Patent Publication Number: US-2016242524-A1

Title: Hair dryer with air outlet arrangement

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of hair dryers, and more specifically to an air outlet arrangement. 
     BACKGROUND OF THE INVENTION 
     A hair dryer is a device for drying hair by blowing heated air over the hair in order to accelerate the evaporation of water. An electrical hair dryer typically comprises a housing with an air inlet and an air outlet, a fan for generating a flow of air from the air inlet to the air outlet and a heating element for heating the flow of air. Nozzles, also referred to as concentrators, are optionally positioned at the air outlet, for example to increase the air speed for styling purposes. The user of a hair dryer with attached nozzle has the impression that the dryer is more “powerful” than without nozzle. 
     For quick drying, a strong heating power is desirable. At the beginning of the evaporation process, the water held by the hair forms a thermal screen which prevents deterioration or burning of the hair or the scalp. As the water evaporates, it will be less and less of a screen. At high temperatures, the hair can be damaged and/or a burning sensation may arise, which may even lead to actual burning. The problem is to achieve fast drying without damaging the hair. 
     Especially when styling hairs with a brush, many users come very close to the hair with the air outlet or nozzle. If they get too close, for example less than 2 cm, the hair may be (partly) shutting off the air outlet area which reduces the air flow significantly. A nozzle does not only affect an increase of air speed but also forms a resistance for the air stream as well. The resistance can cause a reduction of the rotation speed of the fan and decrease the air flow. The decreased air flow corresponds to a lower air mass per unit time which passes the heating element. In consequence, the lower air mass is heated up to a higher temperature. The air temperature can increase up to potentially dangerous, hair damaging values. Implementations according to the prior art suggest adapting the fan speed and/or heating power to solve the problem of overheating. 
     U.S. Pat. No. 5,790,749 discloses an electrical hair dryer with a sensor that measures the temperature of the hair and controls a fan speed of the fan and a heating power of the heating element to prevent overheating and damaging the hair. When the temperature of the hair gets too hot the fan speed and the heating power can be reduced. However, the solution proposed in U.S. Pat. No. 5,790,749 involves additional effort for sensing and control circuitry. This leads to a high complexity of today&#39;s hair dryers and increases the overall costs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a hair dryer that mitigates an overheating of hair and having a reduced complexity and cost. The invention is defined by the independent claims. The dependent claims define advantageous embodiments. 
     In a first aspect of the present invention an electrical hair dryer is presented that comprises a housing with an air inlet and an air outlet arrangement, a fan for generating a flow of air from the air inlet to the air outlet arrangement and a heating element for heating the flow of air. The air outlet arrangement comprises a flow changing device for changing a flow of air at the air outlet arrangement depending on a pressure and/or temperature within the housing. 
     In a further aspect of the present invention a nozzle for a hair dryer is presented that comprises a flow changing device for changing a flow of air at the nozzle depending on a pressure and/or temperature within the nozzle. The nozzle can be part of an air outlet arrangement of an electrical hair dryer. The nozzle can be an integral part of the housing or formed as a separate part that can be attached to the hair dryer. It is to be noted that the term “nozzle” is to be understood as any type of air outlet arrangement. A “nozzle” in the sense of the present disclosure also includes a diffuser or an extension part that is attached to the front part of a hair dryer. 
     Instead of controlling the fan speed or heating power as proposed in U.S. Pat. No. 5,790,749, the inventors have identified an alternative solution to prevent an overheating of hair. The hair dryer according to the present invention comprises a housing with an air outlet arrangement that comprises a flow changing device for changing a flow of air at the air outlet arrangement depending on a pressure and/or a temperature within the housing. Instead of electrically controlling fan speed and heating power, the air outlet arrangement according to the present invention features additional structural elements that modify the flow of air. These structural elements, referred to as flow changing device, alter the flow of air at the air outlet arrangement either by changing the direction of the flow of air or by providing alternative pathways for the flow of air. For example if an obstacle such as a brush with hair blocks an opening of the air outlet arrangement, the flow of air can be guided away from the obstacle by changing the flow direction. This way, the flow of air can freely pass by the obstacle. Alternatively, the flow changing device provides additional pathways. For example, if one pathway is blocked or at least partially blocked, the heated air can exit through an alternative opening of the air outlet arrangement. 
