GB2534984B

GB2534984B - Vacuum cleaner air bleed

Info

Publication number: GB2534984B
Application number: GB1521066.9A
Authority: GB
Inventors: Robert Duncan Christopher; Daniel Lawrence Ian
Original assignee: Numatic International Ltd
Current assignee: Numatic International Ltd
Priority date: 2014-12-01
Filing date: 2015-11-30
Publication date: 2019-08-21
Anticipated expiration: 2035-11-30
Also published as: EP3226736A1; GB201709646D0; GB201521066D0; GB2534984A; WO2016087833A1; GB2558334A

Description

Vacuum Cleaner Air Bleed

The present invention concerns the field of vacuum cleaning, and relates in particular to floor treatment appliances provided with a vacuum suction devices, and in particular vacuum cleaners, in which an air bleed valve may be provided which permits the suction provided at a suction nozzle or work head to be reduced, by providing a bypass for air to be sucked into the machine without passing through the nozzle or work head.

Typical machines are commercial or home vacuum cleaners, carpet cleaners or floor scrubbers provided with suction squeegees. The invention has particular application for upright vacuum cleaners or canister/cylinder vacuum cleaners which have a wand portion (tube) provided with a work head or nozzle for cleaning various surfaces.

Vacuum cleaners equipped with wands can be of the canister and wand tube type (such as the Numatic Henry (RTM) machine) or upright cleaners in which a floor facing work head equipped with a brush bar supports an upright handle portion which carries a collection bag or container. In the case of upright cleaners the wand tube may be drawn out from a stowed position and deployed for cleaning non-flat surfaces such as stairs or curtains.

Vacuum cleaners may be provided with a selectable suction reduction device which in many cases is an openable air bleed orifice or valve, which bleed orifice when open admits ambient air via a partition into portion of the cleaner which is under reduced pressure (providing cleaner suction). In vacuum cleaners with a wand tube portion, the air bleed device may be an orifice formed in the wand tube, with a manually-slidable shutter provided to open and close the orifice. The air bleed valves are typically disposed in the region of a wand grip, so that the user can conveniently select a reduced vacuum level for cleaning delicate surfaces such as curtains, cushions or upholstered furniture rather, than carpeting or other floor surfaces. A problem which arises during use is the objectionable noise generated by the air bleed orifice, when fully or partially open. However since the air bleed valves are generally only used occasionally, manufacturers have been content to allow the end user to put-up with the objectionable sound. The sound is caused by a stream of air entering the orifice, which is accentuated by the usually tubular, elongate nature of the wand. This is particularly so when the bleed orifice is a simple cut-out portion in a wand tube wall.

The present invention seeks in one aspect to provide an air bleed valve in which the sound emitted during use is reduced, to prevent operator noise fatigue and annoyance, and to make the cleaning process more acceptable for the operator.

The invention also seeks to provide a sound damped bleed valve for a vacuum cleaner in which the sound damping material or member may be easily replaced or cleaned.

According to one aspect of the invention there is provided a floor treatment appliance comprising a vacuum suction cleaner for lifting and collecting matter from surfaces, the cleaner comprising a vacuum suction motor in fluid communication with a nozzle or work head through which air-entrained matter can be drawn into the appliance, and a collector in which the air-entrained matter is to be deposited and collected, wherein at least one air-bleed orifice is provided in a wall portion of the appliance which separates an internal region of the appliance which is subject to vacuum suction from an external region which remains at ambient pressure, the orifice providing air bypass communication between said external and internal regions so as to reduce suction at the nozzle or work head, wherein the appliance includes a wand which serves as a vacuum conduit between the nozzle or work head and a vacuum cleaner body in which the suction motor is provided, which wand is provided with the nozzle or work head, and the orifice is provided in the wand, wherein an air-permeable porous material is disposed so as to obturate the orifice, which porous material permits the passage of bypass air through the orifice but restricts the airflow rate therethrough. A closure is provided which is moveable between a bypass configuration in which said orifice is open for the passage of air from the external to the internal regions via the porous material and a closed configuration in which the orifice is blocked so as to prevent or substantially prevent air bypass through the orifice and material. A traveler is attached to the wand and the porous material is provided as a porous membrane disposed in or on the traveler for movement therewith, which traveler can be position so that the porous membrane overlays the orifice, the traveler being movably retained on the wand and including a blocking portion which serves as the closure. The arrangement is such that the traveler may be moved from a bypass position in which the porous membrane overlays the orifice to a closed position in which the blocking portion blocks the orifice against air bypass through the orifice and porous membrane.

