U.S. patent application number 10/590349 was filed with the patent office on 2008-02-14 for method for measuring mass flow of a multi-component gas.
Invention is credited to Christian Moller.
Application Number | 20080034889 10/590349 |
Document ID | / |
Family ID | 34878009 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080034889 |
Kind Code |
A1 |
Moller; Christian |
February 14, 2008 |
Method For Measuring Mass Flow Of A Multi-Component Gas
Abstract
This invention relates to a method of measuring mass flow of a
first gas component in a gas consisting of one or more known gas
components. Typically such methods assume that certain parameters
were constant, such as the gas composition, pressure and/or
temperature, and likewise the heat capacity, density, etc., of the
gas were presumed to be such that they could be determined to have
a constant value. However, it has been found that the determination
of the mass flow is associated with a comparatively high degree of
measurement uncertainty, when it is assumed that the parameters are
constant. The core of the invention relies on this discovery and on
a method wherein all of the gas parameters that are used in the
determination of the mass flow of the first gas component and that
may vary considerably as a function of the gas composition,
pressure and/or temperature are determined continually.
Inventors: |
Moller; Christian;
(Horsholm, DK) |
Correspondence
Address: |
HUNTON & WILLIAMS LLP;INTELLECTUAL PROPERTY DEPARTMENT
1900 K STREET, N.W.
SUITE 1200
WASHINGTON
DC
20006-1109
US
|
Family ID: |
34878009 |
Appl. No.: |
10/590349 |
Filed: |
February 22, 2005 |
PCT Filed: |
February 22, 2005 |
PCT NO: |
PCT/DK05/00118 |
371 Date: |
February 12, 2007 |
Current U.S.
Class: |
73/861.351 |
Current CPC
Class: |
G01F 1/86 20130101; G01F
1/88 20130101; G01F 15/04 20130101 |
Class at
Publication: |
073/861.351 |
International
Class: |
G01F 1/86 20060101
G01F001/86 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2004 |
DK |
PA 2004 00275 |
Claims
1. A method of measuring mass flow of a first gas component in a
gas consisting of one or more known gas components, which gas flows
in a pipe in which one or more measurement devices (440, 450, 460,
461, 462, 465; 540, 550, 560, 561, 562, 565; 635, 640, 660, 661,
662, 665) is/are arranged in connection with the pipe, said method
comprising the following steps: determination of one or more gas
parameters of the gas by means of the measurement device(s) (440,
450, 460, 461, 462, 465; 540, 550, 560, 561, 562, 565; 635, 640,
660, 661, 662, 665), determination of the mass flow of the one gas
component by means of the determination of the one or more gas
parameters, characterised in that the determination of the one or
more gas parameters comprises a continuous determination of all of
those of the gas parameters that are used in the determination of
the mass flow of the first gas component and which may vary
considerable as a function of the gas composition, pressure and/or
temperature.
2. A method according to claim 1, characterised in that in
connection with the pipe a tubular body (410; 510; 610) is
incorporated, which is surrounded by an insulating material (430;
530; 630), and wherein the method further comprises the following
steps: supply of a given amount of energy E to the gas in the
tubular body (410; 510; 610).
3. A method according to claim 2, characterised in that the
measurement device(s) (440, 450, 460, 461, 462, 465; 540, 550, 560,
561, 562, 565; 635, 640, 660, 661, 662, 665), that are used for
determining the one or more gas parameters comprise a volume
percentage measurement instrument (440; 540; 640) and two
temperature measurement instruments (460, 465 ; 560, 565; 660,
665), wherein the volume percentage measurement instrument (440;
540; 640) is arranged in or in immediate vicinity of the tubular
body (410; 510; 610) and wherein the one temperature measurement
instrument (460; 560; 660) is arranged at the inlet of the tubular
body (410; 510; 610) and the second temperature measurement
instrument (465; 565; 665) is arranged at the outlet of the tubular
body (410; 510; 610).
4. A method according to claim ,2 characterised in that wherein the
step of determination of one or more gas parameters by means of
measurement devices comprise: determination of the gas temperature
T.sub.1 at the inlet of the tubular body; and determination of the
gas temperature T.sub.2 at the outlet of the tubular body.
5. A method according to claim 2, characterised in that those of
the gas parameters that are determined continuously and that
partake in the determination of the mass flow consist of the gas
composition and the gas temperature T.sub.1. at the inlet of the
tubular body and the gas temperature T.sub.2 at the outlet of the
tubular body.
6. A method according to claim 1, characterised in that the
measurement devices comprise a pressure differential measuring
instrument (450; 550; 640) and a volume percentage measurement
instrument (440; 540; 640); and in that those of the gas parameters
that are determined continuously and partake in the determination
of the mass flow of the first gas component comprise pressure
differential across a restriction and the volume percentage of the
first gas component.
7. A method according to claim 6, characterised in that the
measurement devices moreover comprise a temperature measuring
instrument (460, 461, 462, 465; 560, 561, 562, 565; 660, 661, 662,
665) and that those of the gas parameters that are determined
continually and partake in the determination of the mass flow of
the first gas component moreover comprise the gas temperature.
8. A method according to claim 6, characterised in that those of
the gas parameters that are determined continually and partake in
the determination of the mass flow comprise the gas density.
9. A method according to claim 1, characterised in that the
measurement devices comprise a hotwire and a volume percentage
measurement instrument.
10. A method according to claim 9, characterised in that the
measurement devices also comprise a temperature measurement
instrument.
11. A method according to claim 9, characterised in that those of
the gas parameters that are determined continuously and partake in
the determination of the mass flow comprise one or more of the
following: the viscosity of the gas, the heat capacity of the gas,
the heat conductivity of the gas, the density of the gas, and the
temperature of the gas, the volume percentage of the first gas
component.
12. Use of the method according to any one of claims 1 to 11 for
the determination of the mass flow of a first gas component being
in saturation state.
