WO2011117659A1

WO2011117659A1 - Steam cooking appliances

Info

Publication number: WO2011117659A1
Application number: PCT/GB2011/050627
Authority: WO
Inventors: Nicholas Edward Gibbs; Marc Gibson Collinson; Steven Anthony Ashton; Andrew Ian Winstanley
Original assignee: Strix Limited
Priority date: 2010-03-26
Filing date: 2011-03-28
Publication date: 2011-09-29
Also published as: EP2552286A1; GB201005195D0

Abstract

A domestic steam cooking appliance (100) comprises a reservoir (112) for water, a closed water boiler (118) for generating steam in an evaporation chamber having a volume much less than that of the reservoir (112), and an electric pump (116) for transferring water from the reservoir (112) to the boiler (118) on demand. The appliance (100) further comprises a steam supply means (122) connected at one end to an outlet of the water boiler (118) to receive steam and connected at the other end to a steam guide member (190). The steam guide member (190) acts to control the velocity of steam therethrough and to distribute the steam supply between a plurality of vents arranged to exhaust steam into one or more steam cooking chambers (128a-c). The flow path through the steam guide member (190) is arranged such that steam must substantially change the direction of flow at least once before being released out of the vents.

Description

Steam Cooking Appliances This invention relates to an appliance for cooking food using steam.

Steaming has been used as a common cooking technique for many years in Asian cultures. In more recent times it has become popular in Western cultures as a healthier way of cooking food without using oil, as compared to frying and baking, and while retaining nutritional content.

Commonly, traditional steamers comprise a water boiling pot that can be placed on a stove and one or more steamer baskets that sit on top of the pot together with a lid. Electric steam cooking appliances are also available, typically configured as stand-alone small domestic appliances.

While currently available electric steam cookers can allow a user to control or automate the steam cooking process to a greater degree than may be possible with stove-top steamers, often the steam generation process is basically the same. In many appliances a bulk reservoir of water is heated up to boiling point so as to produce steam that rises up into the cooking chamber. The water in the reservoir must be kept at boiling point for as long as steam is required. It can take a significant amount of energy to bring the volume of water in the reservoir up to boiling point and to then maintain steam generation. There is also a time delay involved in heating up the water so steam cannot be generated very quickly.

As well as the problems outlined above, in conventional steam cookers there is little control over the passage of steam from the boiler to the baskets where food is supported. Typically the food baskets or other cooking chambers are simply provided with perforations or slots in the base that allow steam to diffuse upwardly therethrough. There is no substantial pressure gradient to drive the flow of steam and thus it merely "wafts" up into the cooking baskets. In some proposals a steam nozzle may deliver steam into a space below the baskets at a certain flow rate, but it is then left to disperse. Another problem is that flakes of scale (usually calcium or magnesium carbonate) formed in the boiler and any steam delivery nozzle, precipitated from hard water during boiling, can be carried with the steam as it passes up into the baskets. These flakes of scales can then become trapped by the slots or perforations in the lowermost basket and clog them up, reducing the area available for steam to flow through. It is also undesirable for scale particles to be carried by the steam and deposited in contact with food in the baskets.

Steam is generated in the above described conventional food steamers essentially at ambient atmospheric pressure. The cooking process is therefore less effective than in pressure cookers, for example, where the cooking chamber is held at an elevated pressure around 70-100 kPa above atmospheric pressure, which results in an elevated water boiling temperature and the generation of steam at a temperature above 100 'Ό. However domestic pressure cookers have experienced limited popularity, perhaps partly due to consumer reluctance to use a pressurised cooking vessel in the home and concerns about the risk of the cooking chamber boiling dry. It can be a drawback that it is not possible, due to the pressurised nature of the cooker, for a user to easily remove the lid during cooking and inspect the food as it cooks. The cooking chamber can also be difficult to clean after use as food can tend to stick or burn on during the intensive, high temperature cooking process.

Pressure cookers are generally more complicated and expensive to manufacture than steamers as they must include a high duty seal and interlock to keep the lid closed against the pressure and thick walls to withstand the internal pressure. They must also be provided with a steam regulator valve in the lid to controllably release excess steam. Due to the high pressures involved, a further pressure relief valve may also be provided by way of a safety back-up should the regulator fail.

The present invention seeks to provide an improved appliance for cooking food using steam. When viewed from a first aspect the present invention provides a steam cooking appliance comprising a reservoir for water, a water boiler for generating steam, means for transferring water from the reservoir to the boiler, and means for supplying steam generated by the boiler to one or more steam cooking chambers. It will be understood that in accordance with the invention a dedicated steam generating boiler is provided separately from the water reservoir. Instead of heating the water in the reservoir to generate steam, water is transferred to the boiler where it may be heated and evaporated in regulated and possibly smaller quantities. Accordingly steam may be generated more quickly upon switching on the appliance than in known appliances. As well as being beneficial in minimising the delay in steam generation, steam can be produced "on demand" for as long as water is being transferred to the boiler but then turned off when not required. The steam cooking process may therefore be controlled with a high degree of precision.

The Applicant has recognised that a potential problem with providing a dedicated water boiler, especially one that may heat relatively small volumes of water in a short space of time to produce a high rate of steam output, is that there may be an increased tendency for scale to form, not only in the water boiler but also in the steam supply means downstream of the boiler. One way of combatting this may be to provide the water boiler with a mesh or other means to trap scale before the steam exits. This may have the additional benefit of acting as a steam separator that prevents large water droplets being entrained in the exiting steam. However, it has been appreciated that the steam passing through the steam supply means may continue to precipitate mineral deposits after it has left the boiler. For example, scale may be formed on the walls of a steam supply pipe and flakes may break off that will be carried by the steam flow into the cooking chambers. It would be desirable to eliminate such flakes of scale before they can reach the cooking chambers, where they will likely cause clogging and/or contaminate the food inside.

In a preferred set of embodiments the means for supplying steam generated by the boiler comprises a steam guide member comprising a plurality of steam vents and a flow path arranged such that steam guided along the flow path is caused to substantially change its direction of flow at least once before being released out of the steam vents. It will be understood that such a steam guide member is advantageously able to control the flow of steam before it is supplied to the cooking chamber(s), unlike conventional steamers that simply allow steam to "waft" out of a water boiling chamber. The steam guide member, in particular the pattern of its steam vents, can be arranged to ensure that steam is released so as to be evenly distributed in the cooking chamber(s). This can help to ensure uniform cooking.

Preferably the steam supply means is terminated by the steam guide member, so that it provides a final encounter for steam before it is supplied to the cooking chamber(s). Where multiple cooking chambers are provided, a single steam guide member may supply the chambers together (e.g. stacked one on top of another in serial fluid connection) or a separate steam guide member may supply each respective chamber (e.g. arranged in parallel relative to the steam supply).

An effect of the changing direction in the steam flow path is to cause solid scale particles entrained in the steam to be left behind and to collect in the steam guide member. Preferably the steam guide member is a removable part of the steam supply means, so that it can be detached and cleaned periodically to remove the scale collected therein. By using a steam guide member it can be ensured that no scale particles are carried by steam into contact with the cooking chamber(s) and the food inside. The steam should therefore be able to pass through the cooking chamber(s) unhindered, without any blockages being formed, thereby helping to ensure an even distribution of steam for cooking. While arranging for the flow path to change the direction of flow at least once can provide the effect of trapping scale in the steam guide member, in some embodiments it is preferred that the flow path is arranged such that steam guided along the flow path is caused to substantially change its direction of flow at least twice before being released out of the steam vents. For example, the flow path through the steam guide member may initially extend in a vertical direction, then change direction to guide steam substantially horizontally, and then change direction again to guide steam substantially vertically to the steam vents. Multiple change of flow direction may be provided. In at least some embodiments the steam guide member may provide a tortuous flow path.

The Applicant has appreciated that the flow path through the steam guide member may be designed to have an additional function. For a given flow rate of steam from the water boiler, the overall cross-sectional area of the flow path(s) in the steam guide member can determine the velocity of steam exiting the vents. The overall flow area in the steam guide member may be the same as, larger than, or smaller than, the flow area of steam in the supply means when it enters the steam guide member. In one set of embodiments, by arranging the flow path in the steam guide member such that the overall flow area does not change then the steam velocity will be unaffected and the steam guide member may simply function to remove scale and direct the steam to the vents.

In another set of embodiments the flow path in the steam guide member may be arranged to increase the overall flow area and therefore allow the steam to expand, i.e. acting as a diffuser. An increase in the flow area could be achieved by the flow path expanding in the steam guide member. Of course, the flow path through the steam guide member may not comprise a single flow path. One flow path may split into multiple flow paths (which may each have a cross- sectional area that is the same, larger, or smaller) overall providing an increase in the total flow area. Where pressurised steam is produced by the water boiler and it is desired to supply steam to the cooking chamber(s) at or near atmospheric pressure, for example, this type of steam guide member can be used to provide the desired reduction in steam velocity.

On the other hand, in another set of embodiments the flow path in the steam guide member may be arranged to reduce the overall flow area and therefore increase the steam velocity, effectively acting as a steam nozzle. The flow path could narrow and/or split into multiple small area paths. Increased velocity jets of steam may then be released from the vents.

Accordingly it will be understood that the design of the steam guide member can be used to control the nature of the steam supply to the cooking chamber(s), as well as acting to trap scale deposits. Preferably the steam guide member is removable not only for cleaning purposes, but also to allow one steam guide member to be interchanged with one another. Thus a user may attach a different steam guide member to the steam supply means depending on what is being cooked, for example a steam diffuser head for gentle steaming of fish, or a steam nozzle head for faster steaming of dense foods such as whole potatoes. Or the steam guide member may be chosen according to the type of cooking chamber(s) to which it supplies steam. The steam cooking appliance may therefore be provided in the form of a kit comprising two or more different steam guide members for connection between the steam supply means and the cooking chamber(s).

Preferably the steam cooking appliance is a domestic appliance. It is therefore preferred that the size of the water reservoir is appropriate for a domestic appliance, such as a counter-top kitchen appliance. The volume V of the reservoir is preferably 0.5 L < V < 3.0 L, more preferably 1 .0 L < V < 2.5 L.

Water may be transferred from the reservoir to the boiler by any suitable means. The transfer may take place intermittently, for example in bursts or pulsed transfer, or it may be a continuous transfer. The transfer means may be capable of generating a pressurised supply of water to the boiler. In preferred embodiments a pump is provided to transfer water from the reservoir to the boiler. Although in a broad sense the use of thermal energy to create a driving force for the movement of water may be considered as a "pump", it is preferable that the pump is a mechanical pump, e.g. powered by an electric motor, which does work on the liquid in order to transfer it from the reservoir to the boiler. In one set of embodiments the pump is arranged to operate continuously whilst the appliance is switched on, or at least after an initial heat-up period as is described below. In other words, it is preferable that water is supplied continuously to the boiler while the appliance is being used, or at least while a steam supply is required. This ensures that there is always a ready supply of water to the boiler and steam can be

continuously produced when required.

