Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
. Field of the _invention .
Thi-~ invention relates. to a ~lame detecting apparatu.~,
more paraticulary to a novel- flame detecting apparatus in which
a flame detecting electrode and an.auxiliary electrode are disposed
in a flame.
.
. DESCRIPTION OF THE PRIOR ART
. .
In many application~ domestic, commercial a~d industrial,
flame combustion of a fuel is used as a source of heat. It is
e~sential, in the intere~ts o~ ~afety, that there should at no time
!
1C~7~
.
be an accwnulation of unburnt gases in the combustion chamber (such
as will occur on flame failure) which may be accidentally ignited
and cause an explosion. Therefore, it is necessary to have some
means for detecting and giving an indication of flame failure, the
means preferably stopping the supply of fuel to the combustion
chamber. Many types of flame detector are available to indicate
flame failure and prevent the build-up of this potentially hazardous
condition. These flame detectors are usually based on one of the
following principles:
.O 1) thermostat e~fect;
2) action of light sensitive thermionic tubes; and
3) electrical properties of flame gases.
The flame detectors based on either of the first principles are
subject to fundamental problem~. Th~rmostats hav~ ~ slow response
time because of the finite time between flame failure and the
detection of cooling which gives an indication of the flame failure.
On the other hand, light sensitive thermionic tubes require delicate
and expensive amplifying means. These devices also require accompa-
nying fault detection equipment to ensure that they are operating
O properly.
The third principle on which flame detectors have been
based involves making use of the electrical properties inherent in
flame gases, for example elec~rical conductivity or rectification;
A flame detector using the electrical conductivity or rectification
action of a flame has a ~oltage of several hundred volts AC applied
between a flame detecting electrode and a burner, and a minute
current which is caused to flow through the flame between the
detecting electrode and the burner is amplified by an amplifier
circuit of high input impedance which employs a field-effect tran-
0 sistor or the like. Alternatively, a light emitting device, such
- 2 -
as a neon tube, is caused to emit light by the use of the minute
current, and a photo-conductive device is operated by the emitted
light. The applied voltage may, for example be 250 volts AC,
with the detected current only being some 4 microamps.
In another type of flame detector using the third prin-
ciple, only the detecting electrode inserted into the flame is
used. This type uses the fact -that some of the atoms or mole-
cules in a flame are thermally ionized by the high temperature,
that is, there are many positive ions of H30+ in the top region
of the flame and many negative ions of Hor in the bottom region
of the flame. Also many electrons produced by the the~mal ioniza-
tion are present in the middle region of the flame. I~ may be
said, therefore that the flame is an electrical conductor, al-
though with a very large impedance. When an electrocle is dis-
posed in the flame, the electrons are caught by the electrode
one by one, so that a current flows through the flame, the elec-
trode being charged to negative potential. This negatlve poten-
tial is used as a detecting signal. However, generally, the
detecting voltage is very small, for example, 0.6 to 0.8 volts,
and the current is also very small, for example 50 to 120 nano-
amps.
With the known flame detectors therefore a quick and
reliable response cannot be obtained, because the detecting sig-
nals are so small.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide
a flame detecting apparatus which is free from the disadvantages
of the known apparatus described above.
Another object of the invention is to provide a flame
detecting apparatus in which a detecting and an auxiliary elec-
trode are disposed in a flame, and one of electrodes in biased
., .
-- 3 --
7~
to a DC potential different from tha-t of the other electrode.
Still another object of the invention is to provide
a flame detecting apparatus in which a detecting and an aux~
iliary electrode are disposed in a flame, and a negative biasing
means is connected to the auxiliary electrode, whereby certain
flame detection and quick response are obtained.
Still anoth~r object of the invention is to provide
a flame detecting apparatus in which a detecting andan aux-
iliary electrode are disposed in a flame, and a biasing means
is provided such that the DC potential of the detecting elec-
trode is higher than that of the auxiliary electrode, whereby
a relatively large detecting voltage and current is obtained.
More particularly, there is provided: flame detecting
apparatus comprising a detecting electrode disposed in a flame
position; an auxiliary electrode disposed in said flame position ?
at a spacing from said detecting electrode; first means for
biasing said detecting electrode and said auxiliary electrode
with different DC potentials; and secand means to derive a
signal from said detecting electrode which signal indicates
whether a flame is present in said flame position or not, where-
in said first means energizes said auxiliary electrode with
a positive potential relative to said detecting electrode.
