Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARAlUS FOR HEAT FLOW MEASUREMENT
TECHNICAI FIELD
The present invention relates to the measurement of
heat flow of fuel gases, and more particularly to a method i
and apparatus whereby the Wobbe Index and/or BTU value of ll
a fuel gas can be determlned. ~¦
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BACKGROUND ART
In the use of fuel gases many combustion problems
result from the di-Eficulty in maintainlng a constant air/gas.
ratio which arises primarily because of variations in the
specific gravlty of the gas~ Typlcally, great emphasl I is
placed on the calorif~c value of gases, but little attention
ls pald to specific gravity, which i8 a governing factor in
the design of jets a`nd injectoxs. In an effort to consider
both the calorific value and the speclfic gravity of a fuel
gas, many manufacturers are adopting the Wobbe Index as a
measure of the true heat flow to a piece of equipment.
The ~obbe Index of Rate of Thermal Delivery, or simply
Wobbe Indes, is defined as the calorific value of a gas
dlvided by the square root of lts speciflc gravity, and the
thermal output of a gas can be expressed by the equation:
Thermal Output = K~ calorific value
~/specific grav~tyJ
where K is a constant depending on the size of the orifice
contrDlllng the discharge of the gas and the differential
pressure across the orifice~ No name has been given to the
units in which the Wobbe Index is expressed, and the nu~erical
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value depends upon the units chosen for the calorific value
and specific gravity.
To maintain accur~te control over the heat input to a
pxocess and thus maintain hlgh e~ficiency, it can be seen
that the Wobbe Index must be controlled. It follows then
that means must be provided to determine the Wobbe Index of
the fuel gas supplied to a facility, such as a power plant or
a processing plant.
A known form of Wobbe Index Recorder vents hot combustion
products through two concentric tubes. The differential
expansion of the two tubes has a direct relationship to the
thermal input of the gas being burned. The disadvantage of
such a system is heat exchange with its surroundings, particu-
larly where the instrument might be heated unequally.
Another known system measures the air flow necessary to
maintain the temperature of the products of combustion at a
constant value. The air is used as a heat transfer medium
controlling a thermal expansion element to vary the air flow.
~lere again, the dlsadvantage of this system is heat exchange
with its surro~mdings.
Because of the foregoing, it has hecome desirable to
develop a method and apparatus to accurately and easily
determine the Wobbe Index of a fuel gas.
SUMMARY OF T~E INVENTION
The present invention solves the aforementioned problems
associated with the prior art as well as other proflems prefer-
ably by providing a method and apparatus for determining the
Wobbe Index of a gas by using a zirconium oxide oxygen analyzer
to determine the amount of oxygen required for combustion. It
is known that the oxygan required for combustion is propor-
tional to the calorific value of 2 gas. ~ccordingly, by using
an orifice at the gas inlet to maintain a constant gas flow,
a dependence on the square root of the specific gravity of
the gas can be obtalned, making it possible to use the measure-
ment o~ the oxygen required for combustion as a determinant of
the Wobbe Index.
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A constant flow of air greater than that required
for complete combustlon i8 mixed with a gas maintained at
a constant pressure. The mixture then flows through the
zirconium oxide oxygen analyzer where complete eombustion
takes place and the remaining oxygen in the flow ls measured.
Some gas mlxtures may require passing through a heated
catalytic chamber to assure complete combustion, The known
total oxygen supplied minus the remainlng oxygen equal~ the
oxygen required for complete combùstion of the gas belng
tested, ~hleh, as previously dlscussed, is proport$onal to
the calorlfie value of the gas times the volumetrie ~low.
Thus, using the known gas flow equationss
V ~ 16.05 ~v ~ ~ and
2 ~ (C X V)
where: ~ = Proportionality constant
V = Volumetrie flow
v ~ Caloriflc value
Kv ~ Valve flow characteristie derived from flow
characteristics of the valve orifiee
A P ~ Pressure drop
SG e Specifie gravity of the gas eompared to air
T ~ Temperature
If KV,~P and T are held eonstant, then
V ~ Constant, and
~ ;
2 = Kl X Constant X Cv, or
. ~
2 = K2 X X Wobbe Index, and
Wobbe Index ~ K2 X 2
Accordingly, since the total 2 is known, and inasmuch
as the above constants can be calculated from known or eon-
~ trolled systems parameters, the remaining oxygen, as determined
;~ 35 by the ~irco~um oxlde oxygen analyze~ can be converted direct-
: ly into a Wobbe Index numberO
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In another embodiment of the.present invention, the output
of the zirconium ox~de ana~yzer.is used as the input to a
controller which oper~tes a valve positioner having known
positions versus air flow characteristics so that the
analyzer output is held constant at some predetermined oxygen
level. The position of the valve positioner is an indication
of the Wobbe Index and can be calibrated for direct cor-
relation. With respect to this embodiment of the present
invention, ii~ air is added to the fuel to regulate its
heating value, an accurate Wobbe Index determination can be
made.
Furthermore~ with respect to this latter embodiment of
the present invention, since the analyzer output is held
constant, the position of the gas valve can give an indica-
tion of the BTU value of the gas by calibrating the devicein units of Bq~/cu. ft. As in the latter embodiment, this
BTV value can be determined even if air has been added to
the fuel.
