Abstract:
A portable fan includes a casing having an air inlet and a first connector, and an air outlet having a second connector. A filter unit includes a third connector, which is substantially the same as the second connector, for co-operating with the first connector to removably connect the filter unit to the casing, a filter which is located upstream from the air inlet when the filter unit is connected to the casing, and a fourth connector, which is substantially the same as the first connector, for co-operating with the second connector to removably connect the air outlet to the filter unit. This can allow the air outlet to be connected either directly to the casing, or to an optional filter unit which is connected to the casing.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of United Kingdom Application No. 1004814.8 filed Mar. 23, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a portable fan. Particularly, but not exclusively, the present invention relates to a floor or table-top fan, such as a desk, tower or pedestal fan. 
     BACKGROUND OF THE INVENTION 
     A conventional domestic fan typically includes a set of blades or vanes mounted for rotation about an axis, and drive apparatus for rotating the set of blades to generate an air flow. The movement and circulation of the air flow creates a ‘wind chill’ or breeze and, as a result, the user experiences a cooling effect as heat is dissipated through convection and evaporation. The blades are generally located within a cage which allows an air flow to pass through the housing while preventing users from coming into contact with the rotating blades during use of the fan. 
     The use of fans in hospitals to keep patients cool is widespread, both in general wards and in isolation wards. For example, depending on the medical condition of the patient it may be preferable to reduce the body temperature of the patient using a fan rather than by using pharmaceuticals. When a fan is assigned to a patient, generally that fan is treated as an item of medical equipment and so, like other medical equipment, will require frequent cleaning by a nurse or other hospital employee. The cleaning of bladed fans can be time consuming for the employee, as the cage housing the blades of the fan needs to be disassembled before the blades of the fan can be cleaned. This disassembly usually requires the use of a screw driver, which cannot be carried by a nurse on a hospital ward. Often, it can be more convenient for the hospital to engage a specialist cleaning company to clean the fan off site, although this can be very expensive. 
     WO 2009/030879 describes a fan assembly which does not use caged blades to project air from the fan assembly. Instead, the fan assembly comprises a base which houses a motor-driven impeller for drawing a primary air flow into the base, and an annular nozzle connected to the base and comprising an annular slot through which the primary air flow is emitted from the fan. The nozzle defines a central opening through which air in the local environment of the fan assembly is drawn by the primary air flow emitted from the mouth, amplifying the primary air flow. 
     The time required to clean off the external surfaces of this type of “bladeless” fan is much shorter than that required to clean a fan having caged blades, as there is no requirement to dismantle any parts of the fan to access any exposed parts of the fan. For example, the external surfaces of the fan may be wiped clean using a cloth. While this level of cleaning may be sufficient for bladeless fans which are assigned to patients on general wards, when the bladeless fan is assigned to a patient in an isolation ward or infection containment ward there remains a need to keep the internal components of the base clean to avoid cross-contamination when the fan is assigned to another patient. 
     SUMMARY OF THE INVENTION 
     The present invention provides a portable fan comprising a casing having an air inlet and a first connector, and an air outlet comprising a second connector. A filter unit includes a third connector, which is substantially the same as the second connector, for co-operating with the first connector to removably connect the filter unit to the casing, a filter which is located upstream from the air inlet when the filter unit is connected to the casing, and a fourth connector, which is substantially the same as the first connector, for co-operating with the second connector to removably connect the air outlet to the filter unit. 
     This can allow the air outlet to be connected either directly to the casing, or for an optional filter unit to be connected between the casing and the air outlet. The type of connection made between the filter unit and the casing, and between the air outlet and the filter unit, is the same as the type of connection which is made between the air outlet and the casing in the absence of the filter unit. This facilitates the connection of the filter unit to the casing and the air outlet, as the technique for connecting the air outlet to the casing is the same as that for connecting the filter unit to the base, and for connecting the air outlet to the filter unit. The filter unit is preferably manually connected to the casing and the air outlet to allow a user to attach the filter unit to the fan, and subsequently detach the filter unit from the fan, without the need for a tool. 
     The filter unit is preferably in the form of a disposable filter unit which can be replaced when, for example, the fan is assigned to a different patient, when the fan is moved with the patient from an isolation ward to a general ward, or when the filter has reached the end of a prescribed usage period. This can significantly reduce the costs associated with the use of the fan, as the frequency with which the fan may need to be taken off site for cleaning can be significantly reduced. 
