The present invention relates to water filter units, particularly water filter units for use in domestic steam generating appliances such as steam irons, steam cleaners, wallpaper strippers and other hand-held steam generating appliances. Water filter units according to the invention may also find use in beverage making appliances comprising a steam generator, such as pressurised coffee makers. Water filter units according to the invention may even find use in domestic water heating appliances which do not include a pressurised steam generator.

The presence of calcium in tap water is known to develop scale in domestic water heating and steam generating appliances. In appliances which are designed to generate a large volume of steam, such as steam irons, there is a great potential for scale to be deposited on the internal boiler surfaces as all the water passed into the boiler unit is evaporated into steam. The build-up of scale can lead to blockages and a reduced level of boiler performance. It is therefore desirable to remove the minerals which cause scale from water before it is passed to a steam generating boiler unit.

It is well known to use ion exchange resins in water filters to reduce the levels of calcium and other alkaline anions in water. Ion exchange resins work by exchanging contaminant ions in the water for those present in the resin, thereby reducing or removing dissolved minerals that lead to scale, such as calcium and magnesium. Due to the prevalence of calcium in water, particularly in areas suffering from hard water, it is important to remove this mineral as fully as possible. There are available ion exchange resins which can remove calcium and add water softening sodium in its place. However, such resins are not ideal for use in appliances where scale build-up is a problem, as sodium-based precipitation may also cause scaling. Hydrogen-based ion exchange resins may be used for such applications. Although hydrogen-based ion exchange resins provide treated water which has a lower pH value than is produced by other resins, scale deposition is greatly reduced.

Water filter systems for steam generating appliances have previously been proposed. However the filter performance is not always sufficient to reduce calcium levels in water to a low enough degree that scaling in the water boiling unit is no longer a problem. Furthermore, it has proved difficult to provide an effective filter in a compact unit suitable for domestic appliances, especially portable and/or handheld steam generating appliances. Pressurised steam generating appliances such as steam irons which use a high throughput of water are particularly prone to scale build-up.

It is an aim of the present invention to provide an improved water filter unit for removing scale-forming substances such as calcium and which can be used in domestic steam generating appliances such as steam irons, and also in other compact devices which heat water and generate steam.

When viewed from a first aspect the present invention provides a water filter unit for a domestic steam generating appliance, comprising a plurality of

compartments containing a water treatment medium, wherein the flow path through the unit is arranged to pass through the compartments in series and the

compartments provide a space for the water treatment medium therein to expand in use.

In accordance with the invention the filtering performance is enhanced by providing space in each compartment for the water treatment medium therein to expand. The Applicant has realised that water treatment media, in particular ion exchange resins, undergo a swelling phenomenon as they become exhausted with use. For a predetermined total volume of ion exchange resin, this swelling increases the pressure drop required to drive water through the volume of treatment medium and causes the flow rate to decrease with use. In accordance with the invention the total volume of water treatment medium is split into a number of smaller volumes, each of which is provided with a space in its compartment to swell without impacting on the treatment medium contained in other compartments. Thus the overall pressure drop across the filter unit will therefore not increase as much during use as when e.g. there is only one main compartment containing the treatment medium.

Moreover the Applicant has appreciated that the filtering performance is also improved by splitting a given amount of water treatment medium between separated compartments in a water filter unit. This means that the water flowing through the unit is constrained to flow through a greater proportion of the water treatment medium, whereas in the absence of such compartments water will generally flow through the unit along a path of least resistance. By forcing the water flow through a plurality of compartments in series the degree of contact between the water treatment medium and the flow is increased. The unit can be arranged, for example, to provide an effective filtering path length which is much greater than the dimensions of the unit. By increasing the overall path length for water passing through the filter, the removal of unwarranted impurities such as calcium is made more effective.

A water filter unit according to the invention can thereby achieve a higher filtering performance per unit volume of water treatment medium than in known filters.

In preferred embodiments the water treatment medium comprises an ion exchange resin, preferably a hydrogen-based ion exchange resin. The ion exchange resin is designed to remove calcium ions which can cause scale to be deposited from heated water.

The Applicant has appreciated that such ion exchange resins can theoretically swell by up to 20% in volume when fully exhausted, having absorbed a maximum amount of calcium. In a typical water filter use cycle, swelling of approximately 15% by volume can be expected. It is therefore preferred that each compartment in the water filter unit is filled with no more than 90% by volume of water treatment medium. Preferably each compartment is only filled to

approximately 80% of its volume. There is preferably provided 10-20% by volume of free space in each compartment, thus allowing the medium such as an ion exchange resin to swell in the compartment without blocking flow.

