WO1999033326A1

WO1999033326A1 - Electrical heater element

Info

Publication number: WO1999033326A1
Application number: PCT/GB1998/003821
Authority: WO
Inventors: Nigel Morris
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-12-22
Filing date: 1998-12-18
Publication date: 1999-07-01
Anticipated expiration: 2000-06-22
Also published as: GB9828072D0; AU1678199A; GB9727046D0; GB2333466A

Abstract

An electrical heater element has a ceramic former (1) and an electrical conductor (2) located on or in the former. The ceramic former is porous to a fluid to be evaporated so that when fluid is introduced into the former it is absorbed by the former and transported to the electrical conductor by the capillary action within the former. The fluid is rapidly evaporated on contact with the electrical conductor. The electrical heater element may be used in a smoke machine.

Description

ELECTRICAL HEATER ELEMENT

This invention relates to an electrical heater element and particularly, although not exclusively, to an element for rapidly vaporising fluids, for example in a theatrical smoke machine; in an industrial smoke generator, for example as a security device, wherein a building can be rapidly filled with smoke upon detection of an unauthorised person; in an industrial humidifying or cleaning system; in an insecticide sprayer; for horticultural or agricultural applications in spraying crops or greenhouses, or providing watering systems in greenhouses; for fumigation of buildings; or for aromatherapy, for example in factory, office or supermarket environments.

In a known smoke machine a large aluminium block is used as a heat store and "smoke fluid" (a mixture typically including water and glycol) from a fluid container is passed through the block in copper or stainless steel tubing, or the aluminium block is machined to give, for example, a 33 μm (13 thousandths of an inch) gap in which the fluid may vaporise. The aluminium block is heated by, typically, a nickel chrome wire wound around a magnesium oxide former, within a steel tube, the structure, for example, in one embodiment being swaged to form a cartridge heater. The swaged cartridge heater is inserted (e.g. following drilling or reaming) into the aluminium block or the aluminium block is moulded around the swaged cartridge heater.

The copper or stainless steel tubing is typically 3 to 6 metres long. Longer lengths of tube require powerful pumps to force adequate volumes of fluid through the spiral, so the length of tube is limited both by pump capability and by manufacturing considerations in the aluminium block fabrication. The fluid is heated to the vapour phase within the first few centimetres of the spiral, while further heating of the vapour to fogging temperatures occurs within the subsequent several metres of the spiral. Compromises in the design and length of the tube normally limit thermal transfer and considerable thermal gradients exist in the system which impede conversion rate and efficiency. For any given formulation of smoke fluid, there is an optimum vaporisation temperature at which the process is most efficient (about 280- 290°C) . Temperatures above the optimum temperature do not increase the volume of smoke produced, but run the risk of overheating the fluid and producing toxic smoke.

Temperatures much below the optimum produce lower volumes of inferior quality smoke which may be 'wet' or contain hot but unvaporised fluid in the output of the machine. Smoke is produced in an output 'burst' of a machine, during which the block temperature falls linearly with time, so that the block is only at the optimum temperature for a short time within the burst. Again this limitation impairs machine efficiency, since the majority of the fluid to vapour conversion occurs at non-optimal temperatures.

Since the spiral is of small diameter and great length, considerable pressure is produced by the rapid vaporisation of fluid, so that the conversion of vapour at 100°C to smoke at 280°C occurs under conditions of high pressure. Several metres are required for this part of the heating phase to occur since the smoke has a high thermal resistance. A high pressure oscillating pump (which is noisy and expensive) is therefore required in the prior art machines to introduce fluid into the spiral in order to overcome the back pressure generated by the vaporisation process. The smoke is ejected at an extremely high velocity, which is very noisy. There is also a danger of the cyclic hot jet of smoke burning personnel in the vicinity of the generator, particularly in a theatrical environment .