     The term “air outlet arrangement” as used within the context of the present invention is to be understood as a part of the housing of the hair dryer preferably a front part of the housing. Optionally a nozzle is part of the air outlet arrangement. The air outlet arrangement comprises one ore more air outlet openings through which air can exit the housing. The simplest example of an air outlet arrangement is one air outlet opening of disk shape through which the entire flow of air from the air inlet exits the housing. Alternatively, the air outlet arrangement comprises a plurality of air outlet openings. One or more of the air outlet openings can be located on a side of the housing. Further, the air outlet arrangement can comprise a flow guiding device for guiding the flow of air to desired air outlet openings. A “nozzle” as used within the context of the present invention can comprise similar features. 
     Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed nozzle has similar and/or identical preferred embodiments as the claimed hair dryer and as defined in the dependent claims. 
     According to a first embodiment of the present invention, an effective cross section of the air outlet arrangement for letting out air depends on the pressure and/or the temperature within the housing. The term “effective cross section” as used within the context of the present invention relates to the sum of the cross sections or areas of the one or more openings of the air outlet arrangement for letting out air. 
     According to a variant of this embodiment above a pressure threshold and/or a temperature threshold the effective cross section is automatically increased. If an obstacle (partially) blocks an opening of the air outlet arrangement, the air flow that can exit the air outlet arrangement is reduced. The decreased air flow corresponds to a lower air mass per unit time which passes the heating element. In consequence, the lower air mass is heated up to a higher temperature. To prevent damaging the hair, the air outlet area of the air outlet arrangement is increased above a temperature threshold of, for example, 100° C. Experiment of the applicant have shown that thermal damages of the hair may already occur at a temperature of 100° C. At higher temperatures up to 200° C. heavy burnings of the hair and the scalp can be caused. 
     Since the air outlet area is increased by increasing the effective cross section of the air outlet arrangement, a larger air mass passes the heating element and exits the hair dryer. This larger air mass can only be heated to a lower temperature given that the heating power remains unchanged. Below the damage threshold, there is no need to change air flow at the air outlet arrangement. Alternatively, a pressure threshold is selected. The pressure within the air outlet arrangement is indicative of the flow of air that exits the air outlet arrangement and also indicative of the air temperature. 
     An air outlet arrangement with a nozzle typically has a limited effective cross section during normal operation in order to ensure a fast, powerful stream of air. When styling hairs with a brush, a user may come too close to the hair with the nozzle and block a substantial part of the cross section. In consequence the air flow that exits the housing through the air outlet arrangement reduces even more. As explained above, the reduced amount of air that now passes the heating element is heated up to a higher temperature. Furthermore, the pressure within the housing increases as the obstacle increases the resistance for the flow of air. In the present embodiment, the flow changing device of the air outlet arrangement change the flow of air that exits the outlet arrangement by increasing the effective cross section of the air outlet arrangement. In consequence, the flow of air increases. As the effective cross section of the air outlet arrangement is increased, the flow of air is not limited to exiting the air outlet arrangement through a narrow space besides the obstacle (e.g. a brush) but has a pathway of larger effective cross section. The heating power of the heating element heats up the air mass that passes the heating element. As the flow of air and thus the air mass is increased, the air temperature is reduced. 
     In this example, the effective cross section remains unchanged until reaching a pressure threshold and/or a temperature threshold. In a first example, the effective cross section has a first value below the pressure threshold and/or the temperature threshold and a second effective cross section above the pressure threshold and/or the temperature threshold. Alternatively, the effective cross section has a first value below the temperature threshold and/or the pressure threshold and increases continuously above said threshold. In a further alternative, the effective cross section increases in discrete steps above said threshold. 