The Applicant has appreciated that it may be desirable to be able to vary the rate at which steam is generated by the boiler. This can allow for food to be steam cooked at different rates or in different times, e.g. fast/slow steaming. It may also be useful if there is a change in the number or volume of the cooking chambers being supplied with steam. It is therefore preferable that the appliance comprises means for controlling the amount of steam that is supplied to the cooking chamber(s) in use, i.e. in terms of the volume of steam supplied per unit time. The amount of steam generated may be controlled in one or more different ways, as will be described in more detail below. While the pump or other transfer means may itself be arranged to supply water to the boiler at a suitable rate, preferably there is further provided means to control the flow of water into the boiler, preferably independently of the transfer or pump rate. Means for controlling the water flow rate may therefore be provided in the flow path between the pump or other transfer means and the boiler. Such water flow control means can advantageously adjust the flow rate of water entering the boiler and thus the rate of steam production. For example, the water flow control means may act in response to a feedback signal concerning the demand for steam. The water flow control means may be controlled in response to the signal(s) from one or more sensors connected to the steam cooking chamber(s), as will be described in more detail below.

The flow path from the pump to the water boiler may include a pressure-compensating constant flow valve so that water is supplied to the boiler at a uniform flow rate regardless of the input flow rate from the pump and any fluctuations in pressure. Such an arrangement is described in more detail in the Applicant's co-pending UK patent application no. 1 102971 .7 filed 21 February 201 1 , the disclosure of which is hereby incorporated by reference.

The appliance may also be provided with means for diverting the flow of water from the pump or other transfer means before it reaches the boiler. This is preferably in addition to the water flow control means. The diverting means may be provided either upstream or downstream of the water flow control means in the flow path to the boiler. In at least some embodiments it is preferred that the diverting means is provided downstream of the water flow control means, i.e. between the water flow control means and the boiler. The flow control means may therefore provide the primary means for controlling the normal flow of water into the boiler. In the event that the flow cannot be controlled adequately then the diverting means may be employed. In an abnormal operating condition, e.g. if there is a problem with the boiler such as a blockage, the diverting means may then become the primary response to any backed-up water flow. The diverting means may even act in a normal operating condition, e.g. when there are no cooking chambers connected to the steam supply means and steam generation is not required. A bypass path arranged to deliver water back to the reservoir may form part of the diverting means to direct water away from the boiler e.g. when a supply is not required, and/or the flow rate and/or pressure is too high, and/or the boiler is not functioning. Where the transfer means is a pump this can advantageously prevent the pump from stalling or drawing excessive current. The diverting means preferably further comprises a water pressure relief valve arranged to divert water to the bypass path when an over-pressure is sensed e.g. because the steam exit from the boiler and/or the steam supply means and/or the steam chamber(s) is/are blocked or the boiler is failing to generate steam at its design rate. The water pressure relief valve may be connected to the water flow control means, but it is preferred for them to be independent. They may then provide separate controls of the water flow to the boiler. The water pressure relief valve may be a simple one-way valve that opens when the water pressure reaches a certain predetermined threshold value, preferably configured at a value that is higher than the normal operating pressure of the appliance but lower than the pressure that would cause the pump to stall.

In addition, the pump's stalling pressure is preferably configured so as not to exceed the pressure at which the appliance's water transfer, boiler and steam supply systems would fail. More preferably the pump's stalling pressure may be configured to operate within relevant standards, as an additional appliance pressure safety feature to compliment the water pressure relief valve in the event that the pressure relief valve itself were to operate abnormally or malfunction.

Although the boiler will typically be thermostatically controlled, it is preferably arranged such that it is allowed to reach a higher operating temperature when there is no water flow (e.g. when the pump is off or the water flow control means blocks the flow or the diverting means is operating) than when water is being supplied to the boiler. This means that the boiler can store additional thermal energy in its thermal mass, further reducing the time to first produce steam after the water supply is turned on because the water initially supplied to the boiler can then be heated more rapidly. In a set of preferred embodiments, the useable energy which the boiler is adapted to store, that is the amount of heat energy available to generate steam, is more than 20 kilojoules, more preferably greater than 35 kilojoules and more preferably greater than 50 kilojoules.

In some embodiments there may be provided means to delay operation of the transfer means e.g. pump until the boiler has reached a predetermined operating temperature. For example, a temperature sensitive control means may be arranged to provide an electrical connection to the pump only when it is detected that the operating temperature of the boiler has been reached. The pump and the boiler may be arranged electrically in series or in parallel with a switch arranged in the circuit to act in response to the thermally sensitive control means. Alternatively a timer could be programmed to delay the operation of the pump or other transfer means until such time that the boiler is expected to have heated up. Such arrangements can ensure that the boiler is hot enough when water is pumped into it that steam generation starts rapidly.

Advantageously the start-up time may be reduced. ln other embodiments the boiler may be configured to have a minimal or reduced thermal mass, as compared to the aforementioned configuration, so that on start-up of the appliance the time for the boiler to reach the operating temperature at which steam is generated with a finite energy input is reduced or minimised, this configuration thereby reducing the time to first produce steam after the boiler is turned on and water supplied. In such embodiments the delay in supplying water to the boiler may be reduced or minimised. Preferably water is supplied simultaneously as the boiler is turned on and only optionally before the boiler is turned on.

In a preferred set of embodiments the steam generating boiler comprises a water inlet connected to the pump or other transfer means, an electrically heated evaporation chamber and a steam outlet connected to the supply means. As the boiler comprises an evaporation chamber arranged for the express purpose of heating and evaporating a selected volume of water to create steam, steam may be generated more quickly in this water boiler than it can, for example, in a steamer appliance that heats the water in a bulk reservoir. It will be understood that the evaporation chamber is preferably closed apart from any water inlet(s) and steam outlet(s) and thus may be able to generate steam at pressures much higher than atmospheric pressure. Pressurised steam can reduce the cooking time. Furthermore, the steam generated (whether or not it is pressurised) can raise the temperature within the cooking chamber(s) to the desired cooking temperature in less time than in common domestic electric steam cookers, for example reaching full temperature in less than two minutes and as quickly as 30 seconds. This is due to a high steam output being available from the boiler right from the start, without the heating-up delay associated with conventional steamers.

Whereas water is transferred e.g. pumped to the inlet of the boiler, preferably there is no pumped flow through the boiler. It is the steam pressure built up in the evaporation chamber that preferably pushes steam out through the outlet. This is quite different to many other water heaters or boilers, such as flow-through heaters or devices that create steam as soon as water is sprayed onto a heated surface. The Applicant has appreciated that there is an advantage in allowing the boiler to generate steam in its own time. As the evaporation chamber is heated and water is transferred therein, it will take a little time for the water first introduced to boil and evaporate and for the steam pressure to build up. Hence the output of steam from the boiler, and thus into the cooking chamber(s) via the steam supply means, preferably is gentle to start with and may then increase to a steady rate as steam is supplied to the cooking chamber(s). The slow build up of steam has been found to be desirable as it can avoid any initial spurts of steam which may damage the food in the cooking chamber(s). The natural flavour of the food may therefore be retained. The Applicant has realised that the temperature and pressure of the steam used in the cooking process is an important factor. The boiler may be arranged to boil water and generate steam at 100 ^QC (or whatever is the boiling point at ambient pressure where the appliance is used). This has the benefit that the steam in the cooking chamber(s) is at ambient pressure so there is no need for a pressure interlock and the chamber(s) can be accessed easily without requiring de- pressurisation.

However, in at least some embodiments the boiler is configured to produce pressurised steam. What is meant by pressurised steam is steam that is produced at a pressure above atmospheric and thus at an elevated temperature compared to the atmospheric boiling point.

Such an arrangement is considered novel and inventive in its own right and thus when viewed from another aspect the invention provides a steam cooking appliance comprising a water boiler for generating steam at a temperature above the atmospheric boiling point and means for supplying the steam generated by the boiler to one or more steam cooking chambers.

This aspect of the invention shares many advantages with the first aspect of the invention discussed above, such as quicker and more controlled steam generation as the steam is produced in a boiler separate from the chamber(s) in which the food is steam heated /cooked. Any of the essential or preferred features of the first aspect of the invention set out above may be applied to this aspect of the invention. Thus in one set of embodiments the appliance comprises a water reservoir and transfer means such as a pump for supplying water from the reservoir to the boiler. It will be appreciated that appliances in accordance with this aspect of the invention cannot only produce steam more quickly than is possible in conventional steamers and benefit from a reduced start-up time, where a dedicated boiler is provided, but can also cook foods more quickly as the steam entering the cooking chambers is hotter. The overall cooking time may be reduced significantly compared to atmospheric pressure steamers. Preferably the boiler generates steam at a temperature up to about 20 ^QC higher than the atmospheric boiling temperature, i.e. at a temperature of 100-120 ^QC at sea level.

It will be understood that such an appliance differs from a common domestic pressure cooking appliance in that there is provided a dedicated water boiler to generate the pressurised steam used for cooking, whereas in a pressure cooker it is the steam generated by boiling liquid in the cooking chamber itself that builds up a pressure in the cooking chamber. Moreover, in a pressure cooker the chamber must be pressurised up to about 100 kPa above atmospheric to achieve steam temperatures above the atmospheric boiling point. According to this aspect of the invention, on the other hand, a steam temperature above the atmospheric boiling point is achieved in a dedicated boiler and then supplied to the cooking chamber(s). The cooking chamber(s) may not be pressurised, or at least not to such a high degree.

In one set of embodiments the cooking chamber(s) are configured to be pressurised by the supply of steam. The chamber(s) may be substantially sealed from the atmosphere and could be provided with a pressure relief valve - such features are described in more detail below. Preferably the boiler generates steam at a pressure of around 0.5 bar (50 kPa) above atmospheric pressure. This mild degree of pressurisation helps to speed up the cooking process but the pressure is not so high that the cooking chamber(s) need to be closed with a high duty seal. A rubber gasket on the lid closing the cooking chamber(s) may be sufficient to hold the pressure without a complicated pressure interlock. A user should therefore be able to access the cooking chamber(s) easily without de-pressurisation being required. The costs and complexity involved in manufacturing an appliance according to the invention may also be less than for conventional pressure cookers, which are typically designed to operate at pressures of 70-100 kPa above atmospheric. The cooking chamber(s) of the appliance may therefore be lighter than a conventional pressure cooker vessel having thick pressure-resistant walls and a heavy sealing lid with an interlock.

The Applicant has found that supplying pressurised steam (i.e. steam at a temperature above the atmospheric boiling point) can improve the cooking process even if the cooking chamber(s) are not configured to maintain the steam pressure. Thus in one set of embodiments the cooking chamber(s) are vented to the atmosphere. This means that the steam supplied to the chamber(s) will have an elevated temperature and pressure initially, but the steam will quickly de-pressurise and cool as it is vented to atmosphere. The net result is a slight over-pressure in the cooking chamber(s) compared to atmospheric pressure, but without the need for the chamber(s) to be sealed or provided with a pressure relief valve. Such embodiments therefore represent an improvement over conventional steam cookers in terms of performance but without any additional complexity being required in the design of the cooking chambers.

Appliances in accordance with both of the above aspects of the invention may also represent an improvement over conventional pressure cookers as the steam is generated externally from the cooking chamber(s). This means that the steam flow rate and/or pressure may be controlled and regulated before the steam enters the cooking chamber(s). The invention may therefore be considered to provide a new hybrid between a conventional steam cooker and a conventional pressure cooker with a mixture of the benefits available from each. As mentioned above, it is preferred that the appliance comprises means for regulating the steam pressure before it is provided to the cooking chamber(s). Firstly, the appliance preferably comprises means for controlling the steam flow rate before the steam is supplied to the cooking chamber(s). A steam flow control means may be provided in the steam path between the boiler and the cooking chamber(s). Additionally or alternatively, a steam pressure relief valve may be connected in the steam path between the outlet of the boiler and the cooking chamber(s).

Where a water reservoir is provided, the steam pressure relief valve may conveniently be arranged to direct steam back into the reservoir where it can condense and be recycled.