     The filter preferably comprises a high energy particle arrester (HEPA) filter. The filter may also comprise one or more of a foam, carbon, paper, or fabric filter. The filter preferably has a surface area in the range from 0.5 to 1.5 m 2  which is exposed to the air flow generated by the fan. To minimize the volume of the filter, the filter is preferably pleated to form a filter which is substantially annular in shape for surrounding the air inlet of the casing. In this case, the filter unit may comprise two annular discs between which the filter is located. These discs can be easily wiped clean during use of the fan. Each disc may comprise a raised rim extending towards the other disc for retaining the filter between the discs. The filter may be readily adhered to the discs during the construction of the filter unit. The discs may together be considered to form at least part of a filter unit to which the filter is adhered during construction of the filter unit. 
     The filter unit may comprise an outer cover comprising a plurality of apertures through which air enters the filter unit. This outer cover can provide a first, relatively coarse filter to prevent airborne objects such as insects or large particles of dust from coming into contact with the filter, and can prevent the filter from being contacted by a user, particularly during the attachment of the filter to the casing, and so prevent damage to the filter. The outer cover is preferably transparent to allow a user to see the amount of dust or debris which has been captured by the filter. 
     In a preferred embodiment the filter unit is in the form of a sleeve which is locatable about an external surface of a casing. The casing may be in the form of a base, which may be free-standing on a floor, desk, table or other surface. 
     The filter unit preferably comprises at least one seal for engaging an outer surface of the casing. This can ensure that the air flow generated by the fan passes through the filter to the air inlet, and not around the filter. 
     The air inlet may extend at least partially about the casing, and may comprise an array of apertures. For example, the casing may comprise a base surface and a side wall, with the air inlet being located in the side wall of the casing. The casing may be substantially cylindrical in shape. The casing may house means for generating an air flow from the air inlet to the air outlet. The means for generating the air flow preferably comprises an impeller driven by a motor. A diffuser is preferably located downstream from the impeller. The filter unit may comprise a first seal for engaging the casing of the fan, and a second seal for engaging the air outlet of the fan so that an air flow is drawn through the filter unit between the seals and through the filter. 
     The air outlet may comprise an interior passage for receiving an air flow and a mouth for emitting the air flow. The interior passage may extend about an opening through which air is drawn by the air flow emitted from the mouth. 
     The first and third connectors may comprise co-operating screw threads to allow the filter unit to be attached to, and subsequently detached from, the casing. Alternatively, the first connector may be arranged to releasably engage the third connector to inhibit rotation of the filter unit relative to the casing. The first connector is preferably in the form of, or comprises, a wedge. The third connector preferably comprises an inclined surface which is configured to slide over an inclined surface of the wedge as the filter unit is rotated relative to the casing to attach the filter unit to the casing. The third connector may also be in the form of a wedge. Opposing surfaces of the first and third connectors subsequently inhibit rotation of the fan unit relative to the casing during use of the fan to prevent the filter unit from becoming inadvertently detached from the casing. The first connector is preferably arranged to flex out of engagement with the third connector, for example due to the user applying a relatively large rotational force to the filter unit, to detach the filter unit from the casing. Thus assembly and disassembly can each be performed in one operation or twist movement, and could be performed by an unskilled user of the fan. 
     The first connector may be located on an outer surface of the casing, and the third connector may be located on an inner surface of the filter unit. The first connector may be located in a recessed portion of the outer surface of the casing. The filter unit may comprise a fifth connector, and the casing may comprise a sixth connector for co-operating with the fifth connector to inhibit movement of the filter unit away from the casing when the filter unit is connected to the casing by the first connector and the third connector. Similarly, the filter unit may comprise a seventh connector, and the air outlet may comprise an eighth connector for co-operating with the seventh connector to inhibit movement of the air outlet away from the filter unit when the air outlet is connected to the filter unit by the second connector and the fourth connector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a front view of a fan; 
         FIG. 2  is a perspective view of the base of the fan of  FIG. 1 ; 
         FIG. 3  is a perspective view of the air outlet of the fan of  FIG. 1 ; 
         FIG. 4  is a lower perspective view of a portion of the air outlet of the fan of  FIG. 1 ; 
         FIG. 5  is a sectional view of the fan of  FIG. 1 ; 
         FIG. 6  is an enlarged view of part of  FIG. 5 ; 
         FIG. 7  is a side view of an accessory for attachment to the fan of  FIG. 1 ; 
         FIG. 8  is a perspective view, from above, of the accessory of  FIG. 7 ; 
         FIG. 9  is a sectional view of the accessory of  FIG. 7 ; 
         FIG. 10  is a perspective view of the fan of  FIG. 1  with the accessory of  FIG. 7  attached thereto; and 
         FIG. 11  is a sectional view of the fan of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a front view of a fan  10 . The fan  10  is preferably in the form of a bladeless fan  10  comprising a base  12  and an air outlet  14  connected to the base  12 . With reference also to  FIG. 2 , the base  12  comprises a substantially cylindrical outer casing  16  having a plurality of air inlets  18  in the form of apertures formed in the outer casing  16  and through which a primary air flow is drawn into the base  12  from the external environment. The base  12  further comprises a plurality of user-operable buttons  20  and a user-operable dial  22  for controlling the operation of the fan  10 . In this example the base  12  has a height in the range from 200 to 300 mm, and the outer casing  16  has an external diameter in the range from 100 to 200 mm. 