It will be appreciated that the compartments in the water filter unit may be arranged spatially in many different configurations. Depending on the arrangement of inlets and outlets which communicate flow between the compartments, water may be constrained to move horizontally and/or vertically through the filter unit. The arrangement of compartments preferably results in a tortuous or convoluted flow path through the unit so as to increase the path length. The flow will naturally seek the path of least resistance through the unit. Preferably it is arranged so that the flow through the unit must pass through each of the compartments in series. For example, the outlet of one compartment may provide the inlet to the next compartment and so on.

In order to help increase the contact between the flow and the treatment medium, the length of the local flow path across each compartment is preferably maximised. This may be achieved, for example, by arranging for the inlet and outlet of each compartment to be at opposite distal ends of the compartment. Preferably, the inlet and outlet are not aligned so there can be no straight-through flow.

In order to maximise the treatment path length, the flow path is preferably arranged to substantially reverse direction from one compartment to the next. This can help to make the filter unit more compact as there can be achieved a greater treatment effect for a given volume of treatment medium. This is considered to be novel and inventive in its own right, and thus when viewed from a further aspect the invention provides a water filter unit for a domestic steam generating appliance, comprising a plurality of compartments containing a water treatment medium, wherein the flow path through the unit is arranged to pass through the

compartments in series and to substantially reverse direction from one

compartment to the next. The invention extends to a portable steam generating appliance comprising such a water filter unit.

As is provided by the first aspect of the invention, it is preferred in this further aspect of the invention that the compartments provide a space for the water treatment medium therein to expand in use. The advantages of this have already been discussed above. The provision of a swelling space in each compartment may be even more important when the flow is reversing direction, as the path length is increased, which means that the exhaustion front for the treatment medium, e.g. ion exchange resin, moves progressively through the compartments.

In accordance with embodiments of both aspects of the invention, the compartments may be arranged one after the other in series, with the inlets/outlets for each compartment arranged such that the flow must generally reverse direction as it crosses one compartment after another. It is preferred that the compartments are provided both top-to-bottom and side-by-side, so as to make the unit as compact as possible. It is preferred that the main inlet to the unit is arranged at the same end as the main outlet from the unit. The compartments can then be arranged to direct the flow path from the main inlet at one end of the unit to another distal end of the unit and then back to the main outlet at the same end of the unit. This helps to maximise the flow path length for a unit of given dimensions.

In one preferred set of embodiments the compartments are arranged such that the flow of water is constrained to move essentially vertically through the compartments. In another preferred set of embodiments the compartments are arranged such that the flow of water is constrained to move essentially horizontally through the compartments.

The flow path through the compartments can also be arranged in a number of ways relative to the space provided in each compartment for expansion of the treatment medium. There may be available a flow path through the compartment which can use the expansion space rather than passing through the treatment medium. A proportion of the flow through the compartment will naturally take this path of lower resistance and thus not all of the water passing through the compartment will be treated. However the filter can still provide an improved filtering effect, as the overall path length may be made longer by constraining the flow to pass through multiple compartments in series.

It will be appreciated that optimal treatment may be achieved by ensuring that the flow path through each compartment always includes a proportion of the treatment despite the presence of the expansion space. It is therefore preferred that the treatment medium and expansion space are arranged in a given

compartment such that substantially all of the flow through the compartment must pass through the treatment medium. Of course the whole volume of the filter medium may not be wetted, but the arrangement is such that there does not exist a flow path which bypasses the treatment medium completely.

This is considered novel and inventive in its own right and thus when viewed from a yet further aspect the invention provides a water filter unit for a domestic steam generating appliance, comprising a plurality of compartments containing a water treatment medium, wherein the compartments provide a space for the water treatment medium therein to expand in use, and wherein the water treatment medium in at least some of the compartments is arranged such that substantially all of the flow through the compartment must pass through the treatment medium.

The flow through at least some of the compartments may be arranged to pass through the treatment medium before passing through the expansion space. This may be achieved in preferred embodiments by providing the treatment medium adjacent to the compartment inlet and the expansion space adjacent to the compartment outlet. Thus all of the flow entering the compartment must pass through a volume of treatment medium before it reaches the expansion space, even if it takes the path of least resistance through the compartment. For example, the treatment medium may fill a bottom portion of the compartment while the expansion space fills a top portion of the compartment, and vice versa.

The flow through other of the compartments may be arranged to pass through the expansion space before passing through the treatment medium. In both cases the flow will meet the treatment medium and the expansion space, or vice versa, in series as it passes through each compartment.

