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How do electrostatic air cleaners work?

Discussion in 'Electronic Design' started by mike, Feb 7, 2012.

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  1. mike

    mike Guest

    There seems to be some ambiguity in terms, so, for the purpose
    of this discussion...

    An ELECTRONIC air filter uses kilovolts to create an electric
    field that charges particles so they get attracted to a surface.

    An ELECTROSTATIC air filter is a passive device that generates
    the electric field by passing air across some dielectric medium.
    The advertising is full of vague marketing speak with dimensionless
    graphs and no real statements as to how they work for pollen particles.

    Most pleated furnace filters have some electrostatic features.
    For some vendors, it's their primary focus. The Filtrete line
    mentions almost as a footnote.

    I can't imagine much voltage being generated, so I set about to
    measure it. I inserted electrodes under the outer polyethylene
    (I think it's poly) mesh on either side and blasted air thru it.
    I measured zero volts.
    Ok, maybe not much current.
    I put a .1uF high voltage cap across the electrodes and let it charge
    for a while. I touched a scope probe to the cap with a trigger
    level of 1V. I expected to see the peak voltage and the decay
    of the .1uF and the 10M scope input.

    I saw zero.

    Is my measurement technique faulty?
    Am I measuring the wrong thing?
    Or are electrostatic filters made of snake oil?

    Thanks, mike
  2. Your brain is faulty if you are trying to take a voltage reading from

    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.
  3. mike

    mike Guest

    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
    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

  4. Tim Williams

    Tim Williams Guest

    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.

  5. gregz

    gregz Guest

    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.

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