Accordingly the steam pressure relief valve is preferably arranged to divert steam from the steam supply path onto a bypass path arranged to deliver fluid back to the reservoir. Where both steam controls are provided, one may be provided either upstream or downstream of the other. Steam control is discussed again in more detail below. The generation of steam, including pressurised steam, and the control of steam delivery

(including steam pressure) external to the cooking chamber(s) can be particularly advantageous where the appliance comprises multiple cooking chambers, each supplied with steam from the boiler. The steam generated by the boiler can be directed as needed to the different cooking chambers and possibly supplied at different pressures and/or flow rates as required. Thus in accordance with both of the above aspects of the invention, it is preferable that the appliance comprise a plurality of cooking chambers. For example, two, three or four cooking chambers may be arranged in the appliance.

This is considered novel and inventive in its own right and thus when viewed from a further aspect the invention provides a steam cooking appliance comprising a water boiler for generating steam and means for supplying the steam generated by the boiler to a plurality of separate steam cooking chambers.

Any of the essential or preferred features of the other aspects of the invention set out above may be applied to this further aspect of the invention. Thus in one set of embodiments the appliance comprises a water reservoir and transfer means such as a pump for supplying water from the reservoir to the boiler. The boiler may be arranged to generate steam at a temperature above the atmospheric boiling point. When multiple steam cooking chambers are provided the means for supplying steam from the boiler is preferably arranged to share the steam between the different cooking chambers that are in use. Preferably each cooking chamber is provided with an independent connection to the steam supply. The steam supply means may comprise a separate steam outlet for connection to each steam cooking chamber. The steam supply means may be arranged to deliver steam on demand to a particular cooking chamber while other chambers are not being used. This makes it possible to selectively use different chambers at different times, and for different foodstuffs. Such an appliance provides greater flexibility over a common domestic steamer wherein multiple steamer baskets are simply stacked on top of one another. In such

conventional steamers the lid and one or more baskets must be lifted if it is desired to add or remove a basket and inevitably steam is lost from all of the baskets while cold air is allowed to enter. Usually the upper basket(s) must be lifted if it is desired to inspect the food cooking in a lower basket. This is because the baskets share a common steam source, namely the pot or reservoir of boiling water arranged below the baskets.

The Applicant has also appreciated that it can be easier to seal closed a plurality of preferably smaller cooking chambers, rather than a single large cooking chamber, when steam is supplied under pressure, for example steam at a temperature above the atmospheric boiling point. De- pressurisation may also be less of a problem.

In some embodiments the steam cooking chambers may each comprise means for regulating the flow and/or pressure of the steam supplied from the boiler, for example at the steam inlet to the chamber. However, where the steam supply means shares the steam generated by the boiler between a plurality of cooking chambers, it can be advantageous to control the steam supply centrally rather than for each cooking chamber to comprise its own steam control means. Advantageously, a central regulator can control the steam pressure before the steam is supplied to each of the chambers, preferably via their individual connections. A central regulator in the form of a steam pressure relief valve has already been described above. Furthermore, a central steam flow control means may be provided between the boiler and the various cooking chambers, as is also described above.

There will now be described some preferred features that are applicable to all of the foregoing aspects of the invention.

In a preferred set of embodiments at least some of the cooking chamber(s) is/are removable from the appliance. The appliance may comprise a mixture of removable and non-removable cooking chambers. The removable chambers can be emptied and filled away from the appliance, thereby enhancing the flexibility of use of the appliance. In addition such

removability can help to reduce the risk of liquid and foodstuffs being spilt on or in the appliance during use. Even so, the appliance may comprise collection means, such as a tray, arranged below the cooking chamber(s) that can catch any spills. A further advantage of such a collection means is that it can collect condensate from the steam exiting the cooking chambers.

Collection of the condensate is advantageous as it contains flavour and nutrients from the cooking process and may be used to make stock or gravy.

Where a cooking chamber is removable it is preferable that it is provided with a connector for connecting with the steam supply means. Where multiple chambers are provided,

advantageously each chamber can be independently removed and connected with the appliance as desired. For example, food may be placed in one chamber and while it is cooking another chamber may be removed to be filled, emptied, and/or cleaned before it is replaced, without interrupting the cooking process in other cooking chambers. If the steam supplied to the cooking chamber(s) is at ambient pressure then the cooking chamber(s) may be directly connected to the steam supply means, for example without an intervening valve. The steam supply means may simply comprises a pipe directing steam into each cooking chamber. The cooking chamber(s) can be opened to atmosphere without an interlock being necessary. If a cooking chamber is removed, for example to dispense food, then steam will simply be vented to atmosphere.

However, the Applicant has appreciated that it is desirable for steam to only be supplied to a cooking chamber while the cooking chamber is in use. This can be particularly important where the cooking chamber is removable, as it would be wasteful for steam to be released when the cooking chamber is not in position. Thus in a preferred set of embodiments there is provided a steam connector between the or each cooking chamber and the steam supply means. This is considered novel and inventive in its own right and thus when viewed from a yet further aspect the present invention provides a steam cooking appliance comprising means for generating steam and means for supplying steam to one or more removable steam cooking chambers, wherein a steam connector is provided to connect the or each removable steam cooking chamber to the steam supply means.

Any of the essential or preferred features of the other aspects of the invention set out above may be applied to this further aspect of the invention. Thus in one set of embodiments the appliance comprises a water reservoir and transfer means such as a pump for supplying water from the reservoir to a steam generating boiler. The boiler may be arranged to generate steam at a temperature above the atmospheric boiling point. The steam connector preferably comprises a flow control valve. The valve in the steam connector is preferably arranged to selectively supply steam to the or each cooking chamber only when it is connected to the steam supply means. It is preferred that the valve in the steam connector is arranged to shut off the steam supply means when there is no cooking chamber connected or a chamber is removed. A user can therefore remove and replace a cooking chamber without being exposed to steam. The valve may be arranged to operate automatically in response to connection or removal of a or the chamber.

Furthermore, where steam is supplied at a pressure above ambient, according to some of the preferred embodiments and an aspect of the invention described above, it would not be desirable for pressurised steam to be ejected when a cooking chamber is removed. A valve connector is preferably provided, as described above. Thus pressurised steam will not be allowed to escape when a chamber is not connected to the appliance. However the Applicant has appreciated that even with a valve connector between the or each cooking chamber and the steam supply means, there is a risk that the chamber may be forcibly ejected if the valve only closes at the same time as the chamber is disconnected and removed, due to the pressurised steam entering the chamber.

In a preferred set of embodiments the or each removable cooking chamber is arranged so as to have two stages of disconnection. Preferably the steam connection for each cooking chamber comprises a physical connection in addition to the valve connection. Furthermore, the additional physical connection is preferably arranged such that the chamber cannot be physically removed until the valve connection has closed. In one set of embodiments the valve is closed in a first stage of disconnection, for example when the chamber is partway through its physical removal. This shuts off the supply of pressurised steam and ensures that all steam has entered the chamber before it is removed. Then the chamber is preferably fully removed in a second stage of disconnection. The two stages of disconnection may be provided by a bayonet-type connection, e.g. requiring a twisting action that acts to shut off the valve before a lifting or pulling action to remove the chamber.

The steam connector may be arranged to connect to any suitable part of a steam cooking chamber. However it may be preferred to supply steam at either one end or the other so as to encourage a flow through the chamber. The steam connector may be arranged at either an upper or lower end of a steam cooking chamber. In one set of embodiments the cooking chamber is arranged to connect to the steam supply at a lower end, such as through the base of the chamber, such that the chamber can be placed down on the connector for ease of use. Steam supplied at a lower end of the chamber may then disperse upwardly into the chamber. Some steam will condense in the cooking chamber and carry with it liquid from the foodstuffs being cooked. Typically a condensate outlet means may be provided in a lower end of the chamber to release such liquids and prevent the food from stewing and getting soggy. If the steam connector is at the same end of the chamber as the condensate outlet then there is a risk that the upward flow of steam may hinder the downward flow of condensate, with the cooler condensate causing thermal losses in the steam.

In another set of embodiments the cooking chamber is arranged to connect to the steam supply at an upper end, such as through the ceiling or lid of the chamber. Steam supplied at an upper end of the chamber may then flow downwardly into the chamber and travel substantially in the same direction as the condensates that are flowing under gravity to be discharged from a lower end. The steam flow can assist the removal of condensates from the cooking chamber. It may be beneficial for the steam and condensates to flow in the same direction along a common thermal gradient so as to minimise thermal losses and maximise the energy transferred to the food to be cooked. Such flow will also follow the general convective pattern in the cooking chamber, with colder vapour at the bottom than the top. It may therefore be preferred for any steam vent(s) or steam pressure relief valve to be arranged at a lower end of the cooking chamber, opposite to the steam supply coming in at an upper end. In one set of embodiments, the appliance may be provided with means for reducing the pressure of the steam supply while a user is connecting, and optionally disconnecting, a cooking chamber. There are of course many ways in which this could be done. For example, manual intervention by a user, e.g. to press a button, could be used. Preferably however the appliance is arranged to detect when a cooking chamber is brought into engagement with a connector to the steam supply. The detection means may be based e.g. on optical, magnetic, or capacitance sensors, etc. Preferably a simple micro-switch is arranged to sense when a chamber has been connected to the appliance and its steam supply. Such detection means may also provide feedback signal(s) allowing for control of the steam supply depending on the number or type of cooking chambers connected.

In response to either a user input or a sensor signal, there is preferably provided control means to adjust the steam flow rate and/or pressure before it reaches the cooking chamber(s). This may be achieved by controlling the steam flow from the boiler and/or the water flow to the boiler. Preferably the signal from the chamber connection sensor, such as a micro-switch, is arranged to control one or more of: (i) a steam flow control means provided between the boiler and the cooking chamber(s); (ii) a water flow control means provided between the pump or other transfer means and the boiler; and/or (iii) the pump or other transfer means supplying water to the boiler.

The steam flow control means may comprise a manifold connecting the steam outlet from the boiler to multiple cooking chambers. The manifold may be used to reduce or shut off the steam supply to a designated cooking chamber while it is being connected and/or disconnected. It may also shut off the steam supply completely when a chamber has been removed. Control of a manifold or similar steam flow control means downstream of the boiler may be preferred, especially where there are multiple cooking chambers, as it prevents the change in steam flow/pressure from affecting other cooking chambers that may be in use. The manifold may include means for throttling the flow of steam to the cooking chamber(s).

If all of the chambers are removed at once when the appliance is switched on then it would be undesirable for steam to continue to be produced. To cope with this situation, the appliance preferably comprises control means responsive to the removal of the cooking chamber(s), e.g. receiving signals from the chamber connection sensors, and arranged to turn off the steam supply when it is sensed that all steam chambers have been disconnected. This may be achieved by the water flow control means diverting water back into the reservoir before it reaches the boiler, by the pump or other transfer means being switched off and/or by switching off the power supply to the boiler.

It is described above that the steam supply to a particular chamber may be cut off when the chamber is removed, preferably by automatic operation of a valve connector. However the boiler will continue to generate steam as long as it is supplied with water. In the event that there are no chambers connected then the steam pressure may start to back-up in the steam path from the boiler. As described above, control means may be arranged to reduce or halt steam generation when not required by the cooking chamber(s), e.g. in response to signals received from chamber connection sensors. However there may be other reasons for an over-pressure in the steam supply line, for example due to a blockage in the steam connector(s) or in the steam path or in the manifold that controls the flow of steam to the cooking chamber(s). Then the steam produced by the boiler is preferably released by an independent means, for example through a pressure relief valve as described above.