     According to a further embodiment of the present invention the flow changing device comprises a mechanical shutter. For example, a mechanical flap acts as a shutter that at least partially covers an air outlet opening of the air outlet arrangement. At low pressure, the flap can be in closed state, whereas the flap continuously opens with increasing pressure. Alternatively, a slider covers an outlet opening of the air outlet arrangement at low temperature and slides back to an open state at higher temperatures, for example when the temperature exceeds a temperature threshold. Any type of shutter can be used including flaps, slides or rotational shutters. The shutter is operable with a folding, sliding or rotational movement respectively. It goes without mentioning that any type of mechanical shutter can be used and that the operation of said shutters can be continuous, in small steps, or a binary change between opened and closed state. The mechanical shutter can also be combined with non-mechanic elements and further electrical, mechanical or electro-mechanical actuators. 
     According to another embodiment of the hair dryer according to the present invention, the flow changing device further comprises a spring element for exerting a closing force on the mechanical shutter that counteracts an opening force on the mechanical shutter, which opening force is caused by the pressure within the housing. In this particular embodiment, the flow changing device change a flow of air at the air outlet arrangement depending on a pressure within the housing. For example, if an obstacle (partially) blocks an opening of the air outlet arrangement, the pressure within the housing increases. The pressure within the housing causes an opening force that presses against the mechanical shutter. The spring element of the flow changing device exerts a closing force on the mechanical shutter that tries to close the mechanical shutter or hold the mechanical shutter closed against the opening force. When the opening force caused by the pressure within the housing exceeds the closing force, the mechanical shutter opens. As described above, an opening of the mechanical shutter can increase the effective cross section of the air outlet arrangement. Hence, the flow of air can (partially) pass through the airway that was opened by the shutter. As the flow of air exits the housing, the pressure within the housing decreases. When the closing force exceeds the opening force, the shutter has returned to its closed position. The pressure threshold when the shutter opens can be set by the spring constant of the spring element. 
     In an alternative embodiment according to the present invention, the flow changing device further comprises a thermal actuator for operating the mechanical shutter wherein said thermal actuator is adapted to change its form depending on the temperature. For example, the thermal actuator is a bi-metallic strip. Bi-metallic strips are known from simple thermometers wherein the bi-metallic strips cause a temperature indicator to move over a temperature scale depending on a temperature. In the present example, the mechanical shutter could be operated by a bi-metallic strip. When the temperature within the housing increases, the deformation of the bi-metallic strip causes the mechanical shutter to open up. For example, the bi-metallic strip exhibits a bending movement and the mechanical shutter is directly coupled to the bi-metallic strip. Alternatively, the bi-metallic strip causes a pushing or pulling movement that is then translated onto the mechanical shutter by further mechanics. Alternatively, the thermal actuator is an element that elongates with increasing temperature and shortens with decreasing temperature or vice versa. This movement can again be translated to an opening or closing of a mechanical shutter. A temperature threshold for the shutter to open depends on the characteristics of the bi-metallic strip. 
     According to an alternative embodiment, the hair dryer further comprises a pressure sensor and/or a temperature sensor and an electro-mechanical actuator for operating the flow changing device. 
     According to another embodiment of the present invention, the air outlet arrangement comprises a main air outlet and a bypass air outlet. The bypass air outlet is an alternative pathway to the main air outlet for the air stream to exit the air outlet arrangement. The bypass can be activated by an increase in pressure within the housing, for example when the air outlet is blocked. Alternatively, the bypass air outlet can be activated by an increase of temperature within the housing. 
     The bypass air outlet can be arranged at any part of the air outlet arrangement other than the main air outlet. For the example of an electrical hair dryer, the bypass air outlet can be arranged at a main part of the housing or at the nozzle. In any case, the bypass air outlet is preferably arranged close to the main air outlet i.e. it is part of the air outlet arrangement. 
     According to another embodiment of the hair dryer, the bypass air outlet is adapted to act as a safety valve. The term “safety valve” refers to a device that ensures that a temperature and/or pressure within a system does not exceed a predetermined threshold and is adapted to release for example a stream of air from a housing. This additional release path ensures that the temperature and/or the pressure within the housing return to a value below said threshold. 