The traveler may comprise a sleeve or clip which is slidably retained on the wand for axial or rotational travel with respect to the orifice between full bypass and closed positions. A stop block may be provided to limit the extent of travel of the sleeve/clip with respect to the wand orifice.

In a preferred arrangement, the traveler is mounted for movement with respect to the wand and its bleed orifice so as to open and close the bleed orifice, which traveler has an orifice and the obturating porous membrane retained thereon, wherein movement of the traveler between a bleed valve open position and a bleed valve closed position, and optionally one or more intermediate valve partially-open positions, is governed by engagement of a fixed feature associated with the wand with a one or more counterpart features associated with the traveler, so that the traveler is constrained to valve open position a valve closes position and optionally one or more discrete valve partially open positions.

In some embodiments the traveler is a sleeve mounted for rotation with respect to the wand and one engagement feature is a valley and another feature is a pointer which may be accommodated in the valley in one of the said positions.

There are preferably at least two valley features associated with at least two said positions, and one pointer feature; and most preferably three valley features associates with three said positions and one pointer feature.

The pointer may be fixed with respect to the wand , and the valley or valleys move with the traveler with respect to the pointer.

In another arrangement of the invention the traveler is an axially moveable shuttle member which is arranged for sliding movement over the wand orifice to open or close said orifice.

Nub and detent features (or other mutually engaging features) may be provided operative between the shuttle member and a base portion attached to the tubuiar member, which features co-operate to retain the shuttle member in the bleed vaive open position, or the bleed vaive dosed position, and optionally said one or more discrete partially open positions therebetween.

The shuttle member is preferably retained on the base portion by means of inward facing recesses and counterpart ribs accommodated therein, which permit sliding of the shuttle with respect to the base portion.

The traveler may be provided with a housing for retaining the membrane, whilst allowing bypass airflow through the membrane and into the bypass orifice. The housing may comprise a rigid support frame on which the porous materiai or membrane is seated. The support frame may be formed as a series of bars a grid, mesh or a perforated plate. The frame should in any case allow bypass airflow therethrough.

The housing may further comprise a cover piece which includes a protective frame which restricts external finger access to the underlying membrane. The cover piece is preferably removable for access to and replacement or cleaning of the membrane. The cover piece may be a snap fit on the housing, or have a latch and or hinge.

The porous material has a silencing effect and is thought to help prevent turbulence at the orifice so as to reduce noise (as compared to a machine absent the porous material). The material also serves to reduce the airflow rate and improve uniformity of flow across the orifice.

The porous material may comprise open cell foam, a felt material, a porous laminate, gauze or perforated sheet. The porous material is provided in the form of a membrane. Preferably the porous material is an open cell foam membrane, most preferably formed of resilient polymeric material. A particularly preferred open cell foam is one having 25 to 35 pores per inch (PPI), and most preferably 30 PPI. This provides a good balance of silencing effect whilst still permitting substantial bypass flow through the bleed orifice.

The porous material provides a silencing effect, whilst allowing the desired bypass effect. A certain amount of simple trial and error may be required to arrive at the optimum porosity, thickness and area of material. A combination of materials may be used, such as a laminate of membranes of the same or different material. A closure is provided which is moveable between a bypass configuration in which said orifice is open for the passage of fluid from the external to the internal regions via the porous material and a closed configuration in which the orifice is blocked so as to prevent or substantially prevent air bypass through the orifice and material.

The appliance includes a wand tube with nozzle or work head, wherein the orifice is provided in a vacuum cleaner wand tube which serves as a vacuum conduit between the nozzle or work head and a vacuum cleaner body in which the suction motor is provided. Thus the appliance may be a wand and cylinder/canister type vacuum machine or an upright vacuum cleaner which has a wand (typically a retractable wand). The wand tube is typically circular in cross section but could be another cross section such as oval, rectangular, triangular, hexagonal or trapezoidal or some other polygonal shape.

The orifice may be formed as a cut-out in a wall portion of the wand tube. The tubular wand portion is typically a rigid elbow portion of a wand assembly, which assembly further includes an elongate straight tubular conduit attached to one end of the elbow portion. A distal end of the tubular conduit may be provided with a work head/nozzle for lifting and entraining detritus from a floor/carpet surface (an example is shown in figure 1). A flexible hose may be attached to an opposite end of the elbow portion and leads to a suction port in the vacuum cleaner cylinder/canister, which leads to an internal vacuum chamber or conduit.

