U.S. patent application number 15/446783 was filed with the patent office on 2017-06-22 for method and measuring apparatus for determining physical properties of gas.
The applicant listed for this patent is MEMS AG. Invention is credited to Andreas KEMPE, Philippe PRETRE, TOBIAS SUTER.
Application Number | 20170176405 15/446783 |
Document ID | / |
Family ID | 48536667 |
Filed Date | 2017-06-22 |
United States Patent
Application |
20170176405 |
Kind Code |
A1 |
PRETRE; Philippe ; et
al. |
June 22, 2017 |
METHOD AND MEASURING APPARATUS FOR DETERMINING PHYSICAL PROPERTIES
OF GAS
Abstract
A method to determine a physical property or a quantity of gas
related to combustion including: flowing a gas from a reservoir
through a critical nozzle and past a microthermal sensor wherein
the mass flow of the gas through the critical nozzle is the same as
the mass flow through the microthermal sensor; measuring the
pressure drop in the reservoir as a function of time; deriving a
first gas property factor based on a time constant of the pressure
drop; determining a second gas property factor which depends from a
flow signal generated by the microthermal sensor; determining a
thermal conductivity of the gas; and determining the physical
property or quantity based on a correlation between the physical
property or quantity, and the first and/or second gas property
factors and the thermal conductivity.
Inventors: |
PRETRE; Philippe; (Dattwil,
CH) ; KEMPE; Andreas; (Zurich, CH) ; SUTER;
TOBIAS; (Kilchberg, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MEMS AG |
BRUGG |
|
CH |
|
|
Family ID: |
48536667 |
Appl. No.: |
15/446783 |
Filed: |
March 1, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14282562 |
May 20, 2014 |
9612229 |
|
|
15446783 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 7/00 20130101; G01N
33/0062 20130101; G01N 25/18 20130101; G01N 33/225 20130101; G01N
25/005 20130101; Y10T 29/49321 20150115; G01N 25/36 20130101 |
International
Class: |
G01N 33/00 20060101
G01N033/00; G01N 25/18 20060101 G01N025/18; G01N 33/22 20060101
G01N033/22; G01N 7/00 20060101 G01N007/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2013 |
EP |
13002708.9 |
Claims
1. A method for determining physical properties and/or quantities
relevant to combustion of a gas and/or a gas mixture, the method
comprising: flowing the gas and/or gas mixture from a gas reservoir
through a critical nozzle and past a microthermal sensor, with the
same mass flow is applied to the critical nozzle and the
microthermal sensor; measuring a pressure drop in the gas reservoir
as a function of time; determining a first gas property factor
(.GAMMA.*), which is dependent on a first group of physical
properties of the gas and/or gas mixture, on the basis of the
measured pressure drop; determining a second gas property factor
(.GAMMA.), which is dependent on a second group of physical
properties of the gas or gas mixture, from a flow signal generated
by the microthermal sensor; determining the thermal conductivity
(.lamda.) of the gas and/or gas mixture using the microthermal
sensor; and determining a physical property and/or quantity
relevant to combustion from the first and/or second gas property
factor (.GAMMA.*, .GAMMA.) and the thermal conductivity (.lamda.)
through correlation.
2. The method according to claim 1, in which the starting point is
an exponential decline of the measured pressure and the first gas
property factor (.GAMMA.*) is derived from the time constant of the
pressure drop, in which case the first gas property factor is
formed in particular by measuring the temperature (T) and by
omitting all gas-unrelated variables.
3. The method according to claim 1, in which the second gas
property factor (.GAMMA.) contains the quotient of heat capacity
(c.sub.p) divided by thermal conductivity (.lamda.) of the gas or
gas mixture, or is dependent on the same, and in which the second
gas property factor is formed by measuring the temperature (T)
additionally and by omitting all gas-unrelated variables.
4. The method according to claim 1, wherein the gas property
factors (.GAMMA.*, .GAMMA.) are validated by comparing the values
for the total volume of released gas or gas mixture by: measuring
the pressure and temperature in the gas reservoir at the start and
end of the pressure drop reading and by determining the released
standard volume by reference to the known volume of the gas
reservoir; summing the standard flow measured with the microthermal
sensor during a time interval between the start and end of the
pressure drop reading; comparing the released standard volume to
the summed up standard volume; and in case of a discrepancy, by
adjusting the first and/or the second gas property factor
(.GAMMA.*, .GAMMA.) by adjusting the pressure signal or the
standard flow variable of the microthermal sensor.
5. A method for determining physical properties and/or quantities
relevant to combustion of gas or gas mixtures, the method
comprising: the gas or gas mixture flows under pressure through a
critical nozzle and past a microthermal sensor into a gas
reservoir, with the same mass flow being applied to the critical
nozzle and the microthermal sensor; measuring a pressure increase
in the gas reservoir as a function of time; determining a first gas
property factor (.GAMMA.*), dependent on a first group of physical
properties of the gas or gas mixture, by reference to the measured
values of the pressure increase; determining a second gas property
factor (.GAMMA.), dependent on a second group of physical
properties of the gas or gas mixture, from the flow signal of the
microthermal sensor, with the second gas property factor
containing, or depends on the heat capacity (c.sub.p) of the gas or
gas mixture; determining a thermal conductivity (.lamda.) of the
gas or gas mixture using the microthermal sensor; and determining a
physical property or quantity relevant to combustion from the first
and/or second gas property factor (.GAMMA.*,.GAMMA.) and the
thermal conductivity (.lamda.) through correlation.
6. The method according to claim 5, where the starting point is a
linear increase of the measured pressure and the first gas property
factor (.GAMMA.*) is derived from a proportionality constant of the
pressure increase, and in which case the first gas property factor
is formed by measuring, for example, in addition the temperature
(T) and the nozzle inlet pressure (pNozzle), and by omitting all
gas-unrelated variables.
7. The method according to claim 5, in which the second gas
property factor (.GAMMA.) contains the quotient of heat capacity
(c.sub.p) divided by thermal conductivity (.lamda.) of the gas or
gas mixture, or is dependent on the same, and in which the second
gas property factor is formed in particular by additionally
measuring the temperature (T) and by omitting all gas-unrelated
variables.
8. The method according to claim 5, where the gas property factors
(.GAMMA.*,.GAMMA.) are validated by comparing the values for the
total volume of the gas or gas mixture fed into the gas reservoir
by: measuring the pressure and temperature in the gas reservoir at
the start and end of the pressure increase reading and by
determining the released standard volume fed into the gas reservoir
by reference to the known volume of the gas reservoir; summing up
the standard flow measured with the microthermal sensor during the
time interval between the start and end of the pressure increase
reading; comparing the standard volume fed into the gas reservoir
to the summed up standard volume and based on the comparison
determining if a discrepancy exists; and in response to the
determination of a discrepancy, adjusting the first and/or the
second gas property factor by adjusting the pressure signal or a
flow variable of the microthermal sensor.
9. The method according to claim 5, wherein the method includes
calibrating the flow signal of the microthermal sensor, by:
calibrating the flow signal of the microthermal sensor for a
specific calibration gas or gas mixture; determining a ratio
(.GAMMA./.GAMMA.*) of the second gas property factor, to the first
gas property factor for an unknown gas or gas mixture on the basis
of the flow signal of the microthermal sensor; and comparing the
standard volume values from the reading of the pressure drop or
pressure increase and the reading of the summed up standard flow of
the microthermal sensor, which are then used to adjust a ratio of
the second gas property factor to the first, and to adapt the value
for the second gas property factor (.GAMMA.).
10. The method according to claim 1 where the physical property is
a density or the thermal conductivity or the heat capacity or the
viscosity of the gas or gas mixture, and/or where the quantity
relevant to combustion is the energy content or the calorific value
or the Wobbe index or the methane number or the air requirement of
the gas or gas mixture.
11. The method according to claim 1, where the certain physical
property or the quantity relevant to combustion (Q) is determined
by aid of a correlation function which is: f.sub.corr(.GAMMA.,
.GAMMA.*, .lamda.)=const.GAMMA..sup.r.GAMMA.*.sup.s.lamda..sup.t,
wherein r, s and t are exponents, and const is a constant.
12. The method according to claim 1, where the pressure in the gas
reservoir is higher at the start of the pressure drop reading than
the critical pressure (perit) of the critical nozzle and the
external pressure downstream of the critical nozzle is less than
half of the critical pressure, or where the pressure in the gas
reservoir at the start of the pressure increase reading is lower
than half of the critical pressure (perit) of the critical nozzle
and the pressure upstream of the critical nozzle is higher than the
critical pressure.
13. A measuring apparatus for determining a physical property
and/or quantity relevant to combustion of gas and/or gas mixtures
comprising: an analyzer unit configured to carry out a procedure in
accordance with claim 1 and a gas reservoir equipped with a
pressure sensor, a critical nozzle and a microthermal sensor to
measure the flow and thermal conductivity, in which case the gas
reservoir is connected to the critical nozzle and the microthermal
sensor.
14. A method to use a gas reservoir and a critical nozzle for
determining physical properties and/or quantities relevant to
combustion of gas or gas mixtures, the method comprises: flowing
the gas or gas mixture under pressure from the gas reservoir
through the critical nozzle; measuring a pressure drop in the gas
reservoir as a function of time; determining a gas property factor
(.GAMMA.*), dependent on a physical property of the gas or gas
mixture, based on the measured values of the pressure drop; and
determining a desired physical property or quantity relevant to
combustion based on the gas property factor (.GAMMA.*) through
correlation.
