Patent Description:
Roofing products, such as bitumen, are used in the sealing of roof structures. During the application of roofing membranes, bitumen-based products are melted using gas-powered roofing torches, and these are used in order to seal the membranes to the roof structure. Prior to applying the membrane, the torch may be used to prepare the area by drying the surface where the membrane is to be laid and/or to ready the membrane.

These gas-powered roofing torches come in many forms, but are generally in the form of a hand-held device comprising a lance with a nozzle at the end. The lance is coupled to a gas source, for example, a cylinder of propane or butane. The gas is burnt at the nozzle to produce a hot naked flame and generate heat, which is then used to melt the bitumen-based roofing product and/or prepare the surface beforehand. The bitumen-based materials might be incorporated into a roofing membrane or they might be heated and applied separately during sealing of a membrane.

However, the use of naked flames during the construction or repair of a building poses a tremendous fire risk and there is plenty of evidence of instances where a fire has started through the use of a naked flame from such a roofing torch. It is not only the presence of flammable gases but in such construction environments there will usually be exposed, combustible parts of the roof structure as well as combustible debris collected in the working area. In addition to safety, there are also moves to burn fewer fossil-based resources.

As a result, it would be desirable to provide an improved roofing torch, particularly one which avoids the use of a naked flame and reduces carbon consumption.

There have been a number of developments recently with electric powered roofing torches. In one known example, a backpack is provided comprising an electric fan to generate a flow of air which is directed via a flexible tube into a handheld torch provided with an electric heater matrix to heat the air. The resulting flow of hot air is then directed via a lance or nozzle to where it is needed in order to apply heat to a roofing product, e.g., a roofing membrane being used on a roof structure. This electric roofing torch solution, while offering many benefits through avoiding naked flames and reduced carbon consumption, is however quite bulky and heavy for the operator to manoeuvre, and improvements in performance are also desirable.

<CIT> discloses a heat-expelling device with a wind guide comprising a shell, a heat-generating body disposed within the front portion of the shell and a wind-blowing unit disposed within the rear portion of the shell.

<CIT> discloses an air heating device having a fan, an air temperature control device and a heater arranged in sequence.

<CIT> discloses an axial fan unit comprising fan blades extending from a hub provided with a deflector ring at the air delivery end of the rotational axis.

According to one aspect, there is provided an electric roofing torch as claimed in claim <NUM>.

Thus the fan unit is mounted in the tubular body to drive a flow of air through the tubular body at velocity, pressure and volume that is sufficient for roofing operations.

The collection of features provided by at least the preferred embodiments work together to result in an electric roofing torch that is:.

For example, the heater tube may be capable of heating the flow of air to temperatures in excess of <NUM>, more preferably in excess of <NUM>. The heater tube may comprise a complex matrix 'super heater' that is mounted in the tubular body to heat the flow of air as it passes through the tubular body.

The fan unit may be able to generate volumes of air flow from a nozzle of the electric roofing torch that are in excess of <NUM><NUM>/h, more preferably in excess of <NUM><NUM>/h. This may be with speeds of air flow in excess of <NUM>/h or even more than <NUM>/h. In a preferred embodiment, air flow speeds of greater than <NUM>/h, for example, <NUM>/h or greater, are achievable from the electric roofing torch at such volumes.

The fan unit may be a high frequency, three-phase AC, electric vaneaxial fan.

Through a selective choice of materials and construction, it is possible to provide a light-weight solution for an operator. Thus in preferred embodiments it may be possible to keep the overall weight of the electric roofing torch down to just a few kilograms (e.g., less than <NUM> and preferably less than <NUM>) through an appropriate choice of components. For example, the fan unit may have a weight of less than <NUM>. The tubular body may weigh less than <NUM>, preferably less than <NUM>, and more preferably still less than <NUM>. Any part where weight can be minimised will help to reduce the weight that the operator has to carry for potentially extended periods, as well as helping to improve the torch's general usability.

The impeller may be provided with a plurality of blades. The blades may each comprise a leading edge and a trailing edge, and wherein the trailing edges of the blades may have been machined back to provide a cylindrical cut-away profile. For example, the trailing edge of each blade comprises a radially outer, rectangular cut-away portion and a radially inner, extended rib portion that blends to a domed outer surface of a hub of the impeller. Such a configuration, helps to facilitate the generation of a powerful flow of air when the impeller is rotated at high rotational speeds.