13. Use of the method according to claim 12, wherein the first gas
component being in saturation state is water vapour.
14. Use of the method according to any one of claims 1 to 11 for
the determination of the mass flow of a first gas component in a
biogas.
15. A mass flow measurement device for measuring a first gas
component in a gas consisting of one or more known gas components,
wherein the mass flow measurement device performs the method
according to any one of claims 1 through 11.
16. A mass flow measurement device according to claim 15, wherein
the mass flow measurement device comprises a tubular body (410;
510; 610) surrounded by an insulating material (430; 530; 630),
which tubular body (410; 510; 610) is configured for being
connected to a pipe in which a gas flows, which tubular body (410;
510; 610) has an inlet (411; 511; 611) and an outlet (412; 512;
612) for the flowing gas; means (420; 520; 620) for supplying
energy to gas in the tubular body (410; 510; 610), a temperature
measurement instrument (460; 560; 660) at the inlet of the tubular
body (411; 511; 611), a temperature measurement instrument (465;
565; 665) at the outlet of the tubular body (412; 512; 612) and a
volume percentage measurement instrument (440; 540; 640).
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method of measuring mass flow of
a first gas component of a gas consisting of one or more known gas
components, which gas flows in a pipe, wherein one or more
measuring devices are mounted in connection with the pipe, said
method comprising the following steps: determination of one or more
gas parameters by means of the measuring device(s); and
determination of the mass flow of the one gas component by means of
the determination of the one or more gas parameters. The invention
moreover relates to use of the method and a mass flow measuring
device.
BACKGROUND OF THE INVENTION
[0002] When determining the mass flow of one or more gas components
in a gas composed of one or more gas components it is common to
assume that some of the gas parameters, such as temperature, gas
composition, density, etc, are unchanged in order to enable
determination of the mass flow of individual gas components in the
composite gas.
[0003] However, this may give rise to high measurement
uncertainties, since typically such gas parameters may vary over
time and hence influence the determination of the mass flow of one
or more of the individual gas components. Of course, it is
inconvenient since, eg in fermentation processes in breweries,
putrefaction processes in putrefaction tanks, gas outlets from
biogas plants, etc, it is expedient to be able to determine the
individual gas mass flows accurately--either to enable monitoring
of the process or to be able to impose users, if any, a tax in
response to the mass flow of the one or more of the gas
components.
[0004] Therefore there is a need for a method of measuring the mass
flow of a first gas component in a composite gas, wherein the
method is associated with considerably reduced measurement
uncertainties compared to the known methods.
BRIEF DESCRIPTION OF THE INVENTION
[0005] It has been found that the above object is accomplished by a
method of the type mentioned above and which is characterised in
that the determination of the one or more gas parameters comprises
a continuous determination of all those of the gas parameters that
are used in the determination of the mass flow of the one gas
component and which may vary considerably as a function of the
composition and/or temperature of the gas. Hereby a determination
is accomplished of the mass flow of a first gas component which is
associated with relatively low measurement uncertainty, since those
of the gas parameters that are used in the determination of the
mass flow and that may vary considerably as a function of the gas
composition are now determined rather than being set to a
predetermined value.
[0006] In case of known methods of measuring the mass flow of a
first gas component in a composite gas it was assumed that the gas
composition was constant over time. The term "gas composition"
covers the make-up of the various known gas components in the gas.
Such make-up may be given in volume percentages or weight
percentages. However, it is not necessarily the case that the gas
composition is constant over time, and the method as described
above thus takes it into account that the gas composition may vary
over time in the determination of the mass flow, which may hence be
performed with a much higher degree of accuracy than was provided
by the known methods. The method according to the invention also
continuously takes into account changes in the gas viscosity,
temperature and other of the gas parameters, whereby the
measurement of the mass flow takes place with a degree of accuracy
that cannot be obtained by the prior art.
[0007] Preferably, in connection with the pipe, a tubular body is
incorporated which is surrounded by an insulating material, and
preferably the method comprises a further step of supplying a given
amount of energy to the gas in the tubular body. Hereby
determination of changes in the gas parameters as a function of the
supplied energy may contribute to determining the mass flow of the
first gas component. The energy E can be supplied as energy
supplied to the tubular body, eg by feeding, or by energy supplied
to the gas itself, eg by a heater element or a heater rod in the
tubular body and in direct contact with the gas. The term "a
tubular body incorporated in connection with the pipe" is intended
to cover both that the tubular body may constitute a part of the
pipe in which the gas flows and that the tubular body may
constitute a branching of the pipe, whereby the gas is conveyed
from the pipe, through the tubular body and back into the pipe.
Finally the term may also cover a specific part of the tube, in
connection with which part measurement devices are associated for
measuring the mass flow of a gas component in the gas.
[0008] According to a preferred embodiment of the method the
measurement device(s) that are used in the determination of one or
more of the gas parameters include a volume-percentage measuring
instrument and two temperature measuring instruments, wherein the
volume-percentage measuring instrument is arranged in or in
immediate proximity of the tubular body, and wherein the one
temperature measuring instrument is arranged at the inlet of the
tubular body and the other temperature measuring instrument is
arranged at the outlet of the tubular body.
[0009] According to yet a preferred embodiment of the method the
step of determination of one or more gas parameters by means of
measuring devices comprise determination of the gas temperature at
the inlet of the tubular body and determination of the gas
temperature at the outlet of the tubular body. Moreover the volume
percentage of the first gas component of the total gas can be
measured by means of the volume-percentage measuring instrument,
and hereby the values of the volume percentage of the first gas
component and the gas temperature at the inlet and the outlet of
the tubular body can be determined continuously and their current
values can therefore be used in the determination of the mass flow
of the first gas component.