     As shown in  FIG. 3 , the air outlet  14  has an annular shape and defines an opening  24 . The air outlet  14  has a height in the range from 200 to 400 mm. The air outlet  14  comprises a mouth  26  located towards the rear of the fan  10  for emitting air from the fan  10  and through the opening  24 . The mouth  26  extends at least partially about the opening  24 , and preferably surrounds the opening  24 . The inner periphery of the air outlet  14  comprises a Coanda surface  28  located adjacent the mouth  26  and over which the mouth  26  directs the air emitted from the fan  10 , a diffuser surface  30  located downstream of the Coanda surface  28  and a guide surface  32  located downstream of the diffuser surface  30 . The diffuser surface  30  is arranged to taper away from the central axis X of the opening  24  in such a way so as to assist the flow of air emitted from the fan  10 . The angle subtended between the diffuser surface  30  and the central axis X of the opening  24  is in the range from 5 to 25°, and in this example is around 15°. The guide surface  32  is arranged at an angle to the diffuser surface  30  to further assist the efficient delivery of a cooling air flow from the fan  10 . The guide surface  32  is preferably arranged substantially parallel to the central axis X of the opening  24  to present a substantially flat and substantially smooth face to the air flow emitted from the mouth  26 . A visually appealing tapered surface  34  is located downstream from the guide surface  32 , terminating at a tip surface  36  lying substantially perpendicular to the central axis X of the opening  24 . The angle subtended between the tapered surface  34  and the central axis X of the opening  24  is preferably around 45°. The overall depth of the air outlet  14  in a direction extending along the central axis X of the opening  24  is in the range from 100 to 150 mm, and in this example is around 110 mm. 
       FIG. 5  illustrates a sectional view through the fan  10 . The base  12  comprises a lower base member  38 , an intermediary base member  40  mounted on the lower base member  38 , and an upper base member  42  mounted on the intermediary base member  40 . The lower base member  38  has a substantially flat bottom surface  43 . The intermediary base member  40  houses a controller  44  for controlling the operation of the fan  10  in response to depression of the user operable buttons  20  shown in  FIGS. 1 and 2 , and/or manipulation of the user operable dial  22 . The intermediary base member  40  may also house an oscillating mechanism  46  for oscillating the intermediary base member  40  and the upper base member  42  relative to the lower base member  38 . The range of each oscillation cycle of the upper base member  42  is preferably between 60° and 120°, and in this example is around 90°. In this example, the oscillating mechanism  46  is arranged to perform around 3 to 5 oscillation cycles per minute. A mains power cable  48  extends through an aperture formed in the lower base member  38  for supplying electrical power to the fan  10 . 
     The upper base member  42  may be tilted relative to the intermediary base member  40  to adjust the direction in which the primary air flow is emitted from the fan  10 . For example, the upper surface of the intermediary base member  40  and the lower surface of the upper base member  42  may be provided with interconnecting features which allow the upper base member  42  to move relative to the intermediary base member  40  while preventing the upper base member  42  from being lifted from the intermediary base member  40 . For example, the intermediary base member  40  and the upper base member  42  may comprise interlocking L-shaped members. 
     The upper base member  42  has an open upper end, and comprises an array of apertures  50  which extend at least partially about the upper base member  42 . The apertures  50  provide the air inlet  18  of the base  12 . The upper base member  42  houses an impeller  52  for drawing the primary air flow through the apertures  50  and into the base  12 . Preferably, the impeller  52  is in the form of a mixed flow impeller. The impeller  52  is connected to a rotary shaft  54  extending outwardly from a motor  56 . In this example, the motor  56  is a DC brushless motor having a speed which is variable by the controller  44  in response to user manipulation of the dial  22 . The maximum speed of the motor  56  is preferably in the range from 5,000 to 10,000 rpm. The motor  56  is housed within a motor bucket comprising an upper portion  58  connected to a lower portion  60 . The motor bucket is retained within the upper base member  42  by a motor bucket retainer  62 . The upper end of the upper base member  42  comprises a cylindrical outer surface  64 . The motor bucket retainer  62  is connected to the open upper end of the upper base member  42 , for example by a snap-fit connection. The motor  56  and its motor bucket are not rigidly connected to the motor bucket retainer  62 , allowing some movement of the motor  56  within the upper base member  42 . 
