Patent Description:
Hand dryers are often installed in public washrooms as an alternative to paper towels.

There are three main types of hand dryer on the market: "warm-air" hand dryers, "high speed" hand dryers and "air-knife" hand dryers.

Warm air hand dryers are very well known. They are invariably low flow, low speed machines which rely on heating the air to promote an evaporative drying effect at the surface of the hand. Examples include the Model-A Series of hand dryers manufactured and marketed by World Dryer Corporation, which is illustrated in <FIG>. The heated airflow is typically discharged through a single nozzle a and the drying action is a "hand-over-hand" action, requiring the user to rub the hands together under the nozzle a with the aim of encouraging the evaporative drying effect.

High speed hand dryers, as the name suggests, use high speed airflow (><NUM>/s) to provide a momentum-drying effect at the surface of the hands. Examples include the Xlerator® hand dryer manufactured and marketed by Excel Dryer Inc. and the Airforce manufactured by World Dryer Corporation. Again the airflow is typically discharged through a single relatively large nozzle - or a cluster of smaller nozzles - and the mode of use is somewhat similar to the "hand-over-hand" action of the warm air dryer, with the hands being held or cupped together underneath the nozzle to dry them. However, instead of being evaporated, the majority of the water on the surface of the hands is instead driven or blasted from the hands by the high-momentum airflow. The airflow tends not to be heated, though waste heat from the motor may in some cases be used to heat the airflow to a degree.

The third general type of hand dryer is the air-knife hand dryer, examples of which include the Dyson Airblade range of hand dryers manufactured by Dyson (UK) Limited and the Jet Towel range of hand dryers manufactured by Mitsubishi Electric Corporation.

Air-knife hand dryers use an "air-knife" - effectively a sheet or curtain of fast-moving air - which moves across the surface of the hand and, as it does so, removes water mechanically by scraping or stripping the water from the surface of the hand.

In certain models of Dyson Airblade hand dryer - and also in the Mitsubishi Jet Towel range of hand dryers - two opposing, stationary air-knives are used: one for each side of the user's hand. The hands are inserted between the air-knives and then withdrawn slowly to effect the required relative movement between the hands and the air knives.

In the Mitsubishi arrangement - shown in <FIG> - the air-knife is discharged through opposing rows of individual discharge apertures (only the rear row b is visible in <FIG>): here, the individual jets combine to produce the air knife downstream of the discharge apertures.

In the Dyson arrangement - which is described in UK Patent No. <CIT> (hereafter, <CIT>) and reproduced in <FIG> - the air knives are instead discharged through narrow, continuous slots (only the rear slot c is visible in <FIG>), each less than <NUM> wide. The slots are ergonomically designed to mirror the anatomic shape of the user's hands. Thus, the front slot is straight to reflect the relatively flat palms of the user's hands and the rear slot c incorporates a pair of concave portions intended to conform better to the slightly-rounded back of the user's hand.

Later models of Dyson Airblade hand dryer - one of which is described in European Patent No. <CIT> (hereafter, <CIT>) and reproduced in <FIG> - provide a single-sided air-knife drying action and the user dries the hands one side at a time. Again, the air-knives (one for each hand) are discharged each through a narrow, continuous slot e less than <NUM> wide.

Document <CIT> discloses a hand dryer comprising a compressed-air container connected via a pipe to two annular chambers which are flattened out to form a hand drying space. The inside of each annular chamber has a ring of air outlet nozzles aligned inclined to the axis of the ring away from the side of the access opening for the hands. Any water remaining on the skin is driven in drops to the finger tips where it is mechanically spun off.

The present invention seeks to provide an improved air-knife hand dryer, in particular one having an improved air-knife outlet design.

According to the present invention, as defined in claim <NUM>, there is provided a hand dryer, the hand dryer comprising a nozzle, the nozzle comprising an elongate air-knife discharge outlet for discharging an air-knife across the width of a user's hand held flat in front of the outlet, thereby mechanically to scrape water from the hand as the hand is moved lengthwise relative to the air-knife, the nozzle comprising a straight part for facing the palm or back of the user's hand, the nozzle comprising a curved wrap-around end part for extending at least part way around the side of the user's hand, the elongate air-knife discharge outlet extending along the straight part of the nozzle and around the curved end part of the nozzle.