The prior art machines are heavy and expensive. They consume continuous power from the mains supply, the power requirements varying between 1 and 10 kilowatts, depending on the size of the machine. A large volume of metal has to be kept at a constant temperature of just below 300°C, the aluminium block storing up to 16 kWh. The heated chamber has temperature sensing devices and thermostats to maintain it at the correct temperature . When the block is at working temperature the fluid pump is enabled and smoke generation occurs until the block temperature falls below some lower limit, typically 220°C. The pump operation is then inhibited until the block temperature has risen once more to the upper temperature limit. The smoke output from the machine is therefore intermittent, with the duty cycle of smoke output to reheat time being set by the mass of the aluminium block — larger blocks will provide longer bursts of operation but at the expense of longer re-heat times. Typical machines require warm up times of 4 to 10 minutes with output durations of 30 to 100 seconds. Freedom to choose a more powerful heater to shorten warm up time is limited by constraints on the electric current which may be drawn from a domestic power supply.

The pumps and heating are controlled electronically. However, it is known for the machines to come out of control and overheat with the dangers of personnel being burnt, of fire or even the melting of the aluminium block.

Substantial insulation is required to keep the heat in, however a high proportion of the stored energy is lost through the insulation. A steel casing surrounds the insulated aluminium block, secured to the aluminium block by steel rivets and steel screws.

The method cf manufacture of the prior art machines is time consuming, inefficient and costly.

The inefficiency of the prior art machines generally arises from thermal lag, because the fluid is remote from the heating winding.

The stainless or copper tubes tend to clog if an inappropriate fluid is used or other materials such as perfume are introduced, and quite often the pumping system seizes. When the aluminium chambers get blocked it is very expensive and time consuming to change them - possibly costing 80% of the value of a new machine for the replacement parts. Moreover, because of the large thermal mass of the machines, they may take 5-6 hours to cool sufficiently for maintenance work to be carried out.

The prior art machines are physically large and the requirements of the large size aluminium block and fluid container limit the freedom of design.

The present invention seeks to mitigate the foregoing disadvantages .

According to a first aspect of this invention there is provided an electrical heater element comprising a ceramic former and an electrical conductor means formed on or in the former, wherein the ceramic former is porous to a fluid to be evaporated such that said fluid introduced into the former may be absorbed by the former and transported to the electrical conductor by capillary action within the former and thereby rapidly evaporated on contact with said electrical conductor.

Preferably, the former has an annular cross- section.

In a first embodiment, the conductor is conveniently wound in a spiral such that the longitudinal axis of the spiral is substantially tangential to the surface of the ceramic former.

Preferably the conductor is a coil provided around and coaxial with the ceramic former and conveniently the conductor is at least partially embedded in the ceramic former and may be etched on the surface of the ceramic former.

In a second embodiment, the ceramic former comprises a plurality of interconnectable annular elements having mating end faces, each annular element having an associated electrical conductor and an electrical connection being provided between the conductors on adjacent annular elements.

Conveniently the electrical conductors are etched on the surface of the annular elements and preferably are etched on the mating faces and the circumferential outer surface of the annular elements. Conveniently the electrical conductor etched on the mating face is in the form of a spiral coaxial with the ceramic former.

In a third embodiment the electrical conductor etched on the mating face is in the form of a radially-extending petal-like pattern centred on the longitudinal axis of the ceramic former.

Preferably the mating faces of the annular elements have rough surfaces and opposing mating faces of adjacent interconnected annular elements make an interference fit.

Conveniently the annular elements are interconnectable by co-operating externally threaded axial projections and internally threaded axial bores, the annular elements being provided with electrical contacts for interconnecting the electrical conductors on adjacent interconnectable annular elements. Conveniently transverse bores are provided between the axial bore and the surface of the axial projection.

In a fourth embodiment, the former is a planar substrate and, preferably, a silica/ceramic layer is secured to said substrate, said layer carrying at least one electrical conductor and, advantageously, an underside, in use, side of said substrate supports fluid proof means which in turn supports an insulating means which may comprise respective layers of a metallic foil and an insulating blanket.

The fourth embodiment advantageously includes a peripheral silicone rubber grommet surrounding the substrate, said grommet incorporating means for mounting the heater element in a housing.

