Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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GAS HOB
This inven~ion relates to a gas hob. An appliance
with such a hob is used for delivering heat to the base of
cooking utensils and is known variously as a "hob unit",
"boiling top", "hot plate", or "boiling table", being used
either domestically or in commercial cateri~ng food
preparation.
Recent developments of electrically heated hob units
have led to the extensive use of ceramic glass cover
plates, of a type known as "Ceran", a Trade Mark of SCHOTT
GLASSWERKE, OF MAINZ, WEST GERMANY, over the various types
of heating elements in use and with the heat being
transferred to the pan through the glass by radiation and
also by conduction from the glass directly to the base of
the pan.
Attempts to utilise gas heating for similar ceramic
glass top hobs have been less successful because whilst the
advantages of easy cleaning etc, as for instance described
in U.X. patent no. 1,419,499, can be achieved, the speed of
heating, ease of control and thermal efficiency, when
compared to an open flame gas burner used in a traditional
type gas hob, are quite inferior.
It has been shown when utilising tests commonly used
to compare gas hobs, as for instance detailed in Bxitish
Standard 53~6 part 3, that thermal efficiencies of only 32%
can be obtained with ceramic glass tops, compared to open
flame cookers which easily exceed 50%. Similarly when
heating speeds were compared, although the ceramic glass
gas heated top achieved roughly similar times to
electrically heated hobs, it was at best 25% slower than
would be accepted for a conventional gas heated hob.
One of the most important functional aspects of a
gas hob is its ability to be acljusted quickly and easily to
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vary the heat input to a wide range of cooking
requirements, and this has in the past been seen as a
significant advantage for the open flame hob over all
others. The amount of heat being supplied can be easily
judged by the appearance of the flames.
It is widely known that radiant heating burners will
not operate well at gas inputs much less than their
optimum, due to flame instability. A minimum heat input no
less than 60% of the maximum is the least that can commonly
be achieved. It is also widely known that the radiant
output of such burners reduces rapidly when their gas input
and operating temperature is reduced only slightly. This
is due to the reduction in primary aeration- when gas
pressure is reduced and has led to such partial solutions
as multiple injector jets (British patent application GB
2201 506A).
Because of these factors, and because a cooker hob
burner must have a very wide range of outputs so that small
pans can also be controlled at a low temperature, it has
been common practise to use a pulsed on/off system whereby
~he ratio of burner time on (at full rate) to burner time
off sets the heat input.
Since cooker hob burners are usually adjus~ed to
below their maximum heat outputs for most cooking demands,
it follows that they are nearly always operating at a much
reduced efficiency and that the lower they are set, the
lower the efficiency becomes.
When comparing the controllability of various types
of hob units, it is found that the ability rapidly to
'lose' heat input to the pan is quite as important as the
ability to increase heat input quickly. In this respect,
since the ceramic glass hob has considerable thermal mass
when particularly compared with open flame burners, it is
at some disadvantage, although the perforrnance is similar
to electric heated ceramic hobs, which in turn have
somewhat poorer performance than conventional tubular
element electric heated hobs.
The amount of heat that is transmitted from one body
to another is greatly affected by the distance between
them. However, with gas burners in closed hobs this
minimum is set by the space above the burner necessary for
the gaseous products of combustion to escape and also for
any secondary combustion air to reach the burner flames.
This factor also limits the maximum area burner which can
be used.
An object of the present invention is to provide a
gas hob burner system which overcomes or at least
significantly reduces these problems and enables a gas
heated hob to compete much more effectively with the
traditional open flame burner type and still to maintain
the good appearance, ease of cleaning and other advantages
of the electric heated glass ceramic hob.