~et another object of the invention is to provide a
; flame detecting apparatus in which two electrodes are disposed
,
in a flame, and a biasing voltage source is provided, so that
the flame detecting apparatus is easily constructed and a stable
output is obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
.
Further objects, features and advantages of the inven-
tion will become apparent from the following description given
by way of example, with reference to the accompanying drawings,
; in which: ;;;
X, - ~L - '~ '
" "' . ':, ~, ~' ~ :
1~37(J7~3
Figures 1 and 2 are diagrammatic views respectively of
first and second embodimen-ts o~ the
invention;
Figures 3 and 4 are graphs of detected voltages and
current plotted against gas flow for
the first and second embodiments re-
spectively; . .
Figures 5 and 6 are diagrammatic views respectively
of third and fourth embodiments of
the invention; and
Figure 7 is a diagrammatic view with a block
diagram of a fifth embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODI~ENTS
-
Referring now to the drawings, Figure 1 shows a Elame
detecting apparatus in which a detecting electrode 1 is disposed
in a flame 2 from a grounded burner 3. An auxiliary electrode 4
is also disposed in the flame 2, but positioned below the de-
tecting electrode 1. The DC potential of the detecting elec-
trode 1 is maintained higher than that of the auxiliary elec-
; trode 4, the auxiliary electrode 4 being biased to a negative
potential by a DC voltage source 5. An indicator such as a volt-
meter 6, or an ammeter, is connected between the detecting elec-
trode 1 and ground.
Some of the atoms or molecules in the flame 2 are
thermally ionized by the high temperature, so there are many
positive ions of H30+ which have lost electrons in the top ;~
region of the flame 2 and many negative ions of HO in the bottom
region of the flame 2. Also, there are many electr~ns el pro-
duced by the thermal ionization in the middle region of the flame
: 2. The electrons el are caught by the detecting electrode 1
.
- 4a -
. ~ ' , '
7~7~
so that a current Il flows through the flame 2 :Erom the elec-
trode 1 to the burner 3, which means that the detecting elec-
trode 1 is charged to a negative potential. Also, since the
auxiliary electrode 4 is substantially heated by the flame 2,~
many thermal electrons e2~ are discharged therefrom, and are
caught by -the detecting electrode 1. Thus, a second current
I2 flows through the flame 2 from the detecting electrode 1 to
the axuiliary electrode 4, so -the detecting electrode 1 is still
further charged to a negative potential. Thus a large current
flows through flame 2 and a large voltage is developed at the
detecting electrode 1.
Figure 2 shows a second embodiment. In this case,
detecting and auxiliary electrodes 11 and 14 are disposed in a
flame 12 as in the first embodiment, but the de:tectlng electrode
11 is positioned under the auxiliary electrode 14, which is
biased with a negative potential relati.ve to the detecting
electrode 11 by a DC voltage source 15. Numerals 13 and 16 ~:~
designate the burner and an indicator, respectively.
Figure 3 shows graphs of the detected negative volt~
age plotted against the gas flow in litres per minute with a
constant air supply for the first and second embodiments shown
in Figures 1 and 2. Curves (a), (b) and (c) show changes of
detected voltage when the source 5 (Figure 1) has various volt-
ages (E), namely; E = -40, -30 and -20 volts. Curves (d), (e),
(f), (g) and (h) show changes of detected voltage when the :
; source 15 (Figure 2) has various voltages (E), namely; E = -50, ~ ~.
-40, -30, ~20 and -10 volts. Additionally, a curve (i) shows
changes of a detected voltage in a prior art apparatus in which
only the detecting electrode is inserted into a flame.
Figure 4 shows graphs of the detected current plotted
against the gas flow for the first and second embodiments.
; . . .
. , .
79~3
Curves (b), (d), (e), (f), (g), (h) and (i) correspond to
curves (b), (d), (e), (f), (g), (h) and (i) in Figure 3. It
can be seen from the curves (b) shown in F:igures 3 and 4, that
when -30 volts is applied to the auxiliary electrode 4 in the
first embodiment, -16 volts and 17 microamps is detected.