It is preferable that a pressure relief valve is provided between the steam outlet of the boiler and the cooking chamber(s), preferably between the steam outlet of the boiler and any steam flow control means. This means that there is always a way of releasing excess steam pressure so that the boiler is not damaged, independently of any steam flow control means. This can also provide a safety back-up in the event that a user forgets to switch off the appliance when not in use. As mentioned above, the pressure relief valve is preferably connected to a bypass path arranged to exhaust excess steam into the water reservoir. In one set of embodiments already mentioned above, the or each cooking chamber is preferably sealed substantially closed to prevent the escape of steam. A sealing lid may be provided. The lid may be closed by any suitable sealing means, such as a screw mechanism, a toggle clamp, a bayonet connection, or an external clamp. The lid may possibly be provided with interlock means for de-pressurisation prior to removal, although this should not be necessary at mild pressures of ~ 0.5 bar above atmospheric. Although steam, heat and pressure may be lost from a particular cooking chamber when a lid is removed, for example to inspect the food being cooked, it will be appreciated that this will not affect any other cooking chambers connected to the appliance in embodiments where multiple steam cooking chambers are supplied separately with steam. Advantageously each cooking chamber may quickly re-attain its steam cooking temperature/pressure as it may represent a relatively small volume compared to the total cooking volume of the appliance. As soon as the lid is replaced the chamber will quickly heat back up and, in the case of pressurised steam, re-attain an elevated temperature.

Furthermore, opening one or more of the cooking chambers will not affect steam generation by the boiler. Whereas in a conventional steamer removal of the lid or one of the steamer baskets results in the loss of heat from the water boiler and may interfere with steam generation, the temperature of the separate boiler is of course unaffected in appliances according to the invention. The same is true when a cooking chamber is removed and the steam supply to that chamber is shut off. The appliance is therefore very efficient and provides high performance in combination with flexibility of use.

Although each cooking chamber is sealed substantially closed in use, it is preferably provided with a pressure relief valve that opens when pressure in the cooking chamber exceeds a threshold. This helps to regulate the pressure in the cooking chamber and preferably keeps it at around 0.5 bar (50kPa) above atmospheric or less. A conventional pressure relief valve venting to the atmosphere could be provided - e.g. similar to those found on traditional espresso coffee makers. In preferred embodiments however the pressure relief valve is configured to vent excess pressure into an unpressurised part of the interior of the appliance, for example when multiple chambers are enclosed by a shared cover. This may be considered to be safer in essentially eliminating the risk, however unlikely, that steam will be vented, especially at pressure, near a user. The Applicant has appreciated that once steam is no longer being supplied to a cooking chamber, the water vapour in the chamber will cool fairly rapidly and condense, which may create a significantly lower pressure in the chamber. This vacuum could make it more difficult to remove the lid and/or disconnect the chamber. It is therefore preferred that the valve is configured to open when there is a pressure differential across it in either direction. Preferably it is configured to open at a lower pressure differential in one direction than the other. This allows it to function as described above more effectively since the vacuum set up in the cooking chamber at the end of the steam cooking cycle will typically represent a lower pressure differential to atmospheric than the over-pressure at which pressure relief is required. Such a bi-directional valve is described in more detail in the Applicant's published PCT application WO 2009/081 159.

In a preferred set of arrangements the valve described above comprises a domed resilient diaphragm having at least one slit defined therein. The domed shape gives the asymmetric pressure characteristics mentioned above. As the pressure on the concave side of the diaphragm becomes increasingly greater than on the convex side, e.g. because a vacuum is created on the convex side, the slit in the diaphragm is forced open, thereby allowing fluid communication through it. This functioning makes it suitable for admitting air into the cooking chamber when a vacuum is formed as the steam therein cools. In preferred embodiments therefore the valve is arranged with the concave side of the diaphragm facing the exterior of the cooking chamber. Should pressure in the cooking chamber approach a dangerous level at any stage, the pressure on the convex side will become sufficient to reverse the curvature of the diaphragm in a 'snap' action which causes the slit to open and so allow a reduction in pressure in the cooking chamber.

More than one slit may be provided - e.g. to form a cross or star shape. Preferably the diaphragm is made of silicone or similar inert, heat and water resistant/tolerant material. This is advantageous over the conventional metal pressure relief valves for prolonged contact with water. The valves described herein have also been found to have a more controllable maximum operating pressure which allows them to be safer as they can be designed to vent at a lower pressure in a fault condition whilst still avoiding leakage in normal use.

The pressure relief valve may be positioned anywhere in the cooking chamber so as to provide a fluid connection between the steam-containing volume of the chamber and the exterior. As the steam is supplied from a source outside the cooking chamber, there may not be any substantial volume of liquid in the chamber. Instead solid food may be supported in the steam- filled space in the chamber. This means that there is much more flexibility in the positioning of the pressure relief valve as compared to a pressure cooking vessel, for example, where most of the vessel is filled with liquid and the pressure relief valve must be in the lid to communicate with the steam space above the liquid. In embodiments of the invention the pressure relief valve may be provided in a lid of the cooking chamber or in a wall of the chamber. The pressure relief valve is preferably positioned so as to vent any escaping steam away from a user of the appliance.

However it has been recognised that there are certain advantages in positioning the pressure relief valve in the base of the cooking chamber, either in a lower part of a side wall or in a base wall. This is only possible because substantially the whole of the volume of the cooking chamber is filled with steam. The food being cooked is preferably supported so that it is substantially not in contact with the base and/or walls of the cooking chamber, which can promote steam circulation and increase the steam contact area with the food. When the pressure relief valve is provided in a lower part of the cooking chamber it can be arranged to exhaust steam onto an external collection means, such as the tray described above. The steam condensate that is collected will include flavour from the food in the cooking chamber(s) that can be used to make gravy or stock.

This is seen to be a particularly advantageous feature, and thus in embodiments where the cooking chamber is pressurised there is preferably included a pressure relief valve that allows steam, condensates and water to drain from the cooking chamber in addition to maintaining a regulated internal pressure. A bi-directional pressure relief valve comprising a flexible diaphragm may be particularly preferred as being tolerant of food debris and easy to clean. Such a valve may be designed so that the internal cooking pressure regulation, i.e. internal to external direction of flow, operates at a pre-determined higher pressure than the atmospheric venting regulation, i.e. external to internal direction of flow.

To comply with some regulations for pressurised cooking vessels it may be a requirement to incorporate more than one pressure relief means to increase safety. To fulfil such requirements additional devices can be employed. However, in a further embodiment of the domed resilient diaphragm pressure relief valve described above, the pressure relief device preferably incorporates an additional pressure relief function so that a single device provide the necessary fail safe. In a set of embodiments the device can operate to relieve pressure at two

predetermined pressures (in addition to operating in reverse when a vacuum is formed e.g. upon cooling). The operation of the valve at a first predetermined pressure, as described above, may be configured to be a lower pressure than a second predetermined pressure.

Operation of the pressure relief means at a second preset pressure is preferably arranged to eject the device from its position in the cooking chamber, for example in the event that the first stage of pressure relief is not effective - such malfunction could be caused by food debris interfering with the valve or by damage to the valve. In a preferred set of embodiments the pressure relief valve comprises a diaphragm supported in an aperture in a cooking chamber wall or lid by flanges arranged to secure the diaphragm in the aperture. The flanges may be made from the same deformable but resilient material as the diaphragm and are preferably configured to deform, allowing the diaphragm valve and the supporting flanges to be ejected from the aperture when exposed to a predetermined second, higher pressure. In a further configuration of the pressure relief valve described above, there may be provided means allowing for manual operation of the valve to vent the cooking chamber to atmosphere on demand, for example to facilitate removal of a lid for the chamber. The valve may be provided with a manual actuator, preferably arranged so as not to interfere with the normal pressure regulating function of the valve and the exiting of steam and condensates. Preferably the manual actuator is integrally moulded with the valve, for example connected by a living hinge or the like. The manual actuator is preferably biased out of contact with the diaphragm of the valve but may be pressed against the diaphragm to open it when operated by a user. Such an actuator can have the additional function of being used to clear the valve if it becomes blocked, e.g. because food debris has become trapped over the diaphragm.

The appliance may be provided with one or more control means. A timing control means is preferably provided so that a user can select a time period for steam cooking and the control will switch off the appliance after the selected time has elapsed. The power supply to the boiler may be cut after a predetermined time has elapsed, while the residual steam and heat may be used to cook the food for the remainder of the selected cooking period. The timer may also control the steam flow control means, where provided, so as to change the rate at which steam is supplied and/or the supply to different cooking chambers. An electronic control means including a microprocessor may be provided. This could enable more complicated cooking programs to be inputted or selected from those stored in a memory.

The steam cooking chamber(s) may be of any suitable form for containing food and enabling steam to come into contact with the food to be cooked. The or each chamber may include one or more components such as trays, spacers, separators, etc. to increase the contact area between the food contained in the chamber and the steam therein. Additionally or alternatively, the or each chamber may include means for directing and/or distributing steam, such as is discussed above. A nozzle may be connected with the steam inlet if a pressurised jet of steam is desired. Steam may be directed through one or more injection needles onto which food products can be impaled to inject steam into the interior of the product. Steam may be diffused through a mesh, foraminous plate, or the like to encourage an even distribution in the chamber. Any of these features may be integrated with a steam guide having the scale collecting function as outlined hereinabove.

The Applicant has devised one particularly advantageous component for supporting food in a steam cooking chamber and bringing steam into contact evenly with the food therein. In a preferred set of embodiments one or more of the cooking chambers is/are provided with a generally conical food supporting member that is perforated to allow steam to pass

therethrough, for example through holes or slits. The conical member may be located over the connector that supplies steam to the chamber. The conical member is preferably flared at its base with walls that extend inwardly and upwardly towards a central cone or dome. This shape has been found to funnel the steam as it passes up from the connector and to bring it evenly into contact with the supported food, thereby improving the steam cooking efficiency.

The cooking chamber(s) may optionally have removable internal sub chambers to divide and separate different foods arranged within the same cooking chamber.

Sub chambers may be employed as holders to impart the flavour of their contents to the food within the cooking chamber, e.g. flavours such as garlic, lemongrass, thyme etc. Thus a further advantage of a steamer appliance according to embodiments of the present invention is the individual flavouring of food to the preferred preference of the user.

In a further optional set of embodiments, the cooking chamber(s) may be configured so that they are multi-functional and can also be used as storage containers for food. This means that a cooking chamber may also be used to keep food that has been prepared or previously cooked in a refrigerator, as well as being used as a cooking chamber and optionally used to serve cooked food at the table.

Where multiple cooking chambers are provided, the appliance is preferably provided with a cover or lid that encloses all the chambers. This can help to trap an insulating layer of air around the chambers, to help keep them warm, as well as ensuring that any steam released from the chambers is cooled and condensed within the appliance rather than causing a risk of scalding to a user. Multiple cooking chambers may be stacked one on top of another, as is conventional, or provided separately e.g. side by side. There may also be provided different types of steam chambers, and possibly one or more chambers comprising multiple steam connectors. Thus it is envisaged that multiple cooking chambers could be removed and replaced with a larger chamber or cooking means that connects with several steam supply ports. The cooking means may not even be a closed chamber, especially where the appliance is provided with an overall enclosure. Thus the cooking chambers could be replaced by a cooking means simply comprising steam connectors and a food support member. Such an arrangement could be used, for example, to steam cook a large piece of food such as a whole fish in the enclosed volume of the appliance at ambient steam pressure.