Of course the filter unit may comprise a mixture of compartment

arrangements, so that the flow through some of the compartments always meets the treatment medium before/after the expansion space, whereas other

compartments may possibly offer a flow path which can bypass some or all of the treatment medium in that compartment. The flow path arrangement through the compartments could be adjusted to provide the unit with a desired overall flow resistance.

The following preferred features of a water filter unit can be applied to any of the aspects of the invention described above.

It will be understood that the overall flow rate through the filter unit will depend on the pressure driving the flow and the flow resistance presented by the filter unit. The greater the number of compartments, the larger the flow resistance is likely to be. The Applicants have therefore found that there is a balance between providing a large enough number of compartments to increase the effective flow path on the one hand, without unduly increasing the flow resistance on the other hand. Tests have shown that a greater number of smaller cross-sectional area compartments (effectively elongating the water flow path length) will increase the efficacy of the filter, however this must be balanced with the increased pressure required to drive water through the unit and the complexity of filling and sealing the filter unit.

Water filter units for use in pressurised water heating and steam generating appliances may comprise a relatively large number of compartments as the pressurised flow can be forced through the unit even if it presents a relatively high flow resistance. In such appliances the number of compartments may be optimally chosen to provide a high filtering performance, although the manufacturing demands in producing a large number of smaller compartments may still need to be considered. In practice, a compact water filter unit sized to fit into a domestic steam generating appliance, including portable/hand-held appliances, may provide a suitable filtering performance using around 10 compartments. A substantial benefit is achieved as soon as a given volume of water treatment medium is split between a plurality of compartments due to the increased flow path and contact with the treatment medium, especially when combined with the swelling space provided by each compartment.

According to a set of preferred embodiments the water filter unit comprises at least four, five or six compartments. Preferably no more than 20 compartments are provided. Of course the number of compartments can be designed depending on factors such as the overall size of the filter unit, the flow resistance of the treatment medium, and the flow pressure.

As water enters the filter unit it is constrained to flow from one compartment to the next in the series. A certain path length of water treatment medium is required to effectively treat a given volume flow rate of water. The overall lifetime of the filter depends on this treatment path length relative to the total path length through the filter unit. In initial use only the treatment medium in the first few compartments (i.e. the treatment path length) will be required to actively remove a given concentration of impurities from a flow as it passes through the filter. After a time, the treatment medium in the initial compartments will have become exhausted of ion exchange function and those compartments further along the flow path will become active for ion exchange and filtering. The front of resin exhaustion will progressively move along the flow path in the unit until the point in time at which the remaining treatment medium path length is no longer sufficient to maintain the previous degree of water treatment e.g. calcium ion removal. Beyond this point in time the performance of the filter will therefore decline as all of the treatment medium in the filter becomes exhausted of its ion exchange function. During this process, the volume of treatment medium contained in each subsequent compartment will sequentially expand into the space provided until the medium in the final compartment has swelled.

It is advantageous for the water filter unit to comprise means to indicate to a user when the filter should be replaced, for example when the water treatment medium has been exhausted of its filtering function. The water filter unit may include a timer which can be activated upon installation. However, the Applicant has realised that the exhaustion of a water treatment medium such as an ion exchange resin, and the swelling which accompanies exhaustion of the resin, may advantageously be used to indicate when the filter has reached the end of its lifetime. The water filter unit preferably further comprises means to detect a degree of water treatment medium expansion characteristic of filter exhaustion. The unit preferably further comprises means to indicate an exhaustion condition to a user.

The monitoring of filter medium exhaustion is made easier to achieve by the presence of a plurality of water treatment compartments in the filter unit. Rather than monitoring an overall degree of swelling, it may only be necessary to sense when the treatment medium in the final compartment, i.e. adjacent to the outlet from the unit, has expanded into the free space provided therein. It is envisaged that the local resin swelling in this final compartment could preferably be used to indicate filter exhaustion, for example by the expansion of the resin activating a flap seal, to block the flow of water out of the filter unit so a user must replace the unit before the appliance will start to function again. Alternatively, the local swelling in the final compartment could be arranged to activate a microswitch or other sensor which causes a visual and/or audible indicator to alert a user that the filter has reached the end of its life. Many other suitable detection arrangements can be envisaged.

In preferred embodiments the water filter unit is provided in a domestic steam generating appliance such as a steam iron, steam cleaner, wallpaper stripper or other steam generating device. The appliance may be portable and/or designed for hand-held use. Such appliances typically include a relatively small water boiling unit which is particularly susceptible to the build-up of scale and blockage problems, especially when there is a high throughput of water. As mentioned above, such appliances may be provided with a pressurised water supply which helps to force water through a filter unit involving a number of compartments. The pressure of the water supply to the filter and water boiler/steam generator is preferably greater than 0.5 bar, for example more than 1 bar and even up to 3 bar or more. It will be understood that water filter units in accordance with the invention are preferably suitable for a pressurised steam generating appliance. A gravity flow of water is unlikely to provide a sufficient water pressure to force a flow through the plurality of compartments in the filter.