Similarly, curves (d) shown in Figures 3 and 4, show that when
-50 volts is applied to the auxiliary electrode 14 of the
second embodiment, -15.5 volts and 13.8 microamps is detected.
Thus, the detected voltage and current are very large compared
with those shown by the curve (i) for the prior art apparatus.
As shown in Figures 3 and 4, the detected voltage
and current with the first embodiment in which the auxiliary
electrode 4 is placed under the detecting electrode l are
larger than those obtained wi-th the second embodiment. It is
thought that, in the first embodiment oE Figure 1, two kinds
of electrons el and e2 flow towards the top region of the
flame 2. That is, both kinds of electrons el and e2 flow
easily towards the positive ions which exist in the top region
of the flame 2, and are assisted by the flow of gases from the
bot-tom to the top of the flame 2.
Figure 5 shows a third embodiment. Detecting elec-
trode 21 and auxiliary electrode 24 are disposed in a flame 22.
The auxiliary electrode 24 is disposed under the detecting
electrode 21, but is energized by a positive voltage source 25.
Numerals 23 and 26 designate the burner and an indicator,
respectively. In this embodiment, when +30 vol-ts is applied
to the auxiliary electrode 24, -~3 volts is detected at and 0.5
microamps flows to the detecting electrode 21 with a gas flow
of 13 litres per minute.
Figure 6 shows a fourth embodiment. In this case, an
auxiliary electrode 34 is positioned above a de-tecting electrode
7~313
31 and is energized by a positive voltage source 35. An
indicator 36 is provided. When the auxiliary electrode 34
is biased with +30 volts, -~4 vol-ts is detected atSand 0.5
microamps flow to the detecting electrode 31~
Thus, in the third and fourth embodiments, a low
positive voltage is detected at the respective detecting elec-
trodes. It isthought that, in the third embodiment shown in
Figure 5, the detecting electrode 21 is charged with negative
potential as mentioned above, but orl the other hand, a current
I2 flows through the flame 22 from the auxiliary elec-trode
24 to the detecting electrode 21, as indicated, due -to the
positive source 25, so that a positive potential is developed
at the detecting electrode 21. This positive potent:ial over-
comes the above-mentioned negative potential, so that a small
positive potential appears at the detecting electrode 21.
A preferred ma-terial for the detecting electrode ;;
comprises 0.1% barium, 0.2% magnesium, 0O1% carbon and the
balance nickel. It is possible to use more than one element,
having work function less than 3 eV from group IIa of the
periodic table, for example, magnesium, calcium, strontium
or barium. The work functions of all these elements is less -
than 3 eV, so an electrode comprising one or more of these
elements can easily gather the electrons el~ produced by
thermal ionization, so that a substantial negative potential
is developed at the detecting electrode. Other examp].es of
materials for a detecting electrode are as follows~
1. Chromium 12 to 15%, silicon less than 0.5%, carbon
:,~ .
less than 0.15%, strontium 0.1%, copper 0.1% and the
balance nickel.
2. Chromium 23%, aluminium 6%, cobalt 2%, carbon less
than 0.1%, strontium 0.1% and the balance iron.
7~37~l3
3. Chromium 18%, nickel 8~, silicon less than 1%, man-
ganese less than 2~, carbon less than 0.03%, stronium
1%, calcium 1% and the balance iron. All the above
percentages are by weight.
Figure 7 shows a fifth embodiment. In this case,
detecting and auxiliary electrodes 41 and 4L4 are disposed in a
flame 42 from a burner 43, the auxiliary electrode 44 being
biased with a negative potential by a negative DC voltage
source 45. In this embodiment, an additional biasing source
47 is connected to the detecting electrode 41 so as to bias
~he detecting electrode 41 with a positlve DC potential. With
this embodiment a large detected signal is obtained and is
applied to an amplifier 48, the output of which is supplied to
a drive circuit 49 which may include a suitable switching
element and relay. The outpu-t oE the drive circuit 49 controls
a valve 50 in a gas supply pipe 51. If the flame 42 goes out,
no output is applied to the valve 50 and no gas is supplied to
burner 43.
Although described in relation to a gas flame, the
invention can of course be used with flames produced by other
~uels.
Moreover, other modifications and variations will be
apparent to those skilled in the art and are included in the
scope of the invention which is defined by -the appended claims.