There will now be described some further preferred features of the water supply and steam generating system that are applicable to all of the aspects of the invention discussed above. Although the appliance and its reservoir, where provided, may be connected to a mains water supply, it is preferable that the appliance comprises means for a user to refill the reservoir manually. The reservoir may be removable so that it can be filled at a tap and replaced. The water reservoir should be designed to hold a sufficient volume of water to provide steam for at least one cooking operation before it is necessary for it to be refilled. Whether or not the reservoir is removable, means are provided to access the reservoir for refilling.

The reservoir may be provided with a water level gauge and/or refill indicator. The appliance may be provided with sensing means to detect the water level in the water reservoir so as to turn off the pump and/or the boiler and/or provide some indication that the reservoir requires filling before the cooking process can proceed.

Although the water reservoir may be provided anywhere in the appliance, in a preferred set of embodiments the water reservoir comprises a tank provided in the base of the appliance. This can advantageously add to the stability of the appliance, for example on a counter-top in a domestic setting. The base is preferably provided with a fixed cord mains electrical connection.

In a preferred set of embodiments the water reservoir is connected to a filter. Preferably the water is filtered before being supplied to the pump. Alternatively or additionally, a filter may be provided between the pump and the boiler. The filter helps to ensure that impurities, such as scale-forming minerals, are removed from the water prior to steam generation so that scale does not build up in the boiler and detract from its performance. The filter may comprise an ion exchange resin as the water treatment medium. There is preferably provided a water filter unit 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. Such a filter arrangement advantageously provides a relatively long treatment path length in a compact unit. Such an arrangement is particularly advantageous when the treatment medium comprises an ion exchange resin, as the resin has been found to swell in use, and is described in more detail in the Applicant's co-pending UK application GB 0917007.7 and PCT application PCT/GB2010/051614. The steam generating boiler will now be described in more detail. In some preferred

embodiments the boiler has a temperature of between 100 and 500 ^QC, more preferably between 105 and 380 ^QC. Preferably the pump is arranged to supply pressurised water to the inlet of the boiler. The pressure of the water supply is preferably greater than 0.5 bar, e.g. more than 1 bar and up to 3 bar or more. The boiler may be pressurised. Preferably the internal steam pressure generated within the boiler should not be greater than that of the water pressure entering it, else water will be prevented from entering the device, resulting in a subsequent drop in steam flow rate and unwanted fluctuation in steam output.

One preferred form of electric water boiler is described in the Applicant's published PCT application WO 2010/089561 . The water boiler preferably comprises a water inlet, an electric heater, a steam outlet and an evaporation space bounded by at least one surface in thermal contact with the heater, wherein the evaporation space is configured to present an expanding cross-sectional area in a direction away from the water inlet. According to one set of embodiments the water boiler may comprise a conical water boiling chamber. The increasing internal volume in the evaporation space and a corresponding increase in surface area during the advancement and a corresponding rise in temperature of the water and steam. In accordance with such arrangements, the evaporation space can start off relatively small to give good intimate contact between the water and the heated surface(s) of the evaporation space to give efficient evaporation of the water, whilst at the same time allowing the steam so generated to expand into the increasing volume as it flows away from the water inlet e.g. towards the steam outlet.

In some example embodiments, the evaporation surface is convex, concave or conical. Other substantially two or three dimensional forms such as fans, deltas, hemispheres, parabolas, prisms, pyramids and other suitable forms can be employed to provide the required increasing volume and surface area. Of course, other, more complex, shapes could be used to give the same effect, both internally to enhance surface area and so evaporation efficiency and externally to minimise the space required for the boiler in the appliance. Preferably the evaporation space simply comprises an open chamber.

A boiler as described above, or indeed one which only has some of the features set out, which might include the feature of expanding cross-sectional area, is advantageous as it allows for the very rapid production of steam from when water first enters the water inlet as compared, for example, with a more traditional boiler in which a heating element is used to heat a body of water. The heated surface bounding the evaporation space (hereinafter referred to as "the evaporation surface") is preferably non-planar. This facilitates maximising the surface area available in a given volume occupied by the boiler within the appliance. In a set of preferred embodiments, the surface area of the evaporation surface (measured prior to the application of any surface enhancing coating) is more than 1 .5 times the maximum planar projection of the surface (i.e. the footprint), more preferably greater than 1 .75 times, more preferably greater than twice.

The evaporation space may of course have more than one evaporation surface. This might be the case as a result of the distribution of the heating element, the provision of multiple heating elements, or simply by the close thermal connection between a surface which is directly heated and another surface.

Preferably the evaporation space is empty until it is filled with water/steam. However, in some example embodiments a lattice or mesh structure may be provided. In some circumstances this can enhance the efficiency of steam generation by increasing the effective area which is heated and can also help to reduce the Leidenfrost effect (whereby small droplets of water are separated by an insulating layer of steam when water comes into contact with a very hot surface). In one exemplary set of embodiments, a woven metal mesh is located within the evaporation space. The mesh structure could be lightly compressed against the evaporation surface so that the extremities of the mesh's major surfaces, those created by the alternating under and over relationship of the mesh's woven elements, contact the evaporation surface and the confronting surface in a corresponding alternating arrangement. Filling the evaporation space with a lattice or mesh can help to restrict the flow of water particles, but allow the freer passage of steam, so increasing the evaporation efficiency of the boiler. In one set of embodiments a woven stainless steel mesh or meshes is employed which is advantageously corrosion resistant. However other configurations can provide a similar advantageous effect, e.g. an expanded mesh, a perforated material, a fibrous material, etc..

Alternatively, or additionally, the evaporation surface could be provided with a texture, structure or coating to increase its surface area at a microscopic level and/or to mitigate the Leidenfrost effect. For example the surface could comprise steps, tessellations or texture creating a myriad of channels or small structures for increased surface area and turbulence to flow within the evaporation space. Preferably the evaporation surface is hydrophilic, at least at its normal operating temperature. In an exemplary set of embodiments the normal operating temperature is greater than 140 °C. This might be a natural characteristic of the material used for the evaporation surface, it might be achieved or enhanced by a suitable surface treatment and/or it might be achieved or enhanced by a suitable heat resistant coating material. Where the evaporation surface is made hydrophilic by a surface treatment or coating the treated or coated surface should be hydrophilic at a temperature at which the Leidenfrost effect would otherwise occur on the untreated or uncoated surface.

In a set of embodiments the evaporation surface is coated with a coating comprising zeolite particulates. Preferably said zeolite particles are of a nano and micro scale. In a set of embodiments the coating comprises aluminosilicate particles. In an exemplary set of embodiments the particles have the CAS number 1318-02-01 or similar. Preferably such a coating comprises a binder which acts as a carrier medium to facilitate the application as a thin film between 3 microns to 100 microns in thickness but more preferably between 3 microns and 50 microns in thickness. The binder is preferably formulated not to saturate the structure of the zeolite particles and to facilitate a functional film layer with micro-porous properties, improving surface wetting and exhibiting minimal surface tension in contact with water.

Prior to drying or curing to form the functional coating, the zeolite particulates are held in suspension within the binder. Upon hardening to form the functional coating the zeolite particles are thereafter encapsulated or partially encapsulated by the binder to create a nano and or micro scale structured open cell syntactic matrix where the zeolite particulates act as scaffolds with interlinking nano and or micro scale voids creating a partially open and partially closed cell structure. Prior to the application of the functional coating, the internal surfaces of the boiler may be prepared via surface roughening and degreasing, where the surface is abraded and a defined texture results to assist in mechanical bonding of the functional coating to the prepared surface but also to further impart a texture. Such a texture can influence the heat transfer surface of the functional coating. The preferred surface roughening method would be high pressure grit blasting or blasting with any other suitable substrate to create the preferred finish however other suitable methods may be employed.

In a set of embodiments, at least part of the evaporation space is configured so as to present an interrupted flow path. Advantageously, such a structure could be provided at least in a portion nearest to an exit of the evaporation space, i.e. furthest from the water inlet. Such

arrangements have been found to enhance the evaporation of water which has not been evaporated and also to physically separate unevaporated droplets of water from the steam. The Applicant has found that a similar effect can be found by throttling or otherwise restricting the flow of steam. Steam may simply be allowed to leave the boiler once it has passed through the evaporation space, for example supplied straight to a cooking chamber. However, in a set of preferred embodiments the boiler comprises means for collecting the steam. This allows it, for example, to be channelled into one or more pipes for delivering it to one or more cooking chambers of the appliance. The means for collecting steam may comprise means for trapping unevaporated droplets of water. For example this might be a protruding outlet tube encouraging steam channelled by the walls of the chamber to undergo a change of direction leading to expulsion of entrained droplets. Additionally, or alternatively, the boiler may comprise a mesh arranged below the steam outlet so as to prevent any particulate material from exiting the boiler with the steam.

The present invention relates to steam cooking appliances for domestic use. It will be appreciated that the flow rate of steam supplied may vary from one appliance to another, but according to preferred embodiments the steam output may be selected from one or more of: (i) 0.5-1 .0 L/hr; (ii) 1 .0-1 .5 L/hr; (iii) 1 .5-2.0 L/hr; (iv) 2.0-2.5 L/hr; (v) 2.5-3.0 L/hr; (vi) 3.0-3.5 L/hr; (vii) 3.5-4.0 L/hr; (viii) 4.0-4.5 L/hr; (ix) 4.5-5.0 L/hr; (x) 5.0-5.5 L/hr; or (xi) 5.5-6.0 L/hr.

As is mentioned above, the water boiler is preferably designed to heat and evaporate a selected volume of water that is pumped from the reservoir on demand so as to reduce the start-up time of the appliance and allow for the rapid production of steam. The steam generated by the boiler is supplied through a steam guide member so as to provide for control of the steam flow before it reaches the cooking chamber(s). The present invention therefore extends to a domestic steam cooking appliance comprising a reservoir for water, a closed water boiler for generating steam in an evaporation chamber having a volume less than 10% of the volume of the reservoir, and an electric pump for transferring water from the reservoir to the boiler on demand, the appliance further comprising a steam supply means connected at one end to an outlet of the water boiler to receive steam and connected at the other end to a steam guide member, the steam guide member acting to control the velocity of steam therethrough and to distribute the steam supply between a plurality of vents arranged to exhaust steam into one or more steam cooking chambers.

Some preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 a is a front perspective view of a steam cooking appliance in accordance with a first embodiment of the invention; Fig. 1 b is a back perspective view of the steam cooking appliance;

Fig. 2 is sectional view through the main components of the steam cooking appliance;

Fig. 3 is another sectional view through the main components of the appliance;

Fig. 4 is a plan view of the components inside the base of the appliance;

Fig. 5 is a perspective view of the base of the appliance with the cooking chambers removed; Fig. 6 is a cross-sectional view through one of the cooking chambers;

Fig. 7 is a perspective view showing the operation of a manual actuator on a bi-directional pressure relief valve; Fig. 8 is a perspective view of a boiler according to one example embodiment of the invention; Fig. 9 is a perspective view of a boiler according to a preferred embodiment of the invention; Fig. 10 is an exploded view of the boiler of Fig. 9;

Fig. 1 1 is a cross-sectional view through the boiler of Fig. 98;

Fig. 12 is a schematic sectional view of a steam cooking appliance in accordance with a second embodiment of the invention;

Fig. 13 is a schematic sectional view of a steam cooking appliance in accordance with a third embodiment of the invention;

Fig. 14 is an exploded perspective view of a steam guiding head for use in a steam cooking appliance according to some embodiments of the invention;

Fig. 15 is a sectional view showing a flow path through the steam guiding head of Fig. 14; Fig. 16 is a perspective view of part of another steam guiding head for use in a steam cooking appliance according to some other embodiments of the invention; and Fig. 17 is a perspective view of part of yet another steam guiding head for use in a steam cooking appliance according to some other embodiments of the invention;

Fig. 18 is a sectional view showing a tortuous flow path through the steam guiding head of Fig. 16 or 17; and

Figs. 19a-19c are schematic diagrams showing the steam flow through cooking chambers according to different embodiments of the invention.