15. A method to determine a physical property and/or a quantity
relevant to a combustion gas or gas mixture using a gas reservoir
and a microthermal sensor, calibrated for a specific calibration
gas or gas mixture, the method comprises: flowing the gas or gas
mixture under pressure from the gas reservoir past the microthermal
sensor; determining a flow rate (vx) of the gas or gas mixture
using the microthermal sensor; determining a summed-up volume flow
(vxA) based on the flow rate (vx); comparing the summed up volume
flow to a gas volume released from the gas reservoir; determining a
gas property factor (S/v'x), dependent on the physical properties
of the gas or gas mixture, based on the comparison, wherein the
quantity (v'x) represents a flow rate based on the released gas
volume, and wherein the gas property factor comprises, for example,
the heat capacity (c.sub.p) of the gas or gas mixture or is
dependent on the same; and a desired physical property or quantity
relevant to combustion is determined on the basis of the gas
property factor through correlation.
16. A method to determine a physical property of combustion
comprising: flowing a gas from a reservoir through a critical
nozzle and a microthermal sensor wherein the mass flow of the gas
through the critical nozzle is the same mass flow through the
microthermal sensor; measuring a pressure drop in the reservoir as
the gas flows through the critical nozzle; determining a first gas
property factor based on the measured pressure drop; determining a
second gas property factor based on a flow signal generated by the
microthermal sensor; determining a thermal conductivity of the gas
using the microthermal sensor, and determining a physical property
of the combustion based on a correlation of the thermal
conductivity and at least one of the first and second gas property
factors.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/282,562 filed May 20, 2014, and claims
priority to European Patent Application No. 13002708.9 filed May
24, 2013, the entirety of both applications are incorporated by
reference.
BACKGROUND OF INVENTION
[0002] The invention relates to a method and a measuring apparatus
for determining physical properties and quantities relevant to
combustion of gas and gas mixtures. Physical gas properties mean in
particular the density, thermal conductivity, heat capacity and
viscosity as well as correlatable quantities relevant to
combustion, such as the energy content, calorific value, Wobbe
index, methane number and/or air requirement of the gas or gas
mixture.
[0003] In gas-fuel firing control systems it is important to keep
the load in the burner constant even at changing fuel gas
qualities. The Wobbe index, formed from the calorific value and the
root of the density ratio between air and this gas, is the
appropriate index for displaying the interchangeability of gases.
An identical Wobbe index will then result in a constant thermal
load in the burner.
[0004] When regulating (natural) gas motors, knowledge of the
calorific value at varying (natural) gas qualities is necessary to
achieve an increase of performance or efficiency, while for gas the
methane number--by analogy to the octane number for gasoline--is
used to assess ignition behaviour (knocking effect or
misfiring).
[0005] An optimal combustion process requires a correct mixing
ratio between fuel gas and air--, known as "air requirement". Soot
(flue gas) usually forms if there is too little air, and this may
damage fuel cells in particular. Too much air during combustion
results in reduced performance. The optimal value depends on the
application concerned, but changes again with varying gas
qualities.
[0006] Correlation methods for calculating quantities relevant to
combustion have been described in academic literature, see for
example U. Wernekinck, "Gasmessung and Gasabrechnung" (Gas metering
and gas billing), Vulkan publishers, 2009, ISBN 978-3-8027-5620-7.
The following combinations of measured variables are used in this
connection: [0007] A. Dielectric constant, sonic velocity, CO.sub.2
content [0008] B. Sonic velocity at 2 pressures, CO.sub.2 content
[0009] C. Thermal conductivity at 2 temperatures, sonic velocity
[0010] D. Thermal conductivity, heat capacity, dynamic viscosity
[0011] E. Thermal conductivity, infrared absorption (not
dispersive) [0012] F. Infrared absorption (dispersive)
[0013] There are currently only a few commercially available
devices that are approved for calorific value readings, e.g. the
EMC500 device by RMG-Honeywell (Type D plus CO.sub.2 content) or
the Gas-lab Q1 device by Elster-Instromet (Type E plus CO.sub.2
content). However, due to the high acquisition costs, none of these
devices is suitable for mass distribution.
[0014] Integrated CMOS hot-wire anemometers are able to take a
microthermal measurement of thermal conductivity as well as of mass
flow. For this technology, reference is made to the publication of
D. Matter, B. Kramer, T. Kleiner, B. Sabbattini, T. Suter,
"Mikroelektronischer Haushaltsgaszahler mit neuer Technologie"
(Micro-electronic household gas meter using new technologies),
published in Technisches Messen 71, 3 (2004), pp. 137-146. It
differs from conventional thermal mass flow meters by taking the
measurement directly in the gas flow and not from the outside on a
metal capillary tube that encompasses the gas flow.
[0015] EP 2 015 056 A1 describes a thermal flow sensor for
determining a quantity relevant to combustion, based on a thermal
conductivity reading if the mass flow is basically known. A
critical nozzle is used to keep the mass flow constant, and the aim
is to correct the gas type dependence of the critical nozzle by
means of the thermal conductivity. However, the information on the
correlation of quantities relevant to combustion is limited to two
more or less independent measured variables and thus does not
permit validation of the measured data.
[0016] WO 2004/036209 A1 describes a sensor for determining a
quantity relevant to combustion where the mass flow is kept
constant and where a value that is proportional to the heat
capacity is identified by means of a thermal measurement. Since the
described sensor is not a microthermal sensor, it is not possible
to draw conclusions regarding thermal conductivity; this means that
the determination of the heat capacity and the quantities relevant
to combustion derived therefrom is only possible up to one
proportionality factor. As a result, an additional calibration with
known gas compositions is required. In addition, the information on
thermal conductivity, and thus the means to correlate thermal
conductivity .lamda. with a quantity relevant to combustion is
omitted. Furthermore, the accuracy of this method is limited by the
occurring variations of the inaccessible thermal conductivity
.lamda..
SUMMARY OF THE INVENTION
[0017] Hence the invention is based on the objective of presenting
a method and a measuring apparatus to determine physical properties
of gas and gas mixtures in order to achieve a higher degree of
accuracy than the sensors from the above referenced patent
documents; in addition, the objective is to produce the measuring
apparatus at a lower cost than the devices commercially available
that are approved for calorific value readings requiring
calibration.
[0018] The concept of the invention is to determine physical gas
properties, based on measuring the pressure drop of a specified
volume of gas through a critical nozzle in combination with a
microthermal sensor able to measure the flow as well as thermal
conductivity. Both the measurement of the pressure drop and of the
flow can be validated for consistency, since the same mass flow for
the critical nozzle is also applied to the microthermal sensor.
[0019] From these three measured variables it is possible to
determine additional values through correlations.
[0020] Measuring the Drop in Pressure of a Defined Volume of Gas
Using a Critical Nozzle:
[0021] The mass flow th through a critical nozzle is described
by
m . = C d p A * .psi. max M T R m , ( 1 ) ##EQU00001##
[0022] in which case C.sub.d represents the "discharge
coefficient", i.e. the loss factor of an actual critical nozzle
compared to an ideal critical nozzle, p the inlet pressure, A* the
nozzle cross-section, T the inlet temperature, R.sub.m the
universal gas constant, M the molecular weight of the gas and
.quadrature..sub.max the maximum value of the critical flow factor.
The latter is a function of the isentropic coefficient
.quadrature..quadrature.=c.sub.p/c.sub.V (ratio of isobaric to
isochoric heat capacity),
.psi. = .gamma. ( .gamma. + 1 2 ) .gamma. + 1 2 ( 1 - .gamma. ) . (
2 ) ##EQU00002##
[0023] If the gas of a known volume V of gas is released from high
pressure through the critical nozzle (e.g., from 9 to 4 bar), then
according to the ideal gas law, pressure in the volume depends on
the time t as follows:
p ( t ) = m ( t ) R m T M V . ( 3 ) ##EQU00003##
[0024] Therefore, the rate at which the pressure changes results
in
dp ( t ) dt = d m ( t ) dt R m T M V = m . ( t ) R m T M V ( 4 )
##EQU00004##
[0025] and together with equation (1) as
dp ( t ) dt = C d p A * .psi. max M T R m R m T M V = C d A * .psi.
max V R m T M p ( t ) . ( 5 ) ##EQU00005##
[0026] Accordingly, if the course of the pressure is measured in
dependence of the time, then the time constant r of the related
exponential function obtained by integration can be defined as:
1 / .tau. = C d A * .psi. max V R m T M . ( 6 ) ##EQU00006##
[0027] If the measuring process additionally delivers the value for
temperature T, a gas property factor can be defined by omitting all
gas-unrelated variables
.GAMMA. * := C d .psi. max 1 M . ( 7 ) ##EQU00007##
[0028] Conversely, if the gas is released from a higher pressure
level through the critical nozzle into a known volume V (e.g., from
ambient pressure to vacuum), the equation (5') for the pressure
increase in volume V reads as follows:
dp ( t ) dt = C d p Nozzle A * .psi. max M T R m R m T M V = C d A
* .psi. max V R m T M p Nozzle , ( 5 ' ) ##EQU00008##
[0029] in which case the pressure before the nozzle p.sub.nozzle
nozzle remains constant, which leads over time to a linear pressure
increase in volume V with
C d A * .psi. max V R m T M p Nozzle ( 6 ' ) ##EQU00009##
[0030] being a proportionality constant. If, in addition, the
values of the temperature T and the nozzle inlet pressure
p.sub.Nozzle are obtained by the measurement, it is possible to
define in turn the gas property factor
.GAMMA. * := C d .psi. max 1 M ( 7 ' ) ##EQU00010##
[0031] by omitting all gas-unrelated variables.