     According to a further embodiment of the hair dryer, the bypass air outlet comprises a mechanical shutter, wherein the mechanical shutter is adapted to operate the bypass air outlet. In this example, the mechanical shutter at least partially closes the bypass air outlet. In one embodiment, the main air outlet does not feature a mechanical shutter and only the bypass air outlet features a mechanical shutter. Alternatively, both the main air outlet and the bypass air outlet comprise a mechanical shutter. Furthermore, a bypass air outlet can comprise a plurality of mechanical shutters or alternatively one shutter can be used for a plurality of bypass air outlets or a bypass air outlet and a main air outlet. 
     According to an alternative embodiment of the present invention, the bypass air outlet is configured to guide heated air substantially in direction of the main air outlet. This embodiment is particularly desirable to avoid a waste of heating power. Instead of reducing the heating power of the heating element as proposed in the prior art, this embodiment of the present invention maintains the heating power and distributes the heated air through a bypass air outlet. Even though the main air outlet may be covered, for example with hair on a brush, heated air that exits in direction of the main air outlet still serves to dry the surrounding hair. For example, an opening of the bypass air outlet is oriented in direction of the main air outlet. Hence, air that exits through this opening is guided in direction of the main air outlet. Alternatively, the mechanical shutter of the bypass air outlet acts as an air guide that guides the flow of air in direction of the main air outlet. For example a flap acts as an air guide. Alternatively, the air outlet arrangement features additional air guiding elements or baffles adapted to guide the air in substantially direction of the main air outlet in order to dry the surrounding hair. 
     In an alternative embodiment, the bypass air outlet is configured to guide heated air substantially away from the direction of the main air outlet. In this embodiment, the stream of air that exits through the bypass air outlet is guided away from the hair of the user in order to avoid disturbances that could negatively affect the hair style. For example, if the user already fixed their surrounding hair beside the main air outlet it is not desirable to have a flow of air onto said surrounding hair. 
     According to an alternative embodiment of the hair dryer according to the present invention, the heating element and/or the fan is configured such that an air temperature of the flow of air is limited to a maximum value, when the flow of air exits through the bypass air outlet only. Assuming that the main air outlet is completely blocked, the entire flow of air will exit through the bypass air outlet. Even in this extreme scenario, the air temperature does not cause damage to the hair or discomfort to the user. The power of the heating element and/or the fan speed is dimensioned such that even when the entire flow of air exits through the bypass air outlet only, an overheating is prevented. A bypass air outlet with large effective area will allow a higher maximum heating power. 
     According to yet another embodiment of the present invention, a direction of letting air out of the air outlet arrangement depends on the pressure and/or temperature within the housing. As an alternative to changing the effective cross section of the air outlet arrangement for letting out air, the flow direction can be manipulated. For example, if an obstacle blocks the air outlet, a baffle can be used to steer the flow of air away from the obstacle. Similar to the above, said baffle can be operated by a spring element, a thermal actuator or an electro-mechanical actuator. A mechanical shutter or flap can act as a baffle. Alternatively a nozzle or part of a nozzle can swivel to the side away from the obstacle. 
     According to a further embodiment of the hair dryer, the flow changing device further comprises a fixation device for fixing the flow changing device in a desired position. This additional feature enables the user to overrule the automatic flow changing mechanism of the air outlet arrangement. This feature is particularly helpful for a skilled person who wants to have more control, for example over the air temperature. The operation of said fixation device can be continuous, in small steps, or a binary change between opened and closed state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings 
         FIG. 1  shows a general technical concept of an electrical hair dryer, 
         FIG. 2 a    shows an air outlet arrangement according to the present invention during operation with closed flow changing device, 
         FIG. 2 b    shows the air outlet arrangement according to the present invention during operation with open flow changing device, 
         FIGS. 3 a  and 3 b    show a second embodiment of the air outlet arrangement according to the present invention, 
         FIGS. 4 a  and 4 b    show a third embodiment of the air outlet arrangement according to the present invention, 
         FIGS. 5 a  and 5 b    show a fourth embodiment of the air outlet arrangement according to the present invention, 
         FIGS. 6 a  to 6 c    show a fifth embodiment of the air outlet arrangement according to the present invention, and 
         FIGS. 7 a  and 7 b    show a sixth embodiment of the air outlet arrangement according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows an electrical hair dryer  100  comprising a housing  110  with an air inlet  111  and an air outlet arrangement  112 . In this particular embodiment the air outlet arrangement  112  also features a nozzle  113 . The nozzle  113  is optional. In the housing  110  there are a fan  120  and a heating element  130 . The fan  120  generates a flow of air from the air inlet  111  to the air outlet arrangement  112 . Any type of air flow generating device can be used as a fan. The heating element  130  heats the flow of air. Different types of heating elements are available for example coils of wire with high electric resistivity such as nichrome wires, infrared heaters and ceramic heating elements. The fan speed is controlled via a fan control switch  122  that allows to set the fan speed. The heating power of the heating element  130  is controlled via a heat control button  132 . In this embodiment it is possible to control fan speed and heating power independently from each other. However, in a different embodiment there can be a joint control element or a simple on/off button. 