The geometry of the air-knife discharge outlet in accordance with the invention has been found to create a flow profile which targets the side of the hand with a relatively high-velocity jet of air. This high-velocity jet of air - which arises from the interaction of the air exiting the straight part with the air exiting the curved end part - is particularly effective for drying the area around the thumb of a user, which has often been a problem-area in prior art air-knife hand dryers: even those such as the one described in <CIT> which utilise ergonomically-designed nozzles.

The nozzle may comprise two curved wrap-around end parts: one at each end of the straight part of the nozzle. In this arrangement, the discharge outlet may extend around both the curved end parts of the nozzle. This provides better drying of both sides of the user's hand rather than just one preferential side. In any event, the overall length of the nozzle, measured between the curved ends, is preferably between <NUM> and <NUM> for encouraging the user to hold the fingers of the hand closer together rather than splayed open. Thus, the wrap-around curved ends of the outlet - and the length of the straight section - combine to encourage a more closed hand position, which has the benefit of further reducing dry-time.

The radius of curvature of the curved end(s) is in the range <NUM> - <NUM>.

The angle subtended by the curved end(s) is preferably in the range <NUM> - <NUM> degrees.

According to the invention, two such air-knife outlets are provided, one for each hand. The air-knife outlets are arranged in a V-configuration when viewed from the front of the hand dryer.

The elongate discharge outlet may be a continuous slot less than <NUM> in width.

<FIG> shows a wall-mountable hand dryer <NUM>, here illustrated in its normal wall-mounted orientation. <FIG> shows the principal interior components of the hand dryer <NUM>, in highly schematic form viewed from the front of the hand dryer.

The basic configuration of the hand dryer <NUM> is similar to the hand dryer described in <CIT>. The hand dryer <NUM> comprises a main casing <NUM>, which houses a ducted fan <NUM>. A motor <NUM> is provided inside the main casing <NUM> to drive the fan <NUM>, which draws air through the intakes <NUM> on either side of the main casing <NUM> (only one intake <NUM> is visible in <FIG>) and forces the air at high speed ((><NUM>/s) out through two nozzles : a left-hand nozzle <NUM> on the left-hand side of the dryer and a right-hand nozzle <NUM> on the right-hand side of the dryer.

Each nozzle comprises a respective discharge outlet <NUM>, <NUM> positioned on an underside of the dryer <NUM>. The discharge outlets <NUM>, <NUM> run along a front lower edge of the main casing <NUM> so that they are spaced from the wall.

The discharge outlets <NUM>, <NUM> each comprise a continuous slot - less than <NUM> in width - arranged so that it runs generally plane-parallel with the wall. Because the discharge outlets <NUM>, <NUM> are very thin the drying air is discharged at high speed in the form of a thin sheet or curtain of air: a so-called "air-knife".

In use, the hands are inserted lengthwise front-to-back underneath the discharge outlets <NUM>, <NUM> and the air-knives are directed down onto the hands to 'scrape' water from the hands as they are subsequently withdrawn underneath the discharge outlets <NUM>, <NUM>.

The hands are dried one side at a time, in similar manner to the hand dryer in <CIT>: first, the user passes his (or her) hands forth and back underneath the discharge outlets <NUM>, <NUM> with the palm facing up towards the discharge outlets (the "standard pass"). Then - after turning over the hands - the user passes his (or her) hands forth and back underneath the discharge outlets <NUM>, <NUM> with the back of the hands facing up towards the discharge outlets (the "reverse pass"). The "standard pass" and "reverse pass" may each be repeated, as required, and carried out in any order.

A conventional sensor arrangement (not shown) can be used to turn on the motor <NUM> in response to the detection of a user's hands. The same sensor arrangement may be used subsequently to turn the motor <NUM> off in response to a null detection, or else the motor <NUM> may be operated on a timer. Use of a sensor arrangement is not essential: the dryer <NUM> may alternatively be arranged for manual operation.

The air-knife discharge outlets <NUM>, <NUM> are arranged in a V-configuration when viewed from the front of the dryer. This is intended to make the dryer more comfortable to use by allowing the user to bank his hands accordingly during both the standard pass and the reverse pass - similar to the manner described in <CIT>.

The hand dryer differs from the one described in <CIT> in relation to the geometric profile of the air-knife discharge outlets <NUM>, <NUM>.