Advantageously, a mains operated electrical power supply is provided, which power supply is an isolated switch mode power supply incorporating an isolating transformer.

Conveniently, in any of the above embodiments, the fluid is a mixture including water and glycol .

Preferably an injector is provided for the introduction of a fluid to the bore of the ceramic former and conveniently the injector is a perforated tube, preferably of stainless steel. Alternatively the fluid may be simply introduced to one end of the former.

Conveniently the electrical conductor is nickel chrome .

Conveniently the former includes a metal filled epoxy ceramic portion such that the metal filling forms the electrical conductor means.

In a second aspect of the invention, there is provided a heater element including at least one interconnectable, substantially cylindrical, ceramic annular element, said annular element comprising: an electrical conductor on or in the annular element; an axial projection on a first major planar face; a corresponding axial bore extending from a second major planar face opposing said first face; electrically conductive means on the major planar faces connected to the electrical conductor whereby a plurality of annular elements may be interconnected by means of said cooperating axial projections and said axial bores on adjacent annular elements and the electrical conductors on adjacent annular elements are electrically interconnected, to form a heating element.

Conveniently the axial projection is externally threaded and the axial bore is co-operarably internally threaded.

Preferably the annular element is provided with an axial bore for the introduction of fluids and a transverse bore between the axial bore and an external surface of the axial projection.

Preferably, in this second aspect of the invention, the ceramic former is porous to a fluid to be vaporised.

In a third aspect of the invention, there is provided a heater element including a planar porous ceramic substrate having a silica/ceramic layer thereon in or on which is provided at least one electrical conductor having means for attachment to a mains electricity power supply.

In a fourth aspect of the invention, there is provided a smoke generator comprising: a heating element as described above; a fluid container, fluid injection means for injecting fluid from the fluid container into the axial bore of the heating element, fluid control means for controlling the supply of fluid to the fluid injection means, sensing means for monitoring a condition of the electrical conductor, processing means to control the supply of electrical power to the electrical conductor dependent on the sensed condition thereof so as to control the temperature of the conductor within a predetermined range of temperatures .

Conveniently the condition is electrical resistance of the electrical conductor.

Preferably the smoke generator further includes air movement means for passing air over the heating element and control means for controlling the air movement means to adjust the density of smoke emitted by the smoke generator.

Conveniently the air movement means produces a bypass airflow over the heating element such that an outer layer of air bypasses the heating element and eliminates condensation from an associated smoke generator housing.

Preferably the smoke generator is capable of substantially continuously converting all of a quantity of fluid in the fluid container to smoke.

Conveniently the fluid to be vaporised is pre-heated. An important feature of the present invention is that the heating wire is at least partially submerged (e.g. 50%) in the fluid. In this way the smoke fluid is in direct contact with the heat source. In order to vaporise the fluid it is necessary to overcome the surface tension, which is achieved by absorbtion of the fluid by the ceramic former. For optimum efficiency, it is also important to maximise the surface area of the heater conductor which interfaces with the fluid by providing an expanded area in contact with the fluid.

With the present invention, it is possible to dispense with the aluminium block of the prior art . This allows much greater design flexibility, for example the use of a injection moulded plastic case to house the machine, making the machine considerably less expensive to manufacture and assemble than the prior art metal- cased machines. The heating elements of the present invention are relatively inexpensive to manufacture and take only seconds to replace.

The present invention also dispenses with the need for separate temperature sensing devices since the invention can use the temperature coefficient of the conductor material to monitor the temperature, thereby reducing the cost and complexity of the product .

A smoke generator based on the heating element of this invention may include a heating element and a processor to control fluid consumption via pulse modulated valves, compression of the gases to produce a satisfactory output and control the smoke output from 0 to 100%. Thus by control of the air flow and/or the liquid flow it is possible to produce smoke varying in density from a light mist to a dense fog, using the same smoke fluid.