       FIG. 2 a    shows an electrical hair dryer and more particularly an air outlet arrangement  220  according to the present invention during operation. 
     A flow of heated air  230  is directed onto the hair  201  of a user  200 . The flow of air  230  exits the air outlet arrangement  220  through a main air outlet  221 . Further, the air outlet arrangement  220  according to the present embodiment of the invention features an upper flow changing device  223  and a lower flow changing device  224 . Alternatively only a single flow changing device  223  or a plurality of more than two flow changing devices can be used. The flow changing devices  223 ,  224  are implemented as flaps. 
     In  FIG. 2 a   , the flap remains in closed position. The main air outlet  221  is located at a distance from the hair  221  of the user  200  such that the flow of air  230  can freely exit the air outlet arrangement  220  through the main air outlet  221 . Hence, the flow of air  230  can exit the air outlet arrangement  220  without being blocked and thus without an increased dynamic pressure P 1  within the air outlet arrangement  220  as a part of the housing of the hair dryer. In consequence, the amount of air that flows by the heating element does not get overheated and stays in a safe temperature range. 
     In  FIG. 2 b   , the user  200  uses a round brush  210  for styling, in particular for curling hair ends. When the air outlet arrangement  220  gets too close to the brush  210 , the brush  210  acts as an obstacle that shuts off at least a part of the main air outlet  221 . In consequence the flow of air  231  at the main air outlet  221  is blocked. In consequence the pressure P 2  of the air within the air outlet arrangement  220  increases. Furthermore, less air passes by the heating element such that also the temperature increases. In addition to the main air outlet  221 , the air outlet arrangement  220  features an upper bypass air outlet  225  and a lower bypass air outlet  226  that are covered by an upper flow changing device  223  and a lower flow changing device  224  respectively. The flow changing devices are adapted to act as a safety valve. The flow changing devices  223 ,  224  are implemented as a mechanical shutter in form of a flap that covers the bypass openings  225 ,  226 . 
     In  FIG. 2 a   , with low pressure P 1  within the air outlet arrangement  220 , these flaps are in closed position, entirely covering the bypass air outlets  225 ,  226 . In  FIG. 2 b   , with higher air pressure P 2  within the air outlet arrangement  220 , the flaps open and allow the flow of air  232  to exit through the bypass air outlets  225 ,  226 . In consequence the air flow through the hair dryer from the air inlet to the air outlet arrangement increases and also the amount of air passing the heating element increases. Thereby, the heating power of the heating element is absorbed by a larger air mass that is heated to the lower temperature. In particular the temperature is below a temperature value that would cause damage to the hair  201  and/or discomfort to the user  200 . 
     In other words, when both the main air outlet  221  and the bypass air outlets  225 ,  226  are open, the effective cross section of the air outlet arrangement  220  for letting out air is increased. 
     In one embodiment, the size of the bypass air outlets  225 ,  226  is selected such that the air temperature is limited to a non-damaging maximum temperature value when the flow of air  232  exists through the bypass air outlets  225 ,  226  only. Alternatively, the size of the single air outlet  225  can be selected accordingly to prevent an overheating. 