[0010] It is preferred that those of the gas parameters that are
determined continuously and that partake in the determination of
the mass flow consist of the gas composition and the gas
temperature T.sub.1 at the inlet of the tubular body and the gas
temperature T.sub.2 at the outlet of the tubular body. The gas
composition is determined by means of the volume-percentage
measuring instrument that determines the volume percentage of the
first gas component; based on that determination the gas
composition can be determined. Based on the composition of the gas
and its current temperature the heat capacity of the gas can be
determined continuously and used for accurately determining the
mass flow of the first gas component.
[0011] According to an alternative, preferred embodiment of the
method the measuring devices comprise a pressure differential meter
and a volume-percentage measurement instrument, and those of the
gas parameters that are determined continuously and that partake in
the determination of the mass flow of the first gas component
comprise differential pressure across a restriction and the volume
percentage of the first gas component. Thus the method according to
the invention can be used in connection with a pressure
differential meter and a volume-percentage measuring instrument,
which are common measuring devices for measuring the mass flow.
[0012] It is preferred that the measurement devices used in the
method according to the invention also include a temperature
measuring instrument and that those of the gas parameters that are
determined continuously and partake in the determination of the
mass flow of the first gas component also comprise the gas
temperature. When the pressure, temperature and composition of the
gas are known, its density can be calculated. The gas density
partakes in the calculation of the mass flow of a gas component by
use of a pressure differential measuring instrument, but
conventionally it was assumed that the density of the gas remained
constant. However, typically the density of the gas varies
considerably as a function of the gas temperature and the gas
composition, and therefore a continuously determined density yields
a far more accurate determination of the mass flow of the gas
component compared to conventional methods. As described above, it
is thus preferred that those of the gas parameters that are
determined continuously and that partake in the determination of
the mass flow comprise the density of the gas. However, it is to be
noted that typically the density is determined on the basis of
knowledge of the temperature, pressure and composition of the gas.
Moreover the Reynolds number for the restriction could also be
measured, since it may also vary and since the Reynolds number
influences pressure-loss coefficient across the restriction and
hence the determination of the mass flow of the first gas
component. However, the variation of the pressure-loss coefficient
is comparatively limited.
[0013] According to yet an alternative method the measuring devices
comprise a hotwire and a volume-percentage measuring device. The
hotwire is a simple flow measuring device comprising an electric
conductor with temperature-dependent electric resistance arranged
in the flow of gas being measured. A voltage is applied to the
hotwire and the amperage through it is measured and can be related
to the temperature and hence to the energy emitted by the hotwire
to the flow of gas. When the hotwire is combined with a
volume-percentage measuring instrument determining eg the volume
percentage of the first gas component, current values of the gas
viscosity, the heat capacity of the gas, the heat conductivity of
the gas, the density of the gas, and the temperature of the gas can
be determined currently. Thus considerably more accurate
measurements of the mass flow of a gas component can be obtained
compared to a scenario in which only a hotwire was used.
[0014] In an alternative embodiment the measurement devices
moreover comprise a temperature measuring instrument, whereby an
accurate value of the gas temperature can be accomplished.
[0015] According to a preferred embodiment of the method those of
the gas parameters that are determined continuously and that
partake in the determination of the mass flow comprise one or more
of the following: the viscosity of the gas, the heat capacity of
the gas, the heat conductivity of the gas, the density of the gas,
and the temperature of the gas, the volume percentage of the first
gas component. Hereby an accurate determination of the mass flow of
the first gas component is accomplished, as outlined above.
[0016] According to a preferred embodiment the method lends itself
for use in the determination of the mass flow of a first gas
component which is saturation state. When it is known that a first
gas component is in saturation state and when pressure and
temperature of the gas in which the first gas component partakes
are known, the volume percentage of that first gas component can be
determined by calculation. In that case it is not necessary to
measure the volume percentage of that gas component. If it is known
for a two-component gas that a first gas component is in saturation
state, the volume percentage of the second component can also be
calculated. If, for a gas with three components, it is known that
the first gas component is in saturation state, the volume
percentage of one of the remaining gas components can be measured
and the last volume percentage can be calculated. Thus, when the
method is used for a gas about which it is known that one of its
components is in saturation state, it is not necessary to measure
the volume percentage of the saturated gas component. A
particularly preferred use of the method is for gases, in which the
first gas component--being in saturation state--is water vapour.
Such gases may be eg biogases. As described above, it is
particularly advantageous to be able to measure accurately on
biogases since taxes may then be calculated correctly with a high
degree of accuracy.
[0017] Moreover the invention relates to a mass flow measuring
device for measuring mass flow of a first gas component of a gas
consisting of one or more known gas components wherein the mass
flow measuring device performs the method as described above.
[0018] According to a particularly advantageous embodiment of the
mass flow measuring device, it comprises a tubular body surrounded
by an insulating material, which tubular body is configured for
being connected to a pipe in which a gas flows, said tubular body
having an inlet and an outlet for the flowing gas, means for
supplying energy to gas in the tubular body, a temperature
measuring instrument at the inlet of the tubular body, a
temperature measuring instrument at the outlet of the tubular body,
and a volume-percentage measuring device. The energy supplied to
the gas is preferably a given amount of energy E, whereby the mass
flow of the various gas components can be calculated
accurately.
[0019] In the above it was assumed that the composite gas is a gas
composed of known gas components, ie gas components with known
parameters. In the present description such parameters are also
designated "the parameters of the gas" or "gas parameters", and
these terms cover parameters of the gas such as density, heat
capacity, viscosity, heat conductivity. Examples of such known gas
components may be H.sub.2O, CO.sub.2, CH.sub.4, etc, the parameters
of which are disclosed in reference books.
[0020] It is to be noted that the term "vary considerably" is to be
understood as "vary by more than 5%", preferably "vary by more than
10%".
