Abstract:
A hairdryer comprises a housing, a fan mounted in the housing, driven by a battery powered motor, creating an airflow through the housing. A burner is in the housing in the airflow passage. The burner has fuel supply. An aerofoil-sectional sleeve baffle element surrounds the burner to separate the airflow passage into a first passage in contact with the burner, whereby the airflow in the passage is heated, and a second passage separated from the burner. A coned exit baffle is arranged to mix the airflows exiting hairdryer. An electronic control means pulses a solenoid valve to control fuel supply to the burner and thus the heat out-put therefrom.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of International Application No. PCT/GB2002/005498 filed Dec. 6, 2002. The International application published in English on Jun. 12, 2003 as WO 2003/047387. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to cordless hairdryers of the type comprising a rechargeable battery to power a motor that drives a fan, and a gas canister to supply gas to a catalyst for flameless combustion and which provides heat to the air driven by the fan.  
         BACKGROUND OF INVENTION  
         [0003]    The concept of such a hairdryer is well known, (see for example, U.S. Pat. No. 5,009,592, U.S. Pat. No. 4,635,382, U.S. Pat. No. 4,800,654, U.S. Pat. No. 4,903,416 DE-A-3103843) but no commercial product has yet been launched on the market. The reasons for this are complex and are connected with, among other things, the inefficiency of batteries and the inability to convert the potential energy in the gas to heat in the air in an efficient way.  
           [0004]    One of the problems addressed by the invention is to ensure that electronic or electrical failure does not compromise product safety, especially in view of the proximity of the gas reservoir and electrical components.  
           [0005]    Burners for such hairdryers are preferably of the flameless type, employing a catalyst that supports combustion of the fuel at a lower temperature than would be the case in the absence of the catalyst.  
           [0006]    Catalytic burners are safer, but sensitive, however, to changes in airflow and are easily extinguished; Preferably, two burners are present so that if one goes out the other can relight the extinguished one, before any safety mechanism cuts-off the fuel supply and necessitates restarting of the hairdryer.  
           [0007]    However, the very nature of hairdryers means that their airflow is frequently disturbed, in use, by the manipulations imposed by the user. Sometimes the outlet of the hairdryer will be blocked,,temporarily, by being held too close to the head, or even to thick wedges of hair. At other times, all exit resistance is suddenly removed, so that gushes of air pass through the hairdryer. Unfortunately, to get the air hot for hair drying, it needs to come into contact with the burner. Indeed, the residence time of the air in the hairdryer is very short, given that the quantity of air required by a satisfactory hair dryer is in the order of 10 to 20 litres per second.  
           [0008]    One approach is for the burner to heat a good heat conductor which is shaped into fins so that the air, passing over the fins becomes heated. However, there are few conductors that can conduct the heat fast enough for this application. Ideally, the air needs to come into direct contact with the burners and the hot exhaust gases to get sufficient heat quickly enough. But that is the nub of the problem: because instantaneous significant variations in airflow will over-cool, over-heat, or extinguish the burner, it is difficult to achieve the desired result without rendering the hairdryer too sensitive to such fluctuations.  
           [0009]    Another approach is to heat plates disposed in front of the burners as disclosed in U.S. Pat. No. 5,155,925, but this disturbs the air flow significantly.  
           [0010]    It is an object of the present invention to provide a hairdryer that overcomes the difficulties experienced, or at least mitigates their effects.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with the invention there is provided a hairdryer comprising:  
           [0012]    a housing, defining an air passage;  
           [0013]    a battery, adapted to power a motor and a fan driven by the motor to drive air down the air passage;  
           [0014]    a burner, in the air passage and in the form of a sleeve enclosing at its open end a catalyst; and  
           [0015]    a combustible gas supply flow to the burner;  
           [0016]    wherein a first, coned baffle is provided to deflect combusted gas exiting the burner so as to mix with air after flowing around the burner; and  
           [0017]    a second baffle is provided to shroud the burner and prevent air that flows in the passage around the second baffle and burner from cooling the burner.  
