Ultrasonic Drying – Not Yet but Possible???

There has been a lot of buzz lately on the internet regarding work at the Oak Ridge National Laboratory to develop a dryer that uses ultrasonics instead of heat to dry things.  The major thrust seems to be to replace the conventional domestic clothes dryer (which uses heat to evaporate water) with one that uses ultrasonics to atomize instead of vaporize water to dry clothes.  Claims include drastically reduced energy consumption and shorter drying times.  As a result, there has also been some buzz about using the same idea (ultrasonics) to dry parts after ultrasonic cleaning.  The logic being that the ultrasonic transducers are already there, why not put them to double use? But, whoa, wait a minute, let’s not get carried away.

What Oak Ridge has been doing is drying tiny pieces of fabric less than 1″ square by actually contacting them with ultrasonic transducers.  In the videos I’ve been able to find on line, a small piece of wet, saturated fabric (cut round to fit perfectly into an indentation in the transducer) is placed directly in contact with the output face of a high intensity ultrasonic transducer.  When the ultrasonic energy is activated an impressive cloud of water vapor is generated as the liquid is atomized by the ultrasonic vibrations.  The same thing, of course, would happen if there was just water and not fabric in contact with the ultrasonic transducer (just like an ultrasonic vaporizer).  After about 15 seconds, production of the atomized water vapor cloud stops and the fabric is declared dry.  Maybe, but maybe not.  Atomization of water will only continue as long as there is a liquid conductive path between the transducer and the wet fabric.  Although there is a possibility that airborne ultrasonic energy in such close proximity might be sufficient to continue to atomize liquid until the fabric is totally dry, I believe the odds are against it.  In the videos, no measure of the actual dryness was offered.  I would, at least, like to see the fabric pressed against a blotter.

Can this technology be scaled to practicality for drying a typical load of laundry in a domestic setting?  In my opinion it is highly doubtful.  The claimed energy savings is due to the fact that the liquid is only being broken into small droplets and does not actually undergo the energy-hungry phase change required for evaporation.  But you still have to prevent the atomized liquid from re-depositing onto the fabric.  How do they propose doing this?

There is a lot to think about here. So, let’s quickly look at what would be required to make ultrasonic drying applicable to drying parts cleaned in an industrial parts cleaner.  There are two possibilities that I can think of.  The first would be to provide a sufficiently powerful ultrasonic sound field in the air surrounding the part to actually atomize the liquid on the part surface.  The only means of delivering this energy would be through air and air, for all practical purposes, does not conduct ultrasonic energy.  The amount of power required to deliver substantial enough power would be huge! Maybe it would work on a small scale where the power requirement would be minimal, but certainly not anything larger than a few millimeters in size.  The other possibility would be to vibrate the part ultrasonically (much like a dog shaking off water after a swim).  This, of course would require contacting the part with an ultrasonic transducer and establishing a good enough conductive path to vibrate the part at the ultrasonic frequency.  Although this is not quite what Oak Ridge is doing, it is similar in that they are contacting and vibrating the liquid directly instead of the fabric.  Again, it may be possible to work this out on a small scale or under specialized conditions but I don’t see drying a 50 pound basket of steel parts using this concept. For the time being, I think the manufacturers of industrial parts cleaning equipment would do well to concentrate on other methods of improving drying technology.  I have offered several seeds for thought in the blog and invite you to look them over again if you haven’t already.  Meanwhile, I don’t think ultrasonic parts drying is in our future – at least the immediate future.

–  FJF  –