The invention will now be described by way of example with reference to the accompanying drawings in which :

Figure 1 shows a first embodiment of a heater element in accordance with the invention;

Figure 2 shows a second embodiment of a heater element in accordance with the invention;

Figure 3 shows an expanded view of the embodiment of Figure 2 ;

Figures 4a-f show a third embodiment of the invention, wherein Figures 4a and 4d show respective mating faces of the annular elements; Figures 4b and 4c show perspective views of annular elements which may be interconnected to form a heater element; Figure 4e shows an alternative arrangement of a mating face and Figure 4f shows four annular elements in cross-section;

Figure 5 shows a perspective view of a fourth embodiment of a heater element in accordance with the invention having a planar substrate as a former,

Figure 6 shows a vertical cross-section of the fourth embodiment,

Figure 7 shows a profile of the upper surface of the fourth embodiment,

Figure 8 shows a seal used in connection with the fourth embodiment; and

Figure 9 shows a schematic diagram of a smoke generator incorporating a heater element in accordance with the invention.

In the Figures, like reference numerals denote like parts.

Referring to Figure 1, the heater element comprises a hollow cylindrical porous ceramic former 1

around which is located a spiral nickel chrome conductor 2, the spiral being coaxial with the former. The conductor may be in the form of a simple nickel chrome wire or, in order to increase the surface area of the conductor, the wire may first be wound in a small diameter spiral before being wound in a large diameter spiral about the former, so that the longitudinal axis of the small diameter spiral is tangential to the surface of the former. The surface of the ceramic former may conveniently be ridged to accommodate the conductor. The conductor may be partially (typically up to 20%) embedded in the surface of the former. Alternatively, the nickel chrome conductor may be etched into the surface of the former.

For use in a smoke machine, a hollow stainless steel injector tube 3, having a diameter less than the inner diameter of the former, is provided, the tube having longitudinally spaced apart apertures 4 radially extending through a wall thereof.

In use, the injector tube 3 is inserted into the bore 5 of the former 1. An electric current is passed through the conductor 2 to heat the conductor 2. "Smoke fluid" is introduced into the tube 3 and via the tube apertures 4 into the former bore 5. Alternatively fluid may be introduced directly to one end of the former without the use of an injector tube. The smoke fluid is carried by capillary action from the surface of the bore 5 to the outer surface of the porous former 1, so that the heated electrical conductor is at least partially submerged (say 50%) in the fluid so that the fluid is rapidly vaporised and converted to "smoke" by the heated electrical conductor. As the former absorbs the smoke fluid, the fluid is broken down into microscopic particles, overcoming undesirable surface tension effects. The element is thus fed with a continuous stream of droplets under high capillary pressure. Since these droplets are of very small size they possess a high surface area to volume ratio and extremely low thermal mass and so are heated virtually instantaneously to fogging temperatures. In this way the water, which acts as a carrier for the glycol, and the glycol, evaporate at about the same time.

Figures 2 and 3 illustrate a second embodiment of the invention in which the former 1 abuts in use axially against a second pre-heater former 6 about which is spirally wound an electrical heating coil 7, the two formers being shown axially separated in Figure 3. The pre-heater former 6 may be formed from a ceramic or china material known per se and the pre-heater electrical heating coil 7 may be nickel chrome wire . The fluid injector 3 may be inserted in use in the pre-heater former 6.

As seen in Figure 3 , there may also be provided a transverse bore 8 in the porous former 1 passing from the surface to the central bore 5 of the porous former 1, which may act as an alternative location for the injector tube 3.

In this embodiment "smoke fluid" is injected through the injector 3, or alternatively is introduced directly to an end of the former, as in the previous embodiment and is transferred by capillary action through the porous former 1 to the electrical conductor 2 to be vaporised. In addition, air is passed over the former 1 and conductor 2, in the direction of arrow-headed line A in Figure 2, the air having been heated by the heating coil 7. This air flow avoids condensation forming on an associated housing 105 (see Figure 5) and maximises the efficiency of the heater element.