Thus, in accordance with one aspect of the present
invention there is provided a method for determining the
oxygen required for complete combustion of a gas under
controlled flow and pressure conditions to enable a heat
flow characteristic of the gas to be determined, comprising
the steps of providing a constant flow of the gas at a constant
pressure, providing a constant flow of pressurized air at a
flow rate greater than that required for complete combustion
of the gas, combining said gas flow and said air flow to define
a gas/air mixture, subjecting said gas/air mixture to complete
combustion, and determining the amount of oxygen remaining in
said gas/air mixture after complete combustion as a determin-
ant of the oxygen required for complete combustion thereof.
In accordance wlth a further aspect of the present
invention there is provided apparatus ~or determining the
oxygen required for complete combustion of a gas under
controlled flow and pressure conditions to enable a heat
flow characteristic of the gas to be determined, comprising
means supplying a flow of said gas under pressure, means f~r
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maintaining said gas at a constant flow rate and constant
pressure, means supplyin~ a flow o~ air under pressure, means
~or maintaining said air at a constant flow rate greater than
the flow rate required for the cGmplete combustion of said
gas, means combining said gas and air flows to define a gas/
air mixture, means subjecting said gas/air mixture to complete
combustion, and analyzer means receiving said gas/air mixture
after complete combustion thereof and operable to determine
the amount of oxygen remaining in said mixture as a deter-
minant of the oxy~en required for the complete combustionthereof.
BRI~F~DESCRIPTION OF IHE DRAWINGS
. _ _ . . _ . . .
Figure 1 is a schematic diagram of the apparatus of the
present invention.
Figure 2 is a schematic diagram of an alternative
embodiment of the apparatus oE the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
.... _
Referring now to the drawings where the illustrations
are for the purpose of describing the preferred embodiment of
the invention and are not intended to limit the invention
hereto, Figure 1 is a schematic diagram of an apparatus 10
for measuring the oxygen required for combustion of a sample
of fuel gas, under controlled flow and pressure conditions,
thus enabling the Wobbe Index of the gas to be determined.
The apparatus 10 includes a catalytic combustion device 12
and a zirconium oxide oxygen analyzer 14. A gas sample from
a source 16 flows through a first conduit 18 and mixes with
air flowing through a second conduit 20. The air-fuel mixture
then flows through a conduit 22 to the catalytic combustion
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device 12 and thcn to the oxygen analyzer 1l~. The oxygen
analyzer 14 can be of the type shown in U. S. Patent
No, 3,960,500, and the catalytic combustion device 12 can
be a chamber filled with platinum, palladium, or slm$1ar
catalysts, usually deposited on an inert ceramic support
material, such as alumina, The catalykic chamber must be
heated to an activatlon threshold temperature of approximately
400 F
The gas sample is maintained at a constant flow by
means ~ an orlfice 24 and a pressure regulator 28 in conduit
18. Alr flow through conduit 20 is regulated by means of a
regulator 26 which maintains a constant flow of air greater
than xequired for complete combustion. When the air-fuel ~,
mixture flows through the catalytic combustion device 12~
complete combustion occurs so that the oxygen in the exhaust
gas flowlng through the oxygen analyzer 14 is excess oxygen
only. Since both the air flow and thus the oxygen flow into
the analyzer are known, the oxygen reading obtained by the
oxygen analyzer can be subtracted from the known oxygen
! quantity to determine the oxygen required for complete com-
bustion of the same gas. This value can then be used to
calculate the Wobbe Index using the equation: ~obbe Index
K2 X 2' as described above.
Referring now to Figure 2, there is illustrated an
alternative embodiment of the present invention which not only
can be used to directly determine the Wobbe Index of an un-
diluted fuel gas supply, but which can also be used to determine
the Wobbe Index of a gas which is mixed with air. The alterna-
3o tive system 100 includes a catalytic combustion device 112, a
;~ zirconium oxide oxygen analyzer 114, a gas source 116, an
orifice 124, and a pressure regulator 128, the latter t~o
elements being located in a conduit 118 from the gas source
116. Air flow to the catalytic combustion device 112 is
controlled by a control valve 126 which is operated by a set
point controller 130 which is connected to the output of
the oxygen analyzer 114. The controller is operable to
maintain the output of the unburned oxygen irom the anaiyzer
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constant by regulating the volume of a~r mixed with th~
gas mixture from the source 116 and entering the catalytic
combustlon device 112,
A valve position indicator 132 indicates the position
of the valve 1~6. Since the volume of air entering through
the condult 120 ls proportional to the oxygen required for
combustion, as previously discussed, the valve position
lndicator 132 can be calibrated to provide a direct reading
of the Wobbe Index of the gas being tested. Furthermorej
inasmuch as the oxygen required for complete combustion is
proportional to the calorific value of the gas, the valve
position indicator 132 can also be calib~ated to give
readings in BTU/cu. ft. or other heat flow un~ts. --
CeI~ain modifications and improvements will occur to
those skilled in the art upon reading the foregoing des-
cription. It will be understood that all such improvements
and modifications have been deleted herein for the sake of i
conciseness and readability but are properly within the sco2e
of the following clalms.