Following is a description by way of example only and with reference to the figures of the drawings of modes for putting the invention into effect.

Figure 1 is a view of a cylinder and wand tube-type vacuum cieaner in accordance with one embodiment of the present invention.

Figure 2a is a three quarter perspective view of the wand tube of figure 1.

Figure 2b is an enlarged view of air bleed device shown in figure 2a.

Figure 3a is a perspective view of the bare wand tube showing a bleed orifice.

Figure 3b is and exploded side view of the air bleed device showing the component parts.

Figures 4a and 4b are perspective views of a sieeve forming part of the air bleed device.

Figure 5a and 5b are perspective underside and top views of a cover piece forming part of the air bleed device.

Figure 6 is a three quarter perspective view of a wand tube of a second embodiment which is suitable for use with the vacuum cleaner of figure 1.

Figure 6a is an enlarged view showing detail of figure 6.

Figure 7 is a three quarter perspective exploded view of the wand of the second embodiment.

Figure 8 is a side view of the assembled wand of the second embodiment.

Figure 9 is a three quarter perspective view of a wand tube of a third embodiment which is suitable for use with the vacuum cieaner of figure 1.

Figure 9a shows detail from figure 9.

Figure 10 is a perspective view of the wand showing the bleed valve in an open configuration.

Figure 11 is a three quarter perspective exploded view showing the components of the wand of the third embodiment.

First specific embodiment in figure 1 a cylinder and wand tube type vacuum cieaner is shown generally at 10. The machine includes a housing 11 in the form of a cylindrical open ended coiiection container. A iower base region of the coiiection container is provided with wheels 12 (two castor wheels under the base region and two side by side wheels which share an axle). An upper region of the cleaner is provided with a cylindrical lid portion 18 in which is provided internally (not visible In figure 1, but see figure 8 for an example) with an electric suction fan. The lid portion upper surface is provided with a handle 1S with on/off and power level toggle switches 20, 21. The lid is attached to the collection container by means of two diametrically opposed damps (one visible) 22. A side wall of the collection container is provided with an inlet block 17 which is connected to and fed by a flexible vacuum hose 18. The hose is connected to one end of a wand assembly 13 which includes a rigid elbow portion 14. The assembly further includes an elongate straight tubular conduit 23 attached to the other end of the elbow portion. A distal end of the tubular conduit is provided with a work head 15 for lifting and entraining detritus from a fioor/earpet. A bleed valve assembly 24 is located on one arm of the elbow portion 14. in figure 3a the bare elbow portion 14 is shown as an elongate steei tube with a straight end region 38 having a cut out bleed orifice 37. in figure 2a the tubular elbow portion 14 is shown along with the bleed vaive device 24 mounted over the orifice 37. The bieed vaive device is shown in separate pieces in figure 3b. The device comprises a generally cylindrical sleeve portion 25 (which is best seen in figure 4) and a cover piece 31 (see figure 5).

The sleeve has a generally rectangular upstanding rim 27. The rim extends around a membrane support frame or grid 26 which defines six rectangular, radiused apertures 29. A porous open cell foam membrane 30 (not shown in figure 4, but see figure 3b) sits on the frame 26. The membrane 30 is for example 30 pores per inch (PPI) open cell foam provided as a rectilinear plate which is sized to be a snug fit within the confines of the rim 27. A sleeve interior surface is provided with an axially extending elongate ridge 9 (see figure 4a) which is just shorter than the length of the cut-out orifice 37 (figure 3a).

The ridge is provided to one side of the sleeve apertures 29. During assembly the sleeve is slid onto the straight tube portion 38 until the ridge snaps into the bleed orifice 37. The sleeve is then able to rotate with respect to the tube and bleed orifice in a circumferential direction, to an extent or travel delimited by the abutment of the sleeve internal ridge 26 with opposite sides of the bleed orifice 37. The ridge prevents any substantial axial travel of the sleeve along the tube.