[0032] Mass Flow Measurement by Means of a Microthermal Sensor:
[0033] The starting point for describing the microthermal mass flow
measurement is that of the one-dimensional thermal conductivity
equation describing the microthermal system (Kerson Huang:
Statistical Mechanics, 2nd volume, John Wiley & Sons, New York
1987, ISBN 0-471-85913-3):
c p .lamda. .rho. v x d dx T = .gradient. 2 T + 1 .lamda. .THETA. ,
( 8 ) ##EQU00011##
[0034] in which
[0035] v.sub.x represents the component of the mean flow rate
(velocity vector) {right arrow over (V)} in the direction of x,
i.e. in the direction of the gas flow,
[0036] T represents temperature,
[0037] d/dx T represents the temperature gradient,
[0038] c.sub.p represents the heat capacity of the gas at constant
pressure,
[0039] p represents density,
[0040] .lamda. represents the thermal conductivity of the gas,
[0041] .gradient..sup.2T represents the Laplacian operator, applied
to temperature T, in which
.gradient. 2 = ( d d x ) 2 + ( d dy ) 2 + ( d dz ) 2 .
##EQU00012##
[0042] Since the gas (gas flow) flows only in the direction x, the
components v.sub.y and v.sub.z in direction y, respectively
direction z of the mean flow rate {right arrow over (V)} are taken
to be zero. .THETA. with the unit Watt/m.sup.3 describes the source
term of the heat element. In the microthermal method, the source
term is the heating wire of a miniaturised, integrated hot-wire
anemometer, which feeds thermal energy into the system. From the
solution of equation (8), which describes the temperature
distribution in the microthermal system, it is possible, by
measuring this temperature distribution, to determine the factor
S,
S := c p .lamda. .rho. v x = c p .lamda. m . A , ( 9 )
##EQU00013##
[0043] wherein A means the cross-section of the flow channel past
the microthermal sensor. In combination with the critical nozzle,
i.e. by arranging the microthermal sensor after the critical
nozzle, the mass flow is provided by equation (1), therefore by
c p .lamda. .rho. v x = c p .lamda. C d p A * A .psi. max M T R m .
( 10 ) ##EQU00014##
[0044] Measuring pressure p and temperature T, and omitting again
all gas-unrelated variables, delivers a second gas property
factor
.GAMMA. = c p .lamda. C d .psi. max M . ( 11 ) ##EQU00015##
[0045] The omission of all gas-unrelated variables in equation (7)
and equation (11) is done implicitly, by putting .quadrature. and
.GAMMA.* in relation to .quadrature. and .GAMMA.* of a known
(calibration) gas. See also FIG. 4.
[0046] Measuring Thermal Conductivity by Means of Microthermal
Sensor:
[0047] It should be noted that the thermal conductivity .lamda.,
due to the source term .THETA., has an additional, separate impact
on the solution of equation (8). The same applies in reverse: the
thermal conductivity can be determined if the microthermal sensor
is measured without an applied mass flow (v.sub.x=0 or {dot over
(m)}=0). The related differential equation for temperature
distribution then simply reads
.gradient. 2 T = - 1 .lamda. .THETA. . ( 12 ) ##EQU00016##
[0048] Validation of the Gas Property Factors .quadrature. or
.GAMMA.*:
[0049] The ratio of the two gas property factors .quadrature. and
.GAMMA.* results in
.GAMMA. / .GAMMA. * = c p .lamda. M .varies. c p .lamda. .rho. norm
, ( 13 ) ##EQU00017##
[0050] since the molecular weight is proportional to the standard
density (density at standard conditions 1013.25 mbar and 273.15 K),
due to the fact that for most gases, the mol volume is almost
identical. Thus, in equation (9), the flow rate v.sub.x and, in
conjunction with the flow channel cross-section A, the standard
volume flow .phi..sub.norm=v.sub.xA can be extracted from the
factor S, measured with the microthermal sensor. The integration of
this volume flow over time, i.e. the time interval t.sub.2-t.sub.1,
should then correspond with the released gas volume calculated on
the basis of the corresponding pressure and temperature values:
.intg. t 1 t 2 .phi. norm ( t ) dt = ! ( p ( t 2 ) - p ( t 1 ) )
1013.25 mbar 273.15 K T V . ( 14 ) ##EQU00018##
[0051] If these two values do not match, the standard volume flow
or the pressure signal can be adjusted, depending on which value
can be measured less accurately, to the point that equation (14) is
satisfied. In the case of a standard volume flow adjustment for
v.sub.x=.phi..sub.norm/A, the right side of the equation (13) is
also adjusted through the measured factor S in equation (9), and
thus also the gas property factor
.quadrature..quadrature..quadrature. again by aid of equation (13)
.quadrature..quadrature. In the case of a pressure signal
adjustment, the time constant .tau. in equation (6), respectively
the proportionality constant in equation (6'), is adjusted, which
in turn leads to an adjustment of the gas property factor .GAMMA.*
in equation (7) or (7'). In this way, .quadrature. and .GAMMA.*
have been defined consistently, because the mass flow through the
nozzle is the same as the mass flow with which the microthermal
sensor is supplied.
[0052] Correlation of Quantities Relevant to Combustion:
[0053] By measuring the gas property factors .quadrature. and
.GAMMA.* as well as thermal conductivity .lamda., three independent
measured variables are obtained, with which it is now possible to
correlate quantities relevant to combustion Q by aid of a function
f.sub.corr:
Q.sub.corr=f.sub.corr(.GAMMA., .GAMMA.*, .lamda.). (15)
[0054] For example, for correlating the density ratio
.rho..sub.corr/.rho..sub.ref at 0.degree. C. and 1013.25 mbar, as
shown in FIG. 4, the following correlation function
.rho..sub.corr/.rho..sub.ref=f.sub.corr(.GAMMA., .GAMMA.*,
.lamda.)=.GAMMA..sup.r.GAMMA.*.sup.s.lamda..sup.t (16)
[0055] is obtained, with exponents r=-0.2, s=-1.8 and t=-0.2 and a
typical H-gas used for reference purposes.
[0056] Method and Measuring Apparatus According to the Present
Invention
[0057] In the method for determining physical properties and/or
quantities relevant to combustion of gas and gas mixtures according
to the present invention:
[0058] the gas or gas mixture flows from a gas reservoir through a
critical nozzle and past a microthermal sensor, with the same mass
flow being applied to the critical nozzle and the microthermal
sensor;
[0059] the pressure drop in the gas reservoir is measured as a
function of time;
[0060] a first gas property factor .GAMMA.*, dependent on a first
group of physical properties of the gas or gas mixture, is
determined on the basis of the measured values of the pressure
drop, with the first gas property factor being derived, for
example, from a time constant of the pressure drop;
[0061] a second gas property factor .GAMMA., dependent on a second
group of physical properties of the gas or gas mixture, is
determined by the flow signal of the microthermal sensor, with the
second gas property factor containing, for example, the heat
capacity c.sub.p of the gas or gas mixture, or being dependent on
the same;
[0062] the thermal conductivity .lamda. of the gas or gas mixture
is determined with the aid of the microthermal sensor; and
[0063] a desired physical property or quantity relevant to
combustion is determined by the first and/or second gas property
factor .GAMMA.*, .GAMMA. and thermal conductivity .lamda. through
correlation.
[0064] The method is advantageously based on an exponential decline
of the measured pressure and derives the first gas property factor
.GAMMA.* from the time constant of the pressure drop, in which case
the first gas property factor is formed, for example, by measuring
additionally temperature T and by omitting all gas-unrelated
variables.
[0065] The second gas property factor (.GAMMA.) typically contains
the quotient of the heat capacity c.sub.p, divided by the thermal
conductivity .lamda. of the gas or gas mixture, or is dependent on
the same, with the second gas property factor being formed by
measuring in addition, for example, the temperature T and by
omitting all gas-unrelated variables.
[0066] According to an advantageous embodiment of the method, the
gas property factors .GAMMA.*, .GAMMA. are validated by comparing
the values for the total volume of the released gas or gas mixture;
this is done by measuring the pressure and temperature in the gas
reservoir at the start and the end of the pressure drop reading and
by determining the released standard volume at a known volume of
the gas reservoir, by accumulating the standard flow measured with
the microthermal sensor across the time interval between the start
and end of the pressure drop reading, and by comparing the released
standard volume to the accumulated standard flow. In case of a
discrepancy, the first and/or the second gas property factor is
adjusted, e.g. by adjusting the pressure signal or the standard
flow value of the microthermal sensor.
