Evaporation With Electrostatics

March 21, 2001
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Figure 1. Schematic drawing of the experimental apparatus for evaluating the Asakawa effect.


Figure 2. Comparison of the water evaporation capability of electrically assisted ecaporation and natural evaporation.
Getting water all charged up for better drying gets Stuart A. Hoenig, P.E., Ph.D., all charged up as well. A professor in the Department of Electrical and Computer Engineering at the University of Arizona, Tucson, AZ, Hoenig has been interested in “the effect of electrostatic fields on liquids, particularly water, for some years,” he says.

Once when he was in Japan, he met with a Professor Asakawa who “had done some very interesting work with evaporation of water,” says Hoenig, pioneering the use of electrostatics for drying about 30 years ago. Because relations between corporations and universities don’t exist in Japan, Asakawa’s work was never implemented.

The Asakawa system is illustrated in Figure 1. Power applied to the mesh screen creates ions that charge the water molecules in the air and push them away from the liquid surface. Drops of water at the surface can also be charged, releasing them up and away into the air.

Applying electrostatics removes water quickly and effectively, states Hoenig. With thermal evaporation, for example, “many of the water molecules that escape from the liquid go right back in. With the Asakawa system, the molecules acquire a charge that repels them from the liquid water.”

Figure 2 shows a comparison of electrically assisted evaporation vs. natural evaporation from Asakawa’s data.

In drying operations, Hoenig notes, using normal methods, there are dissolved particles that often are left behind. By using an electrostatic system, water molecules will be charged and removed so that the majority of the dissolved particles are eliminated.

In Hoenig’s own work on the effect of electrostatics on the drying of fabric, applying the electrostatic field did produce a significant advantage in drying time. For example, with voltage off (no electrical charge), drying of cotton toweling took about 47 min. With voltage on (electrical charge applied), drying time was reduced to approximately 31 min.

In fact, in his experiments, Hoenig asserts, with proper lighting you can actually see water droplets “leaping off the surface.”

The experimental humidity controller consists of metal rods with needles soldered on at the inlet end and a metal screen at the outlet end. Water molecules are normally electrically neutral, but the molecule is positive at one end and negative at the other, says Hoenig. In an electrostatic field produced by holding the metal rods at -15 kV and a metal screen at ground, the water molecules in the air will move to the needles.

“It is interesting to note that the negative ions produced by the needles have been shown to destroy bacteria and cut down on dust,” he states.

In work done by the U.S. Department of Agriculture (USDA), electrostatic charge systems used in commercial chicken houses were shown to remove airborne dust, thereby reducing the potential for transmitting salmonella and other bacteria. The effectiveness of the electrostatic system in removing dust was similar to that of a 95% media filter, but without the problem of plugging in this heavy dust application.

Experiments also show that the electrostatic charge has a sterilizing effect that kills salmonella.

An interesting sidelight is that research has shown that negative ions can have a biological impact which can be beneficial to people.

In removing water vapor by cooling, Hoenig notes that filtering and bacteria control are important, and “condensers are wet and warm in spots and an ideal area for bacteria to grow.” He suggests that the combination of an electrostatic system with an air conditioner might be the best way to go.

Working on a way to build a commercial system, Hoenig is currently talking to a manufacturer about producing his electrostatic device. For all the years he’s put into it, he’d certainly get a charge out of seeing it in action.

For more information, Hoenig can be reached at 520-887-3815; 520-887-9727 (fax); hoenig@ece.arizona.edu (e-mail).

Publication date: 03/26/2001

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