[0021] In the above the use of the method according to the
invention was described for determination of the mass flow of a
first gas component of a gas. It is to be understood that, of
course, the method can also be used for determining the mass flow
of other gas components of the gas, be it simultaneously or
concurrently.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The invention will now be disclosed in further detail with
reference to a drawing, whose figures show:
[0023] FIG. 1 is a general flow chart of the method according to
the invention;
[0024] FIG. 2 is a flow chart of an embodiment of the method
according to the invention, wherein the method is used with a mass
flow measuring device in accordance with the principle of heat
capacity;
[0025] FIG. 3 is a flow chart of an embodiment of the method
according to the invention, wherein the method is used with a mass
flow measuring device having a pressure differential measuring
instrument;
[0026] FIG. 4 is a flow chart of an embodiment of the method
according to the invention, wherein the method is used with a mass
flow device having a hotwire;
[0027] FIGS. 5 through 7 show three different embodiments of a mass
flow meter according to the invention and in accordance with the
principle of heat capacity.
[0028] FIG. 1 shows a general flow chart of the method according to
the invention. The method is used for measuring the mass flow of a
first gas component of a gas consisting of one or more gas
components; however, it can be used for simultaneously determining
the mass flow of other gas components in the gas. The flow chart
starts in step 10 and in step 20 the gas is conveyed into a pipe,
where--in connection with the pipe--one or more measuring devices
is/are arranged. These measuring devices are used in step 30 for
continuously determining one or more gas parameters, eg volume
percentage, temperature, pressure. It is to be noted that those of
the gas parameters that partake in the determination of the mass
flow of the first gas component are determined continuously, and
hence current values will also be used for those parameters that
may vary considerably as a function of the gas composition,
pressure and/or temperature. In step 40 the mass flow is determined
on the basis of common calculation methods and by means of the
current values of those of the gas parameters that are used in the
determination of the mass flow. The flow chart ends in step 50.
Typically the measuring devices will be connected to a storage
unit, either by wires or wirelessly, whereby the measurements
performed by them are stored automatically in the storage unit and
typically a calculator unit will be connected to the storage unit
and will perform the calculations in accordance with the
method.
[0029] FIG. 2 is a flow chart of an embodiment of the method
according to the invention, wherein the method is used with a mass
flow measuring device in accordance with the principle of heat
capacity. The flow chart of FIG. 2 thus shows an embodiment of the
invention shown in FIG. 1. The flow chart starts in FIG. 110 and in
step 120 the gas is conveyed into a tubular body which is
incorporated in connection with the pipe and is surrounded by an
insulating material. In that embodiment a first temperature
measuring instrument is arranged at the inlet of the tubular body
and a second temperature measuring instrument at the outlet of the
tubular body. Alternatively the first temperature measuring
instrument could be arranged before the inlet of the tubular body,
for instance the inlet of the tubular body could be connected to a
boiler in a brewery and the first temperature measuring instrument
could be arranged within the boiler rather than within the tubular
body, if a temperature drop occurring between the boiler and the
inlet of the tubular body can be disregarded.
[0030] In step 130 the temperature T.sub.1 of the gas is measured
by means of the first temperature measuring instrument, ie at the
inlet of the tubular body, and in step 140 a known amount of energy
E is supplied to the gas. That energy supply can occur eg by a
heating rod being inserted into the tubular body which is to be in
direct contact with the gas that flows within the tubular body and
emit heat to the gas. Alternatively the heat supply to the gas may
take place by electric supply to the tubular body. In step 150 the
temperature T.sub.2 of the gas is measured by means of the second
temperature measuring instrument, ie at the outlet of the tubular
body. The first and the second temperature measuring instrument may
advantageously be thermometers.
[0031] In step 160 the volume percentages of the various gas
components of the gas (ie the gas composition) are determined. If
the gas is a one-component gas, the volume of that gas is, of
course, 100. If the gas is a two-component gas, the gas composition
can be determined by measuring the volume percentage of the one
gas. If it is known that the one gas is in saturation state, the
volume percentage of the gas component can alternatively be
determined by calculation (if the pressure and temperature of the
gas are known); hereby the need is eliminated for measuring the
volume percentage of any of the gas components. If the gas consists
of three components of which the one is in saturation state, the
gas composition can be determined by measurement of the volume
percentage of one of the gas components and determination of the
volume percentages of the remaining gas components by calculation.
If neither one of the gas components of the three-component gas is
in saturation state, the volume percentages of two of the gas
components are to be measured in order to enable determination of
the gas composition. This can be generalized such that the gas
composition of a gas having X components can be determined by
measurement of the volume percentages of X-1 gas components, if
neither of the gas components are in saturation state, or the
volume percentages of X-2 gas components if it is known that one
gas component is in saturation state.
[0032] Based on the measurements of T.sub.1, T.sub.2 and E and the
measurement and/or the determination of the volume percentages of
the gas components in the gas, it is possible to continuously
determine (step 170) a current value of the weight percentage of
the first gas divided by the specific heat capacity of the total
gas (weight %/C.sub.p) as a function of the current temperatures
and the volume percentage of the first gas component. Based on
that, the mass flow of the first gas component can be determined
(step 180). The flow chart ends in step 50.
[0033] An example of a gas with several gas components of which it
is desired to determine the mass flow of the one gas component may
include a biogas, ie a gas consisting primarily of CO.sub.2,
H.sub.2O and CH.sub.4 and optionally some N.sub.2 (all in gas
form). Biogas is used for heat or heat and energy production and
may be obtained from biogas plants. It is desired to be able to
accurately measure the amount of methane in the biogas, on the one
hand to be able to monitor the putrefaction process in the biogas
plant and, on the other, to be able to provide a precise value for
the energy/effect yield represented by the methane.
[0034] Typically it is desired to determine the mass flow of
CH.sub.4 and if the biogas is water-vapour saturated the volume
percentages of the individual gas components can be determined
(step 160) by measurement of the volume percentage of CH.sub.4 and
calculation of the volume percentages, since the volume percentage
of the water can be determined directly on the basis of the
knowledge that the gas is water-vapour saturated, and the volume
percentage of CO.sub.2 can be calculated since CO.sub.2 thus
constitutes the remainder of the gas. When current values of the
volume percentages of the gas components are known, the gas
parameters, here in the form of its specific heat capacity and the
weight percentages of the gas components, can be determined on the
basis of knowledge of the specific heat capacity and gas constants
(R.sub.CH4,R.sub.CO2 and R.sub.H2O) of the individual gas
components. Thus the composition of the gas is known continuously
and therefore the mass flow of one single or several gas components
can be determined considerably more accurately than by conventional
methods in which it is assumed that the composition of the gas and
hence specific heat capacity are constant.
