[0001] The invention relates to a gas burner particularly, but not exclusively, for a gas
cooker with spark ignition.
[0002] Spark ignition by means of an in situ spark unit is a very desirable method of lighting
gas burners, but the ratio of gas to air in the primary mixture normally supplied
to a gas burner is controlled according to stoichiometric considerations in the interest
of" efficiency of combustion, and. such a mixture is too gas-rich for ready ignition
by an electric spark.
[0003] It has been proposed to place the spark unit at a position spaced from the burner
head in a direction of the issuance of gas-air mixture from a burner port, at which
position the primary mixture has become so diluted with the so- called "secondary"
air of the atmosphere as to form a more explosive or ignitable mixture. However, when
the position has been exterior of the burner head, the spark unit has been susceptible
to gross contamination from accidental spillage resulting in the unit not being able
to pass a spark. This problem has been mitigated, in the case of a toroidal burner,
by placing the spark unit interiorly of the burner and by providing an ignition port
in the burner head to produce a jet of primary gas-air mixture, in the direction of
the unit. Contamination of the spark unit has not been completely eliminated however
and the further problem has not hitherto been solved that under low gas-flow conditions
the mixture in the region of the spark unit may not be sufficiently gas-rich for ignition.
[0004] Hitherto, a variety of materials have been used for the manufacture of gas burner
bodies including, for spark ignition burners, electrically insulating materials, but
the search for new materials for this purpose has concentrated on such as have good
resistance to thermal shock because of the high risk of spillage of relatively cold
liquids onto a hot burner body, and it was totally unexpected to find a material could
be used for a burner body which had poor resistance to thermal shock.
[0005] According to one aspect of the invention there is provided a gas burner head comprising
a burner body made at least substantially, and preferably of between 75% and 98%,
of alumina. Preferably the head also comprises at least one pair of electrodes spaced
apart by the body and providing a spark gap therebetween. One of said electrodes may
be covered with a cap of ceramic glass so that a user of the burner may be protected
from electric shock.
[0006] According to a further aspect of the invention there is provided a spark ignition
gas burner comprising a burner head body of electrically insulating material formed
with a plurality of passageways for the passage of combustible air-gas mixture therethrough
and each terminating in a flame port, and first and second electrode means connected
or connectible to spark generating means and positioned relative to said body whereby
in use of the burner a spark may be produced in the vicinity of one or other of said
ports.
[0007] The burner head body may comprise a ring of ceramic material such as alumina defining,
at least in part, a wall of a chamber connectible to a source of gas and air, said
passageways comprise vertically elongate slots opening at the upper surface of the
body and extending radially from the interior of the chamber to said ports.
[0008] Preferably the body is supported on the first electrode means and is surmounted by
the second electrode means whereby a spark may pass between the electrode means.
[0009] The burner may further comprise a feed pipe for feeding air-gas mixture from said
source to said chamber and having a restricting throat at which the cross-sectional
area is less than, and preferably only between 35% to 45% of, the sum of the cross-sectional
areas of the said ports.
[0010] Embodiments of the invention will now be described by way of example and with reference
to the accompanying drawings of which:-
Fig. 1 is a vertical section of a burner according to the invention.
Fig. 2 is a perspective and exploded view of the assembly of electrodes and burner
head body of the embodiment shown in Fig. 1 (but showing a smaller number of slots
for simplicity).
[0011] In the drawings the burner comprises a feed pipe 1 and a burner head 2, the pipe
1 being adapted for connection to a source of gas at elevated pressure and being provided
with air inlets 3 whereby air can be introduced into the pipe to provide a primary
mixture of air and gas. The burner head comprises a chamber 4 of which the upper portion
is defined in part by lower electrode 5. The electrode comprises a lower cylindrical
wall 6, an upper low peripheral rim 7 and an intermediate conical portion 8.
[0012] The lower electrode 5 supports a burner head body 9 of annular construction which
is provided with a plurality, in this embodiment, forty, slots 10 which extend radially
from the inside of the annulus to flame ports 19 at the outer edge thereof and downwardly
from the upper surface 11. The radially outer surface 12 of the burner head body and
the rim 7 of the lower electrode taper slightly inwardly and upwardly so tht the body
may be a captive fit within the rim 7 and the lower surface of the body is angled
to correspond with the conical portion 8 so that the body may be supported by the
electrode as shown in Fig. 1. Upper electrode 13 is essentially a disc pressed out
of sheet material and having a peripheral rim 14. The upper electrode also has an
annular depression 16 corresponding in diameter to the inside diameter of the burner
head body 9 so that when assembled as shown in Fig. 1 the depression 16 located against
the radially inner surface of the body.
[0013] The body 9 is made of a material which comprises between 80% and 94% of alumina,
though the proportion may range from about 75% to 98%. Although this material is generally
known to have poor resistance to thermal shock it is surprisingly found that at least
in the construction described it is capable of withstanding very severe shock and
it is thought that this may be due to the radial slots allowing for expansion and
contraction without excessive build-up of strains within the alumina body.