           [0018]    With this arrangement of heating the air by mixing the airflow with combusted gas after its exit from the burner, and by shrouding the burner so that there is no danger of the burner being cooled by the airflow, the risk that flameless combustion might be terminated is eliminated.  
           [0019]    Preferably, the sleeve is finned behind the burner to shed heat, conducted from the burner, through radiation and convection to the surrounding airflow.  
           [0020]    Said second baffle may extend from a fin of the sleeve, ideally the one closest to the burner.  
           [0021]    The burner may include a catalyst in the form of a coil of platinum coated wire.  
           [0022]    The first baffle preferably comprises a ceramic block that leaves an annular space in the air passage through which the combusted gas and air flow. Preferably, the first baffle is truncated at its rear end in the direction of air and combusted gas flow to create turbulence in the flow to ensure complete mixing of the air and combusted gas.  
           [0023]    Preferably, a third baffle is disposed upstream of the burner and creates a tubular flow of the air driven by the fan. Said third baffle may comprise the motor being arranged in the air passage downstream of the fan.  
           [0024]    Preferably, the fan is a radial fan drawing air axially through an end opening of the housing and expelling the air radially from the fan against shrouds that deflect the air into a tubular flow. Said shrouding may comprise the housing itself.  
           [0025]    Preferably, there are a plurality of said burners in the air passage, arranged with their longitudinal axes parallel. In this event, when there are two burners, the first baffle comprises a ridge parallel a line joining said longitudinal axes, and a hip at each end substantially on the longitudinal axis of the respective burner.  
           [0026]    In accordance with another aspect of the present invention there is provided a hairdryer comprising:  
           [0027]    a) a housing;  
           [0028]    b) a fan mounted in the housing, driven by a battery powered motor and creating an airflow in through an air inlet of the housing, along an airflow passage through the housing and out through an air outlet of the housing;  
           [0029]    c) a burner in the housing in the airflow passage;  
           [0030]    d) a gaseous fuel supply for the burner;  
           [0031]    e) a baffle element adjacent the burner to separate the airflow passage into a first passage in contact with the burner, whereby the air flow in said first passage is heated to a first temperature, and a second passage separated from the burner; and  
           [0032]    f) an exit baffle before said air outlet and arranged to mix the airflows exiting said first and second passages so that said mixed air has an exit temperature less than said first temperature.  
           [0033]    Preferably, a shroud is behind the burner with respect to said airflow to protect the burner from direct impacts from said airflow.  
           [0034]    Accordingly, this aspect of the present invention is characterised by the airflow being divided, so that only a proportion of the total airflow passes the burner and, as a result, that air gets much hotter than suitable for drying hair. However, since there is a much smaller flow over the burner, changes in total airflow have much less impact on the airflow over the burner. Moreover, beyond the burner, the hot air mixes with the remaining air and is cooled in the process, while at the same time warming the air that flows down the second passage separated from the burner, so that mixed air at a suitable hair-drying temperature exits the housing.  
           [0035]    Preferably, said baffle element is a baffle sleeve surrounding the burner dividing the airflow passage into said first passage, being annular and delimited internally by the burner and externally by the baffle element, and said second passage, being annular and delimited internally by the baffle element and externally by the housing.  
           [0036]    Preferably, pressure enhancement means increases the pressure of the airflow in the first passage, slowing it down. Said pressure enhancement means may comprise an aerofoil section of said baffle element encouraging more airflow into said second passage than said first passage. When said baffle element is a sleeve, a longitudinal section of the wall of the sleeve is said aerofoil.  