     Returning to  FIG. 2 , the upper end of the upper base member  42  comprises two pairs of open grooves  66  formed by removing part of the outer surface  64  to leave a shaped ‘cutaway’ portion. The upper end of each of the grooves  66  is in open communication with the open upper end of the upper base member  42 . The open groove  66  is arranged to extend downwardly from the open upper end of the upper base member  42 . A lower part of the groove  66  comprises a circumferentially extending track  68  having upper and lower portions bounded by the outer surface  64  of the upper base member  42 . Each pair of open grooves  66  is located symmetrically about the upper end of the upper base member  42 , the pairs being spaced circumferentially from each other. An annular sealing member  69  extends about the outer surface of the upper base member  42 , and is located beneath the tracks  68  of the grooves  66 . 
     The cylindrical outer surface  64  of the upper end of the upper base member  42  further comprises a pair of wedge members  70  having a tapered part  72  and a side wall  74 . The wedge members  70  are located on opposite sides of the upper base member  42 , with each wedge member  70  being located within a respective cutaway portion of the outer surface  64 . 
     The motor bucket retainer  62  comprises curved vane portions  76 ,  78  extending inwardly from the upper end of the motor bucket retainer  62 . Each curved vane  76 ,  78  overlaps a part of the upper portion  58  of the motor bucket. Thus the motor bucket retainer  62  and the curved vanes  76 ,  78  act to secure and hold the motor bucket in place during movement and handling. In particular, the motor bucket retainer  62  prevents the motor bucket from becoming dislodged and falling towards the air outlet  14  if the fan  10  becomes inverted. 
     With reference again to  FIG. 5 , one of the upper portion  58  and the lower portion  60  of the motor bucket comprises a diffuser  80  in the form of a stationary disc having spiral fins  82 , and which is located downstream from the impeller  52 . One of the spiral fins  82  has a substantially inverted U-shaped cross-section when sectioned along a line passing vertically through the upper base member  42 . This spiral fin  82  is shaped to enable a power connection cable to pass through the spiral fin  82  to the motor  56 . 
     The motor bucket is located within, and mounted on, an impeller housing  84 . The impeller housing  84  is, in turn, mounted on a plurality of angularly spaced supports  86 , in this example three supports, located within the upper base member  42  of the base  12 . A generally frusto-conical shroud  88  is located within the impeller housing  84 . The shroud  88  is preferably connected to the outer edges of the impeller  52 , and is shaped so that the outer surface of the shroud  88  is in close proximity to, but does not contact, the inner surface of the impeller housing  84 . A substantially annular inlet member  90  is connected to the bottom of the impeller housing  84  for guiding the primary air flow into the impeller housing  84 . The top of the impeller housing  84  comprises a substantially annular air outlet  92  for guiding air flow emitted from the impeller housing  84  towards the air outlet  14 . 
     Preferably, the base  12  further comprises silencing members for reducing noise emissions from the base  12 . In this example, the upper base member  42  of the base  12  comprises a disc-shaped foam member  94  located towards the base of the upper base member  42 , and a substantially annular foam member  96  located within the impeller housing  84 . 
     A flexible sealing member is mounted on the impeller housing  84 . The flexible sealing member inhibits the return of air to the air inlet member  90  along a path extending between the outer casing  16  and the impeller housing  84  by separating the primary air flow drawn in from the external environment from the air flow emitted from the air outlet  92  of the impeller  52  and the diffuser  80 . The sealing member preferably comprises a lip seal  98 . The sealing member is annular in shape and surrounds the impeller housing  84 , extending outwardly from the impeller housing  84  towards the outer casing  16 . In the illustrated embodiment the diameter of the sealing member is greater than the radial distance from the impeller housing  84  to the outer casing  16 . Thus the outer portion  100  of the sealing member is biased against the outer casing  16  and caused to extend along the inner face of the outer casing  16 , forming a seal. The lip seal  98  of the preferred embodiment tapers and narrows to a tip  102  as it extends away from the impeller housing  84  and towards the outer casing  16 . The lip seal  98  is preferably formed from rubber. 
     The sealing member further comprises a guide portion  104  for guiding a power connection cable  106  to the motor  56 . The guide portion  104  of the illustrated embodiment is formed in the shape of a collar and may be a grommet. The electrical cable  106  is in the form of a ribbon cable attached to the motor at joint  108 . The electrical cable  106  extending from the motor  56  passes out of the lower portion  60  of the motor bucket through spiral fin  82 . The passage of the electrical cable  106  follows the shaping of the impeller housing  84  and the guide portion  104  is shaped to enable the electrical cable  106  to pass through the flexible sealing member. The guide portion  104  of the sealing member enables the electrical cable  106  to be clamped and held within the upper base member  42 . A cuff  110  accommodates the electrical cable  106  within the lower portion of the upper base member  42 . 