[0067] The embodiment of the method described above can be used to
calibrate the flow signal of the microthermal sensor by calibrating
the flow signal of the microthermal sensor for a specific
calibration gas or gas mixture, by determining the ratio
.GAMMA./.GAMMA.* of the second gas property factor, derived from
the flow signal of the microthermal sensor, to the first gas
property factor for an unknown gas or gas mixture, and by comparing
the standard volume values from the pressure drop reading and the
accumulated standard flow of the microthermal sensor, and to use
them to adjust the ratio of the second gas property factor to the
first and to adapt the value for the second gas property factor
.GAMMA..
[0068] In a further advantageous embodiment of the method for
determining physical properties and/or quantities relevant to
combustion of the gas or gas mixture:
[0069] the gas or gas mixture flows under pressure through a
critical nozzle and past a microthermal sensor into a gas
reservoir, with the same mass flow being applied to the critical
nozzle and the microthermal sensor;
[0070] the pressure increase in the gas reservoir is measured as a
function of time;
[0071] a first gas property factor .GAMMA.*, dependent on a first
group of physical properties of the gas or gas mixture, is
determined by reference to the measured variables of the pressure
increase;
[0072] a second gas property factor .GAMMA., dependent on a second
group of physical properties of the gas or gas mixture, is
determined from the flow signal of the microthermal sensor, with
the second gas property factor containing, for example, the heat
capacity c.sub.p of the gas or gas mixture, or being dependent on
the same;
[0073] the thermal conductivity .lamda. of the gas or gas mixture
is determined with the aid of the microthermal sensor; and
[0074] a desired physical property or quantity relevant to
combustion is determined from the first and/or second gas property
factor .GAMMA.*, .GAMMA. and thermal conductivity .lamda. through
correlation.
[0075] The method is advantageously based on a linear increase of
the measured pressure and derives the first gas property factor
.GAMMA.* from the proportionality constant of the pressure
increase, in which case the first gas property factor is formed,
for example, by measuring additionally the temperature T and the
nozzle inlet pressure p.sub.Nozzle and by omitting all
gas-unrelated variables.
[0076] The second gas property factor .GAMMA. typically contains
the quotient of the heat capacity c.sub.p divided by the thermal
conductivity .lamda. of the gas or gas mixture or is dependent on
the same, in which case the second gas property factor is formed,
for example, by measuring additionally the temperature T and by
omitting all gas-unrelated variables.
[0077] According to a further advantageous embodiment of the
method, the gas property factors .GAMMA.*, .GAMMA. are validated by
comparing the values for the total volume of the gas or gas mixture
flown into the gas reservoir; this is done by measuring the
pressure and temperature in the gas reservoir at the start and end
of the pressure increase reading and by determining the standard
volume fed into the gas reservoir at a known volume of the gas
reservoir, by accumulating the standard flow measured with the
microthermal sensor across the time interval between the start and
end of the pressure increase reading, and by comparing the standard
volume fed into the gas reservoir to the accumulated standard flow.
In case of a discrepancy, the first and/or the second gas property
factor is adjusted, e.g. by adjusting the pressure signal or the
standard flow value of the microthermal sensor.
[0078] The embodiment of the method described above can be used to
calibrate the flow signal of the microthermal sensor by calibrating
the flow signal of the microthermal sensor for a specific
calibration gas or gas mixture, by determining the ratio
.GAMMA./.GAMMA.* of the second gas property factor, derived from
the flow signal of the microthermal sensor, to the first gas
property factor for an unknown gas or gas mixture, and by comparing
the standard volume values from the pressure increase reading and
the accumulated standard flow of the microthermal sensor, and to
use them to adjust the ratio of the second gas property factor to
the first and to adapt the value for the second gas property factor
.GAMMA..
[0079] The desired physical property may be, for example, the
density or the thermal conductivity or the heat capacity or the
viscosity of the gas or gas mixture, and the quantity relevant to
combustion may be, for example, the energy content or the calorific
value or the Wobbe index or the methane number or the air
requirement of the gas or gas mixture.
[0080] The desired physical property or quantity relevant to
combustion Q is determined advantageously by aid of a correlation
function Q=f.sub.corr(.GAMMA., .GAMMA.*,
.lamda.)=const.GAMMA..sup.r.GAMMA.*.sup.s.lamda..sup.t, wherein r,
s and t are exponents, and const is a constant.
[0081] The pressure in the gas reservoir at the start of the
pressure drop measurement is typically higher than the critical
pressure p.sub.crit of the critical nozzle and the external
pressure downstream of the critical nozzle is less than half the
critical pressure, or the pressure in the gas reservoir at the
start of the pressure increase reading is typically less than half
the critical pressure p.sub.crit of the critical nozzle and the
pressure upstream of the critical nozzle is higher than the
critical pressure.
[0082] The gas reservoir is typically disconnected from the gas
supply during the measurement, irrespective of the embodiment and
variant. The volume of the gas reservoir can be selected
advantageously in such a way that the pressure inside the gas
reservoir significantly decreases or increases by the end of the
measurement, for example, by at least a tenth or a fifth of the
initial pressure.
[0083] The measuring apparatus for determining physical properties
and/or quantities relevant to combustion of a gas or gas mixture
according to the present invention includes an analyzer unit that
is configured to carry out a procedure in accordance with one of
the embodiments or variants described above, as well as a gas
reservoir, that is equipped with a pressure sensor, a critical
nozzle and a microthermal sensor to measure the flow and thermal
conductivity. In this set-up the gas reservoir is connected to the
critical nozzle and the microthermal sensor for the purposes of
measuring.
[0084] Furthermore, the invention also includes the use of a gas
reservoir and a critical nozzle to determine physical properties
and/or quantities relevant to combustion of a gas or gas mixture;
in this set-up the gas or gas mixture flows under pressure from the
gas reservoir through the critical nozzle, and the pressure drop in
the gas reservoir is measured as a function of time, a gas property
factor .GAMMA.*, dependant on the physical properties of the gas or
gas mixture is determined on the basis of the measured values of
the pressure drop, derived, for example, from a time constant of
the pressure drop; the gas property factor .GAMMA.* then serves to
determine a desired physical property or quantity relevant to
combustion through correlation.
[0085] In another advantageous embodiment, low pressure is
generated in the reservoir, and the gas or gas mixture flows under
pressure through the critical into the gas reservoir; in this
set-up, the pressure increase in the gas reservoir is measured as a
function of time, and a gas property factor .GAMMA.*, dependent on
the physical properties of the gas or gas mixture, is determined
from the measured values of the pressure increase, which then
serves to determine a desired physical property or quantities
relevant to combustion through correlation.
[0086] The above-described use of a gas reservoir and a critical
nozzle to determine physical properties and/or quantities relevant
to combustion of a gas or gas mixture, or the corresponding method
in which a gas reservoir and a critical nozzle are used for
determining physical properties and/or quantities relevant to
combustion of a gas or gas mixture, can also be seen as a distinct,
independent invention, which may additionally include a measuring
apparatus with an analyzer unit, a gas reservoir and a critical
nozzle, in which case the analyzer unit is configured for the use
of the gas reservoir and the critical nozzle to determine physical
properties and/or quantities relevant to combustion of a gas or gas
mixture or to carry out the corresponding method.
[0087] In addition, the invention encompasses the use of a gas
reservoir and a microthermal sensor calibrated for a specific
calibration gas or gas mixtures to determine physical properties
and/or quantities relevant to combustion of gas or gas mixtures; in
this set-up the gas or gas mixture flows under pressure from the
gas reservoir past the microthermal sensor, in which case the
volume flow v.sub.xA, determined by the microthermal sensor
calibrated for a specific calibration gas or gas mixture, is
accumulated and compared to the gas volume released from the gas
reservoir; from the comparison of the two volumes a gas property
factor S/v'.sub.x, dependent on the physical properties of the gas
or gas mixture, is determined, in which v'.sub.x represents the
flow rate of the released gas volume, and in which the desired
physical property or quantity relevant to combustion is determined
from the gas property factor, which may consist, for example, of
S/v'.sub.x=c.sub.p.rho./.lamda. (see equation (9)), through
correlation.
[0088] In another advantageous embodiment, low pressure is
generated in the gas reservoir, and the gas or gas mixtures flows
under pressure past the microthermal sensor into the gas reservoir,
in which case the volume flow v.sub.xA, determined by the
microthermal sensor calibrated for a specific calibration gas or
gas mixture, is accumulated and compared to the gas volume flowing
into the gas reservoir; from the comparison of the two volumes a
gas property factor S/v'.sub.x, dependent on the physical
properties of the gas or gas mixture, is determined, and in which
the desired physical property or quantity relevant to combustion is
determined from the gas property factor, which may be represented,
for example, by S/v'.sub.x=c.sub.p.rho./.lamda. (see equation (9)),
through correlation.
[0089] In another advantageous variant of the embodiment, the gas
flow is generated by moving a piston.
[0090] The above-described use of a gas reservoir and a
microthermal sensor calibrated for a specific calibration gas or
gas mixture to determine physical properties and/or quantities
relevant to combustion of a gas or gas mixture, or the
corresponding method, in which a gas reservoir and a microthermal
sensor calibrated for a specific calibration gas or gas mixture are
used to determine physical properties and/or quantities relevant to
combustion of gas or a gas mixture, can also be seen as a distinct,
independent invention, which may additionally comprise a measuring
apparatus with an analyzer unit, a gas reservoir and a microthermal
sensor, in which case the analyzer unit is configured for the use
of the gas reservoir and the microthermal sensor to determine
physical properties and/or quantities relevant to combustion of a
gas or gas mixture or to carry out the corresponding method.
