Combustion – Timer – programmer – retarder or condition responsive control – By condition of burner feed or feed means
Reexamination Certificate
2000-09-06
2003-03-18
Price, Carl D. (Department: 3743)
Combustion
Timer, programmer, retarder or condition responsive control
By condition of burner feed or feed means
C431S018000, C431S012000
Reexamination Certificate
active
06533574
ABSTRACT:
The present invention relates to a system for active regulation of the air/gas ratio of a mixture of air and fuel gas fed to a burner, using at least one differential pressure measuring system.
BACKGROUND OF THE INVENTION
In many kinds of apparatus and installations in which one or more liquid or gaseous fluids circulate, it is often necessary to be able to measure accurately the flowrate of a working fluid and/or the pressure difference between two different working fluids in order to monitor and/or regulate and/or adjust a process. A differential pressure system is usually employed for this purpose, comprising a differential pressure sensor with two inlets connected to respective pressure ports. In the case of measuring the flowrate of a fluid, the two pressure ports are on respective opposite sides of a diaphragm placed in the pipe in which the fluid flows. In the case of measuring the pressure difference between two different fluids, the two pressure ports are connected to respective pipes in which the two fluids flow. In both cases, the accuracy of the measured flowrate or pressure difference depends on the accuracy of the differential pressure sensor, especially at low flowrates and low differential pressures. For example, in the case of a flowrate measurement, the pressure difference &Dgr;P and the flowrate Q are related by the following equation:
&Dgr;
P=KQ
2
(1)
in which K is a coefficient whose value depends in particular on the density of the fluid whose flowrate is to be measured and on the section of the orifice in the diaphragm placed in the pipe in which said fluid flows. If the instantaneous flowrate of a fluid is to be varied over a wide range, for example in a ratio of 1 to 10, the flowrate of the fluid varies in that ratio but the pressure varies in a ratio of 1 to 100.
In other words, a small variation in flowrate corresponds to a much smaller variation in pressure. The differential pressure sensor used to measure the flowrate must therefore be very accurate and very stable so that it can provide a reliable output value for low flowrates. Differential pressure sensors of this kind exist, but they are extremely costly and therefore cannot be used in apparatus where the total cost of manufacture must remain relatively low, for example in a system for regulating the air/gas ratio of a burner, for example the burner of a boiler for producing domestic hot water and/or central heating hot water.
Also, there are differential pressure sensors which are relatively inexpensive but which are subject to thermal drift and long-term drift which often exceed a few percent. The output signal of such sensors can therefore not be used directly for accurate measurement of the pressure difference over a wide range, for example in a ratio of 1 to 100. If an inexpensive sensor is used, it is therefore often necessary to set the zero of the sensor regularly in order to eliminate the drift referred to above. A conventional solution to this problem uses a measuring system like that shown in
FIG. 1
of the accompanying drawings (see also “Patent Abstracts of Japan”, Volume 009084, date of publication of the abstract Apr. 13, 1985, and Japanese Patent Application JP59212622 in the name of MATSUSHITA DENKI SANGYO, published Dec. 1, 1984).
The differential pressure measuring system shown in
FIG. 1
essentially comprises a differential pressure sensor
1
whose inlet orifices
2
and
3
are respectively connected to a pressure port
4
at which in operation there is a pressure P
1
and to the common channel
5
of a 3-channel valve
6
. The other two channels
7
and
8
of the valve
6
are respectively connected to a pressure port
9
at which in operation there is a pressure P
2
(P
2
≦P
1
) and to the inlet orifice
2
of the sensor
1
via a pipe
11
. In operation the sensor
1
provides at its output
12
a signal which is representative of the pressure difference P
1
−P
2
. That signal is fed to the input of switching means
13
, one output
14
of which is connected to a first memory
15
and another output
16
of which is connected to a second memory
17
. Although two memories
15
and
17
are shown here, the two memories could be separate memory locations of a single memory. The outputs
18
and
19
of the memories
15
and
17
are respectively connected to the positive and negative inputs of algebraic subtractor or adder means
21
which deliver at their output
22
a measurement signal whose value corresponds to the difference between the output signal values from the sensor
1
respectively stored in the memories
15
and
17
.
The valve
6
normally connects the inlet orifice
3
of the sensor
1
to the pressure port
9
and the switching means
13
normally connects the output
12
of the sensor
1
to the input of the memory
15
. Under these conditions, the memory
15
stores the value of the output signal of the sensor
1
, which corresponds to the difference between the pressures P
1
and P
2
. If the pressures P
1
and P
2
are equal, the value of the output signal of the sensor
1
should normally be zero. However, as indicated above, inexpensive differential pressure sensors are often subject to thermal drift and long-term drift. Because of such drift the value of the output signal of the sensor
1
is not always zero when the pressures P
1
and P
2
applied to the two inlet orifices
2
and
3
are equal. Consequently, if the two pressures are different, the value of the output signal of the sensor
1
is subject to an error. That error can be corrected in the following manner. At regular intervals, for example every minute, a control unit
23
sends briefly to the valve
6
and to the switching means
13
, via respective lines
24
and
25
, control signals which momentarily switch the valve
6
to a state such that it disconnects the inlet orifice
3
of the sensor
1
and the pressure port
9
and connects the inlet orifices
2
and
3
of the sensor
1
and momentarily switch the switching means
13
to a state in which they connect the output
12
of the sensor
1
to the input of the memory
17
. Under these conditions, the same pressure P
1
is applied to the two inlet orifices
2
and
3
of the sensor
1
and any measurement error of the sensor
1
is stored in the memory
17
. The subtractor means
21
subtract that error from the value of the output signal of the sensor
1
stored in the memory
15
. Thus the measurement error of the sensor
1
is periodically updated in the memory
17
and a corrected measurement signal is obtained at the output
22
of the subtractor means
21
whose value corresponds to the exact value of the difference between the pressures P
1
and P
2
. The components
13
,
15
,
17
and
22
therefore form a measurement circuit
26
which, in combination with the 3-channel valve
6
and the control unit
23
, enables automatic setting of the zero of the sensor
1
.
The prior art differential pressure measurement system described with reference to
FIG. 1
is entirely satisfactory from the point of view of setting the zero of the sensor. However, it has the drawback of using a 3-channel valve, which is a relatively costly component.
Differential pressure measuring systems of the type described above can be used in systems for regulating the air/gas ratio of a boiler burner. Systems for regulating the air/gas ratio are described in the Japanese Publication already cited, for example, and in the report published by the Association Technique de l'industrie du Gaz en France [French Gas Industry Technical Association], on the occasion of the 113
th
Congress du Gaz [Gas Congress], held in Paris on Sep. 10-13, 1996, “Receuil des Communications” [“Proceedings”], Volume 2, pages 245-251, in the article “Régulation active du rapport air/gaz d'un brûleur” [“Active regulation of the air/gas ratio of a burner”] by C. PECHOUX et al. The system for regulating the air/gas ratio described in the aforementioned Japanese Publication uses a single differential
A Theobald SA
Price Carl D.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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