The invention extends to a pressurised steam generating system comprising any water filter unit as described above.

Many domestic steam generating appliances comprise a water reservoir in a stationary base unit. The base unit may be designed to reside on the floor with a pump provided to force water through an umbilical cord into the hand-held part of the appliance. In such appliances the water filter unit may be provided in the floor base unit, e.g. after the water reservoir, so that water is pumped up through the filter. In such appliances the size of the filter unit may not be such a critical factor. However, even when the water filter unit is provided with a main water reservoir tank e.g. in a floor-mounted base unit, the overall size of the appliance should not be too large in a domestic environment so as to allow the appliance to be easily carried around the home and stored away.

It has also been proposed to provide a water reservoir in the hand-held appliance itself. Here the water filter unit may also be provided in the appliance part and thus it is desirable to make the filter unit as compact as possible. The benefits of the invention may be more fully realised in this situation.

Where the appliance is a two-part cordless unit, comprising a base unit, a mobile appliance and an adaptor for supplying electrical power and water to the mobile appliance, the water filter unit may be provided in either the base unit or the mobile appliance. The size of the water filter unit, and the ability for the user to easily remove and replace the unit, is an important consideration.

The water filter unit may be integrated into the appliance but it is preferred that the water filter unit is removable and replaceable.

Some preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying Figures, in which:

Figure 1 is a perspective view of a water filter unit according to a first embodiment of the present invention;

Figure 2 is an exploded view of the water filter unit of Figure 1 ;

Figure 3A shows the lid component in cross-section and Figure 3B shows the base component in cross-section for the water filter unit of Figures 1 and 2;

Figure 4 shows a vertical cross-section through the water filter unit of the first embodiment when filled with a water treatment medium;

Figure 5 is a schematic view of the water flow path through the water filter unit of the first embodiment;

Figure 6 is a perspective, exploded view of a water filter unit according to a second embodiment of the invention; and

Figure 7 is a horizontal cross-section through the water filter unit of Figure 6. A first preferred embodiment of the present invention will be described with reference to Figures 1 to 5. The water filter 1 is a generally cubical unit defining a volume which is enclosed except for an inlet 2 and an outlet 4. Of course, the filter unit 1 may be of any suitable geometrical shape. As is best seen from Figure 2, the filter unit 1 is assembled during manufacture from a base component 6, a compartment section 8 and a lid component 10. The compartment section 8 comprises ten separate compartments 12. The base component 6, compartment section 8 and lid component 10 are sealingly closed together after the water treatment medium 16 (shown in Fig. 4) has been placed in the compartments 12. In the compartment section 8 each compartment 12 is fully separated and closed off from the other compartments 12 by internal walls. A passage for flow through the water treatment compartments 12 is provided by the base component 6 and lid component 10.

The lid component 10 and base component 6 respectively form the top and bottom enclosures of the individual compartments 12 in the compartment section 8. The base component 6 includes the main inlet 2 and outlet 4 for the filter unit 1 . The inlet 2 and outlet 4 could be connected in-line with a pumped water supply, for example to treat water as it is pumped from a reservoir to a water boiler/steam generator in an appliance.

As is most clearly seen from Figure 3A and 3B, both the lid component 10 and base component 6 provide a series of wall terminating portions which mate with the compartment walls in the compartment section 8. Gaps 14 formed in the walls in the lid component 10 and base component 6 allow for the passage of water between adjacent compartments 12. It can be seen that the gaps 14 are arranged alternately in the lid component 10 and base component 6 from one adjacent compartment 12 to the next.

An exemplary flow path scheme is shown in Figure 5. In use, for example when water is pumped through the filter unit 1 , water is forced vertically upwards from the inlet 2 to the top of the first compartment 12 where a gap 14 is provided in the lid component 10 to allow water to pass into the adjacent compartment 12, from where it must flow downwardly to the bottom of the second compartment before it can pass through a gap 14 provided in the base component 6, and so on. The flow moves alternately up and down, reversing direction from one compartment 12 to the next.