Typical dimensions for the heater element in this embodiment are a 25 mm diameter and 100 mm long porous former 1 and a 35 mm diameter and 75 mm long pre- heater former 6, the electrical conductor 2 being a nickel chrome coil or etched nickel chrome and the pre- heater coil 7 being nickel chrome.

A third embodiment of the invention, comprising heater annular elements 10 is illustrated in Figure 4a-f . As shown best in Figures 4b and 4c, each annular element consists of a cylindrical porous ceramic former 11 having a nickel chrome spiral electrical conductor 12 etched on its outer peripheral surface. Each annular element 10 is provided with an axial, cylindrical, externally threaded projection 13 on one major end face 14 and a corresponding axial, internally threaded bore 16 extending from the opposing major face 17, whereby adjacent annular elements may be threadably interconnected by a one half or three quarter turn about their longitudinal axes. The annular elements 10 are provided with a central axial bore 15, of substantially smaller diameter than the threaded bore 16, such that the central bores 15 of adjacent interlocked annular elements 10 communicate with each other. The annular elements 10 are further provided with a transverse bore 18 in the projection 13 communicating with the central bore 15 and the outer surface of the projection 13.

As illustrated by Figure 4a, which shows an end face 14 of the annular element 10 of Figure 4b viewed in the direction of arrow headed line B, the end face 14 is provided with a spiral 19 of etched nickel chrome coplanar with and etched on the face 14. The spiral 19 is in electrical contact at its outer end with the end of the spiral electrical conductor 12 terminating at the face 14 and in electrical contact with a contact pad (not shown) on the projection 13 at its inner end.

As illustrated in Figure 4d, which shows an end face 17 of the annular element 10 of Figure 4c viewed in the direction of arrow headed line C, the end face 17 is provided with a corresponding spiral 20 in electrical contact with a contact pad (not shown) at the threaded bore 16. The contact pads (not shown) on the projection 13 and on the bore 16 are so arranged that when adjacent annular elements 10 are interlocked the spiral 19 of one annular element 10 is in electrical contact with the spiral 20 of the adjacent interlocked annular element 10.

An alternative arrangement of the etched conductor on the end faces is shown in Figure 4e, where the conductor has a radially-extending petal-like pattern 21. The conductor arrangements 19,20 on the end faces 14,17 are omitted from Figures 4b and 4c in the interests of clarity in the drawings.

The manner of interlocking annular elements

10 is further illustrated in cross-section in Figure 4f. The annular elements 10 are manufactured from tools the faces of which have been spark eroded rather than machined, so that the faces 14,17 of the annular elements are formed with a rough surface, such that they make an interference fit when interlocked with adjacent annular elements 10.

In use, a convenient number of annular elements are axially interconnected to form a composite heating element of the required power rating. A high frequency, low voltage alternating electrical current is passed through the serially connected spirals of electrical conductors from a power supply 106 (see Figure 5) which provides both the precision control required for accurate temperature regulation and the electrical isolation of the device from line current required for safe operation, thereby heating the conductors. The power supply, which includes an isolating transformer, is of a compact design providing power densities of 300 watts per cubic inch and power factor corrected line input to meet world-wide regulations concerning harmonic distortion of line current.

In a smoke machine, "smoke fluid" is introduced into the axial bore 15, and, as in the previous embodiments, is drawn by capillary action through the body of the porous annular elements 10 to the cylindrical outer surface where it is vaporised by the heated conductors . The porous ceramic also acts as a reservoir of fluid. In addition, smoke fluid passes through the transverse bores 18 in the projections 13 to the interference fit between adjacent annular elements 10, whence it is drawn by capillary action towards the cylindrical circumference of the composite heating element, some of the fluid being vaporised on its path to the circumference by the end face heating conductors 19, 20 or 21.

The composite heating element thereby forms a very-high power density element in which a very large area of heater element is in contact with the fluid to be vaporised. Moreover, the power rating can be adjusted by the selection of the number of annular elements 10 to interconnect. Modules of lengths of 20-25 mm with a 1 k rating are found to be convenient, so that a combination of modules making an element 25 mm diameter by 100 mm length is typically rated at 4 kW.