[0091] The advantage of the method and measuring apparatus to
determine physical properties and/or quantities relevant to
combustion of a gas or gas mixture pursuant to the present
invention is that three independent measured variables are
available for correlating quantities relevant to combustion. This
makes it possible, on the one hand, to achieve a comparatively high
level of accuracy for determining quantities relevant to
combustion, which otherwise can only be achieved with substantially
more expensive devices; on the other hand, it is possible to
validate the readings and to adjust any deviations.
[0092] Other advantages are apparent from the following
specification.
SUMMARY OF THE DRAWINGS
[0093] The invention is explained in more detail below with
reference to the drawings. In the drawings:
[0094] FIG. 1a shows an exemplary embodiment of a schematic
configuration of a measuring apparatus according to the present
invention (high-pressure variant),
[0095] FIG. 1b shows a variant of the exemplary embodiment shown in
FIG. 1a,
[0096] FIG. 2 shows a second exemplary embodiment of the schematic
configuration of a measuring apparatus according to the present
invention (low pressure variant),
[0097] FIG. 3 shows an exemplary embodiment of a microthermal
sensor for use in a measuring apparatus according to the present
invention, and
[0098] FIG. 4 shows a graphical illustration of the directly
measured density ratio (ordinate) as a function of the correlated
density ratio (abscissa) for various gas groups at standard
conditions (0.degree. C., 1013.25 mbar).
[0099] FIG. 5a shows an exemplary embodiment of a schematic
configuration of a measuring apparatus according to a second
embodiment of the invention (high-pressure variant),
[0100] FIG. 5b shows a variant of the exemplary embodiment shown in
FIG. 5a,
[0101] FIG. 6 shows a second exemplary embodiment of a schematic
configuration of a measuring apparatus according to a second
embodiment of the invention (low pressure variant),
[0102] FIG. 7 shows a graphical illustration of the directly
measured methane content (ordinate) as a function of the correlated
methane content (abscissa) for a binary raw biogas (methane and
carbon dioxide).
[0103] FIG. 8a shows an exemplary embodiment of a schematic
configuration of a measuring apparatus according to a third
embodiment of the invention with a gas reservoir and a microthermal
sensor (high-pressure variant),
[0104] FIG. 8b shows a variant of the exemplary embodiment shown in
FIG. 8a,
[0105] FIG. 9 shows a second exemplary embodiment of a schematic
configuration of a measuring apparatus according to a third
embodiment of the invention with a gas reservoir and a microthermal
sensor (low pressure variant),
[0106] FIG. 10 shows a graphical illustration of the classification
of natural gas mixtures by reference to thermal diffusivity
(ordinate) with simultaneous knowledge of the thermal conductivity
.lamda. (abscissa).
DETAILED DESCRIPTION OF THE INVENTION
[0107] FIG. 1a shows an exemplary embodiment of a schematic
configuration of a measuring apparatus according to the present
invention in which the pressure in the main gas duct 1 is higher
than the critical pressure for the critical nozzle 6 of the
measuring apparatus (high-pressure variation). In the exemplary
embodiment, the measuring apparatus consists, in addition to the
critical nozzle 6, of an analyzer unit 11, which is configured for
performing the method according to the present invention, a gas
reservoir 4, which is equipped with a pressure sensor 8 and a
microthermal sensor 7 to measure the flow and thermal conductivity,
in which case the gas reservoir 4 is connected with the critical
nozzle 6 and the microthermal sensor 7 for the measurements.
[0108] If required, the measuring apparatus may comprise one or
more of the following additional components: a test line 2, which
leads to the gas reservoir 4, and which may be connected to a main
gas duct 1 during operation, an inlet valve 3, which may be
arranged in the test line 2 to control the gas supply to the gas
reservoir, an outlet valve 5, installed on the outlet side of the
gas reservoir to control the flow of gas from the gas reservoir, an
outlet 10 for discharging the gas released from the measuring
apparatus, an additional pressure sensor 8', which may be installed
on the outlet 10, a temperature sensor 9, which is installed in the
gas reservoir, and a compressor 12', which may be installed on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0109] An exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures according to the present invention is described
below with reference to FIG. 1a. In this method, the gas or gas
mixture flows from a gas reservoir 4 through a critical nozzle 6
and past a microthermal sensor 7, with the same mass flow being
applied to the critical nozzle and the microthermal sensor. The
pressure drop in gas reservoir 4 is measured as a function of time
and a first gas property factor .GAMMA.*, dependent on a first
group of physical properties of the gas or gas mixture, is
determined on the basis of the measured values of the pressure
drop, with the first gas property factor being derived, for
example, from a time constant of the pressure drop. A second gas
property factor .GAMMA., dependent on a second group of physical
properties of the gas or gas mixture, is calculated from the flow
signal of the microthermal sensor 7, with the second gas property
factor including, for example, the heat capacity c.sub.p of the gas
or gas mixture, or being dependent on the same. Next, the thermal
conductivity .lamda. of the gas or gas mixture is determined with
the aid of the microthermal sensor 7, and the desired physical
property or quantity relevant to combustion is determined by aid of
correlation on the basis of the first and/or second gas property
factor .GAMMA.*, .GAMMA. and the thermal conductivity.
[0110] Other advantageous embodiments and variants of the method
are described in the preceding sections of the specification. The
following description provides additional details on the method
that may be used if desired.
[0111] Advantageously, the inlet valve 3 and the outlet valve 5 are
opened first to allow the gas or gas mixture that is to be measured
to flow from the main gas duct 1 through the test line 2 and
through the measuring apparatus to ensure that no extraneous gas
from a previous measurement remains in the measuring apparatus. The
inlet valve and outlet valve can be opened via a control unit. In
individual cases, the analyzer unit 11, too, can control the inlet
valve and the outlet valve, as shown in FIG. 1a. In this case, the
outlet valve 5 closes and the gas reservoir 4, the volume content V
of which is known, fills up until the inlet valve 3 is closed.
Pressure p and temperature T in the gas reservoir can be measured
with the pressure sensor 8 or the temperature sensor 9, to ensure
that the standard volume V.sub.norm of the gas or gas mixture
contained in the gas reservoir can be deduced at any time.
V norm = p 1013.25 mbar 273.15 K T V . ( 17 ) ##EQU00019##
[0112] If the pressure p in the gas reservoir 4 is higher than the
pressure p.sub.crit, which is required to critically operate nozzle
6, the outlet valve 5 can be opened again. By preference, the
pressure p in the gas reservoir exceeds p.sub.crit by several bars,
so that the pressure drop reading can be performed during this
phase of overpressure, while nozzle 6 is always operated
critically. Outlet valve 5 now closes again, which concludes the
pressure drop measurement. By preference, pressure sensor 8 is
installed as a differential pressure sensor relative to the outlet
10 of the measuring apparatus. However, it is also possible to
provide an additional pressure sensor 8' at the outlet.
[0113] During the pressure drop reading, the time-dependent
pressure p(t) and the time-dependent temperature T(t) in the
pressure reservoir 4 has been measured and recorded by the analyzer
unit 11. With these data, the time constant .tau. in equation (6)
or the gas property factor .GAMMA.* in equation (7) is determined
in the analyzer unit. At the same time, flow data have been
measured with the microthermal sensor 7, which were recorded in
turn by the analyzer unit to determine the factor S in equation (9)
or the gas property factor .GAMMA. in equation (11). Since the
inlet valve and the outlet valve close after the pressure drop
reading, no gas flows past the microthermal sensor 7 anymore. Now
the measurement of the thermal conductivity reading .lamda. can
take place. The thermal conductivity .lamda., recorded in turn by
the analyzer unit, is determined with the aid of equation (12).
[0114] Now the (optional) validation of the gas property factor
.GAMMA. or .quadrature..GAMMA.* respectively takes place in the
analyzer unit 11. Thereafter, depending on the desired quantity
relevant to combustion Q, the calculation of this value by aid of
equation (15) with the previously determined correlation function
Q.sub.corr=f.sub.corr(.GAMMA., .GAMMA.*, .lamda.) is made.
[0115] If required, it is possible to provide additionally, as
shown in FIG. 1b, a compressor 12', installed, for example, on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0116] FIG. 2 shows a second exemplary embodiment of the schematic
configuration of a measuring apparatus according to the present
invention, which is based on low pressure in the gas reservoir.
This so-called low pressure variant is advantageous, for example,
for the gas supply to end customers. In the second exemplary
embodiment, the measuring apparatus comprises, in addition to the
gas reservoir 4, a pressure sensor 8 on the gas reservoir, an
analyzer unit 11, which is configured to perform a method according
to the present invention, a critical nozzle 6 and a microthermal
sensor 7 to measure the flow and the thermal conductivity, in which
case the gas reservoir 4 is connected with the critical nozzle 6
and the microthermal sensor 7 for the measurement.