There is shown in Figs. 1 to 5 a steam cooking appliance which embodies several aspects of the invention. From Fig. 1 it can be seen that the appliance 1 generally comprises a base 2 and a steam cooking vessel 4. The base 2 is provided with an electrical power cord 6 for connection to the mains supply. At the back of the base 2 a water inlet 8 allows a user to fill a water reservoir in the base 2. A timer control knob 10 is also provided on the front of the base 2. The control knob 10 could of course be replaced by any other suitable user interface, such as push buttons or a touch sensitive control pad, enabling a user to select a desired steam cooking time or program.

The components of the steam generation system in the base 2 can be seen in Figs. 2 to 4. The water reservoir 12 is connected to a filter unit 14. It can be seen from the sectional views of Figs. 2 and 3 that the filter unit 14 comprises a plurality of compartments containing a water treatment medium. The filter unit 14 is connected by a pipe 15 to the inlet of an electrical pump 16. The pump 16 is connected to the electrical supply provided by the power cord 6. The pump 16 pumps filtered water into a boiler unit 18. The timer knob 10 is connected to the power supply for the pump 16 and turns off the pump 16 when the selected period or program of steam generation has elapsed.

Arranged in the water flow path between the pump 16 and the boiler 18 is a water pressure relief valve 36 and a water flow controller 37. The water pressure relief valve 36 is arranged to divert water from the pump 16 to a bypass pipe 32 leading back into the reservoir 12. This provides a safety function if, for example, there is a blockage in the water supply line or the boiler 18. The water flow controller 37 is arranged to throttle or otherwise control the flow rate of water before it enters the boiler 18. Although the flow controller 37 is shown downstream of the pressure relief valve 36, it can be preferred for the pressure relief valve 36 to instead be provided between the flow controller 37 and the boiler 18, so that any backed-up water pressure is released through the valve 36 before affecting the flow controller 37. The boiler unit 18 includes an electrical heating element 20 that is also connected to the same power supply as the pump 16. Water is heated inside the boiler 18 to generate steam that is collected by an outlet pipe 22. The boiler 18 will be described in more detail below with reference to Figs. 7-10. The boiler 18 is provided with a temperature regulator 19 (seen in Fig. 4) positioned in good thermal contact with its outer surface. The regulator 19 acts to maintain the temperature of the boiler 18 within a desired range, for example an operating temperature range of 105 to 380 °C. A further temperature regulator (not shown) is also provided and forms part of a thermally sensitive control for the electrical supply to the pump 16. This further temperature regulator has normally-open contacts which are not closed until the boiler's preset temperature is reached. These contacts are connected electrically in series with the pump 16 so that the pump 16 will not operate to supply water to the boiler 18 until it has heated up to a minimum temperature. The outlet pipe 22 from the boiler 18 is connected to a manifold 24. From the manifold 24 there are independent pipes 26a, 26b directing steam to two separate steam cooking chambers 28a, 28b. Although only two cooking chambers 28a, 28b are shown, it will be understood that more than two chambers may be provided, with the manifold 24 connected to as many steam supply pipes as required. Each steam pipe 26a, 26b terminates at a steam connector 30a, 30b that provides a fluid connection between the base 2 and the cooking chambers 28a, 28b in the steam cooking vessel 4.

It can be seen from Figs. 2 to 4 that there are two bypass pipes 32, 34 arranged in the steam generation system, both connected to the reservoir 12. Firstly, as described above, a water bypass pipe 32 is connected to the water pressure relief valve 36 provided between the pump 16 and the boiler 18. Secondly, a steam bypass pipe 34 is connected between the steam outlet pipe 22 and the manifold 24. A steam pressure relief valve 38 is arranged to divert steam to the reservoir 12 via the bypass pipe 34, for example if the steam supply to the manifold 24 and thus to the cooking chambers 28a, 28b is to be shut off.

Each removable cooking chamber 28a, 28b comprises a valve connector 30a, 30b. The valve in the connector 30a, 30b shuts off the steam supply to the chamber 28a, 28b when it is disconnected from the base 2. Each cooking chamber 28a, 28b is provided with a micro switch 40a, 40b to sense when it is properly positioned on the connector 30a, 30b. The micro switches 40a, 40b provide feedback signals that may be used to control one or more components of the steam generation system. For example, when it is sensed by a micro switch 40a, 40b that a particular cooking chamber 28a, 28b has been removed then the manifold 24 may be controlled to shut off the steam supply to the relevant steam pipe 26a, 26b. A signal from one of the micro switches 40a, 40b may also be used to control the water flow controller 37 and vary the flow of water into the boiler 18 so as to regulate the generation of steam depending on the number of cooking chambers 28a, 28b that are connected. Similarly, a signal from one of the micro switches 40a, 40b may also be used to control the pump 16 so as to change the amount of steam being produced. When it is sensed by the micro switches 40a, 40b that all of the cooking chambers 28a, 28b have been removed then a feedback signal may be used to disconnect the power supply to the pump 16 and/or to the heating element 20. The physical connection between the cooking chambers 28a, 28b and the base unit 2 is seen most clearly from Fig. 5. A bayonet-type fitting 70 is provided. The steam connector 30a, 30b is provided in the centre of the fitting 70. The micro switch 40a 40b is off-set from the centre and senses when a cooking chamber 28a, 28b is fitted. The base of each cooking chamber 28a, 28b is provided with four spaced lugs that match notches 72 provided on the bayonet fitting. Between the notches 72 there are angled threads 74. To connect a cooking chamber 28a, 28b, a user places it down with the lugs aligned with the notches 72. The cooking chamber 28a, 28b is then twisted to engage along the threads 74. As the threads 74 are angled, the cooking chamber 28a, 28b moves down onto the base 2 as the connection is tightened. The valve in the steam connector 30a, 30b is arranged to be pushed open as the cooking chamber 28a, 28b moves down onto the base 2. Once the steam chamber 28a, 28b is fitted it also presses down on the micro switch 40a, 40b and its connection to the appliance is registered.

It will be understood that the bayonet fitting 70 provides a two-stage connection and

disconnection. As a cooking chamber 28a, 28b is twisted off, it moves up along the threads 74 and first activates the shut-off valve in the steam connector 30a, 30b. Thus the supply of (possibly pressurised) steam is shut off before the cooking chamber 28a, 28b is fully

disconnected. The cooking chamber 28a, 28b is then twisted further until the lugs reach the notches 72 and the chamber 28a, 28b can then be lifted clear of the base 2. The chamber can be safely removed without any forceful ejection under steam pressure.

Fig. 6 shows one of the cooking chambers 28b connected to the appliance base 2. It can be seen from this Figure that a pressure relief valve 80 is provided in the base 82b of the cooking chamber 28b. The pressure relief valve 80 is in the form of a silicone rubber grommet valve. The outer periphery of the valve comprises a pair of annular flanges 84a, 84b which define a channel between them to sealingly receive the edge of a circular aperture in the chamber base 82b in the manner of a conventional grommet. The inner part of the valve 80 is closed by a domed diaphragm 86. The valve 80 is fitted so that the concave face of the diaphragm 86 is on the outside, the convex face being on the cooking chamber side.

The diaphragm 86 has two orthogonal slits through its thickness defining four quadrant flaps which under normal conditions are held together by the inherent resilience of the silicone formed into the dome shape. This therefore provides a fluid-tight seal while food is being cooked by steam in the chamber 28b. If, during cooking, the pressure in the chamber should increase above a first design level, typically ~ 0.5 bar above atmospheric pressure, then the four diaphragm flaps are forced to reverse their curvature and are further forced apart to provide a mechanism for relieving the excess pressure in the cooking chamber 28b and safely exhausting steam onto the base of the appliance (and thus away from the user).

The valve 80 also performs an additional function. The cooking chamber is pressurised by steam during use. However after the steam supply has ceased and the steam in the chamber has cooled and condensed, a vacuum is generated. With a pressure differential across the valve 80 (atmospheric pressure on the outside and a relative vacuum on the cooking chamber side), the four flaps are forced apart against their natural resilience, thereby allowing air from outside to enter the chamber and release the vacuum. This makes it easier to remove the cooking chamber 28b even once it has started to cool down. The valve 80 may also be configured to provide a second stage of pressure safety protection. As mentioned above, the diaphragm 86 is supported in an aperture in a cooking chamber wall or lid by flanges 84a, 84b arranged to secure the diaphragm 86 in the aperture. The flanges 84a, 84b may be made from the same deformable but resilient material as the diaphragm and are configured to deform at second, higher, pressure level than the diaphragm. Upon reaching this second, higher pressure both the diaphragm valve 86 and the supporting flanges 84a, 84b may be ejected from their aperture so as to release the steam pressure, by way of a safefty back-up.

One example of a bi-directional pressure relief valve 80' is seen in Fig. 7. As is described above, the valve 80' is in the form of a silicone rubber grommet valve with the inner part of the valve 80' closed by a domed diaphragm 86'. However in the example shown in Fig. 19 the diaphragm 86' has a single slit across its centre so that it can flex open when the pressure increases above a threshold. This valve 80' has the additional feature of a manual actuator button 87 that enables a user to operate the valve 80' on demand. For example, a user can equalize an internal vacuum formed upon cooling to atmospheric pressure to make it easier to open the cooking chamber 28b.

The valve 80' is configured with means to override the minimum opening pressure required for it to operate, namely the button 87 is provided with a projection or spur with which a user can push against the diaphragm 86' to force it open. As the concave surface of the diaphragm 86' is recessed into the annular flanges of the valve 80' it can not be accessed by a finger. Using the button 87, the projection provides a convex surface to act upon the concave surface of the diaphragm 86'. The button 87 is hinged such that when actuated its projection aligns itself against the concave surface of the diaphragm 86'. The actuator 87 is arranged to spring away from the diaphragm 86' when not in use, so that it does not impede the normal pressure regulating function of the valve 80' and the flow of exiting steam and condensates.

The actuator 87 may be a separate component, however it is preferably an integral part of the valve 80', as shown, so that it can be moulded from the same resilient elastomeric polymer. The properties of such a material may allow the actuator 87 to spring away from the diaphragm 86' automatically when not required. Additionally, if the actuator button 87 manufactured from the same material as the valve 80' then the risk of damage from abrasion as the two parts interact can be minimised.

Turning back to Figs. 1 -5, it can be seen that the cooking chambers 28a, 28b are fitted onto a removable tray 42 that covers the base unit 2. The tray 42 catches any spilled foodstuff and can conveniently be removed for cleaning purposes. Further, the tray 42 collects condensate from any steam released through the pressure relief valves 80 provided in the base of each cooking chamber 28a, 28b. This condensate includes flavour from the food being steam cooked and this juice can be collected to make stock or gravy. The cooking chambers 28a, 28b are each provided with removable lids 44a, 44b, seen in Figs. 3 and 6. The lids 44a, 44b may be sealingly connected to the cooking chambers 28a, 28b. As the chambers 28, 28b are only lightly pressurised during cooking (around 0.5 bar above atmospheric) the lids 44a, 44b can be removed without requiring a de-pressurising interlock and easily replaced, e.g. after inspecting or stirring food in a chamber.