A fourth embodiment of the invention will now be described in connection with Figures 5 - 8 in which, instead of being in circular form, the heater element is planar having an electrical heating coil disposed in a surface of the element.

In Figures 5 - 8, the heater element 50 has a porous ceramic substrate 51 made of silica/sand that is 42% - 45% porous. Located on top of the substrate 51 is a layer 52 formed of a silica/ceramic mix reinforced with knitted stainless steel wire and the layer 52 is formed with a pair of serpentine grooves 72 in which respective conductors 53 are disposed. The conductors 53 are located in the bottom of the grooves 52, which grooves may have a rectangular cross-section, as shown in Figure 7, and the conductors are secured in the grooves by an alumina ceramic adhesive. The opposite major planar surface of the substrate 51 from the layer 52 is located on top of a silicon rubber sheet 54 that in turn is mounted on top of a stainless steel or aluminium foil layer 55. The combination thus far described is located within a peripherally surrounding grommet made, for example, of synthetic rubber 56 having a base 57 which abuts the foil layer 55 and which has an outer side surface bearing substantially parallel ribs 58 which are used for locating the heater element in a smoke machine (not shown) . The interface between the composite heater element and the grommet 56 may incorporate a seal (not shown) .

Preferably, interposed between the base 57 of the rubber grommet and the foil layer 55 is a ceramic fibre blanket 60, whereby the sheet 54 prevents fluid egress from the substrate and the combination of foil 55 and blanket 60 prevent heat loss.

The electrical conductors 53 are typically of square section 0.5mm.

The ceramic substrate 51 incorporates a pair of fluid input apertures 59. The porosity of the ceramic substrate is such that it must be sufficiently porous for fluid to flow through the substrate, but not too quickly since if fluid flow is too fast through the substrate the fluid will not break up into minute globules that are vaporised. If the porosity of the ceramic is too great, then it has been found that the capillary action of the ceramic sucks water out of the adhesive securing the conductors so that the adhesive does not properly cure.

The conductors 53 are connected by leads 61 to a mains electrical supply, e.g. 110 - 250V AC, isolated switch mode power supply 62 incorporating an isolating transformer. The use of an isolated switch mode power supply enables mains electricity to be used with a heater element carrying fluid and it is believed that the present invention is the first to use a heater element that vaporises fluid by using a mains power supply.

In use, air is blown across the heater element in the direction of arrow-headed line A and, by virtue of the air flow, so fluid within the substrate 51 is moved through the substrate.

In a typical but non-limiting embodiment, the ceramic substrate 51 has a square cross-section of side 115mm and a depth of 12mm. The layer 52 has a depth of

2mm and the depth of the grooves 72 is typically 1mm. The silicon rubber sheet 54 has a thickness of 1.5mm and the ceramic fibre blanket a thickness of 1mm.

Such a heater operating from 240V AC is capable of producing a power output of 3kw and rises from ambient to 290°C in 1.5 seconds. Moreover, the heater element rapidly cools to ambient in 10 - 15 seconds.

The heater element of this invention negates the use of a solid aluminium block that was used in the prior art, which block represented 60% - 80% of a smoke machine cost. The heater element of the subject invention is a replaceable low cost consumable component representing approximately 1.5% of the overall cost of a smoke machine .

A smoke generator may be provided with a number of electrically independent heating elements, so that smoke output may be readily varied by energising different elements, rather than by varying the temperature of the conductor.

Smoke machines employing the heating element of the invention typically operate at between 220 °C and 295°C. Temperature feedback between the heating element and the power supply typically maintains the temperature within 5°C of the optimum temperature under all condi- tions of line input voltage and ambient temperature.

Overall system control and a sophisticated user interface is provided by an embedded microprocessor to provide self calibration and diagnostic capability.