At one extent of rotation the sleeve apertures 29 completely overlay the bleed orifice 37 to provide maximum bypass flow. At the other extent the sleeve apertures overlay a blank portion of the tube, to one side of the orifice so that no effective air flow through the apertures is possible. At positions between the two extents the bypass effect is in between the two extremes. In this way the bypass effect can be varied by the user turning the sleeve. A visual indication of bypass effect is provided on an outside surface of the sleeve in the form of + and - signs 39 and direction indicators (see figure 4A). A membrane cover piece 31 has a top region formed with an aperture 32 crossed by protective bars 33. The cover piece is provided with a depending perimeter skirt 34. Opposite end portions of the skirt are provided with cut-out slots 35. In assembling the bleed valve the cover plate is snapped over the sleeve rim 27, with barbs 36 projecting from the sleeve rim engaging with the cover piece slots 35, as shown in figure 2b.

In use the bleed valve is used to admit air into the interior of the wand assembly, thereby reducing the suction, including at the work head or nozzle. The user rotates the traveler member (sleeve portion) with respect to the tubular member, so as to open or close the bleed valve. This allows the work head/nozzle to be used for more delicate tasks than floor or carpet cleaning, In particular for use on curtains, cushions or furniture. The bleed valve also allows the work head to be lifted from flat surfaces such as polished floors which permit a strong suction effect to develop with the work head.

In conventional machines the bleed valve is rather noisy and can emit a high pitched shriek or roar. The foam membrane of the present invention reduces the noise considerably, while still allowing an effective air bypass effect so as to reduce suction.

Second specific embodiment

The second embodiment represents a modified version of the first embodiment and is shown in figures 6 to 8. The vacuum cleaner is essentiaily the same as that shown in figure 1, with a wand eibow portion 114 having a bleed valve device 124 with different configuration as shown in figure 7. in figure 8a the elbow portion comprises an elongate steel tube is shown as 114. The steei tube has a straight end region 138 having a cut out bleed orifice 137. In figure 8 the tubular elbow portion 114 is shown along with the bleed valve device 124 mounted over the orifice 137. The bleed valve device is shown in separate pieces in figure 7. The device comprises a generally cylindrical base portion 100 which has a window 101 formed in a sidewall thereof which is sized to correspond with, and overlay, the cut out orifice 137 of the tube portion 138. The base portion has a raised rim 102 at one end thereof. There is a traveler member which is a sleeve portion 125, which is a sliding fit over the base portion 100. One end of the sleeve abuts the rim 102. There is a cover piece 131 and an open cell foam membrane 130 which is planar and oblong in shape, with a slight curve to match the radius of the sleeve 125.

The sleeve has a generally rectangular upstanding rim 127. The rim extends around a membrane support frame or grid 129 which defines six rectangular radiused apertures. The foam membrane 130 sits on the frame 129 and within the rim 127. The membrane 130 is for example 30 pores per inch (PPI) open ceil foam provided as a rectilinear plate which is sized to be a snug fit within the confines of the rim 127.

An opposite side wail 104 of the sleeve is shown in figure 6a. A sleeve edge region is provided with a cut out 105, which has sloping sides 106 and two hump features 107 disposed side-by-side. The humps are formed of a resilientiy deformable plastics material. The rim 102 of the base portion 100 is provided with an integrally moulded pointer 108 which projects into the cut out 105. The side wail 104 is provided with embossed index numerals 0,1 and 2 which reflect the degree of rotation of the sleeve with respect to the base portion (and tubular elbow portion). The rotation of the sleeve is constrained by the cut out 105 and its humps 107 and sides 106. The pointer 108 can occupy one of three valleys defined on either side and between, the humps 107. To move from one valley to the next the humps must be deformed. In an alternative arrangement the humps are solid and the sleeve 125 is biased by a spring (not shown) towards the rim 102. Thus the humps act as cams which ride over the pointer 106 and shift the sleeve axially slightly against the spring as the sleeve rotates from valley to valley (and index to index).

During assembly the base portion 100 is attached to the straight tube portion 138. The sleeve 125 is slide onto the base portion until it abuts the rim 102, with the point disposed In the cut out 105 (see figure 8). The sleeve is then able to rotate with respect to the tube and bleed orifice in a circumferential direction, to an extent or travel delimited by the cut out hump and valley features.

The overlap of the membrane support (and of course membrane foam) with the bleed orifice is calibrated so that index 2 corresponds to completely open bleed valve, index 1 to half open and index 0 to closed. So at index 2 the sleeve membrane support apertures completely overlay the bleed orifice 137 to provide maximum bypass flow. At index 1 only one side of the apertures overlay the orifice and the sleeve partially closes the orifice. At index 0 the sleeve apertures overlay a blank portion of the tube, to one side of the orifice so that no effective air flow through the apertures is possible.