Within the fan unit, downstream of the impeller there may be a plurality of vanes. There may be between <NUM> and <NUM> vanes, for example, extending between a radially inner surface (radially outer surface of the fan core) and a radially outer surface (radially inner surface of the fan housing) of a core flow path. These may help to pressurise the flow of air.

As a result of the double-walled structure, the operator can be shielded from the heat of the heater tube located within the inner tube. A downstream end of the fan unit may also support an upstream end of the inner tube within the outer tube, facilitating construction.

According to another aspect, there is provided a method of providing a working flow of hot air from an electric roofing torch for use in a roofing operation as claimed in claim <NUM>.

The method of providing a working flow of hot air may comprise using an electric roofing torch as described herein in relation to the first aspect.

Certain preferred embodiments will now be described in greater detail, by way of example only, and with reference to the accompanying drawings, in which:.

As shown in <FIG>, an electric roofing torch <NUM> is provided comprising a tubular body <NUM> having an upstream end 2a and a downstream end 2b. <FIG> is an end-on view looking up towards the downstream end 2b of the tubular body <NUM>, and <FIG> is a side cross-sectional view taken along the central axis A-A. As seen from <FIG>, a fan unit <NUM> is mounted in the upstream end 2a of the tubular body <NUM> to drive a flow of air through the tubular body <NUM>. A heater tube <NUM> is provided within the tubular body <NUM> comprising a heater matrix <NUM>. The heater tube <NUM> is mounted in the tubular body <NUM> to heat the flow of air as it passes through the tubular body <NUM>. In accordance with the present invention, the fan unit <NUM>, which is now mounted within the tubular body <NUM>, is an electric vaneaxial fan (see <FIG>) which is mounted upstream of the heater tube <NUM>.

As shown in <FIG>, an inlet <NUM> of the tubular body <NUM> may be provided at a first, upstream end 2a thereof and an outlet <NUM> provided at a second, downstream end 2b thereof opposite the first end. The tubular body <NUM> is arranged to guide air from the fan unit <NUM> through the heater tube <NUM> towards the outlet <NUM>. The outlet <NUM> is preferably in the form of a nozzle outlet, in particular a conical nozzle <NUM>.

The tubular body <NUM> may have a circular cross-section. The tubular body <NUM> may take the form of a cylindrical housing and is preferably straight in order to aid fabrication.

The tubular body <NUM> comprises a double-walled structure as shown, comprising an inner tube <NUM> and an outer tube <NUM>. The inner tube <NUM> and outer tube <NUM> are arranged as concentric tubes. The inner tube <NUM> may comprise mica and carbon fibre. The inner tube <NUM> provides a dual function of housing the heater tube <NUM> and providing a conduit for the flow of air between an upstream end 2a of the inner tube <NUM> and a downstream end 2b of the inner tube <NUM>. The outer tube <NUM> provides a housing for the electric roofing torch <NUM>, the housing being configured to shield an operator from heat from the heater tube <NUM> during use.

The outer tube <NUM> may be longer than the inner tube <NUM>. In this way, the fan unit <NUM> can be located within an upstream end 2a, <NUM> of the outer tube <NUM>, in a region extending between the upstream end of the outer tube <NUM> and an upstream end of the inner tube <NUM>. A downstream end of the fan unit <NUM> may support the upstream end of the inner tube <NUM> within the outer tube <NUM>.

The tubular body <NUM> may comprise a plurality of spacers <NUM>, preferably annular or ringshaped spacers <NUM>, arranged between an outer surface of the inner tube <NUM> and an inner surface of the outer tube <NUM>. There may be two spacers, for example, as shown in <FIG>.

A mount <NUM>, for example, in the form of a track, may be provided on an upper surface of the outer tube <NUM>. A carriage <NUM> may be fitted to the mount <NUM> as shown in <FIG>. The carriage may be provided with a set of controls <NUM>, for example, in the form of a joystick or throttle through which the operator can carry and operate the torch with one hand. A handle <NUM> may be provided for the operator's other hand, such that the torch is carried in both hands. Other ways of carrying the electric roofing torch and/or controlling the operation of the torch are also possible, for example, a sling that the operator can rest on an operator's arm to transfer the weight of the torch.