     In the example shown in  FIG. 2 b   , the bypass air outlets  225 ,  226  are configured to guide the heated air substantially in direction of the main air outlet  221 . Even though the main air outlet  221  which is directed at the hair  201  of the user  200  is partly or completely blocked, the flow of heated air  232  that exits the air outlet arrangement  220  through the bypass air outlets  225 ,  226  is still guided towards the hair  201  of the user  200 . In many known hair dryers, the heating power and/or fan speed are reduced, when it is detected that the hair  201  is overheating. Hence, the power of the known hair dryer is reduced such that the potential power of the hair dryer is not fully exploited. The hair dryer according to the present invention does not require a reduction of power but opens an alternative pathway for the flow of air  230 ,  232  to be guided in direction of the hair  201  of the user  200 . Thus, the hair dryer according to the present invention can reduce the drying time. 
       FIGS. 3 a  and 3 b    show a more detailed cross section of an air outlet arrangement  300 . The air outlet arrangement  300  comprises a front part  320  of the housing and an optional nozzle or concentrator  310  to increase the air speed. The nozzle  310  is attached to the front part of the housing  320  by a clip mechanism  311 . Alternative attachment devices such as a thread or latches are possible. 
     In this embodiment, the flow changing device is not integrated into the nozzle but into the front part of the housing  320  of the dryer. This way, the flow changing device can change a flow of air at the air outlet arrangement  300  also if the device is operated without the nozzle  310 . 
     In  FIGS. 3 a  and 3 b   , an upper flow changing device is implemented by a flap  331  that acts a mechanical shutter of a bypass opening  334 , a hinge or joint  332  that connects the flap  331  with the sidewall of the front part  320  of the housing. Alternatively, as shown with the lower flow changing device, a thermal actuator  343  can be used for operating a flap  341  that acts as a mechanical shutter of a bypass opening  344 . 
     Referring to the upper flow changing device, the spring  333  exerts a closing force F S1  on the flap  331  that holds the flap closed against a first opening force F P1  on the flap  331 . The opening force F P1  is caused by a pressure P 1  within the housing. In  FIG. 3 a   , the opening force F P1  is smaller than the closing force F S1 . 
     In  FIG. 3 b   , an obstacle  390  blocks a main air outlet  312 . As described above, the pressure within the air outlet arrangement  300  increases to a pressure P 2 . As the pressure within the air outlet arrangement  300  increases, the opening force that acts on the mechanical shutter  331  increases. At the threshold when the opening force F P2  exceeds the closing force F S1  from the spring  333 , the flap  331  opens and enables a flow of air  335  to exit through the bypass air outlet  334 . The flap opens to a state where the opening force F P2  and the closing force F S2  are equal. It the pressure within the housing is reduced, e.g. when the obstacle  390  is removed, the flap  331  closes again. 
     In an alternative embodiment the flap  341  is operated by a thermal actuator  343 . During normal operation without obstacle  390 , the temperature within the air outlet arrangement  300  is at temperature T 1 . The thermal actuator  343  holds the flap  341  in a closed position. When an obstacle  390  blocks the main air outlet  312 , the air inside the air outlet arrangement  300  is heated to a higher temperature T 2  that causes a temperature dependent deformation of the thermal actuator  343 . In this example, the thermal actuator  343  is implemented as bi-metallic strip. The flap  341  is coupled to the thermal actuator  343  such that the flap  341  opens as the thermal actuator deforms. This opens a bypass air outlet  344  that enables the flow of air  345  to exit the air outlet arrangement  300 . In consequence, the flow of air out of the air outlet arrangement  300  increases, the air mass passing the heating element increases and thereby the temperature of the flow of air reduces. This reduction of air temperature prevents or at least reduces damage to the hair of the user. 
     In the example shown in  FIGS. 3A and 3B , the upper flow changing device changes the flow of air at the air outlet arrangement depending on the pressure P within the air outlet arrangement. The lower flow changing device changes the flow of air at the air outlet arrangement depending on the temperature T within the air outlet arrangement. This is particularly beneficial if the temperature and the air speed of the hair dryer can be selected independent from each other. Alternatively, both flow changing devices change the flow of air depending on the temperature. Further alternatively, both flow changing devices change the flow of air depending on the pressure. In another embodiment, a shutter is operated depending on the temperature and the pressure in parallel. The latter can be beneficial for a hair dryer within independent fan speed control  122  and heating element control  132 . 