The membrane cover piece 131 has a top region formed with an aperture crossed by protective bars (as per figure 2b). The cover piece is provided with a depending perimeter skirt. Opposite end portions of the skirt are provided with cut-out slots. In assembling the bleed valve the cover plate is snapped over the sleeve rim, with barbs projecting from the sleeve rim engaging with the cover piece slots, (again all as shown in figure 2b and described with reference to the first embodiment hereinbefore).

The bleed valve is actuated by twisting (rotating) the traveler member (sleeve 125) with respect to the base 100 between positions 0, 1 and 2.

Third specific embodiment

The third specific embodiment represents a modified version of the first embodiment and is shown I figures 9 to 11. The vacuum cleaner is essentially the same as that shown in figure 1, with a wand elbow portion 214 (figure 9) having a bleed valve device 224 with different configuration. In figure 10 the elbow portion comprises an elongate steel tube is shown as 214. The steel tube has a straight end region 238 having a cut out bleed orifice (not visible but as shown as item 137 in figure 7a). In figure 10 the tubular elbow portion 214 is shown along with the bleed valve device 224 mounted over the orifice. The bleed valve device is shown in separate pieces in figure 11. The device comprises a generally cylindrical elongate base portion 200 which has a window 201 formed in a sidewall thereof which is sized to correspond with, and overlay, the cut out orifice of the tube portion 238. The base portion has a raised rim which comprises two parallel axially extending elongate ridge portions 202 which are joined at one end by an arcuate ridge portion 213 which turns through 180 degrees. The ridge portions are formed with an inward facing lip 219 which defines an inward facing recess. The recess formed in each elongate ridge portion has at a central region thereof a nub 220. An opposite end region of the base portion 200 is provided with a raised ramp feature 221. A traveler member 225, is a sliding fit in between the parallel ridge portions 202. The traveler member has at one end region a thumb grip 226 has a series of transversely extending ridges. The grip rises up towards the central region of the traveler member. At the other end region of the traveler member a depressed cut out region 227 is formed which has a curved opposite edges for accommodating a generally planar open cell foam member 230. A lower region of the depressed cut out region is formed with an orifice 228 which has two transverse cross bars. There is a cover piece 231 which is a snap fit onto the upper side of the traveler member 225. The foam member is thus retained in the traveler member in the depressed region. The cover member 231 is provided with an orifice 232 and two transverse cross bars 233.

The cover member orifice and the traveler member orifice are aligned so as to provide a gas flow path through the traveler member and retained foam member. When the traveler member overlays (completely or partially) an air bleed path is formed into the interior of the tubular elbow 214.

The traveler member is provided with two elongate parallel ribs 235 on each side region thereof. Each rib is provided with three axially spaced apart semicircular detents 236. When the traveler member is inserted into the base portion, the ribs occupy the inward facing recesses of the rim portions. The central nub 220 of each recess can be accommodated in each of the detents. Thus the traveler member can be slid to one of three positions in which the nub engages a detent 236. One corresponds to the bleed valve being fully open with all orifices aligned. The next corresponds to the valve being partially (half) open, with the orifices partially aligned. The last corresponds to the bleed valve being closed and an end of the traveler member abutting the curved ridge 213 of the base portion. The opposite end of the traveler member is prevented from sliding out (detachment) of the base portion by the abutment ramp 221 (which is oriented to ease entry of the traveler member, but block its exit).

So in figure 9 the bleed valve configuration is closed whereas in figure 10 the bleed valve is fully open.

This embodiment provides axially sliding bleed valve actuation, which may be conveniently operated by a user’s thumb, and three settings defined by engagement with axially spaced apart detents.

The foam member 230 is a membrane sits on the frame traveler member cross bars and within the depressed region 227 of the traveler member. The membrane 230 is for example 30 pores per inch (PPI) open cell foam provided as a rectilinear plate which is sized to be a snug it within the confines of the traveier member.

