This invention relates to hot gas heating devices.

It has been proposed, inter alia in US Patent Specification 3,167,638 issued January 1965 to J R Hornaday Jr et al, to overcome shortcomings of conventional gas and electric cooking hobs and combine advantages of both by means of an air convection unit forcing air heated by an electric resistance element against the bottom of a cooking pan. Such an arrangement produces a flow of hot air against the pan simulating a gas burner flame, and therefore, at least theoretically, being instantly adjustable after the fashion of a gas burner, and "quicker" than the conventional electric hob unit that relies on conduction and that has a high thermal mass, without the attendant disadvantages of gas, namely that it is poisonous, explosive and requires elimination of combustion products.

There have been other such proposals, but none appears to have been made commercially available, despite the obvious attractiveness of the proposal.

The problem appears to be that the increased complexity of the devices over conventional boiling rings has not been satisfactorily counterbalanced by the improved performance achieved in practice, and, in more recent times, the advent of the ceramic hob, which has advantages in its appearance and easy-care feature, has tended to dictate the trend in electric cooking.

The present invention provides a hot gas heating device which overcomes some perceived problems of the earlier proposals and in particular the proposal of US-3,167,638 and which operates considerably more efficiently than the most efficient electric hob commercially available to date.

The invention comprises a hot gas heating device comprising a heating chamber having a heat access area and including a heating element and a gas circulator circulating gas within the chamber to be heated by the heating element and convectively transferring heat therefrom to the heat access area all without substantial influx and efflux of gas, and the radiant heating element being so arranged as to radiate heat directly to the heat access area.

The heating element may comprise an electric resistance heating element, which may be sheathed.

The gas will usually be air, of course, the chamber being open to atmosphere.

The chamber may be cup shaped with the heat access area at the mouth of the cup which will usually, of course, be uppermost, though as will appear below use of the heating device according to the invention need not be restricted to cooking hobs. The heating element may be disposed around the side walls of the cup, and the circulator disposed at the bottom of the cup.

The circulator may comprise a centrifugal impeller.

The device may comprise electric motor means driving the circulator, and an electric heating element may have element energising means operative only when the motor is energised. The energising current may drive the motor.

The device may be configured as a cooking hob and comprise a pan support at the opening. It _. may, however, be configured as a laboratory heater, substituting perhaps for the Bunsen burner, and comprise a test tube receiving well within the chamber.

Embodiments of hot gas heating devices according to the invention will now be described with reference to the accompanying drawings, in which :-

Figure 1 is a cross-section through a first embodiment of a heating device;

Figure 2 is a plan view of the device of Figure 1;

Figure 3 is a cross-section through a second embodiment of a heating device;

Figure 4 is a plan view of the device of Figure 3;

Figure 5 is a cross-section through a third embodiment of a heating device;

and Figure 6 is a plan view of the device of

Figure 5.

The hot gas heating device illustrated in the drawings comprises a heating chamber 11 having a heat access area 12 and including a radiant heating element 13 and a gas circulator 14 circulating gas within the chamber 11 to be heated by the elements 13 and

convectively transfer heat therefrom to the area 12 all without substantial influx and efflux of gas, the element 13 being so arranged as to radiate heat directly to the area 12.

The heating element 13 comprises an electric resistance heating element which is sheathed. The sheathing makes it safe to operate even if liquid, as from a pan boiling over, enters the chamber 11. Alternatively, spill protection members 20 may be provided as illustrated in Figure 3.

The device is of course configured in these illustrated embodiments as a cooking hob and so the gas is air, the chamber 11 being open to atmosphere.

The chamber 11 is cup shaped in all three embodiments with the heat access area 12 at the mouth of the cup and the element 13 disposed around the side walls 15. The circulator 14 may either be at the bottom of the chamber 11 as in Figures 1 and 5 or may be to one side of the chamber 11, as in Figure 3, in which case air ducts 23 direct air up through the chamber 11. The circulator 14 comprises a centrifugal impeller driven by an electric motor 16. The electric motor 16 may be located below the chamber 11 having a drive spindle 16a projecting into the chamber through a substantially air

tight seal 17, as illustrated in Figure 1, or the electric motor 16 may be located to one side of the chamber 11, as illustrated in Figures 3 and 5.

It will be appreciated that the cup-shaped chamber 11 is airtight, then, except for the heat access area 12 and the impeller 14 merely circulates air within the chamber 11 (aside, obviously, from any initial loss due to expansion on heating up) and in essence creates a circulating "ball" of hot air which scrubs the base of a pan 18 placed atop the chamber 11. The entire base of the pan 18 which is exposed to this hot air is thus available for heat transfer from the air, as compared to the relatively small area which is available for conductive transfer from conventional boiling rings, and the base is shorn by the circulating air of its insulating boundary layer.

The air impinging on the base of the pan 18 will be close to the temperature of the radiant heating element 13 and thus at a temperature similar to that of a gas flame.

In addition, the element 13 radiates heat to the base of the pan, and the combination of convective and radiative transfer is found to result in a very high efficiency of heat transfer. The most efficient cooking

hob currently commercially available is thought to have a heat transfer efficiency of just over 50%, while the efficiency of a device according to the invention can be up to 65%, which represents nearly a 30% improvement. This is of great interest not only from the point of view of energy saving, important though that is, but also because it means faster cooking and a greater degree of controllability. The response of the device to a reduction or increase in power is very fast indeed; the heat capacity of the circulating air is very small, and the heat capacity of the element is fairly small, so the device can very quickly reach whatever working temperature is desired by regulating the current through the element 13 in the usual way.

Having referred to "the usual way", of course, it has to be said that there is a variety of ways in which electric heating elements are conventionally controlled. Common, nowadays, is thermostatic control, in which the element is operated at full power and switched on and off by a thermostat, and such an arrangement is of course suitable for use with the present invention, a temperature sensor 19 being located within the chamber 11, and connected to a current controller with a temperature setting switch. Other control methods may of course be used which do not involve switching the element energising current on and

off at full power but supply continuous power at variable levels, the better to simulate a controllable gas burner.

It can of course be arranged that the element 13 is switched on only when the motor 16 is driving the circulator 14. It may be arranged that the motor is permanently running at full speed when the device is in use, but it may also be arranged that the motor runs at a speed proportional to the heat setting of the device. Or the heat and impeller speed might be independently controllable.

In one arrangement, the element 13 may even be constructed as a coil to produce a rotating field which would drive the circulator inductively.

The embodiments illustrated in the drawings, configured as a cooking hob, have thermal insulation 21 around the chamber 11. A multi-element hob, as is conventional, may have devices of different size to afford larger and smaller heat access openings 12 for different sizes of pan.

The impeller 14 may be made readily demountable from the drive spindle 16a, pressing back on perhaps with a snap fit, to facilitate cleaning the inside of

the chamber 14. The sheathed heating element 13 will of course burn itself clean of any drips that might fall on it. The arrangement may be provided with the usual indicators.

While an electric resistance heating element has been particularly referred to, it may well be the case that other forms of heating element could be used. For example, the element could comprise a quartz-halogen lamp element.

The device may be configured as a laboratory heater as a replacement for the Bunson burner and may be adapted to heat test tubes, by virtue of being formed with a test tube receiving well in the chamber, while at the same time having a beaker/flask supporting rim to the chamber, so that the conventional tripod may be superfluous.