Each cooking chamber 28a, 28b contains a food supporting member, for example a planar member 60 in one of the chambers 28a and a conical member 62 in the other chamber 28b. The food support members 60, 62 are positioned in the base of each chamber 28a, 28b and over the steam connector 30a, 30b. They are removable for ease of cleaning. The food support members 60, 62 are perforated to allow steam to pass through into the rest of the chamber. The shape and size of the food support member 60, 62 can influence the steam contact area of the food. The conical member 62 provides a larger contact area and helps to provide an even distribution of steam into the food.

The steam cooking vessel 4 may also be provided with an overall lid 46, seen in Fig. 1 , to help keep the cooking chambers 28a, 28b insulated by a layer of trapped vapour in the vessel 4. The appliance lid 46 may also be provided with a steam pressure relief valve (not shown). The vessel 4 may itself be used as a steam cooking chamber, for example by removing the two smaller chambers 28a, 28b to create a single large cooking space. A single food support member (not shown) may be fitted onto the steam connectors 30a, 30b, e.g. to hold a large piece of food such as a whole fish.

The boiler 18 is shown in more detail in Figs. 8-1 1 . The boiler 18 comprises a cast non-ferrous, e.g. aluminium, main body 48 which has a conical interior chamber 50 encircled by the embedded sheathed heating element 20. A water inlet 56 allows water to be pumped into the boiler 18 at the coned end of the chamber 50. The ends of the heating element 20 are each provided with a projecting metal connector 21 , known in the art as a "cold tail", to enable electrical connection to be made to the element 20. The element 20 is approximately helical so that it wraps around the conical cavity formed by the body member 48. This ensures an even heat distribution across the tapering wall of the conical chamber 50. It can be seen from Fig. 1 1 that the water inlet 56 introduces water into the tapered bottom end of the conical chamber 50. The inner surface of the conical chamber 50 forms a heated evaporation surface at which steam vapour is produced. As the conical evaporation space has an increasing cross-sectional area in a direction away from the water inlet 56 and towards the steam outlet 22, steam can expand as it is generated and moves up to the outlet 22. When water first enters the chamber 50 it occupies a relatively small volume in the nose of the cone and is rapidly heated. The boiler therefore provides a quick start-up time for steam generation. The conical interior chamber 50 may have a finely ridged surface. The interior surface of the chamber 50 may be treated or coated, e.g. with a zeolite coating, to render it hydrophilic thereby reducing or avoiding the Leidenfrost effect.

A cover member 54 is provided, secured to the main body member 48 in order to close the chamber 50 in a pressure-tight manner. A suitably heat-resistant seal 53 is provided. A centrally disposed outlet in the cover member 54 is connected to the outlet pipe 22 to allow steam to exit the chamber 50. A mesh member 52 may be provided beneath the cover member 54 to prevent particulates from exiting the boiler 18 with the steam. It can be seen from Fig. 8 that in addition to the temperature regulator 19 there is also provided a thermal fuse 58, in a suitably arranged recess in the aluminium body 48, which can operate to permanently disconnect electrical power to the heating element 20 in the event of serious overheating - e.g. if the temperature regulator 19 should fail. Operation of the steam cooker will now be described. When a user is ready to cook food in one or more of the cooking chambers 28a, 28b, the appliance is turned on and power is supplied to the boiler 18 so that the heating element 20 is energised and the steam generating chamber 50 is heated up. Once it is detected that the boiler 18 has reached a predetermined minimum operating temperature, e.g. 120-160 °C, power is connected to the pump 16 and water is pumped from the reservoir 12, through the filter 14 and into the boiler 18. As the boiler 18 has been pre-heated, water is instantly evaporated in the conical chamber 50 to generate steam. The steam is directed through the steam outlet 22 to the manifold 24 and then supplied, as required, to the cooking chambers 28a, 28b. The regulator 19 operates to maintain the temperature of the boiler 18 within a predetermined range by switching the element 20 off when a maximum operating temperature is reached - typically Ι δΟ 'Ό or greater - and back on when the boiler 18 has cooled to a lower threshold temperature - typically 120 ^qC. The heating element 20 can continue to cycle in this manner indefinitely. The temperature range maintained is higher than the reset temperature of the normally-open regulator described above so that the pump runs continuously after the delay when the appliance is initially switched on.

The continuously operating pump 16 will continue to deliver water into the boiler 18, thereby producing steam for as long as required for the cooking process (or until the water reservoir 12 is emptied). The pressure provided by the pump 16 is matched to the maximum rate at which the boiler 18 can produce steam.

While the appliance is in use, the water pressure relief valve 36 or the steam pressure relief valve 38 may operate to divert fluid back to the reservoir 12. Depending on which of the cooking chambers 28a, 28b are connected, and the steam requirements of the chambers 28a, 28b (which may be set by a user or may follow a cooking program), the steam supply may be regulated by controlling the manifold 24, the water flow controller 37 and/or the pump 16. Feedback signals from the micro switches 40a, 40b may be used to determine when cooking chambers are connected and disconnected.

There is shown in Figs. 12 and 13 some alternative embodiments of a steam cooking appliance according to the present invention. It can be seen that in Fig. 12 the appliance 100 comprises a base 102 on top which there is supported a steam cooking vessel 104. However, instead of separate cooking chambers being provided side-by-side, in this embodiment a number of steam cooking chambers 128a, 128b, 128c are stacked one on top of another, similar to the steamer baskets in a conventional steam cooker. Steam is supplied to the chambers 128a-c in the steam cooking vessel 104 by a steam guiding head 190 positioned in the base of the vessel 104.

Lightly pressurised steam, e.g. ~ 0.5 bar above atmospheric pressure, may be supplied by the head 190. If the steam vessel 104 is sealed then the cooking chambers 128a-c may be pressurised. However it is also envisaged that the steam vessel 104 may be vented to the atmosphere and thus the cooking chambers 128a-c will only experience a very small degree of pressurisation. However, such an appliance can still enjoy the benefits of improved steam cooking times because steam generation can be much quicker, and with the steam supplied at increased output rates of 60-75 mg/min, as compared to traditional steam cookers.

The steam generation system shares many common features with that described above, and thus only those features that differ will be described. A water reservoir 1 12, filter unit 1 14 and electrical pump 1 16 are also connected in series to supply water to a boiler unit 1 18. In this embodiment a pressure relief valve 136 and a flow control valve 137 are arranged in parallel in the supply line between the pump 1 16 and the boiler 1 18. The pressure relief valve 136 is part of a bypass path that allows water and/or steam to be vented instead of reaching the boiler 1 18. The flow control valve 137 is a pressure-compensating valve used to supply the boiler 1 18 with a uniform water flow rate regardless of fluctuations in pump pressure. Downstream of the boiler 1 18 a steam supply line 122 leads up to the steam cooking vessel 104 and is terminated by the steam guiding head 190, which will be described in more detail below.

A collection tray 142 is provide underneath the cooking baskets 128a-c, into which condensates can drip. In this embodiment the collection tray 142 feeds into a dedicated container 143 which may be removable, for example so that the collected cooking juices can be used to make gravy. The pressure relief valve 136 may also direct bypass flow into the collection tray 142 and/or collection container 143. ln Fig. 13 it can be seen that the steam cooking appliance 200 comprises a base unit 202 that is arranged substantially laterally of the steam cooking vessel 204 rather than underneath it. Such a design can enable the appliance to have a reduced height, especially where there are multiple steam cooking baskets 228a-c stacked one on top of another in the steam cooking vessel 204. In this embodiment water may flow under gravity from a reservoir 212 to a pump 216 before being pumped to a water boiler unit 218. As before, a pressure relief valve 236 and a flow control valve 237 are provided between the pump 216 and the boiler 218. Steam is supplied by a pipe 222 from the boiler 218 to a steam guiding head 290. A condensate collection tray 242, which may be removable, is provided beneath the cooking baskets 228a-c. The pressure relief valve 236 can directs any backflow of steam and/or bypass flow of water into the collection tray 242.

The steam guiding head 190, 290 shown in Figs. 12 and 13 may take any suitable form.

Typically it comprises a plurality of steam vents arranged to distribute steam across the footprint of the steam cooking vessel 128a-c, 228a-c. One of its functions can be to control the pressure and velocity of the steam before it is released into the cooking vessel 104, 204. If pressurised steam is being generated but it is desired for the steam to be close to atmospheric pressure when it enters the cooking vessel 104, 204, then the steam guiding head 190, 290 can act as a diffuser, allowing the steam to expand and slow down before being vented out.

An important function of the steam guiding head 190, 290 can be to trap and collect particles of scale that are formed in the steam generation system, for example in the steam supply line 122, 222, and to prevent them from reaching the cooking vessel 104, 204. In order to achieve this the steam guiding head 190, 290 comprises a flow path arranged such that steam passing therethrough is caused to substantially change its direction of travel at least once before being released through the vents. This change of direction results in any entrained scale particles being deposited inside the steam head 190, 290 while the steam vapour escapes.

Some possible designs for the steam guiding head 190, 290 are shown in Figs. 14-18. In Fig. 13 the steam head 390 is generally mushroom-shaped and comprises a hollow inlet stem 392 connected to the base part 394 of a diffuser cavity. The diffuser cavity is defined between the base part 394 and an upper part 396 that connects onto the base part 394 using three mounting bosses 397. The diffuser cavity is formed between the base part 394 and the upper part 396 when they are connected together. Vents 398 are provided in the upper part 396, both in the form of holes in the top and slots around the periphery. As can be seen from the flow path shown in Fig. 15, steam that passes up through the inlet 392 is forced to change its direction to spread horizontally in the diffuser cavity before escaping out through the vents 398 in the upper part 396.

Fig. 16 shows an alternative embodiment of a base part 494 for another steam head. In this design the base part 494 is provided with guide channels 495 that radiate out from the inlet stem 492 to control the flow of steam. Fig. 17 shows another alternative embodiment of a base part 594 for another steam head. In this design the base part 594 is provided with a racetrack channel 595 that guides the flow of steam on a spiral path outwardly from the inlet stem 592. It can be seen from the general representation of a steam flow path shown in Fig. 18 that the flow through the steam head may be tortuous, requiring several changes of direction both laterally and longitudinally before the vapour is able to escape through the vents. At each stage of the tortuous path there will be a tendency for any solid particle carried by the steam to impinge on the surfaces inside the steam guiding head and stick there. By removably mounting the base part 394, 494, 594 to the upper part of the steam head (e.g. using bosses 397) the head can be opened up to allow the scale collected therein to be cleaned out from time to time. It will be appreciated that many different designs may be implemented to provide a steam head with the desired steam control and scale-trapping functions.

Operation of a steam cooking appliance comprising a steam guiding head e.g. as described with respect to Figs. 12-18 is essentially the same as described above. Once the steam cooking vessel 104, 204 is in position the appliance is turned on and power is supplied to the pump 1 16, 216 and the boiler 1 18, 218. Water is pumped into the boiler 1 18, 218 to generate steam for as long as there is a demand, for example as set by a user or a selected cooking program. The generated steam is led by the supply pipe 122, 222 into the steam guiding head 190, 290. The inlet stem of the steam guiding head 190, 290 may simply connect onto the end of the supply pipe 122, 222 so that steam passes straight into the steam head 190, 290. However a valve connector may also be provided, for example to prevent steam from being emitted when a steam head 190, 290 is not connected. Although a steam flow control is not shown, this could be provided to regulate the flow rate in the supply pipe 122, 222.