[0117] If required, the measuring apparatus may comprise one or
more of the following additional components: a vacuum pump 12
connected to the gas reservoir 4 to generate low pressure in the
gas reservoir, a test line 2 leading to the gas reservoir 4 and
which may be connected with a main gas duct 1 during operation, an
inlet valve 3, which may be located in the test line 2 to control
the gas supply to the gas reservoir, an outlet valve 5, installed
on the outlet side of the gas reservoir to control the flow of gas
from the gas reservoir, an outlet 10 for discharging the effluent
gas from the measuring apparatus, an additional pressure sensor 8',
which may be located in the test line 2 or main gas duct, and a
temperature sensor 9, which is installed in the gas reservoir
4.
[0118] An exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures according to the present invention is described
below with reference to FIG. 2. In this method, the gas or gas
mixture flows under pressure through the critical nozzle 6 and past
the microthermal sensor 7 into the gas reservoir 4, with the same
mass flow being applied to the critical nozzle and the microthermal
sensor. The pressure increase in the gas reservoir 4 is measured as
a function of time, and a first gas property factor .GAMMA.*,
dependent on a first group of physical properties of the gas or gas
mixture, is determined by reference to the measured values of the
pressure increase, with the first gas property factor being
derived, for example, from a proportionality constant of the
pressure increase. A second gas property factor .GAMMA., dependent
on a second group of physical properties of the gas or gas mixture,
is calculated from the flow signal of the microthermal sensor 7,
with the second gas property factor including, for example, the
heat capacity c.sub.p of the gas or gas mixture, or being dependent
on the same; Next, the thermal conductivity .lamda. of the gas or
gas mixture is determined with the aid of the microthermal sensor
7, and the desired physical property or quantity relevant to
combustion is determined by aid of correlation on the basis of the
first and/or second gas property factor .GAMMA.*, .GAMMA. and the
thermal conductivity.
[0119] Other advantageous embodiments and variants of the method
are described in the preceding sections of the specification. The
following description provides additional details on the method
that may be used if desired.
[0120] In a first step, the pressure in gas reservoir 4 is
advantageously decreased to such an extent, for example with a
vacuum pump 12, that the critical nozzle 6 can be critically
operated; in other words, until the pressure in the gas reservoir
is less than half the pressure upstream of the critical nozzle. No
high vacuum is required. As long as the pressure p and the
temperature T can be measured in the gas reservoir 4, it is
possible to calculate the gas standard volume that has flown into
the gas reservoir. However, it is an advantage if the pressure is
by some factor less than required for critical conditions, because
this means that the measurement can consume more time accordingly,
which makes it possible to determine the proportionality constant
more accurately.
[0121] For further details on the methods, which may be used if
necessary, reference is made to the specification of the first
exemplary embodiment, subject to replacement of the term "pressure
drop" by the term "pressure increase", where appropriate.
[0122] FIG. 3 shows an exemplary embodiment of a microthermal
sensor for use in a measuring apparatus according to the present
invention. For example, the microthermal sensor 7 may be--as shown
in FIG. 3--an integrated microthermal CMOS hot-wire anemometer that
is installed in a section 2' of the test line during normal
operation and that can be supplied with a gas or gas mix flow 2a.
The microthermal CMOS hot-wire anemometer comprises a substrate 13,
which typically contains a membrane 14, which measures only a few
micrometers in thickness. Furthermore, the CMOS hot wire anemometer
consists of two thermal elements 15.1 and 15.2 and a heating
element 16, which can be placed between the two thermo-elements in
the direction of the flow. The two thermo-elements 15.1., 15.2
serve to record the resulting temperature generated due to the heat
exchange 15.1a, 15.2a in combination with the gas or gas mixture
flow 2a.
[0123] For further details on the functioning of the CMOS hot wire
anemometer, reference is made to D. Matter, B. Kramer, T. Kleiner,
T. Suter, "Mikroelektronischer Haushaltsgaszahler mit neuer
Technologie" (Micro-electronic domestic gas meters using new
technologies), in Technisches Messen 71, 3 (2004), pp. 137-146.
[0124] FIG. 4 illustrates the directly measured density ratio
.rho./.rho..sub.ref (ordinate) as a function of the correlated
density ratio .rho..sub.corr/.rho..sub.ref (abscissa) for various
gas groups at standard conditions (0.degree. C., 1013.25 mbar), in
which case the correlated density ratio was determined with a
method or a measuring apparatus in accordance with the present
invention. A typical H-gas was used as a reference gas.
[0125] The measuring apparatus described above for determining
physical properties and/or quantities relevant to combustion of a
gas or gas mixture belongs to a new category, namely "Measurement
of the pressure drop or pressure increase in a gas reservoir,
wherein the gas flows through a critical nozzle, as well as
measurement of thermal conductivity and of flow with the aid of a
microthermal sensor, and data validation by summation of the flow
values". The components used are inexpensive, which makes it
possible to develop new markets, where currently no gas quality
sensors are being used for cost reasons. From an accuracy
perspective, only a few limitations compared to more expensive,
commercially available devices are to be expected, since in this
case, too, at least three independent measured variables are being
used for the correlation.
[0126] Furthermore, the invention comprises in a second embodiment
the use of a gas reservoir and a critical nozzle for determining
physical properties and/or quantities relevant to combustion of a
gas or gas mixture, or a method in which a gas reservoir and a
critical nozzle for determining physical properties and/or
quantities relevant to combustion of a gas or gas mixture are used,
wherein the gas or gas mixture flows under pressure from the gas
reservoir through the critical nozzle; in this case, the pressure
drop in the reservoir is measured as a function of time, a gas
property factor .GAMMA.*, dependent on the physical properties of
the gas or gas mixture, which is derived, for example, from a time
constant of the pressure drop, is determined on the basis of the
measured variables of the pressure drop, and a desired physical
property or quantity relevant to combustion is determined from the
gas property factor .GAMMA.* through correlation.
[0127] The second embodiment of the invention described above can
also be seen as a distinct, independent invention.
[0128] FIG. 5a shows an exemplary embodiment of a schematic
configuration of a measuring apparatus according to the second
embodiment of the present invention in which the pressure in the
main gas duct 1 is higher than the critical pressure for the
critical nozzle 6 of the measuring apparatus (high-pressure
variation). In the exemplary embodiment the measuring apparatus, in
addition to the critical nozzle 6, consists of an analyzer unit 11,
which is configured for carrying out a method according to the
second embodiment of the invention, and a gas reservoir 4, which is
equipped with a pressure sensor 8, in which case the gas reservoir
4 is connected to the critical nozzle 6 for measurement
purposes.
[0129] If required, the measuring apparatus may comprise one or
more of the following additional components: a test line 2, which
leads to the gas reservoir 4, and which may be connected to a main
gas duct 1 during operation, an inlet valve 3, which may be located
in the test line 2 to control the gas supply to the gas reservoir,
an outlet valve 5, installed on the outlet side of the gas
reservoir to control the flow of gas from the gas reservoir, an
outlet 10 for discharging the effluent gas from the measuring
apparatus, an additional pressure sensor 8', which may be installed
on the outlet 10, a temperature sensor 9, which is installed in the
gas reservoir, and a compressor 12', which may be located on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0130] An exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures according to the second embodiment of the invention
is described below with reference to FIG. 5a. In this exemplary
embodiment, the gas or gas mixture flows from the gas reservoir 4
through the critical nozzle 6. The pressure drop in gas reservoir 4
is measured as a function of time and a first gas property factor
.GAMMA.*, dependent on a first group of physical properties of the
gas or gas mixture, is determined on the basis of the measured
values of the pressure drop, with the gas property factor being
derived, for example, from a time constant of the pressure drop.
Furthermore, a desired physical property or quantity relevant to
combustion is determined on the basis of the gas property factor
.GAMMA.* by aid of correlation.
[0131] Advantageously, in the second embodiment of the invention,
binary gas mixtures are analysed in regard to their content of the
two components forming the gas mixture, since the gas property
factor .GAMMA.* is intrinsically a continuous function of the gas
content x % or (1-x %). With the knowledge of content x % or (1-x
%), it is then possible to determine physical properties and/or
quantities relevant to combustion of the binary gas mixture from
sets of tables or by aid of corresponding calculation programs. Of
course, it is also possible to directly correlate these physical
properties and/or quantities relevant to combustion of the binary
gas mixture with the gas property factor .GAMMA.*.
[0132] In an embodiment of the method, it is thus possible to
determine the percentage of a component contained in a binary gas
mixture, in which case the variable to be correlated corresponds
either to the percentage of the component in the composition (x %)
and/or any other physical property of the binary gas mixture.
[0133] Other advantageous embodiments and variants of the method
are described in the preceding sections of the specification. The
following descriptions provides additional details on the method
that may be used if desired.
[0134] Advantageously, the inlet valve 3 and the outlet valve 5 are
opened first to allow the gas or gas mixture that is to be measured
to flow from the main gas duct 1 through the test line 2 and
through the measuring apparatus to ensure that no extraneous gas
from a previous measurement remains in the measuring apparatus. The
inlet valve and outlet valve can be opened via a control unit. In
individual cases, the analyzer unit 11, too, can control the inlet
valve and the outlet valve, as shown in FIG. 5a. In this case, the
outlet valve 5 is closed and the gas reservoir 4, the volume
content V of which is known, fills up until the inlet valve 3 is
covered. Pressure p and temperature T in the gas reservoir can be
measured with the pressure sensor 8 or the temperature sensor 9, to
ensure that the standard volume V.sub.norm of the gas or gas
mixture contained in the gas reservoir can be deduced at any
time.