       FIGS. 4 a  and 4 b    show an alternative embodiment of an air outlet arrangement  400  according to the present invention. The air outlet arrangement  400  comprises the front part of the housing  420  and a nozzle  410 . In this embodiment, the flow changing devices are implemented in the nozzle  410 . Implementing the flow changing devices in a nozzle  410  is an advantage of the present invention over the prior art, because an existing device can be retrofitted with a nozzle  410  according to the present invention. The nozzle  410  comprises a fixed portion  432  and a mechanical shutter implemented as a slider  431 . The fixed portion  432  comprises an opening  435 . The slider  431  comprises an opening  434 . When there is no obstacle  490  in front of the main air outlet  412 , the pressure within the air outlet arrangement  400  is P 1 . This pressure P 1  causes an opening force F P1  on the slider  431 . A spring element  433  connecting the slider  431  to the fixed portion  432  of the nozzle  410  exerts a counteracting, closing force F S1  on the slider. Without obstacle  490  in front of the main air outlet  412 , the closing force F S1  exceeds the opening force F P1 . 
     In  FIG. 4 b    an obstacle  490  partially blocks the main air outlet  412  such that only a reduced flow of air  436  can exit the air outlet arrangement  400  through the main air outlet  412 . In consequence, the pressure P 2  within the air outlet arrangement  400  increases and also causes an increase of the opening force F P2  causing the slider  431  to slide to the side. In this configuration, the slider opening  434  slides over the opening  435  of the fixed portion  432  of the nozzle  410 . Both openings  434 ,  435  now form a bypass air outlet that allows a flow of air  437  to exit the nozzle  410  of the air outlet arrangement  400 . The pressure and temperature within the air outlet arrangement are affected accordingly and thereby prevent overheating. 
     In this embodiment, the flow of air  437  exits to the side of the nozzle  410  and is not guided in direction of the main air outlet  412 . This can be desirable especially during hair styling in order to prevent disturbances that could mix up the hair. 
       FIGS. 5 a  and 5 b    show a similar embodiment with a slider  531  and a fixed portion  532  of the air outlet arrangement  500 . In this embodiment, a flow of air  537  existing through a bypass air outlet  538  is guided substantially in direction of the main air outlet in order to dry the surrounding hair. 
       FIGS. 6 a  to 6 c    show a fifth embodiment of an air outlet arrangement according to the present invention. A single air outlet  612  changes a flow of air at the air outlet arrangement  600  by changing the opening of the air outlet  612  depending on a pressure and/or temperature within the air outlet arrangement  600 . The air outlet arrangement  600  comprises a base  630 , an upper jaw  631  and a lower jaw  632  that are movably attached to the base  630 . Between the upper jaw  631  and the lower jaw  632  there is a wall element  633  that prevents air from leaking out at the side between the jaws. In a first embodiment, the sidewall  633  is made from a material that expands with increasing temperature and contracts with decreasing temperature. Hence, if an obstacle  690  blocks the air outlet  612  and thereby causes an increase in temperature, the sidewall  633  widens and thereby causes the jaws  631  and  632  to separate further. This increases the effective cross section of the air outlet  612  and allows the stream of air to (partially) pass by the obstacle  690 . In consequence, the temperature of the flow of air is reduced. 
     In an alternative embodiment, the upper jaw  631  and the lower jaw  632  are held closed by a spring  640 .  FIGS. 6 b  and 6 c    show a front view of the air outlet arrangement. The pressure P 1  within the air outlet arrangement exerts an opening force F P1  on the jaws that is counteracted by a closing force F S1  from the spring element  640 . As the pressure within the air outlet arrangement  600  increases, the opening force increases to F P2 . The opening of the air outlet arrangement widens as shown in  FIG. 6 c   . As the spring element  640  is stretched, also the closing force F S2  increases until reaching a balance of forces, wherein F P2  equals F S2 . 
     The upper sidewall  634  and lower sidewall  635  at the side of the air outlet  612  prevent air from leaking out to the side. Alternatively, also the sidewalls comprise openings to form an additional air outlet similar to the embodiment shown in  FIGS. 4 a  and 4 b    as they move with respect to each other. 