It can be seen from Figure 4 that the compartments 12 of the water filter unit 1 are filled with approximately 80% by volume of a water treatment medium 16. An expansion space 18 is provided in the top of each compartment 12, above the treatment medium 16. In the depicted embodiments the compartments 12 are filled from the bottom up with the water treatment medium 16, however, the medium 16 could be supported anywhere in the volume of the compartment 12 with the flow through each compartment 12 still being required to pass through both the treatment medium 16 and the expansion space 18. The same treatment medium filling regime may not be used for all the compartments 12. As the water flows vertically alternately up and down through the compartments 12, beneficially all of the flow is forced to pass through the water treatment medium 16 in each compartment 12 and the path length through the filter unit 1 is maximised.

As each compartment 12 is only approximately 80% filled with the water treatment medium 16, for example a hydrogen-based ion exchange resin, the treatment medium 16 is able to swell into the expansion space 18 in each compartment 12 during use.

The inlet 2 and outlet 4 may both be provided in the base of the unit for ease of connection. However, depending e.g. on the geometry of the filter unit and the arrangement of the compartments, the inlet 2 could be in the base component while the outlet 4 could be in the lid component, and vice versa. A common inlet/outlet aperture may alternatively be provided, with means to split the flow between incoming and outgoing flow paths.

In the embodiment shown in Figures 1 to 5 the water filter unit 1 comprises ten compartments 12, each approximately 14x28 mm in cross-section.

Construction of the filter unit 1 is kept relatively simple by using a limited number of compartments 12 and filling is not unduly impeded by the size of the compartments 12.

As described above, the flow of water through the unit 1 is controlled by means of the closed compartment walls and the gaps 14 in the top and bottom plates 6,10 of the assembly. The compartment section 8 comprises two rows of rectangular compartments 12. Woven mesh and seal components (not shown) retain the water treatment medium 16 in the compartments 12. Of course the number of compartments 12 and their dimensions may be adjusted as desired.

A second preferred embodiment of the invention will be described with reference to Figures 6 and 7. A water filtering unit 20 comprises a box 22 and lid component 24. The box 22 is split into a plurality of compartments 26. As is best seen from Figure 7, a gap 28 is formed in the wall separating adjacent

compartments. The position of the gap 28 is alternated from left to right between adjacent compartments 26, thereby extending the flow path across each compartment 26. The compartments 26 are partially filled with a water treatment medium (not shown) so as to leave an expansion space at the top of each compartment 26 beneath the lid component 24. As in the first preferred embodiment, each compartment 26 is filled with approximately 80% by volume of a water treatment medium such as a hydrogen-based ion exchange resin. The lid component 24 is sealingly assembled to the box 22 to close the filter unit 20.

A water inlet pipe 30 and a water outlet 32 are provided in the lid component 24. The inlet pipe 30 is arranged to extend down into the first compartment 26, substantially to the bottom of the compartment 26. This helps to ensure that water is directed into the treatment medium contained in the compartment 6, rather than into the expansion space above. The water flows horizontally back and forth between the compartments 26, reversing its direction after passing through each subsequent gap 28. The outlet 32 is arranged in the lid component 24 at the top of the expansion space in the final compartment 26. Water must therefore be forced up through the treatment medium in this final compartment and through the expansion space before exiting the unit 20.

Despite the arrangement of the inlet 30 and outlet 32, it will be seen that in this embodiment there is a risk that some of the water passing through the compartments 26 may bypass some of the water treatment medium. However, there is still provided the advantage that the flow path is convoluted and extended relative to a straight-through filter unit. A further advantage of this particular embodiment is that the filter unit 20 may be manufactured from two main moulded parts 22,24, which can be simply sealed together during assembly. Two small pieces of mesh (not shown) may be provided over the inlet 30 and outlet 32 to prevent resin from escaping therethrough.

Although not shown, in both of the embodiments the filter unit 1 , 20 may be provided with a filter lifetime indicator. A sensor such as a microswitch may be mounted, e.g. in the final compartment 12, 26 which leads to the outlet 4, 32, so as to detect when the treatment medium has swelled to substantially fill the

compartment 12, 26 and thus determine that the filter material is exhausted. An indicator may signal to a user that the unit needs replacing. Alternatively, blockage of the flow upon full expansion of the treatment medium could be used to trigger shutdown of an appliance until the filter is replaced.

It will be apparent that various changes in the form of the preferred embodiments described above may be made within the scope of the invention. For example, the compartments of the filter unit may be filled to any desired level with a water treatment medium. The Applicant has found that for a hydrogen-based ion exchange resin for calcium removal an expansion space of around 20% by volume is desirable. However, the relative proportions of the fill volume and swell space may be adjusted depending on the requirements of a particular water treatment medium.

The water filter units described here may suitably find use in domestic steam generating appliances such as steam irons, steam cleaners, wallpaper strippers and other portable steam generating appliances.