       FIG. 6  illustrates a sectional view through the air outlet  14 . The air outlet  14  comprises an annular outer casing section  120  connected to and extending about an annular inner casing section  122 . Each of these sections may be formed from a plurality of connected parts, but in this embodiment each of the outer casing section  120  and the inner casing section  122  is formed from a respective, single molded part. The inner casing section  122  defines the central opening  24  of the air outlet  14 , and has an external peripheral surface  124  which is shaped to define the Coanda surface  28 , diffuser surface  30 , guide surface  32  and tapered surface  34 . 
     The outer casing section  120  and the inner casing section  122  together define an annular interior passage  126  of the air outlet  14 . Thus, the interior passage  126  extends about the opening  24 . The interior passage  126  is bounded by the internal peripheral surface  128  of the outer casing section  120  and the internal peripheral surface  130  of the inner casing section  122 . As shown in  FIG. 4 , the outer casing section  120  comprises a base  132  having an inner surface  134 . Formed on the inner surface  134  of the base  132  are two pairs of lugs  136  and a pair of ramps  138  for connection to the upper end of the upper base member  42 . Each lug  136  and each ramp  138  upstands from the inner surface  134 . Thus the base  132  is connected to, and over, the open upper end of the motor bucket retainer  62  and the upper base member  42  of the base  12 . The pairs of lugs  136  are located around the outer casing section  120  and spaced from each other so that the pairs of lugs  136  correspond to the spaced arrangement of the pairs of open grooves  66  of the upper end of the upper base member  42  and so that the location of the pair of ramps  138  corresponds to the location of the pair of wedge members  70  of the upper end of the upper base member  42 . 
     The base  132  of the outer casing section  120  comprises an aperture through which the primary air flow enters the interior passage  126  of the air outlet  14  from the upper end of the upper base member  42  and the open upper end of the motor bucket retainer  62 . 
     The mouth  26  of the air outlet  14  is located towards the rear of the fan  10 . The mouth  26  is defined by overlapping, or facing, portions  140 ,  142  of the internal peripheral surface  128  of the outer casing section  120  and the external peripheral surface  124  of the inner casing section  122 , respectively. In this example, the mouth  26  is substantially annular and, as illustrated in  FIG. 4 , has a substantially U-shaped cross-section when sectioned along a line passing diametrically through the air outlet  14 . In this example, the overlapping portions  140 ,  142  of the internal peripheral surface  128  of the outer casing section  120  and the external peripheral surface  124  of the inner casing section  122  are shaped so that the mouth  26  tapers towards an outlet  144  arranged to direct the primary flow over the Coanda surface  28 . The outlet  144  is in the form of an annular slot, preferably having a relatively constant width in the range from 0.5 to 5 mm. In this example the outlet  144  has a width of around 1 mm. Spacers may be spaced about the mouth  26  for urging apart the overlapping portions  140 ,  142  of the internal peripheral surface  128  of the outer casing section  120  and the external peripheral surface  124  of the inner casing section  122  to maintain the width of the outlet  144  at the desired level. These spacers may be integral with either the internal peripheral surface  128  of the outer casing section  120  or the external peripheral surface  124  of the inner casing section  122 . 
     Referring to  FIGS. 3 and 4 , to attach the air outlet  14  to the base  12 , the air outlet  14  is inverted from the orientation illustrated in  FIG. 4  and the base  132  of the air outlet  14  is located over the open upper end of the upper base member  42 . The air outlet  14  is aligned relative to the base  12  so that the lugs  136  of the base  132  of the air outlet  14  are located directly in line with the open upper ends of the open grooves  66  of the upper base member  42 . In this position the pair of ramps  138  of the base  132  is directly in line with the pair of wedge members  70  of the upper base member  42 . The air outlet  14  is then pushed on to the base  12  so that the lugs  136  are located at the base of the open grooves  66 . The sealing member  69  of the base  12  engages the inner surface  134  of the base  132  of the air outlet  14  to form an air-tight seal between the base  12  and the air outlet  14 . 