In the hand dryer described in <CIT> the discharge outlets are straight. In the hand dryer <NUM>, however, the discharge outlets <NUM>, <NUM> are not straight. Instead, the nozzles <NUM>, <NUM> each comprise a straight part 10a, 12a and two curved, wrap-around end parts 10b, 12b and the discharge outlets <NUM>, <NUM> likewise comprise a straight part 11a, 13a - extending along the straight part of the respective nozzle <NUM>, <NUM> - and two curved, wrap-around end parts 11b, 13b: which extend around the respective curved end parts of the nozzles <NUM>, <NUM>.

The straight sections 11a, 13a of the discharge outlets <NUM>, <NUM> are each arranged for facing the palm or back of the user's respective hand (in this case, depending upon whether the user is engaging in a 'standard pass' or a 'reverse pass'). The curved, wrap-around end sections 11b, 13b are arranged for extending at least part way around a respective side of the hand.

The geometry of each of the air-knife discharge outlets <NUM>, <NUM> creates a flow profile which targets the sides of the hand with a relatively high-velocity jet of air. This is illustrated in <FIG> and <FIG>.

The images shown in <FIG> and <FIG> were each created using an oil-based smoke machine to blow smoke through the discharge outlet. The discharge outlet was located in a dark room and the air and smoke exiting the slot was illuminated using a halogen bulb. The image was captured using a conventional digital camera.

The image in <FIG> is of a discharge outlet <NUM>. It shows two distinct high-velocity jets of air - labelled A and B: one associated with each of the two curved ends 11b of the outlet <NUM>. These localised, high-velocity regions converge at a central region C in front of the discharge outlet.

By way of comparison, the image in <FIG> is of a conventional curved nozzle which is shaped anatomically to follow the back of a user's hand: such as might be found on a conventional hand dryer like the one described in <CIT>. Here, there are no pronounced, localised high-velocity regions within the airflow; instead, the airflow velocity is uniform and the airflow converges at a single focal point, D determined by the radius of curvature of the nozzle.

It is thought that the high-velocity jets of air observed in <FIG> are created at least in part by the local interaction of airflows exiting different parts of the discharge outlet in the region of the curved ends.

In practice, the interaction is complex: but it can be visualised in simplified terms as the vector addition of component airflows.

<FIG> show the same (single) curved end 11b of the discharge outlet <NUM>. In <FIG>, a first airflow A<NUM> is shown exiting the curved end 11b. This first airflow A<NUM> interacts with a second airflow B<NUM> exiting the straight middle section in the region of the curved end. The two airflows A<NUM> and B<NUM> combine to create a higher-speed resultant airflow C<NUM>, where C<NUM> = (A<NUM> <NUM> + B<NUM> <NUM> + 2A<NUM>B<NUM>Cosθ<NUM>)<NUM>/<NUM>. In <FIG>, an airflow A<NUM> is shown exiting a first part of the curved end. This airflow then interacts with an airflow B<NUM> exiting a second part of the same curved end. Again, the two airflows A<NUM> and B<NUM> combine to create a higher-speed, resultant airflow, C<NUM>. In <FIG>, two airflows A<NUM> and A<NUM> are shown exiting different parts of the curved end, which combine to produce an airflow A<NUM>+<NUM>. The airflow A<NUM>+a then interacts with an airflow B<NUM> exiting the straight middle section of the slot to create a higher-speed resultant airflow C<NUM>. The net result of each of these interactions is to create a resultant, relatively high-speed, localised region of airflow which is directed in towards the sides of the user's hand <NUM>, as shown in <FIG>. This represents the high-velocity jet of air A observed in <FIG>, which targets the sides of the user's hands.

By contrast, in the region of the straight section of the discharge outlet, away from the curved ends, the airflow D is discharged uniformly straight and square-on to the user's hand <NUM> and there is no significant interaction between adjacent regions of the airflow D in the manner illustrated in <FIG>. Consequently, the flow profile in this region is generally uniform and the local air speeds are generally lower than in the high velocity jets of air targeting the sides of the hand. In effect, the geometry of the discharge outlet <NUM> creates localised high-velocity airflow regions only where they are required in order to provide an increased drying effect, not across the entire discharge outlet <NUM>.