           [0037]    According to another aspect of the invention, there is further provided a hairdryer comprising a burner, a fuel reservoir storing fuel, an electromechanical, biased closed, solenoid valve for controlling fuel supply from the reservoir, and an electronic valve control circuit, wherein pulse width modulation of the on/off period of the valve regulates the heat level of the burner. Preferably the control circuit includes a valve control element that is electrically isolated from the control circuit; for example it can be opto-isolated. The control circuit may include one or more condition sensors, for example an under-temperature or over-temperature sensor.  
           [0038]    According to another aspect of the invention there is provided a hairdryer, preferably a cordless hairdryer, comprising a burner, a flow passage, and at least one baffle diverting flow in the passage, in which a condition sensor is provided in conjunction with the baffle. Preferably the condition sensor is an under-temperature or over-temperature sensor. Preferably the baffle shrouds the burner, preventing airflow from cooling the burner and/or deflects combusted gas exiting the burner so that it mixes with air flowing around the burner.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    Embodiments of the invention are further described hereinafter, by way of example, with reference to the accompanying drawings, in which:  
         [0040]    [0040]FIG. 1 is an exploded perspective view of the main components of a hairdryer according to the present invention;  
         [0041]    [0041]FIG. 2 is a section through the hairdryer;  
         [0042]    [0042]FIG. 3 shows a control and safety circuit diagram for the hairdryer;  
         [0043]    [0043]FIG. 4 shows user controls and indicators for the hairdyer;  
         [0044]    [0044]FIG. 5 shows one possible component format for the hairdryer;  
         [0045]    [0045]FIG. 6 shows an alternative possible format for the components of the hairdryer;  
         [0046]    [0046]FIG. 7 a  is an exploded diagram showing an alternative format for the hairdryer;  
         [0047]    [0047]FIG. 7 b  is a schematic side sectional view showing the format of FIG. 7 a;    
         [0048]    [0048]FIG. 8 a  is an exploded diagram showing a further alternative format for the hairdryer;  
         [0049]    [0049]FIG. 8 b  is a schematic side sectional view showing the format of FIG. 8 a;    
         [0050]    [0050]FIG. 9 a  is an exploded diagram showing a further alternative format for the hairdryer;  
         [0051]    [0051]FIG. 9 b  is a schematic side sectional view showing the format of FIG. 9 a;    
         [0052]    [0052]FIG. 10 a  is an exploded diagram showing a further alternative format for the hairdryer;  
         [0053]    [0053]FIG. 10 b  is a schematic side sectional view showing the format of FIG. 10 a    
         [0054]    [0054]FIG. 11 a  is an exploded diagram showing a further alternative format for the hairdryer;  
         [0055]    [0055]FIG. 11 b  is a schematic side sectional view showing the format of FIG. 11 a;    
         [0056]    [0056]FIG. 12 a  is an exploded diagram showing a further alternative format for the hairdryer;  
         [0057]    [0057]FIG. 12 b  is a schematic side sectional view showing the format of FIG. 12 b ; and  
         [0058]    [0058]FIG. 13 is a schematic illustration of another hairdryer in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0059]    Referring to FIGS. 1 and 2, a hairdryer  10  comprises a housing  12  which at one end consists of two clamshell halves  14   a,b , a middle, tubular section  16 , and an end nozzle  18 . In the clamshell end  14  is mounted a motor  20  driving a fan  22 . The fan  22  draws air axially through an end aperture  24  (which may be guarded by a grill (not shown) to prevent finger access to the fan  22 ). The fan  22  expels air radially, but the clamshells  14   a,b  are shaped so that air exiting the fan  22  is deflected longitudinally into the tubular portion  16  of the housing  12 . The arrangement therefore provides a tubular-like flow of air through the air passage  26  defined by the housing  12 . Indeed, the motor  20  defines an annular start  26   a  of the air passage  26 .  
         [0060]    A cross element  30  is mounted at the entrance to the tubular part  16  of the housing  12 . Screws  33  secure the cross in place. Two threaded bores  32  are provided in the cross  30 , one above the other, and into each of these is screwed a burner  34 .  