According to the present invention a gas hob
comprises a glass ceramic top plate, at least one gas fired
heat radiating burner unit arranged closely below said top
plate, the or each burner unit having a multiplicity of
chambers over which is disposed a ceramic burner plate
perforated to match the pattern of said chambers,the
chambers of each burner unit being in communication with
respective associated air supply passages,an inlet to each
passage having an orifice of a predetermined size to govern
the amount of air entering the passage,in use, gas supply
means, fan means arranged to supply air through said air
supply passaye inlets to all the chambers of a burner unit
all times when gas is supplied to at least one chamber of
said burner unit,supplied gas and air mixing prior to
entering one or more of said chambers and thereafter one or
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more of said burner perforations, and means for supplying
gas only to the or each burner plate perforation at which
supply of heat is required.
The invention will now be described, by way of
example, with reference to the accompanying drawings, in
which:
Figure 1 is a schematic top plan view of a gas
heated ceramic hob of the invention, with a top glass
ceramic plate removed,
Figure 2 is an enlarged, fragmentary schematic view
on a section through the hob of Figure 1 r
Figure 3 is a similar view to Figure 2, but showing
detail of a gas flow passage to a burner unit, and the
burner unit itself, with the gas flow passage receiving gas
from a control valve,
Figure 4 is an enlarged internal top view of a fluid
flow control valve,
Figure 5 is an enlarged, top view of one of the
burner units, with associated control valve and gas flow
passages, of the hob of Figure 1, and
Figure 6 is a diagrammatic view of an ignition
system, a flame failure system and an overheat system of
the hob of the invention.
A hob unit of the invention, which is shown
schematically in Figure 1, has a body 10 of generally
rectangular configuration, there being at the one of its
longer sides constituting a front lOa of the hob unit three
gas flow control valves 11, 12, 13, with associated control
knobs 14, 15, 16 respectively.
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Within the body are a number of gas-fired heat
radiating surface combustion burner units, in this example
three, namely a small diameter burner unit 17 and two
larger diameter burner units 18, 19 respectively. The
burner units 18, 19 are arranged at the front of the gas
hob adjacent the left and right sides thereof respectively,
being controlled, as will be described, by the control
valves 11 and 13 respectively. The burner unit 17 is
positioned towards the rear of the gas hob and slightly to
the left of the centre of the burner, being controlled by
the control valve 12, which is disposed between valves 11,
13 along the front of the gas hob.
Disposed in the gas hob body below the level of the
burner units is an electrically driven fan 20 of common
type. The fan has its output volume controllable either by
varying its speed or by varying its input or output
orifice. The fan supplies air, in use, to a planum chamber
21 which is in communication with respective sets 22, 23,
24 of burner unit gas/air supply ducts for supplying the
burner units 17, 18, 19.
As best shown in Figures 2 and 3, the hob body 10
has its top closed by a glass ceramic plate 25, the burner
units and the control valves (only one of each shown) being
disposed below the plate 25 on a supporting surface 26.
The burner units are all received closely below the plate
25, e.g. 10-20mm. A vertical wall 27 separates the control
valves from their respective associated burner units. The
hob has means at its rear for removal of gaseous combustion
products.
The fan 20 is disposed between the surface 26 and a
base surface 28 of the hob body and the sets of supply
ducts are at the same level as the fan. As shown in
Figures 1 and 5, each set comprises four or seven parallel
supply ducts in this example. At one end, each duct 29 has
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an orifice plate 30 carrying a plug 30a with an orifice
therethrough to provide communication with the plenum
chamber 21 which extends around each set of supply ducts.
A short way inwardly from ~he plug, each duct has a gas
nozzle 31 extending downwardly into it to supply gas to the
duct from one of the control valves.
Each burner unit is made up of a lower plate 32
formed with a number of concentric chambers 33, for example
seven chambers for the larger burner units. Fitted on top
of the plate 32, via a gasket 34, is a ceramic radiant heat
emitting burner plaque or plate 35 which is perforated to
match the arrangement of the chambers to provide a number
of concentric gas burning rings at its top surface closely
below the glass ceramic plate 25. Burner plaques of this
form are disclosed in West German Auslegeschrift no.