Claims

1. A floor treatment appliance comprising a vacuum suction cleaner for lifting and collecting matter from surfaces, the cleaner comprising a vacuum suction motor in fluid communication with a nozzle or work head through which air-entrained matter can be drawn into the appliance, and a collector in which the air-entrained matter is to be deposited and collected, wherein at least one air-bleed orifice is provided in a wall portion of the appliance which separates an internal region of the appliance which is subject to vacuum suction from an external region which remains at ambient pressure, the orifice providing air bypass communication between said external and internal regions so as to reduce suction at the nozzle or work head, wherein the appliance includes a wand which serves as a vacuum conduit between the nozzle or work head and a vacuum cleaner body in which the suction motor is provided, which wand is provided with the nozzle or work head, and the orifice is provided in the wand, wherein an air-permeable porous material is disposed so as to obturate the orifice, which porous material permits the passage of bypass air through the orifice but restricts the airflow rate therethrough; wherein a closure is provided which is moveable between a bypass configuration in which said orifice is open for the passage of air from the external to the internal regions via the porous material and a closed configuration in which the orifice is blocked so as to prevent or substantially prevent air bypass through the orifice and material, wherein the porous material is attached to the wand and the porous material is provided as a porous membrane disposed in or on a traveler for movement therewith, which traveler can be positioned so that the porous membrane overlays the orifice, the traveler being moveably retained on the wand and including a blocking portion which serves as the closure, the arrangement being such that the traveler may be moved from a bypass position in which the porous membrane overlays the orifice to a closed position in which the blocking portion blocks the orifice against air bypass through the orifice and porous membrane.

2. An appliance as claimed in claim 1 wherein the porous membrane comprises an open cell foam, a felt material, or a porous laminate material, gauze or perforated sheet.

3. An appliance as claimed in claim 1 or 2 wherein the orifice is formed as a cut-out in a wall portion of the wand.

4. An appliance as claimed in any of the preceding claims wherein the traveler comprises a sleeve or clip which is slidably retained on the wand for axial or rotational travel with respect to the orifice between full bypass and closed positions.

5. An appliance as claimed in claim 4 wherein a stop block is provided to limit the extend of travel of the sleeve/clip with respect to the wand orifice.

6. An appliance as claimed in any of the claims 1 to 3 wherein the traveler is mounted for movement with respect to the wand and its bleed orifice so as to open and close the bleed orifice, which traveler has an orifice and the obturating porous membrane retained thereon, wherein movement of the traveler between a bleed valve open position and a bleed valve closed position, and optionally one or more intermediate valve partially-open positions, is governed by engagement of a fixed feature associated with the wand with a one or more counterpart features associated with the traveler, so that the traveler is constrained to valve open position, valve closed position and optionally one or more discrete valve partially open positions.

7. An appliance as claimed in claim 6 wherein the traveler is a sleeve mounted for rotation with respect to the wand and one engagement feature is a valley and another feature is a pointer which may be accommodated in the valley in one of the said positions.

8. An appliance as claimed in claim 7 wherein there are at least two valley features associated with at least two said positions, and one pointer feature.

9. An appliance as claimed in claim 7 or claim 8 wherein there are three valley features associated with three said positions and one pointer feature.

10. An appliance as claimed in claim 7 to claim 9 wherein the pointer is fixed with respect to the wand and the valley or valleys move with the traveler with respect to the pointer.

11. An appliance as claimed in claim 6 wherein the traveler is an axially moveable shuttle member which is arranged for sliding movement over the wand orifice to open or close said orifice.

12. An appliance as claimed in claim 11 wherein nub and detent features are provided to be operative between the shuttle member and a base portion attached to the wand, which features co-operate to retain the shuttle member in the bleed valve open position, or the bleed valve closed position, and optionally said one or more discrete partially open positions therebetween.

13. An appliance as claimed in claim 12 wherein the shuttle member is retained on the base portion by means of inward facing recesses and counterpart ribs accommodated therein, which permit sliding of the shuttle with respect to the base portion.

14. An appliance as claimed in any preceding claims wherein the traveler is provided with a housing for retaining the membrane, whilst allowing bypass airflow through the membrane and into the bypass orifice.

15. An appliance as claimed in claim 14 wherein the housing comprises a rigid support frame on which the porous membrane is seated.

16. An appliance as claimed in claim 15 wherein the support frame is formed as a series of bars, a grid or a perforated plate.

17. An appliance as claimed in any of claims 14 to 16 wherein the housing further comprises a cover piece which includes a protective frame which restricts external finger access to the underlying membrane.

18. An appliance as claimed in claim 17 wherein the cover piece is removable for access to and replacement or cleaning of the membrane.