<FIG> shows a comparative illustration of a curved air-knife nozzle such as might be found on a conventional hand dryer like the one described in <CIT>. Here, the airflows E<NUM>. En converge to a 'focal point' behind the user's hand <NUM> when the user's hand <NUM> is placed next to the nozzle. Consequently, there is no interaction between regions of the air-knife in the manner illustrated in <FIG> before the airflow strikes the hand, and thus no focused drying effect on the sides of the hands. This explains why there is no high-velocity localised jet of air observed in <FIG>. Instead, the airflow speed around the nozzle is generally uniform at the point of impact across the surface of the user's hand.

The velocity of the airflow directed on to the sides of the hands in discharge outlet shown in <FIG> could obviously be increased by increasing the pressure behind the slot, but this would necessarily increase the airflow velocities along the entire slot, which would come at an energy cost. The geometry of the discharge outlet in <FIG> does not provide for the type of flow profile incorporating high-velocity, localised jets of air targeting the sides of the hand in accordance with the invention.

The flow profile of the discharge slot in accordance with the invention is dependent upon both the radius of curvature - or 'focal length'- of the curved end and the angle subtended by the curved end. This is illustrated in <FIG>.

<FIG> illustrate the effect of focal length. In <FIG> the focal length F<NUM> is relatively short. In <FIG> the focal length F<NUM> is relatively long. The angle θ subtended by the curved end part is generally the same in each case. These Figures illustrate then that, for a given angle θ , the focal length determines the minimum operational distance X between the surface of the user's hand and the discharge outlet. In simple terms, the distance X will increase as the focal length F increases.

For very long focal lengths - and consequently a large operational distance X - there may be a broadband reduction in the effectiveness of the air-knife to dry the user's hand: the air-knife will diffuse over distance. Nonetheless, the same essential flow profile characteristics will remain in accordance with the invention as they do for short focal lengths: there will still be a relatively high-velocity airflow targeting the side of the hand and a relatively low-velocity airflow targeting the face of the user's hand.

A preferred radius of curvature/focal length is considered to be in the range <NUM> - <NUM>, preferably <NUM> - <NUM>.

<FIG> illustrate the effect of the angle θ subtended by the curved end. In <FIG> the angle θ<NUM> is relatively large. In <FIG> the angle θ<NUM> is relatively small. The focal length of the curved end is generally the same in each case. These figures illustrate then that, for a given focal length, F, the angle subtended by the curved end 11b determines the angle of attack β of the high velocity jet of air. A larger angle will produce a shallower angle of attack.

A preferred angle θ is considered to be in the range <NUM> - <NUM>°.

The curved end sections 11b, 13b extend at least part way around the respective sides of the user's hand in use: one around the side of the hand where the thumb is located and the other around the opposite side of the hand. The overall length of the nozzle - being the distance Y between the curved ends 11b, 13b (see <FIG>) - has been found to have an effect on the way in which the user holds his or her hand in use. Thus, if the overall length of the nozzle is controlled so that it is in the range <NUM> to <NUM> then it has been found that the user is encouraged to hold his or her hands in a more closed position - with the fingers together rather than splayed open - and that this promotes a more efficient drying action.

Although the invention is described in the context of a one-sided air-knife dryer, the invention is applicable also to two-sided air-knife hand dryers such as the one described in UK Patent No. <CIT>. In this context, the discharge outlet may be used on one side only (and thus in combination with a conventional nozzle on the opposite side) or, preferably, on both sides.

Claim 1:
A wall-mountable hand dryer (<NUM>), the hand dryer (<NUM>) comprising two nozzles (<NUM>, <NUM>), one for each hand, each nozzle comprising:
- an elongate air-knife discharge outlet (<NUM>, <NUM>) positioned along a front lower edge of the hand dryer (<NUM>) so that they are spaced from a wall when mounted to a wall, and for discharging an air-knife across the width of a user's hand held flat in front of the discharge outlet, thereby mechanically scraping water from the hand as the hand is moved lengthwise relative to the air-knife in use;
- a single straight part (10a, 12a) for facing the palm or back of the user's hand; and,
- a curved wrap-around end part (10b, 12b) for extending at least part way around the side of the user's hand and having a radius of curvature in the range <NUM> - <NUM>;
wherein the elongate air-knife discharge outlet (<NUM>, <NUM>) extends along the single straight part and around the curved wrap-around end part of each nozzle; and,
wherein the elongate air-knife discharge outlets (<NUM>, <NUM>) are arranged in a V-shaped configuration, when viewed from the front of the hand dryer (<NUM>) when the hand dryer (<NUM>) is orientated for mounting to a wall.