     To secure the air outlet  14  to the base  12 , the air outlet  14  is rotated in a clockwise direction relative to the base  12  so that the lugs  136  move along the circumferentially extending tracks  68  of the open grooves  66 . The rotation of the air outlet  14  relative to the base  12  also forces the ramps  138  to run up and slide over the tapers  72  of the wedge member  70  through localized elastic deformation of the open upper end of the upper base member  42 . With continued rotation of the air outlet  14  relative to the base  12 , the ramps  138  are forced over the side walls  74  of the wedge members  70 . The open upper end of the upper base member  42  relaxes so that the ramps  138  are generally radially aligned with the wedge members  70 . Consequently, the side walls  74  of the wedge members  70  prevent accidental rotation of the air outlet  14  relative to the base  12 , whereas the location the lugs  136  within the tracks  68  prevents lifting of the air outlet  14  away from the base  12 . The rotation of the air outlet  14  relative to the base  12  does not require excessive rotational force and so the assembly of the fan  10  may be carried out by a user. 
     To operate the fan  10  the user depresses an appropriate one of the buttons  20  on the base  12 , in response to which the controller  44  activates the motor  56  to rotate the impeller  52 . The rotation of the impeller  52  causes a primary air flow to be drawn into the base  12  through the air inlet  18 . Depending on the speed of the motor  56 , the primary air flow generated by the impeller  52  may be between 20 and 30 liters per second. The pressure of the primary air flow at the outlet  92  of the base  12  may be at least 150 Pa, and is preferably in the range from 250 to 1.5 kPa. The primary air flow passes sequentially through the impeller housing  84 , the upper end of the upper base member  42  and open upper end of the motor bucket retainer  62  to enter the interior passage  126  of the air outlet  14 . The primary air flow emitted from the air outlet  92  of the base  12  is generally in an upward and forward direction. 
     Within the air outlet  14 , the primary air flow is divided into two air streams which pass in opposite directions around the central opening  24  of the air outlet  14 . Part of the primary air flow entering the air outlet  14  in a sideways direction (generally orthogonal to the axis X) passes into the interior passage  126  in a sideways direction without significant guidance, whereas another part of the primary air flow entering the air outlet  14  in a direction parallel to the axis X is guided by the curved vanes  76 ,  78  of the motor bucket retainer  62  to enable the air flow to pass into the interior passage  126  in a sideways direction. As the air streams pass through the interior passage  126 , air enters the mouth  26  of the air outlet  14 . The air flow into the mouth  26  is preferably substantially even about the opening  24  of the air outlet  14 . Within each section of the mouth  26 , the flow direction of the portion of the air stream is substantially reversed. The portion of the air stream is constricted by the tapering section of the mouth  26  and emitted through the outlet  98 . 
     The primary air flow emitted from the mouth  26  is directed over the Coanda surface  28  of the air outlet  14 , causing a secondary air flow to be generated by the entrainment of air from the external environment, specifically from the region around the outlet  98  of the mouth  26  and from around the rear of the air outlet  14 . This secondary air flow passes through the central opening  24  of the air outlet  14 , where it combines with the primary air flow to produce a total air flow, or air current, projected forward from the air outlet  14 . Depending on the speed of the motor  56 , the mass flow rate of the air current projected forward from the fan  10  may be in the range from 300 to 400 liters per second, and the maximum speed of the air current may be in the range from 2.5 to 4 m/s. 
     The even distribution of the primary air flow along the mouth  26  of the air outlet  14  ensures that the air flow passes evenly over the diffuser surface  30 . The diffuser surface  30  causes the mean speed of the air flow to be reduced by moving the air flow through a region of controlled expansion. The relatively shallow angle of the diffuser surface  30  to the axis X of the opening  24  allows the expansion of the air flow to occur gradually. A harsh or rapid divergence would otherwise cause the air flow to become disrupted, generating vortices in the expansion region. Such vortices can lead to an increase in turbulence and associated noise in the air flow which can be undesirable, particularly in a domestic product such as a fan. The air flow projected forwards beyond the diffuser surface  30  can tend to continue to diverge. The guide surface  32  extending inwardly towards the axis X converges the air flow towards the axis X. As a result, the air flow can travel efficiently out from the air outlet  14 , enabling rapid air flow to be experienced at a distance of several meters from the fan  10 . 
       FIGS. 7 to 9  illustrate an external accessory for the fan  10 . The accessory is in the form of a filter unit  200  which is detachably attachable to the fan  10  to allow the filter unit  200  to be removed for cleaning or replacement. 
     The filter unit  200  is in the form of a generally cylindrical sleeve which is locatable around the upper base member  42  of the base  12  so that the filter unit  200  is located over the air inlet  18  of the fan  10 , as illustrated in  FIGS. 10 and 11 . This allows the filter unit  200  to remove airborne particles from the primary air flow generated by the fan  10  before the primary air flow enters the base  12  of the fan  10 . 
