opposite side o~ the stack from said source, said beam aligned to impinge said
detector; housing means located inside the stack for enclosing said beam, said
housing means having a plurality of apertures located in the stack permitting said
gas to enter said means, to intercept said beam and to exit from said means; a
plurality of means for enclosing said apertures; and means for purging said gas
from said housing means.
Brief Description of the Drawings
Figure 1 is a side view of a gas measuring apparatus of the present
invention.
Figure 2A is a cross-sectional view-of Figure 1 taken along the plane
2-2, showing the apertures of the apparatus of the present invention, open.
Figure 2B is a cross-sectional view of Figure 1 taken along the plane
2-2, showing the apertures of the apparatus of the present invention, closed by a
plurality of doors.
Figure 3 is a cross-sectional view showing the apertures of the appara-
tus of the present invention with another plurality of doors.
Figure 4 is a perspective view-showing the apertures of the apparatus o~
the present invention with yet another plurality of doors.
~igure 5 is a pictoria~ view of the use of the apparatus of the present
invention in a stack to monitor the exhaust gas from the combustion.
Fi~ure 6 is a graph of the absorption spectrum of a typical gas as a
Eunction oE the frequency.
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DE'I'AILF.D DESCRIPTION OF ~IE DRAWINGS
Referring to FIGURE 1 there is shown a gas measuring apparatus 10
of the present invention. The gas measuring apparatus 10 comprises a first
enclosure 12, a second enclosure 14 and a tube 16. The tube 16 is hollow
inside. The first enclosure 12 is to one side of the tube 16 while the
second enclosure 14 is to the o~her side of the tube 16. A source 20 is in
the first enclosure 12. The source 20 is capable of emitting a beam 22 of
radiation ~shown as dash-dot-dash line~. The beam 22 is aligned, to pass
through inside the tube 16, and to impinge a detector 24 in the second enclos-
ure 14. The tube 16 encloses the beam 22. The tube 16 has a plurality of
apertures ~only 26a and 26b are shown). The apertures 26a and 26b are
located on opposite sides of the tube 16; they permit gas ~o enter the tube
16 via one aperture, e.g. 26b, to intercept the beam 22, and to exit via
another aperture, e.g. 26a. A plurality of doors ~only 18a and 18b are shown
in FIGURE 2A) are positioned to close each of the apertures ~26a and 26b,
respectively) o the tube 16. Each of the doors ~e.g. 18a~ is hinged on a
hinge ~e.g. 38a~ near each of the apertures ~e.g. 26a) and is capable of
pivoting about the hinge 38a to close the aperture 26a. A member, ~e.g. 30a)
is pivotally connected, at one end, to each hinge ~e.g. 38a). A pneumatic
actuator ~e.g. 28a~ is pivotally connected to each member ~e.g. 30a) at the
other 0nd of each member ~e.g~ 30a). Each pneumatic actuator 28a and 28b
is mounted on stand 29 connected to tube 16. The actuation by the pneumatic
actuator 28a would close the aperture 26a, as shown in FIGURE 2B. Blowers
32a and 32b are provided to purge the gas from the inside of the ~ube 16.
In general, any housing means can be used to enclose the beam 22.
Any plurality o closing means can 'be used to close the apertures 26a and
26b of the tube 16. For example, in FIGURE 3 there is shown a cross-section-
al view of another plurality of doors 118a, 118b, ll9a, 119b, to close each
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of tho apertures 126a and 12Gb. At cach aperture ~e.g. 126B) there are two
doors ~o.g. 118a and 119a), to close the aperture 126a. Each of the two
doors 118a and 119a .is hinged on a hinge, 138a and 139a respectively, ~nd
is capable of pivoting about the hinge 138a and 139a to close the aperture
126a. A member ~e.g. 130a) is pivotally connected, at one end, to each
hingo te.g. 138a), A pneumatic actuator Ce.g. 128a) is pivotally connected
to each mer~ber (e.g. 130a) at the other end of each member ~e.g. 130a).
Each pneumatic actuator 128a, 128b is connected to a stand 29'. The pneumatic
actuators 129a and 129b are connected to stand 29.
FIGURE 4 shows yet anothcr plurality of doors 218a and 218b to
close each of the apertures 226a and 226b. Each of the doors 218a and 218b
is adjacent to the tube 216. The door 218a is capable of sliding) along a
track 211a (the track 217b for door 218b is not shown), to close the aperture
226a. A pneumatic actuator 219a is connected to the door 218a to close the
aperture 226 by sliding the door 218a along the track 217a.
One use of the appara~us 10 of the present invention is in monitor-
ing the exhaust gas 34 of combustion from a stack 36, shown in FIGURE 5.