Also visible in <FIG> is flexible power lead <NUM>, and a junction box <NUM> for where the power comes into the electric roofing torch <NUM>. A high power electrical supply may be connected to the electric roofing torch <NUM>, for example a 240V AC and 32A supply. The junction box <NUM> may house electronics for operating the electrical components like the fan unit <NUM> and the heater tube <NUM>. The electronics may raise the frequency of an electrical supply to the fan unit <NUM>.

The fan unit <NUM> is shown in more detail in <FIG>. It comprises a fan core <NUM> comprising an impeller <NUM> and a stator core <NUM>, which are housed within a fan housing <NUM>. The fan housing <NUM> has an upstream end 17a and a downstream end 17b. The impeller <NUM> is arranged at an upstream edge 16a of the stator core <NUM>, with the impeller <NUM> being arranged to rotate about a central axis A-A of the fan unit <NUM>. A motor <NUM> is housed within the stator core <NUM>, the motor having a drive shaft 19a for providing rotational drive to the impeller <NUM>.

The impeller <NUM> may be provided with a plurality of aerodynamically profiled blades <NUM>, for example, as shown in <FIG>. There may be between <NUM> and <NUM> blades <NUM>, more preferably five blades <NUM> as shown, the blades <NUM> extending radially outward from a hub 15a of the impeller <NUM>.

As the fan unit <NUM> is a vaneaxial fan, the impeller <NUM> is arranged upstream of a plurality of vanes <NUM>. These vanes <NUM> extend radially, between a radially inner surface 25a and a radially outer surface 25b of a core flow path <NUM> defined within the fan unit <NUM>.

The radially outer surface 25b of the core flow path <NUM> is provided by an internal surface of a fan housing <NUM>. The radially inner surface 25a is provided by an outer surface of the fan core <NUM>.

There may be between <NUM> and <NUM> vanes <NUM> extending between the radially inner surface 25a and the radially outer surface 25b of the core flow path <NUM>. Preferably there are between <NUM> and <NUM> vanes and most preferably there are <NUM> vanes. The vanes may be spaced equally around the cylindrical surface of the stator core <NUM>.

The vanes <NUM> may be integrally formed with the fan core <NUM> and/or the fan housing <NUM>. In the example of <FIG>, the vanes <NUM> are provided integrally on the cylindrical outer surface of the stator core <NUM>, these vanes <NUM> extending therefrom to slot into position within and against the inner surface 25b of the fan housing <NUM>, in this way suspending the fan core <NUM> within the fan unit <NUM> in a concentric fashion.

The electrical motor <NUM> may be one that is designed, under normal operation, to rotate the impeller <NUM> at speeds in excess of <NUM>,<NUM> rpm, more preferably in excess of <NUM>,<NUM> rpm or even <NUM>,<NUM> rpm. In one example the motor <NUM> is arranged to rotate the impeller <NUM> at <NUM>,<NUM> rpm or higher. The rotational speed of the impeller <NUM> during use may be controllable by the operator, e.g., by using the controls <NUM>.

The impeller <NUM> may comprise a domed hub 15a, from which the blades <NUM> extend. The impeller <NUM> may have an external contour that blends with that of the stator core <NUM>.

The impeller <NUM> may be fabricated through being printed using a 3D printing tool. The 3D printing may form an internal lattice framework within the impeller <NUM> to minimise weight, in particular rotational weight.

Alternatively or additionally, the impeller <NUM> may be machined to final form, for example, from a cylindrical block of material or from a cast blank which is partially pre-formed to an impeller shape.

In one embodiment, the impeller <NUM> is made from aluminium extruded stock (for example, <NUM>-T6) which has been milled to final form. In another, the impeller <NUM> is 3D printed in either aluminium or titanium. Other materials and methods of fabrication are also envisaged.

The blades <NUM> each comprise a leading edge 18c and a trailing edge18d. The trailing edges 18d of the blades <NUM> may have been machined back to provide a cylindrical cut-away profile. The trailing edge 18d of each blade <NUM> may comprise a radially outer, rectangular cut-away portion and a radially inner, extended rib portion 18e that blends to the domed outer surface 18f of the hub 18a. The trailing edge 18d of such a cut-away portion may be provided by a flat, circumferentially extending trailing surface <NUM> that extends between the suction and pressure surfaces 18a, 18b of the blade <NUM>.