[0035] As described above, the measurement devices used for
performing the method shown in FIG. 2 will typically appear in
connection with a storage unit--either via wires or
wirelessly--whereby their measurements are stored automatically in
the storage unit and a calculation unit will typically be connected
to the storage unit and will perform the calculations according to
the method.
[0036] FIG. 3 is a flow chart showing an embodiment of the method
according to the invention, wherein the method is used with a mass
flow measuring device with a pressure differential measuring
instrument. The flow chart of FIG. 3 thus shows an embodiment of
the method shown in FIG. 1. As described in the context of FIGS. 1
and 2, the measurement devices will typically be in connection with
a storage unit--either via wires or wirelessly--whereby their
measurements are stored automatically in the storage unit, and a
calculator unit will typically be in connection with the storage
unit--either via wires or wirelessly--and will perform the
calculations in accordance with the method.
[0037] The flow chart starts in step 210 and in step 220 the gas is
conveyed into a tubular body which is incorporated in connection
with the pipe. That tubular body may merely be that part of the
pipe where the measurement devices are arranged. In the tubular
body a pressure differential measuring instrument may be inserted
in direct contact with the gas; said pressure differential
measuring instrument measuring the pressure drop .DELTA.P across a
restriction (step 230). In step 240 the volume percentages of the
various gas components in the gas are determined. Step 240 is
performed as described above in the context of step 160 in FIG. 1.
In step 250 the current weight percentage is continuously
calculated for the gas component(s) for which the it is desired to
determine the mass flow.
[0038] The density of the composite gas varying considerably as a
function of the gas temperature and composition, it is of
considerable importance to the accuracy of the mass flow
determination that the current gas composition is determined. For
gases with several gas components there is no linear correlation
between the volume percentage and the weight percentage of one gas
component in a composite gas, and thus the weight percentage of one
gas component may vary, albeit the volume percentage of the gas
component concerned is constant, since the volume percentages of
the remaining gas components may vary. If the gas composition (ie
the volume percentages of the gas components) is known, however,
and the density of the individual gas components are known, the
current weight percentage of the individual gas components can be
determined accurately. When the current weight percentage(s) for
the gas component(s) for which it is desired to determine the mass
flow is/are calculated, the mass flow for the gas components
concerned can be determined in accordance with current calculation
methods.
[0039] It should be noted that the density of the gas components
may also vary considerably with variations in temperature.
Therefore the method may be extended (not shown) to also comprise a
measurement of temperature whereby the current values for the
density of the gas components are used in the determination of the
mass flow of the various gas component(s). Alternatively the
temperature of the gas can be entered into the calculator unit if
it is known that it does not vary considerably. The flow chart ends
in step 270.
[0040] As described above the measurement device will typically be
in connection with a storage unit--either via wires or
wirelessly--whereby their measurements are stored automatically in
the storage unit, and a calculator unit will typically be in
connection with the storage unit--either via wires or
wirelessly--and will perform the calculations in accordance with
the method.
[0041] FIG. 4 is a flowchart of an embodiment of the method
according to invention, where the method employs a mass flow
measuring device with a hotwire. Thus, the flowchart in FIG. 4
shows an embodiment of the method shown in FIG. 1. A hotwire is a
flow measuring instrument with an electric conductor, whose
electric resistance depends on temperature. The electric conductor
is arranged in the pipe, perpendicular to the flow direction of the
gas flow. A voltage is applied to the electric conductor
simultaneously with the amperage being measured. The measured
amperage is related to the velocity of the gas flow; reference is
made to tables and databases.
[0042] The flowchart starts in step 310 and in step 320 the gas is
conveyed into a tubular body which is incorporated in connection
with the pipe. The tubular body may merely be that part of the pipe
where the measurement devices are arranged. In the tubular body
there is, as described above, arranged a hotwire in direct contact
with the gas. As described in the context of the preceding figures,
the various measurement devices will typically be in connection
with a storage unit--either via wires or wirelessly--whereby their
measurements are stored automatically in the storage unit, and a
calculator unit will typically be in connection with the storage
unit--either via wires or wirelessly--and will perform the
calculations in accordance with the method. Step 330 consists of
measuring and storing the amperage from the hotwire and comparing
it to a calibration table, whereby the amperage is related to a
given flow velocity of the gas. In step 340 the volume
percentage(s) of the gas component(s) for which it is desired to
determine the mass flow is/are determined. Typically the volume
percentage(s) of one or more of the gas component is/are measured
by means of a volume percentage measurement instrument as described
above, whereby the composition of the gas in volume percentages is
known. When the components of the gas and its composition are
known, the current values for those parameters of the gas that are
to be used for determining the mass flow of one or more gas
components are determined on the basis of knowledge (eg via
reference to tables) of the corresponding parameters for the gas
components. Such parameters that are used in the calculation of the
mass flow in accordance with common calculation methods may be one
or more of the following: viscosity, heat capacity, heat
conductivity, density of the individual gas components or the
gas.
[0043] Moreover, that embodiment may include a temperature
measuring instrument at the tubular body, whereby the measurements
from the hotwire and the volume percentage measuring device are
supplemented with a temperature measurement to increase accuracy
(not shown).
[0044] Again, the novel aspects of the method shown in FIG. 4
compared to conventional methods are that the actual gas
composition is used rather than an estimated gas composition. When
the actual gas composition is known (and so is preferably also the
gas temperature and optionally also its pressure) it is possible to
use current values for heat conductivity, thermal capacity,
viscosity and density, which contributes to a considerably
increased accuracy of the mass flow determination.