V norm = p 1013.25 mbar 273.15 K T V . ( 17 ) ##EQU00020##
[0135] If the pressure p in the gas reservoir 4 is higher than the
pressure p.sub.crit, which is required to critically operate nozzle
6, the outlet valve 5 can be opened again. By preference, the
pressure p in the gas reservoir exceeds p.sub.crit by several bars,
so that the pressure drop reading can be performed during this
phase of overpressure, while nozzle 6 is always operated
critically. Outlet valve 5 now closes again, which concludes the
pressure drop measurement. By preference, pressure sensor 8 is
installed as a differential pressure sensor relative to outlet 10
of the measuring apparatus. However, it is also possible to provide
an additional pressure sensor 8' at the outlet.
[0136] During the pressure drop reading, the time-dependent
pressure p(t) and the time-dependet temperature T(t) in the
pressure reservoir 4 has been measured and recorded by the analyzer
unit 11. With these data, the time constant .tau. in equation (6)
or the gas property factor .GAMMA.* in equation (6') and the gas
property factor .GAMMA.* in equation (7) or equation (7') is
determined in the analyzer unit.
[0137] Depending on the desired quantity relevant to combustion Q,
this value is now calculated on the basis of equation (15) with the
previously determined correlation function
Q.sub.corr=f.sub.corr(.GAMMA.*) in analyzer unit 11.
[0138] If required, it is possible to provide additionally, as
shown in FIG. 5b, a compressor 12', installed, for example, on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0139] FIG. 6 shows a second exemplary embodiment of the schematic
configuration of a measuring apparatus according to the second
embodiment of the invention, which is based on low pressure in the
gas reservoir. This so-called low pressure variant is advantageous,
for example, for the gas supply to end customers. In the second
exemplary embodiment, the measuring apparatus, in addition to the
gas reservoir 4, comprises a pressure sensor 8, installed on the
gas reservoir, an analyzer unit 11, which is configured for
carrying out a method according to the second embodiment of the
invention, and a critical nozzle 6, in which case the gas reservoir
4 is connected to the critical nozzle 6 for measurement
purposes.
[0140] If required, the measuring apparatus may comprise one or
more of the following additional components: a vacuum pump 12
connected to the gas reservoir 4 to generate low pressure in the
gas reservoir, a test line 2 leading to the gas reservoir 4 and
which may be connected with a main gas duct 1 during operation, an
inlet valve 3, which may be located in the test line 2 to control
the gas supply to the gas reservoir, an outlet valve 5, installed
on the outlet side of the gas reservoir to control the flow of gas
from the gas reservoir, an outlet 10 for discharging the effluent
gas from the measuring apparatus, an additional pressure sensor 8',
which may be located in the test line 2 or main gas duct, and a
temperature sensor 9, which is installed in the gas reservoir
4.
[0141] Another exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
and mixtures according to the second embodiment of the invention is
described below with reference to FIG. 6. In this exemplary
embodiment, the gas or gas mixture flows under pressure through the
critical nozzle 6 into the gas reservoir 4. The pressure increase
in the gas reservoir 4 is measured as a function of time, and a gas
property factor .GAMMA.*, dependent on a first group of physical
properties of the gas or gas mixture, is determined by reference to
the measured values of the pressure increase, with the gas property
factor being derived, for example, from a proportionality constant
of the pressure increase. A desired physical property or quantity
relevant to combustion is determined on the basis of the gas
property factor .GAMMA.* by aid of correlation.
[0142] Other advantageous embodiments and variants of the method
are described in the preceding sections of the specification. The
following description provides additional details on the method
that may be used if desired.
[0143] In a preceding step, the pressure in gas reservoir 4 is
advantageously decreased to such an extent, for example with a
vacuum pump 12, that the critical nozzle 6 can be critically
operated; in other words, until the pressure in the gas reservoir
is less than half the pressure upstream of the critical nozzle. No
high vacuum is required. As long as the pressure p and the
temperature T can be measured in the gas reservoir 4, it is
possible to calculate the gas standard volume that has flown into
the gas reservoir. However, it is an advantage, if the pressure is
by some factor less than strictly required for critical conditions,
because this means that the measurement proceeds during more time
accordingly, which makes it possible to determine the
proportionality constant more accurately.
[0144] For further details on the methods, which may be used if
necessary, reference is made to the specification of the first
exemplary embodiment, subject to replacement of the term "pressure
drop" by the term "pressure increase", where appropriate.
[0145] FIG. 7 illustrates the directly measured methane content
n.sub.CH4 (ordinate) as a function of the correlated methane
content n.sub.CH4 corr (abscissa) for a binary raw biogas, composed
of methane and carbon dioxide, at standard conditions (0.degree.
C., 1013.25 mbar), in which case the correlated methane content was
calculated with a method or a measuring apparatus in accordance
with the second embodiment of the invention. A typical H-gas was
used as a reference gas. The desired variable Q (in this case, the
methane content n.sub.CH4 corr in x %) is advantageously determined
with the aid of the correlation function
Q.sub.corr=a+b.GAMMA.*+c.GAMMA.*.sup.2+d.GAMMA.*.sup.3, in the
illustrated example, numerically as a=-7.82, b=22.7, c=-20.4 and
d=6.45.
[0146] The measuring apparatus described above for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures belongs to a new category, namely "Measurement of
the pressure drop or pressure increase in a gas reservoir, wherein
the gas flows through a critical nozzle". The components used are
inexpensive, which makes it possible to develop new markets, where
currently no gas quality sensors are being used for cost reasons.
From an accuracy perspective, only a few limitations compared to
more expensive, commercially available devices are to be expected,
since in this case only one independent measured value, instead of
three, is used for the correlation.
[0147] In addition, the invention encompasses in a third embodiment
the use of a gas reservoir and of a microthermal sensor calibrated
for a specific calibration gas or gas mixtures to determine
physical properties and/or quantities relevant to combustion of gas
or gas mixtures; in this set-up a gas reservoir and a microthermal
sensor calibrated for a specific calibration gas or gas mixture for
determining physical properties and/or quantities relevant to
combustion of gas or gas mixtures are used, with the gas or gas
mixture flowing under pressure from the gas reservoir past the
microthermal sensor, in which case the volume flow v.sub.xA,
determined by the microthermal sensor calibrated for a specific
calibration gas or gas mixture, is summed up and compared to the
gas volume released from the gas reservoir; from the comparison of
the two volumes, a gas property factor S/v'.sub.x, dependent on the
physical properties of the gas or gas mixture, is determined, in
which v'.sub.x represents the flow rate of the released gas volume
and in which a desired physical property or quantity relevant to
combustion is determined from the gas property factor, which may
consist, for example, of S/v'.sub.x=c.sub.p.rho./.lamda. (see
equation (9)), through correlation.
[0148] The third embodiment of the invention described above can
also be seen as a distinct, independent invention.
[0149] FIG. 8a shows an exemplary embodiment of the schematic
configuration of a measuring apparatus in accordance with the third
embodiment of the invention in case the main gas duct 1 is under
pressure (high-pressure variant). In the exemplary embodiment, the
measuring apparatus consists of an analyzer unit 11, which is
configured for carrying out the method in accordance with the third
embodiment of the invention, a gas reservoir 4, which is equipped
with a pressure sensor 8 and a microthermal sensor 7 to measure the
flow and thermal conductivity, in which case the gas reservoir 4 is
connected to the microthermal sensor 7 for measurement
purposes.
[0150] If required, the measuring apparatus may comprise one or
more of the following additional components: a test line 2, which
leads to the gas reservoir 4, and which may be connected to a main
gas duct 1 during operation, an inlet valve 3, which may be located
in the test line 2 to control the gas supply to the gas reservoir,
an outlet valve 5, installed on the outlet side of the gas
reservoir to control the flow of gas from the gas reservoir, an
outlet 10 for discharging the effluent gas from the measuring
apparatus, an additional pressure sensor 8', which may be installed
on the outlet 10, a temperature sensor 9, which is installed in the
gas reservoir, and a compressor 12', which may be located on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0151] An exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures in accordance with the third embodiment of the
invention is described below with reference to FIG. 8a. In the
method, the gas or gas mixture flows under pressure from the gas
reservoir 4 past the microthermal sensor 7, calibrated for a
specific calibration gas or gas mixture, in which case the volume
flow v.sub.xA is summed up and compared to the gas volume released
from the gas reservoir; from the comparison of the two volumes a
gas property factor S/v'.sub.x, dependent on the physical
properties of the gas or gas mixture, is determined, in which
v'.sub.x represents the flow rate of the released gas volume, and
in which the desired physical property or quantity relevant to
combustion is determined from the gas property factor, which may
consist, for example, of S/v+.sub.x=c.sub.p.rho./.lamda. (see
equation (9)), through correlation.
[0152] In an advantageous embodiment of the method, the thermal
conductivity .lamda. of the gas or gas mixture is determined
additionally with the aid of the microthermal sensor 7.