The stator core <NUM> may comprise a range of materials. For example, it may comprise a polymer-based material, a carbon rich nylon or other composite material comprising a polymer-based matrix material, for example, a polyester or epoxy based material. The stator core <NUM> could also be machined from a lightweight metal such as an aluminium alloy.

The fan unit <NUM>, the heater tube <NUM> and the tubular body <NUM> share a common axis. The fan unit <NUM> is housed within the tubular body <NUM>, in particular an inner tube <NUM> of a dual-walled tubular body <NUM>.

The fan housing <NUM> provides a fan inlet (upstream end 17a) which protrudes axially beyond the upstream end 2a of an outer tube <NUM> of the tubular body <NUM> and a fan outlet (downstream end 17b) that is arranged within the inner tube <NUM> of the tubular body <NUM>, upstream of the heater tube <NUM>.

The fan unit <NUM> may be provided with a grille <NUM> at the fan inlet 17a (see <FIG>). The grille <NUM> may be configured to prevent clothing or fingers of the operator from entering the fan unit <NUM> of the electric roofing torch <NUM>, while providing openings that allow a sufficient air flow into the torch <NUM>.

The fan housing <NUM> may comprise a machined sleeve. The sleeve may have been machined from a material like aluminium (for example, hollow stock <NUM>-T6), or it could be made from a polymer-based material, e.g., nylon or a polymer-based matrix material which is reinforced with fibres. In one example this might be a carbon-rich nylon mix. In preference to machining, the fan housing <NUM> may be 3D printed. Whatever the materials, they are preferably chosen to help minimise the overall weight of the electric roofing torch <NUM>.

The inner surface 25b of the fan housing <NUM> may comprise an axially staged profile (see <FIG>). In this way a radius of the inner surface 25b may reduce from the upstream end 17a to the downstream end 17b in a stepwise profile, the steps 17f corresponding in position to the edges of the vanes <NUM>. For example, a step 17f may be provided in the inner surface 17b to locate the vanes <NUM> in position during assembly when the stator core <NUM> is introduced into the fan housing <NUM> from an upstream end 17a of the fan housing <NUM>. The steps 17f may help to locate the stator core <NUM> within the fan housing <NUM> through the engagement of the vanes <NUM>. This is not just during fabrication but also during use when large forces are generated in reaction to the force imparted to the air flow.

The outer surface 17c of the fan housing <NUM> may comprise a bridging section 17d at a downstream end 17b. The bridging section 17d may bridge across an annular space <NUM> between the outer tube <NUM> and the inner tube <NUM> of the tubular body <NUM>. The bridging section 17d may have an outer diameter dimension that reduces in a downstream, axial direction to bridge from the outer tube <NUM> to the inner tube <NUM> of the tubular body <NUM> (see <FIG>). The outer surface 17c may comprise an annular seat 17e of reduced diameter at the downstream end 17b for seating within an inner tube <NUM> of the tubular body <NUM>.

The inner tube <NUM> of the tubular body <NUM> may be provided with a nozzle cone <NUM>, for example, as a separate component that is fitted to the inner tube <NUM>, at the downstream end of the inner tube <NUM> for concentrating the flow of air as it exits the heater tube <NUM>. The nozzle cone <NUM> may comprise stainless steel or other suitable heat resistant material.

The inner tube <NUM> may comprise a polymer-based material. It may comprise, for example, a polymer-based matrix material which is reinforced with fibres, e.g., carbon fibres. The polymer-based matrix material may comprise a polyester or epoxy based material.

The outer tube <NUM> may comprise a polymer-based material. It may comprise, for example, a polymer-based matrix material which is reinforced with fibres, e.g., carbon fibres. The polymer-based matrix material may comprise a polyester or epoxy based material.

The materials of the inner tube <NUM> and outer tube <NUM> are chosen to withstand the operating temperatures of the electric roofing torch <NUM> while minimising overall weight as far as possible.

The heater tube <NUM> may comprise a plurality of electric heater elements 6a. These may be arranged in the form of a matrix <NUM> that the air is passed through to heat the air enroute to a nozzle <NUM> of the electric roofing torch <NUM>.

The heater tube <NUM> may consume more than 18kW during use, preferably more than 20kW, and more preferably still around 22kW or more during use.

