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.

The Airblade generates two thin sheets of fast-moving air to act as a gaseous ‘squeegee’ and is claimed to dry within 10 seconds

The sanitary aspects of the Airblade were always part of the concept, he added. ‘Filtering and disinfection were on the back burner,’ he said. ‘We knew we wanted to collect the water and not throw it over the floor, and we thought about collecting it in a tray. But that would leave it not disinfected, so we started looking for a technology that could help us with that. We didn’t find anything until we started looking at industrial applications of water sterilisation, and we found the iodine resin cartridge, which is quite commonly used in water purification systems in the developing world.’ Once the water has passed through the iodine resin, it’s clean enough to drink, according to Hartley.

The team then had to work out how to dispose of the water. They wanted an automatic system, and turned to technology from humidifiers. ‘We looked at venturi systems to blow air out under pressure, but we would have needed a second pump; and we looked at heating elements, but that would have given us problems with limescale.’

The system they chose uses a small chamber with a piezoelectric disc at the base. ‘The nebuliser part was the biggest control problem for me; it needs to have standing water in it to work, and you can’t run it while somebody is actually drying their hands,’ said electronic design manager Mark Heywood. The team had to determine a method for indicating when there was enough water inside the chamber to operate the system — ‘another method we worked out by experimentation’, said Heywood.

When the chamber is full, a current passes through the disc, causing it to vibrate. ‘There are two frequencies which will vaporise water, and we chose the lower one, 1.6MHz,’ said Heywood. ‘When the vibration starts, a cone forms on top of the water, and it vaporises at the top of the cone.’

The design raised several manufacturing issues, said Hartley. The tight tolerances on the width of the blade slots — accurate to within 0.05mm along their length — meant they had to be made from material that could be formed precisely. ‘We worked with a firm in Singapore which can achieve zero shrinkage rates with its plastics; it makes parts for the digital motor,’ said Hartley. ‘They provided us with a thermoset polymer compound, which also means that it can’t be damaged if someone tries to set fire to it.’

The need for vandalism-proof materials was a major consideration. ‘These things have to go into pub toilets,’ Hartley said. ‘And you know what sort of things they’ll have to put up with.’

The outer shell of the Airblade is therefore made from aluminium, formed in a deep-draw process — the first time Dyson has used large die-cast metal parts. ‘The machine that makes the main panel is a huge, 20-tonne thing,’ said Hartley. ‘There were only two places in the world that could provide them, Germany and China. We’re using the facilities in China.’

The product has attracted strong interest from firms including hotel chains, shopping centres and service station operators, said Hartley.
Dyson is also involved in tests with hospitals. ‘They use paper towels currently, and we’re on a par with those for hygiene. But they have to incinerate the waste towels, which is a huge cost; the Airblade would have an advantage there,’ said Hartley.