How do electrostatic air cleaners work?

Your brain is faulty if you are trying to take a voltage reading from an INSULATOR!

Meters read generated, constant source values, not electrostatics.

Elesctrostatic cleaners use about 6kV typically.

Your electronics training was snake oil.

Two plates. Charge them. They attract each other, and morph the charge of particles within the field between them such that they get attracted to one plate or the other. Pretty simple.

Snake oil. You can easily make an electret, i.e. a dielectric with a frozen-in electric polarization. The poled PVDF film used in billions of microphones and front porch lights is the most common example, but there are lots of others: triglycine sulphate, lithium tantalate, lead zirconate-titanate (PZT), lead lanthanum zirconate-titanate (PLZT) and deuterated triglycine fluoroberylate have all been used fairly widely for piezo- and pyroelectric elements, and there are built-up examples such as PN junctions and floating-gate EPROMs.

All of these work by having a static dielectric polarization inside them with no applied voltage--so far, so good.

_However_, you can't maintain an E field in air that way. It's just like rubbing a balloon on your hair: the triboelectric charge produces an external E field large enough to stick the balloon to the ceiling--for a little while. Then the charge gets neutralized by attracting air ions, conduction via adsorbed water, and also slightly by charged dust. The neutralization process occurs until there's zero net charge on the balloon, at least if we neglect any triboelectric source term due to air friction. (I don't know if this is significant in air filters. It is for raindrops.)

Similarly, an electret attracts free charge to its surfaces until the external E field is cancelled out. But there's more to it than that.

In the electrostatic limit,(*) the voltage between any two points is just the line integral of _E_ dot _ds_ along any path joining them. Thus if the external field is cancelled out, so is the internal field, because otherwise you'd get a different answer for the voltage depending on whether the path of integration went through the material or around the outside, which in electrostatics is impossible.

If you think about the PN junction case, this becomes pretty obvious--even in electrostatics, you can't make a kilovolt just by soldering 800 diodes in series. External charge produces an overall slope that cancels out the effects of all the series-connected bandgaps, and makes all the Fermi levels line up at the terminals.

Cheers

Phil Hobbs

(*) If there's any dB/dt happening, curl E is nonzero, so the integral of _E_ dot _ds_ along a closed path is also nonzero in general. That's how transformers work, after all.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Thanks for the inputs. The stuff on Deutsch-Anderson equation was interesting, but way overkill for what I need and didn't address my concern.

I can restate my question as follows. In a passive electrostatic furnace air filter, (one without any applied power to generate an electric field) where (physical location in the material stack) is the field and how is it generated?

The filter stack is as follows: Metal screen plastic mesh

1/4" grey filter-looking stuff 1/4" blue filter-looking stuff plastic mesh Metal screen.

The two metal screens are electrically connected by the aluminum frame.

There are filters with similar "electrostatic" claims that have plastic frames and no metal screens.

I had it in my head that the electric field was generated by the triboelectric effect of the air flowing thru the plastic mesh. I expected to find kilovolts/cm as in an active electronic air filter. I didn't find it.

Another theory was that the field was permanently "trapped" in the material by solidifying it in a strong electric field to align the polar molecules.

Because of the small physical size of the molecules, it's not hard to imagine localized fields of kilovolts/cm, but the effect would be very localized.

I did some experiments with an incense stick smoke particle source. Placed the filter horizontal and put the incense stick under it. Tried a simple fiberglass filter, the passive electrostatic filter, and a Filtrete allergen filter with "electrostatic properties". In all cases, most of the smoke passed thru the filters.

I was too lazy to take the electronic filter out of the furnace at 4AM and don't want the house to smell like incense.

I did place an air ionizer under the incense smoke generator. In that case, virtually all the smoke was trapped in the simplest fiberglass filter.

I'm unable to experiment with the triboelectric effect. My visual detection of the smoke plume doesn't work with high air flow.

I need a simple particle counter. I'll start another thread on that subject.

Thanks, mike

Talked with a sales rep today, who said he once did a job for a power plant. Their precipitators are large, of course: on the order of 50kV and 5A. They turn them up until corona just starts giving way to sparks, then back them off a few percent. He also said, before the days of low-sulfur coal and all that, the precipitators would consume a small percentage of the total plant power output. That's a *lot* of fly ash.

Tim

-- Deep Friar: a very philosophical monk. Website:

I think some, many, or most plastic tapes use a charge to hold the sticky on. You can even make a electrostatic speaker using two plates, front and back of the tape. It's not very efficient though.

I to have experimented with the various white filters. Believe me, it's starts looking brown in a smokers house. It will also pick up radon particles, and after a couple days It will show elevated radioactivity. I don't know how they work, but the material can be washed with something like greased lightning, and reused, and seems to work just as well.

I have used ion generators around the furnace thinking it would help pick up particles with the filter, and anything else.

The filters with the finer mesh do restrict airflow more than those you can start to see through.

Greg