1116615, to which reference may be made. A spark
electrode 36 is brought up through a centre hole in the
plates 32 and 35, so as to be able to ignite the inner
burner ring as a pilot light.
The other ends of the ducts in each set are upwardly
open to communicate with the concentric chambers 33
respectively. Supply of gas to the sets of ducts is
governed by the control valves~ as will now be described.
Figure 4 shows in detail the control valve 11 of the
control valves 11, 12, 13 shown in Figures 1 to 3. The
valve has a die cast metal body with a separate die cast
metal cover (not shown) which is screwed to the body, with
a gasket therebetween for a gas tight seal. A gas
enclosure is thus formed within the valve.
The body is divided by a wall 37 into a pilot gas
chamber 38 and a larger main gas chamber 39. The two
chambers 38, 39 have respective gas inlets 40, 41 through
the valve body. At the exterior of the wall of the valve
body normal to the inlet 40, there is fitted the control
knob 14 which can be of conventional form. At the inner
end of a spindle 42 rotatable with the knob is a bevel gear
43 in mesh with a further bevel gear 44 on an end of a cam
shaft part carried on a cam shaft centre spindle 45
journalled at and extending through a gas seal at the wall
37. The other end of the cam shaft spindle 45 is
journalled at an end wall 39a of the chamber 39 through
which extends the inlet 41.
In the bottom of the chamber 38 is an outlet in the
form of a gas nozzle 31 previously described. A kinked
spring wire 46 is secured at its one end to the bottom of
the chamber 38 by a screw 47, passes beneath the cam shaft,
and carries at its other, free end a closure element 48
having an outer butyl rubber pad 49 for engaging a seat of
the gas nozzle 31 to close the outlet provided thereby.
Similarly in the chamber 39, a plurality of further spring
wires 50 and associated gas outlet nozzles are provided,
the actual number corresponding to the number of gas rings
in the associated burner unit, less the inner pilot ring.
Accordingly, as shown in Figure 4 there are six kinked
spring wires 50 in chamber 39. The normal bias of each
spring wire is to raise the closure element 48 and pad 49
of the nozzle seat, allowing gas flow therethrough.
As stated, the spindle 45 carries a cam shaft part
in chamber 38 for rotation therewith. In chamber 39 the
spindle 45 carries a cam shaft 45a for rotation therewith.
The cam shaft part and the cam shaft 45a are each of hollow
tubular form. The cam shaft 45a has its one end adjacent
wall 39a closed around its 360 periphery. However, along
its length in a direction away from its said one end, the
tube has a series of adjacent part-annular cut outs
extending through its thickness. The cam shaft part also
has such a cut out. All the cut-outs terminate at the same
angular position around the cam shaft and cam shaft part
3~1
(which can be considered a continuation of the cam shaft),
but they start at regularly angularly staggered positions
along the cam shaft, these positions being 30 of cam shaft
rotation apart, so that the respective angles subtended
between the two sides of the cut out at the axis of the cam
shaft increase regularly by 30 along its length.
Accordingly as the spindle 45, and thus the cam shaft 45a,
rotates from a control knob 'OFF' position, where the
spring wires in the two chambers are all forced by the cam
shaft to seat the pads on the nozzle seats, each spring
wire in turn is allowed to rise under its natural
resiliency to open the nozzle outlet, the respective kinks
in the spring wires being received in the then aligned cut-
outs. The knob is numbered or otherwise marked to
correspond to the number of outlets opened as it is
rotated. A detent spring 51 engages an end of the cam
shaft projecting into the pilot chamber 38 beyond the gear
44, so that the position of the shaft at which each spring
wire is raised can be deduced, and also to tend to stop the
cam shaft rota~ing so that an intermediate position between
open and closed at a gas nozzle is not possible.