Figure 9 shows schematically a smoke generator including a heating element 101 according to the invention. A high frequency, low voltage alternating electrical current is passed through the heating element 101 from a power supply 106, under the control of a microprocessor 107. The temperature of the heating element 101 is controlled by the microprocessor 107 by monitoring the electrical resistance of the electrical conductor of the heating element 101. "Smoke fluid" is supplied from a container 108 to an injector 103 within a central bore of the heating element 101. The flow of smoke fluid is controlled by a valve 109 under the control of the microprocessor 107. Alternatively fluid may be pumped from the container 108 to the injector 103, or the supply may be gravity fed at a pressure typically of 3.4 kPa (0.5 p.s.i.), with or without control by a valve 109.

Air, drawn through an inlet 110 in the hous- ing 105 in the direction of arrow-headed line B, is entrained by a fan 111, also under the control of the microprocessor 107, and transported in the direction of arrow-headed lines A past the heater element 101, such that a low volume of air passes at high velocity over the heater element 101. The air passing over the heating element 101 thereby creates a suction effect to draw fluid through the body of the porous former from the bore to the outer surface thereof. The flow of air is optimised by the microprocessor 107 to produce a required flow of liquid while not unduly cooling the heating element 101.

The airflow is also controlled to control the emission of air and "smoke" from an outlet 112 in the housing 105 in the direction of arrow-headed line C, and also to prevent the development of condensation within the housing. This additional control of the airflow may be provided by suitable ducting (not shown) and a second fan 113, preferably under the control of the microproces- sor 107, and located, for example, adjacent the outlet 112.

Although described in a smoke machine environment, the annular heating elements may be used in other applications, for example in an industrial smoke generator, for example as a security device, wherein a building can be rapidly filled with smoke upon detection of an unauthorised person; in an industrial humidifying or cleaning system; in an insecticide sprayer; for horticultural or agricultural applications in spraying crops or greenhouses, or providing watering systems in greenhouses; for fumigation of buildings; or for aromatherapy, fragrancing systems, for example in factory, office, shops or supermarket environments. It may also be used for fire training of firemen and for training military personnel.

It will be understood, that in any of the embodiments the electrical conductor can be made of suitable materials other than nickel chrome, for example stainless steel.

Moreover, at least part of the former may be formed from a metal-filled epoxy ceramic, and the electrical conductor may be provided by the metal filling of the metal-filled epoxy ceramic.

Claims

CLAIMS :

1. An electrical heater element comprising a ceramic former and an electrical conductor means formed on or in the former, wherein the ceramic former is porous to a fluid to be evaporated such that said fluid introduced into the former may be absorbed by the former and transported to the electrical conductor by capillary action within the former and thereby rapidly evaporated on contact wj th said electrical conductor.

2. An element as claimed in claim 1 wherein the former has an annular cross-section.

3. An element as claimed in claim 1 or 2, wherein the conductor is wound in a spiral such that the longitudinal axis of the spiral is substantially tangential to the surface of the ceramic former.

4. An element as claimed in any preceding claim, wherein the conductor is a coil provided around and coaxial with the ceramic former.

5. An element as claimed in any preceding claim, wherein the conductor is at least partially embedded in the ceramic former.

6. An element as claimed in claim 1 or 2, wherein the conductor is etched on the surface of the ceramic former.

7. An element as claimed in claim 1, wherein the ceramic former comprises a plurality of interconnectable annular elements having mating end faces, each annular element having an associated electrical conductor and an electrical connection being provided between the conductors on adjacent annular elements.

5 8. An element as claimed in claim 7, wherein the electrical conductors are etched on the surface of the annular elements.

9. An element as claimed in claim 8, wherein the 0 electrical conductors are etched on the mating faces and the circumferential outer surface of the annular elements .

10. An element as claimed in claim 9, wherein the 5 electrical conductor etched on the mating face is in the form of a spiral coaxial with the ceramic former.

11. An element as claimed in claim 9, wherein the electrical conductor etched on the mating face is in the 0 form of a radially-extending petal-like pattern centred on the longitudinal axis of the ceramic former.

12. An element as claimed in any of claims 7-11, wherein the mating faces of the annular elements have

25 rough surfaces and opposing mating faces of adjacent interconnected annular elements make an interference fit .