[0153] Advantageously, with the third embodiment of the invention,
natural gas mixtures are examined as to their classification as
H-gases or L-gases (gases with a high (H) or low (L) calorific
value), since the gas property factor, which may consist, for
example, of S/v'.sub.x=c.sub.p.rho./.lamda. (see equation (9)),
corresponds to the reciprocal value of the thermal diffusivity of
the gas mixture, with the aid of which--together with the thermal
conductivity .lamda., which can be measured separately with the
microthermal sensor--a distinction between H-gas group and L-gas
group can be made.
[0154] The classification of a natural gas mixture as belonging to
the H-gas or L-gas group can be determined, for example, by
identifying the gas property factor (S/v'.sub.x) with the
reciprocal value of the thermal diffusivity c.sub.p.rho./.lamda.,
and wherein the classification is made, subject to thermal
conductivity, on the basis of a limit value for the thermal
diffusivity; above the limit value, a gas mixture is classified as
L-gas, and below the limit value, as H-gas.
[0155] Thus, in an embodiment variant of the method, the thermal
conductivity .lamda. of the gas or gas mixture is determined
additionally with the aid of the microthermal sensor 7, and a
classification of the measured gas as H-gas or L-gas is made in
conjunction with the gas property factor
S/v'.sub.x=c.sub.p.rho./.lamda..
[0156] Other advantageous embodiments and variants of the method
are described in the preceding sections of the specification. The
following description provides additional details on the method
that may be used if desired.
[0157] Advantageously, the inlet valve 3 and the outlet valve 5 are
opened first to allow the gas or gas mixture that is to be measured
flow from the main gas duct 1 through the test line 2 and through
the measuring apparatus to ensure that no extraneous gas from a
previous measurement remains in the measuring apparatus. The inlet
valve and outlet valve can be opened via a control unit. In
individual cases, the analyzer unit 11, too, can control the inlet
valve and the outlet valve, as shown in FIG. 8a. In this case, the
outlet valve 5 is closed and the gas reservoir 4, the volume
content V of which is known, fills up until the inlet valve 3 is
closed. Pressure p and temperature T in the gas reservoir can be
measured with the pressure sensor 8 or the temperature sensor 9, to
ensure that the standard volume V.sub.norm of the gas or gas
mixture contained in the gas reservoir can be deduced at any
time.
V norm = p 1013.25 mbar 273.15 K T V . ( 17 ) ##EQU00021##
[0158] The outlet valve 5 can now be opened again. By preference,
the pressure p in the gas reservoir 4 is higher than the downstream
pressure after the gas reservoir by such a rate that the timespan
in which the gas from the gas reservoir 4 flows past the
microthermal sensor 7 is long enough to ensure that the volume flow
v.sub.xA can be summed up with sufficient accuracy. Outlet valve 5
now closes again, which concludes the flow measurement. By
preference, pressure sensor 8 is installed as a differential
pressure sensor opposite outlet 10 of the measuring apparatus.
However, it is also possible to provide an additional pressure
sensor 8' at the outlet.
[0159] Flow data have been measured with the microthermal sensor 7
during the flow measurement and recorded by the analyzer unit 11 to
determine factor S in equation (9). Since the inlet valve and the
outlet valve close after the flow reading, no gas flows past the
microthermal sensor 7 anymore. Now the measurement of the thermal
conductivity reading .lamda. can take place. The thermal
conductivity .lamda., recorded in turn by the analyzer unit, is
determined with the aid of equation (12).
[0160] With these data, the volume flow is summed up in the
analyzer unit 11 to form volume V.sub.sum and to compare it to the
gas volume V.sub.diff released from the gas reservoir. Based on the
comparison of these two volumes, it is now possible to determine a
gas property factor S/v'.sub.x, dependent on the physical
properties of the gas or gas mixture, in which v'.sub.x represents
the flow rate derived from the released gas volume. For practical
reasons, the volumes for the comparison are converted to standard
conditions for the purposes of the comparison by aid of equation
(17), with the result that v'.sub.x consists of
v'.sub.x=v.sub.xV.sub.diff.sup.norm/V.sub.sum.sup.norm (18)
[0161] with the released gas volume V.sub.diff.sup.norm converted
to standard conditions and the accumulated volume converted to
standard conditions V.sub.sum.sup.norm. Thereafter, depending on
the desired quantity Q relevant to combustion, this value is now
calculated in the analyzer unit 11 with the aid of equation (15)
with the previously determined correlation function
Q.sub.corr=f.sub.corr(S/v'.sub.x), or the value of S/v'.sub.x is
being used to classify, in conjunction with the thermal
conductivity .lamda., a natural gas mixture in the category H-gas
or L-gas.
[0162] If required, it is possible to provide additionally, as
shown in FIG. 8b, a compressor 12', installed, for example, on the
inlet side of the gas reservoir 4 to increase the pressure in the
gas reservoir.
[0163] FIG. 9 shows a second exemplary embodiment of the schematic
configuration of a measuring apparatus according to the third
embodiment of the invention, which is based on low pressure in the
gas reservoir. This so-called low pressure variant is advantageous,
for example, for the gas supply to end customers. In the second
exemplary embodiment, the measuring apparatus comprises, in
addition to the gas reservoir 4, a pressure sensor 8 on the gas
reservoir, an analyzer unit 11, which is configured to carry out a
method according to the third embodiment of the invention and a
microthermal sensor 7 to measure the flow and the thermal
conductivity, in which case the gas reservoir 4 is connected to the
microthermal sensor 7 for the purposes of the measurement.
[0164] If required, the measuring apparatus may comprise one or
more of the following additional components: a vacuum pump 12
connected to the gas reservoir 4 to generate low pressure in the
gas reservoir, a test line 2 leading to the gas reservoir 4 and
which may be connected with a main gas duct 1 during operation, an
inlet valve 3, which may be located in the test line 2 to control
the gas supply to the gas reservoir, an outlet valve 5, installed
on the outlet side of the gas reservoir to control the flow of gas
from the gas reservoir, an outlet 10 for discharging the effluent
gas from the measuring apparatus, an additional pressure sensor 8',
which may be located in the test line 2 or main gas duct, and a
temperature sensor 9, which is installed in the gas reservoir
4.
[0165] Another exemplary embodiment of the method for determining
physical properties and/or quantities relevant to combustion of gas
and mixtures in accordance with the third embodiment of the
invention is described below with reference to FIG. 9. In this
exemplary embodiment, the gas or gas mixtures flows at a pressure
that is typically higher than the downstream pressure after the gas
reservoir by such a rate that the timespan in which the gas from
the gas reservoir 4 flows past the microthermal sensor 7 is long
enough to ensure that the volume flow v.sub.xA can be summed up
with sufficient accuracy. The summed-up volume flow V.sub.sum is
compared to the gas volume V.sub.diff released from the gas
reservoir, and from the comparison of the two volumes, a gas
property factor S/v'.sub.x, dependent on the physical properties of
the gas or gas mixture, is determined, in which v'.sub.x represents
the flow rate of the released gas volume, and in which the desired
physical property or quantity relevant to combustion is determined
from the gas property factor, which may consist, for example, of
S/v'.sub.x=c.sub.p.rho./.lamda. (see equation (9)), through
correlation.
[0166] Thus, in an advantageous embodiment of the method, the
thermal conductivity .lamda. of the gas or gas mixture is
determined with the aid the microthermal sensor 7, and a
classification of the measured gas as H-gas or L-gas is made, for
example, in conjunction with the gas property factor
S/v'.sub.x=c.sub.p.rho./.lamda..
[0167] For other advantageous embodiments and variants of the
method, and for further details on the methods, which may be used
if required, reference is made to the preceding sections of the
specification, subject to replacement of the term "pressure drop"
by the term "pressure increase", where appropriate.
[0168] FIG. 10 illustrates how a classification as H-gas or L-gas
can be made by means of known thermal conductivities X (abscissa)
and thermal diffusivities .lamda./(c.sub.p.rho.), also referred to
as temperature conductivities (ordinate). L-gases above the H/L-gas
separation line typically have higher thermal diffusivities than
H-gases with the same thermal conductivity below the separation
line (double arrow at x.apprxeq.1.024). Since the gas property
factor S/v'.sub.x=c.sub.p.rho./.lamda. is essentially equivalent to
the reciprocal value of the thermal diffusivity of the gas mixture,
it is thus possible to make the distinction between H-gas and L-gas
with the aid of the additionally measured thermal conductivity
.lamda.. All values are shown at standard conditions (0.degree. C.,
1013.25 mbar). A typical H-gas was used as reference gas (dashed
line for the coordinate (1.00,1.00)).
[0169] The measuring apparatus described above for determining
physical properties and/or quantities relevant to combustion of gas
or gas mixtures belongs to a new category, namely "Thermal
conductivity and flow measurement with the aid of a microthermal
sensor, cumulative adding of the flow values and a comparison of
the released volume from a reference volume. Thereafter,
classification of natural gases as H-gas or L-gas". The components
used are inexpensive, which makes it possible to develop new
markets, where currently no gas quality sensors are being used for
cost reasons. From an accuracy perspective, only a few limitations
compared to more expensive, commercially available devices are to
be expected, since this apparatus uses only two instead of three
independent measured variables for the correlation.
* * * * *