Typically, the first enclosure 12 and the second enclosure 14 are on opposite
sides of the stack 36, with the tube 16 passing through the stack 36. In
such application, the apparatus 10 is useful for monitoring the exhaust gas
34 to insure compliance with applicable environmental standards, such as the
EPA. In such application, the apparatus 10 may operate as an opacity sensor,
with the source 20 emitting a beam 22 of visible light.
To operate the apparatus of the present invention, the source 20
ernits a beam 22 of radiation at a fre~uency (shown as ~1 in FIGURF 6) which
is absorbed by the gas 34. The beam 22 passes through the gas 34 in the
tube 16 and is absorbed as it travels to the detector 24. The intensity of
the beam 22, recei~ed by the detector 24, is dependent upon the amount of
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absorption, i.e. tho greater the absorption, the lower the intensity of
the beam 22 received by the detector 24, and vice versa. This is shown as
I3 in FIGURE 6. The apertures 26a c~d 26b of the tube 16 are then closed.
The gas 34 is purged f~om the tube 16 by the blowers 3Za and 32bo lhe gas
34 is replaced by a gas, such as atmospheric gas, which permits substa~tially
all of the beam 22 o~ radiatio~ at a fraquency ~1 ~ pass without any absorpk-
ion. The detector 24 measures the radiation after the beam 22 passes
through the tube 16, unimpeded by the gas 34. This is shown as Il in FIGURE
6. The amo~mt of gas 34 that was in the tube 16 is calculated based upon
Il and I3 in accordance with Beer's law, i.e.
I
I3 ~ Ile ~cL or c = ~1 Ln
where ~ - absorption coefficient tppm cm )
c - concentration of gas tppm)
L - path length in gas ~cm)
Typically, the frequency ul is in the infrared region and the
curve shown ln FIGURE 6 is the absorption band of carbon dioxide. The
advantage of th~ apparatus of the present invention is that the reference~
~ measurement ~i.e. the measurement made without ~absorption by the gas 34) is
performed under substantially the same condition as the measurement with
the gas 34. Except for the removal of the gas 34 the referenGe measurement
uses the same source and electrGnics, follows thc same optical path and is
subject to the same environm=nt~as the measurement made with the gas 34.
This provides for greater accuracy and reliability than has been achieved
hereto~ore.
Heretofore, because it has not been possible to make a measurement
with the gas 34 and a measurement without the gas 34, measurements were made
based upon a beam of radlation at t~o different frequencies - one which is
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absorbed by the gas 34 and another l~hich ls not absorbed. In the method
of the prlor art, the sou~ce 20 emlts a beam 22 of radiation at a first
frequency ~1 which is absorbed by the gas 34 and a secolld frequency ~2 which
is not absorbed by the gas 34. The detector 24 receives the beam 22 after it
passes through the gas 34. The detector 24 measures the amount of first
frequency ul received, i.e. I3~ and measures the amount of sccond frequency
~2 received, l.e. I2. Calculation of the amount of gas 34 in ~he tube 16
is made based upon I2 and I3 in accordance with Beer's law, based upon the
assumption that I2 ls the same as Il. However, lt should be noted from
FIGURE 6, tha~ even though the second frequency v2 is chosen such that it is
not absorbed by the gas 34, the amount of second frequency ~2 received may
not be exactly the same as the amou~t of f-irst frequency vl received but
without the gas 34 in the tube 16, i.e. I2 may be necessarily be exactly Lhe
same as Il. There are many posslble causes for this, including drift in
electronics, since ~2 is a frequency different from ~1~ This is clearly a
source of error.
In another method of using the apparatus of the present inventio~,
this error is eliminated by standardi~ing the value of I2, i.e. determining
the quantitative relationship between I2 and Il. To standardlze the value
of I2, the apertures 26a and 26b of the tube 16 are closed. The gas 34 is
removed from the tube 16. The source 20 emits a beam of radiation at a
first frequency ~1 and a second Erequency ~2 The de~ector 24 measures the
amount of radiat~on received at first frequency ~ ~i.e, Il) and the amount
; of radiation received at second frequency ~2 ~i.e. I2). A standardization
factor based upon Il and I2 is determined, i.e.
K I2
Thereafter, in the measurement of the amount oE gas 34 in the tube 16 using
a first frequency ~1 and a second frequency u2, the calculation of the amount
of ~as 34 that ~as m the tube lG is bas~d upon I3, I2 and K in accordance
with
I3 = K I2e ~cL or c - ~1 Ln KI2
where ~, c a~d L are as p~eviously discusscd. In ~his method, the tube 16
need not be closed upon e~ery measuremffnt. Instead, thc closing of the tube
16 is used to standardize the apparatus 10 and to corrclate I2 to Il.