The heater tube <NUM> may comprise a housing provided by a mica cylindrical tube. This may be configured to fit within an inner tube <NUM> of the tubular body <NUM>. The mica will help to insulate the inner tube <NUM> from the high operating temperatures. The heater elements 6a of the heater tube <NUM> may be supported on a mica chassis in a 'complex' formation and arranged to provide a corresponding resistance which when subjected to an electrical load, creates the heat output from the electric roofing torch <NUM>.

The complex formation of the heater elements 6a and the matrix <NUM> ('super heater') may take the form of a set of resistance circuits arranged to provide as uniform as possible heat across the chassis which fills the inner tube <NUM>. There may be three or more resistance circuits. More preferably there are six resistance circuits. In such a set-up, pairs of resistance circuits may be coupled to each phase of a three phase supply. The resistance circuits may be arranged in a hexagonal array within the heater matrix <NUM> to provide a uniform heat across the heater tube <NUM>. The air is forced through the complex matrix super heater <NUM> and when the resistance wires are subjected to an electrical load, thermal energy is transferred from the resistance wires into the air. This process is further accelerated because the flow of air within the electric roofing torch <NUM> is restricted at an exit point via a nozzle <NUM>, for example, a conical nozzle <NUM>.

The fan unit <NUM>, as a result of being a vaneaxial fan, may be able to generate volumes of air flow from a nozzle <NUM> of the electric roofing torch <NUM> that are in excess of <NUM><NUM>/h, more preferably in excess of <NUM><NUM>/h. This may be with speeds of air flow in excess of <NUM>/h or even more than <NUM>/h. In a preferred embodiment, air flow speeds of greater than <NUM>/h, for example, <NUM>/h or greater, are achievable from the electric roofing torch <NUM> at such volumes. The heater tube <NUM> may be capable of heating the flow of air to temperatures in excess of <NUM>, more preferably in excess of <NUM>. The electric roofing torch <NUM> may also have a total weight of less than <NUM>, preferably less than <NUM>, and a size of around half a metre, making it easy for an operator to manoeuvre.

The electric roofing torch <NUM> may further comprise a hanging bracket <NUM> provided on an underside of the electric roofing torch <NUM>. The hanging bracket <NUM> may be configured to provide a foot for when the electric roofing torch <NUM> rests on the ground between roofing operations.

The electric roofing torch <NUM> may operate on a three-phase AC mains supply. Alternatively power can be generated using a mobile generator <NUM> such as that shown in <FIG>. The electric roofing torch <NUM> may have a power rating of <NUM>. 5kW or more, drawing a current of about 32amps.

The electric roofing torch may also comprise a lance or delivery nozzle <NUM>, e.g., as shown in <FIG>. The lance or delivery nozzle may be mountable on a nozzle outlet <NUM> provided at the downstream end 2b of the inner tube <NUM> in order to direct hot air from the nozzle outlet <NUM> of the tubular body <NUM> for use in a roofing operation. The lance or delivery nozzle <NUM> may extend a distance from the operator's hips to a floor level. Thus, the torch lance or delivery nozzle <NUM> may configured to reach the floor level when the electric roofing torch is in use, and preferably when the electric roofing torch <NUM> is being carried by a standing operator.

The lance or delivery nozzle <NUM> may be made of a lightweight material such as aluminium, or more preferably a composite material such as a carbon reinforced matrix material, e.g., as shown in <FIG>, that is able to withstand the high operating temperatures. The composite material may be shaped to define a blade-shaped aperture <NUM> to provide a wide flat jet of hot air for use in the roofing operation.

<FIG> shows an exploded, perspective view of the electric roofing torch shown in <FIG> to help aid understanding.

There follows a brief discussion of the preferred (non-limiting) dimensions for the main components of the electric roofing torch, such as the fan unit, the tubular body, etc..

Exemplary Dimensions for the main components:.

The impeller may comprise an outer diameter with the blades <NUM> included of more than <NUM>, preferably more than <NUM>. In one example, the outer diameter of the impeller is <NUM> or more.

The hub 15a of the impeller <NUM> may have a maximum outer diameter of more than <NUM>. In a preferred embodiment the hub 15a of the impeller <NUM> has a maximum outer diameter of more than <NUM>, more preferably <NUM> or <NUM>.