         [0061]    Each burner  34  comprises a brass collar  36  screwed into the bore  32  and provided with a gas terminal  38 . A gas pipe (not shown) from a gas supply (not shown) via a trigger or other supply control mechanism (not shown) is connectable to the terminal  38 . The terminal  38  is provided with a narrow bore that opens into the interior of the collar  36 , jetting gas therein. A neck  40  of pressed/sintered metal is screwed into the collar  36 . Inside the neck  40  is disposed a plug  42  of porous metal that serves to regulate gas flow through the burner  34 . On exit from the plug  42  gas flows along a bore  44  of the neck  40  where it mixes with air drawn in through a cross bore/port  46 .  
         [0062]    Air is drawn into the port  46  by virtue of the low pressure in the bore  44  caused by the high velocity gas flow therein and controlled by the plug  42 . A stoichiometric volume of gas and air therefore flows along the neck  40  and enters sleeve  50 . Finally, in a broad section  52  of the sleeve  50  is disposed a coiled catalytic wire, coated with platinum (not shown), where the combustible gas mix combusts in the absence of a flame.  
         [0063]    Finally, the combusted gases exit the burners  34  and impinge on a ceramic block baffle  54  to be deflected radially outwardly so as to mix with the tubular air flow caused by the fan  22 .  
         [0064]    A second baffle  56  is provided around the sleeves  50  to further define the tubular flow by defining annular space  26   c  of the air passage  26 . The second baffle  56  prevents the air flow from directly contacting the burners  34  in the region of the cups  52  where the catalyst is disposed. This ensures that the catalyst remains at the requisite temperature to guarantee complete combustion of the combustible gas. As shown, the baffle  56  is mounted on one of several fins  58  of each burner  34 , especially that one closest to the cup  52 . However, it could equally well be mounted on its own spider from the internal surface of housing part  16 .  
         [0065]    The fins  58 , which also extend to the neck  40 , serve to shed heat developed in the burner cup  52  and conducted along the material of the burner  34 . This heat is shed by both convection with the surrounding air flow, as well as by radiation. Indeed, the latter is more significant given a third baffle  60  which is, like the baffle  56 , a disc having a cupped edge. This also defines the tubular air passage by annular space  26   b.  The baffle  60  is held against the cross  50  by the burners  34  passing through apertures (not shown) in the baffle  60 . The baffle  60  guards the ports  46 , and prevents air flow, driven by the fan  22 , from drawing gas out of the ports  46 . Instead, stagnant air behind the baffle  60  is at a relatively high pressure compared with the gas flowing through the bore  44  of the neck  40  of the burners  34 .  
         [0066]    Returning to the second baffle  54 , it is mounted on a rod  62  and a further cross  64  inside the tubular housing part  16 . On the end of the housing part  16 , a nozzle  18  is disposed, which maybe shaped to further enhance mixing.  
         [0067]    As noted already, there are two burners  34 , each expelling hot exhaust gases against the baffle  54 . The baffle  54  has a ridge  66  aligned with the line joining the longitudinal axes of each burner. Most of the outflow from each burner is shed to one side or the other of the ridge  66 . However, a hip  68  is formed on the baffle  54  at either end of the ridge. Gases exiting a sector of each burner remote from the other burner are thus deflected at right angles to the ridge-deflected gases. This spreads the infiltration of the hot gasses into the air flow, so that more effective mixing occurs with less likelihood of hot spots occurring. In FIG.  5 , however, the arrangement is rotated through 90° so that each burner sheds its output to either side of the ridge  66 .  
         [0068]    The baffle  54  is terminated by a blunt end  70 , so that airflow around it is turbulent, further enhancing gas mixing and temperature stabilisation.  
         [0069]    As discussed above, in overview the hairdryer includes a gas burner and gas reservoir and a fan. In addition, as discussed in more detail below, a control circuit and a battery for powering the fan are provided. A first baffle deflects combusted gas exiting the burner so that it mixes with air flowing around the burner. A second baffle shrouds the burner, preventing airflow from cooling the burner.  