Steam emitted from the steam head 190, 290 will rise up through the cooking vessel 104, 204 as colder air falls down. Condensed steam will also drip down to the base of the vessel 104, 204, where it can drip through apertures (for a vessel vented to atmosphere) or pass through a valve (for a pressurised vessel) to be captured in the tray 142, 242.

Figure 19 schematically shows some of the different steam flow arrangements possible for a steam cooking vessel 604. In Figs. 19a-19c the cooking vessel 604 comprises one or more cooking chambers 628, e.g. a single chamber or several stacked on top of one another. A flow valve 680 is provided in the base of the vessel 604 and a vent means 688 is provided in an upper part of the vessel 604. The flow valve 680 may be a bi-directional valve with a manual actuator, as is shown in Fig. 7.

In Fig. 19a steam is supplied into the cooking vessel 604 through a connector 630 without using a steam guiding head. Steam emitted through the connector 630 is free to rise up through the vessel 604, but may not be evenly distributed as it will be influenced by the circulation of cooler air moving down. While the nozzle-like flow of steam out of the connector 630 may be useful for targeted heating of some foodstuffs, it may be less desirable where it is desirable to distribute steam across the area of a cooking vessel.

Fig. 19b shows an alternative arrangement wherein a steam guiding head 690 is used to supply steam into a lower part of the cooking vessel 604. The pattern of vents provided by the steam head 690 can act to distribute steam evenly across the area of the cooking vessel 604 before it flows up through the vessel 604. This may achieve more even heating of food in the vessel 604.

Fig. 19c shows another alternative arrangement wherein a steam guiding head 690 is used to supply steam into an upper part of the cooking vessel 604. Again, the pattern of vents provided by the steam head 690 can act to distribute steam evenly across the area of the cooking vessel 604 before it flows down through the vessel 604. In this example the only way for steam to exit is through the flow valve 680 in the base. Advantageously the flow of steam is in the same direction as the flow of condensates and both can be released together through the flow valve 680.

In Fig. 19 the steam cooking vessel 604 may or may not be pressurised. If the cooking vessel 604 is operated at substantially atmospheric pressure then the upper vent means 688 may be permanently open so the chamber(s) is/are vented to the atmosphere. In this situation the lower flow valve 680 may act to release condensates.

On the other hand, if the cooking vessel 604 is operated at an elevated pressure, e.g. up to 0.5 bar above atmospheric pressure, then the upper vent means 688 may be in the form of a pressure regulating valve. This valve 688 may be configured so as only to operate in response to a predetermined over-pressure condition. The lower flow valve 680 may also be in the form of a pressure regulating valve, configured to operate at a different pressure to the upper valve 688 so that there is provided a double level of safety. The lower flow valve 680 can also have the function of releasing condensates from the cooking chamber 628, and may operate to release fluids at two different pressure thresholds. The flow valve 680 can also be a bidirectional valve, configured to allow air to enter from the atmosphere if the pressure drops below a certain threshold, e.g. due to a partial vacuum being formed due to cooling. Such a valve can act to equalise the pressure in the cooking vessel after the end of a steam cooking operation, making it easier to open the vessel. If the flow valve 680 is provided with a manual actuator, for example as shown in Fig. 7, then a user can operate the actuator to clear the valve if it becomes blocked by food debris or the like. It will be appreciated that various modifications may be made to the embodiments described above. For example, although the cooking chambers are shown as having a pressure relief valve located in the base of the chamber, the valve could instead be provided in a side wall or in the lid. The appliance may be provided with any number of cooking chambers, which may have different shapes, sizes and volumes to accommodate different food stuffs. Multiple cooking chambers can be stacked one on top of another in a cooking vessel to share a common supply of steam, or separate chambers may be provided that each have an independent connection to the steam supply. An appliance could be provided with either or both types of arrangement of the cooking chambers. To increase the flexibility of use of an appliance, various of the components described above, e.g. one or more steam connectors, steam guiding heads, steam supply manifolds, and the various steam cooking vessels, may be provided together with a steam generating base unit in a kit of parts that allows a manufacturer or user to configure the steam cooker to their requirements.

Claims

1 . A steam cooking appliance comprising a reservoir for water, a water boiler for generating steam, means for transferring water from the reservoir to the boiler, and means for supplying steam generated by the boiler to one or more steam cooking chambers.

2. A steam cooking appliance as claimed in claim 1 , wherein the means for supplying steam generated by the boiler comprises a steam guide member comprising a plurality of steam vents and a flow path arranged such that steam guided along the flow path is caused to substantially change its direction of flow at least once before being released out of the steam vents.

3. A steam cooking appliance as claimed in claim 2, wherein the steam supply means is terminated by the steam guide member.

4. A steam cooking appliance as claimed in claim 2 or 3, wherein the steam guide member is removable.

5. A steam cooking appliance as claimed in any of claims 2 to 4, wherein the steam guide member provides a tortuous flow path.

6. A steam cooking appliance as claimed in any of claims 2 to 5, wherein the flow path in the steam guide member is arranged to increase the overall flow area and reduce the steam velocity.

7. A steam cooking appliance as claimed in any of claims 2 to 5, wherein the flow path in the steam guide member is arranged to reduce the overall flow area and increase the steam velocity.

8. A steam cooking appliance as claimed in any preceding claim, wherein the or each cooking chamber comprises means in the base thereof for releasing fluid.

9. A steam cooking appliance as claimed in any preceding claim, wherein the means for supplying steam is arranged to provide steam into an upper part of the or a cooking chamber.

10. A steam cooking appliance as claimed in any preceding claim, wherein the cooking chamber(s) are vented to the atmosphere.

1 1 . A steam cooking appliance as claimed in any preceding claim, wherein the water boiler generates steam at a pressure higher than atmospheric pressure.

12. A steam cooking appliance comprising a water boiler for generating steam at a temperature above the atmospheric boiling point and means for supplying the steam generated by the boiler to one or more steam cooking chambers.

13. A steam cooking appliance as claimed in claim 1 1 or 12, wherein the cooking chamber(s) are configured to be pressurised by the supply of steam.

14. A steam cooking appliance as claimed in claim 15, wherein the cooking chamber(s) can be substantially sealed from the atmosphere.

15. A steam cooking appliance as claimed in claim 13 or 14, wherein the or each cooking chamber is provided with a pressure relief valve.

16. A steam cooking appliance as claimed in claim 13, 14 or 15, wherein the pressure relief valve comprises a bi-directional valve configured to open at a lower pressure differential in one direction than the other.

17. A steam cooking appliance as claimed in claim 16, wherein the bi-directional pressure relief valve comprises a domed resilient diaphragm having at least one slit defined therein.

18. A steam cooking appliance as claimed in any of claims 15 to 17, wherein the pressure relief valve is configured to operate at a first predetermined pressure to vent the or each cooking chamber to the atmosphere.

19. A steam cooking appliance as claimed in claim 18, wherein the pressure relief valve is configured to rupture or be ejected from the or each cooking chamber at a second, higher, predetermined pressure.

20. A steam cooking appliance as claimed in any of claims 15 to 19, wherein the pressure relief valve is positioned in the base of the or each cooking chamber.

21 . A steam cooking appliance as claimed in any of claims 15 to 20, wherein the pressure relief valve is arranged to exhaust condensates from the or each cooking chamber.

22. A steam cooking appliance as claimed in any of claims 15 to 21 , wherein the pressure relief valve comprises manual actuating means enabling a user to operate the valve to vent the or each cooking chamber to the atmosphere on demand.

23. A steam cooking appliance as claimed in any of claims 13 to 22, wherein the cooking chamber(s) are configured to maintain a pressure of at least 0.1 , 0.2, 0.3, 0.4 or 0.5 bar above atmospheric pressure, and preferably a pressure of around 0.5 bar (50 kPa) above atmospheric pressure.

24. A steam cooking appliance as claimed in any preceding claim, comprising multiple cooking chambers supplied with steam from the boiler.

25. A steam cooking appliance as claimed in claim 24, wherein the cooking chambers share a common connection to the steam supply means.

26. A steam cooking appliance as claimed in claim 24, wherein the cooking chambers are separate and each has its own connection to the steam supply means.

27. A steam cooking appliance comprising a water boiler for generating steam and means for supplying the steam generated by the boiler to a plurality of separate steam cooking chambers.

28. A steam cooking appliance as claimed in claim 27, wherein each of the cooking chambers is provided with an independent connection to the steam supply means.

29. A steam cooking appliance as claimed in any of claims 24 to 28, wherein a or each of the cooking chambers is provided with a connector for connecting to the steam supply means, the connector comprising a flow control valve.

30. A steam cooking appliance as claimed in claim 29, wherein the cooking chambers are removable from the appliance.

31 . A steam cooking appliance comprising means for generating steam and means for supplying steam to one or more removable steam cooking chambers, wherein a steam connector comprising a flow control valve is provided to connect the or each removable cooking chamber to the steam supply means.

32. A steam cooking appliance as claimed in claim 29, 30 or 31 , wherein the flow control valve is arranged to shut off the connection to the steam supply means when the or each cooking chamber is removed.

33. A steam cooking appliance as claimed in any of claims 29 to 32, wherein the connector provides for two stages of disconnection from the steam supply means, such that connection to the steam supply means is shut off in a first stage before the or each cooking chamber can be removed in a second stage.

34. A steam cooking appliance as claimed in any preceding claim, wherein the steam supply means comprises a steam flow control means provided in the steam path between the water boiler and the cooking chamber(s).

35. A steam cooking appliance as claimed in claim 34, wherein the or each cooking chamber is provided with a sensor that provides a signal to the steam flow control means.

36. A steam cooking appliance as claimed in claim 34 or 35, wherein the steam flow control means is arranged to adjust the pressure of the steam supplied to a respective cooking chamber.

37. A steam cooking appliance as claimed in any preceding claim, wherein the water boiler comprises a water inlet, an electric heater, a steam outlet and an evaporation space bounded by at least one surface in thermal contact with the heater, wherein the evaporation space is configured to present an expanding cross-sectional area in a direction away from the water inlet.

38. A steam cooking appliance as claimed in claim 37, wherein the evaporation space is defined by a conical water boiling chamber.

39. A steam cooking appliance as claimed in any preceding claim, further comprising water filter means arranged between the reservoir and the water boiler.

40. A steam cooking appliance as claimed in any preceding claim, wherein the means for transferring water from the reservoir comprises an electric pump arranged to provide water to the boiler at a pressure in the range of 0.5 to 4.0 bar, more preferably 1 .0 to 3.0 bar or 1 .0 to 2.0 bar.

41 . A steam cooking appliance as claimed in claim 40, wherein the flow path from the pump to the water boiler includes a pressure-compensating constant flow valve arranged so that water is supplied to the boiler at a uniform flow rate regardless of its pressure.

42. A domestic steam cooking appliance comprising a reservoir for water, a closed water boiler for generating steam in an evaporation chamber having a volume less than 10% of the volume of the reservoir, and an electric pump for transferring water from the reservoir to the boiler on demand, the appliance further comprising a steam supply means connected at one end to an outlet of the water boiler to receive steam and connected at the other end to a steam guide member, the steam guide member acting to control the velocity of steam therethrough and to distribute the steam supply between a plurality of vents arranged to exhaust steam into one or more steam cooking chambers.