Dyson's quick on the dry

Drying your hands in a public washroom isn’t something you’re likely to give much thought to. You wash your hands, you look at the roller towel and decide not to bother, press the button on the hand dryer and rub your hands underneath the hot air for a few seconds, realise it’s never going to dry your hands, and probably wipe them on your trousers.

For James Dyson, however, a minor inconvenience represents an opportunity. ‘James came down and said that he was frustrated with hand-dryers,’ said senior design engineer Marcus Hartley. Hand-drying not only can be done better, Dyson reasoned, it should also be done better. There are definite advantages to doing it better; for health, for the operator of the hand-dryer, and for the environment in general. And there was also an opportunity for Dyson’s trademark of repurposing industrial technology for a completely different environment.

The result of Dyson’s washroom disappointment is the Airblade, a new hand-drying system for commercial and public washrooms. Representing Dyson’s first foray away from the domestic electricals market, the Airblade is a scaled-down, optimised version of an industrial air knife, a well-established production technology for drying or blowing loose material away from surfaces.

Current hand-dryers are far from effective, Dyson argued. Despite using considerable energy to heat the air and operate the fan, they don’t heat the residual water on the hands enough to dry them effectively, even after 30 seconds of drying. Moreover, the air blasted out of the dryer is drawn directly from the washroom, and is laden with bacteria; heating it up is hardly sanitary.

The Airblade uses two thin sheets of unheated fast-moving air as a gaseous ‘squeegee’, wiping the water off the hands. You put your wet hands into the gap at the top of the dryer, between two slots — a straight one, facing the palms of the hands, and a double-curved one, facing the backs of the hands. This triggers an infrared sensor which activates the air jets from both slots, with a flow-rate of 180m/sec. As you pull your hands up past the slots, the airflow wipes the water off the skin and into the machine. The whole process lasts 10 seconds and does indeed leave the skin feeling much dryer than a conventional blower.

It’s also more hygienic, Dyson claims. With a conventional dryer, you rub your hands together under the airstream, which can bring up bacteria from lower skin layers. With the Airblade, the hands stay separate, so the skin is undamaged. Moreover, the air is cleaned by passing through an H13 HEPA-grade filter before it is blasted out through the slots which, according to Dyson, removes 99.97 per cent of its bacterial load. The water removed from the hands runs into the machine, where it passes through an iodine resin micro-filter, which kills the bacteria washed off the hands, and then on to a piezoelectric atomiser, which converts the water into a fine spray and disperses it into the washroom.

The Airblade has a 1.6kW motor, rather than the 2.4kW motor generally found in hand-dryers. This, combined with the shorter drying time and the lack of a heating element, means an Airblade consumes around a fifth of the energy of a conventional dryer, Dyson claims. This may make it an attractive prospect for operators of washrooms.

The X20 digital motor is the central component in the Airblade. It is a brushless switched-reluctance unit, which was originally developed for a compact vacuum cleaner for the Japanese market. The motor was redesigned only slightly for the Airblade to reduce noise, said Frederic Nicholas, senior fluid dynamics engineer. Running at 100,000rpm, compared with 45,000rpm for most switched-reluctance motors, it has a very high ratio of outlet to inlet pressure. ‘The performance comes from speed of the rotation and the design of the impeller,’ said Nicholas. ‘It raises the pressure higher than any other AC motor could.’

Air pressure generated by the motor is translated into air speed by the width of the slots through which the air is pushed. Determining the correct width was a matter of trial and error, Hartley said; the wider the gap, the longer the drying time.

While researching the Airblade concept, and sitting in public washrooms with a stopwatch, the team found most people would not spend longer than 15 to 17 seconds using a hand-drying machine. To develop the prototypes, the team experimented with different gap widths, using shims to vary them. ‘As you open the gap, you get more mass flow but lower velocity,’ said Nicholas.

After experiments with computational fluid dynamics (CFD) models of the airflow, the team settled for 0.3mm as the optimum width for the straight blade, and a varying width for the curved blade, 0.4mm at the sides, and 0.7mm in the middle. ‘We needed to keep flow as close to the hands as possible, which dictated the shape of the blades, but thumbs are further away from the blade than the edge of the hand, so we needed more mass flow in the middle, which meant a wider gap,’ said Hartley.