         [0070]    FIGS.  5  to  12  show various product configurations in schematic form for a hairdryer according to the invention, with the housing and control components not shown, for clarity. Each figure shows a cordless drier designated generally  300  including a fan  302 , a burner  304 , end nozzle  306 , battery pack  308  and gas canister  310 . The fan  302 , burner  304  and nozzle  306  define a generally linear axis  312  with the burner  304  intermediate the fan  302  and nozzle  306 , but various configurations of the remaining components are contemplated. In each case the design avoids obstruction of the airflow through the fan, to keep the power required to drive the fan to a minimum.  
         [0071]    Referring to FIG. 5 the battery pack with larger capacity cells is slung below and parallel with the axis  312 , in a 2×2 configuration forwardly of the fan  302 . The gas canister  310  projects perpendicularly below the axis  312 , between the fan  302  and battery pack  308 , and can be received in a handle significantly spaced from the burners, enhancing safety. In FIG. 6 the positions of the battery pack  308  with larger capacity cells and gas canister  310  are reversed away from the burners, increasing their capacity. In FIGS. 7 a  and  7   b  the arrangement is shown with the gas canister  310  positioned along the axis  312  but rearwardly of the fan  302 , and having a more squat configuration.  
         [0072]    Referring to FIGS. 8 a  and  8   b  the arrangement is similar to that shown in FIG. 6 but with the gas canister  310  placed above rather than below the axis  312 .  
         [0073]    The arrangement of FIGS. 9 a  and  9   b  is similar to that of FIG. 6 but with the profile of the gas canister  310  varied slightly so as to taper from the base up.  
         [0074]    Referring to FIGS. 10 a  and  10   b  the configuration is similar to that of FIG. 7 except that the orientation of the fan is changed. In particular is lies below and perpendicular to the axis  312 , impelling air towards the axis. As a result a suitable deflector would be required to divert the air towards the burner.  
         [0075]    [0075]FIGS. 11 a  and  11   b  is also similar to FIG. 7 except that the gas canister  310  is more elongate, as is the fan  302  housing. Also the battery pack  308  is provided in a 4×1 configuration.  
         [0076]    Referring lastly to FIGS. 12 a  and  12   b  all of the components are aligned with the axis  312 . The gas canister extends along the axis rearwardly of the fan  302  and the batteries are distributed around its circumference. In the embodiment shown the batteries are provided at 90° intervals in grooves provided in the outer wall of the canister.  
         [0077]    Turning in more detail to other aspects of the invention, control and safety circuits control the motor speed, burner level, burner ignition and provide safety shutdown in the case of various failure modes. As the controls and burner are likely to be physically remote, purely mechanical controls and safety features are less desirable. As an electrical power source is available (for the motor), at least part of the gas control system is preferably electrically powered and/or sensed. This provides the most versatile method of control, is the least involved and risky from a development point of view, and allows controls almost identical to a standard corded hairdryer.  
         [0078]    Referring to the control and safety circuit block diagram shown as  399  in FIG. 3, the rechargeable battery pack  400  supplies all electrical power to the hairdryer. The positive supply from the battery is switched through a mechanical on/off switch  402 . In the off position no power is available to energise the normally closed (spring return) solenoid operated gas shut-off valve  404 , comprising a reservoir valve.  
         [0079]    When the switch  402  is in the on position the live connection from the battery connects to a vane type airflow switch  406 . This consists of a plastic moulded vane and microswitch. When the fan is on and the airflow is above a minimum value the switch  406  closes and the battery supply  400  is connected to the over-temperature thermal switch  408 . In an alternative embodiment it is possible to omit the flow switch  406  and rely purely on the over-temperature switch  408  to detect reduced or absent airflow.  