The nozzles 31 lead to the supply ducts already
described and thus by rotating the control knob of a
control valve the flow of gas to the ducts and thus the
number of gas rings in operation at a burner unit can be
varied. The first 'ON' position of the knob is where gas
is supplied to the duct leading to the innermost ring
which, when lit, acts as a pilot light for lighting the
other rings in turn if the knob is then turned to its fully
'ON' position. If the knob is rotated in the opposite
direction from its fully 'ON' position the rings are turned
off in sequence, down to, but normally not including, the
pilot ring. Figures 2 and 3 schematically show a spring
wire in 'OFF' and 'ON' positions respectively of the
control knob. Alternatively provision could be made for
overriding the sequence of supplying gas outwardly from the
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innermost ring and switching rings off towards the
innermost ring.
Figure 6 will now be referred to in order to explain
various other controls of the gas hob. The figure
diagrammatically shows the fan 20 with its central motor,
a main gas supply pipe 52 ~ith three branches, each branch
leading to a flame failure device 53, which is conveniently
a double beat type of valve. Gas outlets 54, 55 are shown
from the devices 53 to the chambers 38, 39 respectively of
a control valve 11, 12 or 13. In an actual hob however,
the flame failure device would probably be secured to the
side wall o~ the control valve having the inlets 40, 41
arranged so that the outlets from the device 53 would
merely be outlets in its side wall communicating directly
with the inlets 40, 41. Each control valve has a spring
loaded push button 56 at its pilot chamber end, although in
Figure 6 the full controls are shown only for control valve
13. This button is, in this embodiment, the control knob
14, 15 or 16 itself. Valves 11 and 12 have identical
controls to those to be described for valve 13, but are not
shown for clarity.
Shown for valve 13 is its associated burner unit 19
and, schematically, the seven supply ducts 29 from the
valve to the burner unit. Schematically shown also are
seven air supply ducts for air from the fan. The flame
failure device 53 has at one end a thruster solenoid 57
controlling gas flow to the pilot chamber 38 and at its
other end a second solenoid 58 controlling total gas flow.
In the path of operation of the knob/push button 14/56 is
trip means 59 of an ignition system 59a for all the burner
units pilots. Flame detection means 60, e.g. a
thermocouple, at the pilot gas ring of unit 19 is connected
to the solenoid 58. An overheat device, in the form of a
thermostat 61 is arranged over or adjacent the burner unit
towards the outer gas rings, and in contact with the lower
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surface of plate 25. The thermostat is connected via an
electric control 62 to the solenoid 57. The fan is also
connected to the ignition system, via a fan pressure switch
63, to ensure that it is switched on at all times when any
one burner unit is in operation. Figure 6 also shows an
electronic timing device 64 of system 59a connected to
solenoid 57 and, via system 59a, to switch 63.
Operation of the burner unit 19 will now be
described, assuming flow of gas only to the flame failure
device 53 initially.
Firstly, to light the innermost pilot ring of burner
unit 19, the push button/control knob 14 (Fig. 4) is pushed
in and turned to the position corresponding to the number
of rings required in use. The pushing in of knob 14, which
is equivalent to pushing in button 56, actuates trip means
59 which causes the fan motor to be energised, air movement
from the fan 20 causing the switch 63 to operate. Such
operation activates the ignîtion system 59a, producing a
spark at electrode 36 at the centre of the burner,
simultaneously operating solenoid 57 of device 53. With
the double beat type valve mentioned, operation of the
solenoid forces a closure member onto an opening to prevent
gas flow between pipe 52 and outlet 55, and forces an
armature off an opening between pipe 52 and outlet 54, to
allow gas to flow to the pilot chamber 38 of control valve
13. Since the knob 14 has been rotated by at least an
amount sufficient to allow lifting of closure element 48 in
chamber 38, gas flows from chamber 38 along the longest of
the ducts 29, mixing with air, to the innermost burner unit
ring, where it ignites.