     Instead of using a dedicated spring element as shown in  FIGS. 6 b  and 6 c   , it is of course possible to use a sidewall  633  made from an elastic material that acts as a spring. Alternatively, a spring can be mounted at the joint  637  similar to the embodiment shown in  FIGS. 3 a    and  3   b.    
     In a further alternative of the embodiment shown in  FIG. 6 a   , the lower jaw  632  is fixed to the base  630  and only the upper jaw  631  is movably attached to the base  630 . In addition to increasing the effective cross section of the air outlet  612 , a temperature and/or pressure dependent movement of the upper jaw  631  also changes a direction of letting air out of the air outlet arrangement  600 . In this embodiment, the flow of air is partly guided over the obstacle  690  when the upper  631  jaw opens. Alternatively, the direction of both upper jaw  631  and lower jaw  632  can be affected, for example by a thermo-actuator that acts on both jaws an causes a rotation of them about the joint  637 . 
     Furthermore, the joint  637  optionally comprises locking mechanism  638  for fixing the upper jaw  631  and the lower jaw  632  in a fixed position. This enables the user to selectively disable the automatic movement of the upper jaw  631  and/or lower jaw  632 . A fixation can be desirable, if the user wants to overrule the automatic adjustment of the flow changing device. 
       FIGS. 7 a  and 7 b    show a sixth embodiment of an air outlet arrangement  700  according to the present invention. This embodiment comprises a temperature sensor  711 , and a pressure sensor  712 . The flow changing device is implemented as a disk  720  that can be rotated with a motor  713  with respect to a front part  730  of the air outlet arrangement  700  depending on a pressure and/or temperature within the air outlet arrangement  700 . As shown in  FIG. 7 b   , the front part  730  comprises an oval main air outlet  732  and a plurality of bypass air outlets  733  that are arranged adjacent to the main air outlet  732 . The disk-like flow changing device  720  comprises a main opening  722  and bypass air outlets  723 . During normal operation when there is no obstacle in front of the main air outlet  732 , the disk  720  is rotated with respect to the front part  730  such that the main air outlet  732  is in front of the main opening  722  of the disk  720 . The disk  720  is arranged inside the front part  730  in close proximity to the surface  735  of the front part  730 . Further, the disk is rotated such that bars  724  of the disk  720  overlap the bypass air outlets  733  of the front part  730  and that the bars  734  of the front part  730  overlap to the bypass air outlets  723  of the disk  720 . Hence, the bypass air outlets are completely blocked. As the temperature and/or pressure within the air outlet arrangement  700  increases, the disk  720  can be gradually rotated with respect to the front part  730  such that the bypass air outlets  733  of the front parts  730  are on top of the bypass air outlets  723  of the disk  720 . Correspondingly, the bars  734  of the front part  730  are on top of the bars  724  of the disk  720 . This way, the openings  723  and  733  form a plurality of bypass air outlets when arranged on top of each other. Alternatively, the openings  723  and  733  are partially rotated on top of each other to partially increase the effective cross section of the air outlet opening of the air outlet arrangement  700 . 
     Of course an electro-mechanical actuator could also be used for operating a mechanical shutter of the air outlet arrangement in form of a flap or a slider depending on a pressure and/or temperature within the air outlet arrangement. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. 
     In summary, advantageous embodiments relate to a hair dryer ( 100 ) and a nozzle ( 113 ,  310 ,  410 ) for a hair dryer ( 100 ), and more specifically to an air outlet arrangement ( 112 ,  220 ,  300 ,  400 ,  500 ,  600 ,  700 ). The air outlet arrangement ( 112 ,  220 ,  300 ,  400 ,  500 ,  600 ,  700 ) comprises a flow changing device ( 223 ,  224 ,  331 ,  341 ,  431 ,  531 ,  631 ,  632 ) for changing a flow of air at the air outlet arrangement ( 112 ,  220 ,  300 ,  400 ,  500 ,  600 ,  700 ), depending on a pressure and/or temperature within a housing ( 110 ), in particular within the nozzle ( 113 ,  310 ,  410 ).