         [0080]    The over-temperature thermal switch  408  of any appropriate type, is placed in close proximity to the burner to detect excessive heating for any reason (excessive burner level, restricted airflow, or failure of the fan/motor). If this switch  408  opens due to excessive temperature, power will be disconnected from the solenoid valve  404  shutting off the gas supply  414 . The hystereses of this switch  408  should be such that it will not close again until temperature is in the range where the under-temperature switch  416  has opened—preventing the possibility of the gas valve  404  being reopened as the unit cools down.  
         [0081]    The next components in the safety circuit  399  are the under-temperature thermal switch  416  and an override switch  418  connected in parallel. Except when the override switch  418  is held in the ignite position by the user, the under-temperature thermal switch  416  (of any appropriate type) will be open whenever the burner temperature is below a pre-determined value (indicating the burner  410  has not been or is not lit). This removes the supply to the gas control valve  404 , switching off the gas supply, if the gas burner  410  is not lit.  
         [0082]    Both the over-temperature and under-temperature switches comprise condition sensors which are provided on the baffle  56  shown in FIG. 1.  
         [0083]    In order to initially light the burner  410  the override switch  418  is moved to the ignite position which momentarily bypasses the under-temperature switch  416  to provide power to open the gas valve  404 . The override switch  418  also provides power to the burner ignition circuit  420  to ignite the gas. As the power supply to an electronic igniter circuit be hardwired through the safety circuit, it is not possible for the igniter circuit to operate unless the switch  418  is in the ignite position.  
         [0084]    Once the gas ignites, after a few seconds the under-temperature switch will close and the power supply  400  will remain connected to the solenoid valve  404  after the override switch is released. An opto-isolatec monitoring circuit  422  determines whether the under-temperature switch  416  has closed in order to illuminate an LED indicating that the burner has lit. This monitoring circuit  422  has no effect on the safety circuit and is provided purely as an indicator  424  to the user that the gas is lit and they can release the ignite/override switch  418 . Failure of this monitoring circuit  422 , whilst it may erroneously indicate whether or not the burner is lit, has no effect on the safety shutdown functions of the gas supply.  
         [0085]    In the case where the gas shut-off valve  404  also provides regulation of heat level using for example pulse width modulation of the on/off period of the valve, an opto-isolated signal via isolator  426  for the electronics switches the power on/off to the solenoid valve via isolator  426 . Failure of the electronics can never switch on the control valve when the other hardwired safety components indicate a failure condition.  
         [0086]    In the case where gas regulation is accomplished using a manual regulator valve  428 , the opto isolated control circuit is omitted and the gas solenoid valve  404  is connected directly to the terminals of the under-temperature and override switches  416 ,  418 .  
         [0087]    Motor control is not critical to the safety of the hairdryer, and therefore can be controlled electronically, as failure leading to overheating is detected by the hardwired over-temperature switch (and airflow switch if necessary). However safety critical components such as the solenoid valve  404  are hardwired using electromechanical components. Furthermore the HT supply  430  to the igniter  420 , and the connection to the solenoid valve are electrically opto-isolated via respective isolations of any appropriate type  432 ,  426  in the electronic control circuit  434 .  
         [0088]    According to this arrangement, the safety critical circuits cannot be compromised by failure of the electronics. The use of optical isolation components to prevent any monitoring or control circuits of the electronics providing power or incorrectly enabling part of the safety circuit ensure this. Failure of the electronic control circuits may prevent operation of the hairdryer, but always in a fail-safe manner.  
         [0089]    The safety circuit shuts off the gas supply in the event of failure to ignite the gas burner (or subsequent extinguishing of the burner, either controlled or due to a failure), an over-temperature condition (due to failure of the fan/motor, blockage of airflow etc.) or airflow less than a minimum level.  
         [0090]    As regards motor control and electronic gas regulation, there are two embodiments presented regarding the gas regulation (to provide different heat settings) One uses a manual regulator valve  428  to provide variable heat control and the other modulates the on/off period of the gas shutoff valve to control the average gas flow.  