Simultaneously with the ignition sequence described
above, the fan pressure switch 63 also staxts the timing
device 64 which is set, for example for 10 seconds, so that
after the thermocouple 60 is sufficiently heated by the
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ll
pilot to maintain the valve armature against solenoid 58,
the device 64 cuts off the electrical supply to solenoid
57, causing the closure member to move off the opening
between pipe 52 and outlet 55, thereby now allowing gas to
flow to chamber 39 as well as chamber 38. Once the flame
is established on the pilot ring, the spark generator
senses this, suppressing the spark in the normal way.
Thus with the pilot lit, the knob 56 can be turned
to switch as many of the remaining burner rings as are
required into, or subsequently out of, use, the gas/air
mixture at each ring being lit by the pilot. As explained,
rotation of the knob opens the no~zles 31 in turn to supply
gas to the ducts 29 where it mixes with the air introduced
by the fan. The gas/air mixture burns to provide an
intensive heat radiation.
If the burner unit heat output, as measured for
example at plate 25, exceeds a predetermined value, this is
sensed by the thermostat 6i which operates the control 62
to reactivate ~energise) the solenoid 57 and thus
extinguish the burner rings except the pilot ring, by
cutting off gas flow to the gas outlet 55. When the heat
output drops sufficiently the process reverses and the
rings operate agaln.
If all the gas flames were to be extinguished with
gas still being suppliedr the flame failure device
(thermocouple) would sense this and operate by de-
activating solenoid 58 and thus prevent gas flowing ou~ of
the device 53 through outlets 54, 55.
The advantages to be gained by way of the invention
when compared to previous gas heated ceramic glass hobs are
as follows.
By using a multiplicity of burner chambers at each
12
of the several burner stations which usually comprise the
hob unit, and by separately controlling these chambers ON
or OFF, then a wide range of control may be achieved
without significant detriment to the thermal efficiency of
heat transfer common in all current designs of gas ceramic
hob. There is no necessity to resort to the ON/OFF cycling
control which is grossly inefficient since the burners
seldom, if ever, reach their maximum effective temperature.
A round domestic burner 200 mm in diameter with
seven annular chambers has been found to allow more than
adequate control for the largest pans commonly used
domestically and when rated at 3 Kw is capable of heating
2 pints (1.137 litres) of water to 100~C from 20C in less
than 7 minutes. At its 2kw setting, independent tests to
British Standard 5386 Part 3 (European Standard EN 30),
have resulted in efficiencies in excess of 49% being
achieved.
By using an electrically powered fan to deliver
combustion air to each of the chambers and by mixing the
gas with the air in the ducts leading to the burner
chambers whilst controlling this separately, many of the
disadvantages inherent in a gas heated ceramic hob have
been overcome. In particular, since the ratio of air/gas
in the mi~ture can be closely controlled, the radiant heat
burner can be operated at aeration levels (typically 100-
110~ stoichiometric) at which its radiant efficiency is at
maximum. Such levels would be impractical, if not
impossible, to obtain in a similar atmospheric aspirated
burner.
The small size of the ducts leading to the burner
chambers also enables a 'thinner' unit to be achieved,
allowing a low level grill to be used if required or
cupboard space below the hob to be fully utilized.
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13
Due to the excellent mixing of air and gas possible
with this method, much higher gas input rates can be
achieved whilst still maintaining good combustion
performance. An increase from 3Kw with normally aspirated
burners to 4Kw with a seven ring 200mm diameter burner
using a fan has been found to be possible.
The clearance of products of combustion is improved
since no reliance need be placed on thermal lift and the
more rapid clearance enables the gap between burner and top
plate to be reduced, increasing radiant transfer
efficiency. Conduction heat transfer is also improved due
to the added turbulence of the flow. By maintaining the
air supply to all burner chambers even when the gas supply
is cut off, a beneficial cooling effect can be achieved
greatly improving the controllability.