[0014] The assembly is surmounted by a cap 17 of ceramic glass such as CERAN or ALPHASTAE
(Trade Marks) or other suitable material to protect the upper electrode from contamination
and to provide electrical insulation for the safety of a user of the cooker. The cap
is formed with a flange 18 which engages the flange 14 of the upper electrode.
[0015] Each of the electrodes is formed of electrically conducting material such as sheet
metal, and the lower electrode 5 is in electrical contact with the burner feed pipe
1 through which it may be connected electrically to the cooker or other appliance
with which it is adapted to be used. The upper electrode is threaded on to a stem
20 (not shown in Fig. 2) which is mounted in a block 21 of insulating material which
in turn is secured by sleeve 22 and nut 15 in a depression la in the lower face of
the feed pipe 1. The stem 20 terminates in terminal 23 which is connectible to a spark
generating device of any suitable type.
[0016] Suitable types of spark generating devices include 1- shot or single-spark, continuous
spark, and re-ignition types each of which may be supplied from electrical mains or
battery and some of which may be alternatively powered by piezo-electrical means.
The spark generator. is connected between terminal 23 and the feed pipe 1 of the burner,
and thus the electrical potential of the generator exists between the upper electrode
13 and the lower electrode 5.
[0017] The closest approach of these electrodes, namely between rim 7 and the foot of flange
14, presents in effect a continuous annular spark gap. In theory, a spark may pass
between the electrode at any position around the burner but in practice it has been
found that sparks normally pass down the outer face 12 of the body 9 intermediate
an adjacent pair of ports 19 where the combination of primary mixture and secondary
air provide an optimum ignitable mixture. Successive sparks may be located in the
vicinity of the same pair of slots but it is more likely that successive sparks will
be found adjacent different pairs. Contamination of surface 12 presents no problem
and indeed tends to produce an even better spark, but should distortion of an electrode
reduce the chance of a spark adjacent one port there are, in this embodiment, a further
39 ports near which a spark can pass. In other embodiments a fewer or even more slots
may be provided around the circumference of the burner head body.
[0018] If moisture were to be deposited on the outer face 12 of the burner body, for example
by condensation, and if the foot of flange 14 were in contact with the upper edge
of face 12, the possibility would arise of a leakage of electricity between the electrodes
via the wet face. Therefore, as shown in Fig. 1, a slight gap 24 is provided between
the underside of electrode 13 and the upper face of body 9 so that the path length
for leakage over a wet surface is increased by the depth d of the gap 24.
[0019] Because the slots are open at the top it is a relatively easy matter to clean the
slots of any contaminants, and for this purpose the assembly of upper and lower electrodes
and the burner head body can be removed readily from the burner. The slots may be
cleaned by brushing.
[0020] In order to ensure a maximum ignition performance, the supply pipe 1 is provided
with a venturi throat 26 of which the internal cross-section is between 35% and 45%
and preferably of the total cross-section area of all the flame ports. The throat
also ensures optimum conditions for aeration when high primary air is called for such
as in the use of liquified petroleum gas.
[0021] It is desirable for the sake of optimum ignition performance that the air inlets
3 are non-adjustable and that gas injector 25 is consistently mounted within gas feed
pipe 1.
[0022] It should be understood that the electrical connections to the electrodes could be
reversed so that sparks pass between the electrodes in the opposite direction.
1. A burner body for a gas burner head, said body being made at least substantially
of alumina.
2. A burner body according to Claim 1 comprising between 75% and 98% of alumina.
3. A burner body according to Claim 1 further comprising at least one pair of electrodes
spaced apart by the body and providing a spark gap therebetween.
4. A burner body according to Claim 3 wherein one of said at least one pair of electrodes
is covered with a cap of ceramic glass.
5. A gas burner with spark ignition comprising a burner head body of electrically
insulating material formed with a plurality of passageways for the passage of combustible
air-gas mixture therethrough and each terminating in a flame port, and first and second
electrode means connectible to spark generating means and positioned relative to said
body whereby in use of the burner a spark may be produced in the vicinity of one or
other of said ports.
6. A gas burner according to Claim 5 wherein the body comprises a ring of ceramic
material such as alumina defining, at least in part, a wall of a chamber connectible
to a source of gas and air.
7. A gas burner according to Claim 5 wherein said passageways comprise vertically
elongate slots opening at the upper surface of the body and extending radially from
the interior of the chamber to said ports.
8. A gas burner according to Claim 5 wherein the body is supported on the first electrode
means and is surmounted by the second electrode means whereby a spark may pass between
the electrode means.
9. A gas burner according to Claim 5 and further comprising a feed pipe for feeding
air-gas mixture from said source to said chamber and having a restricting throat at
which the cross-sectional area is less than the sum of the cross-sectional areas of
the said ports.