     The filter unit  200  comprises a generally annular filter  202  for removing airborne particles from the primary air flow. The filter  202  is preferably in the form of a radially pleated high energy particle arrester (HEPA) filter. The filter  202  has a surface area that is exposed to the incoming primary air flow generated by the fan which is in the range from 0.5 to 1.5 m 2 , and in this example is around 1.1 m 2 . The filter  202  is surrounded by a cylindrical outer cover  204 , which is preferably formed from plastics material, to protect the filter  202  and thus allows a user to handle the filter unit  200  without contacting the filter  202 . The cover  204  is preferably transparent to allow a user to examine visually the state of the filter  202  during use or after a period of use. The cover  204  comprises a plurality of apertures (not shown) through which the primary air flow enters the filter unit  200 , and thus provides a relatively coarse first stage of filtration of the filter unit  200  to prevent relatively large airborne objects or insects from entering the filter unit  200 . The filter unit  200  may further comprise additional filter media between the filter  202  and the cover  204 , or downstream from the filter  202 . For example, this additional filter media may comprise one or more of foam, carbon, paper, or fabric. 
     The filter  202  and the cover  204  are sandwiched between two annular plates  206 ,  208  of the filter unit  200 . Each plate  206 ,  208  includes a circular inner rim  210  and a circular outer rim  212  which both extend partially towards the other plate  206 ,  208 . The filter  202  and the cover  204  are located between the rims  210 ,  212  of the plates  206 ,  208 , and are preferably secured to the plates  206 ,  208  using an adhesive. 
     The upper plate  206  comprises a lower collar  214  which is located radially inwardly from the inner rim  210  of the upper plate  206 . The lower collar  214  extends axially downwards from the upper plate  206 . The inner diameter of the lower collar  214  is substantially the same as the inner diameter of the base  132  of the air outlet  14  of the fan  10 . Similar to the base  132  of the air outlet  14 , the inner surface of the lower collar  214  comprises two pairs of lugs  216  and a pair of ramps (not shown) for connection to the upper end of the upper base member  42  of the base  12  of the fan  10 . The shape of the lugs  216  and the ramps of the lower collar  214 , and the angular spacing between the lugs  216  and the ramps of the lower collar  214 , are substantially identical to those of the lugs  136  and ramps  138  of the base  132  of the air outlet  14 . 
     The upper plate  206  further comprises an upper collar  218  which is located radially inwardly from the lower collar  214 . The upper collar  218  extends axially upwards from the inner circumferential periphery of the upper plate  208 . The outer diameter of the upper collar  218  is substantially the same as the outer diameter of the outer surface  64  of the open upper end of the upper base member  42 . Similar to the upper base member  42 , the upper collar  218  comprises two pairs of open grooves  220  and a pair of wedge members  222 . The open grooves  220  are substantially identical to the open grooves  66  of the outer surface  64  of the upper base member  42 , and the spacing between the open grooves  220  is substantially the same as that between the open grooves  66 . The wedge members  222  are substantially identical to the wedge members  70  of the outer surface  64  of the upper base member  42 , and the spacing between the wedge members  222  is substantially the same as that between the wedge members  70 . A first annular sealing member  224  of the filter unit  200  extends about the outer surface of the upper collar  218 , and is located beneath the circumferentially extending tracks  226  of the grooves  220 . 
     The collars  214 ,  218  are preferably integral with the upper plate  206 , which is preferably formed from plastics material. 
     The lower plate  208  includes a relatively small collar  228  which extends axially downwardly from the inner rim  210  of the lower plate  208 . The collar  228  comprises a circumferentially extending groove located on its inner surface. A second annular sealing member  230  of the filter unit  200  is located within this groove. The collar  228  is preferably integral with the lower plate  208 , which is also preferably formed from a plastics material. 
     To attach the filter unit  200  to the fan  10 , first the air outlet  14  is detached from the base  12 . To detach the air outlet  14  from the base  12 , the air outlet  14  is twisted relative to the base  12  in the opposite direction (anti-clockwise) to that for attaching the air outlet  14  to the base  12 . With a suitable torque applied manually by the user, the upper end of the upper base member  42  is again caused to flex locally radially inwardly. This localized deformation of the upper base member  42  allows the ramp  138  to be rotated over the wedge members  70 , while the lugs  136  are moved simultaneously along the tracks  68  of the grooves  66 . Once the lugs  136  reach the ends of the tracks  68 , the air outlet  14  may be lifted from the base  12 . 