         [0091]    The advantage of the electronic control of the solenoid valve is the reduction of mechanical parts (albeit at the increase of electronic control) and more freedom in the mounting position and type of controls.  
         [0092]    The electronic control circuit (ECC)  434  which is preferably a printed circuit board provides, in addition to the possible PWM control of the solenoid valve; PWM speed control of the fan motor  42 , igniter circuit  430  if desired (see section  3 . 11 ), battery monitoring circuit and low battery display LED  436 , burner lit monitoring and display LED  424 , mounting platform for on/off/ignition switch, mounting platform for burner and motor control switches  438 ,  440 , and mounting platform for gas solenoid valve  404  and a “junction box” for electrical cabling.  
         [0093]    A possible arrangement  499  of the control switches and indicators for a hairdryer according to the invention is shown in FIG. 4.  
         [0094]    A single on/off slide switch  500  controls all power from the battery pack to the hairdryer. When this switch is in the “off” position  502 , all power is removed from the electronics, fan motor and solenoid gas shut-off valve ensuring that the gas source is closed. The on/off slide switch  500  is recessed and must be depressed before it is possible to move it from the off position. This is to minimise the possibility of accidental operation.  
         [0095]    Moving the switch to the “on” position  504  provides power to the electronics and motor control circuits. The fan will run at the speed selected by the “fan speed”control switch  512 . This can be a 2 position switch giving two fan speeds, or a number of speed settings. The gas is still turned off at this point.  
         [0096]    The on/off slide switch  500  has a third momentary spring loaded position  506 . This is the “ignition”position where the user must hold the switch to ignite the gas. Moving the switch to this position opens the gas valve and turns on the HT spark igniter  430  (FIG. 3). Once the burner has lit, a temperature sensor detects this, an LED lamp  508  lights to indicate gas lit, and the user may release the switch  500  back to the “on” position  504 . If the user releases the switch before the gas lights, the gas supply will be switched off, and hairdryer will continue to blow out cold air. The switch must again be moved to the “ignite” position  506  to switch on the gas supply and enable the ignition circuit.  
         [0097]    The heat level is controlled by a switch (or manual regulator)  510  giving two heat levels or a number of heat settings.  
         [0098]    After use the user moves the slide switch to the “off” position  502 . This stops the fan and closes the gas supply valve. To re-ignite gas supply the user must repeat the ignition sequence.  
         [0099]    A “cool shot” options is also possible involving extinguishing and re-igniting the burner. Cold air operation is possible by switching the unit to “off”, then moving the switch to “on” without igniting the gas.  
         [0100]    As regards ignition of the burner, a preferred implementation is the electronic spark ignition system  430  discussed above that repeatedly produces a spark until the gas has been lit or the user releases the ignition button. Such an electronic system can be used in a similar fashion to a conventional mains powered dryer.  
         [0101]    The cheaper alternative is a simple mechanical piezo-ignition system of any appropriate type. The piezo would be triggered once when the user moves the on/off/ignite switch  500  into the ignition position. The disadvantage is that the user will need to repeat the process if the burners fail to light, and this might not be apparent for several tens of seconds if the burners are not visible.  
         [0102]    Further safety and related aspects include the placement of the spark electrodes, chosen to further minimise the risk of ignition of gas products due to the build up of silicone on the spark electrodes.  
         [0103]    Also, to prevent hair ingress into the product a filter or grill is placed at the air inlet. This prevents hair being drawn in and becoming entangled with the fan and keeps other debris out.  
         [0104]    Furthermore, to prevent debris drawn into the hairdryer (eg. fluff, hair, etc.) from being ignited by the gas burners and ejected as burning or very hot particles, firstly the air inlet filter will prevent most particles from being drawn into the hairdryer, and secondly, the risk of any particles that are drawn in, coming into contact with a flame or a very hot surface is minimised by the product design. In particular, the gas burners are catalytic and therefore burn without a substantial flame at temperatures of approximately 500-800 C. (rather than 1300 C.). Furthermore, these hot catalytic surfaces are protected from the intake air to maintain combustion. Thus particles drawn through the product are not exposed to high temperatures capable of posing a hazard.  