[0045] It should be noted that although the flowcharts of FIGS. 1
through 4 show the steps in a specific sequence, these steps will
typically be performed simultaneously and continuously. The
measurement devices used for performing the methods shown in FIGS.
1 through 4 will typically be in connection with a storage
unit--either via wires or wirelessly--whereby their measurements
are stored automatically in the storage unit, and a calculator unit
will typically be in connection with the storage unit--either via
wires or wirelessly--and will perform the calculations in
accordance with the method.
[0046] FIGS. 5 through 7 show various embodiments of a mass flow
measuring device according to the invention and in accordance with
the principle of heat capacity. In all four embodiments the mass
flow measuring device comprises a number of measuring devices and
in all of the four embodiments the various measurement devices are
connected to a (not shown) storage unit--either via wires or
wirelessly--whereby their measurements are stored automatically in
the storage unit, and a calculator unit (not shown) is connected to
the storage unit--either via wires or wirelessly--and will perform
the calculations that are necessary for determining the mass flow.
The measurement devices of the shown mass flow measuring devices
are configured to perform measurements currently/continuously. In
this specification the terms "currently" and "continuously" are to
be perceived as "at small intervals", eg "at intervals of one
second", "at intervals of a minute" optionally "at intervals of 10
minutes", the scope of the meaning of the terms "continually" and
"continuously" having to be seen in relation to the frequency of
variations in the measured values.
[0047] FIG. 5 is a cross sectional view of a mass flow measuring
device 400 that comprises a tubular body 410 with an inlet opening
411 and an outlet opening 412 and configured for measuring the mass
flow of methane in a gas consisting of several components, eg a
biogas consisting of H.sub.2O, CO.sub.2, CH.sub.4.
[0048] The tubular body is configured for being incorporated in a
pipe (not shown), where a gas is introduced into the tubular body
410 via the inlet opening 411, through the tubular body 410 and
discharged through the outlet opening 412, ie in the direction of
the arrows. Around a portion of the external diameter of the pipe
an electric heater element or a heat exchanger 420 is arranged that
may supply heat to the tubular body 410 and hence to the gas that
flows in the tubular body. The pipe section 410 and the heat
exchanger 420 are surrounded by an insulating mat 430 to reduce
loss of heat from the tubular body 410 and the heat element 420 to
the surroundings.
[0049] At the inlet opening 411 of the tubular body 410, a
temperature measuring instrument 460 is arranged that gauges the
temperature T.sub.1 of the gas at the inlet of the mass flow
measuring device 400, and at the outlet opening 412 a temperature
measuring instrument 465 is arranged that gauges the temperature
T.sub.2 of the gas when the gas leaves the mass flow measuring
device 400. Moreover, approximately halfway on the tubular body,
two further temperature measuring instruments 461 and 462,
respectively, are arranged that gauge the temperature T.sub.0 at
the internal wall of tubular body 410 and the temperature T.sub.u
on the outside of the insulating mat 430, respectively. Finally a
volume percentage measuring device 440 gauges the volume percentage
of methane (CH.sub.4). In that embodiment of the mass flow
measuring device 400 a pressure gauge 450 also gauges the pressure
P at the inlet opening 411 of the tubular body 410.
[0050] As mentioned above a given amount of energy E.sub.1 is
supplied to the tubular body via electric feeding by means of the
heat exchanger 420, thereby giving rise to a difference in
temperature (.DELTA.T=T.sub.o-T.sub.u) along the insulating mat
430. Based on that difference in temperature .DELTA.T a loss of
heat E.sub.2 which takes place is calculated. Based on that, the
amount of heat or energy supplied to the gas can be determined as
it constitutes E.sub.1-E.sub.2.
[0051] The composition of the gas can be determined on the basis of
the measurement of the volume percentage of methane and
calculations as described above. Hereby the current density and
heat conductivity of the gas can be calculated. Since currently
updated values of the gas composition, temperature (at inlet and
outlet openings), pressure (at the inlet opening), and the amount
of energy supplied to the gas are known, the mass flow of methane
can be determined accurately.
[0052] FIG. 6 is a cross sectional view of an exemplary embodiment
of a mass flow measuring device 500 comprising a tubular body 510
with an inlet opening 511 and an outlet opening 512 and configured
for measuring the mass flow of methane in a gas consisting of
several components, eg a biogas consisting of H.sub.2O, CO.sub.2,
CH.sub.4.
[0053] The tubular body 510 is configured for being incorporated in
a pipe (not shown), where a gas is introduced into the tubular body
510 via the inlet opening 511, conveyed through the tubular body
510 and discharged through the outlet opening 512, ie in the
direction of the arrows. Inside the tubular body 510 a heater rod
520, is arranged which is in direct contact with the gas that flows
through the tubular body and which may hence transfer heat to the
gas. The tubular member 510 is surrounded by an insulating mat 530
in order to reduce heat loss from the tubular body 510 to the
surroundings.
[0054] The mass flow measuring device also comprises temperature
measuring instruments 560, 561, 562 and 565, respectively, for
measuring the gas temperature at the inlet opening 511, at the
heater rod 520, at the outside of the insulating mat 530 and at the
outlet opening 512, respectively. According to an alternative
embodiment the temperature measuring instrument 562 can be omitted,
since in some cases it can be assumed that the heat emission to the
surroundings is close to zero or that the heat loss can be
estimated. Finally the mass flow measuring device comprises a
volume percentage measuring instrument 540 for measuring the volume
percentage of eg methane, and a pressure meter 550.
[0055] The heater rod emits a known amount of energy to the gas in
the mass flow measuring device; the amount of energy lost through
the insulating material can be calculated on the basis of the
temperature measurement results, and hence the amount of energy
absorbed by the gas can be determined. As described in the context
of FIG. 5, the mass flow of methane can now be determined
accurately since values for the gas composition, temperature (at
inlet and outlet openings), pressure (at inlet opening) and the
amount of energy supplied to the gas are now currently
determined.