13. An element as claimed in any of claims 7-

30 12, wherein the annular elements are interconnectable by co-operating externally threaded axial projections and internally threaded axial bores, the annular elements being provided with electrical contacts for interconnecting the electrical conductors on adjacent interconnectable annular elements.

14. An element as claimed in claim 13, wherein transverse bores are provided between the axial bore and

5 the surface of the axial projection.

15. An element as claimed in claim 1, wherein the former is a planar substrate.

10 16. An element as claimed in claim 15, wherein a silica/ceramic layer is secured to said substrate, said layer carrying at least one electrical conductor.

17. An element as claimed in claim 15 or 16,

15 wherein an underside, in use, side of said substrate supports fluid proof means which in turn supports an insulating means which may comprise respective layers of a metallic foil and an insulating blanket.

20 18. An element as claimed in any of claims 15 to

17, wherein there is provided a peripheral silicone rubber grommet surrounding the substrate, said grommet incorporating means for mounting the heater element in a housing.

25

19. An element as claimed in any preceding claim, wherein a mains operated electrical power supply is provided, which power supply is an isolated switch mode power supply incorporating an isolating transformer.

30

20. An element as claimed in any of the preceding claims, wherein the fluid is a mixture including water and glycol .

21. An element as claimed in any of the preceding claims, wherein an injector is provided for the introduction of a fluid to the bore of the ceramic former .

5

22. An element as claimed in claim 21, wherein the injector is a perforated tube.

23. An element as claimed in claim 22, wherein 10 the injector tube is of stainless steel.

24. An element as claimed in any of the preceding claims wherein the electrical conductor is nickel chrome.

15 25. An element as claimed in any of the preceding claims, wherein the former includes a metal filled epoxy ceramic portion such that the metal filling forms the electrical conductor means .

20 26. An electrical heater element including at least one interconnectable, substantially cylindrical, ceramic annular element, said annular element comprising: an electrical conductor on or in the annular element; an axial projection on a first major planar face; a corre-

25 sponding axial bore extending from a second major planar face opposing said first face; electrically conductive means on the major planar faces connected to the electrical conductor whereby a plurality of annular elements may be interconnected by means of said

30 cooperating axial projections and said axial bores on adjacent annular elements and the electrical conductors on adjacent annular elements are electrically interconnected, to form a heating element.

27. An element as claimed in claim 26, wherein the axial projection is externally threaded and the axial bore is co-operarably internally threaded.

28. An element as claimed in claim 26, wherein the annular element is provided with an axial bore for the introduction of fluids and a transverse bore between the axial bore and an external surface of the axial projection.

29. An element as claimed in any of claims 26-28, wherein the ceramic former is porous to a fluid to be vaporised.

30. An electrical heater element including a planar porous ceramic substrate having a silica-ceramic layer thereon in or on which is provided at least one electrical conductor having means for attachment to a mains electricity power supply.

31. A smoke generator comprising: an electrical heater element as claimed in any preceding claim; a fluid container; fluid injection means for injecting fluid from the fluid container into the axial bore of the heating element; fluid control means for controlling the supply of fluid to the fluid injection means; sensing means for monitoring a condition of the electrical conductor; processing means to control the supply of electrical power to the electrical conductor dependent on the sensed condition thereof so as to control the temperature of the conductor within a predetermined range of temperatures .

32. A smoke generator as claimed in claim 31, wherein said condition is electrical resistance of the electrical conductor.

33. A smoke generator as claimed in claim 31 or 32, further including air movement means for passing air over the heating element and control means for controlling said air movement means to adjust the density of smoke emitted by the smoke generator.

34. A smoke generator as claimed in claim 33, wherein the air movement means produces a bypass airflow over the heating element such that an outer layer of air bypasses the heating element and eliminates condensation from an associated smoke generator housing.

35. A smoke generator as claimed in any of claims

31 to 34 wherein a pre-heater pre-heats the fluid to be vaporised.