The blades <NUM> of the impeller <NUM> each comprise a suction surface 18a and a pressure surface 18b. The suction surface 18a may be profiled with a radius (e.g., when viewed perpendicular to the fan axis) of between <NUM> and <NUM> at a mid-chord position. The suction surface 18a may comprise a radius of between <NUM> and <NUM> at a leading edge thereof. The pressure surface 18b may be profiled with a radius (e.g., when viewed perpendicular to the fan axis) of between <NUM> and <NUM> at a mid-chord position. The pressure surface 18b may comprise a radius of between <NUM> and <NUM> at a leading edge thereof.

Where the blades <NUM> of the impeller <NUM> may have been machined back to provide a cylindrical cut-away profile, the trailing edges 18d of the blades <NUM> may have been machined back by more than <NUM>, more preferably by more than <NUM>. In one example, they have been machined back to provide a cut-away of <NUM>. The trailing edge 18d of each blade <NUM> may comprise a radially outer, rectangular cut-away portion and a radially inner, extended rib portion 18e that blends to the domed outer surface 18f of the hub 15a. The leading edge 18c of each blade <NUM> may be set back from a nose of the hub 15a. This may be by more than <NUM>, more preferably by more than <NUM>, and more preferably still by <NUM>.

The radially inner surface of the fan housing may comprise a diameter of greater than <NUM> at its upstream end. For example, it may comprise a diameter of greater than <NUM>, preferably <NUM> ± <NUM>. The radially inner surface of the fan housing may comprise a diameter of less than <NUM> at its downstream end. For example, it may comprise a diameter of less than <NUM>, preferably <NUM> ± <NUM>.

The radially outer surface 17c of the fan core <NUM> may have a diameter of greater than <NUM>. For example, it may have a diameter of greater than <NUM>, and more preferably it is <NUM> or <NUM> ± <NUM>.

An annular cross-sectional area of the core flow path <NUM> (defined between the radially inner surface 25a and the radially outer surface 25b in the radial direction) may decrease from the upstream end 17a of the fan housing <NUM> to the downstream end 17b of the fan housing <NUM>. A radial separation of the radially inner surface 25a and the radially outer surface 25b of the core flow path <NUM> may be greater than <NUM>, for example, greater than <NUM>. The radial separation of the radially inner surface 25a and the radially outer surface 25b of the core flow path <NUM> may be less than <NUM>, for example, less than <NUM>. Preferably the radial separation is <NUM> ± <NUM>.

The inner tube <NUM> may have an inner diameter greater than <NUM>, for example an inner diameter of <NUM> ± <NUM>. The inner tube <NUM> may have an outer diameter of less than <NUM>, for example, an outer diameter of <NUM> ± <NUM>.

The inner tube <NUM> may have a length of greater than <NUM>. The length of the inner tube <NUM> may be less than <NUM>. In a preferred embodiment the inner tube <NUM> has a length of <NUM> ± <NUM>.

The outer tube may have an inner diameter greater than <NUM>, for example, <NUM> ± <NUM>. The outer tube may have an outer diameter of less than <NUM>, for example, <NUM> ± <NUM>.

The outer tube <NUM> may have a length of greater than <NUM>. The length of the outer tube <NUM> may be less than <NUM>. In a preferred embodiment the outer tube <NUM> has a length of <NUM> ± <NUM>.

Claim 1:
An electric roofing torch (<NUM>) comprising:
a tubular body (<NUM>) having an upstream end (2a) and a downstream end (2b);
a fan unit (<NUM>) mounted in the tubular body to drive a flow of air through the tubular body; and
a heater tube (<NUM>) comprising a heater matrix (<NUM>), the heater tube being mounted in the tubular body to heat the flow of air as it passes through the tubular body,
wherein the fan unit is an electric vaneaxial fan which is mounted upstream of the heater tube, that comprises a fan housing (<NUM>) and a fan core (<NUM>), the fan core comprising a stator core (<NUM>) and an impeller (<NUM>), the impeller being arranged to rotate about a central axis of the electric roofing torch;
wherein the tubular body comprises a double-walled structure comprising an inner tube (<NUM>) and an outer tube (<NUM>), the inner tube housing the heater tube and providing a conduit for the flow of air between an upstream end of the inner tube and a downstream end of the inner tube; and
wherein the fan housing provides a fan outlet (17b) that is arranged within the inner tube upstream of the heater tube; and
characterised in that the fan housing provides a fan inlet (17a) which protrudes beyond an upstream end of the outer tube.