[0056] In the embodiments shown in FIGS. 5 and 6, the mass flow
measuring device may be a tube section configured for being
inserted into a pipe and having essentially the same internal
diameter as the pipe. Alternatively it is an option that the mass
flow measuring device consisting of measurement devices, heat
exchanger and insulating mat is arranged around and/or within an
existing pipe.
[0057] FIG. 7 is a partially sectional view of yet an exemplary
embodiment of a mass flow measuring device 600 that comprises a
tubular body 610 configured for measuring the mass flow of methane
in a gas consisting of several components, eg a biogas consisting
of H.sub.2O, CO.sub.2, CH.sub.4. Around the tubular member 610 an
insulating material 630 is provided and in the tubular body 610 a
heater rod 620 is arranged.
[0058] Like the embodiments shown in the context of FIGS. 5 and 7,
the mass flow measuring device comprises a number of measurement
devices, viz a pressure meter 635 and a temperature measuring
instrument 660 at the inlet of the mass flow measuring device 600,
a volume percentage measuring instrument 640 and a temperature
measuring instrument 665 at the outlet of the mass flow measuring
device 600 and two temperature measuring instruments 661, 662
approximately halfway on the mass flow measuring device 600.
[0059] In that embodiment the mass flow measuring device 600 is
configured for being fitted with a pipe 700 in which a gas flows,
as a branch. The mass flow measuring device 600 is connected to the
pipe 700 via two T-pieces 720 and connecting tubular members 711,
712. A T-piece 720 connects the pipe 700 to a tubular member 711
being in connection with the inlet opening 611 of the mass flow
measuring device 600. The other T-piece 720 connects the pipe 700
to a tube section 712 being in connection with the outlet opening
612 of the mass flow measuring device 600. Barrier or closure
mechanisms 710, such as block valves, regulate whether the gas
flows through the pipe 700 or through the mass flow measuring
device. A barrier mechanism 710 is arranged on the pipe 700 between
the two T-pieces 720 and may thus either block or allow passage of
the gas through the tube section 700 between the two T-pieces 720.
Yet a barrier mechanism 710 is arranged between the upstream
T-piece 720 and the tube section 711. In case that barrier
mechanism is open, the passage of the gas is enabled through the
mass flow measuring device. Moreover yet a barrier mechanism may be
provided between the tube section 712 and the downstream T-piece
720 to avoid that gas is able to flow into the mass flow measuring
device via the tube section 712 and the outlet opening 712 of the
mass flow measuring device 600 when gas passage is allowed through
the barrier mechanism 710 on the pipe 700.
[0060] As mentioned above, the mass flow measurement devices shown
in FIGS. 5 through 7 may be used for measuring the mass flow of one
or more components of a gas containing several components. One
example, which was mentioned above, is measurement of the mass flow
of methane from biogas plants, where the gas typically comprises
saturated water vapour, CO.sub.2 and CH.sub.4 and where only one
measurement of volume percentage is thus required namely that of
CH.sub.4 in the above examples. A further example of a use of the
mass flow measurement devices is for the measurement of the mass
flow of CO.sub.2, from breweries, which measurement can be used for
monitoring the process in the brewery. However, the described
method and mass flow measuring device can be used to measure the
mass flow of any one component of a gas containing one or more
components.
[0061] Below is given an example of a way in which to calculate the
mass flow of methane in a biogas by means of the mass flow
measuring device shown in FIGS. 5 through 7.
[0062] It is assumed that a mass flow measuring device like the one
shown in FIG. 5 is used and that the gas is a biogas consisting of
H.sub.2O, CO.sub.2, CH.sub.4, where the gas temperature at the
inlet to the mass flow measuring device is T. The biogas is
conveyed through the mass flow measuring device and heated during
its passage .DELTA.t [Kelvin] through the heat exchanger 461. A dry
gas is taken as a starting point, ie. vol %.sub.dryCO2=100%-vol
%.sub.dryCH4, wherein vol %.sub.dryCO2 and vol %.sub.dryCH4 are the
volume percentages of CO.sub.2 and CH.sub.4, respectively in a dry
gas. However the biogas is in water-saturated state, where the
saturated water-vapour pressure
P.sub.dm.apprxeq.5,2110.sup.-12(T-178,7).sup.7,12 [Pa].
[0063] When P is the pressure at the inlet of the mass flow
measuring device, the volume percentages of the gas components are
thus given by: Vol %.sub.CH4=Vol %.sub.dryCH4(1-P.sub.dm/P), Vol
%.sub.CO2=Vol %.sub.dryCO2(1-P.sub.dm/P), Vol
%.sub.H2O=100P.sub.dm/P.
[0064] The energy supplied for the heating is E [watt], where the
correlation between the mass flow {dot over (m)} [kg/s] and the
supplied energy E is given by: E={dot over (m)}C.sub.p,gas.DELTA.t,
wherein C.sub.p,gas is the heat capacity of the gas mixture
[J/kgK]. The mass flow of methane, {dot over (m)}.sub.CH4 [kg/s],
may then be determined to be: {dot over (m)}.sub.CH4={dot over
(m)}weight %.sub.CH4/100, wherein weight %.sub.CH4 is the weight
percentage of methane in the gas mixture. Thus the mass flow of
methane is given by: m . CH .times. .times. 4 = E .DELTA. .times.
.times. t weight .times. % CH .times. .times. 4 C p , gas 100 ( 1 )
##EQU1##
[0065] Conveniently weight %.sub.CH4 is expressed as a function of
the gas temperature and the volume percentage of methane in the
composite gas, as they are values that can be measured
directly.
[0066] The correlation between the volume percentage and weight
percentage of methane is first determined. It being well known
that: weight .times. % CH .times. .times. 4 = vol .times. % CH
.times. .times. 4 R CH .times. .times. 4 100 vol .times. % CH
.times. .times. 4 R CH .times. .times. 4 + vol .times. % CO .times.