         [0105]    Yet further, products that might reasonably be used with the hair dryer such as gels and hairsprays do not present a safety risk as a result of various features.  
         [0106]    For example, airflow is controlled through the product so that aerosols etc are less likely to come into contact with surfaces that are sufficiently hot to ignite thenm, the spark ignition source is appropriately placed, the operating temperature of the catalytic burners is reduced, and combustion occurs in a safe fashion with the hairdryer.  
         [0107]    The invention as described herein is hence simple to use and similar in use to existing corded dryers. It provides gas shutdown in the case of failure of the burners to ignite, or extinguishing for any reason, two (or more) levels of heat output and fan speeds, gas ignition, reduced airflow detection and gas shutdown, over-temperature detection and gas shutdown, under-temperature detection and gas shutdown (which may be used to detect ignition failure), battery low indicator and burner on indicator.  
         [0108]    In FIG. 13, a hairdryer  10 ′ comprises a housing  112 , which may be in the form of a clamshell housing commonly used in implements of this type. A motor  114  is mounted in the housing and drives an impeller  116 . The impeller  116  draws air through an inlet  118  at the rear of the housing  112 , which inlet is guarded by a grill  120  to prevent the ingress of particles or, indeed, hair. The motor  114  is powered by a rechargeable battery  122  disposed at the base of a handle  124  of the hairdryer  10 ′. Also in the handle  124  is a butane gas supply or canister  126 . A gas conduit  128  leads to a catalytic burner  130 .  
         [0109]    A baffle element  132  surrounds the burner  130  and divides the airflow from the fan  116 , which exits an outlet  34  at the front of the housing  112 . The airflow is divided into first and second passages  136 , 138 . Both passages are annular, the first passage  136  being defined by the burner  130  and baffle element  132 , whereas the second passage  138  is defined by the baffle element  132  and the housing  112 .  
         [0110]    Behind the burner  130  is a shroud  140 , which shroud serves to prevent airflow from the fan  116  from directly impinging the burner  130 . Instead, an opening  142  is provided between the sleeve baffle  132  and the shroud  140 .  
         [0111]    Longitudinally, the sleeve baffle  132  has an aerofoil section, so that most of the airflow is directed through the second passage  138 . This has the effect of slowing the flow through the first passage  136  and increasing its relative pressure.  
         [0112]    The burner  130  is a catalytic burner and heat both radiates from the burner, as well as entering the airflow in the first passage  136  through the exhaust gas products of the combustion. In either event, the air in the first passage  136  is heated to a temperature substantially greater than the desired temperature of the output from the hairdryer  10 ′. However, the air passing around the sleeve  132  in the second passage  138  is not heated at all (at least, not to a significant extent). Air exiting the sleeve  132  at its outlet  144  mixes with the air from passage  138 . An exit baffle  146  is provided in front of the opening  144  to deflect the air from the first passage to ensure mixing with the air around the baffle  132 . Indeed, although shown diamond-shaped, with a tail  148 , it may be preferred that the tail  148  is omitted, so that a blunt end is provided on the baffle  146 . This ensures turbulent mixing of the two airflows behind the baffle  146 .  
         [0113]    In any event, the air exiting the hairdryer  10 ′through the outlet  134  has a mean temperature much lower than the temperature of the air around the burner  130 . It is at a temperature suitable for drying hair, somewhere between 40 and 80° C., depending on heat settings and fan speeds which are user selectable items.  
         [0114]    It will be appreciated that features and components from the various embodiments can be combined or interchanged as appropriate without departing from the inventive concept. The individual components described, to the extent they are generic or off-the-shelf products, will be well known to the skilled reader and hence have not been described in detail.