As the heat input can be increased, the burner can
be made to heat up much more quickly. As both heating and
cooling are therefore quicker, the controllability is
improved.
The surface temperature of the radiant burner is
such that the glow can be easily seen through any glass
ceramic hob and the heat input can be judged by the number
of rings seen. The number of rings burning can be easily
adjusted to suit the diameters of the pans in use. The
described safety device guards against overheating of the
glass and thus maintains efficiency.
Since all air necessary for combustion is provided
by the fan, no allowance for secondary air is necessary so
that there is no cons~raint on burner size used or any
necessity to feed secondary air centrally through any
burner. Burners to suit very large pans, as for instance
used in commercial catering, can be made.
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14
Many variations from the example of the invention
described and illustrated are possible, and some of these
will be listed hereunder.
Instead of a single glass ceramic plate 25, separate
such plates individual to each burner unit can be used
together in multiples to form a multiple burner hob unit.
Instead of a single ceramic plate 35, the top of the burner
unit could be made up of two or more separate sections
which fit together to provide a suitably perforated ceramic
burner plaque matching the pattern of the chambers.
As described, at one end of each set of supply ducts
29 is an orifice plate. This can provide for entry of air
to each duct merely by having orifices therethrough or it
can receive plugs with respective orifices through them, as
described and illustrated. The volume of air supplied to
each duct is, with either arrangement, determined by -the
size of the orifice. This is dependent on the type of gas
in use and is predetermined for each duct and thus allows
simple conversion from one fuel gas type to another, i.e.
natural gas to L.P.G.
The overheat device can be arranged to cut off only
some of the rings, rather than all but the pilot ring, at
a predetermined ceramic glass temperature.
The fan can have multiple outlets to feed combustion
air to some or all of the burner units simultaneously,
rather than the single outlet and single plenum chamber
described. A further alternative is a branched conduit
conveying air to the burners from a single outlet of the
fan. The fan, or a second fan, could be used to draw out
the gaseous products of combustion. Alternatively, a fan
could be used to dra~ combustion air through the hob, pre-
mixing it with gas before it reaches the supply ducts.
Moreover, in a closed top hob as described, the or each of
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the fans can be used both to supply air for combustion and
to remove the products of combustion. In a further
variation, the or each fan or combination of fans has
sufficient capacity to allow air to be bled from the
combustion system for secondary uses, such as diluting the
hot flue gasses, cooling the glass top rapidly after use,
or reducing the temperature of the ducts and/or the burner
control units.
The fan may be capable of operating at two or more
fixed speeds. The fan may be arranged to continue to
operate after the burner units are extinguished, under
either manual or automatic control, so as to continue to
carry out its secondary functions, for example using the
heat built up during the cooking process to maintain the
temperature of a warming drawer or cupboard. A fan can be
used to facilitate the flow of gases through the hob.
The overheat device could be a thermocouple or a
flame rectification electrode to engage the flames issuing
from the smallest burner ring.
~ ith a gas cooker incorpora-ting a hob as described,
the control knobs 14, 15 and 16 would project from the
front of the cooker as in Figure 1. However, if the hob
were to be arranged as a separate item at a worktop, the
knobs 14, 15, 16 would be arranged to be vertical through
similar right-angled gearing as that shown in Figure 4, but
with the knob a~is turned upright by 90. It is considered
that the gearing between the knobs and associated
respective cam shafts may advantageously be other than 1:1.
The supply ducts 29 could be formed by machined
channels in a material block, as suggested in Figure 3.
However ~o reduce costs and ease manufacture the ducts
could be provided by extruding each set 22, 23, 24 with the
ducts automatically thereby formed therein, or by merely
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16
having a plate bent to provide seven upwardly open channels
which are closed by wall 26. Instead of an orifice plate
at the front of each of whichever form of set 22, 23, 24
used, each duct could merely have a plug 30a, as suggested
in Figure 3.