.times. 2 R CO .times. .times. 2 + vol .times. % H .times. .times.
2 .times. O R H .times. .times. 2 .times. O ( 2 ) ##EQU2## wherein
[0067] R.sub.CH4=518,7 J/kgK is the gas constant of methane; [0068]
R.sub.CO2=189,0 J/kgK is the gas constant of carbon dioxide; and
[0069] R.sub.H2O=461,5 J/kgK is the gas constant of water; and
wherein Vol %.sub.CO2 and Vol %.sub.H2O are the respective volume
percentages for carbon dioxide and water.
[0070] The specific heat capacity for the composite gas,
C.sub.p,gas, can be expressed as: C p , gas = C p , CH .times.
.times. 4 vol .times. % CH .times. .times. 4 R CH .times. .times. 4
+ C p , CO .times. .times. 2 vol .times. % CO .times. .times. 2 R
CO .times. .times. 2 + C p , H .times. .times. 2 .times. O vol
.times. % H .times. .times. 2 .times. O R H .times. .times. 2
.times. O vol .times. % CH .times. .times. 4 R CH .times. .times. 4
+ vol .times. % CO .times. .times. 2 R CO .times. .times. 2 + vol
.times. % H .times. .times. 2 .times. O R H .times. .times. 2
.times. O ( 3 ) ##EQU3##
[0071] In combination equations (3) and (4) give: weight .times. %
CH .times. .times. 4 C p , gas = 100 vol .times. % CH .times.
.times. 4 C p , CH .times. .times. 4 vol .times. % CH .times.
.times. 4 + C p , CO .times. .times. 2 vol .times. % CO .times.
.times. 2 R CH .times. .times. 4 R CO .times. .times. 2 + C p , H
.times. .times. 2 .times. O vol .times. % H .times. .times. 2
.times. O R CH .times. .times. 4 R H .times. .times. 2 .times. O
##EQU4## wherein it is commonly known that: C p , CH .times.
.times. 4 = 1180 + 3 , 464 ( T + .DELTA. .times. .times. t / 2 )
.times. [ J .times. / .times. kg K ] ( 4 .times. a ) C p , CO
.times. .times. 2 = 1541 - 3 , 452 10 5 T + .DELTA. .times. .times.
t 2 + 4 , 410 10 7 ( T + .DELTA. .times. .times. t 2 ) 2 .times. [
J .times. / .times. kg K ] ( 4 .times. b ) C p , H .times. .times.
2 .times. O = 4614 - 3 , 452 10 5 ( T + .DELTA. .times. .times. t 2
) 0 , 5 + 9 , 684 10 5 ( T + .DELTA. .times. .times. t 2 ) 2
.times. [ J .times. / .times. kg K ] ( 4 .times. c ) ##EQU5##
[0072] When the values for the gas constants are included in
equation (4) it follows that: weight .times. % CH .times. .times. 4
C p , gas = 100 vol .times. % CH .times. .times. 4 C p , CH .times.
.times. 4 vol .times. % CH .times. .times. 4 + C p , CO .times.
.times. 2 vol .times. % CO .times. .times. 2 2 , 744 + C p , H
.times. .times. 2 .times. O vol .times. % H .times. .times. 2
.times. O 1 , 124 ( 5 ) ##EQU6##
[0073] As approximated function the following can be used: weight
.times. % CH .times. .times. 4 C p , gas = K 1 T .alpha. vol
.times. % CH .times. .times. 4 .beta. ( 6 ) ##EQU7##
[0074] In order to determine the values of the constants in
equation (6), ie for K.sub.1, .alpha. og .beta., a typical field of
operation is determined:
[0075] It is assumed in the following that .DELTA.t=10.degree. C.,
pressure P=1,03310.sup.5 Pa and that the lowest temperature,
T.sub.min, and the highest temperature, T.sub.max, are 281 K
(=8.degree. C.) and 328 K (=55.degree. C.), respectively. Thus the
expression T+.DELTA.t/2 equals 286 K and 333 K, respectively. The
values for the specific heat capacities for the individual gas
components are given by equations (4a) through (4c). Thus
C.sub.p,CH4 equals 2171 J/kgK at 286 K and 2333 J/kgK at 333 K;
C.sub.p,CO2 is 840 J/kgK at 286 K and 878 J/kgK at 333 K; and
C.sub.p,H2O is 1889 J/kgK at 286 K and 1859 J/kgK at 333 K.
Moreover it is assumed that the volume percentage vol %.sub.dryCH4
may be equal to 50% or 70%, and thus four measurement points are
obtained (viz T=281 K and vol %.sub.dryCH4=50%; T=281 K and vol
%.sub.dryCH4=70%; T=328 K and vol %.sub.dryCH4=50%; and T=328 K and
vol %.sub.dryCH4=70%).
[0076] When the values of these four measurement points are
included in equation (5) and when equation (5) is set to be equal
to equation (6), the following approximation for equation (6)
applies: weight .times. % CH .times. .times. 4 C p , gas = vol
.times. % CH .times. .times. 4 T 0 , 25 544 ( 7 ) ##EQU8##
[0077] If equation (7) is included in equation (1) it applies that:
m . CH .times. .times. 4 = E .DELTA. .times. .times. t vol .times.
% CH .times. .times. 4 T 0 , 25 544 ( 8 ) ##EQU9##
[0078] Thus equation (8) expresses the mass flow of methane
expressed by the energy E supplied to the gas, the difference in
temperature of the gas between the inlet and the outlet of the mass
flow measurement device, the temperature and the volume percentage
of methane. It is noted that the above calculation example serves
merely as an example of a way in which to determine the mass flow
of methane based on calculations on the basis of measurements.
Other calculation methods may also be applicable as long as they
take into account the current temperature and composition of the
gas.
* * * * *