     Although the detachment of the air outlet  14  from the base  12  requires a greater force to be applied to the air outlet  14  than the force required for attachment, the resilience of the upper base member  42  is selected so that the detachment of the air outlet  14  may be performed manually 
     The user then attaches the filter unit  200  to the base  12 . The technique for attaching the filter unit  200  to the base  12  is essentially the same as that for attaching the air outlet  14  to the base  12 . The user locates the open lower end of the collar  228  of the lower plate  208  over the open upper end of the upper base member  42 , and lowers the filter unit  200  around the base  12 . When the bottom end of the lower collar  214  of the upper plate  206  is located immediately above the open upper end of the upper base member  42 , the user rotates the filter unit  200  until the lugs  216  of the filter unit  200  are located directly in line with the open upper end of the open grooves  66  of the upper base member  42 . In this position the pair of ramps of the filter unit is directly in line with the pair of wedge members  70  of the upper base member  42 . The filter unit  200  is then pushed further on to the base  12  so that the lugs  216  of the filter unit  200  are located at the base of the open grooves  66  of the base  12 . To secure the filter unit  200  to the base  12 , the filter unit  200  is rotated in a clockwise direction relative to the base  12  so that the lugs  216  move along the circumferentially extending tracks  68  of the open grooves  66 . The rotation of the filter unit  200  relative to the base  12  also forces the ramps to run up and slide over the tapers  72  of the wedge members  70  through localized elastic deformation of the upper base member  42 . With continued rotation of the filter unit  200  relative to the base  12 , the ramps are forced over the side walls  74  of the wedge members  70 . The upper base member  42  relaxes so that the ramps are generally radially aligned with the wedge members  70 . Consequently, the side walls  74  of the wedge members  70  prevent accidental rotation of the filter unit  200  relative to the base  12 , whereas the location the lugs  216  within the tracks  68  prevents lifting of the filter unit  200  away from the base  12 . 
     As shown in  FIG. 11 , when the filter unit  200  is attached to the base  12  the second sealing member  230  of the filter unit  200  is located beneath the air inlet  18  of the base  12 , and engages the outer surface of the base  12  to form an air-tight seal between the base  12  and the filter unit  200 . As also shown in  FIG. 10 , the buttons  22  and user operable dial  22  of the base  12  remain accessible by the user when the filter unit  200  is attached to the base  12 . 
     The air outlet  14  is then attached to the filter unit  200 . The attachment of the air outlet  14  to the filter unit  200  is essentially the same as the attachment of the air outlet  14  to the base  12 . The base  132  of the air outlet  14  is located over the upper collar  218  of the filter unit  200 , and the air outlet  14  is aligned relative to the base  12  so that the lugs  136  of the base  132  of the air outlet  14  are located directly in line with the open upper end of the open grooves  220  of the filter unit  200 . The air outlet  14  is then pushed on to the filter unit  200  so that the lugs  136  are located at the base of the open grooves  220 . The first sealing member  224  of the filter unit  200  engages the inner surface  134  of the base  132  of the air outlet  14  to form an air-tight seal between the filter unit  200  and the air outlet  14 . Again, to secure the air outlet  14  to the filter unit  200  the air outlet  14  is rotated in a clockwise direction relative to the filter unit  200  so that the lugs  136  move along the circumferentially extending tracks  226  of the open grooves  220  of the filter unit  200 . The rotation of the air outlet  14  relative to the filter unit  200  also forces the ramps  138  to run up and slide over the tapers of the wedge members  222  of the filter unit  200  through localized elastic deformation of the upper collar  218 . With continued rotation of the air outlet  14  relative to the filter unit  200 , the ramps  138  are forced over the side walls of the wedge members  222 . The upper collar  218  relaxes so that the ramps  138  are generally radially aligned with the wedge members  222 . Consequently, the side walls of the wedge members  222  prevent accidental rotation of the air outlet  14  relative to the filter unit  200 , whereas the location the lugs  136  within the tracks  226  of the grooves  200  prevents lifting of the air outlet  14  away from the filter unit  200 . 
     The assembled combination of the fan  10  and the filter unit  200  is shown in  FIGS. 10 and 11 . The air-tight seals that the filter unit  200  makes with the base  12  and the air outlet  14  force the primary air flow to pass through the filter  202  of the filter unit  200  to remove airborne particulates from the primary air flow before it enters the base  12 . In addition to purifying the air in the local environment of the fan  10 , the removal of airborne particulates from the primary air flow before it enters the base  12  can significantly reduce the rate at which dust and debris can build-up on the internal components of the fan  10 , thereby reducing the frequency at which the fan  10  needs to be cleaned. The filter unit  200  may be easily replaced for cleaning or replacement by detaching the air outlet  14  from the filter unit  200 , which is performed in the same manner as the removal of the air outlet  14  from the base  12 , and subsequently detaching the filter unit  200  from the base  12 . This can be performed quickly and easily without the use of any tools. When the use of the filter unit  200  is no longer required, the filter unit  200  can be rapidly removed from the fan  10  by detaching the filter unit  200  from the base  12 , and re-attaching the air outlet  14  directly to the base  12 .